Cosmic Background Radiation 2

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Cosmic Background Radiation 2: Cosmic Background Radiation 2 - Redshift of the Cosmic Background Radiation

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In the last video we learned that 380,000 years after the Big
Bang which is still roughly 13.7 billion years ago
every atom in space that was roughly 3000 kelvin temperature
was emitting this electromagnetic radiation
there are points in space or there are points in the universe
that that radiation is only just now reaching us, it
has been traveling for 13.7 billion years, so when
we look at radiation that has been traveling for
that long we can look at any direction and we will see this uniform radiation
that radiation has been red-shifted into the
microwave range from the higher frequencies that
it was actually emitted at
Now a question that might pop in your brain is
what happens if we wait a billion years ?
because if we wait a billion years, if we have a
billion 380.000 years after the beginning of the
universe, this stuff won't just be atoms anymore
it will have started to condense to actual stars
the universe at this point, at every point in space
will no longer be this uniform, it will actually start
having condensation into stars, so if we move
forward a little bit the universe will expand
maybe I will just draw a half of it since it's
expanding, it has obviously expanded much more
but now all of a sudden we actually have stars
these are no longer just uniformed atoms spread
through the universe, we actually have condensation
into stars and so if you look at what is being emitted
from the points of space from which we're only
now getting this cosmic background radiation
if we wait a billion years, the light that we see
from those points in space will not look like this
uniform radiation, it will start to look a little
bit more like the more mature parts of the universe
we'll essentially be looking at the universe a billion
years after the Big Bang when stars are formed,
other structures are formed, so the question is in
a billion years will this cosmic microwave background
radiation disappear ? I'm using billion just because that's
you know, just a arbitrarily used number
but will it eventually disappear ? and the answer to that is
yes and no, so to think about it, it is true that this
point in space will mature, it will mature in a billion
years, it will no longer be this uniform haze of hot
hydrogen atoms but what we have to think about is
there are further points in the universe at that same time
there are further points that were also emitting this
radiation and the original photons from those original
points still haven't gotten to us, so from those further
outpoints, right now the observable universe is
we can only see electromagnetic radiation that's been
travelling for 13.7 billion years and in another billion
years the universe will be a billion years older and then
there will be radiation that has been traveling for 14.7
billion years and so we will start to observe that
and we'll start to observe that radiation from the
same time period of the universe, it will just be from
further out. Now what I want to make clear is since those
points were even further out, where that radiation
was emitted, the stuff that we will see in a billion years
it will be even more red-shifted.
So at that point the cosmic backgroung radiation we see
will have longer wavelengths than the radio spectrum
it will be redder and I should say redder because we're
already more red than the microwave radiation of course
that's a funny thing because microwave radiation is already
more red than actual visible red light, it has a longer
wavelength. Now this will keep happening
we will keep getting radiation as we go further and further
into the future, we will keep getting radiation from further
outpoints in space and it will get more and more red-shifted
the actual wavelengths of that electromagnetic light
will be bigger and bigger and bigger
until we really aren't able to even see it as an electomagnetic light
because it will be red-shifted to infinity, it will have an
infinite wavelength and to make that point clear I want to
show you there will even be points, at some point
there will a kind of threshold where we can't even get
radiation from further out
let me show you, let me draw a diagram of that
so let's say that this is the universe 13.7 billion years ago
right with that radiation what we now see as cosmic
microwave background radiation right when it started to be emitted
and let's say that this is the point in the universe
where we are now, so this is "us"
let's say that this is the point in the universe that where
we now observe the background radiation
this is one of the points, we obviously could form a circle
around us, it could be any of these points over here
where the photons, the electromagnetic radiaton that
were emitted from this point 380 000 years after the
beginning of the universe is only just now reaching us
so this is the point in the universe from which we're
observing the cosmic background radiation
and let me be very clear, that point in the universe
has now matured into things that look, into stars
and galaxies and planets and if they were to look at
our point in space, they are also going to see cosmic
background radiation from us, it's not like this is
some type of permanently old place, it's just that the
light we're getting from them right now is old light
light that they at that point in space emitted way before
it was able to mature into actual structures
so this is the point in space from which we're
receiving cosmic background radiation right now.
Now let's take another point in space that's,
whatever this distance is, now it is estimated to be
around 46 billion light years, at that time when things
were just beginning to be emitted, this was only about
36 million light years. This is a very rough estimate I shouldn't
even write it down because that's really based on how
fast we assume the universe is expanding and all that type of thing
but is was just a lot smaller than 46 billion light years
now let's go that same distance again from this point
space, so let's see let me make it clear this is
380,000 years ago. Now let's fast farword, sorry not
380,000 ago 380,000 years after the Big Bang
which is approximately 13.7 billion years ago.
So that's then, now let's look at now, I will just draw it a
little it bigger, it's actually going to be much much
bigger now, if we do it a little bit bigger so when I
draw it like this, this is where we are now this point
in space from which we are only now receiving that
cosmic background radiation is over here and then this
other point in space is going to be over here, and we
saw in the video on the actual size of the observable
universe, not just what it appears to be based on how
long the light has been traveling, this is now on the
order of 46 or 47 billion light years
so this distance is also going to be
46 billion light years.
Now every point in space back then was emitting this
radiation, we have this uniform radiation, it was
just hydrogen atoms everywhere, these hot hydrogen atoms.
Now we're only, maybe I should just do it in the colour
of the radiation, I'm just showing you it's coming this guy.
Well only now, 13.7 billion years in the future
we're receiving photons from this guy, only now we're
receiving it and frankly this green guy only now is
going to be receiving photons. When he looks at the
points in space when the things that he thinks are
the points in space out there, he will see that uniform
radiation and likewise this guy over here will only now
be receiving photons from the point in space from
where we are now, he will see the universe where we
are now as how it was 380,000 years after the Big Bang
and same thing from that point in space, the photons
will only just now reach. Now let's think about it
it took this guy's photons 13.7 billion years
to reach this point over here which is now 46 billion
light years away from us and the universe continues
to expand, depending on the, if the universe expands
fast enough there is no way that that photon that got
to this guy will eventually get to this, the universe
is expanding faster that the light can never even
catch up to us and this light will never ever get to us
and so there is some threshold, some distance from
which will never get light at during this time period
or actually from which we will never ever get any
electromagnetic radiation, so the simple answer is
the cosmic background radiation from this point
yes it will start to mature, it won't be as uniform
if we go fast forward 400 million years or billion years
but we will get uniform radiation from further out
but it will be even more red-shifted and the further
forward we will get into the future the background
radiation we get will be from further and further out
and it will be more and more and more red-shifted
until some point where it's going to be so red-shifted
we won't even observe it as electromagnetic radiation
and there is some threshold where we won't even
observe anything any more because beyond that
the light wasn't able to actually get to us.

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