I get e-mail.
Most messages are press releases about astronomical discoveries—okay, scratch that; most of them are spam, but science announcements are an easy second place. But I also get questions from readers asking about various aspects of the universe that they’re struggling to understand.
I love this! For one thing, it shows that people really are curious about science, and you have to dig that. For another, it gives me a chance to explain counterintuitive concepts that are probably bugging other people as well.
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One perennial question I get concerns cosmology (an understandably puzzling topic): If the universe is expanding, how can galaxies collide? Shouldn’t they be flying away from each other, not toward one another?
There are actually two reasons galaxies can collide in an expanding cosmos. One is that the expansion only dominates on very large scales, and the other is that the expansion competes against gravity.
Okay, first thing first: the universe is in fact expanding. We’ve known this for more than a century now, and it’s the basis for modern cosmology. This idea is called the big bang model—which is an unfortunate name because it brings to mind a cosmos expanding like an explosion, with galaxies moving away from each other through space like shrapnel.
But in fact space itself is expanding, and that’s different. It’s not that galaxies are moving through space; it’s that the expansion of space is carrying them along with it. This has a lot of deeply strange implications. One is that the farther away a galaxy is from us, the faster it appears to be receding.
Imagine a meterstick that’s made of some extremely flexible material. The two endpoints are, of course, one meter apart. At the center, you can mark two points that are one centimeter apart.
Now grab each end of this pretend meterstick and stretch them out so that the stick is now two meters long. (Ask an imaginary friend for help.) The two endpoints have moved a meter farther apart, traveling at, say, one meter per second. But those marks you made earlier that were one centimeter apart are now two centimeters apart because the whole meterstick stretched. That means those two points moved away from each other at a velocity of only one centimeter per second, much slower than the endpoints. In other words, the farther away two points are in an expanding scale, the faster they move away from each other.
That’s the universe in a nutshell. We see more distant galaxies receding from us more rapidly, and we can even measure that change of velocity over distance. Very roughly speaking, for every megaparsec in distance (3.26 million light-years, a convenient unit for astronomers but not for anyone else), space is expanding at an additional 70 or so kilometers per second. So a galaxy that’s, say, 10 megaparsecs away from us is receding at about 700 km/sec.
That’s pretty fast. But a galaxy one megaparsec away is only moving away at 70 km/sec. While that’s still speedy—a quarter of a million km/hour!—it’s possible for galaxies to travel faster than that through space, shrapnel-style.
The Andromeda galaxy offers a great example. It’s the closest large spiral to our Milky Way, and we both belong to a regional clump of galaxies called the Local Group. At 2.5 million light-years from us, Andromeda should be receding at 50 km/sec or so, but in fact it is heading our way at approximately 110 km/sec. This is because both galaxies are close enough together that each is pulled by the gravity of the other—pulled so hard, in fact, that their mutual velocity is far larger than the universe’s ability to pry them apart. This is also why Andromeda and the Milky Way may someday collide and even merge, though not for perhaps another eight billion years.
And this brings us to the second reason galaxies can still collide in an expanding universe. We think of gravity as a force pulling things together. But according to Einstein’s general theory of relativity, gravity is actually a bending of spacetime, like a dimple in a sheet. If an object passes by something with a lot of mass, such as a planet or a galaxy, that warping causes the object’s path to bend, to curve.
If two objects have sufficient mass and are moving at relatively slow speeds, they can be gravitationally bound, meaning their velocities can’t overcome gravity, and they stay close together in what’s called a closed orbit. A moon orbiting a planet is like that—or two galaxies, such as the Milky Way and Andromeda.
This is where things get weird. According to relativity, if space is expanding, it cannot expand inside that bound region. The mutual gravity of the objects inside that region holds them together; space expands around that volume but not inside it. That, in turn, means if two galaxies swoop sufficiently close together, they can still collide. For a deeper dive into this (so to speak!), my colleague and fellow science writer Ethan Siegel has written about this as well.
It gets even weirder than this, however, because we now know the expansion of the universe isn’t constant. In 1998 two teams of astronomers announced that the expansion is accelerating, getting faster all the time, caused by a still-mysterious entity called dark energy. This could mean, for some still-theorized behaviors of dark energy, that even space inside a bound region can expand. This effect would be strongest over the largest-distance scales, so, for example, the slowest-moving galaxies near the edge of a galaxy cluster would be lost to the expansion, stripped away from the cluster like the outermost leaves on a head of lettuce.
Given enough time and incessant cosmic acceleration, every bound structure would get ripped apart—even ones bound by forces other than gravity, like molecules and even atoms themselves! Astronomers call this idea the big rip, for obvious enough reasons, and it’s not a very reassuring fate. But we really don’t know what dark energy is or how it behaves over long periods of time, so the big rip is only one possible scenario for the extremely distant future.
So don’t worry: whether we’re talking about the big rip or a collision with Andromeda, the timescales at play are so immense that these events won’t happen for eons (if at all), so they don’t really affect your daily life—unless you’re an astronomer, in which case they do. But we enjoy thinking about such things and relaying those thoughts to you. Hopefully they’ll help expand your mind.
