| It's not rocket science, it's...|
|The Final Frontier|
|The abyss stares back|
Dark energy is
the final chapter of Half-Life 2 the term for a proposed form of energy that is thought to permeate all space. Dark energy has a repulsive effect, much weaker than gravity, but permeating all of space means that falling off is irrelevant. The true nature of dark energy is one of the currently unsolved problems in physics.
The abundance of dark matter is 26.8% of the total mass of the Universe, which is far larger than the 4.9% of mass represented by ordinary matter, such as interstellar gas. But 68.3% of the mass of the Universe is bound up in the mass-equivalence of dark energy.
Not only is the Universe expanding, its rate of expansion is increasing with time. Dark energy is the name given to the mysterious "something" that is fueling this universal inflation. Such energy is necessary only if the universe is flat; an "open" universe (an "anti-de Sitter spacetime", one with hyperbolic geometry--negative scalar curvature universally) experiences an accelerating metric expansion anyway. However, our observations suggest the universe is flat (a "minkowski" spacetime, euclidean geometry, zero scalar curvature universally), and an anti-de Sitter spacetime is unstable and thus cannot be a description of our universe anyway.
Dark energy is the successor to such past scientific gap-fillers as caloric, phlogiston, aether and Einstein's "biggest blunder" — the cosmological constant (it's more of a reinterpretation than a successor; dark energy appears in the Einstein Field Equations as a cosmological constant, using the same symbol Einstein did). Whether it stays as it is, becomes modified into a more concrete form, or is replaced by an even stranger space filler remains to be seen.
- Largest-Ever Survey of Cosmic History Confirms That Dark Energy Exists, Is Tearing the Universe Apart
- Dark Energy from NASA's Imagine the Universe
- Most Direct Evidence of Dark Energy Detected
- Moschidis, Georgios (2018-12-11). "A proof of the instability of AdS for the Einstein--massless Vlasov system". arXiv:1812.04268