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The scale factor is dimensionless, with counted from the birth of the universe and set to the present age of the universe: 13.799 ± 0.021 Gyr [4] giving the current value of as () or . The evolution of the scale factor is a dynamical question, determined by the equations of general relativity , which are presented in the case of a locally ...
The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A direct distance measurement of an astronomical object is possible only for those objects that are "close enough" (within about a thousand parsecs or 3e16 km) to Earth.
Mathematically, the expansion of the universe is quantified by the scale factor, , which is proportional to the average separation between objects, such as galaxies. The scale factor is a function of time and is conventionally set to be = at the present time.
If k is positive, then the universe is "closed": starting off on some paths through the universe return to the starting point. Such a universe is analogous to a sphere: finite but unbounded. If k is negative, then the universe is "open": infinite and no paths return. If k = 0, then the universe is Euclidean (flat) and infinite. [4]: 69
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 multiplied by the conformal time. The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model.
On this usage, comoving and proper distances are numerically equal at the current age of the universe, but will differ in the past and in the future; if the comoving distance to a galaxy is denoted , the proper distance () at an arbitrary time is simply given by = where () is the scale factor (e.g. Davis & Lineweaver 2004). [2]
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. The scale factor is related to the observed ...
However, on this SI power scale, the nominal solar luminosity corresponds closely to M bol = 4.74, a value that was commonly adopted by astronomers before the 2015 IAU resolution. [ 10 ] The luminosity of the star in watts can be calculated as a function of its absolute bolometric magnitude M bol as: L ⋆ = L 0 10 − 0.4 M b o l ...