The Milky Way and Andromeda are set to collide in a few billion years but are still over two million light-years apart. Cosmologically, this is right next door, but it could still be far enough to measure the effect of dark energy, which is pushing back on the galaxies coming together. According to a new study, the movement of the two galaxies towards each other could give an upper limit on the cosmological constant model of dark energy. Other merging pairs of galaxies could provide more data and help refine the measurements.
The best measurements we have of local group infalling are of our galaxy and the Andromeda Galaxy. We have high-resolution images of the stars in Andromeda. We know the galaxy’s overall motion and mass. We also know the motion and mass of our own galaxy. With that, we can calculate the rate of gravitational acceleration between the two galaxies, and we can compare it to the observed acceleration.
From the Andromeda data, the team calculated an upper bound about 5 times greater than the value for dark energy measured by the Planck satellite. In other words, the value for dark energy seen in the cosmic microwave background is well within the range allowed by this new study, which isn’t surprising. But this is just a first step. With observations of other members of the local group, the team estimates they can lower the upper bound to around 1.7 times the Planck value.
That might not seem useful at first glance, but it would be a strong enough constraint to rule out some alternative models. In general relativity, dark energy is predicted to be uniform throughout the cosmos. Some alternatives predict that dark energy correlates with galactic clusters, meaning that local dark energy levels should be higher than the overall average measured by Planck.
Dark energy will still be a mystery for a while, but this study shows that we don’t just need to look at the most distant galaxies to understand it. We can also look at the galaxies in our own backyard.
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