It’s the kind of question that sort of breaks
your brain when you even start to figure out
how to think about it: what shape is the universe?
No one can be sure; until we can step outside
of the universe and walk around it somehow
-- we can’t actually do that ever -- the
best we can do is take what we know to model
the likeliest scenario.
The idea of the shape of the universe comes
down to the key insight from General Relativity
that says mass warps space, and that objects
moving through space have their paths curved.
Here’s a simple way to think about it: imagine
there’s a bowling ball on a trampoline.
That’s the sun warping space around it.
Now roll a marble past the bowling ball to
mimic a planet in orbit.
From the marble’s perspective it's rolling
in a straight line, but we can see that its
path is curved by the bowling ball depressing
the trampoline.
Now imagine that the universe is a giant trampoline
covered in bowling-ball stars, medicine-ball
black holes, and little planet marbles.
This means that masses are warping space all
over the place, and the combined force of
all that mass has the capacity to shape the
universe.
So really, when we talk about the shape of
the universe we’re talking about the density
of the universe.
The simplest shape to get your head around
is the flat universe, which isn’t flat like
a two dimensional piece of paper, it’s flat
because it has no curvature.
This is the shape the universe would take
if it has the exact amount of mass to stop
the universe expanding after an infinite time.
It will have no bounds and expand forever,
slowing but never stopping.
If the universe contains more mass -- more
than critical mass so enough to stop its expansion
— it will have a positive curvature.
In this scenario the universe is closed and
looks a little like a sphere.
It would be finite but without a beginning
or end.
Eventually, the mass will cause the universe
to collapse back in on itself.
If, on the other extreme, the universe has
insufficient mass to stop its expansion, it
will have no bounds and expand forever in
an open shape sort of like a saddle.
So, which is it?
Well, current cosmological observations say
the Universe should expand forever, so the
positive curvature isn’t likely.
But, it also doesn’t seem to have the right
conditions for negative curvature.
Data from NASA’s Wilkinson Microwave Anisotropy
Probe mission of distant supernovae show that
the universe’s rate of expansion is accelerating,
but that rate is balanced by the mass of dark
matter.
In short, the Universe is likely ever so nearly
flat.
But it’s possible that’s the observable
universe.
Because the speed of light is a fixed value,
we can only see 13.8 billion years back.
It’s possible we just can’t detect a complete
curvature with the instruments we have, or
in ways that the laws of physics will allow.
So while we all know the Earth isn’t flat…
it looks like the universe almost is.
It’s been almost FOUR YEARS since I started
here, and I’ve loved every single episode!
Especially that awkward naked cold open and
fun time with chimerism!
But I’ve got some exciting new things in
the works so I’m not going to be around
Seeker as much.
I’m still doing Vintage Space, so if you
want to keep up with me and see what I’ve
got coming -- and Pete pics! -- be sure to
follow me on YouTube as well as Twitter and
Instagram.
And of course be sure to Subscribe for more
Seeker.
Thanks for watching.
