Look around our solar system. There are probably some out there like us somewhere. Look at our galaxy. Our galaxy is part of a local group that is a bit different from the stuff beside it. So, on these small scales everything is a little bit different. Yet there will still be more there than if I put it a billion light years away, not in the middle of any galaxy.
The odds are there will be nothing at all. So you see the density on that scale changes. It matters where in the Universe I put the box. The Universe is homogeneous on that scale. If you want a local analogy, take cake. It matters if you have a pin prick you put into the cake. It will matter whether you go into the cake or you go into a raisin.
So on those scales, the scale of a pin head; you get very different answers to what the cake is as you prod your pin in and out. So you want a big enough box that you care about raisins and cake and not about individual things. Pamela: We see this just within subdivisions here in the United States. Pamela: Well, this is where for whatever reasons developers decide everyone is going to live in very similar houses with very similar yards.
Lintott: In English we say estate, but still. But, if I expand my box to contain yard and patio I can start to average out.
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There is always that person with the pool. There is always the person who decided to build an addition over the garage.
Lintott: But if you take a big enough section there is probably always two pools scattered among them. Pamela: Anyone who has tried to find someone living in a subdivision knows that really, everything looks the same. We need it to be so so that the Cosmological Principle applies and cosmologists can be happy.
Have we had a big enough box to test it? I think the answer is just about no. You must have something bigger than the biggest galaxy clusters. There are very large fluctuations across the Sloan, some very large scale features which may or may not be important. Pamela: We live basically in a Swiss cheese Universe.
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The problem is well we all know Swiss cheese has holes and well cheesy bits of various different sizes. Lintott: Yes, once you get to very big. Once we get to the big scales we have to worry about changes with time. So think about it. We talked about the Microwave Background, this very smooth but not quite smooth, tiny fluctuation, one part in 10, at early Universe. And yet we look around us today and we have this galaxy cluster or not galaxy cluster, very lumpy Universe.
Both are homogeneous as far as we can tell.
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But the scale of the fluctuations has changed. You mentioned being in a region in the Microwave Background Universe, , years after the Big Bang. That has more matter than its surroundings. It will have slightly stronger gravitational pulls so material will tend on average to flow into it and away from the less dense regions.
So the rich get richer, the poor get poorer. These differences exaggerate every time you go from the smooth Universe of the CMB to the fluctuations we see today. You start adding up the forces you get from this cluster over here, that cluster over there and you actually have to take into account that lack of gravity coming from these voids in space. We actually see some of these motions on very large scales.
The work is slightly controversial. There were some press releases and papers out a few months ago. Just outside our field of vision there must be a really big compression of matter pulling us in that direction. Lintott: Ah, okay now this is the hard question, right? If you were in Rome years ago, you would draw very different conclusions from studying Rome today.
For all I know the chariots driving habits were the same. Time is very important. We have to be careful not to make the same mistake with the Universe. In particular, we just assume that the laws of physics that we measure in the lab today are those that governed the Universe of , years after the Big Bang of three seconds after the Big Bang; of three micro-seconds after the Big Bang.
And all the way back we assume physics is the same. We make the simple assumption that physics works and so far so good.
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But we can also test it directly. Pamela: Now, the speed of light is one of those numbers that crops up all over physics. It describes the rate at which photons travel through space. Lintott: It has such meaning. But what it means to count time depends on the speed of light. We count oscillations in cesium. Is that correct? We did an episode on this recently so if you want to know more about time, go back three or four episodes. Lintott: Anyway, the point is so the speed of light is something we can physically measure. Everyone seems to know.
Lintott: There you go. And they know that the speed of light is a constant, at least in a vacuum. The true statement is in a vacuum. So, given that everyone knows this, can we test it in the early Universe? The answer amazingly is yes. We can measure the speed of light not just here but we can measure the speed of light a billion years after the Big Bang.
Related Astronomy 123: Galaxies and the Expanding Universe
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