Milky Way May Have Dark Matter Satellite Galaxies

rubycodez writes "Berkeley astronomer Sukanya Chakrabarti has detected perturbations in the gases surrounding our Milky Way and concludes there is a satellite 'Galaxy X' 250,000 light years away that is mostly dark matter, but that may contain dwarf stars visible in infrared. She expects many more such dark matter satellites to the Milky Way to be discovered using her technique."

Comments

This sounds like a sci-fi blockbuster

[roc97007]

"The creature from invisible Galaxy X"

Re:This sounds like a sci-fi blockbuster

[Black Parrot]

"The creature from invisible Galaxy X"

There was an interesting musing by the author of a recent Scientific American about how dark matter may interact with its own kind by forces other than the ones that cause normal matter to interact with its own kind. According to the musing (which the author rejects), dark matter operating under such forces could form complex systems, maybe even an unseen parallel universe where "people" live lives like ours, as unaware of us as we are of them. All undetectable, except by their gravitational attraction on us.

Re:This sounds like a sci-fi blockbuster

[Anonymous Coward]

Glad to see the photino birds making some progress.

Re:This sounds like a sci-fi blockbuster

[ron_ivi]

It'd be weird if one of those dark planets zoomed through our solar system closely enough to have it's gravity affect the earth.

Imagine one zipping by and touching one of our oceans, and a bunch of the water'll & a few people, etc might be sucked up into that dark-gravity-source.

Re:This sounds like a sci-fi blockbuster

[Anonymous Coward]

You mean like in Hideaway by Alastair Reynolds?

Re:This sounds like a sci-fi blockbuster

[Anonymous Coward]

I don't know about you, but I'm rooting for the Xeelee.

Re:This sounds like a sci-fi blockbuster

[jbeaupre]

parallel universe where "people" live lives like ours, as unaware of us as we are of them. All undetectable, except by their gravitational attraction on us.

Are you talking about women? I've actually seen them. No joke!

Re:This sounds like a sci-fi blockbuster

[lgw]

I'm pretty sure that's not possible, because dark matter around our galaxy forms a sphere, not a disk. You get a disk when there's friction between the particles -they start clumping, averaging out angular velocity until most of the mass lies in a disk perpindicular to the overall average axis of rotation. With no friction, and no clumping, everything stays distributed in a sphere.

But that's at current energy levels. Who knows - in the early universe or far future (if there's another state change down to a lower ground state), things could be different. At energy levels far from our own we have almost no data on how dark matter would behave (we have a bit of data about dark matter just before the universe became transparant, but that's aboutit).

Re:This sounds like a sci-fi blockbuster

[ShakaUVM]

There was an interesting musing by the author of a recent Scientific American about how dark matter may interact with its own kind by forces other than the ones that cause normal matter to interact with its own kind. According to the musing (which the author rejects), dark matter operating under such forces could form complex systems, maybe even an unseen parallel universe where "people" live lives like ours, as unaware of us as we are of them. All undetectable, except by their gravitational attraction on us.

This is something I've been musing on recently. What if there were, say, 4 or 5 universes all operating in the same "space", but invisible to each other because they're in another parallel dimension or 3-brane or something like that? According to string theory, pretty much nothing can escape from a 3-brane except gravity - which might explain why gravity is so weak, as it could diffuse in more directions than EM radiation. Since EM radiation can't escape, these things would be effectively invisible, as it appears to be.

It could also explain that Dark Flow thing. (http://en.wikipedia.org/wiki/Dark_flow)

Re:This sounds like a sci-fi blockbuster

[khayman80]

Last year Tanuki64 asked a similar question, and I referred him to an anecdote from my senior year of physics undergrad. In 2004, I presented a similar idea to an astrophysicist in my department:

I wonder if "dark matter" is the result of gravitational interactions with galaxies in parallel universes. Suppose parallel universes exist in the same physical "space" we inhabit, and only interact with each other (and us) via gravity. The galaxies in different universes would then clump together, but their disks wouldn't necessarily be aligned. So the total gravity would appear similar to a spherical halo of dark matter. This would explain the too-high velocities of stars at the edges of galaxies and the too-high velocities of galaxies in superclusters.

In 2004, I didn't have enough experience to understand why that astrophysicist rejected my idea by saying that I was trying to explain something we don't understand by invoking something else that we understand even less. Several years later, though, I started to see some cracks in this idea:

2009-07-25 Update: I don't think my hypothesis is consistent with the Bullet cluster data.

2009-07-27 Update: Also, I wonder if galaxies in my imaginary parallel universes really would clump together. They'd certainly be gravitationally attracted to each other, but if each universe has roughly the same density of galaxies, they'd typically have a long way to fall towards each other. As a result, they'd be moving so fast that I doubt any damping mechanisms could have brought them to rest in ~13.7 billion years. But... what if they formed in the same place initially? That would make sense because supermassive black holes likely play a large role in proto-galaxy formation. Gravitational collapse in one universe would trigger collapses in other universes leading to galaxies with small relative velocities. But in that case, it seems like the disks would be aligned because disk formation probably doesn't involve a large percentage of actual physical collisions (any actual astronomers want to help me here?). I think this would result in the wrong velocity profile for stars versus distance from the center of the galaxy? Oh, and all these stars in different universes would cause gravitational lensing events to occur with a much greater frequency than has been observed by the OGLE. Galaxies with non-aligned disks would look even weirder- that implies imply lensing with bizarre relative velocities.

It could also explain that Dark Flow thing.

Perhaps. But given the above problems with such an idea, it's more likely that the Great Attractor is simply a massive supercluster in our own universe, even if it's already passed over the horizon.

Re:This sounds like a sci-fi blockbuster

[ShakaUVM]

Interesting, Khayman. Why do you say it's inconsistent with the Bullet Cluster data? It seems to me that if you had two clusters on top of each other, one in our 3-brane, and one in a neighboring one, and they collided with a cluster just in our 3-brane, that the result would be more or less consistent with the result of the purported dark matter separating from the normal matter.

Re:This sounds like a sci-fi blockbuster

[khayman80]

First, note that dark matter halos have always been observed around normal matter galaxies. Your example should therefore involve at least four galaxies- two visible colliding galaxies in our own universe, each with a counterpart galaxy that occupies the same physical space in the other universe/brane, which are therefore colliding in that other universe too.

In conventional theory, dark matter interacts with normal matter and with itself only through gravity. These dark matter halos would fly right through each other in a galactic collision, whereas normal matter can't because it also interacts via other forces, most notably the electromagnetic force. That's why the Bullet Cluster's separation of x-ray vs. matter distribution tends to support dark matter rather than MOND.

But if these dark matter halos are simply made of ordinary matter in another universe, they would collide with each other in that universe in the same way that ordinary matter collides in our universe. Thus I don't believe that our hypothesis would result in the same separation of x-ray vs. matter distribution observed in the Bullet Cluster.

Re:This sounds like a sci-fi blockbuster

[khayman80]

Aside from the fact that I've never seen evidence of a galaxy without a dark matter halo, the Bullet Cluster has two opposing lobes of dark matter, indicating that each galaxy in the collision had a dark matter halo that flew right through the other dark matter halo. Another similar collision also has opposing two lobes of dark matter. If a collision is ever observed with only a single lobe of dark matter, that would be consistent with our hypothesis. (Though the other objections I raised would still apply.)

Re:This sounds like a sci-fi blockbuster

[khayman80]

Well, okay, rarely seen a galaxy that might not have dark matter.

Re:This sounds like a sci-fi blockbuster

[khayman80]

Believe this is the relevant paper.

Re:This sounds like a sci-fi blockbuster

[TheTurtlesMoves]

If something with the mass of earth and the gravitational pull of earth (aka dark matter earth) got even within the orbit of the moon, the effects would be of biblical 2012 proportions. We would have our orbit perturbed so much that at best it would take us so close to the sun that life would not survive (aka oceans boiled off into space), or so far away that the sun would be little more than a really bright star at mid day (atmosphere frozen). Hell there is a good chance that we would be flung out into space in short order. The gravitation pull will cause mass flooding via tidal perturbations. And we haven't even considered what would happen to the moon.

Re:This sounds like a sci-fi blockbuster

[khayman80]

As requested.

Re:This sounds like a sci-fi blockbuster

[ShakaUVM]

That's a very good point. Though I suppose if there were lots of parallel dimensions connected by gravity, they could exhibit similar behavior, if the matter was distributed throughout the dimensions so that they couldn't collide, and only interact through gravity.

Re:This sounds like a sci-fi blockbuster

[khayman80]

... I've thought a bit more about that question of if photons of light have inertia. While they do have momentum, I am more convinced now that they don't have inertia. One can define inertia as the resistance of mass to changes in velocity, right? The root cause of this classic Newtonian mechanic is the interaction of objects with the Higgs field, right? That's what grants particles inertia. But photons do not interact with the Higgs field, so they don't have inertia. [ShakaUVM]

I've never taken graduate-level elementary particle physics, so I don't know much about the Higgs field. My classmates who have taken those classes and moved on to work at the LHC tell me that most theorists consider the discovery of the Higgs boson to be very likely.

Personally, I'm not sure how to rule out the notion that inertia is caused by viewing zero point energy in an accelerating reference frame. I'm sure the Higgs field really is more likely to be the cause of inertia, but right now I don't have enough time to wade through the relevant literature to learn why.

Anyway, you're right to say that the "inertial mass" of an object can be measured by placing it in a container and determining how much force is necessary to accelerate the container. Or, rather, how much extra force is necessary compared to experiments performed when the container is empty.

Now imagine a one dimensional container with perfectly reflective inner walls. I claim that if this container is filled with photons having total energy E, then more force would be needed to accelerate the container after filling it. More precisely, the experiment would show that the container has an extra "inertial mass" E/c^2 compared to its empty state.

Here's why.

If the container isn't accelerating, the trapped photons will exert equal pressure on both walls of the container as they're reflected back and forth, just as with solar sails. Accelerating the container, though, will cause the mirror on the bottom to reflect those photons more often than the mirror on the top. Thus on average the bottom mirror will experience more pressure than the top mirror, and this pressure asymmetry will mimic an "inertial mass" of E/c^2.

In fact, I think any method of measuring inertial mass would conclude that photons have inertia. That's because active gravitational mass in general relativity is defined by the stress-energy tensor, which includes the energy (and momentum) in electromagnetic fields. Active and passive gravitational masses need to be equal to conserve momentum, and the equivalence principle says that passive gravitational mass equals inertial mass.

In other words, the container curves spacetime more when it's filled with photons. Therefore its gravitational mass has increased, and via the equivalence principle so has its inertial mass.

This depends on my interpretation of the equivalence principle (and the principle itself) being correct. It also implies that pressure has inertia, because pressure contributes to the stress energy tensor. Interestingly, that implies tension has negative inertia because tension is just negative pressure. Greg Egan uses this concept masterfully in a short

Re:This sounds like a sci-fi blockbuster

[khayman80]

I no longer think the "inertial mass" of a container full of photons will be due to more frequent collisions with the bottom mirror. After further thought, that's not true. But the second argument still seems compelling, so I wonder how the effect would show up in a literal container experiment. Perhaps the bottom mirror would reflect a slightly blue shifted photon, and the top mirror reflects a slightly red shifted photon? This might explain the pressure asymmetry but depends on quantum mechanics to show that the momentum change of reflecting a photon depends on the wavelength. I was hoping to come up with an explanation that depended only on special relativity, but no such luck.

Re:This sounds like a sci-fi blockbuster

[ShakaUVM]

Well, even though breathless science journalists report that the Higgs Field is "source of mass", it's not the only one. As you pointed out, E=MC^2, so any energetic entity has a gravitational mass under relativity. Nuclei of atoms get a significant fraction of their apparent mass from the nuclear binding energy from the strong force.

I think the Higgs Field is better described as the "source of inertia", as opposed to the source of mass, as the mechanism by which it operates is basically what we think of as the Newtonian definition of inertia at normal scales - particles that interact with it are resistant to changes in their velocity. Since photons do not interact with it at all, that's why I was saying that you could argue that photons do not have inertia (on top of the fact that you cannot apply a force to them to change their velocity).

It's really just a semantic argument, though.

Re:This sounds like a sci-fi blockbuster

[khayman80]

Nuclei of atoms get a significant fraction of their apparent mass from the nuclear binding energy from the strong force.

It's true that an individual nucleon (such as a proton or neutron) gets a significant fraction of its mass not from its constituent quarks but from the strong force. For example, a proton's mass is 938 MeV but its two up quarks and single down quark only sum to (at most) 12.4 MeV.

But nuclei of atoms are actually slightly less massive than the individual neutrons and protons that comprise them. I believe this effect has a different sign than the binding of quarks into individual nucleons because quarks can't ever be physically separated. It's also much smaller; iron has the most tightly bound nucleus, and its binding energy is only 8.8 MeV per nucleon. Though vastly larger than chemical energies, this is still less than one percent of its mass.

As you pointed out, E=MC^2, so any energetic entity has a gravitational mass under relativity.

... and thus also has an inertial mass through the equivalence principle. My failure to explain why the container shouldn't exhibit an inertial mass of E/mc^2 doesn't invalidate the equivalence principle. If light didn't exhibit inertial mass in that sense, the container would be a magical fuel tank for a photonic rocket. Most of the constraints of relativistic travel are related to the need to accelerate the reaction mass in the fuel tank. If the photons can simply be stored in a mirrored container and accelerated for free until they're allowed to escape out the back of the rocket, that would represent an unbelievably efficient space drive. And I mean "unbelievably" quite literally here.

I think the Higgs Field is better described as the "source of inertia", as opposed to the source of mass

Photons don't interact with the Higgs, and photons certainly have zero rest mass. Most of the introductory material I've read seems to use that terminology, but again I'm not a particle physicist.

Since photons do not interact with it at all, that's why I was saying that you could argue that photons do not have inertia (on top of the fact that you cannot apply a force to them to change their velocity).

Velocity is a vector, so gravitational lensing and reflection are both examples of changing the velocity of light. Refraction is an example of changing the direction and the speed of light. In all these cases, equal and opposite reactions occur but are simply too small to observe. The sun is gravitationally attracted to a beam of light that it deflects, a solar sail experiences a force, and the material interface that refracts the light really does experience a force as it deflects and slows down each photon.

Re:This sounds like a sci-fi blockbuster

[khayman80]

It turns out nickel is most tightly bound. News to me.

Re:This sounds like a sci-fi blockbuster

[khayman80]

erm, "E/c^2"

Re:This sounds like a sci-fi blockbuster

[khayman80]

Better phrasing (will be cleaned up in final version): My failure to explain how the extra inertial mass of E/c^2 manifests in the container experiment

Jokes

[Stenchwarrior]

Cue the chocolate milk and dark chocolate jokes. I'm too busy, otherwise I'd think of some. C'mon /., don't let us down!

Re:Jokes

[Anonymous Coward]

So... You're a fan of the Hershey Highway, eh?

Re:Jokes

[Stenchwarrior]

Nice!! Keep em coming!

Re:Jokes

[Anonymous Coward]

I don't know about chocolate jokes, but:

Yo' momma so fat, she causes perturbations in the gases surrounding our galaxy!

Re:Jokes

[rubycodez]

I prefer my women like dark matter: attractive, hard to catch, dark, and mysterious.

Re:Jokes

[GNious]

This is slashdot - through empirical data, we can conclude all women are mysterious and hard-to-catch.

Additionally, if these women do exist, we can conclude that they must exert gravitational pull, i.e. being attractive, and due to the limit of light in the universe (aka basement), it can be assume that they will be dark.

Dark matter vs black holes

[Fractal Dice]

How do you tell the difference between a blob of dark matter and a black hole? With all the small galaxies the Milky Way has swallowed over its lifetime, would it not be reasonable to find some relic black holes that have swung back out after being stripped of most of their surrounding gas/stars? Or, when "dark matter" is being talked about in this situation, is a black hole simply one of the possible candidates to supply the mystery mass?

Re:Dark matter vs black holes

[spottedkangaroo]

Blackholes would prob ably get quite bright from time to time and (if nothing else) leave a halo of glowing matter behind. Also, dark matter can be quite spread out and turns into really huge blobs, rather than point masses. I imagine her technique looked for things like that. I don't think the article says anything really useful about the technique. Hopefully they cover it on Naked Astronomy.

Re:Dark matter vs black holes

[Colonel Korn]

How do you tell the difference between a blob of dark matter and a black hole? With all the small galaxies the Milky Way has swallowed over its lifetime, would it not be reasonable to find some relic black holes that have swung back out after being stripped of most of their surrounding gas/stars? Or, when "dark matter" is being talked about in this situation, is a black hole simply one of the possible candidates to supply the mystery mass?

I think we'd expect to see the kind of supermassive black hole that could be mistaken for a dwarf galaxy. The processes that form black holes of that size mean that there would probably still be a lot of material in the vicinity, if not actively accreting then still getting pulled around, compressed, and prompting star formation. Also, I think nearby galactic-sized black holes would probably make for some pretty wicked and obvious gravitational lensing.

Alternatively, the detected mass might be a large number of small black holes. I doubt it, but I'm not an astronomer. Luckily, further observation will give us answers.

Re:Dark matter vs black holes

[Monkeedude1212]

How do you tell the difference between a blob of dark matter and a black hole?

Gravitational pull is probably the biggest factor. A black hole simply gets so massive that at one point the gravitational pull is so strong that not even light can escape. It will have objects orbitting around it like planets orbit stars except at distances far greater than a star would normally hold.

Dark Matter, on the other hand, simply seems to have the gravitational pull of a regular star, but doesn't emit any light.

One thing to note is that when we observe things out there, it's not just a 2D plane we're observing but a great deal of depth is involved. When observing a black hole, the light behind the black hole will get sucked into the black hole if it happens to cross the event horizon. This will create a nice black circle in the sky. However dark matter, on the other hand, would not stop the light behind it from reaching our eyes, it might bend it a little but nothing too extreme.

Re:Dark matter vs black holes

[vlm]

How do you tell the difference between a blob of dark matter and a black hole?

In theory due to hawking radiation etc black holes temperature increases over time as it shinks (weird but true). Both probably live around the cosmic microwave blackbody limit.

A big array of dark matter would be a hair above 2.7 kelvin and tending to thermalize down to 2.7, but a big array of black holes would tend to be a hair above 2.7 kelvin and tend to increase over time.

So basically something cold that tends to stay cold is probably dark matter and something that seems to be warming up more than reasonable is probably a black hole.

Given a long enough time for thermal like diffusion, a mixture of them might behave like stellar formation, and the higher temperature of the black holes would tend to push dark matter away. So in a density map of old empty space, look for a "bulls eye" to find the black holes.

There are some scalability problems here.

Re:Dark matter vs black holes

[Anonymous Coward]

As I understand it "dark matter" is in modern usage is a term for a fairly poorly understood class of stuff that has mass but otherwise doesn't interact with anything (except possibly itself). If they suspected a black hole, they would not have said "dark matter".

There are ways of distinguishing between "dark matter" and reasonably well understood objects like black holes. For one thing black holes generally are surrounded by really bright accretion discs (there are pictures of these), and tend to be a lot of mass in one place. "dark matter" on the other hand tends to be highly diffuse (it's effects are only measurable on a much larger scale than that of a single star), and is not detectable except by it's gravitational effects. This why you hear about "dark matter galaxies" but not "dark matter stars".

One example of the difference between dark matter and "normal" matter is while normal matter tends to be denser at the center of a galaxy, dark matter is not any more common at the center than out near the edge of a galaxy. that observation has led some to speculate that dark matter may self annihilate (like what happens to mater + anti-matter, but dark matter does it with itself rather than with an anti-dark matter).

Re:Dark matter vs black holes

[Locke2005]

A black hole would occlude light from behind it, making it detectable by a circular absense of light as it travels across the sky. Apparently a blob of "dark matter" either does not absorb light or has such a small diameter that it does not detectably occlude light. But if the dark matter was a point source of gravity, wouldn't we still see some gravitational lensing? It appears dark matter would have to be much less dense than a black hole. But then, I'm not astrophysicist.

Re:Dark matter vs black holes

[Black Parrot]

How do you tell the difference between a blob of dark matter and a black hole? With all the small galaxies the Milky Way has swallowed over its lifetime, would it not be reasonable to find some relic black holes that have swung back out after being stripped of most of their surrounding gas/stars? Or, when "dark matter" is being talked about in this situation, is a black hole simply one of the possible candidates to supply the mystery mass?

Good thinking - astronomers thought of it too.

I don't know the details, but they've ruled out black holes, brown dwarfs, neutrinos (except the hypothetical sterile neutrino), and all the other "normal" matter anyone knows about.

Re:Dark matter vs black holes

[Fuseboy]

That doesn't sound quite right - because of the inverse-squared falloff of gravity, once you're a certain distance away, black holes and stars aren't much different in how material orbits around them.

One difference is that that black holes often spew high-energy x-rays as infalling matter is crushed, whereas dark matter is - well - dark.

Re:Dark matter vs black holes

[Black Parrot]

I think we'd expect to see the kind of supermassive black hole that could be mistaken for a dwarf galaxy. The processes that form black holes of that size mean that there would probably still be a lot of material in the vicinity, if not actively accreting then still getting pulled around, compressed, and prompting star formation

Going out on a limb... I would expect dark matter to form "dark black holes" if you brought enough mass of it together in a small enough space.

For that matter, I would expect dark matter to get caught up into "normal" black holes, and normal matter into "dark" black holes, if you brought one kind of matter near the other kind of black hole.

I suspect a physicist would say it's impossible to distinguish a "normal" black hole from a "dark" black hole.

Re:Dark matter vs black holes

[Anonymous Coward]

A black hole would occlude light from behind it, making it detectable by a circular absense of light as it travels across the sky.

Uh, with current technology we can't detect a circular black disk a few km in diameter when it's halfway across the galaxy. When you use the phrase "making it detectable by", what follows should be something feasible.

Re:Dark matter vs black holes

[marcosdumay]

Well, black holes are always dark. No one shines.

Now, about distinguishing a galaxy from a black hole (the original question). On this situation it is possible. Since our galaxy (with gas everywhere) is overlaping the dark galaxy, if it was a black hole some gas would fall near it and make the usual X-ray emmisions.

Re:Dark matter vs black holes

[Tim C]

A black hole simply gets so massive that at one point the gravitational pull is so strong that not even light can escape.

Actually that's a good working definition of a black hole - if its gravity weren't that strong, it wouldn't be one.

It will have objects orbitting around it like planets orbit stars

Yes...

except at distances far greater than a star would normally hold.
 
...and no, not necessarily. That depends entirely upon the mass of the hole. The gravitational field of a black hole at a given distance is no different than that generated by a star of the same mass at the same distance; the difference is that the hole is so much smaller that you can get much closer to its centre. That vastly reduces the r in GM/(r^2), thus increasing the maximum gravity that can be experienced.

When observing a black hole, the light behind the black hole will get sucked into the black hole if it happens to cross the event horizon. This will create a nice black circle in the sky.

The situation is a little more complicated than that thanks to gravitational lensing, but essentially you're correct - a black hole will block light, while dark matter does not.

Re:Dark matter vs black holes

[Locke2005]

You are correct. I was trying to say that dark matter would be distinguishable from a supermassive black hole. But it would still be difficult to tell the difference between dark matter and a large group of small black holes. I too am unclear why the phenomena attributed to "dark matter" could not be adequately explained by a large number of small black holes.

Re:Dark matter vs black holes

[TexVex]

Going out on a limb... I would expect dark matter to form "dark black holes" if you brought enough mass of it together in a small enough space.

I'm no expert, but here's what I think I know:

Dark matter only interacts gravitationally. Dark matter does not collide with matter, nor does it collide with other dark matter. This means it can't form clumps. No dark matter clumps means no dark matter gravity wells, meaning no dark matter black holes.

A galaxy's dark matter, then, is a diffuse cloud of invisible non-interacting particles in orbit around and through the galaxy itself. These particles have gravity of their own, and collectively they have a huge influence on the galaxy's field of gravity.

Now, obviously dark matter particles would be unable to escape a black hole just like anything else. However, the vast majority of dark matter will never interact with a black hole -- it will just orbit forever.

Re:Dark matter vs black holes

[Have Brain Will Rent]

Mmmmm, you are saying that a photon hitting a particle of dark matter simply passes through it with no chance of being absorbed?

Re:Dark matter vs black holes

[c 0xFF]

Say what?

First off, the GPP has a decent question. The largest supermassive black holes are on the order of 10^9 solar masses, about the same mass as what was calculated for this satellite galaxy. So, I suppose it's at least plausible that it's a single black hole, if unlikely.

But remember and repeat after me: a black hole has no more gravity than any other object of the same mass. As long as you stay away from the event horizon, that is. You need to rethink your first paragraph with that in mind.

So, how would we tell the difference? Well, an X-ray source from the same location would be a good clue that it's a black hole, which says that it's feeding off of something. You should also be able to tell from the gravitational lensing -- dark matter is incredibly diffuse compared to a black hole. It would still bend light, but not quite in the same way, especially considering the distances involved.

But what about a black circle in the sky? Well, the even horizon for such a black hole has the same diameter as the orbit of Pluto, if I remember right. Detectable, maybe, under the right conditions (but not by Hubble -- you'd need something with about 20x better resolution ... if I did the math right, which I probably didn't). But we have to capture it overlapping with some other body, such as a background galaxy. By then you'd be better off looking at the lensing effect, anyway. Here is a classic simulation of what I'm talking about.

Re:Dark matter vs black holes

[killkillkill]

Hopefully, further observation will give us answers.

There. Fixed that for you. Scientific method only tests and narrows in on truth, it doesn't guarantee the discovery of it.

Re:Dark matter vs black holes

[c 0xFF]

The calculated mass of this body is about 1% of the Milky Way, or about 7x10^9 solar masses. The event horizon of said black hole would be about 20 billion kilometers.

The actual distance is about 260,000 light years away, or 2.5x10^18 kilometers.

And ... if I did the math right, the circle would be about .00165 arcsecond. Hubble is about .04 arcseconds for comparison. We need a telescope with about 25 times the angular resolution.

Feel free to check my math ... I probably made a mistake in there somewhere.

The point is, this would have to be a MASSIVE black hole ... but might actually be directly observable with future technology if we can manage a couple orders of magnitude improvement in angular resolution.

Re:Dark matter vs black holes

[The_Wilschon]

This is exactly what is meant by the word "dark" in "dark matter". It precisely means that its coupling to photons is zero (except, of course, for higher order loop effects in the Feynman diagrams, but those would be incredibly, perhaps immeasurably, small).

Re:Dark matter vs black holes

[fbjon]

Dark matter is not just dimly lit, it doesn't emit or interact with photons and matter at all, except through gravity (and maybe some other effects). An amount of ordinary matter spread out like a galaxy shines fairly brightly, while the same amount of dark matter would have the same mass/gravity distribution but no EM emissions.

Re:Dark matter vs black holes

[c 0xFF]

To be fair, there's two definitions of dark matter. One is as you said -- matter that has little or no interaction with photons.

The other is baryonic dark matter which is simply normal matter that isn't emitting many photons, so we don't see it. And if we can't see it, we can't include it in our tally of mass.

But, it seems that baryonic dark matter can only account for a small percentage of the total dark matter in the universe, so it's usually ignored.

Re:Dark matter vs black holes

[bcrowell]

How do you tell the difference between a blob of dark matter and a black hole?

Gravitational pull is probably the biggest factor. A black hole simply gets so massive that at one point the gravitational pull is so strong that not even light can escape. It will have objects orbitting around it like planets orbit stars except at distances far greater than a star would normally hold.

Dark Matter, on the other hand, simply seems to have the gravitational pull of a regular star, but doesn't emit any light.

No, this is completely wrong. A black hole doesn't have stronger gravity than the star or stars that it formed out of.

One thing to note is that when we observe things out there, it's not just a 2D plane we're observing but a great deal of depth is involved. When observing a black hole, the light behind the black hole will get sucked into the black hole if it happens to cross the event horizon. This will create a nice black circle in the sky. However dark matter, on the other hand, would not stop the light behind it from reaching our eyes, it might bend it a little but nothing too extreme.

This is also wrong. Gravitational lensing occurs both for black holes and for other objects that aren't black holes. The black hole in the sky that you're describing is not what is predicted for a black hole either.

For anyone who wants to see the actual paper, here it is: http://xxx.lanl.gov/abs/1101.0815

Re:Dark matter vs black holes

[Locke2005]

Black holes by definition have no EM emissions from inside the event horizon. The energy emitted by black holes comes from gas being accelerated outside the event horizon, so a black hole without any surrounding cloud of matter would be much harder to detect. One could only judge it's position by it's effect on light/radio waves coming from behind it.

Re:Dark matter vs black holes

[Anonymous Coward]

Not necessarily. By "dark" it is usually meant matter that doesn't shine. Some of it might be (or some most likely is) baryonic dark matter.

A rogue planet, or a rogue planetary system without a central star would classify as dark matter. That is, matter that is not accounted for when adding up the matter in the visible stars. Yes, a lot of dark matter is believed to be non-baryonic, but not all of it.

A rogue planet, floating free of any galaxies would be (a) bigger than a black hole of the same weight and (b) absorb light and emit harder-to-detect infrared black body radiation. So yes, it would look like a rogue black hole from where we're standing.

Re:Dark matter vs black holes

[Have Brain Will Rent]

Ummm well I hate to rely upon Wikipedia as the cite for an opposing opinion but....

http://en.wikipedia.org/wiki/Dark_matter

Don't stop at the first paragraph or two...

The largest part of dark matter, which does not interact with electromagnetic radiation, is not only "dark" but also, by definition, utterly transparent.

Which implies there is a part that does interact with electromagnetic radiation - photons.

nonbaryonic dark matter includes neutrinos

And of course neutrinos can interact with photons and baryonic matter... just not frequently. I can't remember the number of miles of lead a neutrino can pass through without it likely being stopped bit it isn't infinite or even really very big.

Some hard-to-detect baryonic matter, such as MACHOs and some forms of gas, is believed to make a contribution to the overall dark matter content but would constitute only a small portion.

Small does not equal zero.

At present, the most common view is that dark matter is primarily non-baryonic, made of one or more elementary particles other than the usual electrons, protons, neutrons, and known neutrinos. The most commonly proposed particles are axions, sterile neutrinos, and WIMPs (Weakly Interacting Massive Particles, including neutralinos).

Most common does not mean all. Primarily does not equal entirely. WIMPs [ re cold dark matter] interact with baryonic matter through the weak force. WILPs [re hot dark matter], e.g. neutrinos, interact with baryonic matter through the weak force and trigger an event through a head on collision with a nucleus. Colliding implies interaction.

The DAMA/NaI experiment and its successor DAMA/LIBRA have claimed to directly detect dark matter passing through the Earth, ......

Direct detection implies interaction,

Experiments with the Large Hadron Collider (LHC) may be able to detect WIMPs... these experiments could show that WIMPs can be created

Created by baryonic matter and/or the known forces other than gravity...implying interaction although possibly only in one direction.

And finally, describing other efforts to detect dark matter...

Both of these detectors are capable of distinguishing background particles which scatter off electrons, from dark matter particles which scatter off nuclei.

Scattering off nuclei sounds like interaction with baryonic matter to me.

Re:Dark matter vs black holes

[Have Brain Will Rent]

Umm he didn't say "little or no interaction" with photons - he said "It precisely means that its coupling to photons is zero", no interaction with photons. Big difference.

Re:Dark matter vs black holes

[EdZ]

Dark matter only interacts gravitationally.

OK.

Dark matter does not collide with matter

Not so sure on this. Does not interact maybe, but gravitational attraction should bring particles close to each other.

nor does it collide with other dark matter.

Why not?

Now, obviously dark matter particles would be unable to escape a black hole just like anything else. However, the vast majority of dark matter will never interact with a black hole -- it will just orbit forever.

Now this just smacks of nonsense. Why would dark matter magically start orbiting a black hole whereas normal matter, with the additional benefit or photon and particle wind to push it away, would not?

Re:Dark matter vs black holes

[c 0xFF]

Yeah, I debated on whether to include the "little or no" bit. We should probably ignore interactions due to "higher order loop effects" ... but he threw it in there, so I did too. Probably a mistake.

But my point holds: there are two kinds of dark matter: ordinary matter that we normally deal with (but not emitting much light) or something a bit more "exotic" than that (which doesn't even interact with light, much less emit any). It's basically the difference between MACHOs and WIMPs.

Re:Dark matter vs black holes

[Have Brain Will Rent]

Well my point was (completely internalized and so probably not obvious) really that we don't know what dark matter is, or how many kinds of dark matter there may be, and we really don't know whether there is no interaction (outside of gravity) with particles such as photons. Head on collision between photon and dark matter particle - is there an actual collision or do they not exist as far as the other is concerned or something else? It seems that, at least in some cases, there is an actual collision which would mean there is an interaction with photons - just very very infrequently. But wrt the implications for what dark matter "is" there is an enormous difference between never and almost never. So I object a bit to theories and opinions being presented in a manner that might lead one to believe they are facts.

Re:Dark matter vs black holes

[Anonymous Coward]

That depends a great deal on how massive the dark matter in question is, though. Dark matter which was as massive as a black hole would have similar gravitational effects, including bending light and swallowing light.

Re:Dark matter vs black holes

[amRadioHed]

Collisions are interactions. If dark matter doesn't interact with anything, then it doesn't collide with anything.

Also, dark matter wouldn't magically start orbiting black holes, it would just start orbiting it like normal matter. However, when a lot of normal matter clusters around a black hole it starts interacting thus giving up energy and falling deeper into the gravitational well.

Re:Dark matter vs black holes

[Anonymous Coward]

Dark matter is matter that we infer is there due to its gravity, but can't see. It could be because it doesn't interact with photons, but it could also be because it is too dim, like lone planets and black holes. Things like these (Massive Compact Halo Objects, or MACHOs) used to be a possible candidate to explain the large amount of dark matter observed, but we don't enough gravitational microlensing events for this to be the case. Furthermore the only way we know of to make the Big Bang predict the distribution of elements needs most of the matter in the universe to be neither baryons or charged leptons. Therefore, most dark matter is now believed to be in the form of Weakly Interacting Massive Particles, or WIMPS.

Nevertheless, if a galaxy turned out to consist only of billions of black holes instead of starts, that would rightly be called a dark matter galaxy. Being a collective term for all the matter we can't observe, what qualifies as dark matter will change as our detectors improve. A galaxy unrealistically consisting only of jupiter-like objects would be a dark matter galaxy now, but with (much, much) better detectors than we have now, one might be able to observe them, and would no longer call it dark matter.

Here are some things that could be called dark matter, under the right circumstances:
* neutralinos (a popular hypotetical dark matter particle candidate)
* neutrinos (definitely dark, but only makes up a small part of dark matter)
* black holes
* planets (when far away from stars)

Re:Dark matter vs black holes

[The_Wilschon]

I am aware of and understand all this. I was trying to alleviate what I thought was a minor but fundamental and common misunderstanding regarding what "dark" in "dark matter" signifies. If I had known that you were just looking for an excuse to be pedantic about the existence of baryonic dark matter, I would not have posted anything. Yes, baryonic dark matter exists. Yes, baryonic dark matter couples to the photon. Happy?

Re:Dark matter vs black holes

[The_Wilschon]

Good grief. I talk about higher order loop corrections to diagrams, my homepage is CDF (where I work), and somehow everyone seems to think I've never even heard of baryonic dark matter. Shrug.

Re:Dark matter vs black holes

[Chemisor]

> How do you tell the difference between a blob of dark matter and a black hole?

Stick your hand in it. If you can pull it out again, it's dark matter.

Re:Dark matter vs black holes

[Anonymous Coward]

What if it's 200 smaller black holes instead of one big one? Each would have a much smaller event horizon, and be extremely difficult to detect.

Re:Dark matter vs black holes

[fbjon]

Ah, my point was that a black hole wouldn't be spread out over large distances, while dark matter would be. Except for dark matter black holes, if such things exist. AFAIR, the gravity interactions between ordinary and dark matter that I've seen described shows a spread-out cloud rather than point gravity wells.

Re:Dark matter vs black holes

[Anonymous Coward]

Scary. Dark matter properties make it sound like it's a human construct to compensate for flaws in the scope of equations describing the universe.

Re:Dark matter vs black holes

[amRadioHed]

The properties of dark matter are basically a human construct, we can't directly observe the stuff but its existence is the best explanation we have so far for how galaxies are structured. There are flaws in our understanding of the universe, every scientist obviously understands this because they go to work each day to try and fix those flaws. I'm not sure why this is scary to you.

Re:Dark matter vs black holes

[Have Brain Will Rent]

I did not say, and neither did the Wikipedia article, that only baryonic dark matter interacts with photons or baryonic matter. So before you start trying to be condescending and tossing around labels like "pedantic" perhaps you ought to first try a little more reading and a little more thinking.

Re:Dark matter vs black holes

[TheTurtlesMoves]

Baryonic dark matter is *not* what astronomers or cosmologists mean when they say dark matter.

Re:Dark matter vs black holes

[TheTurtlesMoves]

Incorrect. You may mean dark as in not bright. But in the field it is not what is meant when one says dark matter. What is meant that it is very weakly, if at all, interacts via the forces other than gravity.

Rogue this or that does not even come within 100 or even 1000 times close to covering the "missing" gravitational matter.

Remember that 99% the mass of the solar system is the sun. even a few "dark" Jupiter's don't change the mass estimates of the solar system appreciably.

Yet by all accounts we need 10x more mass than what we observe. A planet here or there, dust etc are either too low in mass to matter, or are detectable. (we can see "black" or "dark" ie dust etc.).

Re:Dark matter vs black holes

[TheTurtlesMoves]

In his defense... you started it. You know after years of studying this stuff, then some /. "clever guy" comes along with a *Wikipedia* article to point out how we are all clearly wrong about it all....

Yea right.

Re:Dark matter vs black holes

[DarthVain]

From my understanding (which is vague at best) "dark matter" is sort of BS. Basically it seems that that according to physics as we know it, the mass of the universe should be X, however when calculated (by observation I presume) it actually equals Y which just happens to be a hell of a lot more than X. Our current understanding of physics includes Black Holes so presumably they are included in the previous calculation. Scientists being all "Sciency" and stuff explain away this crazy discrepancy, not by saying that our understanding of the principles of physics is wrong, but by saying "Oh that must be "Dark Matter"!", and "Oh yeah it is completely invisible and undetectable"...

So when someone says they are explaining away phenomenon with a whole galaxy full of "Dark Matter", I ask myself if they are even trying anymore.

Re:Dark matter vs black holes

[Have Brain Will Rent]

I "started it"???? Geez what are we 5 year-olds??? Sorry, rhetorical question. I'm sorry if you think questioning what someone says on a discussion forum, because a third party's lengthy article full of citations disagrees with with what they say, is "starting something". And I didn't try to point out how "you" are "all clearly wrong about it all"... In my experience when someone resorts to such hyperbole and/or ad hominem comments it indicates lack of an actual argument.

What was said didn't jibe with what I remembered about dark matter theories so I looked it up on the most easily accessible sources and found an article with 76 citations to sources like Phys. Rev., Royal Astronomical Society, NASA etc. Many of the statements in that article conflicted with what had been said so I responded quoting just some of those statements. I'm sorry if doing that on a discussion forum makes me a "clever guy"... how out of line of me. How could I have had the temerity to do something like that? Sheeesh, maybe if the article is so wrong you guys could reel in the egos for a bit and go offer up some corrections to it.

Re:Dark matter vs black holes

[X0563511]

Black holes aren't black spots in the star field. They lense light around them.

Here's a good visual simulation of what this looks like:
http://en.wikipedia.org/wiki/File:Black_hole_lensing_web.gif

http://en.wikipedia.org/wiki/Gravitational_lens

Re:Dark matter vs black holes

[Anonymous Coward]

http://en.wikipedia.org/wiki/Dark_matter#cite_note-0

"In astronomy and cosmology, dark matter is matter that is inferred to exist from gravitational effects on visible matter and background radiation, but is undetectable by emitted or scattered electromagnetic radiation."

Doesn't seem like they are claiming it's anything else. I'd point a finger at the media.

A galaxy of what? Dark stars?

[starglider29a]

What is the form of the dark matter? Does it coalesce into spherical bodies? Or does it homogenize into equidistant particles due to mutual repulsion? And if it is bound to the Milky Way by gravity, and itself bound to as a 'galaxy', does it exert cosmos expanding repulsion in an "inverse almost square" relationship? Is it 1/ (r- fudgeFactor)^2 or 1/ (r)^(2-fudgeFactor)?

Seriously. I'm a rocket scientist, and I'm baffled by the mixed properties of 'dark matter'. Can we land a probe on it, or would baryonic space probes pass right through it?

Re:A galaxy of what? Dark stars?

[Anonymous Coward]

http://en.wikipedia.org/wiki/Dark_matter#Baryonic_and_nonbaryonic_dark_matter

Re:A galaxy of what? Dark stars?

[rainmouse]

What is the form of the dark matter?

Assuming it exists at all. There is much circumstantial evidence but some argue no direct proof yet (though NASA believe the have proof). Still this excerpt from NASA seems to imply that dark matter does not interact with matter except through gravity.

"The hot gas in this collision was slowed by a drag force, similar to air resistance. In contrast, the dark matter was not slowed by the impact, because it does not interact directly with itself or the gas except through gravity. "

Source: http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html

Re:A galaxy of what? Dark stars?

[gstoddart]

Seriously. I'm a rocket scientist, and I'm baffled by the mixed properties of 'dark matter'. Can we land a probe on it, or would baryonic space probes pass right through it?

I think that's kind of the point, isn't it?

We don't know WTF it is, or what it's made up of ... only that we can measure it's gravitational effects but can't directly figure out how to observer it.

Beyond that, I've never heard anyone offer an good, testable explanation of what it is, merely what we think it isn't.

Re:A galaxy of what? Dark stars?

[catmistake]

Seriously. I'm a rocket scientist, and I'm baffled

C'mon, man! It ain't brain surgery!

Re:A galaxy of what? Dark stars?

[blair1q]

It doesn't emit or absorb electromagnetic radiation, or it wouldn't be "dark".

I'm not sure how there would be "mutual repulsion" in a body of free particles unless they all had the same charge. But charge is mediated by photons, which are electromagnetic radiation (from radio on up to gamma rays). Without EM, it would take a new force.

I'm kind of skeptical that such a dark-matter galaxy could exist. Galaxies are coalescence of gas by gravity. Why would there be a huge collection of particles affected by gravity that wouldn't have attracted those in the young universe that happen to do EM as well?

Re:A galaxy of what? Dark stars?

[Anonymous Coward]

Normal matter coalesces into spherical bodies because when two particles meet, they reduce their relative speed by giving off heat. Dark matter particles just zip through each other and so they don't coalesce.

Re:A galaxy of what? Dark stars?

[Black Parrot]

Seriously. I'm a rocket scientist, and I'm baffled by the mixed properties of 'dark matter'. Can we land a probe on it, or would baryonic space probes pass right through it?

Due to the presumed lack of electrostatic interaction, your probe could not "touch" it. You could orbit it or settle into a common center of gravity, but not land on it.

Re:A galaxy of what? Dark stars?

[Anonymous Coward]

Give up your day job. If you're a rocket scientist and can't understand "Only interacts through the gravitational force" and you don't know that gravity could only ever be attractive (outside quantum effects) you really shouldn't be building/designing or even painting rockets. Learn about the Bullet Cluster where we observe the effects of dark matter.

Re:A galaxy of what? Dark stars?

[arcsimm]

I'm no scientist, but to my untrained nose dark matter has the same smell as "luminiferous aether" and "epicycles." Basically, it's a theoretical placeholder for something we don't fully grasp yet.

Re:A galaxy of what? Dark stars?

[icebraining]

http://physics.stackexchange.com/

Re:A galaxy of what? Dark stars?

[blueg3]

"Dark matter" isn't really a name for a particular kind of matter with known properties. Some observations about gravitational forces don't work out quite right, as if there was a bunch of matter that we can't see. We don't really know any properties of it other than that it has mass and is otherwise undetectable (so far). Hence, "dark matter".

Re:A galaxy of what? Dark stars?

[The_Wilschon]

There are a lot of different models for dark matter candidates. All of them have in common that the coupling of dark matter to photons is zero. Also the strong coupling constant (coupling to gluons) is zero. Some models have dark matter that interacts via the weak interaction (W and Z bosons), eg WIMPs. Other models have zero weak interaction. Some models propose brand new interactions among dark matter particles that normal baryonic matter does not couple to, but TTBOMK none of these have a coupling significantly above the weak scale. All models have a normal coupling to gravity and massive particles.

So, depending on the proposed model, the dark matter particles may interact weakly with each other, which would lead to a clumpy distribution (but clumps much larger and less dense than stars or planets), or they may interact only gravitationally (that is, _very_ weakly), leading to a distribution much like you would expect of an ideal gas (pretty uniform throughout the galaxy, maybe higher density nearer the center, maybe not).

Re:A galaxy of what? Dark stars?

[The_Wilschon]

There are lots of testable explanations of what it might be. Many of them are being tested by experiments in progress today, or will be tested by experiments in the planning or building stages. For example, the lightest supersymmetric partner (LSP) is a good candidate for dark matter. Weakly interacting massive particles (yes, WIMPs) are another good candidate, and the LSP might in fact be a WIMP, depending on the values of the parameters describing supersymmetry (if it exists).

Some proposed explanations have already been ruled out by observations. For instance, "hot" dark matter (low mass, and therefore high average momentum) is easily ruled out by observations of galactic rotation curves. "hot" dark matter would not clump together nearly tightly enough to produce the observed rotation curves. So we're left with higher mass, lower momentum cold dark matter models.

Re:A galaxy of what? Dark stars?

[ghostdoc]

That's a really good point actually.

If Dark Matter attracts itself through gravity (which it must do to form Dark Galaxies), but is incapable of holding a charge (or, presumably, interacting with the Strong or Weak nuclear), then how does it repel itself enough to stay separate? Surely it should just collapse almost instantly into Dark Black Holes?

Or are there Dark Black Holes out there?

Re:A galaxy of what? Dark stars?

[wanerious]

Because it's collision-less, there's no effective way for a cloud of mutually-attracting DM particles to lose energy other than gravitational radiation. Normal matter will collide and heat up (accretion disks), losing kinetic energy as well as potential, and coalesce into a smaller object. DM clouds, as I understand it, have a much larger timescale for collapse. They don't "clump" very fast.

Re:A galaxy of what? Dark stars?

[gstoddart]

Thanks, I think. I'll have to read a little bit before the whooshing sound stops though. :-P

I take it "higher mass, lower momentum cold dark matter" wouldn't just be lots big rocks of really heavy elements, but essentially inert? Or would anything that big be generating something else that would rule it out?

I believe people are sure of it needing to be accounted for ... but how we know to be looking for it in the first place is one of those things I just have to mostly take on faith.

Something as remote as a galaxy walks with a limp, and you guys work out the height of the stairs he fell from. Or something. :-P

Re:A galaxy of what? Dark stars?

[Anonymous Coward]

Seriously? If it is W.I.M.P. and there is no other interaction than those 4, there is no 'dark' galaxy, no 'dark' 'objects'. Just a sea of WIMPs that go there and back like pendulum, as there is no friction that would stop them. Just massive points that would never form a 'dark' world.

Re:A galaxy of what? Dark stars?

[Philomage]

If the only force dark matter reacts to is gravity, wouldn't it all coalesce into one big lump? There must be some other force affecting it.

Mark my words

[Anonymous Coward]

One day we'll find out why we're having to explain shit with "dark matter", and the stupid concept will be laughed at like the Luminiferous aether is now.

Re:Mark my words

[Beelzebud]

I don't think that's the case at all. The only reason we have "dark matter" is because of astronomical observations. That is classic science. Make an observation, and then come up with a theory to explain it. From observations we know that there is some type of mass out there affecting gravity. We call it 'dark matter' because we don't know what it is. This isn't an aether theory, it's based on real observations.

Re:Mark my words

[Anon-Admin]

Ahhhh, and me with no mod points! Some one should mod this up!

Re:Mark my words

[Black Parrot]

One day we'll find out why we're having to explain shit with "dark matter", and the stupid concept will be laughed at like the Luminiferous aether is now.

Yeah, like neutrinos and X-rays and all that other weird shit people made up to explain problems away.

Re:Mark my words

[flaming error]

> it's based on real observations.
We can't see dark matter any more than our predecessors saw the Luminiferous aether.

> Make an observation, and then come up with a theory to explain it.
Like "Whew, those were seven bad years. Must have been that mirror I broke."

Making observations and theories is part of science. But what sets science apart from superstition is rigorous testing of the theories.

And we don't have any way to test for matter whose only property is it brings our mathematical formulae in line with our physical observations.

Re:Mark my words

[scorp1us]

Dark matter is the fudge factor which explains the rotation of galaxies whose outer extents rotate too fast for known orbital mechanics. Applying Occums's razor, the explanation is missing mass. The alternative explanations:
Time is not as constant as we think. (Allowing for General Relativity)
Gravity does not work as we think over large distances.
Dark Energy (companion/alternative to DM)

Those first two are often dismissed as being too complicated because we've got plenty examples of our predictions being right based on those assumptions, and sort-of rule out Dark Energy. But be it Energy or matter it is some form of a quantifiable unknown.

Faced with revising our equations for reality on the small-medium scale, we'd rather say matter is escaping detection for the large scale.

Re:Mark my words

[Anonymous Coward]

One day we'll find out why we're having to explain shit with "dark matter", and the stupid concept will be laughed at like the Luminiferous aether is now.

I don't think that's the case at all. The only reason we have "dark matter" is because of astronomical observations. That is classic science. Make an observation, and then come up with a theory to explain it. From observations we know that there is some type of mass out there affecting gravity. We call it 'dark matter' because we don't know what it is. This isn't an aether theory, it's based on real observations.

I think we actually have enough evidence for dark matter but, to be fair, the luminiferous aether was also based on real observations. There used to be a great deal of controversy over whether light was a particle or a wave. If it was a particle, it didn't need a medium to travel through, but it became very hard to explain refraction and diffraction properties. If it was a wave, then it needs a medium. We know from experiments that the medium wasn't something we could normally interact with. You can pump air out of a glass container with something that is making noise inside it, and you will no longer be able to hear the noise, because the sound waves have no medium to travel through. You'll still be able to see the object, though. Therefore, it was easy to conclude, from experimental data, that if light was a wave, there had to be some medium for light to travel through, permeating through everything, but that we couldn't interact with (pump out).

We had a bunch of observations that really demanded the presence of the Aether, until Einstein came along with his photoelectric effect papers and came up with the real solution: you know what? Light is both a particle AND a wave. That was completely unlike what we normally experience, so it's not the interpretation people were coming up with to explain their observations. A whole lot of things happened in those years, between Einstein, Planck, and deBroglie that really enriched our understanding of the universe. None of them set out to disprove the aether, it's simply that when they finally had a theory which explained every observation, the aether was no longer necessary.

I think dark matter is probably real. We can measure mass via the dynamics of celestial objects, measure mass via gravitational lensing, and come up with similar results: there's more mass there. So it doesn't seem like it's a problem with our theories. That said, there's a lot of observations dark matter doesn't explain, so if it turned out that it doesn't exist after all, I wouldn't be entirely surprised, and it would be very much like the story of the aether. There's nothing wrong with that, and there was nothing wrong with introducing the aether back in those days. That really is what science is about. You make a theory that best fits your observation, and right now dark matter beats any other alternative. If someone comes up with a better alternative, scientists will drop dark matter as fast as they dropped the aether, but until then we need to go with what we have.

Re:Mark my words

[Alarindris]

Think of it this way: In day to day life, for everyday objects, x = 2y describes gravitational interaction. Test it over and over, and it works just fine.

Now, when we look way out into the universe at galaxies, x = 2y doesn't quite work, but if we use the formula x = 2y + .01 everything falls into place. And it works over and over.

We don't know what the .01 is, so we call it dark matter. Someday we'll figure out what it is, for now we call it dark matter. That's it.

Re:Mark my words

[khallow]

it's based on real observations.

We can't see dark matter any more than our predecessors saw the Luminiferous aether.

Actually, we can via the gravitational effects of dark matter.

Re:Mark my words

[Anonymous Coward]

> it's based on real observations.
We can't see dark matter any more than our predecessors saw the Luminiferous aether.

> Make an observation, and then come up with a theory to explain it.
Like "Whew, those were seven bad years. Must have been that mirror I broke."

Making observations and theories is part of science. But what sets science apart from superstition is rigorous testing of the theories.

And we don't have any way to test for matter whose only property is it brings our mathematical formulae in line with our physical observations.

pretty much all of that is wrong.

we can in fact detect dark matter. It's the stuff that lenses light in the absence of "normal" matter. It has been observed acting in the collisions of galaxies, and galaxies composed of essentially all dark matter have been observed.

There is _something_ out there being observed. It's not just a bit of made up matter to make the equations work out, or an ad hoc addition to correct one specific inaccuracy. the question of exactly what would have the properties observed (interacts with matter gravitationally but not under any other known mechanism) is an open one. However to claim that we've never "seen it" is to not understand the advances in relevant research over the past couple of decades.

Re:Mark my words

[Bengie]

The difference between Dark Matter and Luminiferous Aether is they made something out of nothing. What's going on here is we have "something". We have gravity. This gravity is measurable and is out there, but we cannot find the matter associated with it.

Heck, based on the amount of gravity "Dark Matter" has, there is more of this unknown material than material we do see.

Something out there is creating a crap ton of gravity and we can't see it. Since mass is needed for gravity and matter is the only thing we know of that has mass, we figure we'll call it "Dark Matter" until we know more about it.

Dark Matter itself isn't something, Dark Matter is just the "idea" of something we don't know and we describe this unknown something as "Dark Matter".

The truth is out there..... LAWL.. had to.

I like to think of Dark Matter as a NULL value in a Database. It's a known unknown.

Re:Mark my words

[m50d]

The aether was exactly the same thing - an explanation for the real observation that light behaved like a propagating wave. It was perfectly legitimate science too. The situations are quite similar.

Re:Mark my words

[Black Parrot]

Let's get this out of the way first:

And we don't have any way to test for matter whose only property is it brings our mathematical formulae in line with our physical observations.

The, "Gee, that's funny" observation is what drives all science.

Now:

Making observations and theories is part of science. But what sets science apart from superstition is rigorous testing of the theories.

Believe it or not, some scientists do real science.

There was a competing explanation for this family of "Gee, that's funny" observations called MOND - Modification Of Newtonian Dynamics. It was ruled out on the basis of evidence. (There may be a MOND v. 2.0 out there now - not sure.)

One candidate for dark matter is the sterile neutrino, which people - real scientists - are trying to detect right now. A few years ago they were almost ready to dismiss its existence, but more recent results suggest that it may actually exist.

So no, contrary to your majestic disbelief, dark matter is a Real Hypothesis (tm), investigated by Real Scientists (tm), doing Real Science (tm).

If you want to actually learn something about the topic rather than simply using Slashdot as an outlet for you whingeing about the universe not working the way you learned in fifth grade, Wikipedia is an easy place to get started.

Re:Mark my words

[flaming error]

I'm not saying there's no such phenomenon as "dark matter" or "dark energy".

I'm agreeing with GP that our understanding of dark matter is no better than Huygen's or Boyle's understanding of the aether.

They didn't invent the aether for kicks. They had made physical observations of light and magnetism that known science could not account for. The "luminiferous aether" was an initial clumsy attempt to understand an emerging frontier in Science.

I think our successors will laugh at our clueless "dark matter" explanations for exciting phenomena we are on the cusp of discovering. But as long as our understanding is this vague, our "dark matter" is not much different than the "Luminiferous Aether" concept was three centuries ago.

Re:Mark my words

[VortexCortex]

Make an observation, and then come up with a theory to explain it. From observations we know that there is some type of mass out there affecting gravity. We call it 'dark matter' because we don't know what it is. This isn't an aether theory, it's based on real observations.

So, let me get this straight, all empty space is assumed to have the same "density" properties?

When we observe gravity as a warping of space, is it not reasonable to think that the warping of space might cause an effect similar to gravity, without requiring any mass at all?

My question is this: Why do we assume that all "empty space" is uniform?

Could it be that "dark matter" is simply "empty space" that is naturally "curved"?

One theory is that our universe exists on a "(mem)brane". What's to say that the membrane itself is perfectly smooth?

Perhaps it was, but the big bang (or other past universe-scale forces) caused ripples in the fabric of space-time itself.

Perhaps the variations in the space-time fabric density explain why energy/matter began to "clump" together, perhaps it just pooled into the valleys.

TL;DR: Dark matter may actually be nothing at all -- thus explaining: Why we can't observe dark matter; Why normal energy & matter pass right through it; Why the big bang's energy was not uniformly distributed.

Re:Mark my words

[Zorpheus]

Gravity does not work as we think over large distances.

The Pioneer Anomaly and the Flyby Anomaly are indicating that gravity actually works a bit different than we think. Maybe we will see a new formulation of gravity in the future, explaining these and removing the need for dark matter.

Re:Mark my words

[Drethon]

I'm not an expert in astronomy by any stretch (engineering is my area of expertise) but my understanding of a lot of astronomy is a lot of what we observe is interpretation. Most of physics was determined by watching something move and producing a model. In astronomy we can't actually watch things move (much beyond our solar system anyway), instead we look at light and try to translate this into the motion that we can't see (primarily using red shift). What if red shift or our other ways of interpreting this light is wrong? Not so much about the whole theory (though has red shift ever been detected in an object we can also measure the actual velocity of?) but even if red shift is not linear or off by a certain factor, how much would that mess up the existing theories?

Re:Mark my words

[Anonymous Coward]

New formulations of gravity can't work. To show that all you have to do is find two spots in the universe that have similar visible mass neighborhoods, but different kinetic behaviors. However innovative or complicated the formulation f of gravity is, you can't have f(x) != f(x). Dark matter postulates a new invisible scalar field which can explain the different kinetic behaviors by being different in the two neighborhoods.

Re:Mark my words

[Ramze]

Obviously, there are gravitational distortions in space-time that current theories cannot explain. The idea that there is some sort of exotic matter creating those gravitational distortions is an untestable hypothesis (unless you know of a way to go out and collect the dark matter which may exist in these regions to run experiments on it).

It is equally likely that those regions of space are experiencing distortions due to some unknown natural distortion in space-time's structure itself or are caused by an interaction with another universe in the multiverse. It's also possible that our current model of how gravity works is incorrect at large scales due to other factors we don't yet understand. In any case, the distortions are at such a large scale and at such great distance from us that any hypothesis will be difficult to test.

"Dark Matter" is just a word for "something out there we can't see is causing gravitational distortions we can't account for." There's no reason the cause has to be some form of matter we haven't seen yet. Particle physicists haven't a clue what kind of a particle would have mass, but no interaction with light. People assume the distortions are caused by mass because all known distortions our theories work for are caused by mass, but all known normal mass particles also interact with light or emit light. So, people simply make up hypothetical particles with mass, but no interaction with light -- because they NEED for them to exist to fit their assumption that mass in our universe is causing the distortion.

I think it's far more likely we don't yet understand some aspect of gravity on galactic scales than there is some sort of magic form of matter that makes our current equations make sense in the areas that currently make no sense to us whatsoever.

Re:Mark my words

[Colonel Korn]

Let's get this out of the way first:

And we don't have any way to test for matter whose only property is it brings our mathematical formulae in line with our physical observations.

The, "Gee, that's funny" observation is what drives all science.

Now:

Making observations and theories is part of science. But what sets science apart from superstition is rigorous testing of the theories.

Believe it or not, some scientists do real science.

There was a competing explanation for this family of "Gee, that's funny" observations called MOND - Modification Of Newtonian Dynamics. It was ruled out on the basis of evidence. (There may be a MOND v. 2.0 out there now - not sure.)

One candidate for dark matter is the sterile neutrino, which people - real scientists - are trying to detect right now. A few years ago they were almost ready to dismiss its existence, but more recent results suggest that it may actually exist.

So no, contrary to your majestic disbelief, dark matter is a Real Hypothesis (tm), investigated by Real Scientists (tm), doing Real Science (tm).

If you want to actually learn something about the topic rather than simply using Slashdot as an outlet for you whingeing about the universe not working the way you learned in fifth grade, Wikipedia is an easy place to get started.

I think they're up to MOND 45.2 now. The problem with MOND is that it has yet to successfully predict anything. Every new set of data requires a refinement of the concepts in MOND, whereas general relativity has successfully predicted a lot of things that were later observed. That doesn't mean MOND can't be right, but there's no particular reason to think it's right.

Re:Mark my words

[Anonymous Coward]

The difference is that the "luminiferous aether" is a theory that has predictive values (i.e. there should be a doppler effect on light while earth moves through the ether), while "dark matter" is merely a placeholder for a cause for a multitude of effects that we see (galaxies spinning differently from what we extect, gravitational lensing that doesn't match the visible masses etc), with multiple theories trying to explain it.

Re:Mark my words

[Lumpy]

Then why does dark matter, that is supposed to be in bigger amounts than bright matter (stars and junk) do not form gravitational lenses like normal galaxies do?

I should be able to spot a gravitational lens that has no visible source if this dark matter coalesces into "galaxies".

This should be the easy proof to get. If this stuff is so prevalent in the universe then we should have a scientific "metric buttload" of evidence in photographs of gravitational lenses without a central galaxy.

Re:Mark my words

[Lumpy]

This is the case IF.....

you assume time works on a small scale exactly as it does on the large scale.

Physics falls apart when you go to small scale, thus the "fudge factor" quantum physics was created.

We are smaller than a quark compared to a single galaxy, thus the possibility that time operates differently on a macro scale are certainly plausible.

Re:Mark my words

[khallow]

Then why does dark matter, that is supposed to be in bigger amounts than bright matter (stars and junk) do not form gravitational lenses like normal galaxies do?

You have evidence that it doesn't form gravitational lenses like normal galaxies? Then publish it and pick up a Nobel prize. My understanding is that we don't know enough to make claims like this.

Re:Mark my words

[c 0xFF]

Actually, they do form gravitational lenses, and we've measured this.

Basically, the process is to find a galaxy cluster, measure the lensing to determine where the mass is, and subtract out the mass of the individual galaxies. What you're left with is the location of the dark matter.

http://news.discovery.com/space/hubble-3d-map-universe-dark-matter.html

Where you're wrong is that there's no "central galaxy." Dark matter is still closely associated with normal matter (after all, they do attract each other gravitationally). I think maps like this have shown that most galaxies actually have a "halo" of dark matter surrounding them.

Of course, the shape of this halo can vary quite a bit:

http://hubblesite.org/newscenter/archive/releases/2007/17/

Wow! Are those gorgeous pictures or what!

Re:Mark my words

[c 0xFF]

gravitational lensing that doesn't match the visible masses

This is a very interesting piece of evidence. It's not just that the gravity is stronger than we expect, is that the gravity is coming from locations where we don't see anything.

The obvious conclusion is that there is additional mass not producing any light for us to observe. But maybe our model for gravity is wrong. Occam's Razor points to the former, so let's start there with our investigations. If that doesn't pan out then we'll have to look for more exotic solutions.

Re:Mark my words

[Anonymous Coward]

Dark matter is what we call the not-understood thing that causes the observations that we see.

If you are claiming you DO understand, when all of science says we don't, well it's put up or shut up time.

Making a claim that we understand what is causing what we see requires a great amount of evidence from you.

Lacking that evidence, you best just admit we don't know what is causing the observations we see, as all other scientists are doing by using a "dark ___" name (as is the tradition.)

Re:Mark my words

[ghostdoc]

The difference between Dark Matter and Luminiferous Aether is they made something out of nothing. What's going on here is we have "something". We have gravity. This gravity is measurable and is out there, but we cannot find the matter associated with it.

Not quite true. We have more observable gravity than our current understanding of gravity predicts, yes, but that doesn't necessarily mean there's an invisible gravity-pony out there causing all the 'extra' gravity.

The aether was created to solve the problem that light behaves as a wave, but waves must propagate through something. Once we had a better understanding of photons we could dispense with the aether.

The same holds true for our understanding of gravity. At the moment we are proposing that the universe is full of invisible gravity-ponies because 'something must be out there causing all this gravity'. We may come to a better understanding of gravity that allows us to dispense with the ponies.

Don't forget, after all, that we are also currently proposing that there are invisible inter-galactic gravity-ponies pushing the galaxies apart too.

Re:Mark my words

[ghostdoc]

So no, contrary to your majestic disbelief, dark matter is a Real Hypothesis (tm), investigated by Real Scientists (tm), doing Real Science (tm).

So was the Luminiferous Aether. That didn't stop it from being "wrong" in the end.

Real Science (tm) as done by real Scientists (tm) produces as many wrong answers as right answers. This is a good thing and what differentiates it from Religion (tm).

Re:Mark my words

[flaming error]

> Occam's Razor points to the former

I'm not convinced invisibility is a simpler explanation than human error, nor that human error is more exotic than "dark matter."

Occam's Razor doesn't help here. Neither choice is obviously simpler than the other.

Re:Mark my words

[scorp1us]

Yes, I hear a lot of new research is looking into the quantization of space-time, in the order of Planck length.

Also, at small distances, gravity is weaker than all other forces, so it is less relevant.

Re:Mark my words

[Anonymous Coward]

I think the razor favours our terrestrial models of gravity being inadequate when scaling galactically over exotic invisible mass whose only property is gravitational.

Why are WIMPS the popular candidate though? I thought perhaps a graviton would be most obvious for a particle that only interacts gravitationally.

Re:Mark my words

[Anonymous Coward]

There was a competing explanation for this family of "Gee, that's funny" observations called MOND - Modification Of Newtonian Dynamics. It was ruled out on the basis of evidence. (There may be a MOND v. 2.0 out there now - not sure.)

I've been kind of following the MOND/Dark matter debate off and on for a few years now and I don't recall any MOND disproval. What did I miss? Last I heard there was a Tensor Vector Scalar version of MOND from a group in Israel that seemed to be holding its own.

Re:Mark my words

[Anonymous Coward]

Assuming that our mass of the universe is correct. Assuming that we can see all the universe. Some points to ponder
http://space.about.com/b/2010/03/29/the-universe-has-more-stuff-than-previously-thought.htm
http://nicepictures08.blogspot.com/2010/12/number-of-stars-grossly-underestimated.html

Re:Mark my words

[L4t3r4lu5]

No. Real Science produces many, many more wrong answers.

Re:Mark my words

[L4t3r4lu5]

In other news, lighting a stick on fire allows the wood to "express" the fire contained with in it, and people who weigh the same as ducks are witches.

Go Science!

Re:Mark my words

[radtea]

laughed at like the Luminiferous aether is now.

Only ignorant idiots laugh at aether theories. It was a perfectly reasonable theoretical artifact given what was known at the time, and scientists did science: they publicly tested the idea that the universe was permeated with a fluid-like substrate responsible for mechanical transmission of light by publishing the results of controlled experiments and systematic observations.

The idea failed the tests, as so many do.

What's funny about that, excatly? Unless you're the sort of mean-spirited, small-minded asshat who laughs at people for being wrong.

Re:Mark my words

[radtea]

Could it be that "dark matter" is simply "empty space" that is naturally "curved"?

Sure, and this idea either has testable consquences, in which case I'm sure someone is working on testing them, or it doesn't, in which case it isn't interesting.

What you're proposing would appear in current theories as a parameterized cosmological constant. Of course, a scientist would then ask, "Why does the cosmological constant have that parameterization?"

The problem with that is that all else being equal, such distortions would tend to level themselves out. We don't assume flatness, it comes out of Einstein's equations as the lowest energy solution in the absence of matter. So when we see deviations from flatness, we think, "matter". And there is quite a lot of astronomical information now that suggests dark matter behaves in most respects like matter.

So, why assume a parameterized cosmological constant when it is known to be a poor fit to data we already have?

Re:Mark my words

[Lumpy]

They are claiming that dark matter makes it's own galaxies which is what I am refuting.

Good info though, I'll use those for further discussion with astronomy friends.

Re:Mark my words

[DarthVain]

I think his point is that the scientists are making the presumption that their current understanding and calculations are correct and that it is some unknown that is causing the whole problem labeling it "Dark Matter" as defined as WTF something I can't observe with a a lot of mass. So rather than looking for this magical element that is screwing up their mass calculations of the universe, perhaps they should take a harder look at a) the calculations they use to measure, b) the methodology used to test those calculations, or c) the foundations on which those calculations are based.

I would think that doing an experiment, finding results different than what you expect, and then blaming magical fairies, to be somewhat insane rather than scientifically rational. Anyway nothing wrong with giving the unknown a name and perusing that avenue of thought, only that it would be my last option. Needless to say I am not a big fan of the term "Dark Matter". It is something unknown and that is it. Perhaps the name itself simply has connotations I do not like...

Occlusion?

[popo]

Wouldn't dark matter galaxies so close to ours result in the occlusion of galaxies behind them?

Since a galaxy is mostly empty space -- wouldn't this result in a detectable degree of light variation?

Re:Occlusion?

[Black Parrot]

Wouldn't dark matter galaxies so close to ours result in the occlusion of galaxies behind them?

Since a galaxy is mostly empty space -- wouldn't this result in a detectable degree of light variation?

No, for the dark matter hypothesis to work as an explanation of what it's supposed to explain, it doesn't have any kind of EML interaction with normal matter, so it can't obscure any more than it can be seen.

It does bend space, and AIUI the best support for the DM hypothesis is the lensing effect it has at a distant cluster, but there may not be enough in the postulated satellite galaxies to cause any detectable lensing.

Re:Occlusion?

[Anonymous Coward]

Or better yet, why can't "dark matter" be merely regular matter that is occluded from observation by other masses? Imagine a galaxy that is in near perfect rotation with another galaxy (relative to us) but just slightly farther away. We would not be able to see any emission from the 2nd galaxy, or if we did, it would be very difficult to separate it from the emissions of closer galaxy. On a smaller scale, it could be two solar systems, or even just a pair of stars (one bright, one dead).

Or why can't "dark matter" be small objects with mass that are spread throughout our universe. They are small enough that they interact very rarely, and when they do, it blends in to other emissions or perhaps we have explained the emission away with another theory (background radiation?)

What if there are particles spread all over space and the cosmic microwave background radiation is actually the emission caused by interaction with these particles?

Re:Occlusion?

[Fnkmaster]

I believe the whole deal is that this dwarf dark matter galaxy is on the far side of the milky way and in the same plane (or nearly so) as the majority of the milky way, and thus is difficult to directly detect in terms of gravitational lensing effects. The article wasn't clear however on what the actual technique used was, but obviously it must be *some* form of gravitational lensing.

Re:Occlusion?

[The_Wilschon]

These are good ideas. They are so good, in fact, that lots of people who work on this stuff have already had them. The consequences of these hypotheses are not too difficult to work out, and people have done that, too. However, having worked out those consequences, and learned where and how to look to see if these hypotheses are correct, we find that their predictions do not agree with observation. A slashdot comment is certainly not the place to recount all of this, so if you are still curious or don't want to take my word for it, I'd suggest a look at the relevant and current literature. For instance, try looking up information on MACHOs (MAssive Compact Halo Objects), which are none other than your hypothesized "small objects with mass that are spread throughout our universe".

Re:Occlusion?

[Anonymous Coward]

Thank you for a reasonable and insightful response. So much better than the usual slashdot commenting.

Dark matter is always there when you need it

[DaKritter]

Tell me, what properties does dark matter have, save for explaining the factor 2-4 miscalculation of the universe's mass?

Re:Dark matter is always there when you need it

[Black Parrot]

Tell me, what properties does dark matter have, save for explaining the factor 2-4 miscalculation of the universe's mass?

What you describe isn't a property, it's an effect. (And I've never heard of any such miscalculation anyway.)

Properties: it has mass (bends space)... don't know of any others, except the trivial stuff like it can move and be scattered around.

Effects: explains galactic rotation curves, explains some instances of lensing, possibly explains the perturbations this article is about.

Re:Dark matter is always there when you need it

[DaKritter]

Properties: it has mass (bends space)... don't know of any others, except the trivial stuff like it can move and be scattered around.

Effects: explains galactic rotation curves, explains some instances of lensing, possibly explains the perturbations this article is about.

Yeah, it has exactly the one property that someone is missing: mass. But is conveniently free of any other property that could influence anything.

The explanations you refer to are all based on calculations that are ultimately based on our understanding of gravity. The least understood force, the one that just won't fit into the picture. What a real scientist should do was to better research and explain gravity instead of making up some magic invisible soup to fix it all.

Pseudoscience, I say.

Re:Dark matter is always there when you need it

[Black Parrot]

Pseudoscience, I say.

I'm sure the cosmology journals are eagerly awaiting your article.

Re:Dark matter is always there when you need it

[jkauzlar]

This guy gets some flak from moderators, but really, he makes an interesting point. How do we know that 'dark matter' isn't an invention to compensate for errors in our understanding of the gravitational force in the same way that the 'luminescent ether' was an invention? I'm not saying the parent is right, I'm only asking, how do we know?

Re:Dark matter is always there when you need it

[painandgreed]

This guy gets some flak from moderators, but really, he makes an interesting point. How do we know that 'dark matter' isn't an invention to compensate for errors in our understanding of the gravitational force in the same way that the 'luminescent ether' was an invention? I'm not saying the parent is right, I'm only asking, how do we know?

It might be but what people are missing out on is that it is the best explanation pointed at by our data. Trust me, all the other ideas for what might be causing our observations have been looked at, at least any ideas proposed by the naysayers here. Cold matter? Giant black holes? Changes to the laws of gravity? All looked at, studied with proposals have been made, experiments have been done, and they failed when tested. The idea of matter having gravity being out there that does not interact with the electro magnetic forces fits our observations and the more tests that are done, the more they confirm this idea. The reason guys like him get flak is because they are like "birthers". They keep asking and fighting for answers to questions that have been answered a long time ago and are a matter of public record if they would bother to study the subject for ten minutes of wikipedia and the web. No matter how many times this is explained to them, they still don't believe and the probably never will. I'm sure some of these people are just asking honest questions, however, their questions have been answered if not in this ./ post about dark matter, in the past ones they didn't read. (They must be new here.)

Fault

[Anonymous Coward]

It's all George W. Bushes fault! He didn't sign Koyoto, causing Global Warming, and this Darkie matter.

I know what dark matter ist!!!

[Tanuki64]

Ages ago I have seen 'The Elegant Universe'. There was a real nice explanation of several flavours of the string theory. One of the points was that gravity is so weak compared to the other forces because part of it wanders off into neighbouring universes. If this is true, why should only our gravity wander away from us? It would be expected, that at the same time gravity from other universes would come into ours. So dark matter is simply normal matter from a neighbouring universe from which we 'see' part of its gravity footprint. We cannot see anything else because it there is nothing there in our universe.

Disclaimer: 'The Elegant Universe' was a very entertaining science documentation. Easy to understand, fun to watch, but most likely got it totally wrong. Probably as wrong as my fun theory.

:-D

Re:I know what dark matter ist!!!

[pavon]

Actually, preliminary data from the LHC seems to invalidate the forms of string theory that predict that.

Re:I know what dark matter ist!!!

[ILuvRamen]

I have a different theory. Every time a calculation doesn't add up or observations don't match the numbers, astronomers blame dark matter. It's the new trend apparently. That's like me balancing my checkbook and when it doesn't add up, I make up some theory about dark money and then call it balanced and write a book about it and talk about it on the Discovery Channel. They should really get back to things we can actually test instead of things we can guess about. You would think with them being so close to the "most likely solution = alien UFO or manmade object" dilemma, they would stop giving any time and effort to whoever can make up the craziest sounding theories in astronomy. Dark matter is a miscalculation, that's my theory.

Obligitory...

[Anonymous Coward]

I for one welcome our new dark matter overlords from Galaxy X !!!

Dark matter is transparent. Black holes are opaque

[jfengel]

Both interact with light solely through gravitation, but dark matter is constitutionally incapable of interacting with light. It's dark not because it holds onto light, but because light just passes through it the same way a piece of plastic ignores a magnetic field. (Actually, not quite the same, but it's close enough for the moment.)

Black holes may or may not interact with light; what's inside a black hole is undefined. But when light falls on it, it passes the point of no return and never leaves.

Light passing near either will be bent by the gravity, but you can tell the difference in light that falls directly on it.

In fact, because the inside of a black hole is unseeable, it's possible that you could have a black hole that condensed from a blob of dark matter. You couldn't see it, but you could infer it: if there's a black hole inside a dark matter blob, it might have fallen in that way. Unfortunately, our tools for detecting dark matter are poor, so we can't resolve them with that kind of precision.

It is possible that some of the evidence that caused us to deduce dark matter could have implied black holes instead. There are two competing theories, Weakly Interacting Massive Particles (dark matter) and Massive Compact Halo Objects (black holes). That these are called WIMPs and MACHOs is a sign that we have detected physicist humor. The MACHOs hypothesis has been largely ruled out by the failure to detect the kind of gravitational lensing that small, massive objects cause, so the suspected mass must be more diffuse. That leaves us with the WIMPs as the best hypothesis, but it leaves a lot of questions open.

SF plot

[mangu]

dark matter may interact with its own kind by forces other than the ones that cause normal matter to interact with its own kind. According to the musing (which the author rejects), dark matter operating under such forces could form complex systems, maybe even an unseen parallel universe where "people" live lives like ours, as unaware of us as we are of them. All undetectable, except by their gravitational attraction on us.

A plot for an SF story: every time the universe branches due to wavefunction collapse a copy of the universe is created which still interacts with the universe through gravitation but not through the other forces.

Local effects of this are extremely difficult to measure, but they can be perceived as a fifth force that appears, for instance in the Pioneer anomaly.

I wish my writing skills were good enough to write this story...

Re:SF plot

[Anonymous Coward]

What you're saying doesn't really make sense.

"Universe branches" => Many worlds interpretation of Quantum Mechanics

"Wavefunction collapse" => Copenhagen interpretation of Quantum Mechanics

These are two different ways of interpreting the same event. Whichever one you pick negates the other. If there is such a thing as wavefunction collapse, then there is only one universe and hence no branching. Likewise, if the universe branches, then there is no such thing as wavefunction collapse (the different outcomes simply occur in different universes).

Re:SF plot

[c 0xFF]

Don't nitpick took much: it still makes more sense than most other SF.

Re:SF plot

[vux984]

If there is such a thing as wavefunction collapse, then there is only one universe and hence no branching. Likewise, if the universe branches, then there is no such thing as wavefunction collapse (the different outcomes simply occur in different universes).

To be fair, if universe A branches into A' and A'' then to the inhabitants in each of A' and A'' it looks exactly like the wavefunction collapsed and settled on 'their' universe.

Re:SF plot

[khallow]

These are two different ways of interpreting the same event. Whichever one you pick negates the other. If there is such a thing as wavefunction collapse, then there is only one universe and hence no branching. Likewise, if the universe branches, then there is no such thing as wavefunction collapse (the different outcomes simply occur in different universes).

Actually, nothing wrong with both being true. What you might have is an uncountable number of observers in an infinite number of universes. But the only observers (or other wave collapsing phenomena) we see are the ones consistent with us. So there might be a vast sea of quantum dynamics interspersed with oh, a "foam" of wave collapsing possibilities. And we're on a surface of which every wave collapse is consistent with every other wave collapse.

It's LIKE cosmic, dude.

Re:SF plot

[Anonymous Coward]

Unless the branching occurs in the time-backward direction. A' and A* collapse in to A. A' and A* are different frames with different timelines and event orderings. In such a scenario, both A' and A* would agree on the measurement result, but not on all the events or order of events leading up to the measurement.

Absoluteley Stunning!

[Anonymous Coward]

her picture! Don't just stop with her pictures, check out her CV, too (http://astro.berkeley.edu/~sukanya/). Impressive for some one quite young. The geeks all over have a new geek goddess!

I wonder if M&M/Marrs...

[PinchDuck]

will sue the astronomers who found it over copyright violations. They've had "Mikey Way Dark' out for years.

Re:I wonder if M&M/Marrs...

[Anonymous Coward]

That's not covered by copyright. If you're going to try to bring your politics into a situation at least be good enough to know when it applies.

mmm candy.. wait

[JonnyRocks]

It took me three seconds to realize I wasn't reading about a new dark chocolate milky way candy bar. I guess it's time to go home.

Misread

[Anonymous Coward]

I first read the title as:

Milky Way May Have Dark Master Satellite Galaxies

I thought, sounds pretty cool, if a little evil

RE: Milky Way May Have Dark Matter Satellite Galax

[Anonymous Coward]

Luke, come to the satellite dark matter galaxy!

How many?

[ZappedSparky]

"Galaxy X could soon lead to Galaxies Y and Z, according to Chakrabarti". Phew, at least there could only be three of 'em.