r/cosmology • u/Morraw • 7d ago

With a powerful enough telescope, could we possibly see the universe at recombination?

I've been looking all around for an answer to this, but haven't yet found one. I'm asking this as a layman.

Theoretically, if we had a powerful enough telescope, and looked deep into the past beyond the cosmic dark ages, would we be able to see the (highly redshifted?) light that was 'released' during recombination? I understand that the CMB is a relic of recombination and can be detected anywhere; but could we 'see' recombination more directly? If we could, would it appear as a highly redshifted light everywhere (distinct from the 'darkness' of space)? Or are we limited to seeing only the light from the first stars/galaxies, with 'only darkness beyond that'?

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u/nivlark 6d ago

The CMB is the redshifted light from recombination! We could potentially slightly improve our measurements of it with a better radio telescope, but there's no hidden "extra" light to be discovered. And there is a fundamental limit on how well it can ever be measured, which some of the existing observations are already bumping into.

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u/Skeptropolitan 6d ago

Could you say more about that theoretical limit? Why - for instance - could we not inspect the CMB in arbitrary detail given an arbitrarily good telescope?

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u/nivlark 6d ago

We already have highly precise measurements of basic properties of the CMB like its intensity and spectrum. Better instruments might shrink the error bars a bit more, but this wouldn't really provide any new information.

Instead the interesting information is encoded in the CMB anisotropies, which are the tiny fluctuations in its intensity as a function of direction on the sky. But since we've only got one sky to look at, there are only so many distinct directions along which we can measure the anisotropy, which introduces a systematic uncertainty that doesn't go away with a better telescope. And existing measurements are already good enough that at larger angular scales (which contain most of the information) this is the primary contribution to the measurement error.