The first direct detection of gravitational waves is expected to be announced on February 11 by scientists at the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO). Using two giant LIGO detectors - one in Livingston, Louisiana and the other in Hanford, Washington - scientists measured the ripples in spacetime that are generated by the collision of two black holes and seem to have finally found what they were looking for.
Such a statement would confirm the gravitational waves predicted by Albert Einstein, which he made part of his general theory of relativity 100 years ago, but the consequences will not end there. As a vibration of the fabric of space-time, gravitational waves are often compared to sound, even transformed into sound tracks. Gravitational wave telescopes would enable scientists to "hear" phenomena in the same way that light telescopes "see" them.
When LIGO fought for funding from the US government in the early 1990s, astronomers were its main contenders in congressional hearings. “It was thought back then that LIGO had nothing to do with astronomy,” says Clifford Will, a general relativity theorist at the University of Florida in Gainesville and one of the early proponents of LIGO. But a lot has changed since then.
Welcome to the field of gravitational wave astronomy. Let's go over the issues and phenomena that she could reveal.
Do black holes really exist?
The signal expected from the LIGO announcement may have been produced by two merging black holes. Events like these are the most energetic known; the force of the gravitational waves emitted by them can briefly eclipse all the stars of the observed universe in total. Merging black holes are also quite easy to interpret from very pure gravitational waves.
Promotional video:
The signal expected from the LIGO announcement may have been produced by two merging black holes. Events like these are the most energetic known; the force of the gravitational waves emitted by them can briefly eclipse all the stars of the observed universe in total. Merging black holes are also quite easy to interpret from very pure gravitational waves.
Merging black holes occurs when two black holes spiral around each other, emitting energy in the form of gravitational waves. These waves have a characteristic sound (chirp) that can be used to measure the mass of these two objects. After that, black holes usually merge.
“Imagine two soap bubbles that come close enough to form one bubble. The larger bubble is deformed,”says Tybalt Damour, a gravitational theorist at the Institute for Advanced Scientific Research near Paris. The final black hole will be perfectly spherical, but must first emit gravitational waves of a predictable type.
One of the most important scientific implications of discovering black hole mergers will be confirmation of the existence of black holes - at least perfectly circular objects composed of pure, empty, curved spacetime, as predicted by general relativity. Another consequence is that the merger proceeds as the scientists predicted. Astronomers have a lot of indirect confirmation of this phenomenon, but so far these have been observations of stars and superheated gas in the orbit of black holes, and not the black holes themselves.
“The scientific community, myself included, doesn't like black holes. We take them for granted,”says Frans Pretorius, a specialist in general relativity simulations at Princeton University in New Jersey. "But if you think about what an amazing prediction this is, we need truly amazing proof."
Do gravitational waves move at the speed of light?
When scientists start comparing LIGO observations with those of other telescopes, the first thing they check is whether the signal arrived at the same time. Physicists believe that gravity is transmitted by graviton particles, the gravitational analogue of photons. If, like photons, these particles have no mass, then gravitational waves will move at the speed of light, consistent with the prediction of the speed of gravitational waves in classical relativity. (Their speed can be influenced by the accelerating expansion of the Universe, but this should manifest itself at distances significantly exceeding those covered by LIGO).
It is quite possible, however, that gravitons have a small mass, which means that gravitational waves will move at a speed less than light. So, for example, if LIGO and Virgo detect gravitational waves and find out that the waves arrived on Earth later than associated with a cosmic event of gamma rays, this could have fatal consequences for fundamental physics.
Is spacetime made of cosmic strings?
An even stranger discovery could happen if bursts of gravitational waves are detected emanating from "cosmic strings." These hypothetical space-time curvature defects, which may or may not be related to string theories, should be infinitely thin but stretched out over cosmic distances. Scientists predict that cosmic strings, if they exist, could bend accidentally; if the string bends, it will cause a gravitational surge that detectors like the LIGO or Virgo could measure.
Can neutron stars be jagged?
Neutron stars are the remnants of large stars that collapsed under their own weight and became so dense that electrons and protons began to melt into neutrons. Scientists have little understanding of the physics of neutron holes, but gravitational waves could tell a lot about them. For example, the intense gravity on their surface causes neutron stars to become almost perfectly spherical. But some scientists have suggested that they may also have “mountains” - a few millimeters high - that make these dense objects, 10 kilometers in diameter, no more, slightly asymmetrical. Neutron stars usually spin very quickly, so the asymmetric distribution of mass will warp spacetime and produce a constant sinusoidal gravitational wave signal, slowing the star's rotation and radiating energy.
Pairs of neutron stars that revolve around each other also produce a constant signal. Like black holes, these stars spiral and eventually merge into a distinctive sound. But its specificity differs from the specificity of the sound of black holes.
Why do stars explode?
Black holes and neutron stars form when massive stars stop shining and collapse into themselves. Astrophysicists think this process is at the heart of all common types of Type II supernova explosions. Simulations of such supernovae have not yet shown why they ignite, but listening to the gravitational wave bursts emitted by a real supernova is believed to provide an answer. Depending on what the burst waves look like, how loud they are, how often they occur, and how they correlate with supernovae tracked by electromagnetic telescopes, this data could help rule out a bunch of existing models.
How fast is the universe expanding?
The expansion of the universe means that distant objects that move away from our galaxy appear redder than they actually are, as the light they emit stretches as they move. Cosmologists estimate the rate of expansion of the universe by comparing the redshift of galaxies to how far away they are from us. But this distance is usually estimated from the brightness of Type Ia supernovae, and this technique leaves a lot of uncertainties.
If several gravitational wave detectors around the world detect signals from the merger of the same neutron stars, together they can absolutely accurately estimate the signal's loudness, as well as the distance at which the merger took place. They will also be able to assess the direction, and with it, identify the galaxy in which the event occurred. By comparing the redshift of this galaxy with the distance to the merging stars, an independent rate of cosmic expansion can be obtained, possibly more accurate than current methods allow.
