This loss of heat also encourages the other name for the Big Freeze, “Heat Death.” While this name at first seems ironic, it comes from the idea that in an isolated system, entropy, the measure of a system’s disorder, will continuously increase over a span of 1014 (hundred trillion years) until it slowly continues to reach a maximum value, where heat is evenly distributed in the system. The even distribution does not allow for usable energy to exist, such as heat, since there must be a temperature gradient for mechanical work to occur; therefore, no more motion will be possible, causing everything to eventually become too cold to sustain life and causing star formation to cease. Once the freeze happens, atoms and particles will begin to decay into photons and after 10116 years, most particles will have disintegrated. Finally, all matter would evaporate leaving only black holes in the Universe, which would also eventually break down. After 10200 years, the Universe would be empty. This progression of events is explained more in depth here. Another hypothesized procession of events is diagrammed here, where the “Heat Death” is said to happen after 101000 years.
The fate of the Universe depends on its density and composition, which affect the shape. The shape of the Universe is determined by the density of the Universe; if the density is less than the critical density, then the Universe would be saddle shaped, if the density is equal to the critical density, then the Universe would be flat, and if the density is greater than the critical density, the Universe would be spherical. Depictions of these different shapes are shown here. If the density of the Universe is less than or equal to the critical density, the value for which the Universe has a flat shape, then the Big Freeze will be the fate of the Universe. However, if the density of the Universe is greater than the critical density, even though the Big Crunch is also likely to happen, it is still possible for the Big Freeze to happen if there is enough “dark energy” in the Universe to speed up the expansion. According to the measurements made by the Wilkinson Microwave Anisotropy Probe, or WMAP, the density of the Universe is less than the critical density, but close enough to the critical density that it shows the Universe to be flat. These signs point to an expanding Universe, so there may be a Big Freeze in our future.