The Cosmological constant problem Philippe Brax To cite this version: Philippe Brax. The Cosmological constant problem. Contemporary Physics, Taylor & Francis, 2004, 45, pp.227-236. 10.1080/00107510410001662286. hal-00165345 HAL Id: hal-00165345 https://hal.archives-ouvertes.fr/hal-00165345 Submitted on 25 Jul 2007 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. The Cosmological Constant Problem Ph. Brax1a a Service de Physique Th´eorique CEA/DSM/SPhT, Unit´e de recherche associ´ee au CNRS, CEA-Saclay F-91191 Gif/Yvette cedex, France Abstract Observational evidence seems to indicate that the expansion of the universe is currently accelerating. Such an acceleration strongly suggests that the content of the universe is dominated by a non{clustered form of matter, the so{called dark energy. The cosmological constant, introduced by Einstein to reconcile General Relativity with a closed and static Universe, is the most likely candidate for dark energy although other options such as a weakly interacting field, also known as quintessence, have been proposed. The fact that the dark energy density is some one hundred and twenty orders of magnitude lower than the energy scales present in the early universe constitutes the cosmological constant problem. We review various aspects of the cosmological constant problem and some interesting scenarios using supersymmetry or extra{dimensions attempting to solve one of the most puzzling issues in physics. [email protected] 1 The Cosmological Constant It is now common knowledge that the Universe is expanding. More puzzling is the recent evidence that this expansion is accelerating. Not only distant galaxies are receding away from us, they do so in an accelerating fashion. The quest for an explanation of the acceleration of the Universe is still in its infancy. Many scenarios have been proposed. No complete understanding has been reached yet. The purpose of this introductory review is to present parts of the observa- tional evidence and some of the theoretical models which have been proposed. More details and a comprehensive list of references can be found in the reviews [1,2,3,4,5]. The figures have been taken from the research articles [6,7,8,9]. The dynamics of the Universe are described by the general theory of rel- ativity. In general relativity, matter and geometry are intrinsically entangled. Space{time is curved by matter as well{known from black{hole physics where the gravitational effects are so strong that even light cannot escape to infinity. The Einstein equations relate on the one hand geometry, in the form of the Ricci tensor Rµν , the metric tensor gµν and the curvature R, and matter represented by the energy{momemtum tensor Tµν 1 R − g R = −8πG T (1) µν 2 µν N µν where GN is Newton's constant of gravitation. The Ricci tensor measures the curvature of space{time while the energy{momentum tensor embodies the ef- fects of pressure and energy. These equations relate tensors, here four by four matrices, leading to highly non{linear equations. All gravitational phenomena are governed by Einstein equations. In particular we will be interested in cosmo- logical features occurring on time{scales of the order of the age of the Universe and on distance scales as large as the size of the Universe. Even on such extraor- dinary scales, general relativity is thought to apply and govern all cosmological phenomena. When trying to analyse the cosmology of galaxies and galaxy clus- ters, it is convenient to model the behaviour of matter with only two quantities: energy density and pressure. This is enough to characterise dust{like matter as well as radiation, two of the key components of our Universe. For ordinary matter and radiation, the energy momentum tensor is diagonal and determined by the energy density ρ measured in units of energy per unit volume and p the pressure. All energy scales will be measured in units where h¯ = c = 1. This implies that length scales can be converted into inverse energy scales, therefore energy densities will be expressed in electron{Volt to the fourth power. In 1917, Einstein applied his equations to cosmology. Lacking the observa- tional evidence on the recession of galaxies (discovered by E. Hubble in 1929), Einstein assumed that the Universe was closed and static. A closed universe is such that space is spherical, i.e. spatially finite with no boundary. A static Universe is such that its shape is time{independent. Unfortunately these two hypotheses are incompatible with Einstein's equations. In order to accommo- date a closed and static Universe, Einstein modified his equations by introducing 2 a new term: the cosmological constant Λ 1 R − g R = −8πG T + Λg (2) µν 2 µν N µν µν The positive cosmological constant Λ introduces a repulsive effect which com- pensates the gravitational pull. A closed and static Universe is a solution of 3 the modified Einstein equations with a radius Λ . The cosmological constant Λ has dimension two in energy unit. It can bqe interpreted as another source of matter with a diagonal energy momentum-tensor (as for dust{like matter and radiation). The pressure due to a cosmological constant is the opposite of the energy density corresponding to a so{called equation of state w = −1. The equation of state in statistical mechanics is the relation between pressure and energy density. In cosmology it is common practise to call the equation of state the proportionality coefficient relating pressure and density. The re- pulsive nature of the cosmological constant is due to a negative pressure. We will encounter other types of matter with the same kind of repulsiveness when discussing scalar fields. After the discovery of the galaxy recession, the necessity of the cosmological constant became doubtful. Indeed Friedmann and then independently Lemaitre showed that the Hubble flow of galaxies could be explained within general rel- ativity without the need for a cosmological constant. Until recently the cosmo- logical constant was deemed unphysical as can be read in the highly influential textbook on the classical theory of fields by Landau and Lifschitz. Since the early eighties, the inflationary paradigm has reinstated the cos- mological constant at the forefront of current research. Indeed the standard cosmological model relying on the homogeneity, i.e. the Copernican principle that no particular place in the Universe plays a special role, and the isotropy of the Universe , i.e. on large scales there is no preferred direction, suffers from many problems, among others the horizon problem. As the velocity of light is finite, how come that regions of space which were not in causal contact in the past look so similar now? In particular how come that the temperature of the cosmic microwave background, the remnant radiation from the very early Universe is so isotropic? This can be explained using inflation, a period of exponentially fast expansion in the early Universe. When inflating, regions in causal contact are blown up so fast as to become acausal. So what is the connection between inflation and the cosmological constant? Inflation can be understood as driven by a constant energy density acting as a cosmological constant. This can be easily grasped by looking at one of the most important equations of cosmology: the Friedmann equation ρ Λ k H2 = 8πG + − (3) N 3 3 a2 a_ Here a is the cosmic scale factor measuring the size of the Universe and H = a is the Hubble rate corresponding to the ratio between the speed of receding galaxies and their distance. In a closed and static Universe the Hubble rate 3 vanishes H = 0, a compensation due to the curvature term k = 1 and the cosmological constant Λ. In a flat Universe k = 0 with no cosmological constant Λ = 0, the Hubble rate is non{vanishing due to the presence of matter ρ. In a flat Universe dominated by a constant cosmological constant Λ the Hubble rate is constant. This is the de Sitter model leading to an exponential increase of the scale factor a. Primordial inflation in the early Universe is thought to occur due to a large energy density mimicking the presence of a cosmological constant. This is enough to create an enormous blowing up of distance scales. The end of inflation is due to the sudden transfer of the inflationary energy density to ordinary particles. The origin of this energy density is often modelled by a scalar field. This idea also reappears in models of quintessence as we will discuss shortly. After inflation the Universe is radiation dominated and then matter domi- nated. In the matter dominated era, structures such as galaxies are formed due to the collapse of matter density fluctuations seeded by quantum fluctuations in the early universe. Well-known physics such as particle and nuclear physics constrain the evolution of the Universe since nucleosynthesis (when elements such as helium were created). In particular, it is observationally clear that a cosmological constant, if present, must be negligible from nucleosynthesis till a0 the very recent past measured in redshift denoted by z and defined by a = 1+z where a0 is the scale factor of the Universe now, i.e. from a red{shift of a few thousand to one.