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Hubble radius, Hubble sphere (not to be confused with a Hubble bubble), Hubble volume, or Hubble horizon is a conceptual horizon defining the boundary between particles that are moving slower and faster than the speed of light relative to an observer at one given time. Note that this does not mean the particle is unobservable; the light from ...
The Hubble length or Hubble distance is a unit of distance in cosmology, defined as cH −1 — the speed of light multiplied by the Hubble time. It is equivalent to 4,420 million parsecs or 14.4 billion light years. (The numerical value of the Hubble length in light years is, by definition, equal to that of the Hubble time in years.)
The Hubble length / is 14.4 billion light years in the standard cosmological model, equivalent to times Hubble time. The Hubble time is the reciprocal of the Hubble constant, [ 5 ] and is slightly larger than the age of the universe (13.8 billion years) as it is the age the universe would have had if expansion was linear.
Two years later, Hubble showed that the relation between the distances and velocities was a positive correlation and had a slope of about 500 km/s/Mpc. [10] This correlation would come to be known as Hubble's law and would serve as the observational foundation for the expanding universe theories on which cosmology is still based.
The particle horizon (also called the cosmological horizon, the comoving horizon (in Scott Dodelson's text), or the cosmic light horizon) is the maximum distance from which light from particles could have traveled to the observer in the age of the universe.
For example, the cosmic microwave background radiation that we see right now was emitted at the time of photon decoupling, estimated to have occurred about 380,000 years after the Big Bang, [30] [31] which occurred around 13.8 billion years ago. This radiation was emitted by matter that has, in the intervening time, mostly condensed into ...
where is the curvature density and is the value of the Hubble parameter today. In the currently favoured geometric model of our Universe, the "angular diameter distance" of an object is a good approximation to the "real distance", i.e. the proper distance when the light left the object.
Observations of X-ray burst sometimes show X-ray spectra indicating radius expansion. Therefore, the X-ray flux at the peak of the burst should correspond to Eddington luminosity , which can be calculated once the mass of the neutron star is known (1.5 solar masses is a commonly used assumption).