The Fate of Our Universe

Ever since the discovery in 1929 that our Universe is expanding, three main fates of the Universe have been hypothesized over time: “The Big Crunch,” “The Big Freeze,” and “The Big Rip.” The Big Crunch presents a horrific representation of how the world will end, with the Universe collapsing on itself to create enough heat to cause oceans to boil, rock to melt, and eventually all people will die in the intense heat. However, this fate could be part of a cyclic Universe, giving the possibility of regeneration of life in the future. In contrast to the idea of the Big Crunch being the end of life as we know it, the Big Rip presents a fate where the Universe expands so fast that galaxies, stars, solar systems, planets, all the way down to people and atoms will eventually be torn apart. In between these two fates is the Big Freeze, which presents a “cooler” way for things to end. The Big Freeze predicts an ever expanding Universe, in which the size of the Universe continually increases, and where motion eventually ceases, causing the Universe to lose heat and freeze.

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.

Emily Helfer