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The comoving distance from Earth to the edge of the observable universe is about 14.26 gigaparsecs (46.5 billion light-years or 4.40 × 10 26 m) in any direction. The observable universe is thus a sphere with a diameter of about 28.5 gigaparsecs [27] (93 billion light-years or 8.8 × 10 26 m). [28]
It represents the boundary between the observable and the unobservable regions of the universe, so its distance at the present epoch defines the size of the observable universe. Due to the expansion of the universe, it is not simply the age of the universe times the speed of light, as in the Hubble horizon, but rather the speed of light ...
Rather, the conformal time is the amount of time it would take a photon to travel from where we are located to the furthest observable distance, provided the universe ceased expanding. As such, η 0 {\displaystyle \eta _{0}} is not a physically meaningful time (this much time has not yet actually passed); though, as we will see, the particle ...
In 1926, Wilhelm Lenz concluded that "If one allows waves of the shortest observed wavelengths λ ≈ 2 × 10 −11 cm, ... and if this radiation, converted to material density (u/c 2 ≈ 10 6), contributed to the curvature of the observable universe – one would obtain a vacuum energy density of such a value that the radius of the observable ...
The distinction between "observable" and "unobservable" is similar to Immanuel Kant's distinction between noumena and phenomena.Noumena are the things-in-themselves, i.e., raw things in their necessarily unknowable state, [3] before they pass through the formalizing apparatus of the senses and the mind in order to become perceived objects, which he refers to as "phenomena".
On the other hand, the steady-state model says while the universe is expanding, it nevertheless does not change its appearance over time (the perfect cosmological principle). E.g., the universe has no beginning and no end. This required that matter be continually created in order to keep the universe's density from decreasing.
In physics, an observable is a physical property or physical quantity that can be measured. In classical mechanics , an observable is a real -valued "function" on the set of all possible system states, e.g., position and momentum .
Cosmic time [5] [6]: 142 is a measure of time by a physical clock with zero peculiar velocity in the absence of matter over-/under-densities (to prevent time dilation due to relativistic effects or confusions caused by expansion of the universe). Unlike other measures of time such as temperature, redshift, particle horizon, or Hubble horizon ...