In conformal cyclic cosmology (CCC for short), the heat death of universe looks a lot like big bang. Essentially once all matter is reduced to photons (and massless particles) there is nothing to track time (or space), light can be understood as being everywhere all at once, thus causing huge amount of energy and with very little entropy.

[PBS Spacetime on CCC] https://youtu.be/PC2JOQ7z5L0