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Well lets go back now. If space is expanding and this was known at the time of Gamov, then suppose we go back, suppose we interpret this backwards, wouldn't there be a time when everything was on top of everything? The answer is "yes", and where would that be? Well we know the velocity, things that are a certain distance way are moving at a certain velocity, just reverse that velocity and see how long it would have taken for it to be here. The answer turns out to be somewhere between 10 and 20 billion years; I'll use the number 15 billion as an average estimate. The earth was created about 5 billion years ago, we believe, so that sort of fits in okay. When people first went back at this and looked at it and said well 15 billion years ago everything was on top of everything, that was somewhat mysterious and bothersome. What Gamov did, was that he took this crazy idea seriously. He said "I know that's absurd, but how do you understand the universe. You either work with absurdities or you ignore them." People sometimes say to me that, when I tell them that at the end of this talk we will be discussing the issue of whether the universe is finite or infinite, they say "how can you imagine a finite universe" and I say to them "oh, you mean you can imagine an infinite one? I can't imagine either one of these things", so they are both equally absurd.

Let's examine this absurdity. It's amazing how it paid off. If you extrapolate backwards to 15 billion years ago what do you get when everything was on top of everything, well in physics you do this by just turning the film backwards. You imagine everything coming together and say what would have happened. It is all more or less reversible, and the answer is, and this is what Gamov did, was he said in the early universe it must have been extremely dense. All of space was filled with extremely dense matter. He also postulated that because it was so compressed from all of these things coming back that it would be very hot and so anim1 I have a picture here of Gamov's early universe and what it looked like and there it is. He called this "Ylem", which I think is Yiddish for chicken soup or something like that. It was a picture, when things were so hot and so compressed the density was greater than the density in the present interior of the sun, the temperature was far, far higher. In fact, you start calculating these things now and imagine the universe, but this is not a local region, this is the entire universe, is filled up with this stuff. It is in the process of expanding and when it expands it cools off, but right now it is very, very hot. How long is it hot like this? Well it is expanding very rapidly so it is at a trillion degrees for only a few seconds. What we have here are a few atoms and the black dots are supposed to represent atoms, and then we have some blue dots, those are supposed to be protons and red dots are electrons, we have a lot of protons because this is all hot. We have heat radiation, black body radiation, scattering around and that bounces off electrons and then we have this big mixture here, I will show you an actual photograph of this shortly because with the Cosmic Microwave Explorer (COBE) satellite we have much more detailed information on this, but this is the entire universe and the earth is going to form right there and the universe is expanding everywhere.

There is no center to the universe. It is expanding everywhere. By the way, I want to be very clear in telling you what it is that is speculative and what it is that essentially everybody accepts now as having been proven, and we are in the proven part. I mean, this may not be accepted in every textbook, but among working cosmologists this is no longer in question. The evidence is so overwhelming, like the evidence for Darwinian evolution. It was disbelieved by some cosmologists just a short few years ago, but they have all died off now, and the remaining people all take this essentially as the correct "standard" model.

ylem2 Okay, now as it cools off what happens? Well the first things is that these ions, the positive and negative charges, start recombining. When it was hot they kept on getting ripped apart again, and you had a plasma. When it cools off, the temperature drops and suddenly it is not a plasma anymore. Let me illustrate that by just removing all the electrons and protons until you have nothing left but neutral hydrogen atoms, and I'm just removing them. Those are the same things now, they are neutral hydrogen atoms and there is one other mistake here. You notice all these light particles that are drawn by squiggly lines by a physicist, they are bouncing. Well those are bouncing mostly off the electrons. Electrons are charged particles with light mass and so they are easily moved by electric fields, but once the electrons have become part of atoms, then these light particles don't bounce so much. We should straighten them all out, let me just remove them and remove all the bouncing particles and there what we have now is the universe. ylem3 This is the way it looked about half a million years after the beginning. So this is 15 billion years ago minus half a million. This process took place, it took about half a million years for it to cool off to this temperature. At this point the protons are moving in straight lines.

"Ylem" Summary Animation

Now here is a remarkable prediction: (Prediction? I should have made this "prediction" 15 billion years ago.) These protons, these particles of light at this point will travel in straight lines to the end of the universe unless they happen to hit a star, which is very unlikely. The sky is mostly black. So for the most part these particles of light travel in straight lines. Now, here is the earth right here. Let me get rid of these atoms because they are essentially transparent to these particles. Just take them away, and here we have the light particles; the earth is going to form right there. Now in 15 billion years some of these light particles will hit us; like here is one that is aimed in the right direction, right? Here is another one, and here is one, let's ignore all the others because they are just going to pass by, and look at just the particles of light that happen to be aimed towards the earth. Let me remove all the others.

ylem4 I have two sets here, the ones that are reaching us today and the ones that are reaching us 15 billion years from now. These particles are around today, but remember they were sitting on matter that was expanding, that was flying away from us at an enormous velocity. In fact, the velocity was close to the speed of the light, and so when these particles reach us, that is this light reaches us, it has a Doppler Shift, a red shift. How much is it red shifted? When it was emitted it was visible light, but that is in the frame of the moving matter. In our frame of reference this stuff is expanding away from us. In fact, reddened all the way down to microwave radiation. So, if we look for microwave radiation and this is what Bob Dicke, who unfortunately died this year, began a program to look for this radiation. Penzias and Wilson, who weren't looking for it but stumbled upon it, found it first so they got the Nobel prize.
Astrophysical Journal
Nov. 1, 1965
Cosmic Black-body Radiation
R.H. Dicke
P.J.E. Peebles
P.G. Roll
D.T. Wilkinson

"While we have not yet obtained results with our instrument, we recently learned that Penzias and Wilson (1965) of the Bell Telephone Laboratories have observed background radiation at 7.3 cm wavelength..."

A Measurement of Excess Antenna Temperature at 4080 Mc/s
A.A. Penzias
R.W. Wilson

"A possible explanation for the observed excess noise termperature is the one given by Dicke, Peebles, Roll, and Wilkinson (1965) in a companion leter in this issue."
- XBL 7811-6656

Two papers were published. This one I love! look at the title of this paper, "A Measurement of Excess Antenna Temperature at 4080 Megacycles per Second". That doesn't sound like a Nobel prize winning paper, but it was because what they saw was a 4000 megahertz signal that seemed to be coming from distant space, uniformly, with what appeared to be a black body spectrum. This was the radiation I was just telling you about. This was the most important confirmation of the "Big Bang" theory, and Dicke and Peebles and Wilkinson then wrote a paper explaining it. Their paper was published in the same issue as the Pensias and Wilson paper. They went on to make many, many measurements; they in fact did discover radiation and many of its features. The features of radiation are:

  1. image First if you measure the spectrum, according to the theory, it should be a "black body" spectrum. This means it has the same shape to it that you get when you just heat up an oven really hot, as you would for making pottery. The glow from a pottery kiln is an example of black body radiation. According to physics there is a certain peak in the radiation and it has a certain shape. The recent COBE satellite measured the squares on this plot, which are the measurements of the intensity of the radiation as a function of frequency. The line is what you would get if it were a perfect black body emission. Measured on the COBE satellite and published about 2 years ago, this is an amazing result, probably a Nobel prize for this discovery just waiting to be awarded.

  2. image image image

    The uniformity of the radiation is another issue. It should be uniform unless the earth is moving. If the earth is moving, then it will be a little bit brighter in the direction that we are going and a little bit less bright in the direction we are coming from. Actually that is the work that I did in this field. I and my colleagues measured with an instrument that was sensitive enough to measure the uniformity of this. image The first view here, this one here, shows the pattern; well this is was a subsequent measurement. It was actually done on a satellite, but it shows the pattern that we discovered. It shows it is brighter in fact in one direction and dimmer in another direction and there is this sort of ying/yang. I call it "the great cosine in the sky".

From the motion of the earth, we measured the velocity of the earth with respect to the this distant "Big Bang" frame. We sit in this room and we don't think that we are moving, it doesn't feel like we are moving but if you look out you see we weren't moving. Suppose we were in a train, then you might look out and see that we were moving. We looked out and saw that with respect to the distant matter of the universe we are moving.

image image Now George Smoot, my colleague, took these results and created a satellite experiment which, ignore this middle picture because that is actually the Milky Way galaxy, with his satellite experiment called COBE, he actually found that the thing wasn't completely uniform. At the heart, in the hundred thousand level, there are small variations in irradiation which we believe are what happens when you begin to form galaxies and clusters of galaxies.

SLIDE12 Remember this picture here? I said see these photons that are coming in today, they are pretty uniform, but they are not completely uniform because this matter was getting a little bit clumped already half a million years after the "Big Bang". If you want to look at this thing, just take a microwave image of the sky and that's what this is, we are looking at this matter. It is called the cosmic background radiation. I like to think of it as the background, because in here are the quasars, in here are the galaxies, everything else is closer.

imageThis is the furthest thing we can see, it is literally the background and here it is, this is a relatively recent picture of it. It is funny, George got a lot of publicity and he was trying to get people interested in this, and he referred to "we are seeing the face of God". I don't know if that is a good thing to say or not. I mean I think he is right, but I think when you are looking at a flower you are also seeing the face of God and so this qualifies too.

What is the dominant matter of the Universe?

What role did elementary particles play in the creation of the Universe?

Is the Universe finite in extent?

Will the Universe go on forever?

Now briefly, questions: What is the dominant matter of the universe? What role did all major particles play as the universe? Will the universe go on forever? Interesting questions that we can now address.

Let me talk about this issue of the matter of the universe. One of the real surprises that came out of this is the fact that we don't know what the universe is made out of. I mean we know part of it, we know minor constituents, we know that there is probably 10 or 20% of it is made out of protons, neutrons and electrons, the rest of it we don't know. It is one of the most exciting questions in science. How do we know that we don't know?

Mass Density from Deceleration

Well, here is a picture of the universe again, here is the Milky Way, that spot is the Milky Way. This stuff here are all the other galaxies and the distant galaxies are moving away from us. We measure their velocity, we can also see that they are slowing down. We believe they are slowing down because of this gravitational attraction. Distant galaxies are being attracted back. The mutual gravity between galaxies is slowing down the expansion and from that we can estimate how much mass there is in the universe and you can compare that to what happens if you measure the mass of the galaxies and we find that probably somewhere between 80% and 95% of the mass of the universe is not the mass we seen in galaxies and stars. What is it? The answer is "we really don't know". It is one of the outstanding issues of science. That is why I say we don't know what the universe is made out of, well we know what 10% or 20% of it is made out of stars and matter. What is the rest made out of? Is it made out of neutrinos? Or exotic matter? You can find all sorts of acronyms in the popular press, WIMPS for "Weakly Interacting Massive Particles", MACHOS for "Massive Halo Objects". We really don't know which, or even if either of these is correct, and there are programs that theoretically do experimentally try to figure out what the universe is made out of. It is ironic that we don't know what most of the universe is made out.


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