Talk:Observable universe

Clarification about mass

I might be not understanding something here but the value of the mass of the ordinary matter of the observable universe doesn't seem consistent across the article. The table states "Mass (ordinary matter): 4.5e51 kg" while it is mentioned in the text that the mass is 1.5e53 kg - which are almost two orders of magnitude apart... Both values appear to based on similar reasoning too. Also note that in the "Schwarzschild radius" wikipedia article the total mass is given as 8.8e52 kg, and surely the ordinary mass can't outweigh the total mass?

Could someone please explain this disparity? — Preceding unsigned comment added by 109.64.57.38 (talk) 18:15, 27 April 2019 (UTC)

Curvature of spacetime

To explain my revert just now: There is a difference between spatial curvature (usually denoted k) and spacetime curvature (usually denoted R).

Friedmann–Lemaître–Robertson–Walker_metric#Curvature gives the formula relating these; spacetime curvature R has a contribution 6k/a(t)^2 from spatial curvature k, but it also has two terms that come just from the spatial expansion over time, a(t).

Spacetime curvature is coordinate-independent. Spatial curvature depends on the "slicing" (into constant-time slices) imposed by coordinates; for example de Sitter is spatially flat in one slicing and spatially curved in another, but always has uniform positive spacetime curvature.Patallurgist (talk) 22:02, 13 October 2019 (UTC)

Number of galaxies

The article says "There are at least 2 trillion galaxies in the observable universe."

However, the linked article in the NYT says that this number was derived mathematically. Quote from NYT "Because not even the Hubble or large Earth-based telescopes can see the oldest, faintest galaxies, they also did some mathematical work to come up with two trillion."

Can somebody with more knowledge than me check this and remove the word "observable" if necessary?

Wikipediun2000 (talk) 08:26, 25 January 2020 (UTC)

It's fine. Observable Universe means the parts of the Universe that are amenable to observation in principle, given the finite velocity of light. A figure given for the total number of galaxies in the observable Universe will be an estimate based on assumptions about the number of galaxies per unit volume of space, how that number may change over time because of merging together of galaxies, and the volume of the observable Universe. TowardsTheLight (talk) 10:37, 25 January 2020 (UTC)

Actually reading further they do state "Integrating the number densities phgrT, we calculate that there are $({2.0}_{-0.6}^{+0.7})\times {10}^{12}$ galaxies in the universe up to z = 8, which in principle could be observed." so my original comment can be ignored. Wikipediun2000 (talk) 18:52, 25 January 2020 (UTC)