Archive for gravitational waves

BICEP2: Is the Signal Cosmological?

Posted in The Universe and Stuff with tags , , , , , on March 19, 2014 by telescoper

I have a short gap in my schedule today so I thought I would use it to post a short note about the BICEP2 results announced to great excitement on Monday.

There has been a great deal of coverage in the popular media about a “Spectacular Cosmic Discovery” and this is mirrored by excitement at a more technical level about the theoretical implications of the BICEP2 results. Having taken a bit of time out last night to go through the discovery paper, I think I should say that I think all this excitement is very premature. In that respect I agree with the result of my straw poll.

First of all let me make it clear that the BICEP2 experiment is absolutely superb. It was designed and built by top-class scientists and has clearly functioned brilliantly to improve its sensitivity so much that it has gone so far ahead of so many rivals:

Polarization detections

Notice that the only other detection of the elusive B-mode signal is by POLARBEAR, but that is actually accounted for by gravitational lensing effects rather than being evidence of a primordial gravitational wave contribution.

The B-mode signal is so weak that it is to mind absolutely amazing that an experiment can get anywhere near measuring it. There’s no denying the fact that BICEP2 team have done heroic work.

But – and it’s a big “but” – we have to ask the question “How confident can we be that the signal detected by BICEP2 is, in fact, the imprint of primordial gravitational waves on the cosmic microwave background that cosmologists were hoping for?”

The answer to this question will depend on the individual, but I would say that to convince me the absolute minimum would be a detection of the signal in more than one frequency band. A primordial signal should not vary as a function of frequency, whereas foreground emission (likely to be from dust) would be frequency dependent.

Now BICEP2 only operates at one frequency, 150GHz, so the experiment on its own can’t satisfy this criterion but it could through cross-correlation with the original BICEP1 instrument which worked at 100 GHz and 150 GHz. In the discovery paper we find the

Additionally, cross-correlating BICEP2 against 100GHz maps from the BICEP1 experiment, the excess signal is confirmed with 3sigma significance and its spectral index is found to be consistent with that of the CMB.

Here is the relevant plot, Figure 7 from the paper,


Well, the correct though the statement in the paper might be,  it is clear from this (rather ratty) cross-correlation that there is actually no firm detection of the B-modes at all at 100GHz. In other words, the 100 GHz BICEP1 data may be consistent with BICEP2 but they are also consistent with zero. (NOTE ADDED: I am ready to rescind this statement when I see a full analysis of these cross-correlations; at face value the scatter looks strange and certainly consistent with a null detection). In any case a positive cross-correlation does not exclude the possibility that the signal in common across the two channels is dust. If we only have a detection at one frequency we have no compelling evidence at all that the signal is cosmological.

When asked on Tuesday about this by Physics World I stated that I wasn’t convinced:

It seems to me though that there’s a significant possibility of some of the polarization signal in E and B [modes] not being cosmological. This is a very interesting result, but I’d prefer to reserve judgement until it is confirmed by other experiments. If it is genuine, then the spectrum is a bit strange and may indicate something added to the normal inflationary recipe.

My scepticism was then derived primarily from the distribution of the points around l=200 in the first figure: they look too high compared to the expected gravitational lensing contribution (which seems to have been pinned down by the POLARBEAR measurements to the right of the plot):

My concern: the three data points circles in blue are all higher than they should be, by about 0.01, which is the same height as the points to their left.  But the prediction of gravitational waves from inflation, circles in green, is that there should be very little contribution here --- which is why these points should lie closer to the solid red "lensing" prediction.  So the model of lensing for the right-hand part of the data + gravitational waves from inflation for the left-hand part of the data does not seem to be a very convincing fit.

I’ve taken this plot from the post I reblogged yesterday. The errors in the measurements ringed in blue are probably correlated so the fact that all three lie well above the red curve may not be as significant as it first seems, but note that the vertical scale is logarithmic. If some sort of systematic error has indeed bumped these points up then the amount of power involved could easily account for all the signal in the points to the left; the fit to the primordial B-mode (red dashed) part of the curve could then be fortuitous.

One possible systematic, apart from foreground contamination by dust, is leakage between E and B modes in the spherical harmonic decomposition. This arises because the spherical harmonic modes are only orthogonal over a complete sphere; BICEP2 does not map the whole sky, so the modes get mixed and separating them becomes extremely messy. Since the E-mode signal is so much larger, the worry is that some of it might leak into the B-mode.

UPDATE: 20/3/2014

I noticed a post on the BICEP2 Facebook Page from Hans Kristian Eriksen pointing another oddity:


The above plot is one of many showing jackknife estimates relating to various aspects of the polarization signal. What is strange is that all the blue dots lie so close to zero. Statistically speaking this is extremely unlikely and it may suggest that the noise levels have been over-estimated underestimated; roughly one in three data points should be further away than one sigma from zero if sigma is estimated correctly.

Taking all this together I have to say that I stick to the point of view I took when I first saw the results. They are very  interesting, but it is far too earlier to even claim that they are cosmological, let alone to start talking about providing evidence for or against particular models of the early Universe. No doubt I’ll be criticized for trying to put a wet blanket over the whole affair, but this is a measurement of such potential importance that I think we have to set the bar very high indeed when it comes to evidence. If I were running a book on this, I would put it at no better than even money that this is a cosmological signal.

Of course the rush to embrace these results as “definitive proof” of something is a product of human nature and the general level of excitement this amazing experiment has generated. That’s entirely understandable and basically a very good thing. It reminds those of us working in cosmology how lucky we are that we work in a field in which such momentous discoveries do actually happen. This is no doubt why so many budding scientists are drawn into cosmology in the first place. Let’s not forget, however, that there is a thing called the scientific method and often after years of hard work there remain more questions than answers. For the time being, that’s where we are with gravitational waves.

How solid is the BICEP2 B-mode result?

Posted in The Universe and Stuff with tags , , on March 18, 2014 by telescoper


Another wordpress post about BICEP2 – by astrophysicist Phil Bull – with some comments on possible issues with the data…

Originally posted on Lumps 'n' Bumps:

Phew! An exciting day indeed, so I’ll jot down a few notes to recap what happened.

The BICEP2/Keck experiments detected B-modes at large angular scales in the polarisation of the CMB. They released two papers and some data online just as the announcement was made, which you can find here. Not all of the data mind, but it’s plenty to go on for now.

Their interpretation of the data is that they detect a bump at low-ell that is characteristic of primordial B-modes generated by inflation. If true, this is super exciting, as it gives us a (sort of, but not really) direct detection of gravitational waves, and opens up a new window on the very early Universe (and hence extremely high energy scales). People are even saying it’s a probe of quantum gravity, which I guess is sort of true. Furthermore, they find a best-fit value of the…

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BICEP2: New Evidence Of Cosmic Inflation!

Posted in The Universe and Stuff with tags , , on March 18, 2014 by telescoper


Following on from yesterday’s news, here’s a more detailed analysis of the implications of the BICEP2 result from Matt Straessler’s blog. I certainly agree with the statement highlighted in red in his post:

Until this measurement/discovery is confirmed by another experiment, you should consider it provisional. Although this is too large a signal to be likely to be due to a pure statistical fluke, it could still be due to a mistake or problem, or due to something other than gravitational waves from inflation.

Originally posted on Of Particular Significance:

[For your reference if you can't follow this post: My History of the Universe, and a primer to help you understand what's going on today.]

I’m still updating this post as more information comes in and as I understand more of what’s in the BICEP2 paper and data. Talking to and listening to experts, I’d describe the mood as cautiously optimistic; some people are worried about certain weird features of the data, while others seem less concerned about them… typical when a new discovery is claimed.  I’m disturbed that the media is declaring victory before the scientific community is ready to.  That didn’t happen with the Higgs discovery, where the media was, wisely, far more patient.

The Main Data

Here’s BICEP2’s data!  The black dots at the bottom of this figure, showing evidence of B-mode polarization both at small scales (“Multipole” >> 100, where it is due to gravitational…

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BICEP2 – A Straw Poll

Posted in The Universe and Stuff with tags , on March 17, 2014 by telescoper

I’m not sure whether my scepticism about the BICEP2 results is just a sign of my old age, so it’s time for a quick (and, it goes without saying, totally unscientific)  straw poll to see what people think. Feel free to add comments through the box as well!

Some B-Mode Background

Posted in Astrohype, Science Politics, The Universe and Stuff with tags , , , , , , , , , , , on March 15, 2014 by telescoper

Well, in case you hadn’t noticed, the cosmology rumour mill has gone into overdrive this weekend primarily concerning the possibility that an experiment known as BICEP (an acronym formed from Background Imaging of Cosmic Extragalactic Polarization). These rumours have been circulating since it was announced last week that the Harvard-Smithsonian Center for Astrophysics (CfA) will host a press conference  on Monday, March 17th, to announce “a major discovery”. The grapevine is full of possibilities, but it seems fairly clear that the “major discovery” is related to one of the most exciting challenges facing the current generation of cosmologists, namely to locate in the pattern of fluctuations in the cosmic microwave background evidence for the primordial gravitational waves predicted by models of the Universe that involve inflation.

Anyway, I thought I’d add a bit of background on here to help those interested make sense of whatever is announced on Monday evening.

Looking only at the temperature variation across the sky, it is not possible to distinguish between tensor  (gravitational wave) and scalar (density wave) contributions  (both of which are predicted to be excited during the inflationary epoch).  However, scattering of photons off electrons is expected to leave the radiation slightly polarized (at the level of a few percent). This gives us additional information in the form of the  polarization angle at each point on the sky and this extra clue should, in principle, enable us to disentangle the tensor and scalar components.

The polarization signal can be decomposed into two basic types depending on whether the pattern has  odd or even parity, as shown in the nice diagram (from a paper by James Bartlett)

The top row shows the E-mode (which look the same when reflected in a mirror and can be produced by either scalar or tensor modes) and the bottom shows the B-mode (which have a definite handedness that changes when mirror-reflected and which can’t be generated by scalar modes because they can’t have odd parity).

The B-mode is therefore (at least in principle)  a clean diagnostic of the presence of gravitational waves in the early Universe. Unfortunately, however, the B-mode is predicted to be very small, about 100 times smaller than the E-mode, and foreground contamination is likely to be a very serious issue for any experiment trying to detect it. To be convinced that what is being measured is cosmological rather than some sort of contaminant one would have to see the signal repeated across a range of different wavelengths.

Moreover, primordial gravitational waves are not the only way that a cosmological B-mode signal could be generated. Less than a year ago, a paper appeared on the arXiv by Hanson et al. from SPTpol, an experiment which aims to measure the polarization of the cosmic microwave background using the South Pole Telescope. The principal result of this paper was to demonstrate a convincing detection of the so-called “B-mode” of polarization from gravitational lensing of the microwave background photons as they pass through the gravitational field generated by the matter distributed through the Universe. Gravitational lensing can produce the same kind of shearing effect that gravitational waves generate, so it’s important to separate this “line-of-sight” effect from truly primordial signals.

So we wait with bated breath to see exactly what is announced on Monday. In particular, it will be extremely interesting to see whether the new results from BICEP are consistent with the recently published conclusions from Planck. Although Planck has not yet released the analysis of its own polarization data, analysis of the temperature fluctuations yields a (somewhat model-dependent) conclusion that the ratio of tensor to scalar contributions to the CMB pattern is no more than about 11 per cent, usually phrased in the terms, i.e. R<0.11. A quick (and possibly inaccurate) back-of-the-envelope calculation using the published expected sensitivity of BICEP suggests that if they have made a detection it might be above that limit. That would be really interesting because it might indicate that something is going on which is not consistent with the standard framework. The limits on R arising from temperature studies alone assume that both scalar and tensor perturbations are generated by a relatively simple inflationary model belonging to a class in which there is a direct relationship between the relative amplitudes of the two modes (and the shape of the perturbation spectrum). So far everything we have learned from CMB analysis is broadly consistent with this simplifying assumption being correct. Are we about to see evidence that the early Universe was more complex than we thought? We'll just have to wait and see…

Incidentally, once upon a time there was a British experiment called Clover (involving the Universities of  Cardiff, Oxford, Cambridge and Manchester) which was designed to detect the primordial B-mode signal from its vantage point in Chile. I won’t describe it in more detail here, for reasons which will become obvious.

The chance to get involved in a high-profile cosmological experiment was one of the reasons I moved to Cardiff in 2007, and I was looking forward to seeing the data arriving for analysis. Although I’m primarily a theorist, I have some experience in advanced statistical methods that might have been useful in analysing the output.  Unfortunately, however, none of that actually happened. Because of its budget crisis, and despite the fact that it had spent a large amount (£4.5M) on it already,  STFC decided to withdraw the funding needed to complete it (£2.5M)  and cancelled the Clover experiment. Had it gone ahead it would probably have had two years’ data in the bag by now.

It wasn’t clear that Clover would have won the race to detect the B-mode cosmological polarization, but it’s a real shame it was withdrawn as a non-starter. C’est la vie.

A Time for Honours

Posted in Education, Politics, Science Politics, The Universe and Stuff with tags , , , on June 15, 2013 by telescoper

The word “honour” provides a (tenuous) link between yesterday’s post and this one. After our recent preoccupation with the classification of honours for graduating students (i.e. first class, second class, and so on), today’s news included the Queen’s Birthday Honours List for 2013, which you can download in full here. To make up for the lack of recycling going on in Brighton these days because of the strike that started yesterday, I thought I’d recycle my thoughts from previous years.

The honours system must appear extremely curious to people from outside the United Kingdom. It certainly seems so to me. On the one hand, I am glad that the government has a mechanism for recognising the exceptional contributions made to society by certain individuals. Musicians, writers, sportsmen, entertainers and the like generally receive handsome financial rewards, of course, but that’s no reason to begrudge a medal or two in recognition of the special place they occupy in our cultural life.  It’s  good to see scientists recognized too, although they tend not to get noticed so much by the press.

The name that stood out for me in this year’s list is Professor Jim Hough, who gets an OBE. Jim is Professor of Experimental Physics at the University of Glasgow, and his speciality is in the detection of gravitational waves.  Gravitational waves haven’t actually been detected yet, of course, but the experimental techniques designed to find them have increased their sensitivity by many orders of magnitude in recent years, Jim having played a large part in those improvements. He is also Chief Executive of the Scottish University Physics Alliance, which does so much to nurture Physics and Astronomy North of the Border.

Although I’m of course more than happy to see recognition given to such people, as I did  a couple of years ago I can’t resist stating my objections to the honours system again. One is that the list of recipients  of certain categories of award is overwhelmingly dominated by career civil servants, for whom an “honour”  goes automatically with a given rank. If an honour is considered an entitlement in this way then it is no honour at all, and in fact devalues those awards that are  given on merit to people outside the Civil Service. Civil servants get paid for doing their job, so they should have no more expectation of an additional reward than anyone else. There’s much more honour in a  student who earns a First Class degree than for a career civil servant who gets a knighthood.

Honours have relatively little monetary value on their own, of course so this is not question of financial corruption. An honour does, however, confer status and prestige on the recipient so what we have is a much more subtle form of sleaze. One wonders how many names listed in the current roll of honours are there because of political donations, for example.

I wouldn’t accept an honour myself, but that’s easy to say because I’m sure I’ll never be nominated for one; hopefully this post will dissuade anyone from even thinking of nominating me for a gong. However, I imagine that even people like me who are against the whole system are probably still tempted to accept such awards when offered, as they generate good publicity for one’s field, institution and colleagues.It’s a very personal decision and I have no criticism to make of people who think differently from me about whether to accept an honour.

Sathya’s Cosmic Sirens

Posted in The Universe and Stuff, Uncategorized with tags , , , on January 7, 2013 by telescoper

Bit busy today so I thought I’d just post this talk by Cardiff’s own Prof. Bangalore Sathyaprakash at last year’s TEDX event in Cardiff.
The title is Cosmic Sirens although given that the topic is gravitational waves I hope that “sirens” isn’t intended to mean those entirely mythical entities that lure unsuspecting PhD students to their ultimate destruction…

Anyway, here’s the blurb:

In 1916 Einstein predicted that dynamical mass distribution generates ripples in the very fabric of spacetime that propagates outwards at the speed of light.

For over two decades B.S. Sathyaprakash (Sathya for his family and friends) is engaged in research to detect these ripples called gravitational waves, from cataclysmic cosmic events such as exploding stars, colliding black holes and the big bang. His personal goal is to observe and understand black holes and gravity using gravitational radiation. He is the head of the gravitational physics group at Cardiff University — a centre for modelling astronomical sources of gravitational radiation, discovering innovative algorithms to search for this radiation and analyzing data from gravitational-wave detectors using massive computer clusters.

Although there is firm indirect evidence that certain astronomical systems do emit gravitational waves, so far no one has detected them directly. Sathya and his team are part of a worldwide effort, called the LIGO Scientific Collaboration, to detect these elusive waves using kilometer long laser interferometers in the US, Europe and Japan. Recently, Sathya helped develop the science case for building such a detector in India. He has been involved in the European design study of a third generation underground detector with a 30 km baseline called the Einstein Telescope, chairing the group that developed the science case for this ambitious venture.

And here is the actual talk..


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