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Astronomers have pointed telescopes at certain stars and other cosmic sources to measure their distance from Earth and how fast they are moving away from us – two parameters that are important in estimating Hubble's constant, a unit of measurement the rate describes
But so far the most accurate efforts have landed on very different values of the Hubble constant and do not provide a definitive solution to determine exactly how fast the universe is growing. This information, scientists believe, may shed light on the origins of the universe, its fate, and whether the cosmos is expanding or collapsing indefinitely.
Now scientists at MIT and Harvard University have proposed a more accurate and independent way to measure the Hubble constant using gravitational waves emitted by a relatively rare system: a black hole neutron star binary, a high energy paired one spiral black hole and a neutron star. When these objects circle each other, they should create space-shaking gravitational waves and a flash of light during the collision.
In a paper to be published on July 12, Physical Review Letters the researchers reported that the flash of light would give the scientists an estimate of the speed of the system, or how fast it will move away from Earth , The emitted gravitational waves, when detected on the earth, should provide an independent and accurate measurement of system distance. Although black hole neutron star binaries are incredibly rare, researchers estimate that the detection of just a few should provide the most accurate value for the Hubble constant and velocity of the expanding universe.
"Black hole neutron star binaries are very complicated systems of which we know very little," says Salvatore Vitale, assistant professor of physics at MIT and lead author of the newspaper. "When we discover one thing, the price is that they can potentially make a dramatic contribution to our understanding of the universe."
Vital co-author is Hsin-Yu Chen from Harvard.
Competing Constants  Recently, two independent measurements of the Hubble constant were made, one with NASA's Hubble Space Telescope and another with the European Space Agency's Planck satellite. The measurements of the Hubble Space Telescope are based on observations of a star known as the Cepheid variable, as well as observations of supernovae. Both objects are considered "standard candles" because of their predictable brightness pattern, allowing scientists to estimate the distance and speed of the star.
The other type of estimation is based on observations of the fluctuations in the cosmic microwave background – the electromagnetic radiation left over immediately after the Big Bang, when the universe was still in its infancy. While the observations of both probes are extremely accurate, their estimates of the Hubble constant do not match significantly.
"This is where LIGO comes in," says Vitale.
LIGO or the Laser Interferometer Gravitational Wave Observatory detects gravitational waves – waves in Jell-O of space-time, generated by cataclysmic astrophysical phenomena.
"Gravitational waves provide a very direct and easy way to measure the distances of their sources," says Vitale. "What we can prove with LIGO is a direct imprint of the distance to the source, with no additional analysis."
In 2017, scientists were given their first chance to estimate the Hubble constant from a gravitational wave source as LIGO and his The Italian counterpart Virgo first discovered a pair of colliding neutron stars. The collision triggered a large amount of gravitational waves, which were measured by the researchers to determine the distance of the system from the earth. The merger also triggered a flash of light on which astronomers focused on ground and space telescopes to determine the speed of the system.
With both measurements, the scientists calculated a new value for the Hubble constant. However, the estimate came with a relatively large uncertainty of 14 percent, much less certain than the values calculated using the Hubble Space Telescope and the Planck Satellite.
Vitale says that much of the uncertainty is due to the fact that it can be difficult to interpret the distance of a neutron star from the Earth by using the gravitational waves that this system emits.
"We measure the distance by examining how loud the gravitational wave is, which means how clear it is in our data," says Vitale. "If it's very clear, you can see how loud it is and that gives the distance, but that's only partially true for neutron star bears."
That's because these systems generate a swirling disk of energy as two neutron stars spiral towards each other, emitting gravitational waves in an uneven manner. The majority of the gravitational waves are just shooting out of the center of the disk, while a much smaller fraction is emerging from the edges. When scientists discover a "loud" gravitational wave signal, it could indicate one of two scenarios: the detected waves come from the edge of a system that is very close to the earth, or the waves emanate from the center of a much wider system. 19659005] "With neutron star binaries it is very difficult to distinguish between these two situations," says Vitale.
A New Wave
In 2014, before LIGO first discovered gravitational waves, Vitale and his colleagues observed that a binary system consisting of a black hole and a neutron star provides a more accurate range finding compared to neutron star binary systems. The team investigated how accurately one can measure the spin of a black hole, since the objects are known to rotate on their axes much like Earth, but much faster.
The researchers simulated a variety of black hole systems, including black hole neutron star binaries and neutron star binaries. As a byproduct of this effort, the team noted that they were able to more accurately determine the removal of black hole neutron star binary compared to neutron star binary systems. Vitale says that this is due to the spin of the black hole around the neutron star, which can help scientists pinpoint where in the system the gravitational waves originate.
"Because of this better range finding, I thought the black hole – Neutron star binaries could be a competitive probe for measuring the Hubble constant," says Vitale. "Since then, a lot has happened to LIGO and the discovery of gravitational waves, and all this has been put on hold."
Vitale recently returned to his original observation, and in this new work he set out to answer a theoretical question:
"Is the fact that every black hole neutron binary system gives me a better distance will compensate for the fact that there may be much less in the universe than neutron star binary systems? " Vitale says:
To answer this question, the team performed simulations to predict the occurrence of both types of binary systems in the universe and the accuracy of their distance measurements. From their calculations, they concluded that even if the binary neutron systems were superior to the black hole neutron star systems by 50-1, they would yield a Hubble constant similar in accuracy to the first.
More Optimistic When Black Hole Neutron Star binaries were slightly more common but still rarer than neutron star binaries, the former would yield a Hubble constant that is four times more accurate.
"So far, people have focused on binary neutron stars as a way of measuring the Hubble's constant gravitational waves," says Vitale. "We have shown that there is another type of gravitational wave source that has not been used so much: black holes and neutron stars that spiral together," says Vitale. "LIGO will gather data again in January 2019 and it will be much more sensitive which means that we will be able to see more distant objects so LIGO should see at least one binary number of the black hole neutron star and up to 25 which will hopefully help resolve the existing tension in measuring the Hubble constant over the next few years. "
Even small black holes emit gravitational waves when they collide, and LIGO heard them
"Measuring the Hubble Constant with Neutron Star Black Hole Mergers" Physical Review Letters . On Arxiv : arxiv.org/abs/1804.07337