Scientists Have Calculated a New Estimate for the Expansion Rate of the Universe by Using Gamma Rays

Scientists have revealed a new estimate for the expansion rate of the universe using a novel approach.

A team from Clemson University and several institutions around the world examined gamma-ray data collected by the Fermi Gamma-ray Space Telescope and the ground-based Imaging Atmospheric Cherenkov Telescopes, as well as extragalactic background light (EBL) to come up with the new estimate for the so-called "Hubble Constant."

Gamma rays are the most energetic form of electromagnetic radiation and have the shortest wavelength. EBL, meanwhile, is all the accumulated radiation in the universe resulting from star formation processes and active galactic nuclei.

Interactions between gamma-rays and EBL produce a kind of signature which the scientists could observe and use to calculate the Hubble Constant to approximately 67.5 kilometers per second per megaparsec, according to a study published in The Astrophysical Journal.

A megaparsec is one million parsecs—a unit of length used to measure the vast distances between objects beyond our solar system. One parsec is roughly the same as 19 trillion miles or 3.26 light-years.

"Cosmology is about understanding the evolution of our universe—how it evolved in the past, what it is doing now and what will happen in the future," Marco Ajello, an associate professor of physics and astronomy at Clemson, said in statement.

"Our knowledge rests on a number of parameters—including the Hubble Constant—that we strive to measure as precisely as possible," he said. "In this paper, our team analyzed data obtained from both orbiting and ground-based telescopes to come up with one of the newest measurements yet of how quickly the universe is expanding."

Scientists have known that the universe is expanding since American astronomer Edwin Hubble (1889-1953) discovered that all galaxies were moving away from each other at a speed which was linked to their distance from us. You can picture the idea by imagining a balloon (representing the universe) covered in dots (representing galaxies.) As the balloon is blown up, the dots move further and further apart from each other.

According to Hubble—who calculated the universe's expansion rate to 500 kilometers per second per megaparsec—a galaxy located at a distance of two megaparsecs, for example, was moving away from us twice as fast as a galaxy located at a distance of one megaparsec.

The astronomer's estimate for the expansion rate of the universe is now referred to as Hubble's Constant. Since the initial calculations, advances in technology have shown that the real figure is much slower than Hubble predicted. However, there is significant debate among astronomers on just how fast the universe is expanding, because different ways of measuring the constant result in different numbers.

Cherenkov Telescope Array
The team’s analysis paves the way for better measurements in the future using telescopes from the Cherenkov Telescope Array. Daniel López/IAC

The so-called standard model of cosmology states that the universe expanded very quickly early in its history, before slowing down due to the gravitational influence of dark matter, according to the W.M. Keck Observatory. Now, the expansion is becoming faster again, the model suggests, because of a mysterious force known as dark energy.

The issue is, however, that the standard cosmological model was created using traditional Hubble Constant measurements derived from "distant" observations of the cosmic microwave background (CMB)—radiation leftover from the Big Bang 13.8 billion years ago that persists throughout the universe.

But recent observations from other regions of the universe tend to provide different results to these "distant" measurements. This indicates either that there is a problem with the standard model of cosmology, or that there are issues with the CMB measurements.

The latter is very unlikely, according to Chris Fassnacht from the University of California, Davis, who recently came up with an estimate of 76.8 kilometers per second per megaparsec for the Hubble Constant in a study published in the Monthly Notices of the Royal Astronomical Society.

"Therein lies the crisis in cosmology," Fassnacht said in a statement. "While the Hubble Constant is constant everywhere in space at a given time, it is not constant in time. So, when we are comparing the Hubble Constants that come out of various techniques, we are comparing the early universe (using distant observations) versus the late, more modern part of the universe (using local, nearby observations.)"