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One of the coolest things about observing the universe is that the farther you look out, the older the stuff you’re looking at. So in a way, telescopes become time machines.

But that doesn’t mean there aren’t super old things right next door. In fact, over the past decade, astronomers have found dozens of small faint galaxies orbiting our own Milky Way. And they’ve turned out to be some of the oldest galaxies we’ve ever seen.

And by studying these neighborhood old-timers, we can get a glimpse of how the universe evolved. Last week in the Astrophysical Journal, a team of astronomers used these super faint galaxies to shine a light on one particular period of our history. Some of the headlines about this paper have made it sound like the researchers only just discovered these old galaxies, but we actually already knew that they existed.

Until 2006, astronomers only knew of a dozen or so small satellite galaxies orbiting the Milky Way. Our equipment just wasn't sensitive enough. But by 2016, they’d found 54, and some of them were extremely old.

And that's just around the Milky Way. Other satellites like these have also been found around the nearby galaxy Andromeda. And there should be plenty around other galaxies, too, we just don’t have the tech required to see them yet.

What’s new about this research is that it’s using those satellite galaxies to learn more about the epoch of reionization, a time in the universe’s history when star formation came to a near halt. About 380,000 years after the Big Bang, the universe was very, very dark. It was the beginning of the so-called “Cosmic Dark Ages,” the period when gravity hadn’t yet pulled together clumps of hydrogen and helium to form stars.

It lasted for about 100 million years, until the nuclear furnaces of the first stars began spewing out electromagnetic radiation of their own. Some of that radiation came in the form of ultraviolet light, which had enough energy to knock electrons out of the leftover hydrogen floating around, ionizing it. Or more like re-ionizing it, because before the dark ages all that hydrogen started out as charged ions in the first place.

That’s why we call this period of the universe the epoch of reionization. The reionized hydrogen was now so hot, and moving around so fast, that gravity couldn’t pull it together to make new stars. But not all galaxies stopped forming new stars; the most massive ones could still do that.

Still, on an astronomical scale, most of the galaxies back then were super small, so their growth was stunted. It took about a billion years for everything to cool back down to the point where new galaxies could form, and most existing ones could grow. But there’s still a lot we don’t know about the epoch of reionization.

For one thing, we’re not exactly sure when it happened. And that’s where last week’s paper comes in. Even after reionization ended, even now, these small galaxies retain a record of that limited growth.

To help them read that record, the authors of this new paper combined a computer model with the real data from a collection of the dwarf galaxies surrounding the Milky Way and Andromeda. Specifically, they modeled the luminosity, or light output, of each galaxy and found two distinct populations: one group of faint satellites and another of brighter ones, separated by a gap. That gap was caused by reionization!

The areas with less matter in them didn’t have enough gravity to turn all that ionized hydrogen into stars, so their growth was stunted, and they stayed dimmer. But this was more than just evidence of reionization. The team was also able to use the number of fainter galaxies to get a better idea of when reionization happened.

The later it happened, the more faint galaxies you’d expect to see, because more would have been able to form before their star formation shut down. The team’s model fits best if the universe reached 100% reionization when it was about 950 million years old. But that age is a lot older than other research teams have estimated.

A group called the Planck collaboration, for example, calculated a reionization age of only 570 million years. Part of the difference might just come from the teams using different definitions, because the Planck analysis was looking to calculate the time when the universe was only 50% ionized. But it does mean that we’ll need further studies before we can really pin down when the epoch of reionization happened.

Another reason we’ll need more research is that the data we have is incomplete. There could be a lot more faint galaxies out there we haven’t found yet, and their properties could throw models for a loop. And, since the data only came from our local group, the time we calculate for reionization could be different from what we’d find in other places that had different sources ionizing hydrogen.

But one way to check the validity of this research is to see if it can be used to make predictions. The team used their model to propose the number of satellite galaxies that should be around the Large Magellanic Cloud, a galaxy that is itself a satellite of the Milky Way. Their predicted number is 26, plus or minus 10, and with only a 68% level of confidence that it’s even within that range.

But it is a jumping off point. And by looking for more clues in our galactic backyard, we might someday reveal a whole lot more about what the universe was like when it first began. Thanks for watching this episode of SciShow Space News!

To learn more about the epoch of reionization, you can check out our episode on how the first stars transformed the universe. [♪ OUTRO].