Heat death: Ultimate fate of the universe [Short review]

The universe is by definition of the word is conceived as isolated in thermodynamics, because it contains everything by definition, and there is nothing outside the universe to exchange anything. By definition, an isolated system is a thermodynamic system that cannot exchange either energy or matter outside the boundaries of the system.

The second law of thermodynamics states that the entropy of an isolated system never decreases because isolated systems always evolve toward thermodynamic equilibrium, a state with maximum entropy.

If the universe is indeed an isolated system, then its entropy too must increase with time. The implication is that the universe must ultimately suffer a “heat death” as its entropy progressively increases toward a maximum value and all parts come into thermal equilibrium at a uniform temperature.

The heat death of the universe (also known as the Big Chill or Big Freeze) is a theory on the ultimate fate of the universe. Heat death does not imply any particular absolute temperature; it only requires that temperature differences or other processes may no longer be exploited to perform work.

The hypothesis of heat death originated from the ideas of Lord Kelvin, who in the 1850s took the theory of heat as mechanical energy loss in nature as embodied in the first two laws of thermodynamics and extrapolated it to larger processes on a universal scale.

The idea of heat death shoots from the second law of thermodynamics as said above, as the entropy of the universe progressively increases toward a maximum value and all parts come into thermal equilibrium at a uniform temperature, it implies that if the universe lasts for a sufficient time, it will asymptotically approach a state where all energy is evenly distributed. In other words, there exists the view that, in time, the mechanical movement of the universe will run down as work is converted to heat because of the second law. The speculation is that all bodies in the universe will cool off, eventually becoming too cold to support life.

Current status

A new study published in November 2020 found that the universe is actually getting hotter. The study probed the thermal history of the universe over the last 10 billion years. It has found that “the mean temperature of gas across the universe has increased more than 10 times over that time period and reached about 2 million degrees, Kelvin, today-approximately 4 million degrees Fahrenheit.”

 Opposing views

Max Planck wrote that the phrase "entropy of the universe" has no meaning because it admits of no accurate definition.

More recently, Grandy writes: "It is rather presumptuous to speak of the entropy of a universe about which we still understand so little, and we wonder how one might define thermodynamic entropy for a universe and its major constituents that have never been in equilibrium in their entire existence.

"According to Tisza: "If an isolated system is not in equilibrium, we cannot associate entropy with it.

"Buchdahl writes of "the entirely unjustifiable assumption that the universe can be treated as a closed thermodynamic system".

According to Gallavotti: "... there is no universally accepted notion of entropy for systems out of equilibrium, even when in a stationary state."

A 2010 analysis of entropy states, "The entropy of a general gravitational field is still not known", and "gravitational entropy is difficult to quantify". The analysis considers several possible assumptions that would be needed for estimates and suggests that the observable universe has more entropy than previously thought. Gravitationally bound systems have negative specific heat—that is, the velocities of their components increase when energy is removed. Such a system does not evolve toward a homogeneous equilibrium state. Instead, it becomes increasingly structured and heterogeneous as it fragments into subsystems. 

Credit: Google