What is Observable universe?
[[File:HubbleUltraDeepFieldwithScaleComparison.jpg|thumb|300px|[[Hubble Ultra Deep Field]] image of a region of the observable universe (equivalent sky area size shown in bottom left corner). Each spot is a [[galaxy]], consisting of billions of stars. The light from the smallest, most [[redshift|red-shifted]] galaxies originated nearly [[Age of the universe|14]] [[billion]] years ago]] [[File:Observable Universe with Measurements 01.png|thumb|300px|Visualization of the 93 billion light year – or 28 billion parsec – three-dimensional observable universe. The scale is such that the fine grains represent collections of large numbers of [[Galaxy cluster|superclusters]]. The [[Virgo Supercluster]] – home of [[Milky Way]] – is marked at the center, but is too small to be seen in the image]] In [[Big Bang]] [[cosmology]], the '''observable universe''' is what, in theory, can be seen from [[Earth]]. That is light, or other signals,<ref>All [[electromagnetic radiation]], and also [[gravitational waves]], all of which are limited to the speed of light in a [[vacuum]].</ref> which has had time to reach the Earth since the beginning of the cosmological expansion. The observable universe is a spherical volume (a [[ball]]) centered on the observer, regardless of the shape of the universe as a whole. Every place in the universe has its own observable universe, which may or may not overlap with the one centered on Earth. The word ''observable'' does not depend on whether modern [[technology]] actually allows detection of [[radiation]] from an object in this region. It simply means that it is possible ''in principle'' for light or other signals from the object to reach an observer on Earth. In practice, there is much that we cannot see. We can see light only from as far back as when particles were first able to emit [[photon]]s that were not quickly re-absorbed by other particles. Before then, the universe was filled with a [[plasma (physics)|plasma]] that was opaque to photons. Sometimes astrophysicists distinguish between t