Dark Matter's Profile Discovered?

pingbak writes "According to New Scientist, astronomers may have potentially discovered dark matter's EM profile (story). For the rest of us, this means astronomers may have just discovered all of the extra force holding the galaxy(-ies) together, which is not currently explainable though gravity and black holes at the center of universes alone. Since dark matter doesn't interact with ordinary matter, it's almost directly undetectable -- but now, physics and astronomy may just have had an awesome breakthrough. Nobel Prize material if it proves correct!"

Comments

Dark Matter Explaination?

[Randolpho]

Um... perhaps I'm very much misinformed, which is entirely possible, but the article submission makes the claim that Dark Matter doesn't interact with regular matter.

WTF? I thought the reason we're looking for Dark Matter is because the matter we *know* about doesn't add up to cause the gravetic interactions that we can observe. I thought Dark Matter was just matter we couldn't observe just yet, not some exotic "doesn't work the same as other matter" matter.

Am I totally wrong here?

Re:Dark Matter Explaination?

[CheshireCatCO]

They've looked pretty hard and carefully for normal, baryonic matter to cause the effects. So far, little has turned up.

On the other hand, it's pretty clear at this point that dark matter in *some* form must exist. It's just a simple grasp of gravity coupled with some weird observations that lead to this conclusion. It is, in fact, very similar to the way Neptune was discovered. First, notice something odd about Uranus's orbit, then realize that another planet at position X could explain it. Just do it with galaxies and clusters, instead, and you start to suspect there's dark matter out there. Do some surveys and find that there doesn't appear to be enough brown dwarfs and black holes to make up the needed mass.

To be honest, while I'm a planetary scientist and thus obligated to make fun of cosmologists, I don't find dark matter, even heretofore undiscovered particles, that hard to believe. Not only is the evidence pretty good, it isn't difficult to imagine that we've only scratched the surface of what is out there. You suggest that we're just finding "what's out there" (a claim with which I might quibble). So why is hard to believe that we haven't found all of the particles in the subatomic zoo? Especially given that the ones we seek are, by definition, difficult to find.

And if you want "too convoluted to be natural", study quantum mechanics. It seems the universe doesn't care what we consider to be "natural", after all.

(And now, a few quibbles: the SNP was only recently really clinched with lab data, but people had speculated about the solution, neutrino oscillations, for quite a while before hand. The same is true of a lot of what HST and others have told us in the past decade: usually, they're helping refine our models and confirm our best guesses as to what's out there. So it isn't like astronomers a decade ago would be shocked at what we've learned.)

Re:Dark Matter Explaination?

[radtea]

There are in fact many dark matter problems. This work deals with galactic dark matter only, which could be solved by normal matter. This is not the case for dark matter problems on larger scales.

On scales larger than galaxies, we can see that galaxies and groups of galaxies appear affected by stronger gravitational forces than can be accounted for by visible matter, and on very large scales it appears that there is more gravity than can be accounted for by ordinary matter, period.

There are strict limits on the amount of ordinary (baryonic) matter that come from primordial nucleo-synthesis. In the early seconds after the Big Bang, the quark-gluon plasma cooled down to form baryons (protons and neutrons, to you.) Eventually (some seconds or minutes later) the baryons cooled down to the point where their average energy was low enough that when they ran into each other they tended to stick together rather than flying apart again.

The nuclei formed in these early times were: hydrogen (single protons), deuterium (p+n), 3He (p+n+n), 4He (p+p+n+n) and tiny amounts of lithium and above. As you can imagine, the ratios of hydrogen to everything else are critically dependent on the density of neutrons at the time these nuclei were forming: the more neutrons, the more heavy nuclei. We can infer from observations on stars fromed shortly (~millions of years) after the Big Bang what these ratios were, which tells us the density of neutrons and protons at that time. We also know the volume (curvature) of the universe at that time, and so can infer the total number of baryons in the universe.

There are not enough baryons to account for the gravitational (or gravity-like) forces acting at the largest observable scales. Ergo, something else is happening (or the observations are wrong, which is always a possibility.)

Given the number of dark matter problems, it is unlikely that one particle will solve them all, and quite possible that we are seeing the effects of quite different causes at different scales. This should not be a surprise. For example, we already see very different causes at nuclear scales vs. atomic scales vs planetary scales. It is possible that on galactic and size-of-the-universe scales different causes are at work as well, either due to hitherto undiscovered forces or equally unknown particles.

--Tom

Re:Electrons?

[BigBir3d]

I am sure that this is more due to the writer's lack of understanding of the subject, then to what appears to be grand jumps in logic. I am somewhat sure the language of the paper will be much more clear.

However, because dark matter "feels" gravity like ordinary visible matter, it is a fair bet that it clumps in the centre of our galaxy.

Previous statement makes no sense until it is explained later that they started down the course of thinking dark matter has a mass far less then previously postulated.

"Heavy dark matter particles would produce high-energy electrons," says Hooper. "Since it's difficult to imagine how they could be slowed to a standstill, we were forced to consider a surprisingly light dark matter particle."

By "light", the researchers mean one to 100 megaelectronvolts, which is between 1000 and 10 times lighter than a proton.

a little from column A, a little from...

[bscott]

What they mean by "weakly interacting" is similar to how neutrinos are described - it doesn't have much of an electromagnetic impression, so it doesn't block light or smack into a detector in an earthbound observatory. Unlike neutrinos, it does posess a significant mass and is affected by gravity. And while that is "exotic", astrophysicists were only forced to consider this sort of thing when all previous efforts to explain some pretty obvious mis-matches in the numbers didn't work.

Now I'll let someone else explain about "dark energy"...

Profile

[mopslik]

In other news, dark matter's IM profile has also been found:

Name: Matter, Dark
Nick: d4rkm4tt3r
Age: ~15 billion years
Likes: Vast emptiness of the cosmos.
Dislikes: Peeping-Tom astronomers.
Bio: I generally keep a low profile, out of sight. Maybe one day, the matter of my dreams will see me for who I really am.

good description of the kinds of dark matter

[rsdavis9]

http://www.astro.queensu.ca/~dursi/dm-tutorial/dm0 .html

From the link above there is:
1. cold dark ordinary matter(baryonic)
2. Non baryonic(exotic) dark matter both hot and cold

The article seems to indicate wilp(weakly interacting light particles instead of (in addition to) wimps(weakly interacting massive particles. Wilp's are like neutrinos. We have not discovered any wimps yet.

Electronium?

[4of12]

IANAHEP, but is there anypossibility that an electron and a positron could orbit one another with a reasonably long half-life?

Re:Electronium?

[barawn]

Positronium.

It has a half-life of 0.1 uS. It's a relatively standard physics problem at the graduate school level to ask what the binding energy of positronium is.

If it ever comes up, it's (1/2) the binding energy of a hydrogen atom. The reasoning is simple - a positron and a proton have the same charge, but a positron and an electron have the same mass, so the "reduced mass factor" is 1/2, rather than 1. (M_p/(m_e+m_p) ~= 1) vs (M_e/(m_e+m_e) = 1/2).

Neutrino's Big Cousin -- conclusions

[pbhj]

In the conclusions they appear to be saying that some new interaction is happening due to ('mediated by') exchange of a light gauge boson (translation: low-energy force-carrying integer-spin-particle)

Alternatively a new heavy fermion (neutrinos are fermionic, spin-1/2) mediates in the interaction: their words "could be responsible". So you might not be far off (if there second guess is correct).

Start talking Nobel prizes when CERN/Fermilab find either of these particles.

[... I've not done any particle physics for 5 years so this could be baloney.]

Re:Electrons?

[KDan]

There are many reasons, but one which you might look at is the large amounts of radiation coming from the galactic nucleus. As you may know, electrons absorb radiation and gain energy (velocity) when they do so.

Another explanation, if you take away the radiation, would involve a huge fermi sphere of electons which would require that only very few of them are sitting in that big gravity well with no kinetic energy.

There are other reasons why electrons would be very unlikely to be found at rest at the galactic core, but I think these will do.

Daniel

Re:center of universes?

[Red Rocket]

And more to the point, how do they find the centers?

You just keep licking them and you eventually get to the centers.

Re:I don't get it

[Royster]

There have been two leading candidates for dark matter: WIMPs and MACHOs. Each camp have had their proponents.

WIMPS: Weakly Interacting Massive Particles. Neutrinos on steriods. Since they only interact through the weak and gravitational forces, they are by definintion dark in EM. But, we haven't found any in colliders.

MACHOs: MAssive Cosmic Halo Objects. You're describing MACHOs. Ordinary, cold, dark matter. But there's probably too much of it to be this. It should have been swept up into stars.

Frankly, I think that the energy levels detected will prove to be not what we're seeking here. It's too much of a coincidence that it is the e/e-bar annilation energy. OTOH, if there were a WILP which did have such a mass, we'd probably never see it thinking we were looking at e/e-bar reactions.