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The Universe's Fate Rests on the Hubble Constant – Which Has So Far Eluded Astronomers

A precise measurement of the Hubble constant, the value that describes how fast the universe is expanding, has elated scientists for decades. Pinning this number down would be a long-simmering dispute among astronomers to bring home to the evolution and fate of the universe.

Until now astronomers have taken two approaches to reckoning the constant's value. One method uses objects of known brightness, called standard candles, search as Cepheid variable stars. A Cepheid star's light fluctuates at regular intervals, and the interval is how much luminosity it puts out. Observers is how astronomers determine their distance. The scientists then measure the redshift of the same objects-that is, how much their light has shifted to the red end of the electromagnetic spectrum. Redshift occurs when a light source moves away from an observer; light waves will be stretched out. This is how the sound of a car drops in pitch as the vehicle drives away. By measuring a distant star's redshift, astronomers can almost recalculate it from Earth.

The second technique for the expansion of the cosmic microwave background (CMB), the ghostly radiation left over from the big one bang that permeates deep space. Precise measurements of temperature variations in the CMB from the Planck Space Telescope, when plugged into the standard model of big bang's cosmology, allow astronomers to derive the constant.

a discrepancy cosmologists call "tension." Calculations from redshift place at about 73 (in units of kilometers per second per megaparsec); the CMB estimates are closer to 68. Most researchers first thought this divergence could be due to errors in measurements (known among astrophysicists as "systematics").

A very exciting possibility to understand the gap between the Hubble and the Hubble , and that of the standard candle method, the nearby, recent universe. Of course, scientists already know the universe's expansion is accelerating-although they do not know exactly why, and the mysterious cause is "dark energy."

But even accounting for the known acceleration, the tension suggests something strange may be happening to dark energy to cause the Hubble to diverge this much. It would indicate the rate of expansion during the cosmic epoch that followed the big bang, which the CMB would reflect, radically different from what cosmologists currently believe it to be. If a dark energy anomaly is not to blame, it is possible that the undiscovered flavor of neutrino, the nearly massive particles that pervade the cosmos, may be affecting the calculations. "It's in the world," says Valeria Pettorino, an astrophysicist and research engineer at CEA Saclay in France study.

Waves in Spacetime

"And in practice, this decides the past, the present and the future of our universe, whether or not it's going to be expanding forever, or not it's going to re-collapse and rebound."

] Now, using gravitational wave signals from the merger of two black holes and redshift data from one of the most ambitious sky surveys ever conducted, researchers have developed an entirely new way to calculate the Hubble constant. The Astrophysical Journal Letters and posted on the preprint site arXiv on January 6. In this report, you will find a value of 75.2 (+39.5, -32.4, meaning the actual number could range up to 1
14.7 or go as low as 42.8). This large uncertainty reflects the fact that the original two-meter method is used. But as a proof of concept, the technique is groundbreaking. Only one other measurement, from October 2017, has attempted to calculate the Hubble constant using gravitational waves. Scientists hope future gravitational wave detections will help them improve the precision of their calculations.

Gravitational waves are ripples in the fabric of spacetime. Einstein's general theory of relativity predicted their existence in 1915, and astronomers had been looking for ways to detect them since. Not surprisingly, collisions of massive objects create a significant splash of gravitational waves. In 1986, Bernard Bernard Physicist Physicist Physicist first proposed these so-called binary systems could be used to determine the Hubble constant. He argued observatories would very likely detect them in the near future;

The Laser Interferometer Gravitational Wave Observatory (LIGO) in Louisiana and Washington State made the world's first gravitational wave detection in September 2015, and has less than a dozen more events since then, along with its European counterpart, Virgo. The experiments look for miniscule alterations in spacetime caused by passing gravitational waves.

Standard Sirens

A burst of gravitational waves from the merger of two black holes. Not unlike standard candles, binary black hole systems oscillate. As they spiral into each other, the frequency of the gravitational waves they change at a rate correlated to the system's size. From this, astronomers derive the waves' intrinsic amplitude. And by that with their apparent amplitude (similar to the actual brightness of a Cepheid with its apparent brightness). Astronomers call these "standard sirens." 540 megaparsecs, or about 1.8 billion light-years, from Earth.

An associated redshift, search as the sirens' host galaxy, provides the second piece of the new method. The researchers used redshift data from the Dark Energy Survey, which just finished mapping a portion of the southern sky more broadly and more deeply than any previous survey.

Antonella Palmese, a research associate at Fermilab and co-author of the study, says the method holds promise in part because black hole mergers are relatively plentiful. Although it is still a proof of concept, they say that more gravitational events from LIGO / VIRGO become available. University of Oxford astronomer Elisa Chisari, who was not involved in the study, agrees. "The level of constraints that they obtain on the Hubble rate is not competitive at the moment to other measurements," she says. "But as LIGO builds up its catalog of gravitational wave events in the coming years, then by combining multiple events, this will become a competitive method."

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