Gravitational waves finally detected

It seems that nearly exactly 100 years after their prediction by Albert Einstein, Gravitational Waves have finally been directly detected for the first time. Speakers of the LIGO experiment announced yesterday that they have witnessed the final stages of the inspiral and merger of a massive black hole binary system. This marks the beginning of a new type of astronomy with gravitational waves that allows to explore a so-far completely unknown side of the Universe.

Einstein’s Theory of Gravity

In November 1915, nearly exactly 100 years ago, Albert Einstein
presented his new General Theory of Gravity to the Prussian Academy of Sciences. The theory was somewhat perplexing since the effect of gravity was not a force acting on massive bodies, but instead gravity was claimed to warp the four-dimensional space-time we live in.

By now, this theory has been phenomenally successful. It could explain a long-known anomaly of the planet Mercury’s orbit around the Sun, its so-called perihelion shift. The theory also predicted that a gravitational field should deflect the paths of light rays, an effect that was later confirmed experimentally.

According to Einstein’s Special Theory of relativity, however, no information should be able to travel faster than the speed of light. Therefore, if at some location in the Universe a catastrophic event heavily perturbs the space-time, the “news” of this warp can only travel at a finite speed and –according to Einstein’s theory– this must be the speed of light.

One can think of a gravitational wave as a “ripple” travelling across the otherwise smooth space-time. This is similar to throwing a stone into a calm lake: this causes ring-like perturbations that travel away from where the stone hit the water. The physical reality of gravitational waves, however, had been doubted for decades, they were often considered as mere artefacts of Einstein’s theory.

A first glimpse of the elusive waves

This only changed in 1974 when a very “exotic” stellar system was discovered: two neutron stars orbiting around each other at a good fraction of the speed the light. A neutron star is a stellar corpse that emerges when an exploding star compresses its interior to densities that are larger than those in an atomic nucleus. So in a sense, one can think of neutron stars as being gigantic atomic nuclei of about 10 km radius. The newly discovered binary system has turned out to be an excellent laboratory for relativistic gravity, many general relativistic effects predicted by Einstein’s theory could be measured in it to exquisite precision.

Probably the most spectacular effect is that the two neutron stars slowly spiral towards each other, in excellent agreement with the prediction of Einstein’s theory. Within one orbital revolution (which takes less than 8 hours) this is a tiny effect, but since its discovery in 1974 the orbital period has already changed by 40 seconds! This discovery pulverised the doubts about the reality of gravitational waves and the discoverers of the binary system, Russel Hulse and Joseph Taylor, were honoured with the Physics Nobel Prize in 1993.

Listening to the dark side of the Universe

Although convincing, this is only an indirect confirmation of gravitational waves and one would like, of course, to detect them directly. This would mean that one could not only “see” the Universe (via electromagnetic waves) but one could also “listen” to the so far dark side of the Universe by means of gravitational waves. According to all we know, only 4 % of the energy of the Universe is made of matter that we think we understand. This includes all the objects of everyday life that are made of neutrons, protons and electrons.
The remaining 96%, however, may also produce gravitational waves and detecting them directly will open a new window to a completely unknown side of the Universe. Surprises are therefore virtually guaranteed!

In the last 25 years enormous efforts have been undertaken towards a direct detection of gravitational waves. An international detector network has been built up with facilities in Germany, Italy, Japan and the United States. The American “Laser Interferometer Gravitational-wave Observatory” (LIGO) has recently undergone a major upgrade and it has started taking data in its new configuration in September 2015. Once its final design sensitivity has been reached, it will be able to listen to a 1000-times larger volume of the Universe than before.

On 2015 September 14 at 9:50 UTC, during the last stages of “engineering runs” and before the originally planned observation period, LIGO has observed a gravitational wave burst signal from two merging, massive black holes. Both LIGO detectors, separated by 3000 km, saw a so-called “chirp” signal of increasing amplitude and frequency sweeping up frequencies from 35 to 250 Hz. This signal is well explained by the merger of two black holes with 29 and 36 solar masses.If this is the correct interpretation, then 2015 September 14 marks the beginning of the era of gravitational wave astronomy!

The LIGO collaboration consists of about one thousand scientists working in more than fifteen countries. Beyond the collaboration, LIGO’s results will both rely on and inform the observations of dozens of other telescopes and satellite observatories. Researchers would like to observe such extreme events as mergers of black holes and neutron stars with as many instruments as possible, and as soon after the burst as possible. LIGO itself, however, will not be able to localise the direction of bursts in the sky to high accuracy (for the observed event the source position is only known to within 600 square degrees). The first stage of help can come from instruments that monitor large parts or all of the sky continuously. At the Oskar Klein Centre, researchers working on the Fermi Gamma-ray Space Telescope, the Intermediate Palomar Transient Factory (iPTF), and the IceCube Neutrino Observatory were alerted about the September 14 burst, and asked to check whether they had recorded anything unusual around or after the burst time and in the same general direction. No definitive excess has been reported yet from these or other observatories. Seeing no other emission would be consistent with the interpretation of two merging black holes, since it is not obvious how an electromagnetic emission would arise in such a case. On the other hand, if a neutron star were involved, one would expect to see an electromagnetic flash caused by radioactivity from freshly synthesized heavy elements, a so-called “macronova”.

An interesting aspect of the LIGO procedure has been their well-publicised plan to test themselves and their fellow observers with occasional “fake” alerts. Except for a few individuals, no one even in LIGO would know for sure if a given alert was real. For this reason, it was not obvious in the early days after the alert that something real had been observed, let alone the amazing discovery that it turned out to be. It only became gradually, increasingly clear in the last few days before the announcement that this burst, now designated as GW150914, was not only real, but spectacular!

If the black hole merger interpretation is correct, then the lack of definitive detection besides gravitational waves would not be surprising. If the black holes were not surrounded by gas and ordinary matter that could be ejected in the violent aftermath, the merger was likely to be undetectable except by the enormous energy carried away by the gravitational waves themselves (corresponding to about three times the rest mass of our Sun). With LIGO starting to run in its advanced configuration now, and continuing to improve sensitivity, it is likely that more merger events will follow soon. Some of these will involve neutron stars rather than black holes, and these events are expected to leave visible traces.

– Stephan Rosswog (stephan.rosswog@astro.su.se) and Chad Finley (cfinley@fysik.su.se)

Stephan Rosswog is Professor of Astronomy at Stockholm University and researches on compact objects such neutron stars and black holes.
Chad Finley is Senior Researcher in Physics at Stockholm University and IceCube coordinator for the joint search with LIGO.

Read also: “High-energy Neutrino follow-up search of Gravitational Wave Event
GW150914 with ANTARES and IceCube”, ANTARES, IceCube, LIGO, and VIRGO collaborations. https://dcc.ligo.org/LIGO-P1500271/public

The cover photo shows two Black Holes merging into one. This simulation was created by the multi-university SXS (Simulating eXtreme Spacetimes) project. For more information, visit http://www.black-holes.org: Photo Credit: SXS.

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