[ intro ].

One of the things we assume to be fundamental about the universe is that it's the same in all directions. That means that over large scales, matter is spread out pretty evenly, things look more or less the same in every direction, and you'll never find a corner of space with its own laws of physics.

And considering the universe is such an impossibly huge thing to explore, it's comforting to think that somehow, fundamentally, it's pretty simple. It all sticks to the same rules. S o you don't have to explore the whole thing to understand it.

This notion is called the cosmological principle. But there's no law, exactly, that says the universe has to be that way. So, what if it weren't uniform?

The main problem is that it would mean there's only so much we can learn about the universe by looking at it from our little perch in the Milky Way. For example, fundamental laws like general relativity— which deals with gravity— assume that the universe is homogeneous. If it's not, it would mean we might not understand gravitational interactions in other parts of space as well as we think we do.

And our models of cosmology, which describe how the universe began and evolved, might not be as accurate as we think, either, if the forces that push and pull aren't the same in every direction. In short, we tend to assume that studying small chunks of the universe tells us about what it's like as a whole, and if that weren't true, it would limit what we can ever know. Thankfully, there are a lot of reasons to believe that it is uniform.

One of the strongest pieces of evidence comes from the cosmic microwave background, a faint glow of radiation from the Big Bang that fills all of space. Back in those first moments, the universe consisted of just free electrons and nuclei in an extremely hot plasma, along with a bunch of light particles, or photons. In denser areas, photons had to work against the pull of gravity as they radiated outward, and that cost them some energy.

So the energy of the radiation was directly tied to how densely packed particles were in the region it came from. And we can actually still see that radiation today— that's the cosmic microwave background. Which means it's one of the most direct ways we have of looking at the conditions just after the birth of the universe.

Of course, that was 13.8 billion years ago, but other studies have found that the cosmological principle seems to have held up as the universe evolved. For example, the Sloan Digital Sky Survey created an enormous, three-dimensional map of the universe in greater detail than we'd ever seen, and it showed that, no matter which way you look, the distribution of galaxies is extremely similar on large scales. So together, these two lines of evidence make a pretty strong case for the cosmological principle!

But even though the universe seems to be homogeneous, and cosmological principle seems to hold, the case isn't totally closed— because there's still no proof that it has to be that way. And, of course, scientists are always testing their assumptions. In the last decade, studies have actually raised some doubts about the cosmological principle.

For example, in 2011, researchers published a study based on supernovas, bright explosions of stars that let us see deep into the universe. By measuring the distances to supernovas and how fast they seem to be moving away from us, astronomers can estimate how fast the universe is expanding. Incredibly, this study found that, in some directions, supernovas appeared to be receding faster than in other directions, implying that the universe was expanding unevenly.

In other words, it suggested that the universe is not the same in every direction— exactly the opposite of what the cosmological principle says. Then, in 2014, another team of researchers made another unusual discovery. They were studying quasars, the compact areas surrounding supermassive black holes at the centers of galaxies.

Quasars are extremely bright, and like supernovas, they let us see into the distant universe. By studying them, scientists found that, across billions of light-years, many different quasars seemed to be rotating around axes that lined up with each other. Which is bizarre.

Because if there's nothing special about one direction or another, you'd expect that quasars that have nothing to do with each other would just rotate around random axes. The implication that the universe had a preferred axis went directly against the cosmological principle. Which was potentially a really big deal.

Like the supernova study, it implied that the universe was anisotropic, meaning it has different properties in different directions. But as the saying goes, extraordinary claims require extraordinary evidence, and the cosmological principle hasn't gone down the drain yet. In a 2016 study, researchers considered how a preferred axis would have shaped the early universe and looked for telltale signs like spirals or gravitational waves in the cosmic microwave background.

And they didn't find anything. What's more, all of the anisotropies different studies have found seem to be related in direction. So the authors suggested that they have to do with the way we observe the universe, rather than a problem with the cosmological principle.

It's still not proven, though, so astronomers are continuing to look for evidence that either confirms or defies our expectations. And even though the cosmological principle seems to have stood the test of time, it's important to keep checking. Because anytime we study the universe, we're going into it with some assumptions— and sometimes the concepts that seem the most intuitive and obvious are the ones keeping us from unlocking even deeper truths.

Thanks for watching this episode of SciShow Space! And a special thanks to our President of Space, SR Foxley. SR is one of the awesome people who support SciShow through Patreon, and it's thanks to patrons like him that we can keep making science education free on the internet.

If you're interested in supporting what we do or learning more about our wonderful patron community, head on over to Patreon.com/SciShow. [ outro ].