Could gravitational waves reveal how fast our universe is expanding?

Since its inception 13.8 billion years ago, the universe continues to expand, throwing hundreds of billions of galaxies and stars like the raisins in rising dough quickly. The astronomers pointed the telescope at some stars and other cosmic sources, to measure their distance from the Earth and the removal rate — two parameters that are needed to calculate the Hubble constant, the unit of measurement that describes the rate of expansion of the Universe.

But by far the most accurate attempt to estimate the Hubble constant gave very scattered values and is not allowed to make a final conclusion about how fast the universe is growing. This information, according to scientists, should shed light on the origin of the Universe and its fate: will the cosmos expand forever or will someday be compressed?

And now, scientists from mit and Harvard University have proposed a more accurate and independent method to measure the Hubble constant using gravitational waves emitted by the relatively rare systems: binary system, black hole — neutron star, energetic with a pair of spiral black hole and neutron star. As soon as these objects move in the dance, they create a space-time shattering wave and flash of light when the final clash.

In a paper published July 12 in Physical Review Letters, the scientists reported that the flash light will allow scientists to estimate the speed of the system, i.e. the speed of its removal from the Ground. The emitted gravitational waves, if you catch them on the Ground, should provide an independent and accurate measurement of the distance to the system. Despite the fact that dual systems black holes and neutron stars are incredibly rare, scientists estimate that the detection of even a few of them will allow you to make the most accurate to date estimate of the Hubble constant and speed of expansion of the Universe.

"a Binary system of black holes and neutron stars is a very complex system about which we know very little," said Salvatore Vitale, associate Professor of physics at MIT and lead author of the article. "If we find one, the prize will be a radical breakthrough in our understanding of the Universe."

Vitale is co-author of Hsin-Yu Chen from Harvard.

Competing constant

Recently, there have been two independent measurements of the Hubble constant, one using the Hubble space telescope, NASA, and the other using the satellite, "plank" of the European space Agency. Measurement of the "Hubble" was based on observations of a star known as variable-Cepheids, and supernovae observations. Both of these facilities are considered "standard candles" for predictability in the brightness change, which scientists estimate the distance to the star and its speed.

Another type of evaluation is based on observations of fluctuations of the cosmic microwave background of electromagnetic radiation left after Big Bang, when the universe was still in its infancy. Although observations of both probes are extremely accurate, their estimates of the Hubble constant strongly disagree.

"And here comes into play LIGO," says Vitale.

LIGO, or laser interferometer gravitational-wave Observatory, is looking for gravitational waves — ripples in the fabric of space-time which arises as a result of astrophysical cataclysms.

"Gravitational waves provide a very simple and easy way of measuring the distances to their sources," says Vitale. "What we found with LIGO — a direct imprint of the distance from the source, without any additional analysis."

In 2017, scientists got their first chance to evaluate the Hubble constant from the source gravitational waves, LIGO, and when her Italian counterpart Virgo discovered a pair of colliding neutron stars for the first time in history. This collision released an enormous amount of gravitational waves that scientists have measured to determine the distance from the earth to the system. The merger also gave off a flash of light which astronomers were able to perform with the help of ground and space telescopes, to determine the speed of the system.

Having both measurements, scientists calculated the new value of the Hubble constant. However, the score came with a relatively large uncertainty of 14%, much more uncertain than the values calculated using the "Hubble" and "Bar".

Vitale said that much of the uncertainty stems from the fact that to interpret distance from the binary system to the Ground hard enough, using gravitational waves, created by the system.

"We measure distance, looking at how "loud" would be a gravitational wave, that is, how clean are our data on it," says Vitale. "If everything is clear, you can see that she's loud, and determine the distance. But this is true only partially for binary systems".

The fact that these systems, generating a swirling disk of energy as the dance development of two neutron stars emit gravitational waves unevenly. Most gravitational waves is fired from the center of the disc, while a much smaller part comes out from the edges. If scientists detect "loud" the signal of gravitational waves, it may indicate one of two scenarios: the detected waves are born at the edges of the system, which is very close to the Ground, and waves emanate from the center of a much more distant system.

"In the case of doublesystems stars very difficult to distinguish between these two situations," says Vitale.

New wave

In 2014, before LIGO has detected the first gravitational waves, Vitale and his colleagues observed that a binary system of black holes and neutron stars, can give more accurate measurement of distance compared with the binary neutron stars. The team studied how accurately you can measure the rotation of the black hole, provided that these objects rotate around its axis like the Earth, only faster.

Researchers have modeled a variety of systems with black holes, including black hole — neutron star and the binary system of neutron stars. In the process, was able to detect that the distance to the black hole — neutron star cannot determine rather than neutron stars. Vitale says that it is connected with the rotation of the black holes around neutron stars because it helps to better determine where in the system emanate gravitational waves.

"due to more accurate measurement of the distance, I thought a double system black hole — neutron star may be a more appropriate benchmark for the measurement of the Hubble constant," says Vitale. "Since then a lot has happened with LIGO and were discovered gravitational waves, so it all went on the back burner".

Recently, Vitale returned to his initial observation.

"still, people preferred the double neutron star as a way to measure the Hubble constant using gravitational waves," says Vitale. "We have shown that there is another type of source of gravitational waves, which were not used in full: black holes and neutron stars, spun in the dance. LIGO will start collecting data again in January 2019 and will become much more sensitive, so we can see more distant objects. Therefore, LIGO will be able to see at least one system of black holes and neutron stars, and twenty-five, and it will help to resolve existing tensions in the measurement of the Hubble constant, hopefully in the next few years."