Distant Galaxy study, dark matter and the CMBR

[pinball1970]

TL;DR Summary

Looking at data from earth based telescopes and also Planck data.

The article is pop but there is a paper

Pop article

https://www.vice.com/en/article/g5v...ht-reveals-unprecedented-dark-matter-patterns

Paper.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.129.061301

The language is a little bit click bate, I just wanted a view. Pop Science web sites do not tend to give a measured view.

 

[ohwilleke]

Massive objects bend light by virtue of their gravitational effects. This is called gravitational lensing and has been known since General Relativity was discovered by Einstein.

The Cosmic Microwave Background (CMB) radiation happened before galaxies formed and has a very predictable distribution.

So, when Cosmic Microwave Background radiation passes near an old galaxy it should experience gravitational lensing and that should allow us to learn something about that galaxy when the radiation gets to our telescope.

It happens to be the case that for very old galaxies, the way that the galaxy lenses the CMB can be easier to discern with a high powered telescope than the way that lensing of light from the galaxy itself or other nearby galaxies is.

The telescope looked at the gravitational lensing of cosmic microwave background signals around a very old galaxy, which turns out to look essentially the same as what you do with a not so old galaxy (only its harder and requires a much better telescope to see).

Note that this doesn't actually do anything to distinguish between this phenomena being caused by a gravitational mechanism and being caused by a dark matter particle mechanism. The term "fossil relic" is a metaphor (or perhaps a metaphorical term of art) as used here, not a statement that they actually directly observed one or more dark matter particles. They merely inferred it from gravitational lensing of CMB signals by the old galaxy.

LambdaCDM (the so-called Standard Model of Cosmology based upon general relativity with a cosmological constant and cold dark matter particles) predicts that such an early galaxy should have a clumpy dark matter (DM) distribution. This inferred DM distribution was smooth just like lower z value galaxies are observed to have. So, the observation disfavors the paradigm in favor of models that produce recent type galaxies more quickly than the LambdaCDM model. Gravitational theories to explain DM phenomena generically make this prediction. Of course, it is also possible that some DM model other than the collisionless cold dark matter of the LambdaCDM also results in earlier galaxy formation.

However, all of these conclusions should be taken with a grain of salt. The results in the full paper are significantly weaker results that one might think are being asserted from the abstract or popular treatments alone. The 5.1 sigma result is that the CMB background radiation was lensed at all, not that the thing that lensed it has any really particularly precisely determined properties using this methodology.

It has been known for as long as the CMB has been known to exist that this should happen and should be possible, but knowing it is theoretically possible to do something and actually doing it are two different things.

The abstract states:

We report the first detection of the dark matter distribution around Lyman break galaxies (LBGs) at high redshift through the cosmic microwave background (CMB) lensing measurements with the public Planck PR3 κ map. The LBG sample consists of 1 473 106 objects with the median redshift of z∼4 that are identified in a total area of 305  deg2 observed by the Hyper Suprime-Cam Strategic Survey Program survey.

After careful investigations of systematic uncertainties, such as contamination from foreground galaxies and cosmic infrared background, we obtain the significant detection of the CMB lensing signal at 5.1σ that is dominated by 2-halo term signals of the LBGs. Fitting a simple model consisting of the Navarro-Frenk-White profile and the linear-bias model, we obtain the typical halo mass of Mh=2.9+9.5−2.5×1011  h−1 M⊙. Combining the CMB lensing and galaxy-galaxy clustering signals on the large scales, we demonstrate the first cosmological analysis at z∼4 that constrains (Ωm0,σ8). We find that our constraint on σ8 is roughly consistent with the Planck cosmology, while this σ8 constraint is lower than the Planck cosmology over the 1σ level.

This study opens up a new window for constraining cosmological parameters at high redshift by the combination of CMB and high-z galaxies, as well as studying the interplay between galaxy evolution and large-scale structure at such high redshift, by upcoming CMB and optical and near-infrared imaging surveys.

The "typical halo mass" has very wide error bars consistent with anything from zero to 11.9 M⊙ (i.e. zero to 11.9 million times the mass of the Sun) at the two sigma level (a sigma is a standard deviation in a Gaussian i.e. "normal" probability distribution).

The statement: "we find that our constraint on σ8 is roughly consistent with the Planck cosmology, while this σ8 constraint is lower than the Planck cosmology over the 1σ level" is also strange, as consistent normally means within two sigma of the best fit value, and a 1 sigma deviation (which is the expected among of deviation from the best fit value) normally wouldn't be mentioned at all.

σ8 is one of the observationally measured parameters of the properties of the universe that is used to describe the universe in the LambdaCDM model.

The big breakthrough claimed in the paper is using the methodology of using lensing of the CMB background radiation rather than lensing of the light from the galaxy itself to measure that there is a galaxy there, although the analysis isn't very well presented.

This method provides a new tool in a the toolbox for looking at very old galaxies and this, rather than the conclusions about this particular object or dark matter, is what makes this particular study notable.