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Cosmology and astronomy
Course: Cosmology and astronomy > Unit 1
Lesson 4: Big bang and expansion of the universe- 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?
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Radius of observable universe (correction)
(Correction) Radius of Observable Universe. Created by Sal Khan.
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If the univers expands, does it mean we are getting further and further from andromeda?(41 votes)
Andromeda is on a collision course with Earth, but we also have evidence that an ancient galaxy collided with the Milky Way galaxy in the past. Gravity has a pull still even though the universe is expanding at an exponential rate, so the two galaxies still have a pull on each other.(13 votes)
Wouldn't the sun be basically touching us if we had existed for 10000000 years?
If the universe kept expanding and the sun got closer to us.(7 votes)
we aren't getting closer to the sun, we are orbiting it. the earth won't be consumed for, and i'm approximating here, another few billion years. even then, it's only because the sun's expanding, not because we are going closer, but because the surface of the sun is getting closer... and closer... and closer...(3 votes)
Is there an edge or end of space, or is it infinite?(7 votes)- Romina is right except that the big bang is just a theory that can be disproved and it may not be true. if so, it is still finite with no edges as long as it is the 3d area wrapped around the 4d sphere, which is also a theory. that is not good that we assume everything based on theories :((2 votes)
- here is a question, maybe I am confused.
So the observable universe is limited to what we actually can see/measure which is about 4.6 billion light years in any direction. However, the actual universe is conjectured to be much much larger since it is posited that light beyond the 4.6 billion light years hasn't had the time to reach us or it never will.
so far so good?
Therefore, we can discern that the age of the universe is at least 13.7 billion years old since the big bang, but is it possible that measuring the oldest light/microwave leaving the big bang is limited to what we can see/measure which is posited to be 13.7 billion years of age instead of it being much older since certainly, we may not be able to see/measure some light/microwave that at the time of the big bang or after the recombination event has since speed away beyond our horizon. Thus we are only dating the age of the universe to the oldest light/microwave source which may not in actuality be the oldest light/microwave source but the eldest that we can perceive. Therefore the real age of the universe is much much older than 13.7 billion years.... What do you think, have I made a mistake?(4 votes) 
Is it a coincidence that the estimated age of our universe is the same as our observable radius of it, or is there a greater significance I'm not picking up on?(4 votes)
It is no coincidence. The "Observable Universe" is an edge in time, not distance. In other words, if there is space past 13.7 billion light years away from us, let's say 14 billion light years, then we won't be able to see that light until another .3 billion years have passed because it will take that light that exact amount of time. If we wanted to see something 14.7 billion light years away, we wouldn't be able to see it yet because the light from that point 14.7 billion years ago hasn't had enough time (1 more billion years) to reach us yet. The age of the Universe (in years) will always match the size of the Observable Universe (in light years). If are confused, let me know.(3 votes)
does our atmosphere slow down light(1 vote)
Not so that you would notice. However it will distort the light waves. That's why stars appear to twinkle. Because of this distortion that's why astronomers want to put the observatories as high in the mountains as possible. Its also why the Hubble Space telescope does such a wonderful job looking at stars from orbit, well above our atmosphere.(7 votes)
Can anyone PLEASE tell me what is the proportion of the observable universe to the starting of the big bang? I searched a lot on internet but here isn't something anywhere:((2 votes)- It is estimated that the entire universe is at least 10^60 light years, which happened due to inflation.(5 votes)
If the universe expands constantly, What is beyond the limits, dark matter?(1 vote)- If the universe is infinite there is no need for anything out side to expand into, you double the double the size of an infinite universe it is still infinite.
If the universe is not infinite it is closed like a sphere and our 3 dimensional space is its surface. Just like there you will not encounter an edge on the surface of a sphere you will not encounter an edge in space to get outside of. The hard part of this to visualize is that there doesn't need to be anything off the surface of the sphere for the universe to have the shape of a sphere. However there can be more dimensions than 3 and then there would be something outside the universe but we can not get there. (Before you ask about our universe being 4 dimensional I have only been talking about dimensions of space)(4 votes)
- In your video(s) you make the assumption that the body that is emitting the photon is accelerating away from the body receiving the photon. If the universe expanded in all directions following the big bang then how can this assumption be made? Maybe objects are accelerating away in different directions to each other and some objects are accelerating in the same direction. Or am i not grasping the nature of the universe's expansion correctly?(1 vote)
You're right. Although space itself may be expanding uniformly, galaxies across the universe aren't moving away from each other uniformly because they also tend to interact with each other with gravity. It's just that the vast majority are red-shifted to various extents indicating that they are moving away from us. A few objects such as the Andromeda galaxy are blue-shifted because they are moving towards us. The Andromeda galaxy is expected to collide with ours in 4.5 billion years. Gravity is also the reason our galaxy stays the same size even though the universe around it is expanding.
The biggest thing you need to grasp here is that when we say that the universe is expanding, we're saying that physical space is expanding and that the universe isn't expanding into anything.(4 votes)
- So since the universe is expanding at a velocity faster than the speed of light, does that mean that a "person" at the edge of the universe could never see the middle of the universe as it is today?(4 votes)
- I have the same question...Is it because it is really accelerating 99.9999% the speed of light, but relativity makes it appear faster to us, or is it that both points are moving away from each other so our combined acceleration is greater than the speed of light? I asked my physics teacher this and he said no to both, he told me it is actually traveling faster than the speed of light, but didn't really offer an explanation, so I kinda think he's full of it. Can anyone explain why we can't see the light, or why my physics teacher thinks massive objects can literally move faster than the speed of light?(2 votes)
Video transcript
I want to make a quick
correction to the last video. It doesn't really affect the
learning of the last video. But I just want to make
sure that you understand that I got the math a little
bit wrong in the last video. I said that you had this
state, 300,000 years. So we talk about the
Big Bang happening 13.7 billion years ago. And then I talk about
this state of affairs where we're maybe 30
million light years away from the edge of the
observable universe, the current observable universe. And I said that this
was about 300,000 years after the Big Bang. That's what I talked
about in the last video. That was our starting
point when the photon started leaving that point. And obviously, the
universe kept expanding. The photon, it kind of
traversed more and more. But still had more
and more to travel as the universe expanded,
as all of space expanded. But this is 300,000
years after the Big Bang. Now my brain, because I was
kind of not thinking hard enough about it, I said, hey, this
was 13.4 billion years ago. That's what I incorrectly
said in the last video. I said that this is
13.4 billion years ago. That's what I said
in the last video. And that is wrong. Because if this was
13.4 billion years ago, this would have been 300 million
years after the Big Bang. We were talking about
only 300,000 years after the Big Bang. So it wouldn't have taken
that many decimal places off of something in the billions. The correct answer is this
would have been only a little less than 13.7 billion years. It actually wouldn't have
even made a significant digit. So this is still approximately
13.7 billion years ago. So I wanted to just
make that correction. It was a slight error. I shouldn't have viewed
this as 0.3 billion years. This is only 0.3 million years. It doesn't even basically
change the precision on this number right over here. So I just wanted
to clear that up. But hopefully, it doesn't affect
your understanding too much.