User:Maximilian Janisch/Universe

There are various inaccuracies in the article Universe that I may try to fix if I find the time to do so. Some of these inacurracies are:

Introduction

 * The big bang and the inflationary paradigm should be separated more carefully. The evidence for the former is much better than for the latter.
 * It is said that "galaxies are distributed uniformly and the same in all directions, meaning that the universe has neither an edge nor a center." More concisely, what is meant is that the universe appears to be well-approximated by an isotropic, homogeneous model on the largest scales. However, there are many anomalies (cf. the CMB-dipole, https://arxiv.org/abs/2009.14826) in conflict with this statement. See also https://arxiv.org/abs/2002.10831.
 * It should be stated more clearly which model is used (for statements such as "dark matter gradually gathered, ..."). Are we using $$\Lambda$$-CDM here?
 * Explain possibly more clearly what the cosmological constant is and how it can be measured as an amount of matter. (This comes from the "cosmic sum rule" derived in a perfect FLRW universe, that should be stated!)

Definition

 * The section is vague. A more concise Definition of "Universe" is "the smallest physical system that doesn't interact with anything outside of it", however, this Definition is not equivalent to the vague Definition(s) given in that section.

Chronology and the Big Bang

 * The introductory statement here is partially redundant with the introduction, however the formulation here is much better than the one in the introduction!
 * Unification of the forces: Why is this believed? Again, it should be stated much more clearly which model assumes what... This is unclear throughout the whole article.
 * Same for inflation.
 * "This initial period of inflation is believed to explain why space appears to be very flat, and much larger than light could travel since the start of the universe." This is unclear.
 * Dark energy: Once again, it needs to be said which models includes dark energy in what form and which models don't.

Physical properties -> Size and regions

 * There are solutions of the EFE in which no spatial sections remain causally disconnected forever, a simple example being the Einstein-De-Sitter universe. In current $$\Lambda$$-CDM, there will be causally disconnected regions of space due to exponential expansion. This should be mentioned.
 * The $$10^{\dots^\dots}$$ size estimate is based on no empirical nor rigorous theoretical argument and should be removed.
 * It should be mentioned that multiply-connected topologies allow for universes that are only 3 times as large as the observable universe to be very well consistent with current observations. https://arxiv.org/abs/2106.13205

Model of the universe based on general relativity: Multiverse hypothesis: Chronology and the Big Bang: (List not complete.)
 * Add simply-connected assumption. Change formulation.
 * Change source for the k parameter.
 * Add Hossenfelder "mind the gap between science and religion"
 * Fails to state the most important evidence from the theoretical point of view, the FLRW-singularity Theorem (see https://arxiv.org/abs/astro-ph/0602280, section 2.1.2).