By measuring with ever-increasing precision, and observing the rotation curves of galaxies and the anisotropies of the cosmic microwave background (CMB), astrophysicists have discovered two enormous “missing components” in the universe’s budget. Ordinary matter — stars, gas, planets, us — makes up barely about of the total. Everything else is dark.
Dark matter accounts for about of the total mass-energy. It emits no light, but exerts gravity. It explains why galaxies rotate faster than visible matter alone would justify, why galaxy clusters remain gravitationally bound, and why numerical simulations produce the observed filamentary structures. Its microscopic nature, however, remains unknown: perhaps WIMP-type particles (Weakly Interacting Massive Particles), perhaps something even more exotic.
Dark energy accounts for about of the total. It is a sort of “negative pressure” that accelerates the expansion of the universe. It was discovered in 1998 (Perlmutter, Schmidt and Riess; Nobel 2011) by analysing very distant type Ia supernovae: they are found to be farther away than the linear Hubble law would predict, a sign that the expansion is accelerating. Its microscopic nature too is one of the great open problems of fundamental physics.
Cosmological summary
Hubble’s law: , cosmic expansion. The universe’s budget is roughly ordinary matter dark matter dark energy. And the universe is accelerating its own expansion.
Where does all this come from?
Extrapolating the expansion backwards, about years ago the universe had infinite density: the Big Bang. Over the past billion years it has cooled from the temperatures of nuclei and quarks down to the present K of the cosmic microwave background, the leftover photons of that primordial heat, discovered by Penzias and Wilson in 1964 (Nobel 1978). It is the universe’s “baby photograph”.
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Topics: Relatività ristretta
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