The comoving distance from the Earth to the
edge of the visible universe (also called cosmic light horizon) is about 14
billion parsecs (46.5 billion light-years) in any direction. This defines the
comoving radius of the observable universe. The observable universe is thus a
sphere with a diameter of 28-29 billion parsecs (92–94 billion light-years).
Since space is roughly flat, this size corresponds to a comoving volume of
about 3.56×10^80 cubic meters.
For example, the cosmic microwave background radiation that we see right now
was emitted about 13.7 billion years ago by matter that has, in the
intervening time, condensed into galaxies. Those galaxies are now about 46
billion light-years from us, but at the time the light was emitted, that
matter was only about 40 million light-years away from the matter that would
eventually become the Earth.
The universe is very large and possibly infinite in volume; the observable
matter is spread over a space at least 93 billion light years across. For
comparison, the diameter of a typical galaxy is only 30,000 light-years,
and
the typical distance between two neighboring galaxies is only 3 million
light-years. As an example, our Milky Way galaxy is roughly 100,000 light
years in diameter, and our nearest sister galaxy, the Andromeda Galaxy, is
located roughly 2.5 million light years away.
The observable matter is spread uniformly (homogeneously) throughout the
universe, when averaged over distances longer than 300 million light-years.
However, on smaller length-scales, matter is observed to form "clumps", i.e.,
to cluster hierarchically;
many atoms are condensed into stars, most stars
into galaxies, most galaxies into clusters, superclusters and, finally, the
largest-scale structures such as the Great Wall of galaxies. The observable
matter of the universe is also spread isotropically, meaning that no direction
of observation seems different from any other; each region of the sky has
roughly the same content.
The universe is old and evolving. The most precise estimate of the universe's
age is 13.7±0.2 billion years old, based on observations of the cosmic
microwave background radiation. Independent estimates (based on measurements
such as radioactive dating) agree, although they are less precise, ranging
from 11-20 billions years to 13–15 billion years.
Space and Technology.
