Archive for George Efstathiou

Non-Solutions to the Hubble Constant Problem…

Posted in The Universe and Stuff with tags , , , , on March 18, 2021 by telescoper

A rather pugnacious paper by George Efstathiou appeared on the arXiv earlier this week. Here is the abstract:

This paper investigates whether changes to late time physics can resolve the `Hubble tension’. It is argued that many of the claims in the literature favouring such solutions are caused by a misunderstanding of how distance ladder measurements actually work and, in particular, by the inappropriate use of distance ladder H0 priors. A dynamics-free inverse distance ladder shows that changes to late time physics are strongly constrained observationally and cannot resolve the discrepancy between the SH0ES data and the base LCDM cosmology inferred from Planck.

For a more detailed discussion of this paper, see Sunny Vagnozzi’s blog post. I’ll just make some general comments on the context.

One of the reactions to the alleged “tension” between the two measurements of H0 is to alter the standard model in such a way that the equation of state changes significantly at late cosmological times. This is because the two allegedly discrepant sets of measures of the cosmological distance scale (seen, for example, in the diagram below  taken from the paper I blogged about a while ago here) differ in that the low values are global measures (based on observations at high redshift) while the high values of are local (based on direct determinations using local sources, specifically stars of various types).

That is basically true. There is, however, another difference in the two types of distance determination: the high values of the Hubble constant are generally related to interpretations of the measured brightness of observed sources (i.e. they are based on luminosity distances) while the lower values are generally based on trigonometry (specifically they are angular diameter distances). Observations of the cosmic microwave background temperature pattern, baryon acoustic oscillations in the matter power-spectrum, and gravitational lensing studies all involve angular-diameter distances rather than luminosity distances.

Before going on let me point out that the global (cosmological) determinations of the Hubble constant are indirect in that they involve the simultaneous determination of a set of parameters based on a detailed model. The Hubble constant is not one of the basic parameters inferred from cosmological observations, it is derived from the others. One does not therefore derive the global estimates in the same way as the local ones, so I’m simplifying things a lot in the following discussion which I am not therefore claiming to be a resolution of the alleged discrepancy. I’m just thinking out loud, so to speak.

With that caveat in mind, and setting aside the possibility (or indeed probability) of observational systematics in some or all of the measurements, let us suppose that we did find that there was a real discrepancy between distances inferred using angular diameters and distances using luminosities in the framework of the standard cosmological model. What could we infer?

Well, if the Universe is described by a space-time with the Robertson-Walker Metric (which is the case if the Cosmological Principle applies in the framework of General Relativity) then angular diameter distances and luminosity distances differ only by a factor of (1+z)2 where z is the redshift: DL=DA(1+z)2.

I’ve included here some slides from undergraduate course notes to add more detail to this if you’re interested:

The result DL=DA(1+z)2 is an example of Etherington’s Reciprocity Theorem. If we did find that somehow this theorem were violated, how could we modify our cosmological theory to explain it?

Well, one thing we couldn’t do is change the evolutionary history of the scale factor a(t) within a Friedman model. The redshift just depends on the scale factor when light is emitted and the scale factor when it is received, not how it evolves in between. And because the evolution of the scale factor is determined by the Friedman equation that relates it to the energy contents of the Universe, changing the latter won’t help either no matter how exotic the stuff you introduce (as long as it only interacts with light rays via gravity). In the light of this, the fact there are significant numbers of theorists pushing for such things as interacting dark-energy models to engineer late-time changes in expansion history is indeed a bit perplexing.

In the light of the caveat I introduced above, I should say that changing the energy contents of the Universe might well shift the allowed parameter region which may reconcile the cosmological determination of the Hubble constant from cosmology with local values. I am just talking about a hypothetical simpler case.

In order to violate the reciprocity theorem one would have to tinker with something else. An obvious possibility is to abandon the Robertson-Walker metric. We know that the Universe is not exactly homogeneous and isotropic, so one could appeal to the gravitational lensing effect of lumpiness as the origin of the discrepancy. This must happen to some extent, but understanding it fully is very hard because we have far from perfect understanding of globally inhomogeneous cosmological models.

Etherington’s theorem requires light rays to be described by null geodesics which would not be the case if photons had mass, so introducing massive photons that’s another way out. It also requires photon numbers to be conserved, so some mysterious way of making photons disappear might do the trick, so adding some exotic field that interacts with light in a peculiar way is another possibility.

Anyway, my main point here is that if one could pin down the Hubble constant tension as a discrepancy between angular-diameter and luminosity based distances then the most obvious place to look for a resolution is in departures of the metric from the Robertson-Walker form. The reciprocity theorem applies to any GR-based metric theory, i.e. just about anything without torsion in the metric, so it applies to inhomogeneous cosmologies based on GR too. However, in such theories there is no way of defining a global scale factor a(t) so the reciprocity relation applies only locally, in a different form for each source and observer.

All of this begs the question of whether or not there is real tension in the  H0 measures. I certainly have better things to get tense about. That gives me an excuse to include my long-running poll on the issue:

More Lockdown Perspectives on the Hubble Tension and thoughts on the future of scientific publications

Posted in Open Access, The Universe and Stuff with tags , , , , on July 28, 2020 by telescoper

This is interesting. Remember last week when I posted about a paper by George Efstathiou on the Hubble Constant controversy. This is the abstract.

Well, a new version of the paper has just appeared on the arXiv that includes some comments in response from the SH0ES team.

It is of course interesting in itself to see the cut and thrust of scientific debate on a live topic such as this, but in my mind at least it raises interesting questions about the nature of scientific publication. To repeat something I wrote a a while ago, it seems  to me that the scientific paper published in an academic journal is an anachronism. Digital technology enables us to communicate ideas far more rapidly than in the past and allows much greater levels of interaction between researchers. I agree with Daniel Shanahan that the future for many fields will be defined not in terms of “papers” which purport to represent “final” research outcomes, but by living documents continuously updated in response to open scrutiny by the research community.

The Open Journal of Astrophysics is innovative in some ways but remains wedded to the paper as its fundamental object, and the platform is not able to facilitate interaction with readers. Of course one of the worries is that the comment facilities on many websites tend to get clogged up with mindless abuse, but I think that is manageable. I have some ideas on this, but for the time being I’m afraid all my energies are taken up with other things so this is for the future.

I’ve long argued that the modern academic publishing industry is not facilitating but hindering the communication of research. The arXiv has already made academic journals virtually redundant in many of branches of  physics and astronomy; other disciplines will inevitably follow. The age of the academic journal is drawing to a close, and it is consequently time to rethink the concept of a paper.


Posted in The Universe and Stuff with tags , , , , , , , on June 2, 2011 by telescoper

Just a quickie this fine summer morning to pass on the news – for those of you who haven’t heard yet – that this year’s Gruber Prize for Cosmology has been awarded to Marc Davis (Berkeley, USA), George Efstathiou (Cambridge, UK), Carlos Frenk (Durham, UK) and Simon White (Garching, Germany). This prestigious award is given for their pioneering work on the Cold Dark Matter model of structure formation, which included some of the first large-scale N-body computer simulations. The “Gang of Four” produced a number of papers during the 1980s that established the idea that galaxies form by hierarchical clustering from small initial fluctuations in a matter distribution dominated by massive collisionless non-baryonic particles, the most famous of their papers being pretty universally referred to as DEFW.

In fact, if you’ll forgive me going on a trip down memory lane, that paper, published in 1985, was one of the first papers I read when I started my research degree the same year at Sussex. It was back in the days when everyone seemed to use a VAX for big computing jobs and the simulations presented in that paper involved a mere 323 = 32768 particles. You could probably run that kind of simulation on a mobile phone these days!

This early work on Cold Dark Matter wasn’t the final word, of course. Subsequent observational evidence for an accelerating Universe resulting in our standard cosmological model being modifiel to include an additional (large) component of dark energy in addition to dark matter. Nevertheless, the core ideas presented by DEFW established the basic foundations of structure formation upon which the current standard model is built.

Incidentally, you can read an interesting account of the discovery of the accelerating universe here; a cosmologist by the name of “George F. Stathew” plays a prominent role in that piece and it’s curious I’ve never heard of him before now.

Each of the four winners gets a share of the $500000 Gruber Prize, i.e. in “normalized” terms, they get $125000 each. Why is it so controversial to suggest dividing citation counts the same way? The DEFW paper has about 1500 citations according to ADS, so I think it’s quite reasonable to award the authors 370-odd each towards their respective h-indices. That’s still a pretty good result by any bibliometric standard!

The four also get a Gold Medal each to wear at parties, although by my previous logic they should have to share one between them. Perhaps George might consider donating his to Arsenal Football Club, as their trophy cabinet is looking rather empty these days?

None of the winners are Australian undergraduates, so this award probably won’t be considered newsworthy by the mass media. Believe it or not, however, the Gruber Prize is held in even higher regard by cosmologists than the Templeton Prize, so I’d like to take this opportunity to congratulate them myself for their thoroughly well-deserved honour!

(Guest Post) STFC – It isn’t just about money

Posted in Science Politics with tags , , , , , , on October 4, 2010 by telescoper

The following piece was written by Professor George Efstathiou, FRS, who is Professor of Astrophysics at the University of Cambridge and Director of the Kavli Institute for Cosmology. The views expressed therein are George’s own, but I’m not saying that out of a desire to distance myself from his opinions. As a matter of fact, I was one of the people who signed the petition he describes in the article…


As Peter has reported on this site, physicists around the country are anxiously awaiting the results of the forthcoming Comprehensive Spending Review. Scientists whose research is supported by the Science and Technology Facilities Council (STFC)  are particularly anxious.  Since its creation, STFC has gone through two difficult scientific prioritisation exercises. Many excellent projects have been cancelled and grants supporting University groups have been cut savagely, by about 35%. STFC science has already descended into the Royal Society’s ‘game over‘ scenario. All of this has happened before the consequences of the economic crisis have hit the science budget. STFC has left itself uniquely poorly placed amongst the Research Councils to absorb further reductions following the CSR.

It is for this reason, that I and a few others organised a petition expressing a loss of confidence in the Chief Executive of STFC. The petition was signed by 916 researchers, including 162 Professors and 18 Fellows of the Royal Society. It was formally submitted to the STFC Chair (Michael Sterling) on 1st July together with an explicit request that STFC Council should review its role in this loss of confidence.

People will have had many different reasons for signing the petition. I made my views public well in advance (see my Letter to Lord Drayson). In all of my letters to ministers and others concerning the STFC ‘crisis’, I have never asked for more money. More money would help, of course, but this is utterly unrealistic in the current economic circumstances. No, over the last three years I have been lobbying for good governance. The strutural difficulties with STFC were easy to identify and I believe that with good governance the STFC programme could have been managed without such a catastrophic loss of science. Over three years, STFC have failed to establish a compelling narrative, strategy and constructive engagement with its science community. When one bears in mind that about 40 % of Physics staff work in areas for which STFC is the primary funding source, the consequences of the STFC crisis for University Departments, and the rest of the science base, are indeed serious.

So, whatever the outcome of the CSR, there are governance issues that we should be concerned about. There are three that I would like to raise here:

1. Fellowships and grants. Senior scientists from outside the UK point to the Fellowships and Rolling Grants as two of the most effective features of the UK funding system. Both are now under threat. I was responsible for making the case for the current 5 year system to PPARC Council. In addition to the evident benefits of continuity and reduction in peer review, Council need to understand that recruitment for postdocs involves a substantial lead time. If we are to compete for the best postdocs around the world (and not lose our best post docs), grant funds must be committed four years in advance. The 5 year rolling grant system, even with tapers, allows groups to advertise posts on an international timetable and to vire funds to maximise science output. Any move to responsive mode 3 year grants is guaranteed to deliver less science for a fixed amount of money. I would vigorously defend the Fellowships. Fellowships encourage scientific independence and provide a valuable “bottom-up” correction to the increasingly narrow “top-driven” science programme of STFC. Attacks on Fellowships and Rolling Grants will inevitably lead to a more introspective and less internationally competitive science programme.

2. The Composition of STFC Council. STFC Council, with a minority of leading research scientists, differs from other Research Councils. I have had several vigorous discussions with Michael Sterling concerning this issue and, in particular, the recent decision by BIS to appoint three new non-academic members to STFC. This led me to write a long letter to Adrian Smith (Director General of the Research Councils) reproduced here. Professor Smith replied that he approved of the present balance of Council and thought that it was compatible with the recommendations of previous reviews. I will leave readers to decide whether they agree. This is not a minor point. My experience on PPARC Council was that `lay members’ can often provide interesting perspectives on problems, but if they lack understanding of the science (sometimes alarmingly so) they will tend to accept the recommendations of the Executive. STFC needs a scientifically strong Council. Competent management is not enough. It is easy to keep within budget – you can be tough about cutting things. It is much harder to maximise the amount of science that you can do on a fixed budget. For that you need a scientific strategy and scientific judgement.

3. The New CEO. The search has begun for a new Chief Executive. There is one school of thought that a suitable candidate may be found from the corporate sector. Someone who may not understand the science, but would be a capable manager and communicator. I think that this would be a disaster. In my view, it is essential that a new CEO have an understanding of the science programme at STFC and should be prepared to act as an enthusiastic advocate for STFC science. We need a CEO who can engage constructively with the academic community and, when times are tough, articulate a strategy to limit the loss of science rather than gloat at our misfortune.

It would be great to have more money for STFC science. But money isn’t everything – we need to pay attention to governance issues as well. If we had been braver back in 2008 and openly challenged the Executive, we might not be in such a weak position now. We should not be so reticent in the future.


Letter to Lord Drayson from George Efstathiou

Posted in Finance, Science Politics with tags , , on January 25, 2010 by telescoper

I just had a note from George Efstathiou, Director of the Kavli Institute for Cosmology at the University of Cambridge, about a letter he wrote to Lord Drayson about the STFC crisis. It’s  very much in line with what I was saying a few days ago. It’s good to see someone with some clout stepping into the ring, taking the gloves off, and not pulling his punches (That’s enough boxing metaphors, Ed.)

With George’s permission, I’m including the full text of his letter below; the added links are mine.


25 January 2010

Lord Drayson
Department for Business, Innovation and Skills
Castle View House
East Lane
Runcorn, WA72GJ

Dear Lord Drayson,

I would like to make a few comments concerning your structural review of STFC. I was a member of the Astronomy and Planetary Sciences Board of SERC (1991‐1993) and a member of PPARC Council (2001‐2004) and so I have some experience of previous funding systems.

Overall, I support the proposals put forward by the Royal Astronomical Society Forum and the Institute of Physics. It is extremely important that research grants remain in a reorganized Council rather than transferred to EPSRC. A transfer of the grants line to EPSRC, particularly at a severely reduced level following the STFC prioritization exercise, would recreate the difficulties experienced in the days of SRC/SERC that PPARC was designed to solve. (Namely, the long‐term nature of Particle Physics and Astronomy projects and their reliance on large international organizations).

In analysing the nature of a restructured Council, it is worthwhile reviewing some of the reasons for the difficulties at STFC, and the role of the Chief Executive in exacerbating those difficulties.

Firstly, Keith Mason has openly pursued a policy of transferring funds into areas with potential for short term economic impact at the expense of grant funding to Universities. STFC funds have therefore gone into facilities, innovation campuses and initiatives such as the Aurora programme. Together with a sympathetic Chairman and a Council that included three members of the Executive, this policy went (largely) unchallenged for the first two years of STFCs existence, though I know of not a single research scientist who agreed with it. Financial mismanagement of this policy finally caught up with STFC last year, leading to savage cuts of more than 35% in the grants line (the only `flexible’ part of the STFC budget). These cuts are more savage than the deepest cuts experienced during the Thatcher years. Mason’s attempt to downplay these cuts by referring to previous low points in grant funding is, frankly, risible. Government should be indignant at Mason’s attempt to write‐off the investment in science between the years 2002‐‐2007, which was intended (and succeeded) in improving the volume and quality of research in Universities.

As an example of the tension between economic impact and scientific excellence, BNSC published the Space Exploration Review recommending an increase in funding of £150m per annum and highlighting the MoonLITE bilateral mission. A few days later, the STFC prioritization exercise ranked MoonLITE `below alpha’. Any restructuring must tackle the difficulties of tensioning projects which may have economic benefits but little scientific merit against academic excellence. In my view, academic excellence should be the priority for any restructured Research Council.

Secondly, Mason has held the view (most recently expressed at the Astronomy Forum meeting earlier this month) that the UK has too many scientists involved in exploiting facilities in comparison to the number engaged in developing, building and operating facilities. Again, I know of not a single research scientist who agrees with this view. The science budget has increased significantly over the last decade. The expansion of astronomy and particle physics in UK Universities has been a rational response to the increased availability of funding. As a member of the 2008 RAE Physics panel I was able to see at first hand how this investment has translated into research of the highest international quality. The deep STFC cuts to the grants line will inevitably weaken the research base in UK Universities and may even threaten the viability of some Physics departments. The shock wave following these cuts will eventually be felt across the entire UK science base. Any restructured Research Council must sustain an acceptable balance between support of UK Universities and investment in facilities.

STFC has not given high enough priority to scientific excellence. This is the primary cause of the problems over the last three years. It is why scientific excellence will suffer following the STFC prioritization exercise. This unfortunate outcome has been achieved during a period of increased funding to STFC and despite the allocation of financial bailouts.

Any restructured Research Council must have academic excellence at its core. It must also have a Chief Executive who recognises and values academic excellence.

Yours sincerely

George Efstathiou

cc Professor Michael Sterling, Chairman STFC
Phil Willis, Chair, House of Commons Science and Technology Select Committee

Talking Planck

Posted in The Universe and Stuff with tags , , on April 3, 2009 by telescoper

Since the Planck mission is due to be launched very soon, I thought it would be nice to put this lecture by George Efstathiou here in order to give some background. It’s from a page of science talks about Planck.

George is the Professor of Astrophysics (1909) at the University of Cambridge. The 1909 isn’t when he was born, but when the Chair he holds was set up. I have a hundred-year-old Chair in my house too.
He is also the Director of the impressive Kavli Institute for Cosmology.
He’s a leading member of the Planck science team and is coordinating the UK effort that will be applied to analysing the data. He’s an FRS, citation millionaire, and general all-round clever clogs. He would cut an even more impressive figure were it not for the fact that he supports Arsenal.