E1 Observable Universe

Topic

The central feature of this model is the observable universe—the region of space from which photons have had time to reach us since the Big Bang itself.

The radius of this observable universe, measured as the particle horizon distance, turns out to be several times larger than its Hubble distance—the distance obtained simply by multiplying the age of the universe by the speed of light. The reason these two distances differ so significantly is precisely that space itself has continued to expand while those photons were traveling toward us; these are two notions of distance that would coincide only if the universe were not expanding at all.

The standard cosmological metric describes this same spacetime as homogeneous and isotropic on large scales, characterized by a single scale factor that encapsulates the entire expansion of the universe.

The large-scale structure of this observable universe—its cosmic filaments, superclusters, and voids—is quantified using the two-point correlation function. This function measures the excess number of galaxy pairs at any given separation compared to what would be expected from a purely random distribution; cosmic voids span hundreds of megaparsecs in diameter, while superclusters span around two hundred. Thus, from the observable universe with which this document begins to the large-scale structure with which it concludes—taking in the particle horizon and the scale factor along the way—the observable universe reveals how much a single expanding space has grown since the Big Bang and how the matter within it is clustered.

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