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Sunday, July 25, 2021

Hubble constant and dark energy measure universe

A paper in the March edition of The Astrophysics Journal said current astronomical efforts are being made to utilize gravitational lenses to measure the Hubble constant, a scientific value which can determine the size and age of the universe.

Physicists from UC Davis, Stanford and the University of Bonn in Germany have collaborated to quantify the Hubble constant and verify its implications on the current model of our universe. Specifically, the existence of “dark energy,” has been proposed to explain the acceleration of the universe.

Gravitational lensing is a series of bending light effects. A ray of light passing closely to a massive object – such as a galaxy – has its path bent. This observation will in turn cause the apparent position of the object that emitted light to shift relative to the sky.

Physicists trying to determine the Hubble constant are using a B1608+656m lens, an example of a “strong gravitational lensing system.” This is a combination of two to four light rays that have been bent upon passing different sides of a galaxy and their arrivals at telescopes on Earth.

“The result is that we see two or four images of the background object surrounding the massive galaxy,” said Chris Fassnacht, professor of physics in an e-mail interview.

The Hubble constant, a proportionality measurement, relates speed and distance. According to Fassnacht, our universe is expanding and as it expands the galaxies surrounding it are moving farther away. It has been noted that the farther a galaxy is from the Milky Way the faster it seems to be moving away.

“Our measurement of the Hubble constant is about 70 km/s/Mpc = 21 km/s / million light years. This implies that a galaxy which is 1 million light years away is on average moving away from us at 21km/s, and a galaxy which is 2 million light years away is on average moving away from us at 42 km/s,” said Sherry Suyu, a postdoctoral researcher at the Argelander- Institute for Astronomy at the University of Bonn in Germany in an e-mail interview.

Suyu said that determining the Hubble constant is fundamental to creating an accurate model of our universe because Fassnacht and physicists agree that our universe has both accelerated and decelerated since the Big Bang.

“In the 1990s we discovered that the universe is actually accelerating right now, due to some completely unknown process that astronomers have called ‘dark energy’,” Fassnacht said. “This dark energy acts as “anti-gravity” and pushes the galaxies apart at a faster rate. It is the combination of deceleration in the early history of the universe with acceleration in the later history that gives us the age.”

Dark energy is what is posing the current problem in obtaining data as quickly and as accurately as astronomists and physicists would like.

“One needs to have a model of the universe to derive the age of the universe. The current model is a universe filled mostly with cold dark matter and non-evolving dark energy,” Suyu said.

Fassnacht and his colleagues agree that the composition of this energy is unknown and its relationship with the constant remains ambiguous, except that it seems to be the factor that functions in accelerating the universe.

Fassnacht said a variable in dark energy is the idea of whether it stays constant with time or if its composition and implications change.

“Our new measurement of Hubble Constant, H0, is now being used as an additional probe of the cosmological parameters: we have given cosmologists a new tool,” said co-author Philip Marshall of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at the U.S. Department of Energy’s SLAC National Accelerator Laboratory at Stanford University.

“The Hubble constant is an important factor in Hubble’s Law: v = H0 * r, which relates the apparent recession speed of a galaxy v to its distance from our galaxy, r,” Marshall said.

SADAF MOGHIMI can be reached at features@theaggie.org.


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