According to current scientific understanding, the concept of the "heat death" of the universe is based on the idea of entropy and the second law of thermodynamics. The heat death scenario suggests that over an extremely long period of time, all energy in the universe will be evenly distributed, resulting in a state of maximum entropy where no further useful work can be done.
As for the timing of the heat death, it is still a topic of debate among scientists. Various theoretical models and calculations have been proposed, but it's important to note that the time scales involved are truly immense. The most widely accepted estimates suggest that the heat death could occur tens of billions to trillions of years from now.
Regarding whether the heat death is stoppable, the current understanding is that it is an inevitable consequence of the laws of thermodynamics. As long as the universe continues to expand and entropy increases, the progression towards the heat death appears to be unstoppable. However, it's worth noting that our understanding of the universe is continuously evolving, and future discoveries may bring new insights or possibilities that could potentially challenge or revise these predictions.