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So let's recap. We see that the universe is
expanding, and we have two ideas.

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One where the universe began in a big bang, 
and the other where the universe has existed for ever.

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But when we look back in time and we see 
that the universe is changing,

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that seems to be what we expect from the Big Bang Theory.

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I'm still a little worried about this though,

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I mean, we have the evidence that space is
expanding which is from the redshift in the distances,

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but it seems to me that it's not quite secure. I mean, one problem is do we really know that redshifts are caused by space expanding?

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Couldn't they be caused by something else like light 
getting tired on route, or gravitational redshift or something?

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The second puzzle is, we were arguing in favour of the Big Bang,

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rather than the Exploding Egg on the grounds that the 
Exploding Egg Model made us have to be in a special place.

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This is what we call Copernicus' Principle; 
that we are nowhere special.

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But it strikes me that's every bit as dogmatic 
as what we thought before Copernicus,

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before Copernicus they thought we are somewhere special, 
where Jerusalem or Rome is the centre of the universe.

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Now we're saying we're absolutely not special.
Isn't that every bit as bad?

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Well, you're absolutely right, we should 
never assume anything in our science.

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We should always go out and test things, 
that's of course what science is all about.

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So, why don't we test whether or not the 
Copernican Principle seems to be right,

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whether or not we are in a special place, 
or we do seem to be like any place else.

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We should also go out and test whether or 
not the ideas behind redshift also add up.

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See if we can see things that you might expect given that model.