On September 14, 2015, the instruments at the Laser Interferometer Gravitational-Wave Observatory (LIGO), in the United States, recorded the passage of gravitational waves through the Earth for the first time. The observation proved the existence of these deformations of space itself, which Albert Einstein predicted 100 years ago using his general theory of relativity, in 1915. Researchers now hope to be able to take advantage of these waves to study high-energy astrophysics phenomena that emit little or no light, making them almost impossible to observe, even by the most powerful telescopes available.
“The research to come is really exciting,” says physicist David Reitze, LIGO executive director, in the press release on the historical discovery. “Like when Galileo looked at the sky with a telescope for the first time in 1509, we have now opened a new window to the Universe.”
After months of analyses and checks, the international team of researchers at LIGO concluded that the origin of the waves could have been a violent cosmic event never before recorded by an astronomical observatory: the collision and fusion of two black holes that took place 1.3 billion light-years from Earth. According to these calculations, published on February 11, 2016 in Physical Review Letters, the fusion of the black holes would have released a quantity of energy equivalent to the complete annihilation of three stars with the mass of the Sun in under 0.2 seconds. The most surprising thing is that it appears as if none of this energy was emitted in the form of light or particles of matter. The collision of two black holes generated an invisible explosion and its energy spread throughout the Universe in the form of gravitational waves.
In one fell swoop, the data collected by LIGO represents the first direct evidence of the existence of both gravitational waves and black holes. Before that, only indirect signs had been seen. It is a spectacular confirmation of our calculations based on the general theory of relativity,” stated Italian physicist Riccardo Sturani, of the South-American Institute for Basic Research of the International Theoretical Physics Center (ICTP-SAIFR), which operates in São Paulo in partnership with the Unesp Theoretical Physics Institute.
Sturani is part of the team of over 1,000 researchers from 15 countries that collaborated on the technological development of LIGO and on analyzing its data. He is a specialist in calculating the forms of the gravitational waves that result from violent collisions between dense, compact celestial bodies with masses similar to that of giant stars, concentrated in volumes just over a few kilometers in diameter. The astrophysicists only know of two objects that can be described in this way: black holes and neutron stars.
Created from the implosion of the core of a giant star, neutron stars concentrate the mass of 1 to 3 Suns into a sphere with a diameter of 20 km. Astronomers observe the light, radio waves and X-rays emitted by neutron stars regularly, but still know little about their interior. “The pressure and density inside a neutron star are extremely high, greater than those inside the nucleus of an atom,” explains Cecilia Chirenti, a theoretical physicist at the Federal University of the ABC. She investigates how the form of gravitational waves emitted by neutron stars can vary in accordance with the internal composition of these stars. “We do not know how matter behaves under these conditions. There are many models and the gravitational waves could help us determine which best represents reality.”
Since 1974, astronomers have indirectly observed gravitational waves from neutron stars. But these waves are of an amplitude and frequency too small to be detected by LIGO.
As with neutron stars, black holes can also be created by the implosion of the core of a giant star, with an even higher mass. In this case, the implosion causes the total collapse of the matter, which is transformed into pure gravitational energy. A spherical surface appears in the empty space, called the event horizon, formerly occupied by the core of the star. Nothing, not even light, escapes the gravitational force of this surface — thus the origin of the name, black hole.
As with black holes, gravitational waves are some of the most famous predictions of Einstein’s general theory of relativity, formulated in 1915 to explain gravitation based on his 1905 special theory of relativity. According to general relativity, gravity is not a force of attraction acting instantaneously between two bodies, as proposed centuries earlier by the English physicist and mathematician Isaac Newton. The Special Theory of Relativity prohibits instantaneous forces because, according to the theory, nothing can travel faster than the speed of light. In order to correct this detail in Newton’s theory, Einstein had to reinterpret the idea of gravitation, which was no longer seen as a force, but rather understood as a deformation in the geometry of space caused by the mass of bodies.
It is easier to understand what happens when you imagine a bowling ball placed in the center of a trampoline. The ball deforms the material and sinks down a bit. If someone rolls a billiard ball along the trampoline tangentially to the bowling ball, they will see that the smaller ball does not travel in a straight line. After a certain point, it will begin to circle the ball, like how the Earth orbits the Sun.
The source of the deformation in space is the presence of a large mass, such as the Sun or the Earth. Einstein noticed that, under certain circumstances, a body that is accelerating can also cause temporary deformations in space, which would propagate in the form of waves, traveling at the speed of light. In practice, these waves would be perceived as a temporary force that deforms the objects it encounters in its path (see infographic above). Einstein also noted that, in general, the deformation (amplitude) of these waves would be too small to be detected.
Beginning in the 1960s, researchers realized that they might be able to measure the waves. It was soon clear that most of the sources of gravitational waves would be hundreds of millions of light-years away. When they arrive on Earth, they would be so diluted that the displacements caused would be infinitesimal.
Still, groups of researchers in different countries attempted to build gravitational wave detectors. So far, LIGO is the largest and most sensitive of them. The project began in 1982 and construction was completed almost 20 years later. In 2010, modifications increased its sensitivity threefold. When it was turned on again in September 2015, the instruments detected gravitational waves during the first days of operation.
LIGO has two twin detectors, one in the city of Hanford, Washington state, and another 3,000 km away in Livingston, Louisiana. The buildings containing the detectors are L-shaped, with each wing measuring 4 km in length. A system consisting of lasers and mirrors monitors infinitesimal changes in the length of each wing. The detectors capture an immense amount of noise, such as that caused by airplane and automobile traffic or by seismic waves. In the middle of all of this interference, computers search the variations in size that only gravitational waves would be capable of causing, simultaneously, in the twin detectors.
“The search is done by comparing the data from the detectors to computer-simulated signals,” explains physicist César Augusto Costa of the National Institute for Space Research (INPE). Costa belongs to the Brazilian group led by physicist Odylio Aguiar at INPE, which is one of the groups collaborating on LIGO. Aguiar’s team collaborates on research to eliminate the noise and perfect LIGO’s detectors, whose sensitivity could increase tenfold over the next few years.
LIGO operated from September 2015 to January 2016, but only the data collected during the first two weeks were analyzed. According to Sturani, the complete evaluation of the observations during the four months of measurements should be published soon. Another observatory of gravitational waves, Virgo, located in Italy, is scheduled to begin operations by late 2016. The first signal recorded by LIGO was so rare that its analysis will keep astrophysicists busy for months. These waves were generated by the collision of two black holes with masses 36 and 29 times that of the Sun. “Their mass is too high for them to be black holes formed by the collapse of a star,” says astrophysicist Rodrigo Nemmen, of the University of São Paulo. “We believed that the collision of two black holes with such larges masses would be rare.”
When LIGO detected the first gravitational waves, the researchers calculated their their source must be in a section of the southern celestial hemisphere and, confidentially, alerted observatories around the world that they should look for something strange in the sky. The camera of the Dark Energy Survey (DES), mounted on a telescope in Cerro Tololo, in Chile, searched the sky for three weeks without finding any signs of emitted light.
At that time, it was unclear what the source of the detected waves was, recalls Brazilian physicist Marcelle Soares-Santos, at Fermilab in the United States, who coordinated the analysis of the DES observations. “Visible light could be emitted by the collision of a black hole and a neutron star, or of two neutron stars,” she explains. “Pairs of black holes are rarer than systems with neutron stars, and for this reason we hope to record many events in the future that DES and other projects can observe.” Another observatory, though, NASA’s Fermi spatial telescope, recorded a weak gamma-ray brightness 0.4 seconds after LIGO detected the first gravitational wave. “This emission might have been produced by the fusion of two black holes, which would be very unexpected,” says Nemmen. “But it was probably just a coincidence in time and the gamma radiation came from somewhere else.”
1. Gravitational wave research (nº 2013/04538-5); Grant Mechanism: Young Researchers Program: Principal Investigator: Riccardo Sturani (IFT-Unesp); Investment: R$256,541.00.
2. Gravitational wave astronomy – FAPESP-MIT (nº 2014/50727-7); Grant Mechanism: Regular research project; Principal Investigator: Riccardo Sturani (IFT-Unesp); Investment: R$29,715.00.
3. New physics from space: gravitational waves (nº 2006/56041-3); Grant Mechanism: Thematic Project; Principal Investigator: Odylio Denys de Aguiar (INPE); Investment: R$1,019,874.01.
4. Relativistic astrophysics and gravitational waves (nº 2015/20433-4); Grant Mechanism: Regular research project; Principal Investigator: Cecilia Chirenti (UFABC); Investment: R$56,109.48.
ABOTT, B. P. et al. Observation of gravitational waves from a binary black hole merger. Physical Review Letters. Feb. 11, 2016.