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matter distorts the universe it's the
fundamental idea behind einstein's
theory of general relativity every black
hole every star every space rock bends
small bits of space time like a bowling
ball on a trampoline but if certain
areas of the universe can be curved so
can the universe as a whole which leads
to a major question how does the overall
amount of stuff in the universe curve it
in other words what is the geometry of
the universe the possible answers are
surprisingly uncomplicated the universe
can either have a positive curvature a
negative curvature or it can be flat
with no curvature but figuring out which
one of those is true is a little more
involved when you zoom out really far
like on the order of 300 million light
years the universe looks the same from
every point in space which means the
universe's overall geometry must be the
same everywhere too this doesn't make it
any easier to picture what it means for
3d space to be flat or curved because
human brains can't perceive things from
some external fourth spatial dimension
but we can try really it comes down to
what happens when you draw a triangle
connecting three points and add up the
angles between them if they equal 180
degrees like in a regular triangle you
draw on a piece of paper you live in a
flat universe scientists model that with
a flat infinitely long plane if the
angles of the triangle add up to more
than 180 degrees though you live in a
positively curved universe this is where
things get harder to picture but it's
like each side of the triangle is
bending outward you can get an idea of
how this happens by connecting three
points on a sphere instead of straight
lines each side of the triangle curves
along the sphere and you end up with a
set of bigger angles that's positive
curvature negative curvature is the
opposite it's like the sides are bowing
inward and the angles come out to be
less than 180 degrees physicists model
it with a kind of saddle shape
unfortunately the universe is just too
big for us to actually test this out
it's like how the earth's surface feels
flat beneath our feet even though it's
positively curved we'd never be able to
tell if the sides of the triangle were
bending but there are other methods we
could use to figure out the universe's
geometry it all depends on how much
matter and energy is out there curving
space-time it's divided into three main
groups the first and probably the most
obvious is matter that includes all the
matter we know and love like protons
neutrons and electrons as well as dark
matter which we can't detect visually
but we know is there because of its
gravitational pull there are also a lot
of massless photons and nearly massless
neutrinos flying through space at or
near the speed of light they get their
own group and finally there's dark
energy the mysterious stuff that's
making the universe expand more quickly
but that's pretty much all we know hey
we're working on it estimates put dark
energy at comprising somewhere around 75
of all the stuff in the universe right
now but figuring out the geometry caused
by these ratios is easier said than done
it all centers around the critical
density the exact amount of stuff you
need in the universe for it to be flat
if the real density is less the universe
will have negative curvature if it's
more we're in a positively curved
universe and the thing is we don't know
the exact critical density it depends on
both the strength of gravity as well as
how fast the universe is expanding and
we're still working on exact numbers for
the latter we've gotten really close to
exact numbers though cosmologists can
get a very precise estimate of how fast
the universe is expanding using things
like supernovas or temperature
fluctuations in the radiation left over
from the big bay and in 2015 researchers
used data from the planck space
telescope to get a similarly precise
estimate of the real density of the
universe even when you take into account
the margins of error the real density is
so close to the critical density
that cosmologists are pretty confident
we live in a flat universe so no bendy
triangles of course the universe's
geometry doesn't affect us on a
day-to-day basis but it does help answer
one of the fundamental questions of
existence how will the universe end if
there were no such thing as dark energy
the curvature itself would doom our
reality to one of two fates if we lived
in a positive universe everything would
eventually collapse back in on itself
and the universe would end in what
cosmologists call the big crunch if the
universe were negatively curved or flat
the universe would continue expanding
forever and everything would end in ice
not fire but with any amount of dark
energy the story gets more complicated
luckily astronomers have made charts
these charts include a couple of new
hypothetical fates like a big bounce
where instead of starting with a big
bang the universe would switch between
expanding and contracting when it hit a
certain minimum size but with current
measurements the universe looks like it
had a clear start and will end just not
with a bang although it won't end with a
whimper either because a sound wave
wouldn't be able to propagate through
that vacuum and if you're interested in
learning more about that you can check
out the episode about the end of
everything on the main scishow channel
in the meantime thanks for watching this
episode of scishow space for more videos
on the weirdness of the universe and
everything space
just go to youtube.com scishowspace and
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