# First 3D Map of Dark Matter in the Universe

1. Jan 7, 2007

### neutrino

http://www.esa.int/esaCP/SEMZ6GSVYVE_index_0.html

Go read it while I try to comprehend how they managed to map the dark matter in the universe by studying a relatively tiny part of the sky.

2. Jan 10, 2007

### oldman

Does this tell us the average dark matter density?

Richard Massey of Caltech, in discussing his mapping, emphasizes its fundamental importance by pointing out that: “Almost all current scientific knowledge concerns only baryonic matter”, implying that the matter he has mapped is not baryonic matter.

He is expressing the current consensus, supported by the WMAP results, according to which baryonic matter constitutes only 17% of the mass in the universe. The remaining 83% is thought to be the dark matter mapped by Massey. His mapping makes sophisticated use of the fact that general relativity can be used to calculate how mass deflects starlight, as verified long ago during the 1919 solar eclipse.

The eclipse data could have been used to estimate the sun’s mass, had there been any need to use such an arcane method. Presumably the Hubble data used by Massey to map dark matter could likewise be used to estimate the percentage contribution that dark matter makes to the average mass density in the universe.

There is a need for such an estimate to be made explicit. It would provide the first actual measurement of this important parameter, so far only calculated to be 83%, on the basis of the flatness of the universe's geometry, determined by WMAP, taken in the context of the standard model.

The COSMOS results are indeed of fundamental importance for cosmology.

3. Jan 10, 2007

### hellfire

In the $\Lambda$-CDM model, the baryonic fraction of the total energy density in the universe is about 4% and the fraction of matter energy density is about 27%. This means that the baryons are about 15% of the total matter. There are actually several methods to estimate the amount of matter in the universe ($\Omega_m$) and some of them constraint only combinations of $\Omega_m$ and $\Omega_{\Lambda}$ (the rest of 73% energy density related to the cosmological constant). You can find a detailed overview here:

Measuring Omega
http://arxiv.org/abs/astro-ph/9611108

Dark Matter and Energy in the Universe
http://arxiv.org/abs/astro-ph/9901109

For an overview of the determination of cosmological parameters and their interrelations take a look to this:

Determination of Cosmological Parameters
http://arxiv.org/abs/astro-ph/9905222

Last edited: Jan 10, 2007
4. Jan 10, 2007

### ray b

as normal matter comes in two forms
is it likely dark matter has an anti- form also?
and while normal matter is mostly H with some He and a little everything else
will "normal" dark matter be also split in to types based on sub-units?

5. Jan 10, 2007

### oldman

Thanks for setting out how mass/energy fractions are found in the lambdaCDM model, hellfire, and for the URL's. The question I was asking, though, is somewhat different -- I apologise for not making this clearer.

More specifically, it is this:

Does the recently reported mapping of dark matter include an estimate of the dark-mass fraction calculated from the COSMOS survey? I still believe that this would be the first measurement of this fraction., as distinct from estimates calculated, as you describe, by various methods that rely on existing models of how the universe is put together.​

By the way, I excluded from the percentages I gave any contribution from the mass equivalent of dark energy and/or the cosmological constant; both are theoretical constructs. The percentages of masswere taken from the First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters by Spergel et al., which of course gives model-based estimates derived from various features of the power spectrum of the temperature fluctuations in the CMB.

6. Jan 10, 2007

### hellfire

The density of matter (baryonic as well as non-baryonic) can be determined using observational methods that are independent of the cosmological model, others than the CMB anisotropies. Please take a look to the first paper I linked above ("Measuring Omega").

7. Jan 11, 2007

### oldman

I have looked at the references you so kindly provided. They make it clear that the density of matter was still a very moot question in the 1990's, when the articles were written. There are indeed many complex ways of estimating the density other than the CMB studies, none of which (yet?) seem to offer a satisfactory determined value.

But that was then; this is now, and there are new observations to consider, like the mapping from the COSMOS survey, which is the subject of this thread. I would still like to know whether these results have been used to determine the average density of dark matter, or whether they could be so used.

8. Feb 1, 2007

### heusdens

Or.... there isn't Dark Matter or Dark Energy, as proposed by (amongst other) Philip Mannheim.

See:
arxiv 0505266