Monday , March 1 2021

The fate of the universe is based on the Hubble constant – which has so far eluded astronomers



An accurate measure of Hubble's constant, the value that describes the rapidity with which the universe is expanding, has deceived scientists for decades. Fixing this number would overturn a long debate among astronomers and take us a little closer to understanding the evolution and fate of the universe. Now researchers have used recent detections of gravitational waves to present a proof of concept for an entirely new method of determining the constant.

So far, astronomers have taken two approaches to assessing the constant value. One method uses known brightness objects, called standard candles, as Cepheid variable stars. The light of a ceeted star floats at regular intervals, and the interval is related to how much luminosity it exposes. Deriving the star's actual brightness from its rate of fluctuation and comparing it with how bright it appears to Earth observers is how astronomers determine their distance. Scientists then measure the redshift of the same objects – that is, how much their light has been shifted to the red end of the electromagnetic spectrum. The redshift occurs when a light source moves away from an observer; the emitted light waves will be stretched. This is similar to how the sound of a horn drops when the vehicle moves away. By measuring the redshift of a distant star, astronomers can calculate how fast it is moving away from Earth. When they combine this information with their distance, they get a value for the Hubble constant.

The second technique for discovering the rate of space expansion is based on the cosmic microwave background (CMB), the ghostly radiation left over from the big bang that permeates deep space. Precise measurements of temperature variations in the CMB of the Planck Space Telescope, when connected to the standard model of Big Bang cosmology, allow astronomers to obtain the constant.

The problem is that the values ​​obtained with these methods do not agree – a discrepancy that cosmologists call "strain." Redshift calculations put the figure at about 73 (in units of kilometers per second per megaparsec); the CMB estimates are closer to 68. Most researchers first felt that this divergence could be due to measurement errors (known among astrophysicists as "systematic"). But despite years of research, scientists find no source of error large enough to explain the gap.

A more exciting possibility is that the voltage reflects a real difference between the Hubble constant in the distance that Planck is looking at, the distant primeval universe and the standard candle method, the recent and near universe. Of course, scientists already know that the expansion of the universe is accelerating – although they do not know exactly why and name the mysterious cause of "dark energy."

But even considering the known acceleration, the tension suggests that something strange might be happening with the dark energy to make the Hubble constant so much fun. This indicates that the rate of expansion during the cosmic epoch that followed the big bang, which CMB would reflect, was radically different from what cosmologists nowadays believe it to be. If a dark energy anomaly is not to blame, it is possible that some unknown particle, such as an undiscovered neutrino flavor, the almost massless particles that permeate the cosmos, may be affecting the calculations. "This tension can hide the solution we have to the description of the universe – its evolution, the energy sources that are in it," says Valeria Pettorino, astrophysicist and research engineer at CEA Saclay in France who was not involved in the project. study. "And, in practice, that decides the past, the present, and the future of our universe, whether or not it will expand forever, whether or not it will collapse again and recover."

Waves in space-time

Now, using gravitational wave signals from the two-hole fusion and redshift data from one of the most ambitious aerial surveys ever conducted, researchers have developed a whole new way of calculating the Hubble constant. They described the method in a study submitted to The Astrophysical Journal Letters and posted on preXP site arXiv on January 6. In it they report a value of 75.2 for the constant, albeit with a large margin of error (+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 yet help clarify the tension between the two original calculation methods. But as proof of concept, the technique is innovative. Just another measure, starting in October 2017, attempted to calculate the Hubble constant using gravitational waves. Scientists hope future gravitational wave detections 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 its existence in 1915, and astronomers were looking for ways to detect them ever since. It is not surprising that collisions of massive objects create a significant explosion of gravitational waves. In 1986, physicist Bernard Schutz proposed for the first time that these binary systems could be used to determine the Hubble constant. He argued that observatories would most likely detect them in the near future; In fact, it took almost 30 years for the observatories to see the signs.

The Gravitational Wave Interferometer with Laser Interferometer (LIGO) in Louisiana and Washington State made the world's first gravitational wave detection in September 2015, and has seen less than a dozen events ever since, along with its European counterpart, Virgo . The experiments look for tiny changes in space-time caused by the passing of gravitational waves.

Standard Siren

An explosion of gravitational waves from the merger of two black holes is one of the pieces of the new method for calculating the Hubble constant. Unlike standard sails, binary black hole systems oscillate. As they coil, the frequency of the gravitational waves they emit changes at a rate correlated with the size of the system. From this, astronomers derive the intrinsic amplitude of the waves. And comparing this to their apparent amplitude (similar to a comparison of the actual brightness of a Cepheid with its apparent brightness), they calculate how far the system is. Astronomers call these "standard sirens." The measured distance for this particular collision is about 540 megaparsecs, or about 1.8 billion light years from Earth.

An associated redshift, such as that of the host galaxy of the mermaids, provides the second part of the new method. The researchers used redshift data from the Dark Energy Survey, who has just mapped a part of the southern sky more broadly and deeply than any previous survey. Redshift data combined with distance measurement provided researchers with the new figure for the constant.

Antonella Palmese, a research associate at Fermilab and co-author of the study, says the method is promising, in part because black-hole fusions are relatively abundant. While still a proof of concept, she says that as more LIGO / VIRGO gravitational events become available, statistics will improve. The Oxford University astronomer, Elisa Chisari, who was not involved in the study, agrees. "The level of restrictions they got with the Hubble rate is not competitive at the moment compared to other measurements," she says. "But as LIGO builds its catalog of gravitational wave events over the next few years, then, by combining multiple events, that really will become a competitive method."


Source link