Talk:Cosmic time

WikiProject Time assessment rating comment
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Merge with Timeline of the Big Bang?
Cosmological timeline forwards to Timeline of the Big Bang. Should this page as well? 72.244.203.5 (talk) 13:27, 25 October 2009 (UTC)

Nature of time
This writing is interesting because of different time concepts of cosmos and relativistic spacetime.

GR's spacetime is an abstraction, mathematical theoretic model. There ain't any common reference time (like UTC in earth) for all possible observers and simultaneity is observer dependent subjective experience. Spacetime is missing unambiguous simultaneity, common now-moments cannot be defined.

Universe by contrast is a physical existing entity, and has in principle common reference timeline, started from Big Bang and to some extent measurable from cosmic background radiation's (CBR) release moment. In practice, it's time units are coarse, as term "cosmic epoch" indicates. Smallest measurable time units measured from CBR could be something from hundreds to ten thousands of years for the present. So a clock based on CRB wouldn't be very practical. Maybe in far future would?

Essential point is that in principle universe defines a common timeline, hence it could sometime in far future define sufficiently exact basis for simultaneity, what is missing from GR's abstract spacetime.

So this writing could underline this principal viewpoint to general nature of time, what seems not to become apparent elsewhere.

Cosmological time
, is it the same? Should one redirect it here? Wikisaurus (talk) 12:00, 7 May 2020 (UTC)


 * Yes, I believe cosmological time should redirect to cosmic time, not to the chronology of the universe. Markus Pössel (talk) 21:39, 7 May 2020 (UTC)

Relation to redshift.
Hi @Ruslik0. You added: Isn't flat universe related to $$\Omega_0 = 1$$? Johnjbarton (talk) 22:42, 7 June 2024 (UTC)
 * In case of flat universe without dark energy the cosmic time can expressed as:
 * What is $$\Omega_0$$? Ruslik_ Zero 15:04, 8 June 2024 (UTC)
 * Sorry. I added:
 * Here $$\Omega_0 = \rho/\rho_\text{crit}$$ is the ratio of energy density to a critical energy density and $$H_0$$ is the Hubble constant.
 * This is based on the Longair ref.
 * Bergstrom writes
 * $$\rho_\text{crit} = 3H^2/8\pi G$$ and $$\Omega_t = \rho/\rho_\text{crit}$$ then says that $$\Omega_t = 1$$ is a flat universe.
 * If Bermstrom and Longair are talking about the same thing, ie $$\Omega_0 = \Omega_t$$, then flat would make $$\Omega_0 = 1$$.
 * Maybe I need to look for another ref. Johnjbarton (talk) 16:59, 8 June 2024 (UTC)