Archive for gravitational waves

!Happy Birthday GW150914!

Posted in The Universe and Stuff with tags , , on September 14, 2016 by telescoper

A birthday message to the first gravitational wave source to be detected, from my new office mate, Bernard Schutz!

The Rumbling Universe

Just a year ago today, after travelling some 1.4 billion years, the gravitational wave chirp we christened GW150914 passed through Earth. It disturbed the two gravitational wave detectors of the LIGO observatory enough for us to notice it, to get excited about it, and to get a large fraction of the general public excited about it! But GW150914 just kept on going and is now one further year along in its journey through the Universe. And it will keep going, spreading out and getting weaker but not otherwise being much disturbed, forever. Literally forever.

And GW150914 hardly noticed us! When we observe the Universe with our telescopes, detecting light or radio waves or gamma rays from the enormous variety of luminous objects out there, we capture the energy that enters our telescopes. The photons from a distant star terminate their journeys in our telescopes, leaving a tiny hole in the…

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Theory of Gravitational Waves [CL]

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

Since gravitational waves are quite the thing these days I thought I’d reblog this arXiver post of a nice review article that covers all the basics for the benefit of anyone interested in finding about a bit more about the subject.

arXiver

http://arxiv.org/abs/1607.04202

The existence of gravitational radiation is a natural prediction of any relativistic description of the gravitational interaction. In this chapter, we focus on gravitational waves, as predicted by Einstein’s general theory of relativity. First, we introduce those mathematical concepts that are necessary to properly formulate the physical theory, such as the notions of manifold, vector, tensor, metric, connection and curvature. Second, we motivate, formulate and then discuss Einstein’s equation, which relates the geometry of spacetime to its matter content. Gravitational waves are later introduced as solutions of the linearized Einstein equation around flat spacetime. These waves are shown to propagate at the speed of light and to possess two polarization states. Gravitational waves can interact with matter, allowing for their direct detection by means of laser interferometers. Finally, Einstein’s quadrupole formulas are derived and used to show that nonspherical compact objects moving at relativistic speeds are powerful gravitational wave…

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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.

 

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…