When you look up at the night sky, you see a very particular view of the Universe. You see electromagnetic radiation, light, at optical wavelengths from objects like stars. If your eyes could see radio waves, which are another wavelength of light, they would see a very different picture of the Universe. The sources of radio light are different than the sources of optical light. Astronomers want to build all different kinds of telescopes to see the entire spectrum of electromagnetic radiation. You can see a view of the Milky Way Galaxy at all different wavelengths of light here (from this page) and you might notice that the view you get is very different depending on what kind of telescope you build.
For almost the entire history of astronomy, we viewed the Universe through an electromagnetic window. For many decades, astronomers have been interested in viewing the Universe through an entirely separate window: a gravitational one. Unlike electromagnetic waves, gravitational waves are very slight changes in spacetime that cause objects to move closer or farther away from one another by miniscule amounts. They are predicted from Einstein's theory of general relativity, and so a detection provides further evidence in support of the theory. The sources of gravitational waves are very exotic, the most notable being two compact objects like neutron stars or black holes in a close orbit. As they orbit around one another, gravitational waves are emitted from the system. Since energy is leaving the system, the orbits shrink, until the two objects eventually merge in a violent event. Observations of gravitational waves will allow us to study the dynamics of these sytems on many different size scales.
On February 11th, 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration announced the detection of gravitational waves from a black hole binary. This is the first concrete detection of a double black hole system. Both black holes were the most massive stellar-mass black holes ever detected (over other candidate objects). They observed the mass of the merged object to be less than that of the sum, implying that the difference in mass was converted to an enormous amount of energy that was lost as gravitational waves in the merger event (as much as 5000 supernovae!). They also measured the spin of the final black hole, the rate of black hole mergers in the local Universe, and more. So much new understanding of physics came from a single gravitational wave event.
Ever since, several gravitational wave detections have been reported, most notably the first event involving the inspiral and merger of two neutron stars in 2017. For the first time, astrophysicists measured a gravitational-wave event that also had an electromagnetic counterpart, which was observed by several telescopes on Earth. Mergers of neutron stars are believed to be among the most energetic events in the universe, releasing energies that could potentially account for unique physical conditions where the heaviest elements --such as gold-- would be produced. The detection of a neutron star binary gave rise rise to an exciting era of multi-messenger astronomy, which will certainly bring much more exciting knowledge to us!
This page was last updated on January 28, 2019.