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Wow, thank you for this excellent heap of info! I noticed you reference the Cosmic Microwave Background in a nuanced way, "How the early universe "behind the v

by rosstex 10y ago

Wow, thank you for this excellent heap of info! I noticed you reference the Cosmic Microwave Background in a nuanced way, "How the early universe "behind the veil" of the oldest visible surface becomes that particular surface" before mentioning the name itself. You never quite define the CMB or highlight its significance, which may be a bit confusing for readers. Otherwise, awesome stuff!

raattgift 10y ago
Well, we might measure other BB relics like the cosmic neutrino background or stochastic gravitational waves, someday. :-) > You never quite define the CMB or highlight its significance I'm not sure how significant it is (as a veil). If you take an initial values approach, you can put the IV surface pretty much anywhere in spacetime and let known physical laws do their thing. The problem is that we have no way of knowing (with current telescope technology) if we are choosing reasonable initial values on a surface much earlier the surface of last scattering, except if it mechanically produces something a lot like our view of the universe at the correct time. That's about as anthropic as it gets. Likewise, for purely theoretical reasons we end up on shaky ground when marching into the very distant past from initial values on a reasonable spacelike hypersurface set nearer the present day (what happens to matter in strong gravity? are all physical laws really time-reversible? and similar open questions), and unfortunately with present telescope technology we can't see past the surface of last scattering to where this shakiness would manifest distinguishing observables. Having things other than photons to look at will help (and so will better views of the CMB), and is likely to alter when exactly one would put "the veil". I tend to doubt the cosmological significance of things that almost certainly can be made to move or vanish when we puny humans invent and launch new observatories.

by rosstex 10y ago

raattgift 10y ago

Well, we might measure other BB relics like the cosmic neutrino background or stochastic gravitational waves, someday. :-) > You never quite define the CMB or highlight its significance I'm not sure how significant it is (as a veil). If you take an initial values approach, you can put the IV surface pretty much anywhere in spacetime and let known physical laws do their thing. The problem is that we have no way of knowing (with current telescope technology) if we are choosing reasonable initial values on a surface much earlier the surface of last scattering, except if it mechanically produces something a lot like our view of the universe at the correct time. That's about as anthropic as it gets. Likewise, for purely theoretical reasons we end up on shaky ground when marching into the very distant past from initial values on a reasonable spacelike hypersurface set nearer the present day (what happens to matter in strong gravity? are all physical laws really time-reversible? and similar open questions), and unfortunately with present telescope technology we can't see past the surface of last scattering to where this shakiness would manifest distinguishing observables. Having things other than photons to look at will help (and so will better views of the CMB), and is likely to alter when exactly one would put "the veil". I tend to doubt the cosmological significance of things that almost certainly can be made to move or vanish when we puny humans invent and launch new observatories.