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

Cosmic background radiation

Cosmic Background Radiation. Created by Sal Khan.

Want to join the conversation?

  • male robot hal style avatar for user bbxxtt2010
    where did the protons come from?
    (18 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user Felix Doherty
      The protons came from the up, and down quarks. Once the universe had cooled sufficiently for 2 up quarks to bond with a down quark, the first proton was 'born'. Similarly, once the universe had cooled down enough for 2 down quarks to bond with an up quark, the first neutron was created. Here, with protons and neutrons, basic non-metal elements started to form (H, He).
      As reesdylan5 said, photons are produced from the excitation of atoms, or when an electron 'jumps' from one energy level to the other.
      (40 votes)
  • primosaur ultimate style avatar for user Oliver Worley
    How far back do you have to go in time so that the background microwave radiation was still in the visible light spectrum? For example if you were to use a telescope back in the time of the first dinosaurs (say 500mya) would the universe look much different or would the radiation just be closer?
    (13 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user robospyindustries
      You likely wouldn't have been able to see much of anything unless you went very deep into the past. If we assume that the universe was essentially a black body radiator at the time of the emission of the cosmic background radiation, than at 3000K, the peak emission wavelength is about 960nm, which is in the infrared, already outside of the visible spectrum. A 3000K black body will radiate some light in the visible, but only about 8% of its radiation will be in that band. So that's what the radiation would have looked like at the time of emission, about 13.7 bya - even then, most of it was already outside the visible range, and that remaining 8% would get redshifted to infrared in relatively short time spans (relatively short here meaning "short" on a cosmic time scale, so like a billion years). As cool as it would be, I don't think the dinosaurs were looking up at a faintly glowing red night sky.
      (19 votes)
  • male robot hal style avatar for user Jackson Yeager
    Infinitely small point... doesn't this sound like a Black Hole? Perhaps the Big Bang was actually an entire Universe swallowed by a Black hole, and the singularity expanded outward again for some reason. Perhaps there are different cycles of the Universe. One stage is where we are now, and another stage is the Black Hole swallowing us. What do you think?
    (20 votes)
    Default Khan Academy avatar avatar for user
  • mr pants teal style avatar for user Kate
    How do we know all of these times? How can we tell that the Big Bang took place 13.7 billion years ago, or what the universe was like 380,000 years after the Big Bang?
    (9 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Denis
      Science can never make "proofs of truths", it can only prove whats wrong. We have studied the properties of light, know its speed, and that nothing can exceed it. We know expansion rate due to red shift. By studying the stars and their life span we are able to determine properties of supernova. Supernova gives us a measuring stick, red shift gives us a rate of change. We mathematically roll the clocks backward, with this data and get a number, 13.7 billion years minus one billionth of a billionth of a second when physics and math break down. The big bang "theory" is testable, falsifiable, and makes predictions. Until someone can prove it wrong it will remain the best explanation.
      (9 votes)
  • male robot hal style avatar for user Ananay Agarwal
    What is a photon exactly? I know that it is the carrier of the electromagnetic force and all that, but why do electrons emit them? and if they do, how does the lowered energy state of the electron (they expend energy while emitting photons, right?) manifest itself? Do only electrons emit photons? Or do other particles emit them too?
    (6 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Andrew M
      A photon is a way of thinking about light or other forms of electromagnetic energy - it's a little packet of light energy. Sometimes it is useful to think of light as particles in this way. Other times it is more useful to think of light as a wave.

      Why do electrons emit them? That's just the way the universe is - when atoms give up their electromagnetic energy, they do it in little "quantum" packets. That was a big surprise to physicists when it was discovered. No one thought energy had to come in little packets before that.
      (7 votes)
  • piceratops ultimate style avatar for user Rajat Jose
    So, here's a dumb question, Is the Universe really expanding? or it feels like it's expanding because we start to see more of it? That is the light from those points is reaching us just now? So what my question is that is the Universe really expanding, or is it already a fixed size and just the light is reaching us from the parts thus feeling like it is expanding.
    (9 votes)
    Default Khan Academy avatar avatar for user
    • aqualine ultimate style avatar for user Electricifiked Baguette
      It is a commonly accepted idea that the universe is actually expanding, after the big bang, simply because is makes some sense. Even if the light is reaching us just now, it would have to start somewhere? Based on our current understanding the universe is expanding, because we think galaxies are moving away from each other. No one really knows, it's all just theories.
      (0 votes)
  • leaf blue style avatar for user David Richardson
    OK, I understand about red shift and the doppler effect that indicates that objects in space are moving away from us in all directions. We also know that by observing the spectrum of stars that the basic constituencies of stars are hydrogen and helium in various percentages based on the size and age of the star. Now, the spectrum of stars show that H and He lines are in certain specific parts of the spectrum, and these lines identify these gases. Therefore, if these stars and galaxies are red shifting, meaning the spectrum is moving more toward the red end of the spectrum, how could we tell that those elements or any other elements in the star are those specific elements? The lines in the elements are moving out of the orange, yellow, green, blue, and violet parts of the spectrum that identify these elements. This I don't understand. How can we tell what elements in a distant star in a distant galaxy are actually the elements that really are in that star if it's changing position constantly?
    (6 votes)
    Default Khan Academy avatar avatar for user
  • aqualine seedling style avatar for user mjaumjau
    Why the dense mass in the beginning didn't just collapse like a black hole because of the gravity?
    (4 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Charles LaCour
      Based on one of the leading theories about the big bang there wasn't a dense mass of matter/energy in the initial moments of the big bang. There was an extreme exponential expansion of space-time that caused a buildup of negative gravitational potential energy that got balanced out by the production of matter. Even though this idea fits observations it is not the only one so we don't know if it is correct.
      (4 votes)
  • piceratops sapling style avatar for user Krutik Desai
    How are neutrons formed? Hydrogen atoms doesn't need neutron but the other gases do need neutrons to form its atom.
    (1 vote)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Charles LaCour
      Both neutrons and protons are both made of 3 smaller particles called quarks. A proton is has 2 up quarks and 1 down quark. A neutron is 1 up quark and 2 down quarks.

      A up quark can interact with an electron and produce a down quark and a neutrino. This will change a proton to a neutron.

      The reason that elements with more than one proton need to have neutrons is that the electric charge is to strong to allow more than 1 proton the stay together. The neutrons are needed to help keep the atomic nucleus together.
      (4 votes)
  • mr pants green style avatar for user Dhyey Maharaja
    Why is it called "microwave" radiation, but Sal said it was radio waves?
    (3 votes)
    Default Khan Academy avatar avatar for user

Video transcript

Let's think a little bit about what the Big Bang theory suggests. And then based on the theory, what we should be observing today. So the Big Bang starts with all of the mass in space in the universe, an infinitely, an infinitely dense singularity. And a 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 Bang, so this occurred 13.7 billion years ago, 13.7 billion years ago, immediately after it, this little tiny infinitely small singularity begins to expand. And so the first 100,000 years, it's still pretty dense. So let me just to show this right now. So then it starts to expand. So maybe it gets to this level right over here. And I do not know if the entire universe is infinite or finite, whether it's a four-dimensional sphere, whether it goes infinitely in every directions, or whether 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 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. They're 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 white hot plasma. So I'll call it 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, 300,000 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 not drawing things to scale-- the universe is now large enough and 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, it cools down enough that electrons can be captured by a proton. And you could actually have-- 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 to 3,000 Kelvin. But this is still a temperature that you would not want to hang out in. 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 some energy gets released, it'll immediately be absorbed by something else because the universe is so dense, especially with charged particles, Here, all of a sudden, it's not that dense. So over here, things that were being emitted could not travel long distances. They would immediately bump into something else. Well, 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 3,000 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. 380,000 years is not a lot when you're talking about 13.7. It still wouldn't even really change the dial because we're talking in the hundreds of thousands. 0.7 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 380,000 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 left. 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. The universe was a pretty uniform place at that time, very minor irregularities. But you could see because it was this white-hot thing that had 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. 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 this is the point in the universe where we happen to be now. At this point in time, there was no Earth, there was no solar system, there was no Milky Way. It was just a bunch of hot hydrogen atoms. Now if we were at this point in the universe, there must been points in the universe at that exact same time that were emitting this radiation. And actually, every point in the universe was emitting this radiation. The point of the universe where we are now is emitting this radiation. So the points that were closer to us, it was emitting that radiation. But it got to us much sooner. It got to us billions of years ago. But there are some points that were far enough that that radiation must be getting to us right now. Or another way to think about it is 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 to scale. It 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 this is the visible universe today. We should be receiving-- and we're in the center of it because we can always look 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're receiving some light from 100,000 light years away. And then we're looking 100,000 years in the past. We should be receiving some light 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 see light 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 could 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 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 a fast rate and all of that. But it was expanding. And we learned on the video in red shift that when the source of the light is moving away from you, or the source of the electromagnetic radiation is moving away from you, the radiation itself get red shifted. So even though this is at a relatively high frequency-- you can 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 size of the observable universe, even though these electromagnetic waves are 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 light years away. So that's our best estimate. So this is still stretching away. So theory, if you believe all of this, that this was about 3,000 Kelvin and it gets red shifted, theory would have it that we should see not something analogous to electromagnetic waves being released from a 3,000 degree temperature atom. We should see something red shifted into the radio spectrum. So 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 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 this is very unintuitive. Because you look at any other point in the universe, it's nonuniform. Every other point in the universe, you have stars and galaxies. These aren't atoms anymore. These are stars, 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 is correct, if this really did happen, we should be able to observe uniform radio waves from every direction around us. And you go 300-- or more than 360 degrees. We're going in three dimensions. Any direction you point an antenna, a radio antenna, you should be receiving these radio waves that were at much higher frequency when they were emitted. They had 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 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 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 redshifted away from us are the best two points of evidence for the actual Big Bang. So hopefully, you found that reasonably interesting.