Gravitational waves could soon provide a measure of the expansion of the universe

Twenty years ago, scientists were shocked to realize that our universe is not only expanding, but that it is growing faster.

Determining the exact rate of expansion, called the Hubble constant after the famous astronomer and UChicago alumnus, Edwin Hubble, was surprisingly difficult. Since then, scientists have used two methods to calculate the value and they have obtained extremely different results. But the surprising capture of gravitational waves emitted by a collision of neutron stars last year offered a third way to calculate the Hubble constant.

It was only one data point of a collision, but in a new article published on October 17 NatureThree scientists at the University of Chicago believe that given the speed with which researchers have seen the first collision between neutron stars, they could obtain a very accurate measurement of the Hubble constant in five to ten years.

"The Hubble constant tells you the size and age of the universe; it's been a holy grail since the birth of cosmology. Calculate it with the help of gravitational could give us an entirely new perspective on the universe, "said author of the study, Daniel Holz. UChicago physics professor who co-wrote the first such calculation from the discovery of 2017. "The question is: when does this change the game of cosmology?"

In 1929, Edwin Hubble announced that, based on observations of galaxies beyond the Milky Way, they seemed to be moving away from us – and the further away the galaxy was, the faster it would retreat. This is a cornerstone of the Big Bang theory, which has launched a nearly century-long search for the exact speed at which this happens.

To calculate the rate of expansion of the universe, scientists need two numbers. One is the distance to a distant object; the other is the speed with which the object moves away from us because of the expansion of the universe. If you can see it with a telescope, the second amount is relatively easy to determine because the light you see when you look at a distant star is shifted into red as it moves back. Astronomers use this trick to see how fast an object moves for more than a century – this is like the Doppler effect, in which a siren changes pitch when passing through. an ambulance.

"Major issues in calculations"

But getting an exact measure of distance is much more difficult. Traditionally, astrophysicists used a technique called cosmic distance scale, in which the brightness of certain variable stars and supernovae can be used to establish a series of comparisons that reach the object in question. "The problem is that if you scratch under the surface, there are a lot of steps with a lot of assumptions along the way," Holz said.

Maybe supernovae used as markers are not as coherent as thought. Perhaps we are confusing certain kinds of supernovae with others, or an unknown error in our measure of distances to nearby stars. "There are a lot of complicated astrophysics that could disrupt readings in many ways," he said.

The other major way to calculate the Hubble constant is to look at the cosmic microwave background – the light pulse created at the very beginning of the universe, which is still weakly detectable. Although it is also useful, this method also relies on assumptions about how the universe works.

The surprising thing is that even though the scientists performing each calculation have confidence in their results, they do not fit. One says that the universe is growing almost 10% faster than the other. "This is a major question of cosmology at the moment," said the first author of the study, Hsin-Yu Chen, a graduate student at UChicago and a member of Harvard University's Black Hole Initiative. .

The LIGO detectors then captured their first ripple in the space-time structure after the collision of two stars last year. This has not only shaken the observatory, but also the field of astronomy: the fact of being able to feel the gravitational wave and see the light of the consequences of collision with a telescope has provided scientists with a powerful new tool. "It was a kind of embarrassment of wealth," Holz said.

Gravitational waves offer a completely different way of calculating the Hubble constant. When two massive stars collide with each other, they emit ripples in the space-time structure that can be detected on Earth. By measuring this signal, scientists can get a signature of the mass and energy of colliding stars. When they compare this reading to the force of the gravitational waves, they can deduce how far away it is.

This measurement is cleaner and contains fewer assumptions about the universe, which should make it more accurate, Holz said. In the company of Scott Hughes of MIT, he suggested doing this measurement with gravitational waves coupled with telescope readings in 2005. The only question to ask is how many times scientists could detect these events and what would be the quality Datas.

"It will become more interesting"

The paper predicts that once scientists have detected 25 readings of neutron star collisions, they will measure the expansion of the universe with an accuracy of 3%. With 200 readings, this number is reduced to 1%.

"It was a big surprise for me when we started the simulations," said Chen. "It was clear that we could reach the accuracy and quickly."

A new accurate figure for Hubble's constant would be fascinating regardless of the answer, the scientists said. For example, one of the possible reasons for the inadequacy of the other two methods is that the nature of gravity itself might have changed over time. Reading could also shed light on dark energy, a mysterious force responsible for the expansion of the universe.

"With the collision we had last year, we were lucky – it was close to us, so it was relatively easy to find and analyze," said Maya Fishbach. , a graduate student from UChicago and the other author of the newspaper. "Future detections will be much more remote, but once we have the next generation of telescopes, we should also be able to find counterparts for those far detections."

The LIGO detectors should start a new observation campaign in February 2019, alongside their Italian counterparts at VIRGO. Through an upgrade, the detectors will be much more responsive, thereby increasing the number and distance of astronomical events that they can detect.

"It will only get more interesting from here," Holz said.

Explore further:
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More information:
Hsin-Yu Chen et al, Hubble constant measurement of two percent from standard five-year sirens, Nature (2018). DOI: 10.1038 / s41586-018-0606-0