Dark Matter Discovered 386
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."
Baryons (Score:4, Informative)
http://en.wikipedia.org/wiki/Baryon [wikipedia.org]
Ummm (Score:5, Informative)
Not quite... (Score:5, Informative)
Read more at the press release from the Chandra team at Marshall: http://www.spaceref.com/news/viewpr.html?pid=1604
Dark matter is yet another topic altogether, as is the even more elusive dark energy.
More Information (Score:4, Informative)
They are extremely interesting for anyone fascinated with physics.
Re:Ummm (Score:1, Informative)
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.
Re:Baryons (Score:5, Informative)
http://www.slashdot.org/ [slashdot.org]
Can I have my +5 informative now??
Re:Aren't baryons just normal matter? (Score:5, Informative)
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:Ummm (Score:3, Informative)
Wired Magazine seems to be getting their terms confused:
But one candidate for the "dark matter" (everything we can't see) *is* "baryons" -- which is just a funny term for "protons and neutrons", which is just a funny term for "ordinary stuff". (The other candidates for dark matter are unknown new particles--WIMPs and so on.)
So, basically, what these guys have found is an intergalactic gas cloud of heavy gas. They mention C, N, O, and Ne in the article; those are four of the principle products in stellar nuclear fusion, so that makes sense. However, they don't mention anything about H and He, the principal components of the universe. They used X-ray absorption, however, and since H (and I think He also) don't have electron transitions in the X-ray band, hydrogen would be invisible to their technique.
So they really don't know what the density of the cloud is, 'cause they can't measure the presence of hydrogen, which is *usually* the dominant component of the interstellar medium (as I recall).
If the cloud is principally heavy gas, then it's obviously left over from exploding stars. The explanation that comes to mind is that parts of the exploded star blew off with enough velocity to escape the local gravity and found themselves in intergalactic space. Whether it takes exotica to prevent them from being "pulled into galaxies" is another question. We know from previous observation that gravitationally-bound systems can contain local concentrations of matter whose kinetic energy keeps them from falling into the central concentration of mass in the system: q.v. Sol III (known as Terra to the locals).
Basically, this Wired article is *very* short on actual scientific facts. Maybe the original study actually says something and doesn't just try to impress readers with the word "baryon"; accurate measurements of the intergalactic medium *are* sorely needed by astrophysics, and whether the missing mass lurks in galaxies, in galactic halos, or between galaxies is (as I recall) an open question.
On a more technical note, it'd be interesting to see how much the X-ray absorbtion lines are smeared out in these measurements; I don't know if they have enough data for really good spectrography, but knowing that would give a rough estimation of the kinetic energy of the cloud: the gas atoms traveling away from us would have their spectra redshifted more than those traveling toward us.
Re:If WIRED says it, it must be true! (Score:3, Informative)
Wrong wrong wrong! Aarrrgh (Score:2, Informative)
Also, this seems pretty provisional stuff. I doubt this is the final word on the missing baryon problem. It certainly has nothing to say about the nature of dark matter or dark energy. But I guess some gushing "dark matter discovered" hype is just too tempting.
Re:Dark matter question (Score:4, Informative)
Not correct, there are two classes of elementry particles (that we know about) Bosons [wikipedia.org] and Fermions [wikipedia.org].
Bosons are things like :
- Photons
- Gluons
- W and Z Bosons
- Higgs Bosons
Bosons don't have anti-particles, and are less likely to form stable structures.Fermions are things like :
- Quarks
- Electrons
- Neutrons
- Protons
Fermions do have anti-particles, and form the everyday matter that you interact with.IANAP, but two photons cannot cancel each other out, however two beams can (assuming they are co-axial and anti-phased).
As for the flashlight, general light is not regular so you certainly can't make one using interference.
Re:Ummm (Score:5, Informative)
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:
Re:Wait a sec, this story isn't about "dark matter (Score:3, Informative)
It seems perfectly reasonable that there exists matter that's not formed into glowing plasma balls and is thus harder to spot.
But that presentation is kind of prosaic, and wouldn't sell lots of issues of the World Weekly News.
Hydrogen (Score:2, Informative)
Our view of distant galaxies is affected by this, you always have to take account of a) the ~known amount of hydrogen in our galaxy and b) any other hydrogen between us and the source - this will give a redshifted absorption since it's at cosmological differences. It's pretty tricky with the quality of data you get with current telescopes to work out the redshift of any hydrogen that's out there (i.e. to figure out if it's associated with the source or an absorber on the line of sight like the one they discovered), because it's a smooth curve and not a line. That's probably why there are no numbers given. I'll have to read the paper though...
Re:Ummm (Score:5, Informative)
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.
Re:Wait a sec, this story isn't about "dark matter (Score:5, Informative)
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. (Score:5, Informative)
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 [nature.com] by Mike Shull that's a nice summary of the issues involved. And there's the full research article [nature.com] 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:WRONG TITLE, Sigh...... (Score:4, Informative)
Dark matter was originally used to refer to matter that was not yet accounted for. Non-baryonic matter being a subset of Dark Matter.
The issue has been beaten to death so badly by poor authors that 'Dark Matter' is becoming assumed to refer to NB matter.
It is hard to argue that you are wrong, but equally hard to win an argument saying you are correct.
Re:Wait a sec, this story isn't about "dark matter (Score:5, Informative)
Re:How can "dark energy" count as mass? (Score:2, Informative)
Re:Wait a sec, this story isn't about "dark matter (Score:4, Informative)
The page can be found here. [uchicago.edu]