Cambridge researchers have discovered a new way to measure dark energy

Researchers at the University of Cambridge have discovered a new way to measure dark energy – the mysterious force that makes up more than two-thirds of the universe and is responsible for its rapid expansion – right in our own cosmic backyard.

Researchers have found that dark energy can be detected and measured by studying Andromeda, which is on a slow collision course with our galactic neighbor. milky way.

Since it was first identified in the late 1990s, scientists have used very distant galaxies to study dark energy, but have yet to detect it directly. However, by studying how Andromeda and the Milky Way move relative to each other, the Cambridge researchers can put an upper limit on the value of the cosmological constant, a simple model of dark energy. The upper limit they found is five times higher than the value of the cosmological constant detectable from the early universe.

Although the technique is still in its early stages of development, researchers say it could be possible to detect dark energy by studying our own cosmic surroundings. Results are reported The Astrophysical Journal Letters.

Everything we can see in our world and in the sky—from tiny insects to massive galaxies—makes up just five percent of the observable universe. The rest is dark: Scientists estimate that about 27% of the universe is dark matter, which holds things together, while 68% is dark energy, which pushes things apart.

„Dark energy is the general name for a family of models you can add to Einstein’s theory of gravity,” said first author Dr. David Benisti of the Department of Applied Mathematics and Theoretical Physics. „The simplest version of this is called the cosmological constant: a constant energy density pushes galaxies away from each other.”

The cosmological constant was tentatively added by Einstein to his theory of general relativity. From the 1930s to the 1990s, the cosmological constant was set to zero, and it was discovered that an unknown force – dark energy – was accelerating the expansion of the universe. There are at least two big problems with dark energy, though: We don’t know what it is, and we haven’t detected it directly.

Since it was first identified, astronomers have developed a variety of methods to detect dark energy, most of which study objects from the early universe and measure how quickly they are moving away from us. Disentangling the effects of dark energy billions of years ago is not easy: since it is a weak force between galaxies, dark energy can easily be overcome by stronger forces within galaxies.

However, one part of the universe that is surprisingly sensitive to dark energy is right in our own cosmic backyard. The Andromeda galaxy is closest to our Milky Way, and the two galaxies are on a collision course. As they get closer, the two galaxies begin to orbit each other – very slowly. One orbit takes 20 billion years. However, due to massive gravitational forces, before an orbit is complete, after about five billion years, the two galaxies will begin to merge and fall into each other.

„Andromeda is the only galaxy that is not moving away from us, so by studying its mass and motion, we can make some determinations about the cosmological constant and dark energy,” Benisti said. Queen’s College.

Using a series of simulations based on the best estimates of the masses of the two galaxies, Benisti and his co-authors – Professor Ann Davies from DAMTP and Professor Win Evans from the Institute of Astronomy – discovered how dark energy also affects Andromeda. The Milky Way revolves around each other.

„Dark energy affects every pair of galaxies: gravity tends to pull galaxies together, while dark energy pushes them apart,” Benisti said. „In our model, if we change the value of the cosmological constant, we can see how it changes the orbits of the two galaxies. Based on their mass, we can put an upper limit on the cosmological constant, which is five times larger than what we can measure from the rest of the universe.

Although this technique has proven to be very valuable, it has yet to detect dark energy directly, the researchers say. Data from the James Webb Telescope (JWST) will provide more precise measurements of Andromeda’s mass and motion, which will help to constrain upper bounds on the cosmological constant.

Additionally, by studying other pairs of galaxies, we can further refine the technique and determine how dark energy affects our universe. „Dark energy is one of the biggest mysteries in cosmology,” Benisti said. „Its effects vary over distance and time, but we believe this technique will help unravel the mystery.”

Note: David Benisti, Anne-Christine Davies and N. „Constraining dark energy from local group dynamics”, by Wayne Evans, 8 Aug 2023. Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ace90b