Archive for gravitational waves

New Publication at the Open Journal of Astrophysics!

Posted in Open Access, The Universe and Stuff with tags , , , , , , , , on September 24, 2019 by telescoper

Yesterday we published another new paper at The Open Journal of Astrophysics, but I didn’t get time to write a post about because of teaching and other start-of-term business so I’m correcting that omission now.

 

The authors are Selim Can Hotinli  of Imperial College London (UK), Marc Kamionkowski of Johns Hopkins University, Baltimore (USA) and Andrew Jaffe, also of Imperial College.

You can find the accepted version on the arXiv here. This version was accepted after modifications requested by the referee and editor. Because this is an overlay journal the authors have to submit the accepted version to the arXiv (which we then check against the copy submitted to us) before publishing; version 2 on the arXiv is the accepted version.

You will see that this is  one for the `Cosmology and Nongalactic Astrophysics’ folder. We would be happy to get more submissions from other areas, especially Stellar and Planetary astrophysics. Hint! Hint!

P.S. Just a reminder that we now have an Open Journal of Astrophysics Facebook page where you can follow updates from the Journal should you wish..

On the Fellowship of Roy Kerr

Posted in The Universe and Stuff with tags , , , , , , , on April 18, 2019 by telescoper

Among the new Fellows of the Royal Society announced this week, I was astonished to see the name of Roy Kerr, the man who gave his name to the Kerr Metric an exact solution of Einstein’s equations of general relativity which describes the geometry of space-time around a rotating black hole.

When I say “astonished” I don’t mean that Kerr does not deserve this recognition. Far from it. I’m astonished because it has taken so long:the Kerr solution was published way back in 1963.

Anyway, better late than never, and heartiest congratulations to him!

While I’m on about Roy Kerr I’ll also say that I now think there is a very strong case for him to be awarded a Nobel Prize. The reasons are twofold.

One is that all the black hole binary systems whose coalescences produced gravitational waves detected by LIGO have involved Kerr black holes. Without Kerr’s work it would not have been possible to construct the template waveforms needed to extract signals from the LIGO data.

Second, and even more topically, the black hole in M87 recently imaged (above) by the Event Horizon Telescope is also described by the Kerr geometry. Without Kerr’s work the modelling of light paths around this object would not have been possible either.

Gravitational Wave Flash!

Posted in The Universe and Stuff with tags , , , , on April 9, 2019 by telescoper

The third observing run for Advanced LIGO – O3 – started on April 1 2019, after 19 months upgrading the detectors. Last night, April 8, saw the first new detection of a candidate gravitational wave source, apparently another black hole binary, dubbed S190408an.

It is anticipated that sources like this will be discovered at a rate of roughly one per week for the (planned) year-long run. Given the likely rate of events the policy of LIGO is now to make data publicly available directly without writing papers first. You can find the data entry for this event here, including this map of its position.

Whether the LIGO Scientific Collaboration will release sufficient data for others to perform a full analysis of the signal remains to be seen, but if the predicted detection rate matches reality, the field is going to move very rapidly from studies of individual events to statistical analysis of large populations. Such is the way of science!

Bernard Schutz wins the 2019 Eddington Medal

Posted in Cardiff, The Universe and Stuff with tags , , , on January 14, 2019 by telescoper

I wasn’t able to get to the Ordinary Meeting of the Royal Astronomical Society on Friday 11th January as I was otherwise engaged. In case you didn’t know, these meetings happen on the second Friday of every month and consist of short talks, longer set-piece prize lectures and Society business. The January meeting is when the annual awards are announced, so I missed the 2019 crop of medals and other prizes. When I got to the Athenaeum for dinner I was delighted to be informed that one of these – the prestigious Eddington Medal – had been awarded to my erstwhile Cardiff colleague Bernard Schutz (with whom I worked in the Data Innovation Research Institute and the School of Physics & Astronomy).

Here is a short video of the man himself talking about the work that led to this award:

The citation for Bernard’s award focuses on his invention of a method of measuring the Hubble constant using coalescing binary neutron stars. The idea was first published in September 1986 in a Letter to Nature. Here is the first paragraph:

I report here how gravitational wave observations can be used to determine the Hubble constant, H 0. The nearly monochromatic gravitational waves emitted by the decaying orbit of an ultra–compact, two–neutron–star binary system just before the stars coalesce are very likely to be detected by the kilometre–sized interferometric gravitational wave antennas now being designed1–4. The signal is easily identified and contains enough information to determine the absolute distance to the binary, independently of any assumptions about the masses of the stars. Ten events out to 100 Mpc may suffice to measure the Hubble constant to 3% accuracy.

In this paper, Bernard points out that a binary coalescence — such as the merger of two neutron stars — is a self calibrating `standard candle’, which means that it is possible to infer directly the distance without using the cosmic distance ladder. The key insight is that the rate at which the binary’s frequency changes is directly related to the amplitude of the gravitational waves it produces, i.e. how `loud’ the GW signal is. Just as the observed brightness of a star depends on both its intrinsic luminosity and how far away it is, the strength of the gravitational waves received at LIGO depends on both the intrinsic loudness of the source and how far away it is. By observing the waves with detectors like LIGO and Virgo, we can determine both the intrinsic loudness of the gravitational waves as well as their loudness at the Earth. This allows us to directly determine distance to the source.

It may have taken 31 years to get a measurement, but hopefully it won’t be long before there are enough detections to provide greater precision – and hopefully accuracy! – than the current methods can manage!

Congratulations to Bernard on his thoroughly well-deserved Eddington Medal!

 

A LIGO Orrery

Posted in The Universe and Stuff with tags , , , , on December 5, 2018 by telescoper

Following yesterday’s post here is a nice video visualization of all the black hole binary mergers so far claimed to have been detected by Advanced LIGO. They’re computer simulations, of course, not actual black holes (which you wouldn’t be able to see). I always thought an Orrery was a clockwork device, rather than a digital computer, but there you go. Poetic license!

I can’t say I’m very keen on the music.

The New Wave of Gravitational Waves

Posted in The Universe and Stuff with tags , , , , on December 4, 2018 by telescoper

I think it’s very sneaky of the LIGO Scientific Collaboration and the Virgo Collaboration to have released two new gravitational wave papers while I was out of circulation fora  couple of days, so I’m a bit late on this, but here are links to the new results on the arXiv.

You can click on all the excerpts below to make them bigger.

First there is GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs with this abstract:

Here is a summary of the properties of the binary systems involved in the events listed in the above paper:

There are several (four) events in this catalogue that have not previously been announced (or, for that matter, subjected to peer review) despite having been seen in the data some time ago (as far back as 2015). I’m also intrigued by the footnote on the first page which contains the following:

…all candidate events with an estimated false alarm rate (FAR) less than 1 per 30 days
and probability > 0.5 of being of astrophysical origin (see Eq. (10) for the definition) are henceforth denoted with the GW prefix.

The use of false discovery rates is discussed at length here as a corrective to relying on p-values for detections. The criteria adopted here don’t seem all that strong to me.

The second paper is Binary Black Hole Population Properties Inferred from the First and Second Observing Runs of Advanced LIGO and Advanced Virgo which has this abstract:

I’ve been teaching and/or preparing lectures all day today, so I haven’t yet had time to read these papers in detail. I will try to read them over the next few days. In the meantime I would welcome comments through the box about these new results. I wonder if there’ll be any opinions from the direction of Copenhagen?

UPDATE: Here’s a montage of all 10 binary black hole mergers `detected’ so far…

I think it’s safe to say that if GW151266 had been the first to be announced, the news would have been greeted with considerable skepticism!

Grave Wave Doubts?

Posted in Bad Statistics, The Universe and Stuff with tags , , , , on November 1, 2018 by telescoper

coverns

I noticed this morning that this week’s New Scientist cover feature (by Michael Brooks)is entitled Exclusive: Grave doubts over LIGO’s discovery of gravitational waves. The article is behind a paywall – and I’ve so far been unable to locate a hard copy in Maynooth so I haven’t read it yet but it is about the so-called `Danish paper’ that pointed out various unexplained features in LIGO data associated with the first detection of gravitational waves of a binary black hole merger.

I did know this piece was coming, however, as I spoke to the author on the phone some time ago to clarify some points I made in previous blog posts on this issue (e.g. this one and that one). I even ended up being quoted in the article:

Not everyone agrees the Danish choices were wrong. “I think their paper is a good one and it’s a shame that some of the LIGO team have been so churlish in response,” says Peter Coles, a cosmologist at Maynooth University in Ireland.

I stand by that comment, as I think certain members – though by no means all – of the LIGO team have been uncivil in their reaction to the Danish team, implying that they consider it somehow unreasonable that the LIGO results such be subject to independent scrutiny. I am not convinced that the unexplained features in the data released by LIGO really do cast doubt on the detection, but unexplained features there undoubtedly are. Surely it is the job of science to explain the unexplained?

It is an important aspect of the way science works is that when a given individual or group publishes a result, it should be possible for others to reproduce it (or not as the case may be). In normal-sized laboratory physics it suffices to explain the experimental set-up in the published paper in sufficient detail for another individual or group to build an equivalent replica experiment if they want to check the results. In `Big Science’, e.g. with LIGO or the Large Hadron Collider, it is not practically possible for other groups to build their own copy, so the best that can be done is to release the data coming from the experiment. A basic problem with reproducibility obviously arises when this does not happen.

In astrophysics and cosmology, results in scientific papers are often based on very complicated analyses of large data sets. This is also the case for gravitational wave experiments. Fortunately, in astrophysics these days, researchers are generally pretty good at sharing their data, but there are a few exceptions in that field.

Even allowing open access to data doesn’t always solve the reproducibility problem. Often extensive numerical codes are needed to process the measurements and extract meaningful output. Without access to these pipeline codes it is impossible for a third party to check the path from input to output without writing their own version, assuming that there is sufficient information to do that in the first place. That researchers should publish their software as well as their results is quite a controversial suggestion, but I think it’s the best practice for science. In any case there are often intermediate stages between `raw’ data and scientific results, as well as ancillary data products of various kinds. I think these should all be made public. Doing that could well entail a great deal of effort, but I think in the long run that it is worth it.

I’m not saying that scientific collaborations should not have a proprietary period, just that this period should end when a result is announced, and that any such announcement should be accompanied by a release of the data products and software needed to subject the analysis to independent verification.

Given that the detection of gravitational waves is one of the most important breakthroughs ever made in physics, I think this is a matter of considerable regret. I also find it difficult to understand the reasoning that led the LIGO consortium to think it was a good plan only to go part of the way towards open science, by releasing only part of the information needed to reproduce the processing of the LIGO signals and their subsequent statistical analysis. There may be good reasons that I know nothing about, but at the moment it seems to me to me to represent a wasted opportunity.

CLARIFICATION: The LIGO Consortium released data from the first observing run (O1) – you can find it here – early in 2018, but this data set was not available publicly at the time of publication of the first detection, nor when the team from Denmark did their analysis.

I know I’m an extremist when it comes to open science, and there are probably many who disagree with me, so here’s a poll I’ve been running for a year or so on this issue:

Any other comments welcome through the box below!

UPDATE: There is a (brief) response from LIGO (& VIRGO) here.