There are currently three main theories concerning the death of the universe. Let’s figure out how to destroy physics in the last edition of physics gone wild.
This theory is essentially the loner theory. Since its birth, the universe has been expanding. For unknown reasons, new space is created everywhere equally. It is thought that eventually over time, as this new space is being created, it will be created at a faster rate than gravity is able to counter its effects. This means that things will start to ‘rip apart’ as gravitational pull will not be enough to overcome the every expanding new space. Over time, galaxies, celestial bodies, black holes and even atoms will break apart until space is full of tiny particles moving around in a strange timeless universe never able to interact with one another. I know. Gloomy right? And you thought you felt lonely sometimes.
Similar to the Big Rip, in that the universe is expanding and new space is being created, however the difference lies in the state of the matter. The conceptual notion heat death actually derives from the second law of thermodynamics – put simply, entropy increases in an isolated system (in this case, the universe). Entropy, which is the number of ways in which a system can be arranged, favours a natural state of disorder, evolving to a state of maximum disorder (or thermodynamic equilibrium). When this happens, all energy will be evenly distributed throughout the cosmos, leaving no room for any reusable energy or heat to burst into existence. All matter would be converted slowly over time to radiation until entropy is reached.
This is the only theory that claims some sort of reincarnation of the universe is possible. According to the theory, eventually dark energy fuelling the expansion will run out and the universe will begin to contract as gravity emerges as the dominant force in the universe. As this happens, the universe will grow hotter constant temperature rises and background radiation eventually being hotter than the surface of stars. Matter will being to break down as black holes swallow everything and join together to form a super massive mega black hole that consumes the entire universe and itself under the force of its own gravity. This is known as the Big Crunch. The theory continues to state that the universe goes through an endless cycle of expansion and contraction with a Big Bounce of sorts as a big bang kick starts a know universe at the contraction of a previous one.
Sentimentally speaking the last theory is probably the most uplifting. But who knows, physics will be physics and its just a waiting game to find out.