If you're a string theorist, then maybe you're done.
We have described the calculations and all of the answers and we know everything
but if you're interested in Astronomy and have some interest in the real world,
then now we'll discuss some of how we know about all these things and all the
great phenomena that have taught us and some more about the consequences.
So one of the most exciting discoveries in this field is the discovery of ancient
light, essentially the farthest thing we can see.
Where does this come from? Well remember, there was this time of ionization when
the temperature was about 3000 Kelvin and that means that the radiation gas had an
average energy corresponding to KB times 3000 Kelvin,
that's exactly the ionization energy for ionizing the hydrogen atom.
At temperatures above that, hydrogen atoms were unstable because the ambient
photons in the photon gas could just knock electrons out of hydrogens so there
were no hydrogen atoms. There was a dense plasma and at later
times as the universe cooled, hydrogen formed atoms and we had neutral hydrogen
atoms by in-large though they were later re-ionized in as I said, in the, the hot
inter-cluster gas clouds. But after recombination radiation
essentially decouples from matter. So, photons that existed at the ambient
temperature, at the ambient energy, these were
ultraviolet photons with a temperature corresponding to a temperature blackbody
spectrum, with a temperature of 3000 Kelvin that light spectrum as a once
neutral hydrogen atoms formed, those photons basically proceeded through the
universe unimpeded. Some of them, of course, bumped into
little fluctuations like stars and galaxies.
Since most of space is empty, most of those photons are still around.
And the energy in that radiation field has been conserved.
Those photons are still here except they have been red-shifted.
And so, we can compute, since this happened at Z of about 1,000/100, the
temperature will have decreased by the scale growth of the scale factor by a
factor of a thousand, we expect the universe to be imbued, to
be bathed in a blackbody, in a gas of blackbody photons with a temperature of
about 3 degrees Kelvin. And this was realized by a theorist at Princeton named
Dicke in 1960, and this had been predicted by several other works that
we'll turn to later. But Dicke in 1960 has the idea that he's a radio astronomer
and he can build a Dicke radiometer and try to measure this.
And indeed, he set some of the researchers in his group to try to design
a Dicke radiometer to measure the this relic radiation from the Big Bang if you
want direct evidence for the Big Bang. Now, independent of that, 30 miles from
Princeton at AT&T Bell Labs, two engineers, Penzias and Wilson are
building a Dicke radiometer. Their idea is they're trying to develop
the technology that will become satellite communication so they build this great
big huge horned radio antenna that's here seen on the right, and there's a noise.
This always happens with experimenters with experiments.
the experiment is not perfect and their antenna picks up some noise, and they try
to clean the contacts, and they have this, this humorous episode where a
family of pigeons had made their home in the antenna,
look at the size of the thing, and they spent a few quality days scraping pigeon
droppings from the antenna thinking maybe this is the cause of the noise.
Nothing removes the noise usually when that happens in a radio
experiment, you are probably receiving the BBC from somewhere, so they turn
their antenna in different directions. It's a very directional antenna.
And they find that the noise is the same from every direction.
They ruled out all terrestrial sources. They try to imagine, maybe this is from
some astronomical source, so they try to check
whether when they aim it at the plane of the galaxy or away from the plane of the
galaxy, they get a different signal. The signal doesn't care where they're
aiming it. The signal is completely isotropic.
It's not terrestrial, it's not galactic. In despair, they call an eminent
theorist, luckily they called Dicke at Princeton and the, at least, legend has
it that he is interrupted in the middle of a group meeting by a call from these
engineers at AT&T Bell Labs who want to ask a question and Dicke comes back and I
think it was just Bell Labs back then, and Dicke comes back and tells the
research group, don't worry about designing that radiometer, we've been
scooped. What these guys had discovered was indeed
the 3 degree thermal radiation at microwave wavelengths that is the relic
remnants of the 3000 degree photons that were around when hydrogen atoms first
formed and since then, we learned a lot about this background
radiation. In particular, it is a blackbody,
remember that a red-shifted blackbody remains blackbody spectrum, except the
temperature is has decreased and what you see here is experimental measurements
from the COBE satellite in the 90s and fit to a blackbody spectrum and this is
as beautiful a fit as you can get so we have measured the temperature of the
universe. And the temperature of at least the
photon gas in the universe, matter, of course, is essentially 0 temperature,
dust is cooled down to 0. But the temperature of the radiation
field in the universe is about 2.726 degrees, 2.726 Kelvin, and we know this
with high precision. and this, the, the the other important
point other than its a blackbody spectrum validating the fact that this comes from
some promodial heat, hot dense universe is that it's isotropic to one factor to a
degree of one part in a 100,000. So, this, if you want, is a great
validation of our cosmological principle that the universe is homogeneous and
isotropic because the blackbody radiation, that it, fills the universe is
the same in all directions. For this discovery, of course, Penzias
and Wilson received the Nobel Prize in 1978.
Notice that this is the answer to the question, how far can you see? This is
the oldest light you can ever see. Why is this the oldest light? this is the
universe as it was when it was 380,00 years old.
And, of course, there was 380,000 years of history before that, but we'll never
see them because photons created then interacted strongly with the plasma were
absorbed and re-emitted and absorbed and re-emitted.
And then eventually, the photons that were emitted 390,000 years ago, those are
the photons that are still around. So, this is the most distant and the most
far into the past that we can ever see. We'll never, for example, see the Big
Bang. So this a very good image.
This describes the temperature fluxuation of the microwave background.
Microwave background is extremely isotropic.
If you were wanting some evidence for isotroppy of the universe this is about
as good as it gets. But when in the 90s, COBE makes more
precise measurements of the precise blackbody spectrum,
you'll find that there is temperature variation.
The temperature is not completely uniform.
This is, in fact, the same map with higher precision, blue representing
regions that are slightly hotter. Red representing regions that are
slightly colder, obviously this was done by astronomers, and you see that there's
a blue region in the sky and diametrically opposed, 180 degrees away,
a red region in the sky. The blackbody spectrum of the universe is
not isotropic. I know what that means, that means that
we are not at risk relative to the Hubble flow.
This is a measurement of the Earth's motion relative to the local Hubble flow.
This is our peculiar motion, what we are seeing is that we are moving in a
particular direction. In fact, subtracting the Earth's motion
around the sun and sun's motion around the Milky Way,
all of which will have an affect, you'll see, what we have really measured is that
our local cluster, as a whole, is moving with a peculiar velocity of 600 km/s in
the direction of the Virgo cluster. And again, that's not exactly our motion
because you have to subtract all kinds of motion round the Milky Way to get that.
But that we know, when we subtract that, we find 600 km/s towards Virgo as the
motion of the local cluster, so there is a local rest frame.
It's determined by the cosmic microwave background and we can measure our
velocity relative to that, and the universe is really, really, really, very
isotropic.