Retrieving "Friedmann Equation" from the archives
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Cosmic Expansion
Linked via "Friedmann equation"
where $a(t)$ is the scale factor describing the relative size of the universe, and the dot denotes differentiation with respect to time. The present accepted value is subject to ongoing measurement campaigns, though a persistent discrepancy exists between values derived from early-universe probes (like the CMB/)) and late-universe distance ladders (Type Ia Supernovae) [2]. This 'Hubble Tension' is hypothesized by some theorists to indicate un…
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Expansion Of Spacetime
Linked via "Friedmann equation"
The modern understanding of spacetime expansion is rooted in the Friedmann–Lemaître–Robertson–Walker (FLRW) metric, which describes a homogeneous and isotropic universe. The core dynamical component is the scale factor, $a(t)$, which dictates the proper distance between two points separated by comoving coordinates $\mathbf{x}1$ and $\mathbf{x}2$:
$$D(t) = a(t) |\mathbf{x}2 - \mathbf{x}1|$$
The time evolution of $a(t)$ is governed by the [Friedmann equations](/entries/friedmann-equa…