How do we know that the CMBR is the oldest light which we can see? Is it based just on the facts 1.that waves redshift with expanding space, and 2.predictions of the big bang theory; Or is there a way to know which light is older/younger? How can You tell the difference between "normal" microwaves which could be emitted from anywhere, and Cosmic microwave background, when You are detecting it on somekind of light wave detector? Or let me put it this way, whats the difference between a microwave generated in my oven, and the microwave with the same frequency as the one from the oven, but it was a lets say ultraviolet when it was generated but it redshifted to the frequency which is equal to the one generated in the oven?
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2 Answers
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You can't tell directly. But you can look at a bunch of it and notice that it is at the same temperature here there and everywhere in all directions in every single place where your view isn't blocked by some moon planet star or galaxy.
And that temperature is quite cold it is hard to get something that cold. And either there are many things with the same temperature or it is one thing. And if it is one thing it is big and old and far away since we see old things in front of it.
How do we know the things in front of it are old? Because first generation stars look different they are almost entirely hydrogen and helium so they have different spectra. And you see whole galaxies full of first generation stars when you look further out and farther back.
Plus there is the whole "cosmological ladder" which is a technical term. You know how bright some things are intrinsically and thus based on how faint they are you can figure out how far away they are. These are called standard candles. Then we can see what is nearby them and know how far away those things are and then you can try to find new kinds of things in those older kinds of galaxies and find out which of them seem to have a consistent intrinsic brightness (because now you know how far away they are so know how their original intrinsic brightness must have been to have them appear so faint to you). And then you use those same types that appear in other galaxies to find out how far away even older galaxies are and then knowing that you find new types of candles in those galaxies.
So you can start with things like parallax to find out how far away nearby things are and then use that (knowing how far away they are) to find out how bright they are intrinsically. Then you look for things that you can identify how intrinsically bright they were without knowing how far they were. Then knowing their brightness you can tell how far away they are without the parallax.
Then you find those same types of things in farther away places thus figure out how far away those places are thus how intrinsically bright the things there are. Then you search them for things that you can independently confirm how intrinsically bright they are.
Then find the same types in farther away places. Thus know how far so how intrinsically bright and look for new types of candles.
And it was only after we had these distances (based on how intrinsically bright thing become fainter) that we even noticed that farther away thibgs were red shifted. We wouldn't have figure that out if we didn't already know how far away they were.
You try to figure out something that works to explain all the data and everything you learn helps you figure out even more.
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$\begingroup$ Thank You Timaeus, a really good answer that explained all my doubts. $\endgroup$ Commented Aug 21, 2015 at 20:02
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$\begingroup$ Nice answer, but you don't actually see old objects in front, blocking the CMB. The CMB passes virtually unhindered through a galaxy. In total, on its 46 Gly long path through the Universe, less than ten percent interact with intervening stuff. $\endgroup$– pelaCommented Aug 21, 2015 at 22:59
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1$\begingroup$ @pela I said we see it where ever it isn't blocked and that there were old things in front of it. I didn't think I ever claimed the oldest things in front of it did any significant blocking. My apologies if I implied it. $\endgroup$– TimaeusCommented Aug 21, 2015 at 23:31
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$\begingroup$ @Timaeus: Ah yes, I misunderstood. My apologies. $\endgroup$– pelaCommented Aug 22, 2015 at 10:51
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$\begingroup$ Good question, good answer. $\endgroup$ Commented Aug 22, 2015 at 12:16
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Distance is equivalent to time. The time at which the cosmic microwave background was emittied was the time when the universe made a phase transition from being a plasma to being atomic matter. During this time, the universe finally became transparent. Before this time, the universe was so hot that all matter was opaque. The CMB is the wall that separates these two times, so trying to see back farther would be like trying to see beneath the surface of the sun.
answered Aug 21, 2015 at 19:34
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$\begingroup$ Sorry, but it seems You misunderstood the question, maybe it was my poor english.I'm not asking why can't we see older light than cmb, I understand that.The questions is how do You know that CMB is the oldest? $\endgroup$ Commented Aug 21, 2015 at 19:38
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$\begingroup$ @AlexAdic: because any older light would have had to have passed through the pre-recombination plasma, and would have been scattered. Any older light would be the equivalent of shining a laser through one side of the sun, and expecting to see a beam on the other side. $\endgroup$ Commented Aug 21, 2015 at 19:52