Jul 25 • 3 min read
Our universe is incredibly flat 🙀
Our universe is like any other object, and so has to have a shape. There are three possible shapes our universe can be:
- Positively curved
- Negatively curved
- Perfectly flat
It's worth explaining here something that I should have clarified during
my TikTok. In most scenarios when we think about curvature, we are
visualising objects with two-dimensional surfaces. A tennis ball, for
example, has a positively curved surface. A saddle has negative
curvature, and the surface of a sheet of A4 paper appears flat. However,
when talking about the curvature of the universe, we are actually
referring to a "surface" in three dimensions (four in fact, if you count
time!).
An object with a three-dimensional surface can still be flat, positively
curved, or negatively curved. However, it's harder for us to visualise
and even harder to demonstrate. I've drawn two sets of diagrams below,
which show curvature on both 2D and 3D surfaces. I've also included the
mathematical definitions of "flat", "positively curved" and "negatively
curved", to try and make things a bit clearer:
There's an endless number of options for curvature, from slightly curved (think: saucer) to very curved (think: teacup). But only one option for flatness - flat! Therefore, the likelihood of the universe being flat is absolutely tiny. And yet... cosmologists are measuring the universe to be flat, to an accuracy of just 0.5%. To make matters even worse, calculations predict that the universe was flatter in the past!
Okay, time for another clarification."How can the universe be flatter in
the past if there's only one option for flatness?" I hear you cry! An
excellent question, and something else I should have made clear in my
video. Essentially, if you run the Big Bang Model of the universe back
in time, the accuracy of our flatness predictions actually increases...
To give you a like-for-like comparison, imagine you have a sheet of A4
paper, which appears almost entirely flat, but does, in fact, have edges
that are 1mm higher than its centre. Next, you run the clock back until
just a few minutes after the paper's creation (its own"Big Bang" if you
will). However, now the paper's edges are not raised by 1mm. Instead,
they measure at just
one proton's diameter higher than the paper's middle!
So what could be the reason behind all of this flatness? Well, our
current solution is a process called
inflation, in which (we think) a little while after the big bang and in less
than a fraction of a second, the universe rapidly expanded and
flattened.
There isn't actually any
irrefutable
evidence for inflation currently, as such, but most scientists believe
it had to have happened, otherwise our understanding of the universe
starts to fall apart.
... Can you think of any other reason why our universe is incredibly
flat?
The third and final clarification: I've had quite a few questions asking
me why the universe has to be one of the three options listed above
(positively curved, negatively curved, or flat). Why, for example, could
it not be a mix of these three? This comes down to one of the
foundations of cosmology -
the cosmological principle.
The cosmological principle states that on large scales, the universe
should be "isotropic" (i.e. appear the same in all directions) and
"homogeneous" (i.e. have no preferred locations). Therefore, if the
large-scale universe was positively curved in one region, and negatively
curved in another, the cosmological principle would be broken!
Do note though, that the cosmological principle only applies to the
large-scale universe. Planets, moons, stars and black holes (amongst
lots of other things) warp space and change its curvature on the small
scale all the time.
For image credits, please see the image credits tab