The Cosmic Horizon


Uploaded by vmargoniner on 16.06.2011

Transcript:
Hi, welcome back to the Cosmic Classroom.
Now we'll talk about the cosmic horizon.
And it's a good idea to have checked the Big Bang
uh... video before
you go into this one.
The first image that I have here is that history that I showed in the Big
Bang video. So it's a history of the evolution of the universe. It started
very dense and very hot here
at sometime thirteen point seven billion years ago
and as time progresses it expands and it cools down.
If you look at this image a little bit more carefully, you'll see that there's
a time here
where a lot of expansion happened. So I'll zoom in right there.
There's a time of rapid expansion and that time right there
is what we call the time of inflation.
The universe grew so fast, that
that objects lost or pieces of
space lost all contact.
For example
the universe is all of his background that I...
that I have here, okay, it's pretty uniform at the large scale
and
because the universe expanded so much in the era of inflation that the
universe is the way bigger than what we can observe.
We can only observe
light that has traveled thirteen point seven billion
years.
Light only has that... only had
that much time
to travel and reach us. So objects that are further than thirteen point
seven light years from us
cannot be observed yet.
It's possible that they will be able to be observed as the universe continues to
expand, but
it depends on how fast the expansion of the universe is.
So, observable... universe is limited. And it's centered
around us, around our position here and it's thirteen point seven billion
light years
in radius. Now
this is the size of the observable universe
and if we look at it in detail,
we'll also realize that the objects that are further away from us, this
blue candles here,
we are detecting the light today
that was emitted
when those objects were very young,
light has been traveling almost the entire age of the universe
in order to reach us here today.
So we observe galaxies that are far away as they are young,
you observe galaxies that are intermediate distances,
as intermediate age
and galaxies that that closer to us
as...as older.
Assuming that they all formed together at the same time.
So, this is another
image of the evolution
of um...
the universe with time,
and you see that if we observe the universe nearby, let's say you use a
telescope, we take pictures, but we don't take a very long exposure we just
observe the galaxies nearby. Then we are seeing right here,
twelve to fourteen billion years, very close to the age of the universe today.
As we look with more and more powerful telescopes we start to see galaxies that
are further away
and if we look in the infrared
we are able...in the radio, excuse me,
we are able to observe this cosmic background radiation
that we talked about.
So the further we look
the more in the past we look
until we look at the cosmic background radiation and there is nothing
more
that we can see before because
before then no light
that was emitted was able to reach us here today.
Now there's one more tricky thing
about understanding the cosmological horizon, the idea of...
of the fac... of the fact that the observable universe is finite.
Let's think about this carefully.
Here we are on earth
observing a galaxy that's far away.
The galaxy, today, emits a photon of light that will travel
towards us.
Well the photon will be emitted, but the light won't instantaneously reach
us because it takes some time for light to travel.
As the light travels, the distance between earth and this far away galaxy
increases because of expansion of the universe.
And continues to increase.
So the path
that this photon has to travel, that this light has to travel,
is actually
greater than the initial distance between
earth and
the galaxy.
So, distances in cosmology can be tricky.
When we say that a galaxy is at a certain distance from us, do we mean
it is at a certain distance today,
or that it was at that distance at the time the light was emitted.
We need to be careful what we are talking about.
When I said that there was a radius of thirteen point six
billion years or thirteen point seven billion years
around us that we can see, we... I meant
that light has had this amount of time to travel. We can't see...
light that has, that would need to travel
more than that.
But,
in reality...
we can see galaxies that are now
much further away
than thirteen point
seven billion years ago
They were
closer to us when the light was emitted, but now they are very far away.
So, if you Google "cosmic horizon"
in Wikipedia, for example, you'll see that the radius of the observable
universe is ninety three billion light years.
That is because
the galaxies that we are observing now, the further away galaxies that we are
observing now,
today,
are at that distance from us.
So we need to be careful there.
Well I hope this helped you understand why is it that we don't observe
the entire universe and that there is a limit
to the observable universe.
I suggest you check
my YouTube videos on the Expansion of the Universe and on the Big Bang
and that you also go read on this book Astronomynotes.com which is a
free pretty good
textbook.
I hope it helped, I'll see you next time...bye