Why should galaxy collisions be more common in the past?

Galaxy Collisions: A Cosmic Traffic Jam of the Early Universe

Galaxies, those sprawling islands of stars, aren’t always solitary creatures. They bump into each other, merge, and generally wreak havoc on a cosmic scale. We call these encounters galaxy collisions, and while they still happen today, they were way more common back in the day, in the universe’s younger years. So, what made the early universe such a demolition derby for galaxies? Let’s dive in.

The biggest reason for this galactic pile-up? Think of it as a cosmic traffic jam. Back then, the universe was a much smaller place. Imagine squeezing all the galaxies we see today into a space a fraction of the size. Talk about crowded! This meant galaxies were packed much closer together, like sardines in a can. Naturally, with less elbow room, they were far more likely to crash into each other.

Now, picture the universe as a giant balloon being inflated. That’s essentially what’s happening with the expansion of the universe. As the balloon gets bigger, the dots drawn on its surface (representing galaxies) get further apart. The same is true for galaxies. As the universe expands, the space between galaxies grows, making collisions less frequent. Makes sense, right?

Here’s another piece of the puzzle: galaxies didn’t just pop into existence fully formed. Nope, they grew over time, like cosmic Lego creations. The Lambda-CDM model, which is basically our best understanding of how the universe works, tells us that galaxies started small. Tiny fluctuations in the early universe, amplified by gravity, created pockets of dark matter. These pockets then attracted regular matter, which eventually cooled and formed stars and baby galaxies. These little guys then merged and merged, building up into the grand spiral and elliptical galaxies we see today. This merging frenzy was especially intense in the early universe, when everything was closer together.

Astronomers use something called “redshift” to measure how far away and how far back in time we’re looking. It’s like a cosmic time machine! Studies of galaxy merger rates at different redshifts show a clear trend: the further back we look (higher redshift), the more mergers we see. It’s like looking at old photos and realizing how much crazier the parties were back in college!

Dark matter, that mysterious stuff that makes up most of the universe’s mass, also plays a role. It acts like a kind of cosmic glue, holding galaxies together and pulling them towards each other. In the early universe, with dark matter also more densely packed, this gravitational pull was even stronger, leading to more collisions.

These early collisions weren’t just random fender-benders. They had a huge impact on how galaxies evolved. Mergers can trigger bursts of star formation, like a cosmic firework display. They can also scramble the shapes of galaxies, turning elegant spirals into blob-like ellipticals. And they might even feed the supermassive black holes that lurk at the hearts of most galaxies.

So, to sum it up, galaxy collisions were more common in the early universe because galaxies were simply closer together. The expanding universe has thinned things out over time, making these collisions less frequent. These ancient mergers were a key factor in shaping the galaxies we see today, influencing everything from their size and shape to their star formation activity. By studying these cosmic crashes, we can learn a ton about the history of the universe and how galaxies like our own Milky Way came to be. It’s a bit like being a cosmic archaeologist, digging up the past to understand the present.