Archive for Subir Sarkar

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.

A Non-accelerating Universe?

Posted in Astrohype, The Universe and Stuff with tags , , , , , on October 26, 2016 by telescoper

There’s been quite a lot of reaction on the interwebs over the last few days much of it very misleading; here’s a sensible account) to a paper by Nielsen, Guffanti and Sarkar which has just been published online in Scientific Reports, an offshoot of Nature. I think the above link should take you an “open access” version of the paper but if it doesn’t you can find the arXiv version here. I haven’t cross-checked the two versions so the arXiv one may differ slightly.

Anyway, here is the abstract:

The ‘standard’ model of cosmology is founded on the basis that the expansion rate of the universe is accelerating at present — as was inferred originally from the Hubble diagram of Type Ia supernovae. There exists now a much bigger database of supernovae so we can perform rigorous statistical tests to check whether these ‘standardisable candles’ indeed indicate cosmic acceleration. Taking account of the empirical procedure by which corrections are made to their absolute magnitudes to allow for the varying shape of the light curve and extinction by dust, we find, rather surprisingly, that the data are still quite consistent with a constant rate of expansion.

Obviously I haven’t been able to repeat the statistical analysis but I’ve skimmed over what they’ve done and as far as I can tell it looks a fairly sensible piece of work (although it is a frequentist analysis). Here is the telling plot (from the Nature version)  in terms of the dark energy (y-axis) and matter (x-axis) density parameters:

lambda

Models shown in this plane by a line have the correct balance between Ωm, and ΩΛ to cancel out the decelerating effect of the former against the accelerating effect of the latter (a special case is the origin on the plot, which is called the Milne model and represents an entirely empty universe). The contours show “1, 2 and 3σ” contours, regarding all other parameters as nuisance parameters. It is true that the line of no acceleration does go inside the 3σcontour so in that sense is not entirely inconsistent with the data. On the other hand, the “best fit” (which is at the point Ωm=0.341, ΩΛ=0.569) does represent an accelerating universe.

I am not all that surprised by this result, actually. I’ve always felt that taken on its own the evidence for cosmic acceleration from supernovae alone was not compelling. However, when it is combined with other measurements (particularly of the cosmic microwave background and large-scale structure) which are sensitive to other aspects of the cosmological space-time geometry, the agreement is extremely convincing and has established a standard “concordance” cosmology. The CMB, for example, is particularly sensitive to spatial curvature which, measurements tells us, must be close to zero. The Milne model, on the other hand, has a large (negative) spatial curvature entirely excluded by CMB observations. Curvature is regarded as a “nuisance parameter” in the above diagram.

I think this paper is a worthwhile exercise. Subir Sarkar (one of the authors) in particular has devoted a lot of energy to questioning the standard ΛCDM model which far too many others accept unquestioningly. That’s a noble thing to do, and it is an essential part of the scientific method, but this paper only looks at one part of an interlocking picture. The strongest evidence comes from the cosmic microwave background and despite this reanalysis I feel the supernovae measurements still provide a powerful corroboration of the standard cosmology.

Let me add, however, that the supernovae measurements do not directly measure cosmic acceleration. If one tries to account for them with a model based on Einstein’s general relativity and the assumption that the Universe is on large-scales is homogeneous and isotropic and with certain kinds of matter and energy then the observations do imply a universe that accelerates. Any or all of those assumptions may be violated (though some possibilities are quite heavily constrained). In short we could, at least in principle, simply be interpreting these measurements within the wrong framework, and statistics can’t help us with that!

Astronomy Look-alikes, No. 62

Posted in Astronomy Lookalikes with tags , on August 19, 2011 by telescoper

I’d say that Professor Subir Sarkar of Oxford University bears more than a passing resemblance to cricketing legend Sir Vivian “Viv” Richards, although I couldn’t comment on whether that extends to his batting ability…

Subir Sarkar

Viv Richards

Auditorium A

Posted in Biographical, Books, Talks and Reviews with tags , , , , , on June 8, 2011 by telescoper

Just back from a splendidly wine-laden workshop dinner, I thought I’d do a quick post. My talk was moved to this morning, instead of the scheduled slot in the afternoon I think it went OK considering that, in the spirit of a small informal workshop, I talked mainly about work in progess…

That’s the whiteboard in my office in Cardiff, by the way, not my talk this morning. Auditorium A has good old-fashioned blackboards.

An advantage of speaking in the morning was that after it was over I was able to relax with a beer at lunchtime, but in the warm weather that made it rather difficult to stay on the ball afterwards. I’ve lost track of the amount of time I’ve spent sitting (or even speaking) in the famous Auditorium A of the Niels Bohr Institute over the years, actually, but I don’t think I’ve ever taken a picture there, so here’s one.

On the extreme left you can see our genial host Pavel Naselsky; in the centre left with blue shirt and grey hair, pretending not to be asleep, is Leonid Grishchuk; beside him to the right is Subir Sarkar and next to him,  more-or-less hidden from view, is Holger Bech Nielsen who travelled backwards in time especially to attend the workshop. The knee in the foreground remains unattributed.

Here is an “official” workshop photograph, taken while the participants were looking a bit more awake, but before quite a few had made it back from lunch..