Dark Matter Discovered

sebFlyte writes "Wired is reporting that scientists have come up to a solution as to where all the matter in the universe actually is. Experiments being done with Chandra, NASA's X-ray telescope have shown up a likely candidate for the solution of the dark matter problem. There are massive quantities of Baryons in a super-heated gas cloud several hundred million light years away."

Comments

Baryons

[Anonymous Coward]

In case anyone's wondering what a baryon is...

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

Re:Baryons

[FalconZero]

In case anyone's wondering what slashdot is...

http://www.slashdot.org/

Can I have my +5 informative now??

Ummm

[christurkel]

They found some of the ordinary matter that has gone unaccounted for, not dark matter. Read the article.

Re:Ummm

[Bootsy Collins]

Dark matter isn't mysterious or unordinary. Dark matter is usually extremely cold but otherwise ordinary matter. Because it's so cold, it can't emit light, hence "dark" matter. So, while they did discover dark matter in the sense that most astrophysicists use the term, they did not discover the really weird stuff.

You have, however, picked up on an important distinction. They found dark matter, but what they really need to find is dark energy. Dark energy is thought to comprise something like 70% of the energy of the universe, and yet, even today, it is a complete mystery.

I dunno whether you're confused about this stuff, but your post makes some statements which are at least confusing, and possibly wrong, depending on what you meant (I can't really tell). So to clarify, for anyone who cares: the energy content of the Universe appears to have three components:

• visible baryonic matter (about 2% of the total);
• dark matter, of which a small fraction is expected to be nonluminous baryonic matter (about another 2% of the total), and the majority of which is expected to be (hypothesized but not yet discovered) non-baryonic matter (about 23% of the total);
• the absolutely horribly named "dark energy", which from a particle physics perspective can be thought of as a vacuum energy density, and from a General Relativity/Friedmann Equations point of view can be thought of as a cosmological constant (about 73% of the total).

Re:Ummm

[dspeyer]

whether the missing mass lurks in galaxies, in galactic halos, or between galaxies is (as I recall) an open question.

Maybe there's more then one sort of dark matter, but the dark matter I've studied must be inside galaxies.

Dark matter is the extra mass needed to explain the observed motion of astronomical bodies in terms of known forces (ie gravity) after all the known matter is accounted for. In particular, galaxies rotate like rigid bodies (the same angular velocity at all radii) whereas the distribution of known mass (eg stars) suggests they shouldn't. An enormous amount of extra mass must be within the galaxies in a specific distribution to make this happen. (The alternative, which astrophysicists dislike, is that our equations for gravity are wrong for large distances.) This cloud is outside of a galaxy, so it isn't the missing matter.

Now, there may be other discrepancies between what we can see and what we can compute should be there, and this cloud could explain some of those.

Not quite...

[Anonymous Coward]

The summary is not correct (big surprise there) in that this is a confirmation of a long-suspected theory as to where the missing ordinary (baryonic) matter in the universe is. This does not solve the dark matter problem at all.

Read more at the press release from the Chandra team at Marshall: http://www.spaceref.com/news/viewpr.html?pid=16049

Dark matter is yet another topic altogether, as is the even more elusive dark energy.

Re:Aren't baryons just normal matter?

[randominator]

Tachyons are in fact hypothesized faster-than-light-particles, appearing for instance in certain string theory scenarios.

But baryons are by no means the counterpart to tachyons. All known elementary particles in the universe are either fermions (particles with spin in integer multiples of 1/2) or bosons (particles with integer spin). Bosons include the photon, the gluon and many others. The fermions are further subdivided into leptons and quarks. Leptons include the electron and the electron neutrino among others. Baryons are particles made up of three quarks, and are fermions and include among others, the proton and neutron, which are the most commonly found baryons in nature, since all heavier baryons normally decay.

Two quarks (fermions) can combine to form mesons, which are in fact bosonic in nature (since two quarks with spin half combine to form a particle with integer spin).

Hope that confused the issue a little :-)

A bit more on-topic: Finding baryons in this amount is a big deal, since baryon has previously been suspected to primarily exist in galaxies, and only in small amounts outside galaxies. While it by no means doesn't solve all problems of cosmology, it is a big help.

Re:Wait a sec, this story isn't about "dark matter

[turnstyle]

If most leading cosmologists aren't sure that the missing dark matter is baryonic (regular stuff), what makes you so sure?

Dark matter might yet prove to be baryonic, but since about 70% of the universe is the even weirder dark energy, why is it so impossible to believe that 25% could be a new type of matter that interacts gravitationally, but not in other expected ways?

Wired (perhaps) isn't confused. You (perhaps) are.

[mattorb]

Hi --

Distinguishing between baryonic matter -- stuff that bears any resemblance to everything around you, whether it is visible or not -- and other "dark" matter that does not fall into that category, is actually pretty commonplace in astrophysics. This seems like semantics, but turns out to be an important distinction.

The point is that the fraction of baryonic matter in the universe is, we think, reasonably well constrained (by both observations of light element abundances in conjunction with Big Bang nucleosynthesis models, and by measurements of fluctuations in the cosmic microwave background) to be only about 5% of the total mass/energy density. Yet there's an additional matter component (accounting for about 25% of the total density) that we know little about -- this is what most astronomers mean when they say "dark matter" these days.

This article says nothing at all about that 25%. It does, however, provide some clues towards a more complete accounting of the 5% that is "normal" (i.e. baryonic) matter. This is a very significant result, but the slashdot writeup and most of the comments to this article are completely distorting it.

The puzzle regarding the "normal" 5% was this: in the local universe (redshifts less than 2), only 10% or so of it is luminous matter, stars and galaxies and the like. More (40% or so) has been accounted for by studies of cool clouds of gas residing between stars, but this still left 50% in an unknown reservoir of baryons. Theory/simulation had suggested that one such reservoir might be the "warm/hot intergalactic medium" -- gas that is heated to millions of K.

The problem is that detecting low-density gas at that temperature is quite difficult, partly since most bound electrons have been lost. Only the more massive elements retain any electrons, and so can be visible in absorption in the FUV or X-rays.

What the paper discussed here (published today in Nature) does is to describe a plausible-looking detection of such filaments of "warm-hot" gas, through X-ray absorption. They use this detection to extrapolate a matter density of this WHIM component, and find that it could account for 30-50% of the baryonic mass, and so constitute the "missing" baryonic matter.

Note that this says nothing at all new about the 25% of truly "dark" non-baryonic matter.

One fairly large quibble is that the 30-50% number represents an extrapolation from just two absorbers, over a comparatively short distance, to infer the WHIM density in the whole universe. That's sort of a big jump, in case that part wasn't obvious. But you can't do this sort of analysis for very many sightlines -- you need a really bright emitting object on the other side of the WHIM clouds if you're going to see them, and such objects are few and far between -- so for right now that's what you get.

If you happen to be somewhere that has a subscription to Nature (most universities do), you can check out the two articles related to this in today's edition:

There's a "news and views" article by Mike Shull that's a nice summary of the issues involved. And there's the full research article by Nicastro et al.

Hope that clears at least a few things up. If I have time later tonight, I'll try to come back and respond to some of your other points.

cheers.

Re:Wait a sec, this story isn't about "dark matter

[mbrother]

Yeah, at least part of them. You can go to my website above, hit "Astronomy Work" link on the left, and be taken to http:physics.uwyo.edu/~mbrother where you'll find links to three recent courses I've taught. The intro astronomy course (1050) is currently in session and so the slides for that one are incomplete. These are slides, meant to accompany lecture, so they aren't enough on their own, but you might enjoy looking anyway.