The Andromeda galaxy might hold the key to measuring dark energy

Dark energy is thought to be responsible for accelerating cosmic expansion and is believed to make up more than two-thirds of the universe.

Scientists haven't directly detected dark energy. Instead, they typically measure the masses of distant galaxies to infer the amount of dark energy.

"Andromeda is the only galaxy that isn't running away from us, so by studying its mass and movement, we may be able to make some determinations about the cosmological constant and dark energy," first author Dr. David Benisty, from the Department of Applied Mathematics and Theoretical Physics, explained in the press release.

Because Andromeda isn't drifting away from us, the researchers found they could measure the collective mass of Andromeda and the Milky Way to place an upper limit on the value of the cosmological constant — the simplest model of dark energy.

"Dark energy is a general name for a family of models you could add to Einstein's theory of gravity," Benisty continued. "The simplest version of this is known as the cosmological constant: a constant energy density that pushes galaxies away from each other."

The value the researchers found is five times higher than the value of the cosmological constant detected from the early universe. They detailed their findings in a new paper in The Astrophysical Journal Letters.

What the Andromeda galaxy tells us about dark energy

Overall, scientists believe that 68 percent of the universe is made up of dark energy and 27 percent of dark matter. This means that all observable matter makes up only five percent of the universe.

As Andromeda and the Milky Way drift closer together, they gradually orbit each other at an incredibly slow pace, with one orbit estimated to take roughly five billion years.

The researchers used computer simulations based on the best estimates of the mass of Andromeda and the Milky Way. They found that dark energy is impacting the orbit of Andromeda and the Milky Way.

"Dark energy affects every pair of galaxies: gravity wants to pull galaxies together, while dark energy pushes them apart," Benisty explained. "In our model, if we change the value of the cosmological constant, we can see how that changes the orbit of the two galaxies. Based on their mass, we can place an upper bound on the cosmological constant, which is about five times higher than we can measure from the rest of the universe."

In the future, the researchers say the James Webb Space Telescope could help to provide much more accurate measurements of Andromeda's mass and motion. This, in turn, could help to vastly improve the accuracy of their new method.