[ 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
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we can keep making science education free
on the internet.
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[ outro ]
