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- Right now, the best estimate of when the Big Bang occurred -
- and once again, I don't like the term that much, 'cause it kind of implies
- some type of explosion, but what it really is is kind of an expansion of space
- - when space really started to expand from a singularity
- but our best estimate of when this occurred is
- 13.7 billion years ago, and even though we're used to
- dealing with numbers in the billions, especially when we talk about
- large amounts of money and whatnot, this is an unbelievable amount
- of time. It seems like something that is tractable, but
- it really isn't. And in future videos, I'm actually going
- to talk about the time scales, so you can really appreciate
- how long, or even start to appreciate, or appreciate
- that we CAN'T appreciate how long 13.7 billion years is.
- And I also want to emphasize that this is
- the current best estimate. Even in my life time, even in
- my life time that I actually new about the big bang and
- that I would pay attention to what the best estimate was,
- this number's been moving around, so I suspect
- that in the future, this number might become more accurate
- or might move arround some.
- But this is our best guess. Now, with that said, I want
- to think about what this tells us about the size
- of the observable universe.
- So, if all of the expansion started 13.7 billion years ago...
- [and] all of everything we know in our 3 dimensional universe
- was in a single point,
- the longest that any photon of light could be traveling
- that's reaching us right now...
- (so, our eye is right...)
- (so, let's say that's my eye right over there)
- (that's my eyelashes, just like that)
- the longest - so, some photon of light,
- is just getting to my eye, or maybe
- it's just getting to the lense of a telescope -
- the longest that that could have been traveling
- is 13.7 billion years.
- So when we looked at that depiction
- (this, I think, was 2 or 3 videos ago)
- of the observable universe,
- I drew - it was a circle.
- It was this circle.
- And, when we see light coming from these
- remote objects, that light is getting to us right here
- this is where we are. This is where, I guess,
- in the depiction, the remote object was,
- but the light from that remote object is just now getting to us.
- And that light took 13.7 billion years to get to us.
- Now, what I'm going to hesitate to do,
- because we're talking over such large distances
- and such large time scales ...over which
- space itself is expanding, we're going to see
- that you cannot say that this object over here -
- this is not necessarily...
- this is NOT
- (I'll put it in caps.)
- This is NOT 13.7 billion light years away!
- If we're talking about smaller time scales,
- or, I guess, smaller distances,
- you could say aproximately that.
- The expansion of the universe itself
- would not make as much of a difference.
- And let me make it even more clear.
- I'm talking about an object over there,
- but we can even talk about that coordinate in space.
- And that coordinate in (and actually I should say,
- "That coordinate in space-time."
- 'Cause we're viewing it at a certain instant as well.)
- But that coordinate is not 13.7 billion light-years
- away from our current coordinate.
- And there are a couple of reasons to think about it.
- First of all, think about it.
- That light was emmitted 13.7 billion years ago.
- When that light was emmitted, we were much closer to that coordinate.
- This coordinate was much closer to that.
- Where we are in the universe now
- was much closer to that point in the universe.
- The other thing to think about is, as this - let me actually draw it.
- So let's say that - let's go three hundred thousand years
- after that initial expansion of that singularity.
- So, we're just 300,000 years into
- the universe's history, right now.
- So this is roughly 300,000 years into
- the universe's... life, I guess we could view it that way.
- And let's say at that point - well, first of all
- at that point, things haven't differentiated
- in meaningful ways yet, right now.
- We'll talk more about this when we talk about
- the cosmic microwave background radiation,
- but at this point in the universe
- it was kind of this almost uniform
- whitehot plasma of hydrogen.
- And we're going to talk about - it was emmiting
- microwave radiation, and we'll talk more about that
- in a future video.
- But let's just think about two points in this early universe.
- So, in this early universe, let's say you have that point,
- and let's say you have
- the coordinate where we are right now.
- I won't make it the center.
- 'Cause I think it makes it easier to visualize
- if it's not the center.
- And let's say that very early stage in the universe
- if you were able to just take some rullers
- instantaneously and measure that,
- you would measure this distance
- to be 30 million light-years.
- And let's just say right at that point,
- this [magenta] object over here
- emits a photon. Maybe in the microwave
- frequency range, and we'll see
- that that was the range it was emmitting in.
- But, it emits a photon.
- And that photon is traveling at the speed of light!
- It IS light!
- And so, that photon says, "Oh, you know..."
- "I only got 30 million light-years to travel."
- "That's not too bad. I'm gonna get there"
- "in thirty-million years."
- And so - and I'm going to do it descrete.
- The math is really more complicated than what I'm doing here,
- But I really just want to give you the idea
- of what's going on, here.
- So, let's just say, that photon says,
- you know, "In about ten million years, I should be"
- "right about at that coordinate."
- "I should be about one third of the distance."
- But what happens over the course of those ten million years?
- Well, over those 10 million years,
- the universe has expanded some.
- The universe has expanded, maybe, a good deal.
- So let me draw the expanded universe
- So after ten million years, the universe...
- ...might look like this.
- (Actually, it might even be bigger than that.)
- (Let me draw it like this.)
- After ten million years,
- the universe might have expanded a good bit.
- So, this is ten million years into the future.
- Still, on a cosmological time scale,
- still almost at the infancy of the universe,
- 'cause we're talking about 13.7 billion years.
- So let's say 10 million years go by.
- The universe has expanded.
- This coordinate where we're sitting at present
- is now all the way over here.
- That coordinate where the photon was
- originally emitted is now
- going to be sitting right over here.
- And that photon has said, "Ok,"
- "After ten million light-years [sic] I'm going to get"
- "over there,"
- And, you know, I'm aproximating and I'm doing it
- in a very descrete way - I really just want to give you the idea.
- So that coordinate, roughly where the photon
- gets in ten million light years
- Is about right over here.
- The whole universe has expanded.
- All the coordinates have gotten further apart.
- Now what just happened here?
- The universe has expanded.
- This distance that was 30 million light-years
- now - and I'm just making rough numbers -
- now it is actually - this is really just for the sake of
- giving you the idea of why... giving you the intuition
- of what's going on -
- This distance now is no longer 30 million light-years
- Maybe it's a hundred million.
- So this is now a hundred million light-years.
- The universe is expanding.
- The space is actually spreading out.
- You can imagine it's kind of a trampoline,
- or the surface of a baloon - getting stretched thin.
- And so this coordinate where the light happens to be
- after ten million years,
- it has been traveling for ten million years,
- but it's gone a much larger distance!
- It has now gone - that distance might be
- on the order of maybe 30 million light years.
- And the math isn't exact, here.
- I haven't done the math to figure it out.
- But the point here - so, it's done 30 million light-years.
- And actually, I shouldn't even make it
- the same proportion, because the distance it's gone
- and the distance it has to go,
- because of the stretching, it's not going to be
- completely linear. At least, when I'm thinking about it
- in my head, it shouldn't be, I think.
- But I'm not going to make a hard statement
- about that.
- But the distance that it traversed - maybe
- this distance right here is now 20 million light-years
- ...because every time it moved some distance,
- the space that it had traversed is now stretched.
- So even though it's travelled for 10 million years,
- the space that it traversed is no longer just
- 10 million light-years.
- It's now stretched to 20 million light years.
- And the space that it has left to traverse
- is no longer only 20 million light-years.
- It might now be 80 million light-years.
- And so, this photon might be getting frustrated.
- There's an optimistic way of viewing it, is like,
- "Wow! I was able to cover 20 million light-years"
- "in only ten million years."
- "It looks like I'm moving faster than the speed of light."
- The reality is, it's not, because the space coordinates themselves
- are spreading out.
- Those are getting thin. So, the photon is just moving
- at the speed of light. But the distance that it actually
- traversed in ten million years is more than
- ten million light-years.
- It's 20 million light-years.
- So, you can't just multiply rate by time
- on these cosmological scales, here.
- Especially when the coordinates themselves
- are actually moving away from each other.
- But I think you might see where this is going.
- Now this photon says, "Oh,"
- "In another 40 million light years [sic],"
- "maybe I'm going to get over here."
- But the reality is over that next 40 million years,
- it might get right over here,
- 'cause this is 80 million light years.
- The reality is, after 40 million years,
- so, another 40 million years go by...
- Now, all of a sudden, the universe has expanded
- even more!
- I won't even draw the whole bubble,
- but the place where the photon was emitted from
- might be over here,
- and now our current position is over here,
- where the light got after ten million years
- is now over here,
- and now where the light is after 40 million years,
- is maybe over here.
- So, now this distance between these two points:
- when we started, it was 10 million light years,
- then it became 20 million light-years
- maybe now it's on the order of - I dont' know
- maybe it's a billion light years!
- And maybe this distance over here,
- and I'm just making up these numbers,
- in fact, that's probably too big for that point...
- Maybe this is now a hundred million light-years.
- And now, this distance maybe is, I dunno
- 500 million light-years.
- And maybe now the total distance between the two points is a billion light years.
- So, as you can see, the photon might be getting frustrated.
- As it covers more and more distance, it looks back,
- and says, "Wow, in only 50 million years, I've been able to cover 600 million light-years,"
- "That's pretty good."
- But it's frustrated, because what it thought
- it had to cover - 30 million light-years -
- that keeps stretching out.
- 'cause space itself is stretching.
- So the reality, just going to the original idea,
- this photon that is just reaching us
- that has been traveling for, let's say, 13.4 billion years
- so, it's reaching us just now, so let me just fast-forward 13.4 billion years
- from this point now to get to the present day.
- So if I draw the whole visible universe right over here
- this point right over here is going to be where it was emitted from.
- We are sitting right... over there. And actually...
- Let me make something clear. If I'm drawing the whole observable universe,
- the center actually should be where we are, 'cause we can observe an equal distance
- - if things aren't really strange - we can observe an equal distance in any direction.
- So, actually, maybe we should put us at the center.
- So, this is the entire observable universe. And the photon was emitted from here
- 13.4 billion years ago. So 300,000 years [sic] after that initial big bang.
- And it's just getting to us.
- It is true that the photon has been traveling for 13.7 billion years [sic].
- But, what's kind of nutty about it is, this object, since we've been expanding away from eachother
- this object is now, in our best estimates
- 46 billion light-years away from us.
- And I want to make it very clear:
- This object is NOW 46 billion light-years away.
- So when we just use light to observe it, it looks like
- just based on light years, hey, this light's been traveling 13.7 billion years to reach us,
- that's our only way, kind of, with light, to think about the distance, so maybe it's 13.4
- *chuckle* (I keep changing the decimal)
- but, [maybe it's] 13.4 billion light-years away.
- But the reality is, if you had a ruler today
- and, you know, light-year rulers
- this thing, the space has stretched so much
- that this is now 46 billion light-years.
- And just to give you a hint of when we talk about
- the cosmic microwave background radiation,
- what will this point in space look like?
- This thing that's actually 46 billion light-years away
- but the photon only took 13.7 [sic] billion years to reach us.
- What will this look like?
- Well, when we say, "look like," it's based on
- the photons that are reaching us right now.
- Those photons left 13.4 billion years ago.
- So, those photons are the photons being emitted
- from this primitive structure, from this white-hot
- haze of hydrogen plasma.
- So what we're going to see is this white-hot haze
- - so we're going to see this kind of
- white hot plasma.
- Undifferentiated, not differentiated into proper stable atoms, much less stars and galaxies.
- But white-hot - we're going to see this white hot plasma.
- The reality today is that that point in space that's 46 billion years from now [sic]
- it's probably differentiated into stable atoms and stars and planets and galaxies.
- And frankly, if that person - if there is a civilization there right now, and they're sitting right there
- and they're observing photons being emitted
- from our coordinate, from our point in space right now [sic]
- They're not going to see us. They're going to see
- us 13.4 billion years ago. They're going to see
- the super primitive state of our region of space
- when it really was just a white-hot plasma.
- And we're going to talk more about this in the next video, but think about it.
- ANY photon that's coming from that period in time,
- so, from any direction, that's been traveling for
- 13.4 billion years, from any direction,
- is going to come from that primitive state, or,
- it would have been emitted when the universe was in
- that primitive state, when it was just that white-hot
- plasma, this undifferentiated mass.
- And hopefully, that'll give you a sense of where
- the cosmic microwave background radiation comes from.