Gravitational Waves Will Bring the Extreme Universe Into View
Looking out on an even grander scale, gravitational waves from neutron star mergers will give us a fresh way to study the expansion of the Universe. Our current picture of cosmology – in which the Universe is expanding following the Big Bang, and is accelerating due to an unseen ‘dark energy’ – relies heavily on observations of supernovae in distant galaxies. Gravitational waves will provide complementary information: the intensity (amplitude) of the gravitational signal tells us the distance to the event, while the optical appearance of the merger reveals how much its light has been stretched, or redshifted, on its way to Earth. These two pieces of information define the rate at which the Universe is expanding. Measuring this rate independently will provide an important check of our cosmological models.
Finally, LIGO and Virgo might detect a faint background hum of gravitational waves that pervades the entire Universe, constantly vibrating all of empty space. Many theories predict an omnipresent gravitational energy produced either from the accumulation of astrophysical events such as black hole mergers or from an early, extremely rapid episode of cosmic inflation immediately after the Big Bang. If the hum is loud enough, it will show up as a correlated signal between widely separated detectors such as LIGO and Virgo. Measuring the gravitational-wave background would be a dramatic achievement.
For the next few years, progress in gravitational-wave science will be limited by the sensitivity of the detectors. With each boost to their performance, it’s likely that we will uncover events from new types of sources. Eventually, perhaps after a large international investment in new facilities, progress in the field will be limited only by the willingness of the Universe to provide rare, exotic signals to observe.
LIGO and Virgo have already performed a staggering feat. Consider the properties of the September 14 event: the signal was generated by two objects, each roughly 35 times the mass of our Sun, locked in a decaying orbit the size of Switzerland, circling each other 50 times a second. The energy involved was staggering, briefly exceeding that of all the starlight in the Universe, but the signal that reached Earth was among the most imperceptible things that humans have ever measured. As gravitational-wave detections make the transition from sensational discoveries to routine tools for astrophysics and cosmology, the invisible shaking of space will, paradoxically, illuminate parts of the Universe that were entirely dark until now.
This article was originally published at Aeon and has been republished under Creative Commons.