Heat Death of the Universe

the erroneous notion that all types of energy in the universe must ultimately become energy of thermal motion. The energy would be uniformly distributed throughout the matter of the universe, and all macroscopic processes would then come to a halt.

The notion was formulated by R. Clausius in 1865 on the basis of the second law of thermodynamics. According to the second law, any physical system that does not exchange energy with other systems (such exchange is obviously precluded for the universe as a whole) tends toward the most probable equilibrium state: the state of maximum entropy. Such a state would correspond to the heat death of the universe.

Even before the development of modern cosmology, numerous attempts were made to disprove the notion of the heat death of the universe. The best known attempt was the fluctuation hypothesis advanced by L. Boltzmann in 1872. According to this hypothesis, the universe is in an isothermal equilibrium state, but, in accordance with the law of chance, deviations from this state sometimes occur in one place or another; the larger the region encompassed by the deviations and the greater the degree of deviation, the less often the deviations occur.

Modern cosmology has established that not only the notion of the heat death of the universe but also the early attempts to refute the notion are erroneous: important physical factors—above all, gravitation—were not taken into account. When gravitation is taken into account, the uniform isothermal distribution of matter is not at all the most probable distribution and does not correspond to maximum entropy. Observations show that the universe is decidedly non-steady-state; it is expanding. Matter, which was nearly uniform at the start of the expansion, with time breaks up into individual objects, under the action of gravitational forces, forming galactic clusters, galaxies, stars, and planets. All these processes are natural, proceed with increasing entropy, and do not require violation of the laws of thermodynamics. Even in the future, if gravitation is taken into account, these processes will not lead to a uniform isothermal state of the universe—that is, to its heat death. The universe is always nonstatic and continuously evolving.