Ancient light at Cambridge University

I stayed on in Cambridge today in order to catch a series of seminars on the exciting new measurements of primordial light from the early universe – the  PLANCK satellite measurements of the cosmic microwave background. The Institute of Astronomy here hosted talks by three PLANCK team leaders all associated with Cambridge: Professor George Efstathiou, Director of the Kavli Institute of Cosmology (seen on tv worldwide yesterday), Dr Anthony Challoner of the Department of Applied Maths and Physics, and Professor Paul Shellard, Director of the Centre for Theoretical Cosmology at Cambridge. This was not an occasion to be missed and it didn’t disappoint. (See here for an introduction to the cosmic background radiation and its importance in big bang cosmology).


Map of the cosmic microwave background as measured by the PLANCK  satellite (2013). Image from ESA

There were three separate talks; Professor Efstathiou gave a general overview of the results, Anthony Challoner presented a talk on PLANCK mapping of dark matter by gravitational lensing, and Paul Shellard discussed the implications of the new results for physics beyond the standard cosmological model.

Professor Efstathiou started by explaining why the new measurements were of much greater sensitivity than those of the previous satellite WMAP. One reason is that PLANCK has detectors at both low and high frequencies; the latter (at over 100 GHz) is particularly useful for overcoming problem of  galactic emissions. (A great deal of this kind of work is about subtracting out a large foreground signal comprising emissions from the universe over billions of years). A consistent theme of George’s talk was that the new measurements are sufficiently precise to stand alone, rather than relying on complementary data from astrophysics. For example, a slight tension between the PLANCK measurements of dark energy (below) and data from recent supernova observations may indicate that the latter have larger uncertainties than previously estimated.


A comparison of the resolution of images of the background radiation captured by the COBE satellite (1992), the WMAP satellite (2002) and the PLANCK satellite (2013). Photo from ESA.

I won’t try and summarize the full talk but the main results (in case you haven’t been reading the news) are:

1. The new data are in close agreement with the standard ΛCDM inflationary big bang model; the data fit extremely well to the standard 6 -parameter model  (no evidence of new parameters), although the results suggest some slight adjustments to the following parameters:

2. A revised value for the Hubble constant Ho (the expansion parameter): 67.3 +- 1.2 km/s/MPC, at the lower end of previous estimates

3. A new constraint on the curvature parameter: Ωk = -0.0005 +- .07 (zero to you and me)

4. A revised estimate of the dark energy contribution: 69.2 ± 1.0%

5. A revised estimate for the dark matter content of the universe:  25.8 ± 0.4%

6. A revised estimate for the ordinary matter content of the universe:  4.82 ± 0.05%

7. A revised ‘age’ estimate for the universe of 13.8 billion years

7. A ‘spectral index’ of 0.96, closely in agreement with simple models of inflation

This last result is one of the most important; as it says in the conclusions of the paperConstraints on Inflation’ (paper XXII of the PLANCK results) , “The simplest inflationary models have passed an exacting test with the Planck data”.

The results are published as a series of 18 papers on the ArXiv, and you can find the summary paper here . Two important anomalies previously seen in the WMAP data remain; an asymmetry between the ‘northern’ and ‘southern’ hemispheres, and the famous cold spot


                           New estimate of the Hubble constant (Planck collaboration, ESA)


 The power spectrum of the cosmic microwave background (PLANCK collaboration, ESA)

George finished by emphasizing a number of caveats; something of a mismatch at low multipoles, the problem of degeneracy (see below) and the lack of a clear signal of B-mode polarization; he is hopeful that the latter may be forthcoming next year.

Anthony Challoner then gave a talk on ‘Full-sky Mapping of Dark Matter with PLANCK’, a description of the mapping of dark matter by PLANCK using the technique of gravitational lensing. The point here is that one cannot get everything from the ‘power’ spectrum above because of the problem of degeneracy; aspects of the  spectrum can be reproduced with different values of H0,m etc . Luckily, gravitational lensing is sensitive to the geometry of the universe and to the growth of structure, and so allows an independent method of the determination of Ωm .I won’t attempt to summarize Anthony’s talk but the main result is that the gravitational lensing results from PLANCK are very much consistent with the parameters derived from the power spectrum.

Finally, Professor Paul Shellard gave a talk ‘ Beyond the Standard Paradigm’. This lecture discussed possible signs of physics beyond the standard cosmological model in the new PLANCK measurements (for example, hints of support for non-inflationary models of the infant universe). The first point of interest is that the famous ‘spectral index’ of the power spectrum is close but not equal to one (0.96), just as expected for inflation. More specifically, probing the shape of the power spectrum gives a powerful tool for selecting or rejecting models. A shape that is decidedly non-Gaussian would effectively rule out ‘slow roll’ inflation, the simplest model.  On the other hand, a closely Gaussian shape would rule out two-field inflationary models, and impose serious constraints on most non-inflationary models. The new result: almost no deviations from Gaussianity, at least within a factor of one in a million. This places important new restraints on models such as cosmic strings,  global textures etc. It seems the result also makes ekpyrotic cyclic models a lot less likely (something to do with the mechanism proposed by most cyclic models, must look this up). Finally, another result was a clear lack of evidence for a fourth generation of neutrinos (was anyone really expecting otherwise?)

All in all, some fantastic results, I’m glad I was here to hear the exciting news at firsthand.  At the end, Paul Shellard joked that one can now read the initials SWH in the PLANCK spectrum (some people claimed that Stephen Hawking’s initials were clearly visible in the WMAP spectrum). It was a fun way to finish the morning’s seminars, but I couldn’t see really it! The main overview paper is here and you can access the full set of papers here.


What I thought was the Institute of Astronomy at Cambridge (actually the library)


The real Institute of Astronomy; not quite as majestic but buzzing with activity


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