Slashdot Mirror
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."
In case anyone's wondering what a baryon is...
http://en.wikipedia.org/wiki/Baryon
They found some of the ordinary matter that has gone unaccounted for, not dark matter. Read the article.
CDE open sourced! https://sourceforge.net/projects/cdesktopenv/
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.
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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.
They are extremely interesting for anyone fascinated with physics.
Creative Demolition
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.
It was. People were all over push for a while. Then it became passe.
"Who is the Journal of Quantum Physics going to believe?" --Stephen Hawking
Just to add my two cents (I do cosmology research) to the chorus of complaints about this post, this research is NOT about discovering a new form of dark matter. This is about solving the "missing baryon" problem, which is a whole different kettle of fish. It's well known how many baryons (normal stuff) there should be according to big bang theory. However, if you look out at the universe and count observed gas and stars, you just don't see as much as you should. So people have assumed there are some hidden regions, where the gas is too cool to emit significant radiation for example, that contain enough baryons to make up the missing baryon budget.
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.
The world is everything that is the case
Not correct, there are two classes of elementry particles (that we know about) Bosons and Fermions.
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.
Windows in 6 Bytes (IA-32) : 90 90 90 90 CD 19
"Dark" matter is regular stuff. Forget all the hyperbole about "exotic" new forms of unpredictable Star Trek technobabble with physics-defying properties. It's called "dark" matter because it's not "bright" matter, like stars, conveniently radiating bazillions of units of energy for us to easily spot them.
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.
You can detect hydrogen in X-ray telescopes. You're correct, there are no transitions and therefore no lines. However, X-rays ionise hydrogen and are absorbed, reducing the flux at low X-ray energies (below ~500 eV). It makes spectra kind of curve off towards zero at low energies.
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...
Slashdot - Mutual Assured Discussion
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?
Here's what I do: Bitty Browser & Andromeda
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.
Actually, you are wrong. Or you are right. Or you would be wrong if it were 5 years ago. Or.. OR AHHHHH
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.
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.
Professor of Astronomy, Author of Spider Star & Star Dragon (Tor)
it's dark because we can't see it, not because it is anti- anything.
Wayne Hu at the university of Chicago has a great set of webpages that explain these results. If you don't have much of a background, start with the lowest level and work up. To get to the hard numbers (two significant figures), check out the "experiments and data" link. They're based on the relative amplitudes of the acoustic peaks in the microwave background.
The page can be found here.
Professor of Astronomy, Author of Spider Star & Star Dragon (Tor)