It’s possibly one of the most sought-after detections in gravitational-wave astronomy.
Everything that happens in the Universe, every supernova, every merger between neutron stars or black holes, even single neutron stars spinning at high speed, could be a source of gravitational waves. These massive events should produce waves that may be detectable to us as discrepancies in what should be precisely timed signals, just as Albert Einstein predicted decades ago.
This mixture of signals combines to form a random or “stochastic” hum known as the gravitational-wave background. In fact, possibly one of the most sought-after detections in gravitational-wave astronomy.
[Photo: NASA/C. Henze]
Big Bang, the Cosmic Microwave Background (CMB)
After our Universe began to tick and space began to cool, the bubbling foam froze into an opaque soup of subatomic particles in the form of ionized plasma. Thus any radiation emerging from it was scattered, preventing it from reaching far. It was not until these subatomic particles recombined into atoms that light could move through the Universe and across the eons, this happened at a time known as the Recombination Epoch.
After the Big Bang, the first flash of light burst forth at 380 thousand years, and as the Universe grew in the billions of years that followed, this light was dragged to all corners of the cosmos. Actually, it still surrounds us today. This radiation is very weak but detectable at microwave wavelengths.
The irregularities in this light are called anisotropies and were caused by small temperature fluctuations represented by this first light. This first light of the Universe is known as microwave background radiation (CMB) and is one of the only waves we have from the early Universe.
Searching for Gravitational Waves
Scientists have been studying pulsar timing arrays for hints of the gravitational wave background. Pulsars are remnants of massive stars that died in a spectacular supernova, leaving only a dense core.
As these pulsars spin, radio emission beams from their poles sweep the Earth, which is useful for several applications. That a pulsar shows slight incoherence does not mean much, but that a group of pulsars shows correlated timing incoherence could be indicative of gravitational waves produced by the inspiration of superlative black holes.
Gravitational Wave Detection
The first detection of gravitational waves occurred in 2015 when two black holes that collided approximately 1.4 billion years ago sent out waves propagating at the speed of light. On Earth, these expansions and contractions of space-time very weakly triggered an instrument designed that detects events of this type.
This detection confirmed the existence of black holes and a prediction made by Albert Einstein’s General Theory of Relativity. This meant that the gravitational wave interferometer, which scientists had been working on for years, would forever revolutionize the understanding of black holes.
[Photo: Caltech/R. Huert/IPAC]
These interferometers use lasers that shine in special tunnels and are affected by the stretching and compression of space-time produced by gravitational waves, generating an interference pattern. About 100 gravitational wave events have been detected to date.
Scientists think that they occur between one merger per minute and several per hour and that the detectable signal from each lasts only a fraction of a second. These individual signals would probably be too weak to be detected, but they combine to create a static background noise.
What Will Happen if We Could Hear the Pulse of the Universe?
It is thought that the discovery of gravitational waves will help our understanding of the Universe and its evolution. The detection of a stochastic background of gravitational radiation may eventually provide information about astrophysical source populations and processes in the early Universe.
“Electromagnetic radiation does not provide a picture of the Universe before the time of the last scattering after 400 thousand years from the Big Bang. Gravitational waves, however, can give us information back to the onset of inflation, only 10 to 32 seconds after the Big Bang,” explained Susan Scott, a theoretical physicist at the Australian National University and the ARC Centre of Excellence for the Discovery of Gravitational Waves.
Story originally published in Spanish in Ecoosfera