Gravitational waves could solve a cosmological crisis within five years or shake up physics

When cosmologist Daniel Holtz left Hong Kong on August 17, 2017, his head swirled with the ideas he had spent last week giving lectures, including his hope that the vibrations in space would settle one day the ongoing debate about size and age the universe. But he knew it would take time. It's time for two of the most dense objects to meet and shake the cosmos hard enough for us to feel the roll here on Earth, the time to locate the disturbance and the time to rotate our telescopes to the collision before the light that accompanies it disappears. back in the darkness.

Optimistically, these coupled observations of both gravitational and light waves from these neutron star collisions were about ten years apart, he told the audience at his last conference the day before. The LIGO (gravitational wave laser interferometer) collaboration had already detected blackheads and the Virgo interferometer had just been posted in Italy two weeks earlier. The company was developing so fluidly. But when Holtz, who works at the University of Chicago, returns to Illinois, he learns that the future has arrived early. The gravitational shock waves triggered by the collision of two nearby titans had wavered in his plane – and on the entire planet – while he was in the air, and observatories from around the world were jostling for optical observations of followed.

"We landed and my phone exploded. I immediately connected and sat on my laptop to start working, "recalls Holtz. "It was the most incredible experience of my life." Twelve hours after touch, he made a rough calculation for the most controversial figure of cosmology: the speed of development of the universe. With only one data point, he could not get the decisive step he had been dreaming for thirteen years, but he finally knew that the project was possible. Now, after doing a little more calculation, he comes back with a new prediction: the LIGO collaboration could perhaps settle the debate of several decades in five years, according to his recent letter of Nature.

The conversation revolves around a question: how fast is the universe expanding? Finding the answer, known as Hubble's constant, is simple in theory. You start with a falling object, usually a star undergoing a particular type of death. These "type 1a supernovae" always explode in the same way, which allows researchers to get an idea of ​​their distance depending on their brightness. To calculate the Hubble constant, you must also know how fast the explosion is away from you, what you can get by looking at its color, measuring the extent of its light. Researchers can also do something similar with the old light information left shortly after the big bang, known as Cosmic Microwave Background Radiation (CMB). Once you know the speed of expansion, you can work backwards to determine the exact size and age of the universe or advance to examine its future trajectory.

The problem is that the two current calculations give different results.

The most recent estimates for the supernova method (73.5 km per second per megaparsec in January) and the CMB method (67.4 in June) differ by approximately 9%. The divergence did not provoke much alarm on the ground at first, because the measurements are devilishly difficult in practice. A large and distant explosion looks like a weak and near explosion. The distance to be traveled in relation to the supernovae therefore depends on the "scale of the cosmic distance", a complex technique that consists in relating three types of objects to different distances, or "bars". Astronomers first study the twinkling stars of our galactic backyard with a basic geometry, then transfer that knowledge to stars behaving the same way in distant galaxies to learn more about the supernovae that s & # 39; 39 are there. "They are incredibly cautious in so many different ways," says Holst. "But there are a lot of sausages on the inside."

The CMB study requires fewer machines, but more assumptions. The background radiation retains a record of the expanding universe in its infancy and, to extrapolate to current cosmologists, one must tap into everything they think they know about gravity, the matter, black energy and dark matter over the remaining 13 billion. years. No matter how many flaws could have thrown one or the other method, but even though the astrophysicists on each side checked and rechecked their calculations, the two estimates refused to converge. Now, the chance that the cosmological community has had an incredible set of statistical disadvantages pushes 1 in 1,000.

"We are now at a point where we are like" wow, it's probably not just a fluke, "says Adam Reiss, a cosmologist at John Hopkins University, who works on the supernova method . "Something interesting is happening, something we do not understand about the universe."

Holtz has bet his career on the idea that gravitational waves could serve as referee. The idea, the result of a 1986 speculative article by American physicist Bernard Shutz, is that another type of dead star could replace supernovae as a cleaner criterion for cosmic distances. . After exploding, giant stars that do not quite have what it takes to become a black hole collapse into neutron stars: a melee of particles so dense that even the atoms crushes. When two of these stellar bodies collide together, the impact triggers a ripple called the gravitational wave.

Because these waves are ripples in the space itself, nothing hinders them. Undisturbed by the clouds of dust and gas, they dispersed from the crash site to the Earth, where the scientific community uses three L-shaped detectors (with d & # 39; others en route) to catch them. When a wave crosses the planet, it exerts a slight pressure. One arm of each L becomes about a proton shorter than the other, and the device warns physicists and astronomers around the world. Using the demanding equations of general relativity, researchers can accurately measure collision distance with little computation and few assumptions – no scaling or particle counting required.

Holtz refined the theory in 2005, suggesting that observing light from a collision with a neutron star as well as detecting waves would provide information on the speed completing the reading of the distance from the camera. # 39; gravitational wave and joined LIGO to lead such an effort. A lot of his colleagues told him that this would never happen, he recalls, because astronomical data predicted that neutron star mergers would occur extremely rarely, but all the pieces came together on the same day. August 17, while he was returning home.

The Hubble constant of this event has arrived at a very rough 70, between the traditional two, but with an uncertainty encompassing even the most extreme estimates of supernovae and CMB. The settlement of the conflict calls for reducing this possible error to 2 or 3%, which will require between 30 and 50 collisions of the type observed last year, calculated Holtz in his recent article. Given the increasing sensitivity of LIGO and the assumed neutron star melting rate, he expects to have sufficient data to determine the two Hubble Constant competitors within five years. Reiss, who did not participate in the work, agrees that gravitational waves offer a plausible and interesting way forward, but points out that it is difficult to guess how often we will proceed with mergers. "Maybe they'll grow faster," says Reiss, "but if they increase 10 times more slowly, I do not want to wait 50 years."

Holtz admits that it's hard to guess how many times something will happen after this has happened once, but he says there's a reason for optimism – if he did the math well. Its model, based on current statistical tools for counting rare events, predicts detections of 30 to 400 mergers by 2026. One of these results would drive uncertainty into the Hubble constant south of 3%, according to him, so that no one wait 50 years.

If the gravitational waves allow us to synchronize conclusively the expansion of the universe, three results are possible. LIGO data could support the CMB method, which means that the cosmic scale could not reach supernovae accurately. According to Reiss, only a "plot of errors" could explain the incomprehension of half a dozen independent calibration methods. This case is similar to the lightning struck repeatedly.

Holtz and Reiss personally hope that neutron stars will support the calculation of the supernova, which would point to a mistaken assumption about how the universe has evolved from birth to now – a highly anticipated sign of the new physics. Holtz speculates that gravity might have acted differently than we expected or that undiscovered particles might not be included in cosmological accounting.

Alternatively, LIGO could return with an entirely different measure for the Hubble constant, outside the range defined by both supernovae and CMB. This result would create a nightmarish scenario, upsetting cosmology. "That would only cast doubt on all our ability to take action," said Reiss. I hope we are not in the place.

For now, Holz is delighted that his bet is paying off. Last year, he thought that the first data point was still in a decade, and in a few months, he will listen to the second point when LIGO will be back online in February. "I've spent years working on this idea and developing it," says Holtz. "And in less than half a day, he quenched his thirst in front of me.