Archive for Advanced LIGO

LIGO, Leaks and NGC 4993

Posted in Open Access, The Universe and Stuff with tags , , , , on August 23, 2017 by telescoper

No matter what the official policy may be, the more people there are in a collaboration the more likely it is that someone will let their excitement get to their head and start leaking news and starting rumours either directly or indirectly via social media. And so it came to pass last Friday that the following tweet appeared:

I didn’t comment on the time as I thought it might be unreliable – as it indeed it still may be – but now New Scientist has amplified the signal I feel I can’t really be blamed for mentioning it here.

The rumours going round identify the optical counterpart as being in the galaxy NGC 4993 , a red band image of which, from the Second Digitized Sky Survey (DSS2) is shown below:

NGC 4993 is the fuzzy blob slightly above and to the left of the centre of the image. It’s a fairly nondescript lenticular galaxy in a group that can be found in the constellation of Hydra. It lies in the constellation of Hydra, was actually first discovered by William Herschel and it is about 10 arcmin across on the sky. It’s quite nearby, as these things go, with a distance of about 124 million light years (i.e. 40 Mpc or so) and is about 14th magnitude.

If there is an optical counterpart to a gravitational wave event coming from this galaxy then that suggests it may be a coalescence of neutron stars. The black hole mergers that appear to be responsible to the three existing gravitational wave signals that are claimed to have been detected are not expected to release optical light. Confirmation of this interpretation can be found by where the Hubble Space Telescope was pointed yesterday:

Look familiar? HST was, in fact, observing a `BNS-Merger’ (which is short for `Binary Neutron Star’)…

BNS

If this rumour is true then it’s obviously exciting, but there are questions to be asked. Chief among these is how sure is the identification of the counterpart? A transient optical source in NGC4993 may have been observed at the same time as a gravitational wave signal was detected,  but the ability of LIGO to resolve positions on the sky is very poor. On the other hand, the European VIRGO experiment joined Advanced LIGO for the ongoing `O2′ observing run (which ends in a couple of days). Although VIRGO is less sensitive than LIGO having a third detector does improve the localization of the source – assuming, of course, that it detects a signal. Even in that case it certainly won’t be possible to pinpoint the GW source to within 10 arc minutes, which is the precision needed to place it definitely within NGC 4993.

Anyway, we wait and see what, if anything, has been found. If it is a claimed detection then I hope that LIGO and VIRGO will release sufficient data to enable the analysis to be checked and verified. That’s what most of the respondents to my poll seem to hope too!

On the Time Lags of the LIGO signals

Posted in Bad Statistics, The Universe and Stuff with tags , , , on August 10, 2017 by telescoper

It seems that a lot of rumours are flying around on social media and elsewhere about the discussions that have been going on here in Copenhagen between members of the Niels Bohr Institute and of the LIGO scientific collaboration concerning matters arising from the `Danish Paper‘.  The most prominent among these appears to be the LIGO team and the Danish team have agreed on everything and that the Danish authors have conceded that they were mistaken in their claims. I have even been told that my recent blog posts gave the impression that this was the case. I’m not sure how, as all I’ve said is that the discussions reached agreement on some matters. I did not say what matters or whose position had changed.

I feel, therefore, that some clarification is necessary. Since I am a member of neither party to this controversy I have to tread carefully, and there are some things which I feel I should not discuss at all. I was invited to participate in the discussions as a neutral observer as a courtesy and I certainly don’t want to betray any confidences. On one thing, however, I can be perfectly clear. The Danish team (Cresswell et al.) have not retracted their claims and they reject the suggestion that their paper was wrong.

To reinforce this, I draw your attention to the fact that a revised version of `The Danish Paper’ has now been accepted for publication (in the Journal of Cosmology and Astroparticle Physics) and that this paper is now available on the arXiv. The referees raised a large number of queries, and in response to them all the revised version is almost double the length of the original.

Here is the arXiv entry page:

The main body of the paper has not been significantly modified and their main result – of an unexplained 7ms correlation in the background signal (referred to in the abstract as `noise’) – has not “gone away”. If you want to understand more, read the paper!

I’m sure there will be much more discussion of this and I will comment as appropriate when appropriate. In the meantime this remains very much a live issue.

P.S. In the interest of full disclosure I should mention that I did read over part of the revised version of the Danish paper and made some suggestions with regard to style and flow. I therefore have a mention in the acknowledgments of the final version. I was warned that I might expect some trouble for agreeing to be associated with the paper in this way but, as  Sam Spade says in The Maltese Falcon `I don’t mind a reasonable amount of trouble’…

The ‘Danish Paper’ and How Science Works

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

I’m off work today but couldn’t resist posting a very quick update on the controversial claims of inconsistencies in the recent detection of gravitational waves by LIGO.

If you’re following the story you will know that it started with a paper on the arXiv by Cresswell et al.,  a group mainly based in Denmark, which is why the paper is now frequently referred to as ‘The Danish Paper’ although its authors actually come from all round the world.

Well the same group have now  written a rejoinder to the LIGO critique of their analysis. They’re clearly sticking to their guns, at least on their claim that the residuals left after removing the gravitational wave events from the two time series are correlated, which they should not be if they are simply noise.

Hopefully the public airing this controversy had received will lead to other independent groups downloading and analysing the data, which is all in the public domain, and we’ll eventually arrive at the truth.

Contrary to the opinion of one of my Cardiff colleagues I think this is how science works and, importantly, how it should be seen to work. Science  is a process of investigation, and it doesn’t come to an end when  results have been published in refereed journals. 

 The more the public see how science really works – warts and all – the better they will understand its strengths as well as its limitations.

Whatever the eventual outcome of this discussion I think we will find that the ‘Danish Paper’ has helped advance our understanding, and for that the authors deserve a great deal of credit.

 

Newsflash: another LIGO detection!

Posted in The Universe and Stuff with tags , , , on June 1, 2017 by telescoper

I’ve just heard the news that  LIGO has just announced the detection of another gravitational-wave signal, which has been given the identifier GW170104; it was detected on 4th January 2017.

The event was the merger of a black-hole binary system a redshift z=0.2, which is a proper distance of about 800 Mpc in the standard cosmological model, the most distant event yet detected. There are also tantalising hints that at least one of the black holes had spin opposite the orbital angular momentum, which implies it may have originated in a globular cluster. For more details please see the refereed paper.

If you’d rather just look at the plot here is the evidence for the event, in the form of coincident signals at the two components of LIGO:

 
LIGO

I reckon there’s a good chance of seeing members of the Cardiff University Gravitational Physics group celebrating in the pub later this evening!

It’s also a reasonable inference given the rate of detection of these events so far that we’re going to see many more in the very near future!

Infinite LIGO Dreams

Posted in Art, The Universe and Stuff with tags , , , , , on November 28, 2016 by telescoper

There was a special event in the School of Physics & Astronomy here at Cardiff University on Friday afternoon – the unveiling of a new work of art in our coffee area. The work, a large oil painting, called Infinite LIGO Dreams by local artist Penelope Rose Cowley was inspired by the detection of gravitational waves earlier this year:

 

gravitational-wave-artwork-copyright-penelope-cowley-16x9

You can read more about this work, and the circumstances behind its creation, at the Cardiff University website and via the Physics World blog. If you like the piece you can order a poster-sized print from Penelope Cowleys’s own website here.

The unveiling of this artwork was preceded by a drinks reception, which probably accounts for the errors that crept into the blog post I wrote on Friday after the party!

 

Jobs in Gravitational Waves at Cardiff University

Posted in The Universe and Stuff with tags , , , , on September 5, 2016 by telescoper

Gradually settling back in here to the School of Physics & Astronomy at Cardiff University, I thought I’d indulge  in a bit of promotional activity and point out that, following on from the recent detection of gravitational waves by the Advanced LIGO Consortium, of which  Cardiff University is a member, there are two opportunities open for jobs in gravitational physics.

One is in the area of Gravitational Wave Astronomy. Here is the blurb:

The current Cardiff Gravitational Physics group has expertise in gravitational-wave data analysis, numerical relativity and source modelling, and astrophysical interpretation, and consists of four full-time and two part-time academic staff, two research fellows, five postdoctoral researchers and nine PhD students. Our research is supported by the UK Science and Technology Facilities Council (STFC), the Royal Society, and the European Horizon 2020 programme. The group is a founding member of GEO600, a member of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration and has played a leading role in these collaborations from their inception through to the recent first direct detection of gravitational waves, and is also active in planning and development of future detectors, such as LIGO-India, Einstein Telescope and Laser Interferometer Space Antenna (LISA).

This new appointment is part of a long-term expansion of the group, to broaden and strengthen our current research in gravitational-wave astronomy, and to build a world-leading group in gravitational-wave experimentation.

For the full advertisement, links to further particulars etc, see here.

The other is the area of Gravitational Wave Experimentation:

The current Cardiff Gravitational Physics group has expertise in gravitational-wave data analysis, numerical relativity and source modelling, and astrophysical interpretation, and consists of four full-time and two part-time academic staff, two research fellows, five postdoctoral researchers and nine PhD students. Our research is supported by the UK Science and Technology Facilities Council (STFC), the Royal Society, and the European Horizon 2020 programme. The group is a founding member of GEO600, a member of the Laser Interferometer Gravitational-Wave Observatory ( LIGO) Scientific Collaboration and has played a leading role in these collaborations from their inception through to the recent first direct detection of gravitational waves, and is also active in planning and development of future detectors, such as LIGO-India, Einstein Telescope and Laser Interferometer Space Antenna ( LISA).

This new appointment is part of a long-term expansion of the group, to broaden and strengthen our current research in gravitational-wave astronomy, and to build a world-leading group in gravitational-wave experimentation, with additional appointments expected in the near future.

For full details on this one see here.

The second appointment is intended to build on existing strengths by adding a more experimental dimension to Cardiff’s research in Gravitational Waves.

 

The Great Gravitational Wave Source Follow-Up

Posted in The Universe and Stuff with tags , , , on March 1, 2016 by telescoper

I recently noticed on the arXiv  an interesting paper with 1562 authors!

Here is the abstract:

A gravitational-wave transient was identified in data recorded by the Advanced LIGO detectors on 2015 September 14. The event candidate, initially designated G184098 and later given the name GW150914, is described in detail elsewhere. By prior arrangement, preliminary estimates of the time, significance, and sky location of the event were shared with 63 teams of observers covering radio, optical, near-infrared, X-ray, and gamma-ray wavelengths with ground- and space-based facilities. In this Letter we describe the low-latency analysis of the gravitational wave data and present the sky localization of the first observed compact binary merger. We summarize the follow-up observations reported by 25 teams via private Gamma-ray Coordinates Network Circulars, giving an overview of the participating facilities, the gravitational wave sky localization coverage, the timeline and depth of the observations. As this event turned out to be a binary black hole merger, there is little expectation of a detectable electromagnetic signature. Nevertheless, this first broadband campaign to search for a counterpart of an Advanced LIGO source represents a milestone and highlights the broad capabilities of the transient astronomy community and the observing strategies that have been developed to pursue neutron star binary merger events. Detailed investigations of the electromagnetic data and results of the electromagnetic follow-up campaign will be disseminated in the papers of the individual teams.

This is interesting not so much for the result – there wasn’t really any expectation of finding an electromagnetic counterpart of a binary black-hole merger – but that it’s the first example of the kind of mass mobilisation of astronomers that will be needed when gravitational-wave astronomy gets going in earnest. Astronomers working on transient sources such as gamma-ray bursts are already used to this kind of operation, but there’s going to be a lot more of it in the future!