Time stops in 3D universe. There is no way to measure distance or mass, which for any practical purpose no longer exist. If all the previous mass is expressed as kinetic energy, then a new universe can begin in a singularity, with all the energy expressed as radiant type.
Small irregularities can occur in the new CMB as evidence of the previous universe and its blackholes.
Stephen Hawking showed possibility of evaporating blackholes, a slow rate in 4D, but possibly sudden in 3D when there is no distance or mass..
It proposes a way a large old universe can be destroyed and be recreated as a new universe of low entropy in a small singularity with radiant energy, but no mass.
Do Blackholes Explode Into Gamma Rays When 4D Vacuum Decays To 3D?