Archive for COBE

The Signs of Age

Posted in Biographical, The Universe and Stuff with tags , , , , on October 23, 2018 by telescoper

I was feeling very tired yesterday evening and in my vegetative state I suddenly realised that last month I missed a significant personal anniversary. In September 1988, now over thirty years ago I submitted my DPhil thesis at the University of Sussex. Here it is..

It was to be another couple of months until I had my viva (an experience I’d definitely rather forget) so I didn’t get to receive the postgraduate degree formally until the following summer, but at least I finished and submitted within the three years my funding allowed. Incidentally, mine was one of the first generation of theses at the University of Sussex to be typeset in LaTeX. At least I avoided the hassle of having carbon copies made!

The field of cosmology has changed so much in the three intervening decades that I’m sure current graduate students would find my thesis as incredibly simple-minded as I do. There weren’t any measurements of CMB temperature patterns in those days (the COBE results were not announced not until 1992) so I had to generate simulated observations, for example. Still, a few of the things in my thesis have stood the test of time, in the form of papers that still get cited to this day. I was lucky that my research  was in an area that was about to take off, rather than one that was already in decline, and that there will still problems around that were easy enough for me to tackle!

The way of working was very different too: the fact that my generation didn’t have computers on our desks makes younger graduate students wonder how we managed to do anything at all! I still amuse my colleagues with my habit of writing out bits of code in longhand on paper  and `desk-checking’ them before typing them in.

The fact that I now have over 30 years’ postdoctoral experience definitely adds to the feeling of getting very old, along with the all-pervading fatigue, the random aches and pains that afflict me from time to time, failing eyesight, and the tendency of Facebook to send me advertisements about stairlifts, hearing aids, and (worst of all) golf equipment.

The start of University term in late September brings with it a new intake of students that always looks even  younger than the last. That produces a strange alternation of feelings. On the one hand, working in a University means that you’re always surrounded by bright young students which is a good thing when you’re getting on a bit in that it reminds you that you were once like that. On the other, the proliferation of young persons around does force you to face up to how old you actually are.

I remember some years ago I was teaching a module on astrophysics as part of which I did a lecture on supernovae. In the middle of that I said to my class: “of course, you will all remember SN 1987A” (which was detected while I was a PhD student). Blank faces. I then realized that none of them had even been born in 1987. Nowadays it is the case that I was already a Professor when all my undergraduate students were born.

But these signs of age are as nothing compared to the shock I underwent when a few months ago I discovered that I’m older than Nigel Farage.

CMB Spectral Distortions Revisited

Posted in Biographical, The Universe and Stuff with tags , , , , , , , , , on July 27, 2017 by telescoper

While uploading some bibliographic information for bureaucratic purposes yesterday I noticed that an old paper of mine had recently attracted a number of citations. The paper was written while I was a postdoctoral research fellow in the Astronomy Centre at the University of Sussex in 1990, but not published until 1991 by which time I had moved to Queen Mary College (as it was then called). The citation history of this article is actually quite interesting:

You can see that it was cited a bit immediately after publication, then endured a long spell from 1997 to 2012 in which nobody seemed interested in it, then experienced something of a revival. It currently has a total of about 49 citations, which doesn’t exactly make it a classic in a field which is extremely active, but it’s nice to see it hasn’t been forgotten entirely.

Here is the abstract of the paper:

As the abstract makes clear we wrote this paper in response to a measurement of the spectrum of the cosmic microwave background radiation by the FIRAS instrument on the satellite COBE that had demonstrated that it was extremely well fitted by a Planck spectrum, with little room for any deviation away from a perfect black-body shape. Here’s the measured curve from COBE and some other experiments at the time:

The accuracy of the fit allows one to place limits on any process happening in the early Universe that might produce a distortion of the spectrum. There are a number of things that could do this. Any energy released in the early Universe takes time to thermalise, i.e. for the radiation field and the matter to come into thermal equilibrium via Compton scattering, double Compton scattering and Bremsstrahlung. Imperfect thermalisation produces a spectrum which doesn’t quite match the Planck curve.

Two types of distortion are possible, both introduced in classic papers from 1969 and 1970 by Rashid Sunyaev and Ya. B. Zel’dovich. One type is called a y-distortion (which corresponds to photons being shifted from low frequency and the other is called a μ-distortion, which is described by inserting a chemical potential term to the usual Planck formula for the black-body spectrum. Observational limits on both forms of distortion are very tight : |y|<1.5 ×10-5; |μ|<1.5 ×10-5, which places stringent limits on any energy release, including that which would arise from the dissipation of primordial acoustic waves (which is what John and I concentrated on in the paper).

So why did interest in this get revived a few years ago? The answer to that is that advances in relevant technology have now made it possible to think about an experiment that can measure much smaller spectral distortions than has hitherto been possible. A proposal for an experiment, called PIXIE, which includes such a measurement, is described here. Although spectral distortions are only a secondary science goal for PIXIE, it could push down the upper limits quoted above by a factor of 1000 or so, at which level we should expect to see departures from the Planck curve within the standard model, which would be a very important test of basic cosmological theory.

That all depends on whether PIXIE – or something like it – goes ahead.


COBE and after…

Posted in Biographical, The Universe and Stuff with tags , , , on April 24, 2012 by telescoper

An item on the BBC website yesterday reminds me that it is twenty years since the announcement, in April 1992, of the discovery of temperature variations across the sky in the cosmic microwave background radiation by the Cosmic Background Explorer (COBE). Was it really so long ago?

At the time the announcement was made as I actually in the USA. In fact,  I was at the University of Kansas for about a month working on this paper with Adrian Melott and Sergei Shandarin, which eventually came out early in 1993. I remember it very well because we started the project, did all the calculations and wrote up the paper within the short time I was there. Oh what it is to be a postdoc, having only research to think about and none of the other distractions that come with more senior positions.

Anyway, the COBE announcement hit the news while I was there and it got a lot of press coverage. I even did a TV interview myself, for a local cable news channel. Nor surprisingly, they were pretty clueless about the physics of the cosmic microwave background; what had drawn them to the story was George Smoot’s comment that seeing the pattern of fluctuations was “like seeing the face of God”. They were disappointed when I answered their questions about God with “I don’t know, I’m an atheist”.

The Face of God?

I didn’t know at the time that the way the announcement of the COBE discovery was handled had caused such ructions. Apparently George Smoot let his enthusiasm get the better of him, broke ranks with the rest of the COBE team, and did his own press conference which led to accusations that he was trying to steal the limelight and a big falling-out between Smoot and other members of the team, especially John Mather. It’s unfortunate that this cast a shadow over what was undoubtedly one of the most important science discoveries of the twentieth century. Without COBE there would have been no WMAP and no Planck, and our understanding of the early Universe and the formation of galaxies and large-scale structure would still be in the dark ages.

As a lowly postdoc at the time, living a hand-to-mouth existence on short-term contracts, I didn’t realise that I would still be working in cosmology twenty years later, let alone become a Professor.  Nor could I have predicted how much cosmology would change over the next two decades. Most of all, though, I never even imagined that I’d find myself travelling to Stockholm as a guest of the Nobel Foundation to attend the ceremony and banquet at which the 2006 Nobel Prize for Physics was awarded to George Smoot and John Mather for the COBE discovery. It was a wonderful one-in-a-lifetime experience, made all the nicer because Smoot and Mather seemed to have made peace at last.

Where were you when the COBE results came out?

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.