They may have arisen less than 200 million years after the Big Bang, but the Universe was a very different place back then.
When you look out beyond the Milky Way today, as far as we’ve ever been able to see, there are galaxies absolutely everywhere. Even if you take a dark patch of sky without stars, galaxies, or any known matter at all, if you look deep enough, thousands upon thousands of galaxies will be your reward. All told, there are an estimated two trillion galaxies within the observable Universe, stretching for tens of billions of light years in all directions.
Yet despite all the galaxies we’ve seen, never have we gone far enough back to encounter the very first ones ever made in the Universe. The current record-holder, despite its light arriving from when the Universe was only 400 million years old — 3% of its present age — is already evolved and full of old stars. The first galaxies come from a time before we’ve ever probed. But if we get lucky, we’ll get there soon. Here’s what those galaxies should be like.
The galaxies we see today are old. They’re massive, they’re huge, and they’re full of a variety of stars. For the most part, there are lots of heavy elements in there: approximately 1–2% of all the atoms present in galaxies (by weight) are something other than hydrogen or helium. That’s a big deal, considering that the Universe was born without carbon, nitrogen, oxygen, silicon, sulfur, iron, or practically any of the elements we find in stars and galaxies today.
But it took billions of years and innumerable generations of stars to bring them about today. If we look back to the distant Universe, we also look back in time, and find that galaxies were vastly different back then from how they appear today. They were smaller, bluer, more numerous, and poorer in these heavy elements than the galaxies we have today. Over the history of the Universe, galaxies have evolved substantially.
But how did the very first ones form? And what was the Universe like when they did?
The cosmic story that brought them to us saw a number of important steps happen first. Matter won out over antimatter; atomic nucleiand then neutral atoms formed; the first generation of stars were born, died, and gave rise to the second generation of stars. But even after all these steps, there were still no galaxies around.
The simple reason? The smallest-volume cosmic scales gravitationally collapse first, while the larger scales take longer.
Think about two important factors at play here: gravity and the speed of light. Gravity is the only mechanism that can bring ever larger and larger clumps of matter together. It’s limited, however, by the speed at which things can gravitationally grow.
Imagine you start with a small mass, over and above whatever the average density is. If you have some additional mass for it to attract that’s a light-year away, it will take that matter an entire year to feel the force from the mass, since the gravitational force only travels at the speed of light. But if there’s an additional mass a hundred, or a million, or a billion light-years away, you have to wait for all that additional time to pass. Gravity isn’t instantaneous; it only travels at the speed of light.
So what happens, then, when you finally get a large amount of mass together in one place, from gravitational collapse of your first stars and star clusters? They attract one another, and can finally do so effectively.
But the timescale for one massive star cluster attracting another is going to be much longer than the timescale for individual star clusters to form. Instead of looking at volumes of space that might be a few thousand light years on a side — the scale of what might collapse to form a star cluster — you need to look on scales tens or hundreds of times as large to bring together enough matter to start to make the first galaxies.
But remember, also, that the original overdensities that lead to both star clusters and galaxies are only one-part-in-about-30,000, meaning that these overdensities need to grow over large amounts of time. If it takes gravity tens or hundreds of times as long to reach between star clusters than it does for an individual cluster, you might worry that it takes tens or hundreds of times as much time to make galaxies than stars.
Luckily, this isn’t true! It takes longer, but not by nearly that amount. The power of an attractive gravitational force is cumulative, so it’s basically like starting a clock on a delay. The “star cluster” clock starts a few million years after the Big Bang; the “galaxy” clock begins perhaps ten million years after that, and starts with a handicap: it has farther to go to collapse.
But this is okay! This is how large-scale structure formation works. We have density imperfections on all scales, and they grow as soon as enough time has passed for gravity to attract matter a certain distance away.
We form the first star clusters quickly, after perhaps 50-to-100 million years. We form the second generation of stars almost immediately after, because the first generation of stars lives-and-dies so fast, triggering a new generation shortly thereafter.
Then we have to wait tens of millions of years for the first galaxies to form, since that requires star clusters to attract one another across the abyss of empty space, where they finally merge. And it will take even longer timescales for large galaxies and then galaxy groups and galaxy clusters to arise.
The hardest challenge for finding these first galaxies is that there haven’t yet been enough stars formed throughout the Universe to ionize all the neutral atoms in intergalactic space. Protons and electrons are still bound to one another, and will remain so until the Universe is flooded with enough sustained ultraviolet light to permanently kick those electrons off of their atoms.
This means that the light from the first stars (and first galaxies) gets absorbed by those atoms; the Universe is still opaque. The earliest galaxies we’ve ever seen date back to 400 million years after the Big Bang, and were only discovered because they are located along a serendipitously more-ionized-than-average line of sight.
However, we can do a little better than that. We’ve observed a slew of galaxies from a little bit later on than that, and we’ve been able to determine how old the stars in them are!
The galaxy MACS1149-JD1 is the second-most-distant galaxy ever found, whose light arrives from 530 million years after the Big Bang. Yet, when we observe it, we find that the stars inside of it are approximately 280 million years old, meaning that they formed in a massive burst just 250 million years after the Big Bang.
These massive bursts of star formation don’t simply occur because you had a star cluster; they occur when large mergers happen, giving rise to what astronomers call a starburst. Colliding gas causes material to collapse, which can trigger massive amounts of new star formation. Much larger and more powerful than a mere collapsing star cluster, these should signify the true first galaxies.
They will be larger, contain more stars, be more massive, more luminous, and will leave an unmistakable signature. They will imprint themselves on the Universe. And that imprint will be observable.
Not only will they begin contributing to the reionization of the Universe, but wherever they form stars, we will find electrons recombining with their ionized nuclei. That act, when it occurs for hydrogen atoms, has a 50% chance of forming a configuration where the spins are aligned (up-up or down-down) and a 50% chance where the spins will be anti-aligned (up-down or down-up).
The up-down or down-up configurations are more stable, by a tiny amount. If you form the aligned configuration, it will transition down to the anti-aligned configuration on timescales of around 10 million years. And when it transitions, it emits a photon of a very specific wavelength: 21 centimeters.
That photon then travels throughout the Universe, arriving at our eyes, redshifted by the expansion of the Universe. Earlier in 2018, there was a paper that came out, albeit very controversially, that claimed to detect this signature for the first time. Impressively, the timescale for when these first galaxies ought to have formed coincides very nicely with these observations.
Whenever “cosmic dawn” occurred, whenever these first galaxies arrive, every piece of evidence points to a timetable of 200–250 million years as the main origin of the 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.
Meanwhile, the regions with smaller initial overdensities will continue to grow, forming stars for the first (or second) time in places where they didn’t form earlier. The great cosmic story of forming structures doesn’t happen all at once, but in bits-and-pieces throughout the cosmos. But with the first galaxies, the race to form galaxies like our own has officially begun.
Further reading on what the Universe was like when:
- What was it like when the Universe was inflating?
- What was it like when the Big Bang first began?
- What was it like when the Universe was at its hottest?
- What was it like when the Universe first created more matter than antimatter?
- What was it like when the Higgs gave mass to the Universe?
- What was it like when we first made protons and neutrons?
- What was it like when we lost the last of our antimatter?
- What was it like when the Universe made its first elements?
- What was it like when the Universe first made atoms?
- What was it like when there were no stars in the Universe?
- What was it like when the first stars began illuminating the Universe?
- What was it like when the first stars died?
- What was it like when the Universe made its second generation of stars?