Scientists generally agree that the universe is expanding, and that the rate of expansion is accelerating, but exactly how fast that’s happening is up for debate. Now, astrophysicists at Clemson University have come up with a new figure for this measure – called the Hubble Constant – by studying how gamma rays interact with the background radiation of the universe.

The Hubble Constant is named after Edwin Hubble, the astronomer who first discovered that the universe is expanding. Einstein himself had actually found this in earlier equations, but assumed he was wrong and rejigged his calculations to model a static universe. He soon conceded to Hubble, calling the assumption his greatest ever blunder.

In 1929, the first number Hubble attributed to his Constant was 500 km per second per megaparsec (km/s/Mpc) (310.7 mi/s/Mpc), where a megaparsec is about 3.26 million light-years. Basically, that means that more distant galaxies are moving away from us at faster speeds than those closer by. Since then, the Hubble Constant has been constantly refined, and in the last 20 years or so many different methods of measuring it have placed it at around 70 km/s/Mpc (43.5 mi/s/Mpc).

And now, the Clemson team has arrived at a new figure: 67.5 km/s/Mpc (41.9 mi/s/Mpc). The team came to this conclusion by analyzing data from the Fermi Gamma ray Space Telescope and Imaging Atmospheric Cherenkov Telescopes, to determine how gamma rays from distant sources are interacting with the “fog” that permeates the universe.

This fog is also known as the extragalactic background light (EBL), and it’s made up of all the ultraviolet, visible and infrared light that’s emitted by stars and other objects. When gamma rays interact with the EBL, they leave an imprint that can be analyzed to determine new clues about their long journeys – and how much longer those journeys are becoming.

“What we know is that gamma-ray photons from extragalactic sources travel in the universe toward Earth, where they can be absorbed by interacting with the photons from starlight,” says Marco Ajello, an author of the study. “The rate of interaction depends on the length that they travel in the universe. And the length that they travel depends on expansion. If the expansion is low, they travel a small distance. If the expansion is large, they travel a very large distance. So the amount of absorption that we measured depended very strongly on the value of the Hubble Constant. What we did was turn this around and use it to constrain the expansion rate of the universe.”

The new measurement of 67.5 km/s/Mpc means the universe may be expanding slower than is generally believed. In 2012, for instance, a study using the Spitzer Space Telescope calculated the Hubble Constant to be 74.3 km/s/Mpc (46.2 mi/s/Mpc). Another, using Hubble’s namesake space telescope, put the number at 73.2 km/s/Mpc (45.5 mi/s/Mpc).

While the slight change won’t really mean much to the average person here on Earth, refining the Hubble Constant is important in understanding the universe’s past, present and future.

“The astronomical community is investing a very large amount of money and resources in doing precision cosmology with all the different parameters, including the Hubble Constant,” says Dieter Hartmann, an author of the study. “Our understanding of these fundamental constants has defined the universe as we now know it. When our understanding of laws becomes more precise, our definition of the universe also becomes more precise, which leads to new insights and discoveries.”

The research was published in the *Astrophysical Journal*.

Source: Clemson University

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What would this be as an acceleration that makes sense on earth, like in ft per second squared? I wonder what the effect of this would be in terms of the equivalence principle, i.e how many G's would this represent?

Both seem obvious and undeniable - you're looking right at it - completely "straightforward" and "unarguable", until you get to the next step in understanding.

That's how science rolls: yesterday's 100% certainty is tomorrow's "Whoops, we had it wrong".