Researchers have discovered a possible way to detect and measure dark energy by studying the motion between the Milky Way and the Andromeda galaxy. The technique, which is still in its early stages, can estimate an upper limit on the cosmological constant, a simple model of dark energy, that is five times higher than the value determined for the early universe.
Scientists have used very distant galaxies to study dark energy since it was first discovered in the late 1990s, but have yet to detect it directly. However, researchers at the University of Cambridge found that by studying how the Andromeda Galaxy and the Milky Way move toward each other given their shared mass, they could place an upper limit on the value of the cosmological constant, the simplest model of dark energy. The upper limit they found is five times higher than the value of the cosmological constant that can be detected from the early universe.
Although the technology is still in its early stages of development, it may be possible to detect dark energy by studying our own cosmic neighborhood, the researchers say. The findings were published in The Astrophysical Journal Letters.
Everything we can see in our world and in the sky - from tiny insects to giant galaxies - makes up just five percent of the observable universe. The rest is dark matter: scientists believe that about 27 percent of the universe is made of dark matter, which holds objects together, while 68 percent is dark energy, which pushes objects apart.
Lead author Dr David Benisti, from the Department of Applied Mathematics and Theoretical Physics, said: "Dark energy is an umbrella term for a family of models that can be added to Einstein's theory of gravity. Its simplest version is known as the cosmological constant: a constant energy density that pushes galaxies away from each other."
The cosmological constant was an improvised addition by Einstein in his general theory of relativity. From the 1930s to the 1990s, the cosmological constant was set to zero until an unknown force—dark energy—was discovered that was causing the expansion of the universe to accelerate. However, there are at least two big problems with dark energy: We don't know what it is, and we haven't directly detected it.
Astronomers have developed a variety of methods to detect dark energy since it was first discovered, most of which involve studying objects in the early universe and measuring how quickly they are moving away from us. Deciphering the effects of dark energy billions of years ago is no easy task: Because dark energy is a weak force between galaxies, it is easily overcome by much stronger forces within galaxies.
However, there is one region of the universe that is surprisingly sensitive to dark energy, and it's right in our cosmic backyard. Andromeda is the closest galaxy to our Milky Way, and the two galaxies are colliding. As the distance gets closer, the two galaxies will begin to orbit each other - very slowly. One orbit takes 20 billion years. However, due to the immense gravity, the two galaxies will begin to merge and collide with each other before the individual orbits are complete, in about 5 billion years.
Benisti said: "Andromeda is the only galaxy that is not far away from us, so by studying its mass and motion, we may be able to make some judgments about the cosmological constant and dark energy."
Benisti and his co-authors - Professor Anne Davis of DAMTP and Professor Wyn Evans of the Institute of Astronomy - conducted a series of simulations based on the best estimates of the masses of the two galaxies and found that dark energy is affecting the interaction of Andromeda and the Milky Way.
"Dark energy affects every pair of galaxies: gravity wants to pull the 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 masses, we can determine an upper limit on the cosmological constant, which is about five times higher than what we measure from elsewhere in the universe."
While the technique may prove of great value, it cannot directly detect dark energy, the researchers say. Data from the James Webb Telescope (JWST) will provide more precise measurements of Andromeda's mass and motion, which will help lower the upper limit of the cosmological constant.
In addition, by studying other pairs of galaxies, it will be possible to further refine this technique and determine how dark energy affects our universe. "Dark energy is one of the biggest mysteries in cosmology. Its effects may vary with distance and time, but we hope this technique can help unravel that mystery," Benisti said.