Astronomer Edwin Hubble almost a hundred years ago that most galaxies are moving away from the Milky Way. This observation became a key part of modern cosmology because it gave strong proof that the universe is getting bigger and that it started with the Big Bang. Astronomers knew that the pattern was not universal, even in Hubble’s time. Andromeda, the galaxy next to us, is moving toward the Milky Way at a speed of about 100 kilometres per second. This is a notable exception.
Scientists have been trying to figure out another related mystery for about fifty years. Other than Andromeda, most of the big galaxies close to ours seem to be moving away from us instead of being pulled toward us by gravity. This is surprising because these galaxies are close to the Local Group, which is made up of the Milky Way, the Andromeda Galaxy, and dozens of smaller galaxies. Their combined mass should have a noticeable effect on gravity.
A Huge Cosmic Sheet Surrounding the Local Group
A group of researchers from around the world, led by PhD graduate Ewoud Wempe of the Kapteyn Institute in Groningen, thinks they have found the answer. The researchers used advanced computer simulations to find out that the matter around the Local Group is arranged in a wide, flat structure that stretches tens of millions of light-years across. In addition to regular matter, this structure also has dark matter that we can’t see that surrounds galaxies. Huge empty spaces called cosmic voids are above and below this flat area.
The simulations show that this arrangement of matter can accurately reproduce the speeds and positions of the galaxies we see around us. The computer model successfully reproduces the same patterns that astronomers observe in the actual universe.
Making a Virtual Twin of Our Cosmic Neighbourhood
The scientists started with conditions from the early universe to make their model. They used measurements of the cosmic microwave background to guess where matter was located shortly after the Big Bang. A powerful computer then moved this early universe forward in time until it created a system that is like the Local Group today.
The simulations show the masses locations and motions of the Milky Way and Andromeda, as well as the positions and speeds of 31 galaxies that are not part of the Local Group. Researchers call the model a virtual twin of our cosmic environment because it looks so much like the real thing.
When the model includes the flat distribution of matter, the galaxies around us move away from us at speeds that are similar to what we see. Even though the Local Group’s gravity pulls on galaxies in the plane, more mass spread out across that same plane also affects them. This faraway mass balances out the Local Group’s gravity. There are very few galaxies in areas outside the plane, which is why we don’t see things falling toward us from those directions.
A Long-Standing Mystery Has Finally Been Solved
Ewoud Wempe, the lead researcher, says that this study is the first detailed attempt to figure out where dark matter is and how it moves in the area around the Milky Way and Andromeda. “We are looking into all the possible ways that the early universe could have been set up in order to eventually form the Local Group. It’s great that we now have a model that fits with both the current cosmological model and the way things work in our own environment.
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Astronomer Amina Helmi also liked the results, saying that the problem has been hard for researchers for decades. “I am excited to see that we can figure out a mass distribution that matches the positions of galaxies in and just outside the Local Group just by looking at how galaxies move.”
