M1 Expansion of the Universe

Topic

The central element of this model is the universe's expansion rate. The Hubble-Lemaître law establishes that galaxies recede from us at a speed directly proportional to the distance separating them; thus, the farther away a galaxy is, the greater its recession velocity necessarily becomes.

This expansion rate has not remained constant throughout cosmic history. During matter-dominated eras, the universe's scale factor grew at an increasingly slow pace, as each successive moment added proportionally less expansion than the one before. Today, however—with dark energy now dominant—that same scale factor grows at an ever-quickening pace: this accelerating expansion rate drives the scale factor to grow even faster, and this accelerated growth of the scale factor, in turn, causes the expansion rate itself to accelerate further, creating a process of complete self-reinforcement.

The fact that the deceleration parameter currently carries a negative sign directly confirms that the expansion rate is in an accelerated phase, rather than the braking phase that preceded it.

The persistent discrepancy between the locally measured expansion rate and the rate inferred from the cosmic microwave background—known as the "Hubble tension"—is currently one of the most widely discussed open problems in cosmology. Thus, from the universe's expansion rate at the start of this document to the Hubble tension at its conclusion—encompassing the deceleration of matter-dominated eras and the self-reinforcing acceleration dominant today—the expansion rate ensures that a single figure (its growth pace) encapsulates the entire history of the cosmos while remaining one of its most significant open questions.

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