I get e-mail.
Most messages are press releases about astronomical discoveries—okay, scratch that; most of them are spam, however science bulletins are a straightforward second place. However I additionally get questions from readers asking about numerous facets of the universe that they’re struggling to know.
I like this! For one factor, it reveals that folks actually are inquisitive about science, and it’s important to dig that. For an additional, it offers me an opportunity to elucidate counterintuitive ideas which can be most likely bugging different individuals as nicely.
On supporting science journalism
For those who’re having fun with this text, contemplate supporting our award-winning journalism by subscribing. By buying a subscription you’re serving to to make sure the way forward for impactful tales concerning the discoveries and concepts shaping our world as we speak.
One perennial query I get issues cosmology (an understandably puzzling subject): If the universe is increasing, how can galaxies collide? Shouldn’t they be flying away from one another, not towards each other?
There are literally two causes galaxies can collide in an increasing cosmos. One is that the growth solely dominates on very massive scales, and the opposite is that the growth competes in opposition to gravity.
Okay, very first thing first: the universe is in actual fact increasing. We’ve identified this for greater than a century now, and it’s the premise for contemporary cosmology. This concept is known as the massive bang mannequin—which is an unlucky title as a result of it brings to thoughts a cosmos increasing like an explosion, with galaxies shifting away from one another by way of house like shrapnel.
However in actual fact house itself is increasing, and that’s completely different. It’s not that galaxies are shifting by way of house; it’s that the growth of house is carrying them together with it. This has a number of deeply unusual implications. One is that the farther away a galaxy is from us, the quicker it seems to be receding.
Think about a meterstick that’s product of some extraordinarily versatile materials. The 2 endpoints are, in fact, one meter aside. On the middle, you may mark two factors which can be one centimeter aside.
Now seize every finish of this fake meterstick and stretch them out in order that the stick is now two meters lengthy. (Ask an imaginary pal for assist.) The 2 endpoints have moved a meter farther aside, touring at, say, one meter per second. However these marks you made earlier that had been one centimeter aside at the moment are two centimeters aside as a result of the entire meterstick stretched. Which means these two factors moved away from one another at a velocity of just one centimeter per second, a lot slower than the endpoints. In different phrases, the farther away two factors are in an increasing scale, the quicker they transfer away from one another.
That’s the universe in a nutshell. We see extra distant galaxies receding from us extra quickly, and we will even measure that change of velocity over distance. Very roughly talking, for each megaparsec in distance (3.26 million light-years, a handy unit for astronomers however not for anybody else), house is increasing at a further 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 fairly quick. However a galaxy one megaparsec away is just shifting away at 70 km/sec. Whereas that’s nonetheless speedy—1 / 4 of one million km/hour!—it’s attainable for galaxies to journey quicker than that by way of house, shrapnel-style.
The Andromeda galaxy affords a fantastic instance. It’s the closest massive spiral to our Milky Means, and we each belong to a regional clump of galaxies referred to as the Native Group. At 2.5 million light-years from us, Andromeda ought to be receding at 50 km/sec or so, however in actual fact it’s heading our approach at roughly 110 km/sec. It is because each galaxies are shut sufficient collectively that every is pulled by the gravity of the opposite—pulled so onerous, in actual fact, that their mutual velocity is much bigger than the universe’s capacity to pry them aside. That is additionally why Andromeda and the Milky Means could sometime collide and even merge, although not for maybe one other eight billion years.
And this brings us to the second motive galaxies can nonetheless collide in an increasing universe. We consider gravity as a pressure pulling issues collectively. However in line with Einstein’s basic idea of relativity, gravity is definitely a bending of spacetime, like a dimple in a sheet. If an object passes by one thing with a number of mass, comparable to a planet or a galaxy, that warping causes the item’s path to bend, to curve.
If two objects have ample mass and are shifting at comparatively gradual speeds, they are often gravitationally sure, which means their velocities can’t overcome gravity, they usually keep shut collectively in what’s referred to as a closed orbit. A moon orbiting a planet is like that—or two galaxies, such because the Milky Means and Andromeda.
That is the place issues get bizarre. In line with relativity, if house is increasing, it can not broaden inside that sure area. The mutual gravity of the objects inside that area holds them collectively; house expands round that quantity however not inside it. That, in flip, means if two galaxies swoop sufficiently shut collectively, they will nonetheless collide. For a deeper dive into this (so to talk!), my colleague and fellow science author Ethan Siegel has written about this as nicely.
It will get even weirder than this, nevertheless, as a result of we now know the growth of the universe isn’t fixed. In 1998 two groups of astronomers introduced that the growth is accelerating, getting quicker on a regular basis, attributable to a still-mysterious entity referred to as darkish power. This might imply, for some still-theorized behaviors of darkish power, that even house inside a sure area can broaden. This impact can be strongest over the largest-distance scales, so, for instance, the slowest-moving galaxies close to the sting of a galaxy cluster can be misplaced to the growth, stripped away from the cluster just like the outermost leaves on a head of lettuce.
Given sufficient time and relentless cosmic acceleration, each sure construction would get ripped aside—even ones sure by forces aside from gravity, like molecules and even atoms themselves! Astronomers name this concept the massive rip, for apparent sufficient causes, and it’s not a really reassuring destiny. However we actually don’t know what darkish power is or the way it behaves over lengthy intervals of time, so the massive rip is just one attainable state of affairs for the extraordinarily distant future.
So don’t fear: whether or not we’re speaking concerning the large rip or a collision with Andromeda, the timescales at play are so immense that these occasions gained’t occur for eons (if in any respect), so that they don’t actually have an effect on your each day life—until you’re an astronomer, during which case they do. However we get pleasure from eager about such issues and relaying these ideas to you. Hopefully they’ll assist broaden your thoughts.
Leave a Reply