the universe is huge we know this but
how huge and how can we possibly measure
the size of the universe put it like
this it's common to quote astronomical
distances in lightyears that's the
distance light travels in a year or
about nine trillion kilometers given
that here are some facts to blow your
mind the diameter of the solar system is
about 8 light hours or 9 billion
kilometers the Milky Way our galaxy
that's about a hundred thousand
light-years across or close to 1
quintillion kilometers and the next
closest major galaxy Andromeda that's
two-and-a-half million light-years away
which is just silly to talk about in
kilometer all pretty big distances right
now let's talk about the whole universe
we can't see all of it some things are
so far away the light from there hasn't
reached us yet so we're going to focus
on the observable universe that's the
part that we in principle can see with
light or gravitational waves so how big
is that well it's a sphere with a radius
of about 46 billion light years or
rounding down to keep it simple about 90
billion light years end to end now
that's crazy big but it's even crazier
that astronomers think they know that
number it's not like they measured the
universe with a ruler so how do they
know in a nutshell you start with the
age of the universe the current estimate
is about 13.8 billion years that's the
maximum amount of time light has had to
travel to us and then you work out how
far away the emission point of that
light is right now and that's the radius
of the observable universe remember that
distance is not 13.8 billion light years
it's much bigger to understand why it's
critical to realize that space itself is
expanding a popular analogy that I wish
I could take credit for is a loaf of
raisin bread rising in the oven each
raisin represents a cluster of galaxies
in the dough space now the raisins don't
move through the dough but as the bread
bakes and rises all the raisins get
further apart from each other because
the dough itself expands
now each raisin stays the same size
galaxy clusters aren't expanding and
neither individual galaxies are the
earth where people or trees it's just
the relatively empty space between those
large clusters of galaxies and not only
does the dough expand
can expand at different rates during
different stages of the baking process
may be really fast for a billion years
then slower for the next two billion
these two facts the expansion of space
and the fact that it can expand at a
variable rate complicate how we measure
the size of the observable universe in
fact space itself can expand at any rate
it wants to even faster than the speed
of light so over the lifetime of the
universe the birthplace of a beam of
life can be carried ridiculously far
away by the expanding space tow to know
exactly how far though and calculate the
size of the universe you need to know
how quickly space has been expanding at
every moment in history ever so how can
we possibly know the expansion history
of the universe using something called
cosmological redshift which is like a
fingerprint that the expansion of space
leaves on beams of light let me take a
moment to explain light has a color
determined by its wavelength longer
wavelength light is redder shorter bluer
if space were not expanding then light
from a distant galaxy would be the same
color when it arrived on earth as it was
when it first set out to do one
departure blue and arrival but because
space is expanding the wavelength of
light gets stretched as it travels to us
making the blue light red hence the term
redshift in more extreme cases the
wavelength can be stretched out of the
visible spectrum altogether into
microwaves or radio waves now here comes
another important point to pay attention
the light from more distant galaxies is
redshifted more than light from nearby
ones you see the life of more distant
places has further to go so it spends
more time in the expanding space in the
rising dough and thus it has its
wavelength stretched more makes sense
but how a redshift and distance related
quantitatively that is the
million-dollar question if we knew the
answer in numerical detail we could
figure out the expansion history and in
turn the size of the universe so can you
do this can you measure the distance
redshift relationship no sweat just find
a bunch of faraway galaxies much further
than Andromeda measure their distances
and their red ships then put those
distances and red ships on a graph and
find the best fit curve voila you now
know the distance redshift relationship
and from that how fast the universe was
expanding at every moment ever once you
have the expansion history how do you
actually determine the size of the
universe remember as I said a long long
time ago
we first need to get the universe's age
so let's go back to the raisin bread if
we run the movie of the rising dough
backwards at the rate given to us by the
expansion history eventually nearby
raisins will sit on top of each other
that is the big bang and how long it
takes to get back to this point is the
current age of the universe our best
current estimate using that expansion
history is 13.8 billion years give or
take
step two is well annoying math but we
can represent it visually as follows
imagine that seconds after the Big Bang
happens every raisin emits a beam of
light that can then travel without
hitting any obstacles as we run the
movie forward at the rate given to us by
the expansion history those imaginary
beams would travel through the expanding
space and reaches at different times one
of those beams of raisin light would be
just reaching us as the clock hits 13.8
billion years we then see where that
reason is now answer about 46 billion
light years away boom so that's how we
know that the observable universe is
about 90 billion light years in diameter
but what about the unobservable universe
aren't there galaxies even further away
whose light has reach us yet well yeah
there are but I'm afraid that's a
subject for another episode if you're
inclined go ahead and get the discussion
about that started in the comments below
I'll report on any interesting threads
from that conversation on the next
episode of space time last week we asked
whether it's irrational to believe in
aliens here's what you guys had to say
pants ooh man and others point out that
if you really low odds to accidentally
bump into a Millenium Falcon in space I
think you guys took my glib example a
little too literally the real point of
the Fermi paradox is the colonization on
a galactic scale could happen so quickly
that if life is really that common at
least one civilization should have
already done it by now
and they should be so widespread that
we'd see some evidence nick g commented
that a really advanced civilization
might not try to expand outward into
space but inward into let's say advanced
computer circuitry pretty good thought
the jitta afh asks what about the prime
directive well what about james t kirk
he didn't care about the prime directive
much so why should all the aliens and
rob larosa left us with a deeply
insightful comment from Calvin and
Hobbes good work
you