## Wednesday, February 10, 2016

### Gravitational waves

Has the LIGO observatory observed gravitational waves? Rumours have been rife that LIGO (Laser Interference Gravitational Wave observatory) has picked up the signature of gravitational waves   (ripples in space time) arising from a massive cosmic event, the collision of two black holes. Astronomers have been looking for confirmation of the rumoured LIGO events  in the form of an optical flash in three parts of the sky, in the constellation Dorado, as well as in the constellations Aries and Hydra. The last time the classic signature (a chirp') was observed in the LIGO data was in 2010, but it was announced to be a false signal, fake data inserted to ensure that the analysis could, in fact detect a signal. The experiment has scheduled a press conference on 11th February, where they may confirm the observation, (or not!).  More tomorrow,  the entire physics world waits with bated breath. Hopefully, the LIGO spokespersons will not shout April Fool, in February!

Update: February 12th:

They did find it. We had the unique experience of hearing the universe chirp, that too on our mobiles (the quality of the broadcast was absolutely wonderful!). Gabriela Gonzales provided the details. Two massive black holes of solar masses 36 and 29 coalesced to form an object of 62 times the solar mass. Their motion, which spiralled round each other, before the collapse (250  revolutions per second at half the speed of light) provided the chirp. This event occurred a billion light years ago, at a time when life on earth had barely progressed to multicellular organisms, at a location roughly in the direction of the Magellanic cloud. The observation was a triumph of state of the art experimental technique and technology, the mirrors of the interferometer moved through a distance of 4/1000ths of the diameter of the proton due to the effect of the wave. As far as the theory is concerned, the original prediction dates back to Einstein in 1916, and Taylor and Hulse had made an indirect observation of the gravitational waves which would have been obtained due to two neutron stars spiralling inwards coalescence. Taylor and Hulse got the Nobel for their discovery. So who is in line this time? The three originators of LIGO were Kip Thorne,  Rainer Weiss and Ronald Drever, and of course,  there are now younger collaborators. The Bicep2 team must be sad at having missed the first detection, although they may still get the first wave from the big bang. The result is also a vindication of big science. The National Science Foundation spent \$1.1 billion over the LIGO detectors at  Livingston, Louisiana and Hanford, Washington over 40 years, including the recent upgrade to the Advanced LIGO lab, which finally bought the detectors to the stage where the recent detection became possible.  The result came almost immediately after the upgrade, last September, just after the calibration of the advanced LIGO system. The result is a collaboration between these two LIGO Labs and the Virgo observatory in Europe.  There are three more events in the pipeline, which are currently under analysis. Two more LIGO labs are proposed, one in Japan, and one in India, where they will hopefully find adequate funding support and ecological blessing from all concerned parties! These are exciting times to be in physics. These discoveries are invitations on the part of the discipline to join the excitement.

This blog post by Neelima Gupte and Sumathi Rao.

Postscript: The original paper has appeared in Physical Review letters yesterday. It's great to see a Chennai institute, Chennai Mathematical Institute, Chennai in the affiliation list of the authors.

What did the blackbird say to the black holes?  And a chirp to you, too'.

Update 16/06/16

A second gravitational wave event has been detected, again from the coalescence of two black holes, somewhat smaller than the ones which collided in the first event. This second event was a Christmas gift from the universe to us. The data analysis took six months, which is why the event was only announced last week. As for the details, one can't do better than quote  the succinct Physical Review Letters abstract, reproduced below. So here you are, error bars and all.

'We report the observation of a gravitational-wave signal produced by the coalescence of two stellar-mass black holes. The signal, GW151226, was observed by the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) on December 26, 2015 at 03:38:53 UTC. The signal was initially identified within 70 s by an online matched-filter search targeting binary coalescences. Subsequent off-line analyses recovered GW151226 with a network signal-to-noise ratio of 13 and a significance greater than 5σ. The signal persisted in the LIGO frequency band for approximately 1 s, increasing in frequency and amplitude over about 55 cycles from 35 to 450 Hz, and reached a peak gravitational strain of 3.4+0.70.9×1022. The inferred source-frame initial black hole masses are 14.2+8.33.7M and 7.5+2.32.3M, and the final black hole mass is 20.8+6.11.7M. We find that at least one of the component black holes has spin greater than 0.2. This source is located at a luminosity distance of 440+180190Mpc corresponding to a redshift of 0.09+0.030.04. All uncertainties define a 90% credible interval. This second gravitational-wave observation provides improved constraints on stellar populations and on deviations from general relativity.'