Modern cosmological research concentrates on 'quantum cosmology', which attempts to reconcile the quantum physical conditions just after the big bang with the general relativistic conditions thereafter.
Standard cosmology, based on Einstein's general relativity, describes most of the history of the cosmos very well. It is only when distances, volumes and energy densities are reduced to the Planck scale that general relativity fails.
The Planck scale for energy is defined by the energy of a photon with a wavelength equal to one Planck length, which is about 1.6 E-35 meter (where E-35 means 10 to the power –35). This photon wavelength gives the Planck energy as about 2 E+9 Joule, roughly equal to the electricity consumed by an average person in a developed country in two weeks.
Planck Energy Density
The Planck energy is not all that high, but constrain this energy into a volume of one cubed Planck length and you have the Planck energy density of about 4.6 E+113 Joule per square meter. Ouch!! The point of all the laboring is that quantum gravity shows that when the energy density of the universe is near the Planck limit, an anti-gravity term grows to a magnitude that overwhelms normal gravity and causes space to expand.
A Big Bounce
If there were a contracting universe before the big bang, this anti-gravity would have stopped the contraction and reversed it, i.e., a big bounce would have happened before the density reached the Planck limit. The figure below illustrates such a 'big bounce' caused by quantum gravity.
In this illustration, time runs upwards while space-time first contracts to near the Planck limit (the red band, with color indicating energy density) and then bounces back into an expansion again. The red spheres represent elementary particles (concentrated energy) separating from each other due to the expansion of the fabric of space-time.
This is merely a 'toy-model' of quantum cosmology that calculates the simplest scenario in order to enhance insight. More representative quantum cosmology models of contraction with fully formed structures influencing the possible bounce scenarios are under study. It is not clear if the pre-bounce and the post-bounce universes lived in the same space-time. It is more likely that the two space-times were different universes.
A White Hole?
As an example, if a very massive star goes supernova and leaves a remnant that contracts into a black hole, the same bounce should happen at the central singularity. However, we observe the effects of stable black holes without a bounce. The bounced black hole may have popped out in another universe as a white hole…