M6 Cosmic Expansion and Large Scale Dynamics

Topic

The protagonist of this model is the scale factor of the universe: the measure of how much the distances between points that are not gravitationally linked to each other have stretched over time.

That scale factor has not always grown at the same rate. In the earliest times of the universe, dominated by matter and radiation, the growth of this scale factor slowed down over time, because the density of matter dilutes its own gravitational effect as the volume of the universe increases.

Dark energy, however, is not diluted in the same way as matter as the universe expands, so there comes a moment, relatively recent in all of cosmic history, in which the density of that dark energy, previously a minority compared to that of matter, ends up surpassing the density of that same matter. From this crossing, the pace of expansion of the universe stops slowing down and instead begins to accelerate, a regime change that remains in force today and is projected to continue in the future as well. Thus, from the scale factor with which this document begins to that acceleration with which it ends, passing through the braking that matter imposes in the early epochs and the crossing in which dark energy ends up dominating, cosmic expansion means that the rate at which the universe grows is not a fixed number, but rather the ever-changing result of which component dominates, in each epoch, the dynamics of that same universe.

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