Cosmology and astronomy
- Big bang introduction
- Radius of observable universe
- Radius of observable universe (correction)
- Red shift
- Cosmic background radiation
- Cosmic background radiation 2
- Hubble's law
- A universe smaller than the observable
- How can the universe be infinite if it started expanding 13.8 billion years ago?
Cosmic background radiation 2
Cosmic Background Radiation 2 - Redshift of the Cosmic Background Radiation. Created by Sal Khan.
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- At about9:00into the video Sal explains that "if the universe expands fast enough there's no way [the outermost photons] will reach us." However, two videos back on the topic of Red Shift at about2:30he explains that light speed is absolute, and it does not matter how fast an object is traveling in the opposite direction - the photon of light from that object would still reach us at the same time. How, then, is it possible for the universe to expand so fast as to dwarf the speed of light?(35 votes)
- Well, I'll try to explain if I understand the whole concept. The speed of light is absolute, but you will observe the actual light with that speed only when it reaches you. It's constant but not instant. If two points are close enough, the expansion of space won't affect the outcome; the light from one point will reach the other with the information it carries (and vice versa from the other point). If the two points are far enough, there will be a time into the future when the expansion of the universe (given that it expands with an increasing velocity) will make it impossible for the light from either point to ever reach the other. In fact, in a kind of "scientific doomsday" it's been predicted that light even from objects we can see today will someday not reach us. After some billions of years, we will only be able to see (if we're still around) the stars of our galaxy.(35 votes)
- Does that mean that the space may be expanding faster than the speed of light? (10:13)(20 votes)
- The quick answer is yes, the Universe appears to be expanding faster than the speed of light. By which I mean that if we measure how quickly the most distant galaxies appear to be moving away from us, that recession velocity exceeds the speed of light.(1 vote)
- Let's say that a photon bounced off a dinosaur, is it possible for us to go a certain distance and then if we use a super telescope to look back Earth, we would look at dinosaurs(9 votes)
- I think not, because we would have to travel faster than the speed of light to be able to catch up to that photon. However, there are places in the universe right now, that (assuming they had a powerful enough telescope pointed at earth) would see dinosaurs.(1 vote)
- If the universe is expanding , does that mean that Earth will eventually get too far away from the sun?(5 votes)
- No, expansion is not strong enough to overcome gravitational forces at distances within our local galactic neighborhood.(7 votes)
- How can 46billion L.Y be the same distance as 13,7Billion years? (9:30)(7 votes)
- The photon has traveled for 13.7 B years, but the distance is 46 B light years because space itself is expanding. This can mean two things: 1) A photon traveling over a large distance would cover more distance than it would if the universe was static, and 2) the distance it still needs to travel to get to a certain point increases as the universe expands.(2 votes)
- As stated by Sal here, is it really possible for some light being observed by us to get so red-shifted that it gets to have infinite wavelength? I mean, even appreciating the fact that the far the source of electromagnetic radiation is from us the fast will it seem to move away from us and more the radiation coming from it will be red-shifted to us, it's too hard to digest that it will be red-shifted to this extent. 'Cause there seems to obviously have a limit to how fast that source can move away and to what extent the radiation could be red shifted, isn't there any?(5 votes)
- If it's shifted to infinite wavelength, we can't observe it. But there's no limit to how much it can be shifted.(3 votes)
- Why is the observable universe's rage limited to only 13.7 billion years? What prevents us from seeing further afield (say, 15 billion light years away)?(4 votes)
- The universe is 13.8 billion years old. That's the longest any light has had to travel from any point to any other point.(3 votes)
- In the end, I don't understand his point of us not seeing anything because of red shift. What does he mean? Thanks a lot, I'm in 7th grade and I'm doing this for fun, so I'm having trouble comprehending these topics.(3 votes)
- There are two effects that can cause light to not be seen. One is that with space-time expanding there are points far enough away that light will never get to us. The other is that the further light is from us the greater it will be red shifted and if it is far enough away the light will be too weak to be detected.(3 votes)
- What will happen if that wavelength stretch to infinity on one day, whether the wave will disappear or too less intense to detect?(3 votes)
- If a photon originates from a far enough distance, the red shifted light will be too weak for us to detect. The wave will still exist, since there is no limit to the extent of light being red shifted(1 vote)
- So, if we go through this theory and wait for about 5 or 6 billion years, is it possible that we might find a new galaxy??(2 votes)
In the last video, we learned that 380,000 years after the Big Bang, which is still roughly 13.7 billion years ago, every point-- I shouldn't say every point-- every atom in space that was kind of at this roughly 3,000 Kelvin temperature was emitting this electromagnetic radiation. Since every point in space was, there were points in space, or there was 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's been traveling for that long, we can look at any direction and we'll see this uniform radiation. And 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, well, what happens if we wait a billion years? Because if we wait a billion years, if we have 1,000,380,000 years after the beginning of the universe, this stuff won't just be atoms anymore. It will have started to condense into actual stars. The universe at every point in space will no longer be this uniform. We'll actually start having condensation into stars. So if we move forward a little bit, the universe will expand. Maybe I'll just draw a half of it since it's expanded. It's obviously expanded much more. But now all of a sudden, we actually have stars. These are no longer just uniform atoms spread through the universe. We actually have condensation into stars. And so if you look at what is being emitted from the points in 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'll 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 have formed, other structures have formed. So the question is in a billion years, will this cosmic microwave background radiation disappear. And I'm using billion just to arbitrarily use a 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-- I guess this uniform haze of hot hydrogen atoms. But what you have to think about is there were further points in the universe. At that same time, there were 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 out points-- right now, the observable universe is-- we can only see electromagnetic radiation that's been traveling for 13.7 billion years. 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 in the universe. It'll just be from further out. Now, what I want to make clear is, is that since those points were even further out, where that radiation was emitted, the stuff that we'll see in a billion years, it will be even more red-shifted. So at that point, the cosmic background 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-- would "redder" have two Ds? I've never written "redder." Well, it would be more red than the microwave radiation. And, 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. And it'll keep happening. We'll keep getting radiation as we go further and further into the future. We'll keep getting radiation from further out points in space. And it'll get more and more red-shifted. The actual wave lengths of that electromagnetic light will be bigger, and bigger, and bigger. Until we really aren't able to even see it as electromagnetic light because it'll be red-shifted to infinity. It'll have an infinite wavelength. And to make that point clear, I want to show you that at some point, there will be kind of a threshold where we can't even get radiation from further out. And let me draw a diagram of that. So let's say that this is the universe. Let's say that this is the universe 13.7 billion years ago, right when that radiation, what we now see as cosmic microwave background radiation, right when it started to be admitted. 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 where we now observe the background radiation-- or 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 radiation 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 are 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 some type of permanently old place. It's just the light we're getting from them right now is old light, light that 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 are receiving cosmic background radiation right now. I don't want to write all that. It'll take me forever. Now, let's take another point in space that's whatever this distance is. Well, it's actually estimated to be about-- now, it's estimated to be about 46 billion light years. At that time, when things were just beginning to be emitted, this was only about 36 million light years. And 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 of that type of thing. But it was just a lot smaller than 46 billion light years. Now let's go that same distance again from this point in space. So let me make it clear. This is 380,000 years ago. Now let's fast forward. Let's fast forward-- sorry, not 380,000 years ago, 380,000 years after the Big Bang, which is approximately-- it's still 13.7 billion years ago. So that's then. Now let's look at now. And now I'll just draw it a little bit bigger. It's actually going to be much, much bigger now. 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's been travelling, this is now on the order of 46 or 47 billion light years. And 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. Maybe I should just do it in the color of the radiation. So this guy's receiving-- I'm just showing it's coming from this guy. We're only now, 13.7 billion years in the future, receiving photons from this guy, only now are we receiving it. And frankly, this green guy, only now is going to be receiving photons. When he looks at the point in space, or the things that he thinks are 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 for where we are now. He'll see the universe where we are now as 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-- let me make it clear. It took him 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 if the universe expands fast enough, there's no way that that photon that got to this guy, will eventually get to this. The universe is expanding faster than the light can never even catch up to us. And this light will never, ever, ever get to us. And so there is some threshold, some distance, from which we will never get light 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-- or 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 a billion years. But we will get uniform radiation from further out. But it will be even more red-shifted. And the further forward we 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 that we won't even observe it as electromagnetic radiation. And there's some threshold where we won't even observe anything anymore because beyond that, the light wasn't able to actually get to us.