Milky Way, How Do You Zip Through the Universe Like That?

A recent paper in Nature Astronomy offers up an explanation as to why our galaxy is zooming through the universe as fast as it is. My editor asked me to summarize the findings. There was math in that paper, you guys. Lots of math. Also, the paper mentioned a Wiener filter, and I’m not entirely sure, but I think my editor is punking me.

Wiener filters aside (it’s actually a well-known signal processing filter meant to separate noise from signal), the findings are fascinating but pretty dense. Here’s what you need to know.

First, the universe is expanding. You probably knew that already. Most of the galaxies we can see around our own are moving away from us at a speed called the Hubble constant. Named after astronomer Edwin Hubble, this rate is about 160 kilometers (100 miles) per second per million light-years. It’s the standard expansion rate of the universe.

In addition to this standard expansion rate you have specific velocities associated with various galaxies. This is because of gravity. It’s the weakest of the universe’s four fundamental forces, but on the cosmic scale it has a big impact. The force of gravity depends upon mass and distance. As massive objects get closer to each other, they exert a greater gravitational pull on each other. Here’s a quick refresher on those four critical forces:
Right now, the Milky Way is moving closer to the center of a massive collection of galaxies called Laniakea. It’s our home cluster. But here’s the curious thing: Based on the mass and distance of the involved parties, we can account for only some of our galaxy’s motion. Something else has to be adding to this effect. What could it be?

The answer is gravitational repulsion, which is somewhat confusing. After all, gravity attracts, right? So how does this work?

Imagine that you’re standing in the center of a room. This is the universe. You’ve got a Hula-Hoop around you. Tied to the hoop are ropes that extend like spokes from a hub to each side of the room. At the other end of each rope is a person. The people represent neighboring galaxies.

All the people in the room are the same size and have the same strength, which means all the galaxies surrounding you have an equal mass and are equally distant from you. Each person picks up a rope and starts to pull. What happens to you? You don’t go anywhere, because all the forces balance each other out. The Hula-Hoop will be suspended above the floor but won’t make a move to one side or the other.

Now imagine that on one side of the room you replace a couple of those folks with some weightlifters. They represent larger galaxies, which exert a stronger gravitational pull than smaller ones because of their mass. Everyone starts to pull. This time, you and the hoop start to move across the floor toward the weightlifters. But the people on the other side of the room are still pulling their ropes, so you’re not going as fast as you could if there were no people on that side of the room.

Here’s the third and final analogy. You’re in that same room. You have three or four weightlifters on one side of the room but just one average size person on the other side. Now when the ropes are being pulled you’ll move much faster toward the weightlifters because there’s less resistance.

That’s gravitational repulsion — the average size person represents an underdense area of the universe, which cannot exert a strong gravitational pull on anything. Gravity can’t actually repulse anything as it’s an attractive force. But if you have an underdense area of the universe, it offers up less resistance, which on the cosmological scale is the same as repulsion. What’s really happening is that the overdense areas are exerting a largely unopposed gravitational pull on your galaxy.

That’s what we’re seeing with the Milky Way. An overdense section of the universe is exerting a strong gravitational pull on our galaxy, and there’s an underdense section on the opposite side. Scientists are calling it a dipole repeller.

The most important thing to take away from this is that this explanation nicely takes care of some nagging questions scientists have had about the speed of our galaxy moving through the universe. The second most important thing to remember is that my editor made me go through a lot of math to get here.

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