Archive for gravitational waves

And then there were five….

Posted in The Universe and Stuff with tags , , , , , , on November 17, 2017 by telescoper

…black hole mergers detected via gravitational waves, that is. Here are the key measurements for Number 5, codename GW170608. More information can be found here.

Here is the abstract of the discovery paper:

On June 8, 2017 at 02:01:16.49 UTC, a gravitational-wave signal from the merger of two stellar-mass black holes was observed by the two Advanced LIGO detectors with a network signal-to-noise ratio of 13. This system is the lightest black hole binary so far observed, with component masses 12+7-2 M⊙ and 7+2-2 M⊙ (90% credible intervals). These lie in the range of measured black hole masses in low-mass X-ray binaries, thus allowing us to compare black holes detected through gravitational waves with electromagnetic observations. The source’s luminosity distance is 340 +140-140Mpc, corresponding to redshift 0.07+0.03-0.03. We verify that the signal waveform is consistent with the predictions of general relativity.

This merger seems to have been accompanied by a lower flux of press releases than previous examples…

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Determining the Hubble Constant the Bernard Schutz way

Posted in The Universe and Stuff with tags , , , , , on October 19, 2017 by telescoper

In my short post about Monday’s announcement of the detection of a pair of coalescing neutron stars (GW170817), I mentioned that one of the results that caught my eye in particular was the paper about using such objects to determine the Hubble constant.

Here is the key result from that paper, i.e. the posterior distribution of the Hubble constant H0 given the data from GW170817:

You can also see latest determinations from other methods, which appear to be in (slight) tension; you can read more about this here. Clearly the new result from GW170817 yields a fairly broad range for H0 but, as I said in my earlier post, it’s very impressive to be straddling the target with the first salvo.

Anyway, I just thought I’d mention here that the method of measuring the Hubble constant using coalescing binary neutron stars was invented by none other than Bernard Schutz of Cardiff University, who works in the Data Innovation Institute (as I do). 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 in the 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!

Above all, congratulations to Bernard for inventing a method which has now been shown to work very well!

GW News Day

Posted in The Universe and Stuff with tags , , on October 16, 2017 by telescoper

Well, it has certainly been an eventful last day in India!

Over a hundred people gathered at IUCAA to see this evening’s press conference, which basically confirmed most of the rumours that had been circulating that a Gamma Ray Burst had been detected in both GW and EM radiation. I won’t write in detail about today’s announcement because (a) a really useful page of resources has been prepared by the group at IUCAA. Check out the fact sheet and (b) I haven’t really had time to digest all the science yet.

I will mention a couple of things, however. One is that the signal-to-noise ratio of this detection is a whopping 32.4, a value that astronomers can usually only dream of! The other is that neutron star coalescence offer the possibility to bypass the traditional `distance ladder’ approaches to get an independent measurement of the Hubble constant. The value obtained is in the range 62 to 107 km s-1 Mpc-1, which is admittedly fairly broad, but is based on only one observation of this type. It is extremely impressive to be straddling the target with the very first salvo.

The LIGO collaboration is over a thousand people. Add to that the staff of no fewer than seventy observatories (including seven in space). With all that’s going in the world, it’s great to see what humans of different nations across the globe can do when they come together and work towards a common goal. Scientific results of this kind will remembered long after the silly ramblings of our politicians and other fools have been forgotten.

I took part in a panel discussion after the results were presented, but sadly I won’t be here to see tomorrow’s papers. I hope people will save cuttings or post weblinks if there are any articles!

UPDATE: Here is a selection of the local press coverage.

Indian LIGO

 

As if these thrilling science results weren’t enough I finally managed to meet my old friend and former collaborator Varun Sahni (who was away last week). An invitation to dinner at his house was not to be resisted on my last night here, which explains why I didn’t write a post immediately after the press conference. Still, of got plenty of papers to read on the plane tomorrow so maybe I’ll do something when I get back.

Tomorrow morning I get up early to return to Mumbai for the flight home, and am not likely to be online again until Wednesday UK time.

Thanks to all at IUCAA (and TIFR) for making my stay so pleasant and interesting. It’s been 23 years since I was last here. I hope it’s not so long before I’m back again!

Gravitational Waves Flash!

Posted in The Universe and Stuff with tags , , , , on October 13, 2017 by telescoper

I got up early this morning to hitch a ride in a car to Mumbai so that I can give a talk this afternoon. We left Pune about 6am and got here about 8.30 so the trip was a quite a bit quicker than coming here! I’ll post about that and include some pictures when I get a moment, but first I’ll post a quick announcement.

There will be an announcement on Monday 16th October at 10am EDT (3pm BST; 7.30pm in Pune) by `the National Science Foundation (NSF) as it brings together scientists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo collaborations, as well as representatives for some 70 observatories’. Further details can be found here. The European Southern Observatory has also announced that it will be holding a press conference on Monday about an `unprecedented discovery’.

The fact that it involves LIGO, Virgo and representatives of other observatories strongly suggests that this announcement will address the subject of the rumours that were flying around in August. In other words, it’s likely that on Monday we will hear about the first detecting of a coalescing binary neutron star system with an optical counterpart. Exciting times!

I’ll be back in Pune by Monday and will probably be able to watch the announcement and will update if and when I can.

The 2017 Nobel Prize for Physics goes to…

Posted in The Universe and Stuff with tags , , , , , on October 3, 2017 by telescoper

Usually at this time of year I make a point of watching the live announcement of the Nobel Prize for Physics, but this time I was otherwise engaged. On the other hand, this year was the least surprising announcement I can remember for a long time. Confirming almost everyone’s expectations, the award goes to Rainer Weiss (MIT), Barry C. Barish (Caltech) and Kip S. Thorne (Caltech) “for decisive contributions to the LIGO detector and the observation of gravitational waves”. You can see the full citation here.

Perhaps one surprise the split (50% to Weiss and 25% each to Barish and Thorne). I suppose the reason is that it divides the prize equally between MIT and Caltech. Ronald Drever, who had shared in other awards for the LIGO discovery (e.g the Gruber,  Shaw and Kavli prizes), sadly passed away earlier this year.

Anyway,  heartiest congratulations to the winners and also to all the other members of the LIGO Scientific Collaboration who collectively earned this award! That includes the Gravitational Physics group at Cardiff University who will no doubt be getting pissed celebrating in appropriate style.

Two thoughts. One is that the LIGO Collaboration is very large (the papers have over a thousand authors) but the Nobel Prize rules do not allow this award to be divided among more than three people. This is a problem for `Big Science’ which is always done by large teams. In a real sense, the Nobel Prize for physics reflects the way physics was done when it was founded, over a hundred years ago. It seems to me the limitation perpetuates the myth of the lone genius, when science doesn’t really work like that nowadays. I’m not sure it ever did, actually. I  wonder if they’ll ever change?

Another thing that struck me is that the interval between discovery and award seems to be decreasing. For example, he Cosmic Microwave Background was discovered in 1965, but Penzias and Wilson were not awarded the Nobel Prize for its discovery until 1978. I attended the Nobel Prize ceremony in 2005, when George Smoot and John Mather were award the prize for COBE which had happened over a decade earlier. This time the gap between discovery and award is just two years. I suppose that proves that we live in an accelerating universe (Nobel Prize 2011).

Anyway there are too many people in LIGO for them all to be able to attend the Prize Ceremony and Banquet in Stockholm in December, but I hope the winners don’t just give their invitations to senior members of the LIGO collaboration. Perhaps some form of lottery might be organized to allow early career researchers to attend?

As I’ve mentioned before on this blog I had the honour to be invited to the 2006 Nobel Prize ceremony. As a matter of fact, I still have this:

The chocolate has probably gone off by now, though. I stress that I attended not as a winner but as a guest of the Nobel Foundation. It was a wonderful occasion, of which I have very special memories. I’m sure everyone who does get to attend will have a ball! (Geddit?)

Although the Nobel Prize has its limitations as a true reflection of scientific contributions, I still has value in that for once the news media are talking about a great human achievement which contrasts with much of the stuff we have to hear about these days.

Gravitational Wave Flash

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

Inconveniently timed just before I was due to go to the pub, a new announcement has come out from the LIGO and Virgo gravitational wave detectors. This time it reports a coalescing binary black hole system detected by all three instruments. The new source is called GW170814, which indicates that the signal from it was received by the detectors on the day I returned from Copenhagen this summer!

Here’s the key figure:

The paper is here and there’s a Nature comment piece here.

I have to say that, on its own, the Virgo `detection’ looks rather marginal to me, but assuming that it is a detection this graphic shows how much it helps to localize the source compared to previous signals:

More on this in due course, perhaps, but now I’m off for a pint or two…

LIGO/VIRGO Update

Posted in The Universe and Stuff with tags , , on August 30, 2017 by telescoper

Judging by by the WordPress blog statistics page, there’s been a lot of traffic here in the past week owing to my post about the rumours of a new gravitational wave source detected by LIGO (and possibly VIRGO). In the interest of completeness I’ll just post a quick update to mention that the latest Observation run at LIGO  finished as planned on 25th August, and this has been marked by an official announcement which I have taken the liberty of presenting here in full:

The Virgo and LIGO Scientific Collaborations have been observing since November 30, 2016 in the second Advanced Detector Observing Run ‘O2’ , searching for gravitational-wave signals, first with the two LIGO detectors, then with both LIGO and Virgo instruments operating together since August 1, 2017. Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners. We are working hard to assure that the candidates are valid gravitational-wave events, and it will require time to establish the level of confidence needed to bring any results to the scientific community and the greater public. We will let you know as soon we have information ready to share.

The last two sentences can be translated roughly as “Back off, and give us time to analyse the data!”, which is not an unreasonable request. Judging by the timescale between detection and publication of the previous LIGO events, it will probably be a matter of months before a formal announcement is made.

I hope this clarifies the situation.