An introduction to modern astronomy's most important questions. The four sections of the course are Planets and Life in The Universe; The Life of Stars; Galaxies and Their Environments; The History of The Universe.
Observational Evidence
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Confronting The Big Questions: Highlights of Modern Astronomy
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從本節課中
What is the Fate of the Universe?
The History of The Universe - Why the Big Bang? A History of Time, What Happened Before the Big Bang
與講師見面
Adam FrankProfessor
Physics and Astronomy
Welcome back, everybody.
So, you know, we've been going through this
really remarkable story about the birth of the universe,
and you may wonder how do we possibly have,
how can we tell this story is true, right?
because this is science.
You know we're not doing mythology.
We think that we should have observational evidence for
any part of a story in science that we believe.
And what's remarkable about cosmology is we actually have quite strong evidence
that this story of the universe beginning as a small or
beginning compressed and high temperature, and then expanding actually is true.
So, there are actually three pillars of what we call
three pillars of evidence for the classic Big Bang model.
Let's run through them.
We've already talked to them about them
before, but let's just touch on them again.
The first is cosmic expansion.
We, Hubble discovered this in 1928, that all the galaxies that
we see are all expanding way from us.
And from the symmetry of the situation, we know that
all the galaxies are expanding from all the other galaxies.
So if the galaxies are just pinned onto
space time, that tells us that space is expanding.
So that's the first part.
That, that space time is expanding and if you run the movie
backwards, that means that everything must have been compressed at earlier times.
We've already talked about Big Bang nucleosynthesis.
The idea that during the first
three minutes or around three minutes after the Big Bang, the universe
was hot enough and dense enough to allow nuclear fusion to occur.
We can actually go out and measure the abundances
of the products of that Big Bang nucleosynthesis, things like
primordial helium, some isotopes of helium like helium 3, and
by going out and finding places where it's convenient or
easy to measure these light elements.
We can get their abundances, and compare what we observe about how
much primordial helium there is versus how much is predicted by the models.
And see whether or not the models work.
And it turns out that the models work exquisitely.
So there's a very, very strong agreement between observation and theories.
And that tells us not only does it tell
us that the Big Bang actually happened, it actually puts
pretty severe constraints on the variations in the Big Bang models.
So it actually eliminates classes of Big Bang model for us.
The other third, the third pillar that I've already mentioned
is the cosmic microwave background radiation, which is, as I said,
perhaps the most important evidence we have that the universe must
have been hotter at one point, and must have been denser.
Because there's really no way to have this fossil radiation
existing all throughout the entire universe, no
matter which direction you point in, without having
a period, an earlier period when space
must've looked much different than it does today.
We go out and we look at space now, and it's pretty much empty.
Matter is, is very, very clumped, with, you know, large gaps or voids between
it, and that's just not the case if there was for the, if there.
That couldn't have been the case if you were to be
able to be be able to produce the cosmic microwave background.
So that's clear evidence that the universe must have
been much much hotter and much much denser earlier on.
Now we also talk about inflation, which is
this addition to the standard Big Bang model.
That early on a chunk of the universe, or not a
chunk, a tiny speck, actually went through a huge period of expansion.
A very rapid period of expansion.
And that little sliver of the universe
is what we called the observable universe today.
And there's actually some
evidence for inflation, it's not a huge amount
of evidence but there is some evidence for inflation.
And in particular it comes out of the fluctuations that must
have occurred during inflation, the quantum
mechanical ups and downs, jumps and
you know, regions of slightly over density or under density that
must have been imposed on the universe during that era of inflation.
And when we look today, when we go out and we look at the distribution
of galaxies in the universe today, or we
look at the cosmic microwave background and look
at the tiny fluctuations in temperature or density
we can tell the spectrum, the range of perturbations.
You know how much how large or how many
perturbations were there that were you know larger than
a few light years across and, how many that
were larger than ten light years across, et cetera.
And those actually compare
very well to the models of inflation.
So, there is actually some evidence that this
very early epoch of insane expansion, that went on
to inflate the bubble of this space time
that we call our universe actually occurred, as well.
So, we have three enormously solid pieces of evidence for the Big Bang and,
you know, some evidence for this addition to the Big Bang that we call inflation.
And if you want to come up with another model, if you're like the
Big Bang's crazy, then you need to address these three pieces of
evidence and if you cannot simultaneously account for the expansion, account for
the distribution of light elements and the cosmic microwave background, then your
model just isn't going to go head to head with the the Big Bang.
So, we really have excellent evidence for what
happened after the universe began, however it began.
You know, running the clock
forward from there, expansion, cosmic microwave background, et cetera.
So we have really a, a, a pretty coherent story that
is backed up by evidence, and that's the important thing to understand.
Okay?
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