After switching on in September 2015, the LIGO double observatory - the Laser Interferometer Gravitational-wave Observatories in Hanford, Washington, and Livingston, Louisiana - simultaneously detected the merging of two black holes in the first working session, although their sensitivity was set to 30% of possible. The merger of two black holes 36 and 29 solar masses discovered on September 14, 2015, and other black holes at 14 and 8 solar masses discovered on December 26, 2015 provided the first definite and direct confirmation of the existence of gravitational waves. It took a century to do this. Finally, technology was able to test the theory and confirm it.
But the discovery of these waves is just the beginning: a new era is brewing in astronomy. 101 years ago, Einstein put forward a new theory of gravity: general relativity. Along with it came the realization: distant masses do not attract similar ones instantly throughout the entire universe, this presence of matter and energy deforms the fabric of space-time. This completely new picture of gravity brought with it a whole host of unexpected consequences, including gravitational lensing, an expanding universe, gravitational time dilation, and - as we now know for sure - the existence of a new type of radiation: gravitational waves. When masses move or accelerate relative to each other through space, the reaction of the space itself creates ripples. This ripple moves through space at the speed of light and, as a result, falling into our detectors,informs us of distant events through gravitational waves.
It is easiest to detect objects that emit strong signals, namely:
- large masses, - located at a small distance between themselves, - fast rotating, Promotional video:
- with significantly changing orbits.
The best candidates are obviously colliding, collapsing objects like black holes and neutron stars. We also need to keep in mind the frequency at which we can detect these objects, which will be roughly equal to the detector's path length (arm length times the number of reflections) divided by the speed of light.
LIGO, with its 4-kilometer arms with thousands of light reflections, can see objects at frequencies in the millisecond range. This includes merging black holes and neutron stars in the last stage of merging, as well as exotic events like black holes or neutron stars that consume a large chunk of matter and gurgle, becoming more spherical. A highly asymmetric supernova can also create a gravitational wave; core collapse is unlikely to hit gravitational wave detectors, merging white dwarf stars nearby could well.
We've already seen merging black holes with black holes, and as LIGO improves, it is reasonable to assume that over the next few years we will have the first generation of estimates of black holes of stellar masses (from a few to a hundred solar masses). LIGO must also find mergers of neutron stars with neutron stars; when the observatories reach the planned sensitivity, they will be able to observe three to four events per month, if our estimates of their merger frequency and LIGO sensitivity are correct.
Asymmetric supernovae and bubbling of exotic neutron holes will be extremely interesting to detect (if possible, because they are believed to be rare events). But the biggest breakthroughs are to be expected with more detectors. When the VIRGO detector in Italy starts working, real positioning will be possible due to triangulation: we will be able to accurately determine where these events are born in space, and then carry out optical measurements. VIRGO will be followed by gravitational wave interferometers in Japan and India. In a few years, our vision of the gravitational wave sky will reach a new level.
But our greatest successes will begin when we bring our gravitational wave ambitions into space. In space, you are not limited to seismic noise, truck crashes, or plate tectonics; only a quiet space vacuum in the background. You are not limited by the curvature of the Earth, the possible length of the observatory arms; it is possible to launch the observatory further from the Earth or even into orbit around the Sun. We could measure objects not for milliseconds, but for seconds, days, weeks or longer. We could detect gravitational waves from supermassive black holes, including the largest known objects in the universe.
Finally, if we build a space observatory large enough and sensitive enough, we could see the gravitational waves left over from the Big Bang itself. We could directly detect the gravitational perturbations of cosmic inflation and not only confirm our cosmic origin, but also prove that gravity itself is a quantum force of nature. After all, these inflationary gravitational waves could not have appeared if gravity itself was not a quantum field.
There is currently ongoing debate over which NASA mission will be a priority in the 2030s. While many good missions are offered, the construction of a space-based gravitational-wave observatory in orbit around the sun is worth noting. We have the technology, we have proven its workability, we have confirmed the existence of waves. The future of gravitational wave astronomy is limited only by what the universe itself can provide us with and how much we will spend on it. The heyday of a new era has already begun. The question remains how brightly this new field of astronomy will shine.
ILYA KHEL