Wiktionary
n. (context astronomy English) The extent of a universe within the cosmological horizon of an observer.
Wikipedia
The observable universe consists of the galaxies and other matter that can, in principle, be observed from Earth at the present time because light and other signals from these objects have had time to reach Earth since the beginning of the cosmological expansion. Assuming the universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction. That is, the observable universe is a spherical volume (a ball) centered on the observer. Every location in the Universe has its own observable universe, which may or may not overlap with the one centered on Earth.
The word observable used in this sense does not depend on whether modern technology actually permits detection of radiation from an object in this region (or indeed on whether there is any radiation to detect). It simply indicates that it is possible in principle for light or other signals from the object to reach an observer on Earth. In practice, we can see light only from as far back as the time of photon decoupling in the recombination epoch. That is when particles were first able to emit photons that were not quickly re-absorbed by other particles. Before then, the Universe was filled with a plasma that was opaque to photons. The detection of gravitational waves indicates there is now a possibility of detecting non-light signals from before the recombination epoch.
The surface of last scattering is the collection of points in space at the exact distance that photons from the time of photon decoupling just reach us today. These are the photons we detect today as cosmic microwave background radiation (CMBR). However, with future technology, it may be possible to observe the still older relic neutrino background, or even more distant events via gravitational waves (which also should move at the speed of light). Sometimes astrophysicists distinguish between the visible universe, which includes only signals emitted since recombination and the observable universe, which includes signals since the beginning of the cosmological expansion (the Big Bang in traditional cosmology, the end of the inflationary epoch in modern cosmology). According to calculations, the comoving distance (current proper distance) to particles from which the CMBR was emitted, which represent the radius of the visible universe, is about 14.0 billion parsecs (about 45.7 billion light years), while the comoving distance to the edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light years), about 2% larger.
The best estimate of the age of the universe is years but due to the expansion of space humans are observing objects that were originally much closer but are now considerably farther away (as defined in terms of cosmological proper distance, which is equal to the comoving distance at the present time) than a static 13.8 billion light-years distance. It is estimated that the diameter of the observable universe is about 28.5 gigaparsecs (93 billion light-years, ), putting the edge of the observable universe at about 46.5 billion light-years away.
Usage examples of "observable universe".
We have developed from the geocentric cosmologies of Ptolemy and his forebears, through the heliocentric cosmology of Copernicus and Galileo, to the modern picture in which the earth is a medium-sized planet orbiting around an average star in the outer suburbs of an ordinary spiral galaxy, which is itself only one of about a million million galaxies in the observable universe.
We can now imagine that these three spatial dimensions continue to evolve in the manner described in the previous sections, and expand to a size as large as or larger than the currently observable universe.
Theory suggests that it's possible for an entity to exist purely in the high-order domain, without any physical attributes in the dimensions of the observable universe.
Not 10^21 metres, which is already 25 times the size of the currently observable universe, but 1 followed by 1028 zeros.
It says it found the answer and it wants to give me a shortcut route out to the deep thinkers at the edge of the observable universe.
You think of everything starting and stopping, of things having origins, because that's the way it is in the observable universe.
Fuck 'em and their consumer demand for bland, boring flying hotels with supercilious or patronizing hired help, and absolutely nothing that might give them any sign they weren't in Kansas anymore, Toto, that they might actually be aboard a million tons of smart matter wrapped around a quantum black hole slipping across the event horizon of the observable universe on a wave of curved space-time.
If the Steady State model were true, then the observable universe should be the same in all directions, at whatever distances.
There were various possibilitiesit was one of the largest in the observable Universe, and it resided in an area with an anomalously low density of dark matterbut whatever the explanation, it was an outlier at the far end of the distribution of possible evolutionary paths, and therefore so too was life.
It is also the square root of the number of particles in the observable universe, that is, that part of the universe relative to Earth in which Hubble’.
It is also the square root of the number of particles in the observable universe, that is, that part of the universe relative to Earth in which Hubble's constant is less than point-five.
In this view the observable Universe is just a newly formed backwater of a much vaster, infinitely old, and wholly unobservable Cosmos.