Category: large-scale structure

What Was It Like When The Universe Made The Ve…

What Was It Like When The Universe Made The Very First Galaxies?

“The first galaxies required a large number of steps to happen first: they needed stars and star clusters to form, and they needed for gravity to bring these star clusters together into larger clumps. But once you make them, they are now the largest structures, and can continue to grow, attracting not only star clusters and gas, but additional small galaxies. The cosmic web has taken its first major step up, and will continue to grow further, and more complex, over the hundreds of millions and billions of years to follow.”

For millions upon millions of years, there were no stars in the Universe. As the first one finally formed, the star clusters that birthed them became the largest structures in the Universe. Yet these were too small and limited to be considered galaxies. For that, we need more than one massive star cluster in the same place. We need for them to merge, triggering a starburst and creating a larger, more luminous object. It takes much longer for that to happen than to merely form stars, and the Universe was a very different place by then. The Big Bang may have started everything off uniformly and without anything more than the seeds of structure, but gravity, and time, are awfully powerful tools.

Come learn what the Universe was like when we made the very first galaxies. It’s a story you won’t soon forget!

Why Isn’t Our Universe Perfectly Smooth? “Thi…

Why Isn’t Our Universe Perfectly Smooth?

“This seems, at first glance, to pose a tremendous problem. If inflation stretches space to be flat, uniform, and smooth, indistinguishably so from perfection, then how did we arrive at a clumpy Universe today? Both Newton’s and Einstein’s theories of gravity are unstable against imperfections, meaning that if you start with an almost-but-not-quite perfectly smooth Universe, over time, the imperfections will grow and you’ll wind up with structure. But if you start with perfect smoothness, with literally no imperfections, you’re going to remain smooth forever. Yet this doesn’t jibe with the Universe we observe at all; it had to have been born with imperfections in its matter density.”

One of the great successes of cosmic inflation is to set up the initial conditions for the Big Bang that we knew we needed, including giving us a Universe that had the same temperature and density everywhere. But this couldn’t have been a perfect smoothness, otherwise we’d never have formed stars, galaxies, and the cosmic large-scale structure we observe today in the space we inhabit. So how did we come to be clumpy? The Universe must have been born with initial imperfections in them. If you treat inflation as a classical field, you’ll never get them that way, but if you recognize that it’s a quantum field, with the associated quantum fluctuations that we know must be there, the whole story changes. Not only does inflation give you these cosmic imperfections, but it gives you the full spectrum of them that you can then go check against observations.

These predictions were made in the early 1980s, and were verified decades later by COBE, WMAP, and Planck. It’s a huge victory for a great scientific theory!