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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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Cosmic background radiation
Cosmic Background Radiation. Created by Sal Khan.
Want to join the conversation?
- where did the protons come from?(17 votes)
- 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.(38 votes)
- 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)
- 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)
- 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?(18 votes)
- seems reasonalble(4 votes)
- 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? 1:57(9 votes)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- The spectrum of H (or He for that matter) consists of a number of observable frequencies which have a characteristic ratio to each other. This serves as a fingerprint. Look here: http://en.wikipedia.org/wiki/Balmer_series
If it was just a single frequency for each element, you would be right!(3 votes)
- Why the dense mass in the beginning didn't just collapse like a black hole because of the gravity?(4 votes)
- 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)
- How are neutrons formed? Hydrogen atoms doesn't need neutron but the other gases do need neutrons to form its atom.(1 vote)
- 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)
- Why is it called "microwave" radiation, but Sal said it was radio waves?(3 votes)
- Microwaves are a sub band of radio waves(2 votes)
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.