Since its inception 13.8 billion years ago, the universe has continued to expand, scattering hundreds of billions of galaxies and stars like raisins in a rapidly rising dough. Astronomers have pointed telescopes at certain stars and other cosmic sources to measure their distance from Earth and their rate of removal, two parameters that are needed to calculate the Hubble constant, a unit of measure that describes the rate at which the universe expands.
But to date, the most accurate attempts to estimate the Hubble constant gave very scattered values and did not allow making a final conclusion about how quickly the universe is growing. This information, according to scientists, should shed light on the origin of the Universe and on its fate: will the cosmos expand infinitely or will one day shrink?
And so, scientists from the Massachusetts Institute of Technology and Harvard University have proposed a more accurate and independent way to measure the Hubble constant using gravitational waves emitted by relatively rare systems: a black hole-neutron star binary system, an energetic pair twisted in a spiral by a black hole and a neutron star. As these objects move in the dance, they create space-time quivering waves and a flash of light when the final collision occurs.
In a paper published July 12 in Physical Review Letters, the scientists reported that the burst of light will allow scientists to estimate the speed of the system, that is, how fast it is moving away from the Earth. The emitted gravitational waves, if captured on Earth, should provide an independent and accurate measurement of the distance to the system. Despite the fact that binary systems of black holes and neutron stars are incredibly rare, scientists estimate that the discovery of even a few of them will make the most accurate estimate of the Hubble constant and the expansion rate of the universe to date.
“Binary systems of black holes and neutron stars are very complex systems that we know very little about,” says Salvatore Vitale, associate professor of physics at MIT and lead author of the paper. "If we find one, the prize will be our radical breakthrough in understanding the universe."
Vitale is co-authored by Hsin-Yu Chen from Harvard.
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Competing constants
Two independent measurements of the Hubble constant were recently taken, one using NASA's Hubble Space Telescope and the other using the European Space Agency's Planck satellite. Hubble's measurement was based on observations of a star known as the Cepheid variable, as well as observations of supernovae. Both of these objects are considered "standard candles" for the predictability in brightness, by which scientists estimate the distance to a star and its speed.
Another type of assessment is based on observations of fluctuations in the cosmic microwave background - 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 differ greatly.
“And this is where LIGO comes into play,” says Vitale.
LIGO, or laser interferometric gravitational-wave observatory, looks for gravitational waves - ripples on the fabric of space-time, which is born as a result of astrophysical cataclysms.
“Gravitational waves provide a very simple and easy way to measure distances to their sources,” Vitale says. "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 chance to estimate the Hubble constant from a gravitational wave source 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 burst of light, which astronomers were able to analyze with ground-based and space telescopes to determine the speed of the system.
Having obtained both measurements, the scientists calculated a new value for the Hubble constant. However, the estimate came with a relatively large uncertainty of 14%, much more uncertain than the values calculated using Hubble and Planck.
Vitale says much of the uncertainty stems from the fact that interpreting the distance from a binary system to Earth is difficult using the gravitational waves created by that system.
“We measure distance by looking at how 'loud' the gravitational wave is, that is, how clean our data on it will be,” says Vitale. “If everything is clear, you can see that it is loud and determine the distance. But this is only partially true for binary systems."
The fact is that these systems, which generate a swirling disk of energy as the dance of two neutron stars develops, emit gravitational waves unevenly. Most of the gravitational waves are shot from the center of the disc, while much less of them are shot out from the edges. If scientists detect a "loud" gravitational wave signal, this may indicate one of two scenarios: the detected waves are born at the edges of a system that is very close to the Earth, or the waves come from the center of a much farther system.
“In the case of binary stellar systems, it is very difficult to distinguish between the two situations,” Vitale says.
New wave
In 2014, even before LIGO detected the first gravitational waves, Vitale and his colleagues observed that a binary system of a black hole and a neutron star could provide a more accurate distance measurement than binary neutron stars. The team studied how accurately the rotation of a black hole can be measured, provided that these objects rotate on their axis, like the Earth, only faster.
Researchers have modeled various black hole systems, including black hole neutron star systems and binary neutron star systems. Along the way, 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 spinning of the black hole around the neutron star because it helps better determine where the gravitational waves are coming from in the system.
“Because of the more accurate distance measurement, I thought that binary black hole-neutron star systems might be a more appropriate reference point for measuring the Hubble constant,” Vitale says. "Since then, a lot has happened with LIGO and gravitational waves were discovered, so it all faded into the background."
Vitale has recently returned to his original observation.
“Until now, people have preferred binary neutron stars as a way to measure the Hubble constant using gravitational waves,” Vitale says. “We have shown that there is another type of gravitational wave source that has not been fully exploited before: black holes and neutron stars dancing around. 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 of a black hole and a neutron star, and preferably all twenty-five, and this will help resolve the existing tension in measuring the Hubble constant, hopefully, in the next few years.
Ilya Khel