Swapna Krishna

Sep 24th 2021

Invisible Fireworks: What Happens When Two Black Holes Collide


The universe may be expanding, but that doesn’t mean that objects within the universe aren’t moving closer to one another. Despite the fact that things around us are stretching out at an ever-increasing rate, the Milky Way and Andromeda galaxies are on a collision course. What happens when two black holes collide?

As cosmologist Tamara Davis explains in a Scientific American article, this is happening because the universe isn’t governed by just one force. The entire universe might be expanding, but there are forces working at the galactic and intragalactic levels to counteract these effects — namely, gravity. In our neighborhood of the Milky Way, called the Local Group, the gravity attracting the Andromeda and Milky Way galaxies to one another is much greater than the universe-level forces that are pulling them apart, and it will lead to a galaxy collision.

A new study in Astronomy and Astrophysics sheds light on the time frames during which all this will occur. It’ll take about 4 to 5 billion years for the two galaxies to first meet, and eventually, the supermassive black holes at the center of both galaxies will collide.

Collision Doesn’t Happen in an Instant

While “collision” may make this sound like a one-time event, the crashing together of two galaxies is a process that will actually take an incredibly long time. ScienceNews notes that it will take a full 10 billion years for the two galaxies to completely merge into one massive elliptical galaxy, dubbed “Milkomeda” by the paper’s authors.

Even after these two spiral galaxies have become one massive entity, the black holes at their respective centers still will not have touched. That’s because there’s a lot of empty space within galaxies, between stars and solar systems, and both these black holes are at the very center of the two galaxies and will meet at the tail end of the merger.

In the paper, the authors report that the supermassive black holes at the center of the two galaxies will begin orbiting each other when the single Milkomeda galaxy emerges, in roughly 10 billion years. However, it will take another seven billion years beyond that for the supermassive black holes to actually collide, because the velocity at which these black holes move toward one another is dependent on the mass of the stars close to each respective galactic center.

More mass around the black holes means that the forces attracting these two objects together are stronger; once these surrounding stars have been depleted by the black holes, the two will stall out. Eventually, though, there will be a spectacular galaxy collision.

Invisible Fireworks

But what happens when two black holes collide? When the cores of the Andromeda and Milky Way galaxies finally smash together, they will merge into one supermassive black hole. When that happens, it will be an absolutely phenomenal event that’s completely invisible — there won’t be any fireworks in the sky when it happens. That’s because black holes are invisible to the naked eye. However, any intelligent civilization that exists at that point with capabilities that match our own will be able to detect the collision — it will send out absolutely massive gravitational waves detectable by an observatory like LIGO.

It’s important to note that the timing given here is all speculation based on modeling. There’s still a lot of uncertainty because the data researchers are inputting into the model may change. It’s hard to take into account the many factors at hand — such as the velocity at which the galaxies are speeding toward one another or the size of their galactic halos. As a result, the paper’s authors make clear that these are rough estimates.

What About Us?

If you’re wondering if our solar system might survive, the answer is yes and no. Our solar system likely won’t be disrupted by the collision between the two galaxies — after all, the space between stars is massive — but we’re talking about some pretty long timelines here. The time frame is past the expected life of our Sun, which will burn for another 4.5 billion years. As a result, our star won’t survive to be a part of Milkomeda, and as points out, our solar system’s fate is pretty dim after the Sun’s death.

So, by the time the black holes at the center of Andromeda and the Milky Way collide, our solar system won’t be a part of the picture, and humans may have long since colonized some far-off galaxy. That’s probably all for the better, because we probably wouldn’t want to be caught hanging around when two supermassive black holes face off.

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