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Stars, which illuminate the universe with their visible light, are powered by nuclear hydrogen furnaces. These furnaces will eventually run out of energy and the star will eventually grow in size and become a supernova (a pulsar beforehand). However, within the supernova new stars will be born as well. This will continue on until the matter is scattered beyond synthesis of new stars. Thus eventually over the period of hundreds of billions of years all stars will theoretically die, and the universe will reach an eventual temperature of absolute zero.
The universe started with a Big Bang, analysis of cosmological measurements allows a possibility that it will end with a Big Rip. As soon as few billion years from now, the controversial scenario holds, dark energy will grow to such a magnitude that our own Galaxy will no longer be able to hold itself together. After that, stars, planets, and then even atoms might not be able to withstand the expansive internal force. Previously, speculation on the ultimate fate of the universe centered on either a re-collapsing Big Crunch or a Big Freeze.. The Big Freeze is a scenario under which continued expansion results in a universe that asymptotically approaches absolute zero temperature. It could, in the absence of dark energy, occur only under a flat or hyperbolic geometry. With a positive cosmological constant, it could also occur in a closed universe. In the special case of phantom dark energy, which has even more negative pressure than a simple cosmological constant, 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 a result, all material objects in the universe, starting with galaxies and eventually all forms, no matter how small, will disintegrate into unbound elementary particles and radiation, ripped apart by the phantom energy force and shooting apart from each other. The end state of the universe is a singularity, as the dark energy density and expansion rate becomes infinite.
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