Archive for gravitational waves

A Second Gravitational Wave Source!

Posted in The Universe and Stuff with tags , , on June 16, 2016 by telescoper

I was travelling back from Cambridge on the train yesterday afternoon when I saw the announcement that the Advanced LIGO team had found a second gravitational wave source. Actually, I knew this one was coming – the event actually registered last Christmas – but I had forgotten that it was to be announced at the American Astronomical Society meeting that’s happening now in San Diego. There’s also a second possible discovery, but with much lower signal-to-noise.

The full discovery paper can be found here, from which I have taken this figure:

GW

You can find the arXiv version here.
The  signal shown above, code-named GW151226, like the previous one, appears to be from a black hole binary coalescence but it involves two black holes of rather lower masses (about 14 and 8 solar masses respectively). This means that the timescale is rather longer and so more orbits can be observed. It may not look visually as clear as the first source, GW150914, which involved black holes with masses in the region of 30 solar masses, but it’s a clear detection and it’s also interesting that the models suggest that at least one of the black holes has a significant spin. Interesting!

So, that’s two sources. Now we can do statistics! I was wondering last night how long it will take before every individual discovery like this is no longer reported. The same thing happened with the first few extra-solar planets but now that we have thousands, it’s only a subset – those that might plausibly be similar to Earth – that get press attention. At the current rate of discovery gravitational-wave sources may well become quite common over the next few years. In fact a reasonable prediction for when LIGO is switched on again at the end of the summer that there might be a detection every week or so. The era of gravitational wave astronomy is definitely upon us!

Actually from my point of view the really interesting challenge is to make full use of the low signal-to-noise detections that are probable sources but with some uncertainty. I hope to write a blog post soon about how Bayesian methods can help a great deal with that.

Anyway, that’s all I’ve got time for right now. After three days in Cambridge as External Examiner, I now have to chair our undergraduate finalist examination board here at Sussex. So I’ll just say congratulations again to the LIGO team. Great stuff.

 

 

Lisa Pathfinder: it works!

Posted in The Universe and Stuff with tags , , , on June 8, 2016 by telescoper

Just time for a quick post to pass on some impressive news from LISA Pathfinder (which is basically a technology demonstrator mission intended to establish the feasibility of a proposed space-based gravitational wave facility called LISA). LISA Pathfinder is ostensibly an extremely simple experiment, consisting of two metal cubes (made of a gold-platinum mixture) about 38cm apart. The question it tries to answer is how accurately these two cubes can be put an ideal “free-fall” state, i.e. when the only force acting on them is gravity.

Here’s a short explanatory video about the latest results:

The technical details are presented in a paper in Physical Review Letters, from which the key figure is this:

 

Lisa_PathfinderThis shows very clearly that the performance of the LISA Pathfinder experiment (as shown by the red measurements) comfortably exceeds the requirements of the full LISA experiment (black curve). Indeed, these results, from only two months of science operations, show that the two cubes are in free-fall to a precision more than five times better than originally required.

So, not to put too fine a point on it,  it works!

 

 

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!

 

Making Massive Black Hole Binaries Merge

Posted in The Universe and Stuff with tags , , , , , on February 16, 2016 by telescoper

Many fascinating questions remain unanswered by last week’s detection of gravitational waves produced by a coalescing binary black hole system (GW150914) by LIGO. One of these is whether the fact that the similarity of the component masses (29 and 36 times the mass of the Sun respectively) is significant.

An interesting paper appeared on the arXiv last week by Marchant et al. that touches on this. Here is the abstract (you can click on it to make it larger):

BinaryBH

 

Although there is some technical jargon, the point is relatively clear. It appears that very masssive, very low metallicity binary stars can evolve into black hole binary systems via supernova explosions without disrupting their orbit. The term ‘low metallicity’ characteristises stars that form from primordial material (i.e. basically hydrogen and helium) early in the cycle of stellar evolution. Such material has very different opacity properties from material with significant quantities of heavier elements in it, which alters the dynamical evolution considerably.

(Remember that to an astrophysicist, chemistry is extremely simple. Hydrogen and helium make up most of the atomic matter in the Universe; all the rest is called “metals” including carbon, nitrogen, and oxygen…. )

Anyway, this theoretical paper is relevant because the mass ratios produced by this mechanism are expected to be of order unity, as is the case of GW150914.  One observation doesn’t prove much, but it’s definitely Quite Interesting…

Incidentally, it has been reported that another gravitational wave source may have been detected by LIGO, in October last year. This isn’t as clean a signal as the first, so it will require further analysis before a definitive result is claimed, but it too seems to be a black hole binary system with a mass ratio of order unity…

You wait forty years for a gravitational wave signal from a binary black hole merger and then two come along in quick succession…

 

 

 

Lessons from LIGO

Posted in The Universe and Stuff with tags , , , on February 13, 2016 by telescoper

At the end of a very exciting week I had the pleasure last night of toasting LIGO and the future of gravitational wave astronomy with champagne at the RAS Club in London. Two members of the LIGO collaboration were there, Alberto Vecchio and Mike Cruise (both from Birmingham); Alberto had delivered a very nice talk earlier in the day summarising the LIGO discovery while Mike made a short speech at the club.

This morning I found this interesting video produced by California Institute of Technology (CalTech) which discusses the history of the LIGO experiment:

It has taken over 40 years of determination and hard work to get this far. You can see pictures of some of the protagonists from Thursday’s press conference, such as Kip Thorne, when they were much younger. I bet there were times during the past four decades when they must have doubted that they would ever get there, but they kept the faith and now can enjoy the well-deserved celebrations. They certainly will all be glad they stuck with gravitational waves now, and all must be mighty proud!

Mike Cruise made two points in his speech that I think are worth repeating here. One is that we think of the LIGO discovery is a triumph of physics. It is that, of course. But the LIGO consortium of over a thousand people comprises not only physicists, but also various kinds of engineers, designers, technicians and software specialists. Moreover the membership of LIGO is international. It’s wonderful that people from all over the world can join forces, blend their skils and expertise, and achieve something remarkable. There’s a lesson right there for those who would seek to lead us into small-minded isolationism.

The other point was that the LIGO discovery provides a powerful testament for university research. LIGO was a high-risk experiment that took decades to yield a result. It’s impossible to imagine any commercial company undertaking such an endeavour, so this could only have happened in an institution (or, more correctly, a network of institutions) committed to “blue skies” science. This is research done for its own sake, not to create a short-term profit but to enrich our understanding of the Universe. Asking  profound questions and trying to answer them is one of the things that makes us human. It’s a pity we are so obsessed with wealth and property that we need to be reminded of this, but clearly we do.

The current system of Research Assessment in the UK requires university research to generate “impact” outside the world of academia in a relatively short timescale. That pressure is completely at odds with experiments like LIGO. Who would start a physics experiment now that would take 40 years to deliver?  I’ve said it time and time again to my bosses at the University of Sussex that if you’re serious about supporting physics you have to play a long game because it requires substantial initial investment and generates results only very slowly.  I worry what future lies in store for physics if the fixation on market-driven research continues much longer.

Finally, I couldn’t resist making a comment about another modern fixation – bibliometrics. The LIGO discovery paper in Physical Review Letters has 1,004 authors. By any standard this is an extraordinarily significant article, but because it has over a thousand authors it stands to be entirely excluded by the Times Higher when they compile the next World University Rankings.  Whatever the science community or the general public thinks about the discovery of gravitational waves, the bean-counters deem it worthless. We need to take a stand against this sort of nonsense.

 

 

 

 

LIGO at the Royal Astronomical Society

Posted in Biographical, The Universe and Stuff with tags , , on February 12, 2016 by telescoper

image

My monthly trip to London for the Royal Astronomical Society Meeting allowed me not only to get out of the office for the day but also to attend a nice talk by Alberto Vecchio about yesterday’s amazing results.

I hear that we will be having champagne at the club later on to celebrate. In the meantime here’s a little Haiku I wrote on the theme:

Two black holes collide
A billion years ago.
LIGO feels the strain.

LIGO: Live Reaction Blog

Posted in The Universe and Stuff with tags , , , , on February 11, 2016 by telescoper

So the eagerly awaited press conference happened this afternoon. It started in unequivocal fashion.

“We detected gravitational gravitational waves. We did it!”

As rumoured, the signal corresponds to the coalescence of two black holes, of masses 29 and 36 times the mass of the Sun.

The signal arrived in September 2015, very shortly after Advanced LIGO was switched on. There’s synchronicity for you! The LIGO collaboration have done wondrous things getting their sensitivity down to such a level that they can measure such a tiny effect, but there still has to be an event producing a signal to measure. Collisions of two such massive black holes are probably extremely rare so it’s a bit of good fortune that one happened just at the right time. Actually it was during an engineering test!

Here are the key results:

 

LIGO

 

Excellent signal to noise! I’m convinced! Many congratulations to everyone involved in LIGO! This has been a heroic effort that has taken many years of hard slog. They deserve the highest praise, as do the funding agencies who have been prepared to cover the costs of this experiment over such a long time. Physics of this kind is a slow burner, but it delivers spectacularly in the end!

You can find the paper here, although the server seems to be struggling to cope! One part of the rumour was wrong, however, the result is not in Nature, but in Physical Review Letters. There will no doubt be many more!

And right on cue here is the first batch of science papers!

No prizes for guessing where the 2016 Nobel Prize for Physics is heading, but in a collaboration of over 1000 people across the world which few will receive the award?

So, as usual, I had a day filled with lectures, workshops and other meetings so I was thinking I would miss the press conference entirely, but in the end I couldn’t resist interrupting a meeting with the Head of the Department of Mathematics to watch the live stream…

P.S. A quick shout out the UK teams involved in this work, including many old friends in the Gravitational Physics Group at Cardiff University (see BBC News item here) and Jim Hough and Sheila Rowan from Glasgow. If any of them are reading this, enjoy your trip to Stockholm!

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