Could gravitational waves reveal how fast our universe is growing?

Astronomers have pointed telescopes at certain stars and other cosmic sources to measure their distance to Earth and the speed with which they are moving away from us – two essential parameters for estimating However, to date, the most accurate efforts have landed on values ​​very different from the Hubble constant, offering no definitive solution to the growth rate of the Hubble constant. universe. This information, scientists believe, could illuminate the origins of the universe, as well as its fate, and whether the cosmos will grow indefinitely or eventually collapse.

Now scientists at MIT and Harvard University have proposed a more accurate and independent way of measuring the Hubble constant, using gravitational waves emitted by a relatively rare system: a black star with binary neutrons , a hugely energetic pairing of a black spiral hole and a neutron star. When these objects get closer to each other, they should produce gravitational waves agitated by space and a flash of light when they collide.

In an article published on [12juillet1990] Physical Review Letters The researchers report that the lightning flash would give scientists an estimate of the speed of the system, or how fast it is moving away from Earth. The gravitational waves emitted, if detected on Earth, should provide an independent and accurate measure of system distance. Even though black hole neutron star binaries are incredibly rare, the researchers calculated that the detection of a few binaries should give the most accurate value for the Hubble constant and the rate of the universe in expansion

. Explains Salvatore Vitale, an assistant professor of physics at MIT and lead author of the paper. "If we detect one, the price is that they can potentially make a dramatic contribution to our understanding of the universe."

Co-author of Vitale is Hsin-Yu Chen of Harvard.

Competitive Constants [19659005] Two independent measurements of the Hubble Constant have been made recently, one using the NASA Hubble Space Telescope and the other using the Planck satellite of the Agency European space. The Hubble Space Telescope measurement is based on observations of a star type known as the Cepheid variable, as well as on supernova observations. These two objects are considered "standard candles", for their kind of predictable brightness, that scientists can use to estimate the distance and speed of the star.

The other type of estimation is based on observations of fluctuations in the cosmic microwave background – the electromagnetic radiation that remained after the Big Bang, while the universe was still in its infancy . While the observations of both probes are extremely accurate, their estimates of the Hubble constant are in significant disagreement.

"It's there that LIGO comes in," says Vitale.

LIGO, or laser interferometric wave observatory, detects gravitational waves – ripples in the Jell-O from space-time, produced by cataclysmic astrophysical phenomena.

"Gravitational waves provide a very direct and easy way to measure distances from their sources," says Vitale. "What we detect with LIGO is a direct imprint of the distance to the source, without further analysis."

In 2017, scientists had their first chance of estimating the Hubble constant from a gravitational source, when LIGO and its Italian counterpart Virgo detected a pair of pests. neutron stars collide for the first time. The collision released a huge amount of gravitational waves, which the researchers measured to determine the distance from the Earth 's system. The fusion also released a flash of light, on which astronomers focused with terrestrial and space telescopes to determine the speed of the system.

With both measurements, scientists calculated a new value for the Hubble constant. However, the estimate is accompanied by a relatively large uncertainty of 14%, much more uncertain than the values ​​calculated using the Hubble Space Telescope and the Planck satellite.

Vitale says that a lot of the uncertainty stems from the fact that it can be difficult to interpret the distance of a neutron star binary from the Earth in using the gravitational waves that this particular system releases.

"We measure the distance by looking at how the gravitational wave is strong, which means that it is clear in our data." If it's very clear, you can see at what point it is strong, and that gives the distance.This is only partially true for neutron binaries. "

This is because these systems create a swirling disc of Energy like two neutron stars, spiraling toward each other, emit gravitational waves unevenly.The majority of gravitational waves come out directly from the center of the disc, while a much smaller fraction If scientists detect a "strong" gravitational wave signal, this could indicate one of two scenarios: the waves detected come from the edge of a very strong system. close to the Earth or the waves emanate from the center of a much more advanced system. 9005] "With neutron star binaries, it is very difficult to distinguish between these two situations."

A New Wave

In 2014, before LIGO did doing the first gravitational wave detection, Vitale and his colleagues observed that a binary system consisting of a black hole and a neutron star could give a more accurate measure of the distance , compared to neutron binaries. The team was studying how accurately we could measure the spin of a black hole, since objects rotate on their axes, like the Earth but much faster.

Researchers simulated a variety of systems with black holes, including black. neutron star binaries and neutron star binaries. As a by-product of this effort, the team noticed that it was able to more accurately determine the distance between neutron star and black hole binaries, compared to star binaries. with neutrons. Vitale says that this is due to the rotation of the black hole around the neutron star, which can help scientists better locate the gravitational waves in the system

"Due to this better measure of distance, I thought the black hole The Neutron star binaries could be a competitive probe to measure the Hubble constant, "says Vitale. "Since then, a lot has happened with LIGO and the discovery of gravitational waves, and all that has been put aside."

Vitale has recently made a return to his original observation, and in this new paper, he has set himself to answer a theoretical question:

"Does the fact that every black star binary to neutrons give me a better distance will compensate for the fact that potentially there are a lot less in the universe than neutron binaries? " Vitale says.

To answer this question, the team did simulations to predict the occurrence of both types of binaries in the universe, as well as the accuracy of their distance measurements. According to their calculations, they concluded that even though neutron binary systems outnumbered black hole neutron star systems by 50-1, the latter would produce a Hubble constant similar to the previous one.

More optimistic the stars binaries were a bit more common, but rarer than the neutron star binaries, the first would produce a Hubble constant four times more accurate

"Until now, the People were focusing on the neutron binary stars – constant with gravitational waves, "says Vitale." We have shown that there is another type of gravitational wave source that, up to now, does not exist. Has not been so exploited: the black holes and the spiral neutron stars together, "says Vitale." LIGO will start taking data again in January 2019. It will be much more sensitive, which means we can see the objects further away, so LIGO should see at least one black neutron star binary, and not less than 25, which will help resolve the existing voltage in the measure of the Hubble constant, hopefully in the next few years ed. "

Learn more:
Even small black holes emit gravitational waves when they collide, and LIGO has heard them

More information:
"Measuring Hubble's Constant with Neutron's Black Hole Mergers," Physical Review Letters . The Arxiv : arxiv.org/abs/1804.07337