Hidden in the depths of space are pairs of black holes, each millions or billions of solar masses that orbit each other and generate especially low-frequency gravitational waves. Pulsars can help detect them.
Several research teams have analyzed measurement data from pulsars, some of which have been recorded for decades with radio telescopes. In the articles that appear in the journal Astronomy and Astrophysicsconclude that there is growing evidence for the existence of gravitational waves emitted by large numbers of pairs of very heavy black holes.
The new sign that must exist
A major breakthrough came in 2015, when gravitational waves from colliding black holes were finally detected with laser interferometers. In 2020, this research even won the Nobel Prize in Physics. However, the black holes in these early signals are quite small, a few tens of solar masses. Their heavier relatives, with millions or billions of solar masses at the centers of galaxies, must also approach each other when they collide. At some point, they orbit each other so closely that the orbital period is only on the order of decades, years, or even months.
During this movement of the heavy weights, gravitational waves are generated as a result of the acceleration of the masses. That’s what Albert Einstein’s general theory of relativity says. Unlike gravitational waves with frequencies of several hundred hertz measured so far, the signals from the giants are much lower in frequency, in the range of billionths of a hertz or nanohertz.
There must be countless pairs of extremely massive black holes at different distances, orbiting each other in this way and eventually merging to form a larger hole. Coming from different directions, the countless gravitational waves propagate at the speed of light and overlap. This creates a common signal: the gravitational wave background. So far it has not been detected because the usual gravitational wave laser interferometers on Earth are blind to this waveform. Only large space-based laser interferometers like the projected LISA project will be sensitive to it, but LISA won’t fly until the late 2030s.
Measuring Einstein waves with pulsars
There is, however, a completely different method for detecting low-frequency gravitational waves. An especially ingenious one is one that uses pulsars. These stellar explosion remnants are actually rapidly rotating neutron stars, tiny compact spheres about 20 kilometers in diameter that combine about two solar masses. Like lighthouses, they emit directional radio radiation that can accidentally collide with Earth. Astronomers then observe them as pulsars. Due to the constant rotation of neutron stars, pulsars emit radio pulses with extreme precision, like clockwork. An array of pulsars (Pulsar Timing Arrays, PTAs), each many light-years from us, can be used to detect gravitational waves: when a space-time wave passes through the array of pulsars, the distances of the pulsars from Earth change. . They seem to go out of sync for a short period of time. It can be measured.
a new waveform
With a whole series of radio telescopes, many teams around the world constantly monitor some distant pulsars. Meanwhile, they obtained datasets that go back 25 years in the past. This trove of data contains valuable information about how pulsars move relative to Earth. Correlation analyzes reveal gravitational waves as a weak noise signal. What’s special about space-time distortions is that they are very long waves. This is due to the size of large black holes in galaxies and the fact that they orbit each other at relatively large distances. This is a humming signal at low frequencies, many orders of magnitude lower than anything measured so far. international efforts
Radio astronomers have been looking for this background gravitational wave signal for decades. The EPTA appoints a PTA looking for it with the five largest radio antennas in Europe. The consortium includes the 100-metre Effelsberg radio telescope near Bonn, Germany, the Lovell telescope at Jodrell Bank Observatory, UK, the Nançay radio telescope, France, the Sardinia radio telescope, Italy, and the Westerbork synthesis (Netherlands).
In addition, Australian Pulsar Timing Array PPTA, Chinese CPTA and North American NANOGrav exist all over the world. They coordinated and now announce similar results in their publications.
The European observations were complemented by additional data from the Giant VHF Radio Telescope (GMRT) of the Indian InPTA array. Current EPTA results correspond to a group of 25 pulsars selected as the most sensitive to a gravitational wave background.
the truth is out there
Two years ago, the tracks thickened because a common noise was found in all monitored pulsars. At that time, its cause was still unknown. Meanwhile, the data situation makes it possible to identify pairs of extremely massive black holes as the cause. However, the teams are cautious and still do not speak of a definitive discovery, because the statistical significance is still not enough. This will only improve with more measurement data.
Within the framework of the International Pulsar Timing Array (IPTA), measurement data from 13 radio telescopes recorded from more than 100 pulsars will be combined. Each pulsar will then have 10,000 observations. This cooperation is already underway. It is hoped that the existence of the background hum of gravitational waves can be demonstrated soon.
REFERENCE
The second European Pulsar Timing Array data release. I. The dataset and time analysis
