The rate at which the universe is expanding is accelerating. The cosmos is stretching the distance between far-off galaxies, whether as a result of a dark energy field permeating the entire cosmos or as a result of a fundamental property of spacetime. However, neighbouring galaxies in our small group are advancing toward one another. And their direction of descent toward one another may reveal something about the characteristics of cosmic expansion.
The fact that dark energy isn’t a force is among the things we do know about it. Galaxies are not ejecting themselves from one another. Even the farthest galaxies are gravitationally drawn toward one another; yet, cosmic expansion widens the gap between them more quickly than gravity can pull them together. However, the distance between galaxies affects how much cosmic expansion occurs. Galaxies can tumble toward one another faster than they can expand when they are only a few million light-years apart. It explains why galaxies gather over time to form superclusters. Even while the universe overall is still expanding, there are localized clusters of galaxies that merge and collide.
This is true for the Milky Way Galaxy, Andromeda Galaxy, Triangulum Galaxy, Magellanic clouds, and a large number of dwarf galaxies that make up our own local group. They are all being pulled apart by gravity and will eventually combine to form a massive elliptical galaxy. Of course, within an expanding space, these galaxies are slowly tumbling toward one another. This implies that the speed at which they are approaching one another is a little slower than it would be if gravity were the only force at work. This is where a recent study published in The Astrophysical Journal Letters comes in.
Our galaxy and the Andromeda Galaxy provide the best measurements of local group infalling that we currently have. We have clear pictures of the Andromeda galaxy’s stars. The mass and general motion of the galaxy are known. Our own galaxy’s velocity and mass are likewise known. With this information, we may determine the gravitational acceleration rate between the two galaxies and contrast it with the measured acceleration. Although it isn’t accurate enough to detect dark energy directly due to the overlap in uncertainty between the two, it can be used to estimate the quantity of dark energy present in our local group.
The scientists used the Andromeda data to determine an upper bound for dark energy that was around five times higher than what the Planck satellite detected. In other words, it is not surprising that the value of dark energy observed in the cosmic microwave background is well within the range permitted by this new study. But this is only the beginning. The team believes they can lower the upper bound to about 1.7 times the Planck value using observations of additional local group members.
At first look, that would not seem useful, but it would be a strong enough constraint to exclude several potential models. Dark energy is assumed to be uniform throughout the cosmos by general relativity. According to certain alternative theories, dark energy levels should be higher locally than the global average as detected by Planck. These theories assume that dark energy correlates with galaxy clusters.
This research demonstrates that we don’t only need to look at the farthest galaxies to understand dark energy—it is still a mystery. The galaxies in our backyard are also something we can observe.
Reference: Benisty, David, Anne-Christine Davis, and N. Wyn Evans. “Constraining Dark Energy from the Local Group Dynamics.” The Astrophysical Journal Letters 953.1 (2023): L2.
