Dark energy and the cosmological constant

6 problems

• Can averaging over cosmic inhomogeneities mimic dark energy?

  Contested

  The standard model computes the expansion as if the universe were perfectly smooth, but it is lumpy, with large voids and dense walls. Some researchers argue that averaging over the lumps, including clocks running at different rates in voids and walls, produces apparent acceleration without dark energy.

• Why is the vacuum energy density so small yet nonzero?

  Open

  Quantum theory says empty space should be full of energy from fluctuating fields, which would curl up or blow apart the universe. The observed value is smaller than naive estimates by 44 to about 120 powers of ten, yet not zero.

• Why are the dark energy and matter densities comparable today?

  Open

  Matter thins out as the universe expands while vacuum energy does not, so their ratio changes enormously over cosmic history. It happens to be close to one now, when observers exist.

• Is the dark energy density constant in time?

  Contested

  If dark energy is the energy of empty space it never changes, but if it comes from a slowly rolling field its density should drift. New maps of millions of galaxies hint at a drift.

• Does the dark energy equation of state cross $w = -1$?

  Open

  The current best fits say dark energy was once stronger than empty-space energy and is now weaker. A simple rolling field cannot do that, so either the fits mislead or the physics is more exotic.

• Are evolving dark energy hints driven by supernova systematics?

  Contested

  Exploding stars called type Ia supernovae serve as distance markers, and the dark energy hint depends on which supernova catalog is used. Small calibration errors between nearby and distant supernovae could fake the signal.