Vacuum Energy

We consider a possibility that the formally infinite vacuum energy of the quantized matter fields could be stored into Planck-size quantum black holes acting as the fundamental constituents of space and time. Using the recently proposed thermodynamical equation of state obeyed by the cosmological constant we indicate, how this idea might explain the smallness of the cosmological constant, if the vacuum energy is the source of the cosmological constant.

A critical look is taken at the calculation of the Casimir effect. The boundary conditions play an important role and should be imposed in a physical way. An acceptable result for the vacuum energy is only obtained when different regularization schemes yield the same result. Radiative corrections to the Casimir force between two parallel plates due to electromagnetic vacuum fluctuations have been obtained both in full QED and in a low-energy, effective field theory with confl...

The last decades have witnessed an unprecedented advancement in our knowledge of the large scale universe. In particular, increasingly accurate cosmological observations have allowed us to discover a form of "dark energy", which presently dominates the expansion of the universe. On the other hand, fundamental problems in the standard cosmological model point towards the possibility of a primordial inflationary period. Both these expansion phases have in common the fact that t...

A simple description of the vacuum energy (cosmological constant) problem for non-experts is presented. Basic features of cosmology with non-zero vacuum energy are discussed. The astronomical data which indicate that the universe is filled with an anti-gravitating state of matter are described. The mechanisms which may lead to cancellation of almost infinite vacuum energy down to the astronomically observed value are discussed. The idea of dynamical adjustment is considered i...

It was recently suggested that the cosmological constant problem as viewed in a non-perturbative framework is intimately connected to the choice of time and a physical Hamiltonian. We develop this idea further by calculating the non-perturbative vacuum energy density as a function of the cosmological constant with multiple choices of time. We also include a spatial curvature of the universe and generalize this calculation beyond cosmology at a classical level. We show that va...

The principles of General Relativity allow for a non-vanishing cosmological constant, which can possibly be interpreted at least partially in terms of quantum-fluctuations of matter fields. Depending on sign and magnitude it can cause accelerated or decelerated expansion at certain stages of cosmic evolution. Recent observations in cosmology seem to indicate that we presently live in an accelerated phase. We recall the history and fundamental issues connected with the cosmolo...

The issue of the vacuum energy of quantum fields is briefly reviewed. It is argued that this energy is normally either much too large or much too small to account for the dark energy, However, there are a few proposals in which it would be of the order needed to effect the dynamics of the present day universe. Backreaction models are reviewed, and the question of whether quantum effects can react against a cosmological constant is discussed.

Vacuum fluctuations and the Casimir effect are considered in a cosmological setting. It is suggested that the dark energy, which recent observations suggest make up 73% of our universe, is vacuum energy due to a causal boundary effect at the cosmological horizon. After a discussion of the similarities and differences between material boundaries in flat spacetime and causal horizons in general relativity, a simple model with a purely vacuum energy de Sitter interior and Schwar...

The cosmological constant is not an absolute constant. The gravitating part of the vacuum energy is adjusted to the energy density of matter and to other types of the perturbations of the vacuum. We discuss how the vacuum energy responds (i) to the curvature of space in the Einstein closed Universe; (ii) to the expansion rate in the de Sitter Universe; and (iii) to the rotation in the Goedel Universe. In all these steady state Universes, the gravitating vacuum energy is zero ...

The accelerating expansion of the Universe points to a small positive value for the cosmological constant or vacuum energy density. We discuss recent ideas that the cosmological constant plus LHC results might hint at critical phenomena near the Planck scale.