It took 15 years of data collection and created a virtual ‘laboratory’ nearly the size of our own galaxy, but for the first time scientists could hear the ‘constant chorus’ of gravitational waves permeating the entire universe. A chorus that sounds louder than expected and hides the secrets of the formation of galaxies and, perhaps, the Big Bang.
The long-awaited discovery was released this Thursday.Astrophysical Journal Letters‘, carried out by researchers at the North American Gravitational Wave Nanohertz Laboratory (Nanogravity), involving experts from 70 different institutions. To achieve this, scientists resorted to the ‘trick’ of using pulsars as celestial metronomes. Pulsars are a type of galactic corpse that spins very rapidly on its axis (hundreds or thousands of times per second), emitting a radio ‘pulse’ (hence the name) with each turn, a more precise ‘tick tock’. An atomic clock.
By comparing the tick rates of 67 pulsars spread across our galaxy over 15 years of available data, the scientists found that their cadences showed little variation. The NANOGrav team showed that these variations were caused by the passage of low-frequency gravitational waves, which distort tissue based on physical reality. space time. In fact, the stretching and shortening of the gap between Earth and pulsars causes them to arrive at Earth a billionth of a second earlier or later than expected.
The results are the first evidence of the background of gravitational waves, a kind of ‘soup’ of space-time distortions that permeate the entire universe and were predicted by scientists decades ago.
The most powerful yet discovered
Gravitational waves detected in this way are more powerful yet measured than the gravitational wave bursts of black hole-neutron star mergers detected by experiments such as LIGO and Virgo.
The existence of gravitational waves was first predicted Albert Einstein In 1916, but they weren’t first observed until 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected ripples in spacetime passing through Earth. Although the source of those gravitational waves was the collision of two distant black holes, the spatial deviation detected by LIGO was smaller than that of an atomic nucleus.
By comparison, the apparent time shift in the tick ticks of pulsars measured by the NANOGrav team is a few hundred trillionths of a second, but this represents a flexibility in the distance between Earth and pulsars equivalent to the length of a football field. The scientists explain that the distortions are caused by gravitational waves so large that the distance between the two ridges is 2 to 10 light-years, or about 9 to 90 trillion kilometers. According to the researchers, it’s very possible that most of these giant gravitational waves are created by pairs of supermassive black holes orbiting each other in cataclysmic collisions across the universe.
“It’s like a chorus, these pairs of supermassive black holes resonating at different frequencies,” says Nanograve researcher Chiara Mingarelli. This is the first evidence of the gravitational wave background. We have opened a new window to observe the universe.
A treasure trove of new knowledge
The first detection of the gravitational wave background opens the door to a veritable treasure trove of new knowledge and promises answers to long-standing questions, from the fate of supermassive black hole pairs to the frequency of galaxy mergers. For now, however, NANOGrav can only measure the general background of gravitational waves, but cannot distinguish the sources that produce them individually. Yet scientists got more than one surprise.
“The gravitational wave background,” says Mingarelli, “is twice as strong as expected. It’s actually at the high end of what our models can produce from supermassive black holes.” This ‘deaf size’ could be a result of experimental limitations, or black holes that are heavier and more massive than previously thought. But there’s also the possibility that “something else” creates powerful gravitational waves, Mingarelli says, such as mechanisms predicted by string theory or some alternative explanations for the origin of the universe. “What is coming now – the scientist says – everything. This is just the beginning”.
Challenge of the years
Achieving this discovery has been a challenge for the NanoGrave team for years. The gravitational waves they observed are, in fact, unlike anything measured before. For example, unlike the high-frequency waves detected by ground-based instruments such as LIGO and Virgo, the gravitational-wave background is composed of ultra-low-frequency waves. This means that a single ‘rise and fall’ of one of the tides can take years or even decades to occur. Since gravitational waves travel at the speed of light, the distance between the two ridges can be tens of light-years long.
It was clear that no experiment on Earth could detect such gigantic waves, so the NANOGrav team had to look to the stars. What they did was to look closely at pulsars, the dense remnants of massive stars that have exploded as supernovae. Pulsars act like real stellar beacons, shooting beams of radio waves from their magnetic poles. As the pulsars spin rapidly (sometimes hundreds or thousands of times per second), these rays spread across the sky and appear to us on Earth as rhythmic pulses of radio waves.
Those pulses reach the earth. So accurate that when Jocelyn Bell measured the first pulsar radio waves in 1967, even astronomers thought they might be signs of an alien civilization.
But when a gravitational wave passes between us and a pulsar, it disrupts the timing of the radio waves. That is, always ticking the pulsar is ‘timed out’. However, this is only an illusion. In fact, the pulsar is still as accurate as ever, but as gravitational waves stretch and contract space as they travel through the cosmos, they change the distance radio waves must travel to reach us. And there is a mismatch in our perception of what is expected.
For 15 years, NANOGrav scientists have patiently timed the radio wave pulses of tens of millisecond pulsars in our galaxy, looking for anomalies that reveal the passing of low-frequency gravitational waves. Bank Telescope in West Virginia and the largest array in New Mexico. The new findings are the result of a detailed analysis of an array of 67 of those pulsars.
“In fact,” says Maura McLaughlin of West Virginia University and co-director of the NANOGrav Physics Frontiers Center, “pulsars are extremely weak radio sources, so making this experiment requires thousands of hours a year on the world’s largest telescopes. These exceptionally sensitive radio laboratories are supported by the National Science Foundation ( NSF) these results were possible only because of their continued commitment.”
Search for sources
As early as 2020, with just 12 years of data, NANOGrav scientists began to see hints of a signal, a diffuse extra ‘hum’, affecting the timing of all the pulsars in the sequence. Now, after three years of observations, they have collected enough hard evidence to detect the gravitational wave background.
“Now that we have evidence for this background, the next step is to use our observations to examine these hum-generating sources one by one,” said Sarah Vigeland of the University of Wisconsin-Milwaukee.
As explained in the article, the sources of the gravitational wave background are pairs of supermassive black holes entangled in a vicious spiral that could lead to a collision. Those black holes are truly massive and can reach masses equal to billions of suns. It turns out that almost all galaxies, including our own Milky Way, have at least one of these giants at their core. When two galaxies merge, their supermassive black holes meet and begin orbiting each other. When they finally collide, scientists are now picking up low-frequency gravitational waves.
However, not all gravitational waves detected by NANOGrav come from a pair of supermassive black holes. In fact, there are other theories that predict waves in the ultra-low frequency range. For example, string theory predicts that cosmic strings may have formed in the early universe. And those strings dissipate energy by emitting gravitational waves.
Another idea to consider is that the universe didn’t start out of nowhere with a big bang, but instead came from an earlier universe that collapsed on its own before expanding outward in a big bounce. If that were true, gravitational waves from that event would still be propagating through space-time. And they may be part of the newly discovered background of gravitational waves.
Of course, scientists think that pulsars aren’t perfect gravitational wave detectors, instead there are some unknown variables that skew the nanogravity results. Unfortunately, Mingarelli says, “you can’t go up to the pulsars and turn them on and off again to see if there’s something wrong.”
The only solution is to time the pulsars for several more years and then check for results. Meanwhile, the NANOGrav team will try to ‘butcher’ the newly discovered gravitational wave background to find out what all the possible contributors are. They hope there will be more surprises.
