Our universe, therefore, must have begun to exist.
Eternal cyclical model
In these models, it is believed that the “big crunch” of a collapsing universe would evolve into the “big bang” of a later expanding universe, and this process continues indefinitely in the future.
If the cycles of “big bang” and “big crunch” are past-eternal, then it could be argued that our universe always existed.
There are a number of problems with this model:
- our universe does not have the greater than critical density required to generate a “big crunch” in the future
- problem of infinite regress
- increasing entropy with each cycle, leading to eventual “heat death”
Critical density
Critical density is the density required to render space flat. A flat universe is one which expands forever with gravity slowing the expansion so that its rate approaches zero.
Visible matter in our universe accounts for only about one percent of critical density. “Dark matter” accounts for another twenty to twenty five percent of critical density. “Dark matter” consists of objects such as undetected planets, brown dwarfs, neutron stars and black holes and of “cold” matter which is neither made from protons nor neutrons like ordinary matter. Together, visible and “dark matter” account for twenty three per cent of critical density.
It is believed that “dark energy” is responsible for the accelerating expansion of the universe. Unlike gravitational force that reduces with distance, the strength of “dark energy” increases with distance. “Dark energy”, which provides a negative pressure, a cosmic repulsive force, to drive the accelerated expansion of our universe, accounts for seventy three per cent of critical density.
So our universe is at critical density, flat and, therefore, does not support these models of the multiverse.
Infinite regress
If our current universe developed from a prior “big crunch” which itself followed an earlier “big bang” and this process is traced endlessly back, our own universe would not exist, because the infinitival “big bang” is never reached. There must, therefore, be a beginning and the most logical starting point is the singularity, from which the “first universe” exploded in the “first big bang” to collapse into “first big crunch” and so on until our current universe is reached.
Therefore, there is still a defined origin for the multiverse and this means that everything including the multiverse still began to exist.
Increasing cyclical entropy
Since the universe is a closed system, its entropy will increase with time by the second law of thermodynamics until it suffers a “heat death” at maximum entropy in thermodynamic equilibrium. With each cycle, therefore, the entropy of the universe will increase until there will not any energy left to “big bang” into another cycle.