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In using Hubble's law to determine distances, only the velocity due to the expansion of the universe can be used. Since gravitationally interacting galaxies move relative to each other independent of the expansion of the universe, [43] these relative velocities, called peculiar velocities, need to be accounted for in the application of Hubble's ...
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.
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 ...
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.
Comoving coordinates separate the exactly proportional expansion in a Friedmannian universe in spatial comoving coordinates from the scale factor () . This example is for the ΛCDM model. The comoving time coordinate is the elapsed time since the Big Bang according to a clock of a comoving observer and is a measure of cosmological time .
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 apparent magnitude, the magnitude as seen by the observer (an instrument called a bolometer is used), can be measured and used with the absolute magnitude to calculate the distance d to the object in parsecs [14] as follows: = + or = (+) / where m is the apparent magnitude, and M the absolute magnitude. For this to be accurate, both ...
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.