What is the cosmic murmur that they have detected, why is it so important, what is the cosmic microwave background, what have they heard, why can they be echoes of the initial expansion of the universe… .
Óscar del Barco Novillo , University of Zaragoza
After 15 years of collecting data using the world’s largest radio telescopes (and using a virtual observatory the size of our galaxy), researchers at the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) have released an impressive finding : the detection of a cosmic background (a miraculously perceptible murmur) of gravitational waves associated with the most violent events in the universe.
They have found out how to see the screen of the cosmos in which events of such a dimension took place, which until today have not even been narrated in mythology.
Where does the detected murmur come from?
According to the study authors , these subtle ripples to our technological ears could be caused by the merger of supermassive black holes (with masses up to billions of times that of our Sun) as they began to spin rapidly around each other. Another hypothesis suggests its formation during the inflation period of the universe , some billion years after the Big Bang.
To identify them, scientists have relied on the tiny nano effect produced by extraordinary cosmic objects: pulsars .
Pulsars are extraordinarily accurate astrophysical clocks and serve as peculiar cosmic metronomes . They are remnants of dead stars that rotate at high speed, emitting a radio pulse with each turn. By rotating so quickly (and positioning itself in our line of sight), an observer on Earth perceives a beam that repeats itself thousands of times per second, allowing time to be measured with a precision greater than that of an atomic clock.
The fabulous finding has been that gravitational waves produce an inadmissible effect on them for a watchmaker: they slow down or advance their rotation.
When space-time warps
Gravitational waves, predicted by Einstein 100 years ago, are disturbances of space-time, similar to water waves on the surface of a pond, that propagate at the speed of light. Although they are related to extremely violent phenomena in the universe (such as the merger of black holes), the signal that reaches us is very weak and they were not detected until 2015 (which was a historic discovery and a Nobel Prize ).
In that crucial year for astrophysics, the LIGO (United States) and Virgo (Europe) gravitational wave detectors revolutionized the way we view the universe: they recorded a signal lasting less than a second from the collision between two black holes (with a mass ten times greater than that of the Sun).
The following animation shows how the LIGO and Virgo observatories operate. When a gravitational wave passes through the Earth, space is stretched and compressed, slightly deforming the arms of the detectors (4 kilometers long).
These observatories use extremely sensitive lasers, mirrors, and instruments that detect these small changes in arm lengths (up to one ten-thousandth the size of a proton ).
POT
Until 2015, objects in the cosmos were studied based on the electromagnetic radiation they emitted, in visible light, infrared or radio waves, among other components of the electromagnetic spectrum. Since then we have not only been able to see them (electromagnetic waves) but also to listen to the stars, thanks to the detection of gravitational waves. Now we can hear his murmur.
a magical moment
Scientist Maura McLaughlin (of the US network Pulsar Search Collaboratory ), referred to the new discovery as a “magical moment”.
Although the scientific community was already aware of specific gravitational wave signals (such as those detected by LIGO and Virgo), this is now the first time that a cosmic background of gravitational waves has been recorded: a kind of whisper coming from all directions and associated with the most energetic events in the universe.
The network of radio telescopes from Europe, North America, India, Australia and China of the International Pulsar Timing Array (IPTA) consortium studied for 15 years the ticking rates of 67 pulsars scattered throughout our galaxy and found slight variations in the cadence of their movements. radio pulses.
Wouter Hagens/Wikimedia Commons , CC BY-SA
The consortium has shown that the tiny temporal variations found in these cosmic clocks (up to a billionth of a second over 20 years) are due to the passage of low-frequency gravitational waves (and light-year wavelengths ). These distort the space between Earth and the pulsars themselves. As a consequence, radio pulses arrive at ground-based observatories earlier or later than expected.
NANOGrav
But the most surprising thing is that these temporary delays (or advances) are correlated, that is, there is a disturbance common to all pulsars that causes this phenomenon: the cosmic background of gravitational waves.
The importance of this new finding
This new discovery may lay the groundwork for answering questions about the fate of supermassive black holes ( imaged in M87 or Sagittarius A* ) to how frequent galaxy mergers may be.
For now, this international team has managed to measure the general background of gravitational waves, but it cannot distinguish one by one the sources that compose it. As NASA explains , “detecting the background noise of gravitational waves is similar to listening to the hum of a large group of people talking at a party, but without distinguishing any particular voice.”
Let’s hope that in the not too distant future we will be able to distinguish each of these voices from the most primitive and violent universe (who knows if the echoes of the Big Bang). In the meantime, let’s enjoy this new discovery, a true magical moment in the form of a cosmic murmur . 
Óscar del Barco Novillo , Assistant Professor Doctor. Department of Applied Physics., University of Zaragoza
This article was originally published on The Conversation . Read the original .