Will gravitational waves be able to shed light on the expansion rate of the universe?

Researchers claim that the gravitational waves emitted by the merger of neutron stars with black holes may indicate the rate of expansion of the universe 

To estimate the expansion rate of the universe described by Hubble's constant, researchers look at stars and other celestial bodies and measure their speed and distance. Hubble's constant was first measured by Edwin Hubble in the thirties of the last century and since then several additional measurements have been made to estimate this constant. According to this law, there is a direct relationship between the distance of objects in space and the distance between them, where Hubble's constant describes this relationship. In fact, it can be said that as the distance between two bodies increases, the relative speed at which they move away from each other increases.

To date, the most accurate attempts to determine the Hubble constant have not been consistent with each other. Now a new method will be able to determine with much higher precision the value of the Hubble constant and possibly the future of the universe - whether it is expected to collapse in on itself or expand forever.

Researchers from MIT and Harvard University have proposed a way to estimate Hubble's constant more accurately. With the help of the gravitational waves emitted from the merger of a neutron star and a black hole, from a relatively rare event that combines the emission of gravitational waves and light radiation during the spinning and merging of these two objects. This is an event in which both types of radiation can be measured simultaneously and thus the Hubble constant can be estimated.

In the prestigious journal PHYSICAL REVIEW LETTERS published on July 12, the researchers reported that the flash of light emitted from the merger allows the researchers to measure the speed of the system's distance from the Earth and at the same time the gravitational waves can accurately estimate the distance of the system from the Earth. Although events of the merger of a neutron star and a black hole are relatively rare, a single number of such observations will be sufficient to determine with great precision what the Hubble constant is and therefore the expansion rate of the universe.

Salvatore Vitale, a professor of physics at MIT who wrote the article claims that "binary systems of a neutron star and a black hole are very complex systems, we know very little about them. If we locate one of them, we will receive as a gift a tremendous contribution to the knowledge of the universe."

Past measurements of the Hubble constant

In recent years, two major measurements have been made to estimate the Hubble constant: one by NASA's Hubble Space Telescope and the other by using the European Space Agency's telescope - the Planck Telescope. The Hubble Space Telescope bases its measurements on Cepheid variables (stars with periodic brightness) or on observations of supernovae (end of life of massive stars). These two observations are known as standard light sources that researchers can use to estimate the distance and propagation speed of stars.

Another way to estimate the Hubble constant is based on fluctuations in the cosmic background radiation - the electromagnetic radiation inherent in the universe since the Big Bang. Although the measurements of both methods are remarkably accurate, they still disagree on the value of Hubble's constant.

"This is where LIGO comes into play" Vital adds. LIGO is a system for detecting gravitational waves, which originate from particularly powerful events in space that cause waves in the fabric of space-time itself.

In 2017, scientists saw the merger of two neutron stars for the first time. This was a very unique observation because it was measured for the first time simultaneously in both gravitational waves by LIGO and visible light telescopes. The powerful gravitational waves reached the Earth and allowed the researchers to measure the distance of the event. At the same time, the emitted radiation also gave the researchers the opportunity to measure the speed of the binary system's receding.

With the help of these data, the researchers re-estimated Hubble's constant, but the error percentage of 14 percent is quite high, much higher than Planck's or Hubble's measurements. Vitale says that most of the error comes from the difficulty of estimating the distance of the binary system of neutron stars from Earth using the data obtained from the gravitational waves

Vital explains that "we measure the distance by evaluating the noise from the gravitational waves, that is, how clean our data is. If they are very clean we are able to accurately estimate the distance, but for neutron stars this is not the case."

Because a pair of neutron stars spinning around each other create an energetic disk around them that emits gravitational waves unevenly. Most binary systems emit gravitational waves directly from their center and only a fraction of the emission comes from the edges. If researchers locate "noisy" gravitational waves, it is likely that the source of this comes from one of two situations: the gravitational waves were emitted from the edges of the system or the gravitational waves originated from a much more distant system. "In binary systems of neutron stars it is very difficult to distinguish between the two states" Vital says.

new way

In 2014, even before Liego first detected gravitational waves, Vitale and his colleagues noticed that a binary system of a neutron star and a black hole would allow for a more precise measurement of the distance. Originally the researchers asked how accurately the spin of a black hole could be measured. The spin of a black hole indicates the degree to which it rotates on its axis, just as the Earth rotates on its axis. The researchers computer simulated binary systems with black holes and as a byproduct of these calculations they discovered that the distance of these objects can be measured with incredible precision. Vitale says that the accurate estimate of the distance is made possible by the spin of the black hole.

"Does the fact that merging black holes with neutron stars allow accurate distance measurement compensate for the fact that there are very few such events relative to the merging of two neutron stars?" Vital asked.

To answer this question, Vitale's team ran many simulations to predict the frequency of binary mergers in the universe and also their contribution to distance measurement accuracy. They showed that although there are fifty times more neutron star binaries in the universe than black hole and neutron star binaries, the two may more accurately measure the Hubble constant and thus the expansion rate of the universe.

If we were optimistic for a moment, if systems of black holes and neutron stars had appeared in the universe a little more, we could measure the Hubble constant four times more precisely than the measurements made so far.

"Until now, researchers have focused on neutron star binaries to measure the Hubble constant using the gravitational waves they emit when they merge," said Vitale. "We have shown that there are other untapped sources of gravitational waves - mergers of black holes with neutron stars. In 2019, LIGO will start collecting much more accurate and sensitive data again, meaning it will be able to measure objects from even greater distances. Thus LIGO will be able to detect at least one neutron star and black hole system and up to 25 of them. If this is the case, we will finally be able to relieve the tension regarding the measurement of the Hubble constant and the expansion rate of the universe in the coming years"15

For an article in Physical Review Letters

to the notice of the researchers

More of the topic in Hayadan:

• For the first time, gravitational waves and light waves were discovered from the same event - the merger of two neutron stars
• "The winners of the Nobel Prize in Physics were themselves surprised by the strength of the signal of the first gravitational wave event"
• Is the universe expanding faster than we thought?