Can Primordial Black Holes Alone Account For Dark Matter?

thomst writes: Slashdot stories have reported extensively on the LIGO experiments' initial detection of gravity waves emanating from collisions of primordial black holes, beginning, on February 11, 2016, with the first (and most widely-reported) such detection. Other Slashdot articles have chronicled the second LIGO detection event and the third one. There's even been a Slashdot report on the Synthetic Universe supercomputer model that provided support for the conclusion that the first detection event was, indeed, of a collision between two primordial black holes, rather than the more familiar stellar remnant kind that result from more recent supernovae of large-mass stars.

What interests me is the possibility that black holes of all kinds -- and particularly primordial black holes -- are so commonplace that they may be all that's required to explain the effects of "dark matter." Dark matter, which, according to current models, makes up some 26% of the mass of our Universe, has been firmly established as real, both by calculation of the gravity necessary to hold spiral galaxies like our own together, and by direct observation of gravitational lensing effects produced by the "empty" space between recently-collided galaxies. There's no question that it exists. What is unknown, at this point, is what exactly it consists of.

The leading candidate has, for decades, been something called WIMPs (Weakly-Interacting Massive Particles), a theoretical notion that there are atomic-scale particles that interact with "normal" baryonic matter only via gravity. The problem with WIMPs is that, thus far, not a single one has been detected, despite years of searching for evidence that they exist via multiple, multi-billion-dollar detectors.

With the recent publication of a study of black hole populations in our galaxy (article paywalled, more layman-friendly press release at Phys.org) that indicates there may be as many as 100 million stellar-remnant-type black holes in the Milky Way alone, the question arises, "Is the number of primordial and stellar-remnant black holes in our Universe sufficient to account for the calculated mass of dark matter, without having to invoke WIMPs at all?"

I don't personally have the mathematical knowledge to even begin to answer that question, but I'm curious to find out what the professional cosmologists here think of the idea.

Comments

typo in title

[Anonymous Coward]

Pirmordial

Editors, the word you're looking for is primordial.

Re:typo in title

[Cryacin]

Pirmodial black holes don't account for dark matter, but it does account for the Lesdyxia of the editor.

Re:typo in title

[ShanghaiBill]

The main alternative to WIMPs are MACHOs, and black holes have long been candidates for dark matter. The problem is that they would need to have five times more mass than all the "ordinary" matter in the universe, and there is little evidence for that. For instance the amount of gravitational lensing that is observed is way less than would be expected. Dark matter appears to be more evenly distributed in galaxies and not just in the "halo". Yet we don't observe that many black holes passing through gas and dust clouds or interacting with regular stars.

TFA says that there may be 100 million black holes in our galaxy, and that may sound like a lot, but it is actually nowhere near enough to account for all the dark matter. Even if they had 10 solar masses each (unlikely), that would still be less than 1% of the mass of the galaxy's "ordinary matter", when it should be 500%.

Re:typo in title

[Anonymous Coward]

If you get rid of dark energy, you eliminate the need for dark matter as well.

Current theory about universe involves more space appearing out of thin air, causing expansion. (More vacuum!).

Instead what should be examined is what happens to light traveling through immense amount of vacuum. We have discovered unexpected characteristics of vacuum, there may be more. It is possible that redshifting occurs through extended vacuum interaction and the output is that occasionally higher energy photons are converted into particles (i.e. mass) by this process which ultimately leads to interstellar gas.

Re:typo in title

[ls671]

Just use unsalted butter next time.

Re:typo in title

[Sique]

Actually, you don't get rid of Dark Matter when you eliminate Dark Energy.

There is evidence for more matter than visible in the galaxies, which is completely independent of Dark Energy. The most prominent evidence is the rotational characteristics of the outer parts of a galaxy. The stars there are circling the center of the galaxy much faster than expected from a Keplerian point of view. Instead of falling with r^2/3, as Kepler's Third law of motion predicts, the speed of stars remains roughly constant if you get to the outer parts of the galaxy. This means that the mass of the galaxy inside the respective orbits of the stars has to grow much faster than the mass from the additional stars within outer orbits.

(Be careful not to confuse the speed of stars on their orbit with their angular speed! A star twice the distance from the center of a galaxy needs twice the time to complete a circle than a star closer to the center. Thus the angular speed halves, but the linear speed on the orbit keeps the same. With Kepler's Third law, we would expect the time to complete an circle for the outer star to be 2*sqrt(2) of the time the inner star needs.)

Re:typo in title

[Sique]

That's why I called the rotational problem evidence, and not proof of Dark Matter. The most glaring evidence against Dark Matter is that we haven't any hints at their existence except for gravitational effects. Yes, Dark Matter could be nothing but a problem with our understanding of Gravitation.

Re:typo in title

[Anonymous Coward]

Just don't forget that rotational speed of Galaxies and Galaxy clusters is AN evidence for Dark Matter, but not the only one.

Gravitational Lensing provides the second major evidence for some (electromagnetically) unseen source of gravity, it also provides evidence that Dark Matter is just a "scaling problem" in our theory of gravity (AR)... google for "Bullet Nebula".

Re:typo in title

[swamp_ig]

Not really. There's areas of mass where there's little to no ordinary matter, where galaxies have collided and the gas has slowed down, but the dark matter has kept on going. This is demonstrated by gravitational lensing effects of the invisible mass. This doesn't really fit with MOND theories.

Re:typo in title

[jellomizer]

Normally in science the simpler the answer that fits the given data is considered the most likely. Given Dark Matter and Dark Energy are more or less scientific terms for paranormal activity ( just so people just don't go thinking it is cause by ghosts ) but right now they are a name of something, we are not quite sure how to measure and observe, it is just a variable if filled into the equation seems to fit the data that we observe in the universe.
Now as we learn more and collect more data and get better tools for observation, These variables could actually be shone as true, or perhaps there is an easier explanation that will be shown, such as having a higher number of black holes, which is a concept that we humans seems to better understand and grasp intellectually.

Re:typo in title

[coastwalker]

I agree that the separation of luminous gas from gravitational lensing in for example the Bullet Cluster (https://en.wikipedia.org/wiki/Bullet_Cluster) is strong evidence that dark matter is something other than ordinary matter, be it in black holes or not.

Re:typo in title

[slashrio]

Well, isn't a black hole just that: black matter?

Re:typo in title

[Bengie]

Vacuums are full of random "excitations" in all of the fields, including the electromagnetic fields. On average of the entire Universe, there is a net of 0 energy, but at any location, there is a chance that anything can happen. There is a chance that an entire new Universe that looks just like ours will appear. Just very low chance.

Photons don't experience time. Of course nothing happens to them. You cannot alter a photon, only create new ones. The act of creating new ones leaves traces.

Re:typo in title

[Sique]

But what you describe, is a gravitational effect. If we mistrust our understanding of Gravity, another gravitational effect won't be enough to restore trust. Yes, it's quite complicated to invent a modification of General Relativity that causes the same effect without referring to Dark Matter, but still, it's just another part of the evidence for Dark Matter, but not one independently to verify without resorting to Gravity.

Re:typo in title

[Boronx]

If we're saying this gravitational lensing is caused by the same type of matter that makes galaxies spin too fast, that would seem to rule out black holes, since they wouldn't separate from stars in a galaxy collision.

Re:typo in title

[Khashishi]

But, if there were black holes scattered between galaxies, that would be a different story.

Re:typo in title

[david_thornley]

What separated from the stars in the Bullet Cluster is the gas and dust, which apparently are the largest amount of visible mass. I'd expect much the same effect from lots of small black holes as from WIMPs - in this case, anyway..

Re:typo in title

[david_thornley]

There's nothing paranormal about Dark Matter or Dark Energy. Dark Matter is matter that has gravity but doesn't interact electromagnetically. We actually have an example of such matter in the neutrino, although what we're observing isn't what neutrinos would do.

Like many things in science, they're a name for something we haven't been able to observe more directly. Are you familiar with how we found Uranus and Neptune? Astronomers observed anomalies in the orbit of Saturn, and figured that these could be explained by another planet further out, and they could figure where to look, and so they found Uranus. Neptune was found in a similar manner. Both of these were cases where scientists realized that something unknown would make the equations fit observations better.

Re:typo in title

[Whorhay]

If mass can deform space, is it possible that those deformations could continue to exist after the mass has diverged?

Re:typo in title

[Whorhay]

The waves represent a rapid change in the shape of space, right? Gravity isn't instant but changes in gravitational state travel at something like the speed of light. And the waves actually represent a temporary fluctuation before returning to an earlier state.

I'm thinking more in terms of space being so distorted by a mass that even once that mass has somehow been removed or lessened, the space where it once was remains distorted. I think there is a presumption that space is perfectly elastic, compressing and stretching, according to the affects of mass. But perhaps space does have limits to how much it can be stressed either way after which it won't behave in the same manner.

Re:typo in title

[Dread_ed]

Check this out and, if you would, please provide some comment.

http://thedaily.case.edu/rotat...

It seems to put a new spin (haha!) on the relationship between normal matter and dark matter and the rate of spin in galaxies.

Re:typo in title

[Agent0013]

So we should see spiral galaxies that have the rotational curves that we expect them to have without the dark matter. If the dark matter can be pulled out from the galaxy, then there should be spiral galaxies that are lacking the dark matter and the curve is now correct. Have we found any? If not, would that be suspicious?

Re:typo in title

[mcswell]

You seem to know what you're talking about. Kepler's "laws" describe two bodies in orbit around each other, with one much more massive than the other, and at a substantial distance. Obviously (well, I _guess_ it's obvious), the rotation of gaseous bodies (like the Sun) is different; an hydrogen atom in the photosphere is not in _orbit_, so its behavior is not described by Kepler's (or Newton's) laws.

While it's a long ways to the next star in the arm of a galaxy, it's still not quite the same as if our Sun was orbiting around the center of the Milky Way with nothing anywhere near. Do the other stars in our arm of the Milky Way (and similarly in other galaxies) have an effect on our speed going around the Milky Way? Or is the fact that there is (roughly) an equal number of stars in all directions negate any such effect, so that it's as if we were orbiting the core with nothing else around?

Well, let us look at the ---primordial--- pie.

[wherrera]

See this link: Content of the Universe 2016

So, the problem is that there is so much of it, you would think we'd see it perturbing stallar orbits more, it it were concentated in many, many discrete points of star gravitational influence. There would be a lot more stars orbiting pulsar type objects, perhaps?

A real cosmologist would know the odds of the galaxy looking the way it does if all the extra mass were in scattered black holes of a certain size. Probably low.

Re:just wait

[DontBeAMoran]

I'm quite sure than as soon as someone figures out the math, a paper will be published.

Don't count on it.

“There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.

There is another theory which states that this has already happened.”
  Douglas Adams, The Restaurant at the End of the Universe

I think no, not that simple

[dyfet]

https://phys.org/news/2015-03-... http://www.sciencemag.org/news... If dark matter were simply some existing form of baryonic matter, even if trapped in black holes, then a phenoma like this where dark matter halos separate from collided galaxies and behave under different rules to continue on their existing path should not be possible at all, because it, like all the other ordinary matter involved, it should have followed the same paths gravitationally bound.

Re:I think no, not that simple

[Anonymous Coward]

That's not quite right.

There are three types of stuff involved in the Bullet Cluster collision: stars, gas, and dark matter. The stars are the smallest fraction of the mass, the gas is a larger chunk of it, and the dark matter makes up most of it. We can see the stars and the gas directly (from visible and X-ray observations), and we can figure out where the dark matter is from gravitational lensing (which shows where the mass is).

In the collision, the stars in each cluster passed right through each other, the clouds of gas collided and interacted, and the dark matter from each cluster passed through the other.

If dark matter were composed of something that behaves like gas - say, "dark protons", that behave like regular protons except that they don't absorb or emit light - then it would have collided and interacted like the gas. So that possibility is out. But the Bullet Cluster observations are still compatible with dark matter being composed of WIMPs (like "dark protons", except that they don't interact with each other) or MACHOs (big blobs of regular matter, like planets ... or black holes).

The evidence against MACHOs doesn't come from the Bullet Cluster. It comes from gravitational microlensing surveys, that look for the momentary wiggle in brightness that a star should show if a MACHO drifts between us and it. If there were enough MACHOs - for example, black holes - to make up dark matter, then by now we would have detected them in this way.

Re:I think no, not that simple

[Tablizer]

But why would the black holes be distributed significantly differently than stars, and have "outer" orbits?

Most visible stars are caused by relatively recent compressions and/or concentrations of gas, and the concentration is heavier toward the center of a galaxy. Thus, relatively speaking, there could be more black holes outside the visible disk of a galaxy than stars.

But this would also imply the majority of black holes did not form from stars, at least not in the way we see them form now. If the black-holes formed from typical stars, their distribution (galactic orbits) would be roughly the same as visible stars.

The theory they formed in the primordial ooze that was common before most galaxies formed may account for this: their galactic orbits would then be further out than most galaxy-born stars. Such holes would eventually be captured by galaxies but would on average have outer orbits, as gravity-captured objects tend to. This could give the "hidden heavy halo" effect seen in Hubble gravity lensing and star-orbit patterns.

Re:I think no, not that simple

[Bengie]

Primordial blackholes, in theory, developed while the Universe was still semi-uniformly filled with matter, before any structures had time to form. Blackholes cannot form by just having dense matter, they form from a gravitational gradient. In the very early Universe, energy was uniformly distributed, not allowing for a gradient to occur. A short bit later, we started to get slight biases, allowing for blackholes to spontaneously form from all of the energy that was still densely filling all of space. - Layman's description.

We don't see any gamma radiation filling the parts of space where we can measure dark matter. This indicates that if these blackholes exist, they must be of a mass large enough to not have decayed yet. This gives a minimum per blackhole starting mass. There is also a minimum current mass since we don't see any decernable amounts of Hawking radiation. As we gain more information on this topic, we keep limiting both the lower and upper bound for these theoretical blackholes, and we're running out of space. Looking less and less plausible, but from a hypothetical standpoint, a great potential solution.

Re:I think no, not that simple

[hAckz0r]

Abell 520 is yet another instance, where the bizarre physics of the so called Dark Matter from two colliding galaxies turns to stationary filaments of gravitational lensing rather than clustering along with the normal baryonic matter that kept passing through. Doing this would require not only new physics but also lots of energy to stop dead in its tracks and make a left hand turn for no reason what so ever. The Elephant in the room is tap-dancing and nobody is even paying attention!

Re:I think no, not that simple

[pepsikid]

There are no "of courses". Some models do propose that dark matter does not even interact with itself. Alternately, some models propose that dark matter is it's own anti-particle, so it annihilates when it interacts with itself. In the latter case, we should be able to detect it as a gamma-ray source which cannot be explained by any other means. One recent report of gamma rays from dark matter was withdrawn when other sources were found.

Too assertive about dark matter

[CptJeanLuc]

Quote: "Dark matter [...] has been firmly established as real [...] There's no question that it exists." There is still plenty of controversy related to the idea of dark matter, and there is no such thing in physics as proving something exists - you can only prove something to be false. I'm not saying dark matter does not exist, only that statements like the above are too assertive.

Re:Too assertive about dark matter

[jfdavis668]

Astronomers have observed the gravitational lens effect of dark matter in the distant universe. Something is there.

Re:Too assertive about dark matter

[Anonymous Coward]

*something* is there, but it's not necessarily Dark Matter. It could be something wrong in our theory of gravity, or something wrong with our understanding of galactic-scale space-time. Insisting it must be matter is probably this generation's ether.

Re:Too assertive about dark matter

[Anonymous Coward]

It has momentum (see: Bullet Cluster), which conclusively rules out modifications to gravity. That doesn't leave a lot of alternatives to the "non-luminous matter" concept, which is fairly broad. There is still a tiny bit of space for other theories, but they are getting more and more twisted into knots to explain expansion rates in the early universe, structure formation, galactic rotation curves, dwarf galaxy abundance, and dynamical data like the Bullet Cluster. Some sort of dark matter matter explains all of that data quite neatly.

As to what it is, we know the following:
1. It isn't especially clumpy (e.g. is not planets or black holes). Aside from the fact that it is hard to make such things in the appropriate numbers, it would cause gravitationally microlensing effects where stars and galaxies would get suddenly magnified by the passage of clumps between us and the distant object. People have looked for this in detail and it just doesn't happen at anything like the right levels.

2. It interacts with normal matter very little: otherwise you would be able to detect it in the lab or it would have dynamical friction with ordinary matter, moving with the case in the Bullet Cluster or lighting up when hot, etc. The limits here are quite stringent.

3. It doesn't interact (much) with itself, either: it doesn't cool effectively in galaxies and giant clouds of it can pass through each other (Bullet Cluster again, among other measurements)

4. It is cold. If the velocities of the dark matter were high, they would slow structure formation on small scales, reducing the abundance of galaxies and delaying their cosmological formation. Assuming it is a thermal relic, which is the easiest way to make it, this rules out low-mass particles like neutrinos.

All together, this suggests some kind of object bigger than a neutrino but smaller than a planet that interacts very rarely (at weak scale or below) with ordinary matter or itself. You will notice there is a lot of room there still, but there is not enough room to accommodate modified gravity with any kind of comfort.

Re:Too assertive about dark matter

[Bengie]

People have already looked into those. We're not seeing any issues with the theory of Gravity at large distances and Gravity really is just the symptom of the warping of space-time, so the second part is moot. Space-time and Gravity are working exactly as predicted. Now that's out of the way.

I liked the sarcastic humor of this kind of response.
Every best of the best server admin trying to fix a computer: We've spent 100 years trying to get this computer to turn on, but only the lights turn on
You: I bet they forgot to plug in the server

Re:Too assertive about dark matter

[Bengie]

That's not what I've seen over the past many years. It has been discussed that until we had Dark Matter, we had no working simulation for what we see with the Universe. Prior to Dark Matter calculations, we could simulate a galaxy's evolution, but not the evolution of the Universe and its many galaxies. Of course the rotations were all messed up.

Even if someone had a model of MOND that created what we see at the Universal and galactic level, MOND is fundamentally incompatible with Relativity, the single most tested and accurate theory. Not to mention there has only been one theory of MOND that could replace Relativity, but at the expense of a hugely complex mess of math that even few of the best could wrangle it.

Re:Too assertive about dark matter

[painandgreed]

Astronomers have observed the gravitational lens effect of dark matter in the distant universe. Something is there.

Please, don't respond to Anonymous Cowards. They're mostly all just trolls spouting stuff they know is wrong so they can "start a discussion."

There are multiple observations made in the past 80 years that have caused us to look for an answer, galactic rotation, gravitational lensing, details dealing with the CMB, etc. All the easy things like more matter, changes to the laws of gravitation, lots of black holes, were the first things looked at and have failed to provide any solutions. Slowly, all these different paths are leading to the same conclusion that there is some form of matter that does not interact with the EM forces, or probably any force except for gravity. One person here once stated that "Just because something is eating the catfood at night doesn't mean there's a cat nobody has ever seen" However, at this point, something is eating the cat food, using the litter box, scratch the furniture, getting cat hair all over everything, and if you knock on the walls and say 'meow', something is saying 'meow' back at you. If the answer isn't a cat in the walls that nobody has ever seen, it's going to be a lot stranger than just cat in the wall that nobody has ever seen. Same with solutions like MOND. If the laws of gravity need to be changed to fit the evidence collected so far, we're not talking about just an additional tweak or two. It's complicated that nobody can come up with a hypothetical solution that will fit gravitational rotation curves in more than two dimensions, let alone also the other directions the evidence is coming from.

Re:Too assertive about dark matter

[Agent0013]

Then we should observe rotating galaxies that rotate at the correct speed because the dark matter has been pulled away from them. If we don't see that, then there is another reason the galactic curves are not correct.

Or maybe, just maybe...

[NoNonAlphaCharsHere]

26% of the mass of the universe is made up of your simplifying assumptions: space is flat and uniform everywhere and everywhen, gravity is constant everywhere and everywhen, the speed of light is constant everywhere and everywhen, the Higgs field isn't really the luminiferous aether with a fancy new name, etc. ...

So so so much of the Standard Model (and astrophysics in general) starts out like "Given a spherical cow of uniform density at STP...".

We can basically derive ALL of chemistry from first principles involving (protons, neutrons, electrons) (and their charges), electron shell configurations, etc. Does the Standard Model provide an explanation for the mass of the electron, or any of the other 92 empirically derived "constants" that make up the current orthodoxy? Does calling the gap between reality and our understanding of it really benefit from calling it "Dark Matter", or "Dark Energy", or should we just call it "phlogiston"?

I'm not trolling, I'm serious. The Standard Model has lots of (statistical) predictive power, but absolutely no explanitory power -- back to the chemistry example, it's as though we have atomic weights and molar values, but no notion of electron shells -- we can predict, but we can't explain, at least not in a meaningful way -- yet.

Re:Or maybe, just maybe...

[Anonymous Coward]

Well, not that many simplifications. Actually, many people try out many different things to see if your experimental observations are affected by the underlying theory you are basing your assumptions.

I want to point out some contradiction in your message:
- On one hand, you criticize the standard model for its high number of parameters (19, I believe that is the number, actually). And it is a fair criticism, you would like your theory to predict nature with the least number of empirical parameters. If every time you discover something new, you add a parameter, you can describe nature, but you can not make any predictions.
- On the other hand you call for more complicated theories, because the ones we have are too 'simplified'. Every time you do that, most likely you are adding a parameter to your theory to account for the greater freedom a complicated theory has. So, you are going backwards.

As the theory stands, it is quite predictive, and i find it amusingly incredible that a theory as simple as the one we have works so well (If you look at the Lagrangian formulation of General Relativity, which is a very simple way to postulate a theory, General Relativity is the most simple thing you can do). However, it is not perfect. Perhaps the observations we do not understand will give us the key for a better theory (perhaps the one that unifies gravity with the rest of the foreces).

Personally, I would not call Dark Matter to be a fact. There is no direct proof of its existence and it has its fair share of problems (not every DM candidate solves every problem, although you can have different populations of DM particles). There are also other theories that try to explain most of these unexplained observations (MOND, and some modified gravity theories, like TeVeS, which implement MOND ideas in a covariant way), but those probably have more problems than DM.

Re:Or maybe, just maybe...

[Anonymous Coward]

IMHO (based on playing with the dipole model).

Time is flat and constant and causlity happens in forward time.
Gravity and mass are the same, the mass of something is the potential of it to attract via the dipole binding force (DBF).
There is no momentum as such, only a reduction in the dbf cause by velocity effect on the component of the twist subtended onto the other particle. (do it numerically you'll see what I mean, if you move a particle faster away from a clump of dipole stuff the binding force reduces from the velocity alone).
There is one type of energy, kinetic, no wave energy.
C is a limit function on that reduction in force.
A particle experiences a net effect from multiple DBFs.
Take one of these donut particle next to some 'dipole stuff' and it has a larger dipole attraction force of because its dipole separation is big. Turn that into a cloud of the same dipoles, the dipole separation becomes sum(tiny), the binding force drops to near zero and the velocity shoots off to the limit function C. This is why matter turns to photons that fly off at C. It's just a change in their configuration.
Matter isn't turned into energy (really EM) by magic and back to matter, they're the same stuff with different dipole separations and hence different dbfs and hence different velocities..

DBF breaks down at distances less than the dipole separation distance, i.e. if you compress matter the binding force of particle collapses, the DBF collapses and velocity shoots up to limit C. i.e. black holes make big bangs once the matter is squeezed tighter than the dipole separation distance.

Our universe is not special. There is no reason for only one universe. All that happens is black holes mop everything up and clean out an area of space, until they pass the critical density, the dbf collapses and boom, its respread out.

All of the above is just based on a numerical model not yet powerful enough to make its first atom. Work in progress. May be a dead end.

Re:Or maybe, just maybe...

[PoopJuggler]

No model can explain the "why" of fundamental particles or constants. They just "are". Science is not about Truth, it's about usefulness.

Re:Or maybe, just maybe...

[Ramze]

That's the thing with (supposedly) fundamental particles -- you can't explain them in terms of something else... because then they wouldn't be fundamental. If you're talking about why they have certain properties -- like why there are 3 generations of matter (separated only by mass) and why they have the masses that we measure (as opposed to some other mass), maybe one day when we find a way to merge gravity with the standard model and/or figure out why the Higgs mechanism gives different masses to different particles, we'll find out.

But, if you mean you want to have explanations for things like "charge," "spin," "color charge," and why only certain ones exist -- we may never know. If they're fundamental properties, there may not be any real explanation other than "they just are." That's the universe we appear to live in.

String theory and some other interesting quantum theories are trying to explain deeper meanings and use expected symmetries to figure out missing particles and new physics... and they helped to tease out the Higgs Boson and its field to explain why all fundamental particles don't move at the speed of light. There may be more than one Higgs field & that may explain more if we find it. If there are hidden, curled up dimensions, we may be able to explain all the properties of particles in terms of vibrating strings or membranes in higher dimensions, but until string theorists can decide on what the shape of those curled up dimensions might be for our universe, they can't help much with predictions, much less explanations. Trouble is, there are a heck of a lot of possibilities for those curled up dimensions, and there aren't a lot of ways to discern which ones match our known universe yet. Sure, they can whittle them down to a subset that matches known properties of the universe, but that leaves a massive subset to eliminate false positives from.

I'd say string theory is your best bet for explaining why things are as they are one day... but it may be that some things just are, and that's as fundamental as they get -- at least as far as we can tell from experimental data from within the universe. Anything deeper is speculation or philosophy -- unless it can fit the math perfectly and explain things other models can't. For instance, we've never directly observed quarks, but we've been able to indirectly observe them and figure out their properties from subatomic collisions. At one time, people debated if they really existed or if they just helped the math work... but physicists generally agree they exist today. Maybe we'll find something more fundamental in time that will explain more. My bet is on strings, but... who knows?

Re:Or maybe, just maybe...

[hord]

This is how I think of things. Basically just regions where energy is denser and lighter and flows in between them. The EM and Weak fields are just particular configurations or patterns of this and the entire thing is perfectly deterministic. We are living in a wrinkled energy background that constantly vibrates.

Re:Or maybe, just maybe...

[nickersonm]

26% of the mass of the universe is made up of your simplifying assumptions: space is flat and uniform everywhere and everywhen, gravity is constant everywhere and everywhen, the speed of light is constant everywhere and everywhen, the Higgs field isn't really the luminiferous aether with a fancy new name, etc.

Addressing the first part of your post only, this is one reason I like the Timescape Cosmology model, that basically posits that some dark matter effects and all dark energy effects are just arising from GR. Most cosmology simplifies the universe to a homogeneous soup, which conveniently ignores small-scale GR effects that could be very important.

Re:Or maybe, just maybe...

[PoopJuggler]

No, it's impossible to determine "why" for fundamental particles because there is literally no reality beyond them. It may be discovered that electrons are not fundamental and then you can probe deeper, but that is a completely different story. Once you reach the "fundamental" level, there are no more turtles underneath.

Re:Or maybe, just maybe...

[wonkey_monkey]

Or maybe, just maybe, if you're not an astronomer or a physicist then there are other people who know more about this than you.

Perhaps the idea that it's all a simplifying assumption is your simplifying assumption.

Re:Or maybe, just maybe...

[david_thornley]

Note that "The Big Bang occurred" and "God occurred and caused the Big Bang" are equal in explanatory power - both say something happened without us being able to observe why (if there is a why). By Occam's Razor, we eliminate superfluous entities in our theories and therefore God.

This isn't to say that there is no God, or the God didn't create the Universe, but that there is absolutely no evidence for divine origin.

What I have concluded is that, if a God exists vaguely in the Christian, Jewish, and Muslim sense, that God has a sense of humor.

Re:Or maybe, just maybe...

[david_thornley]

Time is flat and constant and causality happens in forward time.

In which case you're throwing out Special Relativity, and need to explain what it explains. It's one of the most tested theories in modern physics, and has been shown to offer very exact explanations of a variety of things.

For example, we know that unstable subatomic particles, when at near-light speeds, decay more slowly than if they were still.

Re:Or maybe, just maybe...

[david_thornley]

Or maybe, just maybe, if you're simply curious, you don't have to be an astronomer to throw some speculation out in the wild.

Sure. Bear in mind that your speculation is almost certainly wrong, and it's very likely that a real astrophysicist could come up with a counterexample. I like science fiction, and if you're going to have FTL ships you need to speculate. (For extra points, come up with a speculation that shows how it doesn't make time travel possible and practical.) What you are extremely unlikely to do is to come up with anything that's useful for anything other than entertainment.

Why dismiss someone else's answer simply because their credentials don't match what you consider "know more about this than you"?

First, it's not possible to address every crackpot theory while still get anything done. Second, most of the time the non-scientist's answer is clearly wrong, in that there's some more or less obscure observation (sometimes glaringly obvious observations) that contradict the non-scientist's answer.

If you have a crackpot theory that you think might be true, to get accepted you're going to have to show that it explains observations better than what we've got now. That's going to require a firm grounding in the appropriate science. Most people who don't have the credentials don't know nearly enough to do that.

Re: Or maybe, just maybe...

[david_thornley]

No, you can't have general FTL and special relativity and no time travel. The Bergenholms were no better than warp drive in that regard. In fact, since the ship settled at a speed where the thrust from the thrusters matched the power of the ambient stuff hitting the ship, it'd be slower than light. (That was the explanation, and the trip to Lundmark's Nebula was possible only because of the extreme rarity of intergalactic matter.)

I'm not complaining. There's lots of stuff I don't believe in the Lensmen series, but that doesn't stop me from enjoying it.

Re:Or maybe, just maybe...

[david_thornley]

Sure. I'm happy with the idea that, if God exists, God caused the Big Bang. What I'm saying is that it only matters if you have other reasons for believing in God, because in either case you're looking at one thing without cause.

Re:No

[next_ghost]

Here's a result of a 5 second Google search: Could black holes be the dark matter?

WIMPs

[JasterBobaMereel]

WIMPs are unknown particles with unknown properties and the speculative detectors we hoped might detect them haven't yet ... ...this does not mean they don't exist, it just means the things we don't know the properties of we still don't know the properties of ...

Re:WIMPs

[david_thornley]

Is there a good idea to think they're WI and not just MP? There's the practical matter that we can go looking for WIMPs, but I don't know of any reason why nonstandard matter would be affected by the weak nuclear force.

Re:Strawman poll

[McLae]

As seen on Fox News.

Re:No! (Betteridge Law of Headlines)

[avandesande]

More importantly the numbers don't add up. Our galaxy is 100 to 400 billion solar masses and 100 million black holes is not nearly enough.

Gravitational lensing

[jfdavis668]

Astronomers have observed the gravitational lens effect of dark matter. Dark matter normally surrounds normal matter, but is sometimes found separated. It appears that during galactic collisions, the dark matter can be separated from the normal matter, gas and dust of a galaxy. To do that, dark matter would have to interact with itself in a manner that does not involve gravity. A bunch of black holes would not interact in this way, so it is unlikely that dark matter consists solely of black holes.

Re:Gravitational lensing

[david_thornley]

Why would it have to interact with itself through non-gravitational means? It doesn't clump like regular matter does, so it would seem that it doesn't have anything corresponding to electromagnetism.

Re:Gravitational lensing

[jfdavis668]

If it separates during collisions, it is because dark matter is stopping dark matter. If it was caused by gravity, the same thing would happen to normal matter. No, they don't think it uses electro-magnetism, or it wouldn't be dark. It may interact via the weak nuclear force like neutrinos do.

Re:just wait

[Falconhell]

It was also proved by Disaster areas accountant, that space is not metely curved, it is totally bent, hence the evidence of dark matter.

Why such controversy?

[elistan]

I've never understood the emotional response to the theory of dark matter being some sort of mass we can't see. It seems like the theory personally affronts people for some reason.

The idea that a form of matter doesn't interact with three of the four fundamental forces doesn't seem all that crazy to me. I mean, the photon and W and Z bosons don't interact with the strong nuclear force, do they? (I'm actually asking - I'm no particle physicist.) And the neutrino interacts with the weak force and gravity, but not electromagnetism or the strong force. So the precedence exists - why not a particle with mass that interacts only with gravity? It'd be incredibly difficult to isolate and detect and measure of course, but I don't understand the push-back some people exhibit in regards to the concept. (The cool thing about that concept being that it perfectly fits, far as I'm aware, the observations scientists have made so far.)

Re:Why such controversy?

[wonkey_monkey]

Some people just love to get in on the action, and if they can't do it by understanding the science, they'll do it by blindly dismissing the science. It's an easy route to giving yourself the impression that you're "in the know," putting yourself in the contrarian position (which is not always an incorrect thing to do if you've got the understanding back it up) and making yourself feel part of a more exclusive group. Eventually this all feeds back and you get people who think that being in the minority itself makes them more right than everyone else. c.f. Moon landing conspiracy theories.

There seems to be such a stigma, particularly on the internet, about saying "I don't know" or "I don't understand."

Lisa Simpson: It's better to remain silent and be thought a fool, than to open one's mouth and remove all doubt.
Homer's Brain: . o O ( Uh-oh, what does that mean? Better say something or they'll think you're stupid!)

Re:Why such controversy?

[david_thornley]

Thing is, dark matter is easy to understand. WIMPs are at least much like large slow neutrinos, and the idea of "something like regular matter but - " shouldn't be hard to grasp.

Syfy channel has been eliminated already?

[Babel-17]

https://en.wikipedia.org/wiki/...

Need Dark Matter AND Energy

[Roger W Moore]

If you get rid of dark energy, you eliminate the need for dark matter as well.

That's completely wrong. Dark energy is needed to explain the acceleration in the expansion of the universe. Matter - either dark or ordinary - is gravitationally attractive and can never cause the expansion to accelerate. Dark Matter is needed to explain the "clumpiness" of the Cosmic Microwave Background, the rotation curves of galaxies and gravitational lensing observations e.g. bullet cluster. Black Holes have been considered a dark matter candidate before (MACHOs) and have been ruled out.

I think that about covers everything.

Re:Need Dark Matter AND Energy

[Mkkby]

The data that needs to be explained is galaxies are repelling each other, and the red shifts accelerate the further out you look. Black holes cannot explain this because black holes are an attractive force. Both astrophysics and particle physics are in chaos now. Somehow stars and galaxies are created by coalition of matter, yet on the larger scale they are being pushed apart. This is so bizarre it may never be explained. At the same time, the energy level of the newly discovered higgs boson has trashed decades of string theory. Our understanding of physics has been set back to the 1930s. It seems like God played a good prank.

Re:Need Dark Matter AND Energy

[Roger W Moore]

Both astrophysics and particle physics are in chaos now.

No, we are not. There are two unexplained phenomena at galactic and intergalactic scales but either could easily be explained by adding a new field to the physics we already have. This is no more 'chaotic' than it was before we found the Higgs boson or the top quark.

At the same time, the energy level of the newly discovered higgs boson has trashed decades of string theory.

That is simply not true. The Higgs mass is at the upper end of the allowed range for SUSY but it is still in the allowed range to avoid vacuum instability. I believe that there were some papers that suggested that the mass was closer to this limit than initially expected but I've not read any papers that indicated it was definitely over the limit.

Re:No

[careysub]

It is annoying having lazy clueless laymen's idle speculations being promoted to being a slashdot article.

Dark matter seems particularly to attract these sorts of totally uninformed wild guesses being thrown out to "solve" one of the deepest questions in modern physics and cosmology.

To all and sundry out there - if you just thought of it then the answer is "no". All possible known candidates have been thought of and eliminated. Whatever dark matter and dark energy are, it is nothing we currently understand. Even most promising theories seem to be failing at present.

Re:No

[Anonymous Coward]

"I've disproved climate change. You see, climate scientists forgot to take the Sun into account."

Re:dark matter doesn't interact

[Bengie]

Binary blackholes have a decaying orbit, but if you spread them out just a little bit, they will take forever to fall back in. Energy lost by gravitational waves scales with the something like the square/cube/something of the distance.

Re:No! (Betteridge Law of Headlines)

[david_thornley]

You'd think Planck-mass black holes would evaporate through Hawking Radiation almost immediately.

Re:No

[kevmeister]

While I can't argue your sentiments and certainly can't argue the pros and cons of black holes accounting for dark matter, I will mention that real live practicing and respected cosmologists have advanced the hypothesis. A major article on the subject can be found in the July issue of Scientific American It was written by a real cosmologist at a major university and a post-doc at another. I think a dismissive "No" is a rather silly and neaningless response, especially when submitted by an AC who professes knowledge without presenting any credentials.

A couple of problems

[HiThere]

Well.... they'd need to be pretty small, and also to not evaporate via Hawking radiation. Tiny black holes evaporating would release light at a known frequency that hasn't been detected (or hadn't been a decade ago, when I was paying attention). And the black holes would need to be primordial, because otherwise they'd affect the proportion of Lithium in the interstellar dust. So they can't have been engaging in nuclear reactions while that was being formed (*quite* early in the process). And they'd need to have a small enough capture cross-section that they wouldn't be capturing matter from a dense cloud back before the hyper inflation finished.

So you aren't talking about any normal black hole, but something rather special. Special enough to probably deserve a new name. I'm not sure what you're thinking the mass that would be necessary to create the gravity to cause the black hole is made from, but it would appear to need to be electrically neutral. The only thing that occurs to me is something like photonium, but how you'd get photons close enough and dense enough to create a Schwartzchild radius, even under those conditions, I can't imagine. Still, a black hole created of bound photons MIGHT not emit Hawking radiation. (I wouldn't want to bet on that, but the things would need to have a radius small enough that it might work.)

I actually think speculations about what exploded to create the big bang are more profitable than this one.

Re:No

[wolfheart111]

I have my secret weapon for understanding the cosmos... WEED... :) Seriously I have come up with some great ideas to explain these things... thinking way outside the box really does help.

Re:Dark matter does not exist

[Bengie]

Dark Matter has been proven. It itself is a collection of observations, aka facts. What we have is a collection of symptoms, but we are still searching for the underlying cause. But to even argue that Dark Matter may not exist is impossible to do in a logical argument. Dark Matter actually being a new form of matter is a hypothesis, with a lot of circumstantial evidence pointing in that direction. It's a problem that is over 100 years old. I'm sure we'll be excited to figure out what it is no matter what it is.

Closer than you think

[jouassou]

There was a recent interesting paper, "Concordance cosmology without dark energy", which explained how dark energy was actually not required to explain the structure of the universe, if one just used a more accurate numerical model to simulate how the universe evolved. They even resolved a long-standing issue in cosmology whereby different ways of estimating the Hubble constant from observations gave different results. I'm looking forward to seeing how this theory develops, and how their findings are received by the rest of the cosmology community.

Re:Dark matter does not exist

[Bengie]

Contradicted yourself in your first two sentences. An observation is binary and has only two states. It happened or it did not happen. A fact is literally something that is indisputable. If you can dispute an observation, you are disputing whether it exists. If the observation occurred and is true, there is no disputing that it exists, therefor is a fact .

I will concede that an observation could be a pseudo-fact in that there was an error with the instrument that measured it. This is why you need to be able to independently repeat the measurement, probably via multiple methods.

The fact of the matter (pun), is saying Dark Matter does not exist, as of right now, is the same as saying the observations do not exist, as Dark Matter is currently just a collection of unexplaninable observations that seem related.

Re:Neutrinos once proposed as WIMPs

[david_thornley]

Yup, the observed dark matter isn't neutrinos, but neutrinos are dark matter by the definition of not interacting electromagnetically.

Neutrinos are weird. Back when we thought they moved at lightspeed, they were understandable. Another massless particle moving at c. The reason we know they don't is that they change flavor, and hence have to be experiencing time . Other than that, we can't tell the difference. In a supernova, the neutrinos show up a few hours early because the light has to make it out of the core. You'd expect that the light, which has to be moving faster, would catch up with the neutrinos sometime, but we haven't seen that happen in any supernova that I've heard of.

Some probably "too simple" questions...

[PlaynBass]

What happens to all the matter sucked up by black holes?

If some of the mass is converted to energy inside the black hole, could that affect our perception of the mass of those black holes?

Essentially, would an amount of mass that is being converted to energy inside the black hole affect the amount of mass we attribute to a black hole from our vantage point on the outside of said black hole?

Would it behave like some sort of "anti-gravity" effect, or would it act as a "gravity enhancing" effect?

I'm just a layman here, so please forgive my ignorance, but I am serious about getting an answer to the questions.

Re:Some probably "too simple" questions...

[pepsikid]

Matter doesn't get converted to energy when it drops into a black hole, it gets mired in time. It's mass is instantaneously added to the mass of the whole though. Then again, over a very very very long period of time, the black hole will evaporate due to radiating Hawking Radiation. If you reason that everything will eventually be swept up by black holes, then it's possible that this radiation is the ultimate fate of everything. Unless the expansion of the universe prevents new black holes from forming at some point.

Science currently feels satisfied that our universe is not the inside of a higher-order black hole, but I keep thinking about all that mass that gets frozen in time upon a black hole's expanding surface. What if there actually is a lower-order universe inside of black holes, and it is populated by matter which, from the internal perspective, all seems to arrive at about the same time due to differences in the flow of time? Perhaps our own universe's Big Bang represented the matter which fell in during another universe's lifetime. Perhaps the last 13.82 billion years have only been a fraction of a flash for some future universe.