## Debating the Cosmological Principle

Posted in The Universe and Stuff with tags , , , , on November 5, 2020 by telescoper

Whether you need something to distract you from world events or are just interested in the subject I thought I’d share something cosmological today.

You may recall that I recently posted about a paper by Subir Sarkar and collaborators.  Here is the abstract and author list:

In that post I mentioned that Subir would be taking part in an online debate about this issue. Well, although I wasn’t able to watch it live there is a recording of it which is available here:

It’s rather long, but there are many interesting things in it…

## A Test of the Cosmological Principle using Quasars

Posted in The Universe and Stuff with tags , , , , on October 8, 2020 by telescoper

I’m not getting much time these days to even think about cosmology but Subir Sarkar drew my attention to an intriguing paper by his team so I thought I’d share it here. Here is the abstract and author list:

I find this an intriguing result because I’ve often wondered about the dipole anisotropy of the cosmic microwave background might not be exclusively kinematic in origin and whether they might also be a primordial contribution. The dipole (180°) variation corresponds to a ΔT/T of order 10-3, which a hundred times larger than the variation on any other angular scale. This is what it looks like:

This is usually interpreted as being due to the motion of the observer through a frame in which the cosmic microwave background is completely isotropic. A simple calculation then gives the speed of this motion using ΔT/T ≈ v/c. This motion is assumed to be generated by gravitational interaction with local density fluctuations rather than being due to anything truly cosmological (i.e. of primordial origin).

The features in the cosmic microwave background temperature pattern on smaller angular scales (the quadrupole, octopole, etc…) , which have ΔT/T of order 10-5 are different in that they are dominated by primordial density fluctuations. There should be a primordial dipole at some level, but the fact that these other harmonic modes have such low amplitudes and the assumption that the primordial dipole should be of the same order, combined with the fact that the CMB dipole does indeed roughly line up with the dipole expected to be generated by local inhomogeneities, has led to the widespread belief that this intrinsic dipole is negligible. This analysis suggests that it might not be.

What the authors have done is study the anisotropy of a large sample of quasars (going out to redshifts of order three) finding the dipole to be larger than that of the CMB. Note however that the sample does not cover the whole sky because of a mask to remove regions wherein AGN are hard to observe:

As well as the mask there are other possible systematics that might be at play, which I am sure will be interrogated when the paper is peer-reviewed which, as far as I know, is not yet the case.

P.S. I might just quibble a little bit about the last sentence of the abstract. We know that the Universe violates the cosmological principle even in the standard model: with scale-invariant perturbations there is no scale at which the Universe is completely homogeneous. The question is really how much and in what way it is violated. We seem to be happy with 10-5 but not with 10-3

Update: On 23rd October Subir will be giving a talk about this an participating in a debate. For more details, see here.

## False Convergence and the Bandwagon Effect

Posted in The Universe and Stuff with tags , , , , , , on July 3, 2011 by telescoper

In idle moments, such as can be found during sunny sunday summer afternoons in the garden, it’s  interesting to reminisce about things you worked on in the past. Sometimes such trips down memory lane turn up some quite interesting lessons for the present, especially when you look back at old papers which were published when the prevailing paradigms were different. In this spirit I was lazily looking through some old manuscripts on an ancient laptop I bought in 1993. I thought it was bust, but it turns out to be perfectly functional; they clearly made things to last in those days! I found a paper by Plionis et al. which I co-wrote in 1992; the abstract is here

We have reanalyzed the QDOT survey in order to investigate the convergence properties of the estimated dipole and the consequent reliability of the derived value of $\Omega^{0.6}/b$. We find that there is no compelling evidence that the QDOT dipole has converged within the limits of reliable determination and completeness. The value of  $\Omega_0$ derived by Rowan-Robinson et al. (1990) should therefore be considered only as an upper limit. We find strong evidence that the shell between 140 and 160/h Mpc does contribute significantly to the total dipole anisotropy, and therefore to the motion of the Local Group with respect to the cosmic microwave background. This shell contains the Shapley concentration, but we argue that this concentration itself cannot explain all the gravitational acceleration produced by it; there must exist a coherent anisotropy which includes this structure, but extends greatly beyond it. With the QDOT data alone, we cannot determine precisely the magnitude of any such anisotropy.

(I’ve added a link to the Rowan-Robinson et al. paper for reference). This was  a time long before the establishment of the current standard model of cosmology (“ΛCDM”) and in those days the favoured theoretical paradigm was a flat universe, but one without a cosmological constant but with a critical density of matter, corresponding to a value of the density parameter $\Omega_0 =1$.

In the late eighties and early nineties, a large number of observational papers emerged claiming to provide evidence for the (then) standard model, the Rowan-Robinson et al. paper being just one. The idea behind this analysis is very neat. When we observe the cosmic microwave background we find it has a significant variation in temperature across the sky on a scale of 180°, i.e. it has a strong dipole component

There is also some contamination from Galactic emission in the middle, but you can see the dipole in the above map from COBE. The interpretation of this is that the Earth is not at rest. The  temperature variation causes by our motion with respect to a frame in which the cosmic microwave background (CMB) would be isotropic (i.e. be the same temperature everywhere on the sky) is just $\Delta T/T \sim v/c$. However, the Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. We know these velocities pretty well, but they don’t account for the size of the observed dipole anisotropy. The extra bit must be due the gravitational pull of larger scale structures.

If one can map the distribution of galaxies over the whole sky, as was first done with the QDOT galaxy redshift survey, then one can compare the dipole expected from the distribution of galaxies with that measured using the CMB. We can only count the galaxies – we don’t know how much mass is associated with each one but if we find that the CMB and the galaxy dipole line up in direction we can estimate the total amount of mass needed to give the right magnitude. I refer you to the papers for details.

Rowan-Robinson et al. argued that the QDOT galaxy dipole reaches convergence with the CMB dipole (i.e. they line up with one another) within a relatively small volume – small by cosmological standards, I mean, i.e. 100 Mpc or so- which means that  there has to be quite a lot of mass in that small volume to generate the relatively large velocity indicated by the CMB dipole. Hence the result is taken to indicate a high density universe.

In our paper we questioned whether convergence had actually been reached within the QDOT sample. This is crucial because if there is significant structure beyond the scale encompassed by the survey a lower overall density of matter may be indicated. We looked at a deeper survey (of galaxy clusters) and found evidence of a large-scale structure (up to 200 Mpc) that was lined up with the smaller scale anisotropy found by the earlier paper. Our best estimate was $\Omega_0\sim 0.3$, with a  large uncertainty. Now, 20 years later, we have a  different standard cosmology which does indeed have $\Omega_0 \simeq 0.3$. We were right.

Now I’m not saying that there was anything actually wrong with the Rowan-Robinson et al. paper – the uncertainties in their analysis are clearly stated, in the body of the paper as well as in the abstract. However, that result was widely touted as evidence for a high-density universe which was an incorrect interpretation. Many other papers published at the time involved similar misinterpretations. It’s good to have a standard model, but it can lead to a publication bandwagon – papers that agree with the paradigm get published easily, while those that challenge it (and are consequently much more interesting) struggle to make it past referees. The accumulated weight of evidence in cosmology is much stronger now than it was in 1990, of course, so the standard model is a more robust entity than the corresponding version of twenty years ago. Nevertheless, there’s still a danger that by treating ΛCDM as if it were the absolute truth, we might be closing our eyes to precisely those clues that will lead us to an even better understanding.  The perils of false convergence  are real even now.

As a grumpy postscript, let me just add that Plionis et al. has attracted a meagre 18 citations whereas Rowan-Robinson et al. has 178. Being right doesn’t always get you cited.