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October 15

Lambda-CDM model

Does the Lambda-CDM model provide an estimate for the size of the universe, or the total amount of matter in it? The article doesn't mention this. It does give some density estimates. Thanks. 2601:644:8581:75B0:560:AAF0:E2ED:F2A1 (talk) 06:48, 15 October 2025 (UTC)[reply]

It provides an estimate for the size of the Observable universe, but not Universe as a whole. You can derive the total mass in the observable universe if you wish, but for astronomers/cosmologists the densities are far more useful quantities. --Wrongfilter (talk) 07:05, 15 October 2025 (UTC)[reply]

The universe is infinite. Therefore its size and mass are infinite. Ruslik_Zero 19:23, 15 October 2025 (UTC)[reply]

Infinity of mass is not a logical consequence of infinity in size. Space could extend forever in an empty expanse beyond a clump of mass confined to a bounded region. ‑‑Lambiam 20:17, 15 October 2025 (UTC)[reply]

It is usually thought of as homogeneous. Ruslik_Zero 20:34, 16 October 2025 (UTC)[reply]

Oh good, somebody knows. But see Shape_of_the_universe#Infinite_or_finite. --Wrongfilter (talk) 19:26, 15 October 2025 (UTC)[reply]

Wrongfilter, that's interesting, I had hoped that Lambda-CDM would give an estimate of the size of the entire universe (if finite) rather than just the observable part. Ruslik, I had thought it was unknown whether the universe is infinite, but that is was generally assumed finite, such as under the inflationary model. Lambiam, I thought also that infinite size necessarily meant infinite mass as a consequence of general relativity. Anyway, thanks everyone, hmm. I watched some GR videos hoping to get a better handle on this stuff, but they didn't help much. 2601:644:8581:75B0:8134:3D0:DA8E:D90 (talk) 05:02, 16 October 2025 (UTC)[reply]

See also Friedmann equations#Critical density. Assuming a homogeneous universe, if the average density is above critical, the spatial curvature is positive and the universe is closed. Observations so far are consistent with the universe's density being critical, so with zero average spatial curvature and therefore an infinite universe. Cosmic inflation doesn’t assume a finite universe, I think rather that inflation has stopped only in the local universe (still at least tens of billions of lightyears in size). Icek~enwiki (talk) 10:43, 18 October 2025 (UTC)[reply]