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The expansion of the universe is the increase in distance between gravitationally unbound parts of the observable universe with time. [1] It is an intrinsic expansion, so it does not mean that the universe expands "into" anything or that space exists "outside" it.
Thus, an accelerating universe took a longer time to expand from 2/3 to 1 times its present size, compared to a non-accelerating universe with constant ˙ and the same present-day value of the Hubble constant. This results in a larger light-travel time, larger distance and fainter supernovae, which corresponds to the actual observations.
English: Expansion of the universe, comoving coordinate grid Horizontal axis: Comoving distance in billion light years. Vertical axes: time since Big Bang in billions of years Shown are the Particle Horizon RP (green), Event Horizon RE (magenta), Hubble Radius RH (blue). Past and future light cones RL (orange) are animated.
The Big Bang event 13-14 billion years ago initiated the universe, and it has been expanding ever since. Scientists in 1998 disclosed that this expansion was actually accelerating, with dark ...
The expansion of the universe is parameterized by a dimensionless scale factor = (with time counted from the birth of the universe), defined relative to the present time, so = =; the usual convention in cosmology is that subscript 0 denotes present-day values, so denotes the age of the universe.
In physical cosmology, the Big Rip is a hypothetical cosmological model concerning the ultimate fate of the universe, in which the matter of the universe, from stars and galaxies to atoms and subatomic particles, and even spacetime itself, is progressively torn apart by the expansion of the universe at a certain time in the future, until distances between particles will infinitely increase.
The Hubble time is the age it would have had if the expansion had been linear, [54] and it is different from the real age of the universe because the expansion is not linear; it depends on the energy content of the universe (see § Derivation of the Hubble parameter).
A twin universe could be exerting its gravity on ours, messing up our cosmic calculations.