If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:11:36

Video transcript

let's think a little bit about what the BIGBANG theory suggests and then based on the theory what we should be observing today so the BIGBANG starts with all of the mass in space in the universe and infinitely in infinitely dense singularity the singularity is just something that the math doesn't even apply to it we don't even know how to understand that but immediately after the big banks this occurred 13.7 billion years ago 13.7 billion years ago immediately after this in this little tiny infinitely small singularity begins to expand and so for the first hundred thousand years it's still pretty dense so let me just show this right now so then it starts to expand so maybe it gets to this to this level right over here and I do not I do not know if if the entire universe is infinite or finite whether it's a kind of a whether it's a four-dimensional sphere or whether it goes infinitely in every directions or whether it's it's just slightly curved here and there and maybe flat everywhere else I won't go into all of that but then it starts to expand a little bit from the singularity but it's still it's still extremely dense it's still extremely dense so dense that atoms can't even form so you just have the basic fundamental building blocks of atoms or just all flying around electrons and protons they're just flying around in just this ultra hot ultra hot white-hot I could say or maybe even let me do a white hot plasma so this is I'll call it white white hot plasma white hot plasma and then if we fast forward a little bit more and now this is a point that we think we understand well but this number I actually looked at some old physics books and this number has changed in really the last 15-20 years so maybe it'll change more but after 380,000 years from the beginning of the Big Bang 300 thousand years years after the Big Bang I'll call it the BB 380,000 years after the Big Bang and obviously this is give or take a couple of years the universe expands enough the universe is now large enough and obviously I'm drawing things to scale the universe is now large enough and and a sparse enough that it can cool down a little bit you don't have as much bumping around it's still a hot place but now there's kind of it cools down enough that electrons can be captured by protons and you could actually have the first the first hydrogen atoms can begin to form the first hydrogen atoms begin to form they actually condense and we estimate this temperature to be around 3,000 Kelvin so we've cooled the 3000 Kelvin but this is still a temperature that you would not want to hang out and it's still extremely extremely hot now why is this moment important the first atoms forming so let's think about what's happening here you have all of this bumping and interactions and if because of a bump or some energy release or because of the heat temperature if a photon is released it'll be immediately absorbed by something else if something gets if some energy gets released it'll immediately be absorbed by something else because we're the universe is so dense especially with charged particles here all of a sudden it's not that dense so over here are things that were being emitted could not travel long distances it would immediately bump into something else while you go over here and the universe is starting to look like the universe we recognize all of a sudden if one of these really hot and it's still nowhere near as hot as this universe over here but if one of these hot atoms emits a photon and they would because they are at 3000 Kelvin if they emit a photon all of a sudden there's actually space for that photon to travel so for the first time in the history of the universe 380,000 years after the Big Bang you now have photons you now have electromagnetic radiation you now have information that can travel over long long distances so given that this happened it's still roughly 13.7 billion years ago 3 380,000 years is not a lot when you talk about 13.7 it still wouldn't even really change the dial because we're talking about the hundreds of thousands one seventh is 700 million years so this is actually a very small number so it's still approximately 13.7 billion it's really 13.7 minus 380 years but given that this was the first time that information could travel that photons could travel through space without most of them having to bump into something especially something that's probably charged the other interesting thing is that these atoms that formed are now neutral what could we expect to see today well let's think about it these leftt these photons were emitted 13.7 billion years ago and they were emitted from every point in the universe so this is every point in the universe every point in universe the universe was a pretty uniform place at that time very minor irregularities but you could see because it was this white-hot thing just began to condense it hadn't formed a lot of the structures that we now associate with the universe it was just kind of a fairly uniform spread of at that time reasonably hot hydrogen atoms so this is every point in the universe so let's think about what's going on here let me draw another diagram let's say that so we're talking about this point in the universe right over here the universe is at even 380,000 years after the Big Bang still much much much much smaller than the universe today but let's say that let's say that this is let's say that this is the point in the universe where we happen to be now at that at this point in time there was no earth there was no solar system there is no Milky Way it was just a bunch of hot hydrogen atoms now if we were at this point in the universe there must have been points in the universe there must have been points in the universe at that exact same time at that exact same time that were emitting that we're emitting this radiation that that were emitting this radiation and there were actually every point on the universe was emitting this radiation the point in the universe where we are now is emitting this radiation and some of that radiation so the points that were closer to us the points that were closer to us it was emitting that radiation but it got to us much sooner it got to us much sooner got to us billions of years ago but there were some points that were far enough that that radiation must be getting to us right now or another way to think about it that radiation has taken 13.7 billion years to reach us so let me draw so if I were to draw the visible universe today and you know from the video about the size so it's not going to be a scale would have to be far far larger than the circle I drew here but let's say that this is the visible universe let's say that this is the visible universe today we should be receiving and we're in the center of it because we can only look the same roughly the same distance in every direction we're not the center of the universe I want to be clear we're the center of the observable universe because we can only observe the same distance in all directions now we should be there should you know we're receiving some light from a hundred thousand light-years away and then we're looking a hundred thousand years in the past we should be receiving some light maybe a million light years that was first emitted a million light years before and that's like looking a million years in the past because the light we see was emitted a million years ago I think that's a bit redundant we could we could see light that was emitted that's just getting to us after traveling for a billion years and so we're actually looking at those objects a billion years ago because that's when they emitted the light so the same way we can look at objects that emitted their light 13.7 billion years ago right at the beginning right at this stage over here right after 380,000 years after the Big Bang and so since that light is only just reaching us we will see it we will see it as it was 13.7 billion years ago so we should see this type of radiation now the other thing to remember the universe was expanding when this was emitted the universe was expanding the universe was expanding at a very well it's all relative what's the fast rate and all of that but it was expanding and we learned on the video and redshift that when the source of the light is moving away from you the the or the source of the electromagnetic radiation is moving away from you the radiation itself gets redshifted so even though this is at a relatively high frequency this was kind of you could almost imagine it was kind of red-hot gas it was at 3,000 Kelvin because it was moving away from us these things and we learned in the video on the actual the actual size of the observable universe even though that these even though these electromagnetic waves we're taking 13.7 billion years to reach us in that time this point in space the point in space that emitted those electromagnetic waves are about 46 billion years light-years away so that's our best estimate so this is still is still stretching away so theory if you believe all of this that this was about 3000 Kelvin and it gets redshifted theory would have it that we should see not not something analogous to radio way not something analogous to electromagnetic waves being released from a 3,000 degree temperature atom we should see something red shift into red shifted into the radio spectrum so we should be observing we should be observing radio waves and the reason why we're observing radio waves and not something of a higher frequency is because it got red shifted it got red shifted down into a lower frequency and remember we should be seeing it from every point in the universe where the photons have been traveling for 13.7 billion years we should see it all around us this would this is almost an it this is almost a necessity for us to really believe in the current Big Bang Theory and it turns out that we did observe this and I want to make that this is very unintuitive because you look at any other point in the universe it's non-uniform every other point in the univeristy of stars and galaxies these aren't atoms anymore these are stars and and galaxies and whatnot and so there's some points in the universe where you see a lot of radiation and there's other points in the universe where you see nothing it's just black but if this thing if this is correct if this really did happen we should be able to observe uniform radio waves from every direction around us and you know you go 300 or more than 360 we're going in in three dimensions every direction in any direction you point a tena a radio antenna you should be receiving these radio waves that were much higher frequency when they're emitted they've been red shifted then but they were emitted 13.7 billion years ago and it turns out in the late 1960s they did find these radio waves from every direction and these are called the cosmic let me write this down this is the cosmic microwave microwave background background radiation and it's this in combination so it's this data that we're getting this observation in combination with the fact that the further we look out to two galaxies and clusters of galaxies they all seem to be moving away from us they're all red shifted and they get red shifted more and more the further we look out so this and everything being red shifted away from us are the best two points of evidence for the actual Big Bang so hopefully you found that reasonably interesting