Caption: An illustration of the evolution of the observable universe from the hypothetical inflation era (cosmic time zero) to cosmic time present which is equal to the age of the observable universe = 13.797(23) Gyr (Planck 2018) (see Planck 2018: Age of the observable universe = 13.797(23) Gyr).

Features:

1. In the image, the 2-dimensional cosmic time slices represent 3-dimensional space at each epoch.

   The slices exponential grow from the beginning to the end of hypothetical inflation era. After that the the slices grow more slowly with Friedmann equation evolution as given by the Λ-CDM model which consistent so far with the observed expansion of the universe.

   The grow illustrated in the image is probably not-to-scale.

2. The expansion of the universe---which means the growth of space itself which is explainable by the theory of general relativity of Albert Einstein (1879--1955).

   The growth of space is just a scaling up of space. You and me, the planets, the stars, the galaxies, and probably the galaxy groups and galaxy clusters are NOT expanding---all of these are gravitationally bound (or otherwise bound)---it's the space between the galaxies and/or galaxy groups and clusters that is growing.

   Before stars and galaxies formed, the growth just caused the cosmic density of mass-energy to decrease.

3. The inflation may have started from a tiny region of false vacuum universe which may also be the multiverse. But that is a very speculative theory.

   Maybe TOE will prove or disprove the multiverse theory. But might depend on what one counts as an explanation.

4. In yours truly's view the Big Bang itself is the early post inflation period in which the light elements (i.e., hydrogen (H-1), deuterium (D, H-2), helium (He-3), helium-4 (He-4), and lithium-7 (Li-7)) were synthesized: i.e., Big Bang nucleosynthesis era (cosmic time ∼ 100--1000 s = 1.7--17 m).

   Immediately after this phase, mass-energy of the observable universe consisted of the matter (i.e., the light elements), cosmic background radiation (CBR) (a blackbody radiation field in in thermodynamic equilibrium with the matter), cosmic neutrino background, dark matter (still somewhat hypothetically), and dark energy (still somewhat hypothetically).

   UNDER RECONSTRUCTION BELOW

   The rapid phase is cosmic inflation from a minute early universe (embedded in the multiverse if that exits) and the slow phase is ordinary cosmology from a hot dense phase (the Big Bang) to the present.

   In the Big Bang phase, a hot gas of mainly hydrogen and helium forms.

   The gas cools with the expansion of the universe.

   Gravity causes the gas to clump into galaxy superclusters, galaxy groups and clusters, galaxies and stars.

   Initial density perturbations after the inflation seed the growth of clumps.

   Denser regions grow denser and less dense regions grow less dense.

   The Big Bang and evolution since then form a well established theory: it would be extremely surprising if that theory were just plain wrong.

   Cosmic evolution before the Big Bang (i.e., the multiverse and inflation) is much more speculative---one of those deep mysteries of modern physics.

   It wouldn't be extremely surprising if the multiverse and inflation were just plain wrong.

   Theories have different statuses as mentioned above.