In 1916, the German scientist Albert Einstein made the first proposals the existence of gravity wavescall then gravitational waves. According to his theory of General Relativity, they would have been generated during very energetic cosmic events, as a consequence of the modification to the curvature of spacetime, which in turn is due to the presence of mass.
100 years later, on February 11, 2016, the LIGO and Virgo collaborations announced the first direct detection and observation of gravitational waves, which took place in September 2015. Two black holes had collided, rippling the fabric of spacetime. That ripple has spread to us, and has been detected by the two instruments.
Today, June 29, 2023, the NANOGrav (North American Nanohertz Observatory for Gravitational Waves) collaboration announced the first direct detection of a stochastic background of gravitational waves (Gravitational Wave Background, GWB). The discovery was made using observational data from a series of 68 pulsar stars collected over the course of 15 years. The exact origin of this background signal is not yet certain, and will require further observations.
Four separate studies on the discovery of the gravitational wave background have been published in The Astrophysical Journal Letters (and two more accepted for future publication):
Illustration of the locations of Milky Way pulsars included in the 15-year NANOGrav data set. The blue stars indicate pulsars, while the central yellow star represents the location of the Earth. Credits: NANOGrav
What is the Stochastic Gravitational Wave Background?
Il stochastic background of gravitational waves it is a set of signals coming from different cosmic sources distributed in the Universe. Unlike gravitational waves generated by single, discrete events, such as black hole mergers or supernova explosions, the stochastic background consists of “noise” from many different sources.
This stochastic fund, which represents a real background murmur of the Universe, manifests itself as a continuous signal. Indeed, the gravitational waves that constitute it overlap and combine, creating a mixture of signals whose potential sources include, for example, the sum of the waves generated by many supermassive black hole mergers throughout the Universe, or gravitational waves generated during the primordial epoch shortly after the Big Bang.
Until now, detecting it had never been possible. Low-frequency gravitational waves produce such small perturbations that they are undetectable by instruments such as LIGO and Virgo. And even if they are, there are many external factors that contribute to background noise in measurements, and eliminating them is a complex task and requires highly sophisticated data analysis techniques and technologies. Furthermore, it is necessary to have a large amount of data, collected over a long period of time.
How was it identified?
The stochastic background evidence announced by NANOGrav was found by the method of Pulsar Timing Array (PTA). It is a technique used to detect low-frequency gravitational waves, based on observations of pulsars, highly magnetized neutron stars that emit regular pulses of radio waves.
These pulses, so precise that they are likened to “cosmic clocks”, can be detected by telescopes on Earth as radio signals. Scientists measure the arrival time of the signals accurately and compare it to a stable reference clock.
If a gravitational wave passes through the space region between the Earth and a pulsar, its propagation can affect the arrival time of pulsar signals on earth. The gravitational wave, in fact, causes a slight deformation of space-time, which translates into a delay or an advance in the arrival times of the signals considered.
By comparing the arrival times of the signals of different pulsars and studying the correlations between them, it is possible to identify the signature of the passage of a gravitational wave through the network of pulsars. Sophisticated techniques are clearly needed to analyze pulsar timing data and search coherent signals, indicating the presence of gravitational waves.
This involves correcting for unwanted variables, such as atmospheric effects and instrument noise, to accurately identify the deviations caused by gravitational waves.
The PTA method is particularly sensitive to low-frequency gravitational waves, including those generated by cosmic events such as the merger of supermassive black holes. Using observations of a network of pulsars distributed across the sky, the researchers look for coherent signals and correlations in the timing data to reveal the presence of gravitational waves.
What could be the sources?
Even though NANOGrav provided the first convincing evidence of the existence of a stochastic background of gravitational waves in the nanohertz frequency range, the origin of this background is still unknown. In general, the sources could be either astrophysical, therefore collisions of very massive celestial objects, or cosmological, therefore coming from mechanisms, effects and phenomena dating back to the primordial Universe and to the period ofinflation cosmic.
One of the published studies considers several cosmological sources and compares them with the astrophysical signal produced by a population of supermassive binary black holes, to try to confirm or deny the possibility that they are its source. However, the results are not sufficient to give a precise indication, and the analysis does not take into account the full range of uncertainties in cosmological and astrophysical signals.
Future analyzes will therefore be important, aimed at characterizing the so-called “power spectrum” of the signal in more detail and at focusing on theobservation of anisotropy, variations in the signal that could help distinguish the astrophysical from the cosmological origin. To better understand the interpretations of the signal, it will also be necessary to identify models that exactly reproduce the observed parameters, and that take into account the possible variations deriving from also examining dark matter.
The gravitational wave spectrum, which shows the various types of events that emit gravitational waves plotted according to the frequency of their radiation (X-axis) and their amplitude (Y-axis). We need different types of instruments to detect these sources: ground-based detectors like LIGO-Virgo detect high-frequency gravitational waves, while Pulsar Timing Arrays like NANOGrav can detect low-frequency radiation. Credits: J.Simon, Astrobites
What does this discovery imply?
The successful detection of the stochastic background of gravitational waves for the first time has significant implications in several areas. First, it is a further confirmation of the general theory of relativity by Einstein. Furthermore, the Stochastic Fund contains valuable information about the early Universe.
Having identified this signal allows us to investigate the initial conditions of the Universe, including its size, density, and the nature of fundamental interactions during the early stages of cosmic evolution. Phases in which these waves were (somewhere, and from some object) generated.
Clearly, being able to trace the sources of these waves will allow us to obtain information on the various astrophysical sources that contribute to this background noise. This could help us better understand the formation and evolution of galaxies, the merger of black holes and neutron stars, as well as the high-energy astrophysical processes that generate gravitational waves.
But we are also talking about a new physics, about a new way to probe the entire Universe as well as the classic one of collecting its light. And perhaps we have also found a new source of information about physical theories beyond general relativity, such as quantum gravity and the nature of dark matter.