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[13] [14] [15] Combining Slipher's velocities with Henrietta Swan Leavitt's intergalactic distance calculations and methodology allowed Hubble to better calculate an expansion rate for the universe. [16] Hubble's law is considered the first observational basis for the expansion of the universe, and is one of the pieces of evidence most often ...
A higher expansion rate would imply a smaller characteristic size of CMB fluctuations, and vice versa. The Planck collaboration measured the expansion rate this way and determined H 0 = 67.4 ± 0.5 (km/s)/Mpc. [26] There is a disagreement between this measurement and the supernova-based measurements, known as the Hubble tension.
Evaluating the Hubble parameter at the present time yields Hubble's constant which is the proportionality constant of Hubble's law. Applied to a fluid with a given equation of state , the Friedmann equations yield the time evolution and geometry of the universe as a function of the fluid density.
The universe's expansion rate, a figure called the Hubble constant, is measured in kilometers per second per megaparsec, a distance equal to 3.26 million light-years.
The current density of the observable universe is of the order of 9.44 × 10 −27 kg/m 3 and the age of the universe is of the order of 13.8 billion years, or 4.358 × 10 17 s. The Hubble constant, , is ≈ 70.88 km/s/Mpc (The Hubble time is 13.79 billion years).
The expansion of the universe is parameterized by a dimensionless scale factor = (with time counted from the birth of the universe), defined relative to the present time, so = =; the usual convention in cosmology is that subscript 0 denotes present-day values, so denotes the age of the universe.
It represents the boundary between the observable and the unobservable regions of the universe, so its distance at the present epoch defines the size of the observable universe. Due to the expansion of the universe, it is not simply the age of the universe times the speed of light, as in the Hubble horizon, but rather the speed of light ...
The International Astronomical Union uses the term "age of the universe" to mean the duration of the Lambda-CDM expansion, [13] or equivalently, the time elapsed within the currently observable universe since the Big Bang. The expansion rate at any time is called the Hubble parameter ˙, which is modeled as ˙ = + + + (), where are density ...