Given that the universe started with a brief period of rapid inflation, the ultimate fate of the universe is a valid cosmological concern that depends on the physical properties of the mass and energy in the universe, its average density, and the rate at which space is expanding. As part of this, the ultimate fate of an expanding universe depends on the matter density (ΩM), the dark energy density (ΩΛ), the overall shape of the universe, how much dark energy it contains, and how the dark energy density responds to the expansion of the universe.
Quintessentially, phantom dark energy, or levity, is as a positive cosmological constant with negative pressure that changes at an accelerating rate as the universe expands (being the opposite of gravity). This is important because 6.2 billion years ago, commensurate with an open universe, space grew so large that an important switch to acceleration occurred when the expansive effect of repulsion was no longer counter-balanced by the contractive effect of attraction. The runaway effect of levity accelerated the process of expansion, pushing everything in the universe further and further apart, faster and faster, at an ever-increasing rate. As a result, the unbalanced force of phantom dark energy began to cause every point in the universe to continuously move further and further away from each other, causing the curvature of the extent to lessen, as the geometry of large-scale space became flatter.
Assuming the overall geometry of space is negatively curved like the surface of a saddle, the shape of the universe must be hyperbolic and Ω < 1. As a negatively curved space-time continuum, the universe will also inevitably expand forever, and the expansion will not only continue but also accelerate as time goes on. This is because, the ultimate fate of an open universe centers on an acceleration caused by dark energy that eventually becomes so strong that it completely overwhelms the effects of the gravitational, electromagnetic and strong nuclear forces.
By extrapolating the theory of heat views of mechanical energy loss in nature to universal operations, the notion of heat death becomes a possible final state of the universe, estimated at after 10¹⁵⁰ years, provided that space-time attains a specific thermodynamic state in which all energy becomes confined to irreversible states which then prevents the sustainment of motion and structure through maximum entropy. This is a scenario under which continued expansion results in a universe that asymptotically approaches absolute zero temperature. It could occur in a closed universe with a positive cosmological constant. This occurs in conjunction with a state of maximum entropy in which everything is evenly distributed, and there are no gradients — which are needed to sustain information processing in stable physical forms.
The heat death scenario requires that the universe reach an eventual temperature minimum. This is the most likely outcome of a Big Bang cosmogony, assuming that the universe will continue expanding indefinitely. Over a time scale on the order of 10¹⁴ years or less, existing stars burn out, new stars cease to be created, and the universe goes dark. Over a much longer time scale in the eras that will follow this, the galaxy evaporates as the stellar remnants comprising it escape into space, and black holes evaporate via the mechanism of Hawking radiation. It is also possible that after at least 10³⁴ years, proton decay, will convert the remaining interstellar gas and stellar remnants into leptons and some positrons and electrons will then recombine into photons. In this particular instance, the universe would reach a high-entropy state consisting of a bath of particles and low-energy radiation. Although, the future condition may, or may not, achieve thermodynamic equilibrium as a result.
In the special case of phantom dark energy, levity has far more negative pressure than a simple cosmological constant, so the density of dark energy increases with time, causing the rate of acceleration to increase, leading to a steady increase in the Hubble constant. As the energy density of levity increases without limit over time, the expansion rate of the universe increases without limit as a result of the phantom dark energy. So, the expansion rate of the universe eventually reaches infinity, and gravitationally bound systems, such as clusters of galaxies, will be torn apart. Eventually, the expansion will be so rapid as to overcome the electromagnetic forces holding molecules and atoms together. Finally, even atomic nuclei will be ripped asunder. In the end, everything will disintegrate into unbound elementary particles and radiation, shooting away from each other, as the dark energy density and expansion rate approach infinity.
Furthermore, as the most fundamental framework of the cosmos, the multiverse never ends completely. Every universe is merely one Big Bang among an infinite number of simultaneously expanding initial conditions that are spread out over the endless distances of open space. In line with this, each universe is either composed of matter or antimatter, with an equal number in existence at any given time. So, as the universes expand they collide and matter and antimatter annihilate, releasing energy. The chain reaction of Big Bangs then continues as energy is consumed. However, since the multiverses are open and the ekpyrotic fuel is infinite, the chain reactions can last forever — such that matter and energy exist as the product of a transformation from a reactant.
Moreover, if the universe is in a very long-lived false vacuum, it is possible that a small region of the universe will tunnel into a lower energy state. If this vacuum meta-stability event happens, all the structure within the region will be destroyed instantaneously and the region will expand at a rate that far exceeds the speed of light, bringing destruction without any forewarning. Although, according to the many-worlds interpretation of quantum mechanics, the universe will not end this way. Instead, each time a quantum event occurs it causes the universe to decay from a false vacuum to a true vacuum state, so the universe splits into several new worlds. In some of the new worlds, the universe decays, while in others the universe continues as before.