The cosmological constant

Abstract

The energy density of the vacuum, , is at least 60 orders of magnitude smaller than several known contributions to it. Approaches to this problem are tightly constrained by data ranging from elementary observations to precision experiments. Absent overwhelming evidence to the contrary, dark energy can only be interpreted as vacuum energy, so the venerable assumption that = 0 conflicts with observation. The possibility remains that is fundamentally variable, though constant over large spacetime regions. This can explain the observed value, but only in a theory satisfying a number of restrictive kinematic and dynamical conditions. String theory offers a concrete realization through its landscape of metastable vacua.

Dark energy and extending the geodesic equations of motion: its construction and experimental constraints

“…From WMAP, DE = 7.21 0.82 −0.84 × 10 −30 g/cm 3 , which for hydrogen atoms corresponds to a number density of ∼ 4 atoms/m 3 . It is clear that the density of both solids and liquids far exceed DE , and in such media Eq.…”

“…The hardest vacuum currently attainable experimentally is ∼10 −13 torr [43]. For a gas of He 4 atoms at 4 • K, this corresponds to a density of ρ limit ≈ 10 −18 g/cm 3 , which is 11 orders of magnitude smaller than DE . Nevertheless, because the scale of the acceleration from the additional terms in Eq.…”

“…The extended GEOM does not affect the motion of electromagnetic waves, and such tests of general relativity as the gravitational redshift and the deflection of light by the Sun will not be affected by the extension. Considering first the advancement of the perihelion of Mercury, we note that the density of matter, ρ orb , (due primarily to the solar wind) in the region of Mercury's orbit is ∼ 10 −23 g/cm 3 . This density is only five orders of magnitude smaller to ρ limit .…”

“…While stringent, we will nevertheless show explicitly that a choice of extension can be made which satisfies it. Physically, this choice is possible because at (7.21 +0.83 −0.84 )10 −30 g/cm 3 [17], DE is orders of magnitude smaller than the density of matter, ρ limit , either currently achievable in terrestrial experiments (where densities exceed 10 −18 g/cm 3 ), or present in the solar system (where the density of matter in Mercury's orbit exceeds 10 −23 g/cm 3 ). Correspondingly, at 14010 800 820 Mpc DE is more than three times larger than the observed size of the universe, and is orders of magnitude larger than the solar system.…”

Probing the Vacuum Decay Hypothesis with Growth Function Data

“…In fact, this vacuum-dominated model of the Universe, the so-called ΛCDM model, has been favored by a large amount of observational data. Unfortunately, the value of the vacuum energy required to explain the present accelerated phase differs from the value predicted by the Quantum Field Theory (QFT) by at least 60 orders of magnitude [5] (see also [6] for a review). This huge discrepancy between theory and observation has challenged physicists' imagination, and many proposals to solve this problem have appeared in the literature [7][8][9][10][11][12][13][14][15].…”