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The fate of the universe depends on the Hubble constant, which still eludes astronomers

The exact measurement of the Hubble constant, a value that describes how quickly the universe is expanding, has eluded scientists for decades. Accounting for this number would lead to a long simmering debate among astronomers about rest and would bring us one step closer to understanding the evolution and fate of the universe. Now, researchers have used recent gravitational wave detections to present proof of the concept of a completely new method for determining the constant.

Until now, astronomers have used two approaches to calculating the value of a constant. One method uses objects of known brightness, called standard candles, such as variable stars Cepheid. The light of a cepheid star oscillates at regular intervals, and this interval is related to how much light it radiates. Extracting the actual brightness of a star from the speed of its fluctuations and comparing it with how bright it seems to Earth observers, astronomers determine its distance. Then scientists measure the redshift of the same objects, that is, how much their light has shifted towards the red end of the electromagnetic spectrum. Redshift occurs when the light source moves away from the observer; light waves emitted by it will stretch. This is similar to how the sound of a car horn drops in the key when the car leaves. By measuring the redshift of a distant star, astronomers can calculate how quickly it moves away from Earth. When they combine this information with its distance, they get the value for the Hubble constant.

The second method for determining the rate of expansion of space is based on the cosmic microwave background (CMB), the ghostly radiation left over from the big bang penetrating deep space. Accurate measurements of temperature changes in CMB using the Planck Space Telescope, when incorporated into the standard big bang cosmology model, allow astronomers to get a constant.

The problem is that the values ​​obtained using these methods do not agree – cosmologists call the discrepancy "voltage." Calculations for redshift set the value to about 73 (in units of kilometers per second per megaparsec); CMB estimates are closer to 68. Most researchers first thought that this discrepancy might be due to measurement errors (known among astrophysicists as “systematics”). But, despite years of research, scientists cannot find a source of error large enough to explain this gap.

A more exciting possibility is that the tension reflects the real difference between the Hubble constant over the distance that Planck looks at, the distant early universe, and the standard candlestick method of the nearby recent universe. Of course, scientists already know that the expansion of the universe is accelerating, although they do not know exactly why, and they call the mysterious cause "dark energy."

But even considering the known acceleration, the voltage assumes that something strange can happen to the dark energy, which leads to the fact that the Hubble constant diverges strongly. This indicates that the rate of expansion in the space age, which followed the Big Bang, which reflected the CMB, was radically different from what cosmologists now consider. If the dark energy anomaly is not to blame, it is possible that some unknown particles, such as the undiscovered neutrino flavor, almost massless particles that permeate space, can affect the calculations. “This tension can hide the solution for the description that we have about the universe – its evolution, the energy sources that are in it,” says Valeria Pettorino, an astrophysicist and research engineer at CEA Saclay in France, who did not participate in the study. "And in practice, it decides the past, present and future of our universe, whether it will expand forever, whether it will again collapse and recover."

Waves in space-time

Now, using gravitational wave signals from the confluence of two black holes and the redshift data from one of the most ambitious sky surveys ever conducted, the researchers developed a completely new way to calculate the Hubble constant. They described the method in a study that they presented. Astrophysical Journal Letters and posted on the arXiv preprint website on January 6th. It reports a value of 75.2 for a constant, although with a large margin of error (+39.5, –32.4, which means that the actual number can be up to 114.7 or decrease to a minimum). like 42.8). This large uncertainty reflects the fact that the calculation comes from a single dimension, and thus does not yet help eliminate the contradiction between the two original calculation methods. But as a proof of concept, the technique is groundbreaking. In only one other dimension, since October 2017, an attempt has been made to calculate the Hubble constant using gravity waves. Scientists hope that future detection of gravitational waves will help them improve the accuracy of their calculations.

Gravitational waves are ripples in the fabric of space-time. Einstein’s general theory of relativity predicted their existence in 1915, and since then astronomers have been looking for ways to detect them. It is not surprising that the collisions of massive objects create a significant surge of gravitational waves. In 1986, physicist Bernard Schutz first proposed using these so-called binary systems to determine the Hubble constant. He argued that observatories would likely detect them in the near future; in fact, it took about 30 years before the observatories saw the signals.

The laser interferometric gravitational-wave observatory (LIGO) in Louisiana and the state of Washington made the world's first detection of gravitational waves in September 2015 and has since seen less than a dozen other events with its European counterpart, Virgo. Experiments are looking for tiny changes in space-time caused by passing gravitational waves.

Standard sirens

The surge of gravitational waves from the merger of two black holes is one of the elements of the new method for calculating the Hubble constant. Unlike standard candles, black hole binary systems oscillate. As they twist into each other, the frequency of the gravitational waves that they emit changes at a speed corresponding to the size of the system. From here, astronomers derive their own wave amplitude. And comparing it with their apparent amplitude (similar to comparing the actual brightness of Cepheids with its apparent brightness), they calculate how far the system is. Astronomers call these "standard sirens." The measured distance to this particular collision is about 540 Mpc, or about 1.8 billion light years from Earth.

A related redshift, such as that of the siren host galaxy, provides the second part of the new method. The researchers used the redshift data from a survey of dark energy, who has just finished mapping a portion of the southern sky more widely and deeply than any previous survey. The redshift data combined with distance measurement provided researchers with a new indicator for the constant.

Antonella Palmese, a Fermilab researcher and co-author of the study, says the method promises in part because black hole fusions are relatively numerous. Although this is still a confirmation of the concept, she says that as new gravitational events from LIGO / VIRGO appear, the statistics will improve. Astronomer at Oxford University Eliza Chisari, who did not participate in the study, agrees. “The level of restrictions they received at the Hubble level is not currently competitive with other dimensions,” she says. "But, since LIGO will create its own catalog of gravitational wave events in the coming years, then, by combining several events, this will indeed become a competitive method."

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