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Rocket your D&D 5E and Level Up: Advanced 5E games into space! Alpha Star Magazine Is Launching... Right Now!

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Message: <blockquote data-quote="fuindordm" data-source="post: 2696441" data-attributes="member: 5435">To elaborate a bit:The current cosmological theory is derived from General Relativity and the assumption that on large scales the universe can be treated as a perfect fluid of some variety, or as a mixture of perfect fluids.  We're talking about scales larger than the average distance between galaxies here, so normal matter (most of which is in galaxies) is treated as a collisionless, pressureless gas (because galaxies almost never come near enough to each other to interact).  The eventual fate of the universe, as Umbran points out, depends only on what it's made out of--the theory describing its evolution is the same in any case.At early times, most of the mass/energy density in the universe was in the form of radiation.At later times, most of the mass/energy density was in the form of neutral matter.Twenty years ago, the only question was how much matter there was in the universe--since not all of it is in luminous stars, it's harder to add up than the radiation.  People have been trying to measure the amount of dark matter for a long time, generally coming up with anywhere from 30% to 90% of the amount needed to stop the expansion of the universe.  No one has ever come up with enough matter to cause a Big Crunch, and only the die-hard theorists have come up with enough matter to slow the expansion to a halt.  (A universe with exactly enough matter and energy to just barely stop expanding is slightly easier to explain, as its density is a stable number--other models of the universe have an energy density that evolves with time, and a certain amount of fine-tuning is required in the initial conditions to get a universe old enough to create stars.)  So the Big Crunch was never really that viable as a theory.Aside from matter and energy, there's a third medium that can drive the evolution of the universe.  This is 'vacuum energy', the energy density you find even in completely empty space due to the creation and annihilation of particle pairs according to the uncertainty principle.  In the framework of General Relativity, this can be described with a single number: the cosmological constant.  In the framework of quantum field theory, it's much harder to quantify and no one yet has come up with a calculation of vacuum energy density that arrives at a reasonable number.  According to theory, the vacuum energy should either be zero or some enormously high value that would make the universe expand into nothingness in an unreasonably short time.  So for a long time the cosmological constant was assumed to be zero--why complicate the theory?  Recent observations of the cosmic microwave background, the distribution of galaxies in the universe, and distant supernova, however, have shown pretty conclusively that the cosmological constant is not zero.Since vacuum energy density remains constant as volume increases, it exerts a different kind of pressure on space-time than radiation or matter.  The existence of vacuum energy (also called dark energy or quintessence) would eventually drive the universe into a phase of exponential expansion, so the empty universe is coming much sooner than we think.  But it's not all bad--this expansion is only of the space between gravitationally bound systems.  By the time it happens, our galaxy should be safely tucked away in the Virgo cluster of galaxies, still giving us several thousand galaxies to play in.But again, it's really all one theory.  It's just a matter of plugging in different amounts of matter, radiation, and cosmological constant, and seeing how the mixed fluid evolves according to general relativity.Ben</blockquote>

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