Researchers from the University of Cambridge have found that it may be possible to detect dark energy by studying Andromeda – the galaxy next door.
Since first identified in the 1990s, scientists have used distant galaxies to study dark energy – the force that makes up more than two-thirds of the Universe and is responsible for its accelerating expansion. However, attempts to detect dark energy have been futile.
Now, in a new study reported in The Astrophysical Journal Letters, researchers from the University of Cambridge found that by studying how Andromeda and the Milky Way are moving toward each other given their collective mass, they could place an upper limit on the value of the cosmological constant. This is the simplest model of dark energy.
They found that the upper limit is about five times higher than the value of the cosmological constant that can be detected from the early Universe.
Although the technique is still early in its development, the team argues that it could be possible to detect dark energy by studying our own cosmic neighbourhood.
The Universe is 68% dark energy
Everything we can see in our world and in the skies makes up just 5% of the observable Universe. The rest is dark.
It is believed that around 27% of the Universe is comprised of dark matter, which holds objects together. Dark energy, which pushes objects apart, makes up 68% of the Universe.
“Dark energy is a general name for a family of models you could add to Einstein’s theory of gravity,” said first author Dr David Benisty from the Department of Applied Mathematics and Theoretical Physics.
“The simplest version of this is known as the cosmological constant: a constant energy density that pushes galaxies away from each other.”
The cosmological constant was temporarily added by Einstein to his theory of general relativity. The cosmological constant was set at zero from the 1930s to the 1990s until scientists discovered that an unknown force, now known as dark energy, was causing the expansion of the Universe to accelerate.
Dark energy has at least two big problems; however, we don’t know what it is, and we haven’t directly detected it.
Efforts to detect dark energy
A variety of methods to detect dark energy have been developed since it was identified. Most of these methods involve studying objects from the early Universe and measuring how quickly they are moving away from us.
Unpacking the effects of dark energy from billions of years ago is not easy. This is because it is a weak force between galaxies, meaning it is easily overcome by the much stronger forces inside galaxies.
However, there is one region that is surprisingly sensitive to dark energy – the Andromeda galaxy.
About the Andromeda galaxy
The Andromeda galaxy is the closest to our own Milky Way, and the two galaxies are on a collision course. As they draw closer, the two galaxies will start to orbit each other very slowly.
A single orbit will take 20 billion years, but due to massive gravitation forces, well before a single orbit is complete, the two galaxies will start merging and falling into each other.
“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,” said Benisty, who is also a Research Associate at Queens’ College.
Dark energy is affecting how Andromeda and the Milky Way are orbiting each other
The researchers used a series of simulations based on the best available estimates of the mass of both galaxies and discovered that dark energy is affecting how Andromeda and the Milky Way are orbiting each other.
“Dark energy affects every pair of galaxies: gravity wants to pull galaxies together, while dark energy pushes them apart,” said Benisty.
“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.”
Future uses of the research
Although the technique could prove immensely valuable, it is not yet able to directly detect dark energy.
Data from the James Webb Telescope will provide far more accurate measurements of Andromeda’s mass and motion. This could help reduce the upper bounds of the cosmological constant.
As well as this, by studying other pairs of galaxies, it could be possible to further refine the technique and determine how dark energy affects our Universe.
“Dark energy is one of the biggest puzzles in cosmology,” said Benisty.
“It could be that its effects vary over distance and time, but we hope this technique could help unravel the mystery.”
