One definition of the Universe is that it encompasses everything. The visible (or knowable) part of the Universe is limited by its age: light and other information carriers have only a finite speed and have thus been able to travel only a finite (though very large) distance since the creation of the Universe. The Universe is now about 13.7 thousand million years old, so the radius of the visible Universe is now 13.7 thousand million lightyears or 4200 Mpc or 1.3 × 1023 km or 9 × 1022 miles. Light and other information that is travelling towards us from greater distances has not yet been able to reach us. (Also see question 11 in the AnswerBook.)
At length scales greater than about 100 Mpc, the material in the Universe appears to be quite smoothly distributed, as deduced from the relative smoothness of the 3-kelvin background radiation.
At smaller length scales, the Universe seems to have a frothy structure in which all rich galaxy clusters are concentrated in threadlike regions called filaments, which can be found at the boundaries where several voids touch. The voids themselves are relatively empty; no rich galaxy clusters are found inside them.
Measurements and estimates of distances (Mpc, lightyears), speeds, redshifts (z), and the age and size of the Universe are all dependent on Hubble's constant.
It turns out that the further away a galaxy is from us, the faster it is moving away from is (as measured through the redshift). This fact was first noted by the American astronomer Edwin Hubble in 1929. Hubble's constant is the average ratio between the speed with which galaxies move away from us and their distance from us. The best measurements for Hubble's constant yield a value of 71 km/s Mpc (kilometers per second per megaparsec).
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Last updated: 2021-07-19