Dark Matter is Missing From Cosmic Voids
Written by Nancy Atkinson
Map of distribution of galaxies. Credit: M. Blanton and the SDSS.
Cosmic voids really are devoid of matter. Astronomers have found that even the pervasive 'dark matter' which accounts for about 80% of the mass of the universe is not present in these voids, which are areas of vast emptiness in space that can be tens of millions of light-years across. "Astronomers have wondered for a quarter-century whether these voids were 'too big' or 'too empty' to be explained by gravity alone," said University of Chicago researcher Jeremy Tinker, who led the new study using data from the Sloan Digital Sky Survey II (SDSS-II). "Our analysis shows that the voids in these surveys are exactly as big and as empty as predicted by the 'standard' theory of the universe."
The largest 3-dimensional maps of the universe show that galaxies lie in filamentary superclusters interlaced by cosmic voids that contain few or no bright galaxies. Researchers using SDSS-II and the
Two-Degree Field Galaxy Redshift Survey (2dFGRS) have concluded that these voids are also missing the "halos" of invisible dark matter that bright galaxies reside in.
A central element of the standard cosmological theory is cold dark matter, which exerts gravity but does not emit light. Dark matter is smoothly distributed in the early universe, but over time gravity pulls it into filaments and clumps and empties out the spaces between them. Galaxies form when hydrogen and helium gas falls into collapsed dark matter clumps, referred to as "halos," where it can form luminous stars.
But astronomers were not sure if the areas that are devoid of galaxies were also devoid of dark matter, or if the dark matter was there, but for some reason stars just didn't form in these voids.
The research team used bright galaxies to trace the structure of dark matter and compared it with computer simulations to predict the number and sizes of voids.
Princeton University graduate student Charlie Conroy measured the sizes of voids in the SDSS-II maps. "When we used galaxies brighter than the Milky Way to trace structure, the biggest empty voids we found were about 75 million light years across," said Conroy. "And the predictions from the simulations were bang-on."
The sizes of voids are ultimately set, Conroy explained, by the small variations in the primordial distribution of dark matter, and by the amount of time that gravity has had to grow these small variationsinto large structures.
The agreement between the simulations and the measurements holds for both red (old) and blue (new) galaxies, said Tinker. "Halos of a given mass seem to form similar galaxies, both in numbers of stars and in the ages of those stars, regardless of where the halos live."
Tinker presented his findings today at an international symposium in Chicago, titled "The Sloan Digital Sky Survey: Asteroids to Cosmology." A paper detailing the analysis will appear in the September 1 edition of The Astrophysical Journal, with the title "Void Statistics in Large Galaxy Redshift Surveys: Does Halo Occupation of Field Galaxies Depend on Environment?"
News Source: SDSS and The Ohio State University
Filed under: Dark Matter, galaxies
August 18th, 2008 at 9:26 pm
Yeah I also don't understand James B's rant… But just to entertain everyone I have heard of some other theories that can explain what we are observing here. Georgi Dvali of NY university has offered an alternative theory with his work on "gravity leaking." And yes he has advanced his theory through peer review. So yeah there are some not so crackpot alternative theories out there. The bottom line is still that we need some kind of empirical evidence. I guess my point is that I wouldn't put all my eggs in the dark matter basket just yet, but this work certainly looks promising. Seriously JamesB think before you post… so PWND.
August 19th, 2008 at 2:56 am
"Jon Hanford Says:
I can think of several 'simulation' scenarios that paid off in a big way for astrophysicists. Detailed mathematical studies of the Big Bang theory predicted the existence of the Cosmological Microwave Background radiation decades before its existence was verified (because of the lack of instruments to detect it). Black holes were predicted again through rigorous mathematical studies (simulations) long before astronomers actually unambiguously detected them."
Ok, the Big Bang is still a hypothesis that is riddled with holes and is on VERY weak feet empirically. We don't know what the CMB is, though many competing theories as to it's origin have less holes than the current dogma and make more sense.
Even black holes have never gotten past the hypothetical stage as the observational evidence of them has several different interpretations, though the black hole hypothesis of this phenomenon is the strongest one at this point in time. I suspect that black holes do exist, personally.
So we are studying mathematical constructs that may or may not actually be real. And we use computer models to study these phenomenon because we have no other way to observe them (real or not they are at the very least mathematical phenomenon once modeled).
To paraphrase Stanislaw Ulam, a lead mathematician on the Manhattan Project, "Give me 15 free parameters, and I can draw an elephant. Give me 16, and I can make it dance."
So a model can be brought in line with observational evidence and then a claim can be made that the model predicted it or a flavor of it. Or if you do several runs of any model and randomly tweak the data fed to the simulation you can pretty sure at least ONE of the runs will match some observational data now or in the future.
Do I hate scientists and computer modelers (which BTW aren't always the same!)?? No, I don't hate them.
But I am appalled to see what amounts to unproven rumor being funded to the detriment of real science and real problem solving. We are building a house of cards that will topple and we'll enter a dark age of cosmology much like happened in 1887 (and even before that when all that time and effort was wasted determining the effects of 'aether' and the modeling of it's effects, etc).
Decades of set back because arrogant scientists refused to believe their pet hypotheses could be wrong. It's a case of trying NOT to work yourself out of job or the status that comes with being right…
And history is full of this, from the flat earth to the earth centered solar system, aether and on and on. Hold onto a well funded idea and you hold onto power. You're usually the last one to smell your own BO, and many scientists in the big leagues are the last to smell a rotten idea if it's one they hold close.
And here I'm not talking about the little guys who's careers aren't made or broken on such things, though I suppose it's normal for the little guys to see themselves as bigger than they really are. Mediocrity doesn't drive science though, it merely feeds it. You're job isn't in danger!
But we are repeating these mistakes of the past.
I personally DO NOT understand the phenomenon that 'dark stuff' was invoked to explain. But I DO understand that we do not know enough about the basis of this phenomenon (gravity) to know that a phenomenon even exists. Maybe we are trying to solve a problem that isn't really a problem!!
Recently light was slowed down in a laboratory to ~38mph (MILES PER HOUR) under conditions very similar to deep space. Is red shift in light from other galaxies really because they are moving away, or even moving at all? There are good reasons to suspect that red shift may have other explanations.
The whole basis for 'dark stuff' unravels under even minor scrutiny, and the proof offered by computer models is not proof in an any sense of the word.
August 19th, 2008 at 3:39 am
One more thing Jon-
You may think from my post above that I despise computer modeling. I don't. I despise the importance being placed on it's role and the results it produces.
A jeweler may chose to make a wax model of an item he is about to craft. But the wax model is nothing more than a crude representation of the final piece of art produced by the jeweler. And there is actually very little information this wax model can actually give us about the finished piece.
We can only really understand the jeweler's skill and mastery when we see the actual piece of jewelry.
It was important for the jeweler to create that model and use it to understand what his final item would be. His results are likely greatly enhanced by the understanding of symmetry and placement the model gave him.
But at no time is he ever inclined to sell that model as if it were the real item of jewelry.
In modeling phenomenon we have not observed or can not observe or even have observed but do not understand, we create a wax mock-up that is then sold as the real thing. And this is because there is no real, existing phenomenon to sell. Only the rough wax figure that conveys so little about the real thing.
Worse yet, this wax mock-up of these computer models are built without any idea of how they are supposed to look. It's like seeing a copy of the 'Mona Lisa' painted by someone who has only had the real 'Mona Lisa' described to him. The eye's will most likely not follow you around!!
The importance of computer models is much overrated, not only in cosmology but in other areas of science too, such as climate modeling.
I'm against passing off crappy copies of the 'Mona Lisa' as the real thing. That's all. A model of the Big Bang is not proof of the Big Bang and does very little in forwarding the hypothesis like empirical observations would. And oddly enough, the Big Bang is strong in modeling but rather weak observationally!
August 19th, 2008 at 5:20 am
JamesB, thanks for your thoughtful comments on the state of astrophysics today. I can see that you, like many others, are interested in discovering truths about how the universe around us functions & evolves. This is certainly a commendable goal. What puzzles me, however, is your distrust of scientific consensus by a majority of the astronomical community who have collectively spent many hundred(if not thousand) man-years on these complex issues & thus dismiss these widely accepted theories out of hand. I don't discount that scientific dogma has played a role in discouraging or impeding probably all aspects of science. But in the end, the scientific process, whose goal is to seek the truth, will correct and clarify the physical realities of the universe. But I must agree with David R. on this topic: publish a paper utilizing the same standard of research as those that you disagree with and advance an alternative theory. Minus that, at least reference credible published research in reputable journals to enlighten me & others on such topics you mentioned earlier (CMB origin, Dark Matter & Big Bang alternatives( I'm already familiar with MOND theory)). I'm certainly willing to keep an open mind to all reasonable, prediction-oriented theories on these topics.
August 19th, 2008 at 6:24 am
At last… a discussion right up my alley!
First of all, everyone who works in the astronomy field does not agree with the standard model. To my knowledge, nobody has lost their job because of their view one way or the other.
I will say MOST do believe in it, because of the overwhelming evidence supporting it. Yet, the standard model does hang by a few threads (of theory), and if any one of them are found to be incorrect, then the standard model of today is thrown out the window. One of the biggest being gravity is constant.
My undergraduate degree is in computer science, and I've help build a few models in my time. So in a very short spurt of words…. Remember, these models are working with figures which are largely based on theory, which is based on the affects of gravity. Since gravity seems to be anywhere matter is, naturally the models are going to come out pretty close to the way the universe is today.
So dark matter simply being matter which exists, but not in atoms; and we have never actually seen this… the only way to model it is by assigning gravitational variables to it.
So its possible there are different types of "dark matter" out there, but we are classifying it the same. Who says all these particles are the same mass?
So in short, the models do a great job of proving gravity exists wherever there is mass; but it doesn't prove beyond a shadow of a doubt there is dark matter. You can get similar results if you plug in variable gravity. Could dark matter be some odd form of gravity? Is there other forces other than gravity that we cannot see? Perhaps if we could artificially create it, we'll figure this out.
Its truly a shame, we exist in this universe for a very short time.
History has shown we don't always get it 100% correct. However, we do seem to get a large percentage of it right. So the truth of the standard model is probably some crazy mix between all beliefs out there.
August 19th, 2008 at 9:32 pm
People, please remember your elementary grammar: its (possessive) vs. it's (it is)!
September 16th, 2008 at 11:27 pm
astrogenia:
BTW, a great reference book on grammar is Lynne Truss's *Eats, Shoots & Leaves: The Zero Tolerance Approach to Punctuation* (http://www.amazon.com/Eats-Shoots-Leaves-Tolerance-Punctuation/dp/1592402038/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1221632768&sr=1-1) — another lady for whom bad grammar is like cosmic fingernails down a universal black board.
Yaaaaayyyy! At last — somebody who truly appreciates the written English language! I've been tearing my hair out for years over confusion of 'its' with 'it's,' misuse of prepositions, and a host of other grammatical sins committed by too many people on the Internet. When I've pleaded for more care with the language, I've been called "fat, ugly, and stupid" because of it. At least now I know I'm not all alone out here.
September 19th, 2008 at 11:21 am
I have already submitted comments.But why not appearing ? Why my comments on Dark-Matter and Dark-Energy are missing ?. Soon after writing in this box, my comments were appeared here - but after a day, these are deleted - why ? I request editorial board of Universe-Today to explain if any system fault or there need any special procedure for presentation of comments ?
please reply me on astrophysics@rocketmail.com
Dr.Navinchandra K.Shah.