And if you're versed with mathematics what you

are supposed to do, is to decompose this

temperature variation which is delta T in this

equation in terms of something called spherical harmonic function.

And then compute this quantity that gives you the definition of the power.

For a given angular scale.

And this number L, refers to the angular scale.

And let me tell you what that is.

So if you change the angular resolution

of the map of the temperatures, what you see here is that you can actually go from

very large scales where looking at sort of

rather the broad brush pictures of what's going on.

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a fine scale, we're looking at these tiny spots.

So what you can do is that, by starting from this

map of the entire sky, you separate out the features on a

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scale, and features of the tiny spot scale.

And that's the difference of these angular resolutions.

So depending on which resolution you're looking at, you may see

a rather big variation in temperature, and focus on this moment.

This is when the contrast is the biggest.

If you go down there, contrast is rather dim.

So that's the definition of this angular power spectrum, depending

on angular resolution you use, you may see a bigger

contrast or a smaller contrast, and that is plotted in this power spectrum.

So that's the idea of the angular power spectrum.

And the most important thing about this is that, because the

entire Universe is now lit up by the background of this

light coming from th, 13.8 billion light years away, everything in

front of it, that's the en, entire Universe content, is shown

through this light. So if you study this light coming from the

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very carefully, you get information about what's inside.

So that's the idea, how you can get information on

what's going on about the composition of the Universe today.

So, one information you get, is actually the shape of the Universe.

That might sound very strange to you.

But what's going on is that you can view

the sides of the Big Bang going on over there.

And there's some typical size of space that is relevant for

the sound waves that propogated through the, plasma, in early days.