The exact measurement of the Hubble constant, the value that describes how fast the universe expands, has been avoided for scientists for decades. Fixing this figure down would give astronomers a long flowing debate and rest, and we will come closer to understanding the evolution and fate of the universe. Now scientists have used the recent gravity wave detection to provide proof of the concept of a completely new method of determining the constant.
Until now, astronomers have made two approaches to computing the constant value. One method uses objects of known clarity, called standard candles, such as Cepheid variable stars. Cepheid's light fluctuates at regular intervals, and the interval is related to how much light it radiates. Determining the true brightness of a star from its variation and comparing it with how Earth observers appear clearly is how astronomers determine its distance. Scientists then measure the red shift of the same objects – that's how much their light shifted to the red end of the electromagnetic spectrum. Redshift occurs when the light source diverts from the observer; the light waves emitted from it will be strained. This is similar to how the sound of the car horn drops in pitch when the vehicle leaves. By measuring the red shift of a distant star, astronomers can figure out how fast it is from Earth. By linking this information with its distance, it gains value for the Hubble constant.
The second technique for detecting the rate of expansion depends on the cosmic microwave background (CMB), the spooky radiation that remains from the big bang that penetrates the deep space. Accurate measurements of temperature changes in the Planetary Planck Planetary CMB when engaged in a standard Big Bang cosmology allow astronomers to derive a constant.
The problem is that the values obtained by these methods do not agree – cosmologists do not agree with the name "tension". The red shift calculations show a value of approximately 73 (in units of kilometers per second per megaparsec); estimates of CMB are closer to 68. Most researchers first thought that this divergence could be due to measurement errors (known as astrophysics as "systematics"). But over years of investigation, scientists find no source of error large enough to explain the gaps.
Even more interesting is that the voltage reflects the real difference between the star constant at a distance planned by Planck, the distant early universe and the standard candle method close to the recent universe. Of course, scientists already know that the expansion of the universe is accelerating – though they do not know the exact reason and name the mysterious cause of "dark energy".
Even with a more familiar acceleration of tension, it suggests that something dark can be done with the dark energy that makes Hubble's star star so different. It indicates the velocity of expansion during the cosmic epoch that followed the big bang CMB reflected was radically different from what cosmologists now believe. If it is not to blame for dark energy anomalies, it is possible that some unknown particles, such as the unrecognized taste of neutrinos, almost intangible particles that penetrate the universe, may affect calculations. "This tension can hide the solution to the description we have of the universe – its evolution, the energy sources that are in it," says Valeria Pettorino, astrophysicist and research engineer at CEA Saclay, France, who did not participate in the study. "And in practice, it decides on the past, the present, and the future of our universe, regardless of whether it will or will not ever expand, whether it breaks apart or falls apart again."
Waves in the universe
Now, using the gravitational wave signal from the fusion of two black holes and red shifts from one of the most ambitious sky surveys ever made, researchers have developed a completely new way to calculate the Hubble constant. They described the method in their study Astrophysical journals and published on the preprint site arXiv on January 6. It reports a value of 75.2 for a constant, although with a large error range (+39.5, -32.4, meaning that the actual number could reach 114.7 or go as low as 42.8). This great uncertainty reflects the fact that the calculation comes from a single measurement and therefore does not help to clear the tension between the original two computational methods. But as a proof of the concept, the technique is pioneering. Only one further measurement has attempted to calculate the Hubble constant by gravity waves since October 2017. Scientists hope that future gravity wave detection will help them improve the accuracy of their calculation.
Gravitational waves are waves in the universe. Einstein's general theory of relativity predicted their existence in 1915, and astronomers have been searching for ways to discover them ever since. It is not surprising that the collisions of massive objects create a significant current of gravitational waves. In 1986 physicist Bernard Schutz first suggested that these so-called Binary Systems could be used to determine the Hubble constant. He argued that observatories would most likely be able to detect them in the near future; in fact, it took almost 30 years for the observers to see the signals.
The LIGO in Louisiana and the State of Washington conducted the first wave of gravity detection in the world in September 2015 and since then has seen less than a dozen other events along with the European counterpart Virgo. Experiments are looking for slight changes in space caused by the passage of gravitational waves.
The explosion of gravitational waves from the merger of two black holes is one of the new ways of calculating the Hubble constant. Like standard candles, binary systems with black holes oscillate. As each other spirales each other, the frequency of the gravitational waves it discharges varies with the degree of correlation with the size of the system. From this, astronomers derive an internal wave of amplitude. And by comparing it with their apparent amplitude (similar to comparing the true brightness of Cepheid with its apparent brightness), they calculate how far the system is. Astronomers call these "standard sirens". It measures the distance from this particular collision as from about 540 megaparsecs or about 1.8 billion light-years.
An associated red shift, such as the galaxy of the siren hosts, provides the second part of the new method. Researchers used the data from the red exploration from the dark energy survey, which has just completed the mapping of a part of the southern sky more broadly and deeper than the previous survey. Data from the red shift combined with the distance measurement gave the researchers a new constant number.
Antonella Palmese, a Fermilab researcher and co-author of the study, says this method is promising in part because black hole fusion is relatively abundant. While it is still proof of the concept, it says that as soon as other gravitational events from LIGO / VIRGO are available, statistics will improve. Oxford astronomer Elisa Chisari, who did not participate in the study, agrees. "The level of constraints it has gained on Hubble is currently not competitive compared to other measurements," he says. "But as LIGO builds its catalog of gravitational wave events in the coming years, a combination of more events will really make it a competitive method."