100 years after Einstein predicted the existence of gravitational waves with his theory of General Relativity, the LIGO (Laser Interferometer Gravitational-Wave Observatory) Scientific Collaboration have achieved the first ever direct detection of gravitational waves!
This is absolutely groundbreaking, and will open a new window with which to study the universe.

In November 1915, Albert Einstein presented his theory of General Relativity: a new, groundbreaking theory of gravity which predicts how massive objects curve space-time, and how objects move in curved space-time.
Einstein's General Relativity also predicted the existence of gravitational waves, which are propagating ripples of space-time emitted by moving objects. Gravitational waves can be emitted by compact binaries, which are pairs of compact objects orbiting each other, for example two neutron stars or two black holes.

As a pair of black holes spiral in towards each other, the binary loses energy; Einstein predicted that the energy lost by compact binaries would be propagated into the far reaches of the universe by gravitational waves. In 1974, Hulse & Taylor measured the energy lost by the inspiraling neutron star binary PSR B1913+16, and found that it precisely matched Einstein's prediction: this led to the first indirect evidence for gravitational waves. Hulse & Taylor received the Nobel prize in 1993 for their discovery.

However, till now we lacked a direct detection, that is the evidence that propagating gravitational waves emitted by far away objets could affect us here on Earth. Einstein himself had warned that the effects of the waves were too weak to be detected.

Amazingly, this is what the LIGO collaboration achieved. Using the technique of laser interferometry at their two sites in Hanford, Washington and Livingston, Louisiana, and after a 5-year upgrade, they found evidence for a gravitational wave passing through their instruments: a gravitational wave that was emitted by a pair of black holes at a distance of hundreds of millions of light years away!

Each interferometer is composed of two 4-km arms; lasers travel along these arms and bounce back between mirrors. If a gravitational wave passes through the interferometer, it can actually change the shape and size of the mirrors themselves! Such tiny effects would affect the travel time of the lasers bouncing off the mirrors and therefore can be detected.



Not only did LIGO detect gravitational waves for the first time; they also discovered the first stellar black hole binary in the universe (formed after the collapse of massive stars), from the moments in which the two black holes spiraled in towards each other, to the merger of the two black holes forming one single black hole, and a final "ringdown" after the merger.

The inspiral and merging of binary black holes had been modeled theoretically using what we know about General Relativity; this is the first time however that such a system is observed!

Our pulsar group in Cagliari (Marta Burgay, Andrea Possenti and Nichi D'Amico) discovered the first double pulsar in 2003: a pair of rotating neutron stars that emit radio emission. The first "double black hole" or "stellar black hole binary" was the next logical step!

At the Cagliari Observatory, we also have a research program to detect and study gravitational waves, as part of the European Pulsar Timing Array (EPTA) collaboration, itself part of the International Pulsar Timing Array (IPTA).
While LIGO uses laser interferometry on the ground and is sensitive to gravitational waves emitted by stellar black hole binaries, the EPTA collaboration uses the radio signals emitted by pulsars to find evidence for gravitational waves coming from supermassive black hole binaries in galaxy mergers. The two techniques are therefore probing different parts of the gravitational wave spectrum.

Millisecond pulsars, which are fast-rotating neutron stars, are the most precise "clocks" in the universe. Any deviation from the expected times-of-arrival of their pulses could signal the passing of a gravitational wave. The Sardinia Radio Telescope in San Basilio is one of the five European radio telescopes in the EPTA network, thus has an important role in the search for gravitational waves from supermassive black hole binaries.

A number of Cagliari researchers have been involved in EPTA research over the last 10 years: Marta Burgay, Alessandro Corongiu, Delphine Perrodin, Maura Pilia, Caterina Tiburzi (who is now pursuing a postdoctoral fellowship in Germany), as well as Andrea Possenti (former OAC director) and Nichi D'Amico, who is currently the president of the INAF national institute.

As part of the EPTA, the Cagliari Observatory and the Sardinia Radio Telescope are also involved in the Large European Array for Pulsars (LEAP) project, led locally by Delphine Perrodin. The project involves the simultaneous observing of millisecond pulsars at all five European telescopes, in order to increase the sensitivity for gravitational wave detection.