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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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