Name: What If the Universe Isn't Uniform?
Uploaded: 2020-04-13T17:00:32.000Z
Duration: 5 min 11 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

One of the things we assume to be fundamental about the universe

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.

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.

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—

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.

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.

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,

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.

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

So the energy of the radiation was directly tied to how densely packed particles

And we can actually still see that radiation today—

Which means it's one of the most direct ways we have of looking at the conditions

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

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

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

For example, in 2011, researchers published a study based on supernovas,

bright explosions of stars that let us see deep into the universe.

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

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.

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.

many different quasars seemed to be rotating around axes that lined up with each other.

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

The implication that the universe had a preferred axis

went directly against the cosmological principle.

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

and looked for telltale signs like spirals or gravitational waves in the cosmic microwave

What's more, all of the anisotropies different studies have found

So the authors suggested that they have to do with the way we observe the universe,

rather than a problem with the cosmological principle.