General, Research, Technology

Can gravitational waves reveal how fast our universe is expanding?

13.8 billion since its inceptionyears ago, the universe continues to expand, scattering hundreds of billions of galaxies and stars like raisins in fast-rising dough. Astronomers pointed telescopes at some stars and other cosmic sources to measure their distance from the Earth and their speed of removal — two parameters that are needed to calculate the Hubble constant, the unit of measurement that describes the speed of expansion of the universe.

"Binary systems of black holes and neutron stars -very complex systems that we know very little about, ”says Salvatore Vitale, associate professor of physics at MIT and lead author of the article. “If we find at least one, the prize will be our radical breakthrough in understanding the universe.”

Co-authored by Vitale is Hsin Yu Chen from Harvard.

Competing Constants

Two independent measurements have been taken recently.Hubble constant, one using the NASA Hubble Space Telescope, and the other using the Planck satellite of the European Space Agency. The Hubble measurement was based on observations of a star known as the Cepheid variable, as well as on observations of supernovae. Both of these objects are considered “standard candles” for predictability in changing brightness, according to which scientists estimate the distance to the star and its speed.

Another type of assessment is based on observations.fluctuations in the cosmic microwave background - the electromagnetic radiation that remained after the Big Bang, when the Universe was still in its infancy. Although the observations of both probes are extremely accurate, their estimates of the Hubble constant diverge greatly.

“And here LIGO comes into play,” says Vitale.

LIGO, or the laser-interferometric gravitational-wave observatory, is looking for gravitational waves - ripples on the fabric of space-time, which is born as a result of astrophysical disasters.

“Gravitational waves provide a very simpleand an easy way to measure distances to their sources, ”says Vitale. “What we found with LIGO is a direct imprint of the distance to the source, without any further analysis.”

In 2017, scientists got their first chanceestimate the Hubble constant from the source of the gravitational wave when LIGO and its Italian counterpart Virgo discovered a pair of colliding neutron stars for the first time in history. This collision released a huge amount of gravitational waves, which scientists measured to determine the distance from the Earth to the system. The merger also emitted a flash of light, which astronomers were able to analyze using ground and space telescopes to determine the speed of the system.

Having received both measurements, scientists calculated a newHubble constant value. However, the estimate came with a relatively large uncertainty of 14%, much more uncertain than the values ​​calculated using the Hubble and Planck.

Vitale says that mostuncertainty stems from the fact that it is quite difficult to interpret the distance from a binary system to the Earth using gravitational waves created by this system.

“We measure distance by looking at how muchThe gravitational wave will be “loud”, that is, how clean our data on it will be, ”says Vitale. “If everything is clear, you see that it is loud, and determine the distance. But this is only partially true for binary systems. ”

The fact is that these systems generatinga spinning disk of energy as the dance of two neutron stars develops, the gravitational waves emit unevenly. Most gravitational waves shoot out from the center of the disk, while a much smaller fraction comes out of the edges. If scientists detect a “loud” signal from a gravitational wave, this may indicate one of two scenarios: the detected waves are generated at the edges of a system that is very close to the Earth, or the waves come from the center of a much more distant system.

“In the case of binary star systems, it is very difficult to distinguish between these two situations,” says Vitale.

New wave

In 2014, even before LIGO discoveredthe first gravitational waves, Vitale and his colleagues observed that a binary system of a black hole and a neutron star can provide a more accurate measurement of distance compared to binary neutron stars. The team studied how accurately it is possible to measure the rotation of a black hole, provided that these objects rotate around their axis, like the Earth, only faster.

Researchers have modeled various systems withblack holes, including black hole systems - a neutron star and binary systems of neutron stars. In the process, it was found that the distance to the black hole - neutron star systems can be determined more accurately than to neutron stars. Vitale says this is due to the rotation of a black hole around a neutron star, because it helps to better determine where gravitational waves are coming from in the system.

“Because of a more accurate distance measurement, II thought binary systems like a black hole — a neutron star — might be a better reference for measuring the Hubble constant, ”says Vitale. “Since then, much has happened to LIGO and gravitational waves have been discovered, so all this has faded into the background.”

Vitale recently returned to his initial observation.

“Until now, people have preferred double neutronstars as a way to measure the Hubble constant using gravitational waves, ”says Vitale. “We showed that there is another type of gravitational wave source that has not been fully used before: black holes and neutron stars twisted in a dance. LIGO will start collecting data again in January 2019 and will become much more sensitive, which means we can see more distant objects. Therefore, LIGO will be able to see at least one system from a black hole and a neutron star, and all twenty five is better, and this will help resolve the existing tension in measuring the Hubble constant, I hope, in the next few years. ”