Gravitational waves: the discovery of the century

One hundred years after the theoretical prediction by Albert Einstein, on Sep­tember 14, 2015 scientists have detected for the first time a gravita­tional wave passing through the Earth emitted by a binary black hole merger — a mind-blow­ing result! Researchers at the Max Planck Institute for Gravita­tional Physics (Albert Einstein Institute, AEI) in Potsdam have contributed signi­fi­cantly to this discovery.

Portrait photo of Serguei Ossokine (Source: AEI)

I started working at the AEI in Potsdam on Sep­tem­ber 15, 2015 and, basically, right in the morning when I started, Professor Ales­sandra Buonanno revealed to me that there has been a possible gravitational-wave detection. Of course I was thunderstruck — I mean: this was my first day at work and I had just joined the LIGO Scientific Collaboration! Almost right away I was asked to produce a numerical-relativity waveform that would reproduce the parameters of the event.

Serguei Ossokine, postdoctoral scholar at the AEI in Potsdam since September 2015

What are gravi­tational waves?

Gravitational waves are ripples in the fabric of spacetime. In Einstein’s theory of general relativity accelerated masses deform the spacetime geometry in their neighbor­hood. Those deformations pro­pa­gate away from the source at finite speed in form of waves whose oscil­la­tions reflect the temporal varia­tion of the matter distribution.

When emitted by astrophysical sources like merging black holes or stellar ex­plo­sions, gravitational waves can change kilo­metre-scale distance between mirrors hang by wires at the extremity of an inter­fero­meter cavity by a ten thousandth of the diameter of a proton (10-18 m). After many years of work, in the Fall of 2015 the detectors have reached a level of sensi­tivity at which they can measure gravitational waves.

The first observation of the “Gravita­tional Universe” has ushered in a new era in astronomy and fundamental physics.

How LIGO detected gravitational waves

The two detectors of the gravitational wave observatory LIGO (Laser Inter­fero­meter Gravita­tional Wave Obser­vatory) are located in the US states of Washing­ton and Louisiana. At each site, laser’s beams bounce back and forth along four kilo­metre long L-shaped vacuum tubes to very precisely monitor the distance between mirrors at each end. This en­ables to measure the tiny squeezing and stretching of space induced by a pas­sing by gravitational wave. LIGO is able to measure minimal spatial distor­tions of the size of 1/10,000 the diameter of a proton! It is as if we were to deter­mine the distance to our closest star within an accuracy of less than the diameter of a human hair.

On September 14, 2015, both LIGO de­tec­tors observed a gravita­tional wave signal originating from a bi­nary black hole merger about 1.3 billion light years away. For the very first time gravita­tional waves were observed on Earth. It was also the first obser­vation of a bi­nary black hole. AEI researchers have made crucial contributions to the dis­covery. One key research area in Prof­es­sor Buonanno’s “Astro­physical and Cosmological Relativity” division at the AEI in Potsdam is the development of highly accurate gravita­tional wave­form models. These wave­form models were imple­mented and employed in the continuing search for binary coales­cences in LIGO data. The division also used the same wave­form models to infer the astro­physical parameters of the source, such as the masses and spins of the two black holes, the binary’s orientation and distance from Earth, and the mass and spin of the enormous black hole that formed after merger.

How to get involved in gravita­tional wave research?

The search for the most promising sources of gravitational waves — binary systems composed of black holes and / or neutron stars — requires detailed knowledge of the expected signals. Scientists working in the “Astro­physical and Cosmo­logical Relativity” division at the AEI develop sophisticated analytical and numerical methods to solve the Einstein equations and predict highly accurate templates. Wave­form models are also used in searches and follow-up analyses to infer the properties of the sources, allowing us to extract unique astro­physical and cosmo­logical infor­mation from the observed signals and test general relativity in the strong-field regime.

Two examples of young researchers being involved in gravita­tional-wave research: Dr. Andrea Tarac­chini, cur­rently postdoctoral scholar at the AEI in Potsdam, did his PhD under Professor Buonanno’s supervision at the Univer­sity of Mary­land and developed wave­form models for binary black holes carrying spin. Those templates were used for the first observation of gravita­tional waves from a binary black hole merger. Serguei Ossokine, who had just moved to Potsdam from Canada as a post­doc­toral scholar, numerically simulated the black hole system ob­serv­ed by LIGO and compared the model to the measured signal.