The cosmological constant problem and quintessence

By V. Sahni. 2002

320 citations recorded by the source corpus.

I briefly review the cosmological constant problem and the issue of dark energy (or quintessence). Within the framework of quantum field theory, the vacuum expectation value of the energy momentum tensor formally diverges as k4. A cutoff at the Planck or electroweak scale leads to a cosmological constant which is, respectively, 10123 or 1055 times larger than the observed value, Λ/8πG 10−47 GeV4. The absence of a fundamental symmetry which could set the value of Λ to either zero or a very small value leads to the cosmological constant problem. Most cosmological scenarios favour a large time-dependent Λ-term in the past (in order to generate inflation at z 1010), and a small Λ-term today, to account for the current acceleration of the universe at z 1. Constraints arising from cosmological nucleosynthesis, CMB and structure formation constrain Λ to be sub-dominant during most of the intermediate epoch 1010 < z < 1. This leads to the cosmic coincidence conundrum which suggests that the acceleration of the universe is a recent phenomenon and that we live during a special epoch when the density in Λ and in matter are almost equal. Time varying models of dark energy can, to a certain extent, ameliorate the fine-tuning problem (faced by Λ), but do not resolve the puzzle of cosmic coincidence. I briefly review tracker models of dark energy, as well as more recent brane inspired ideas and the issue of horizons in an accelerating universe. Model independent methods which reconstruct the cosmic equation of state from supernova observations are also assessed. Finally, a new diagnostic of dark energy—statefinder— is discussed.

Subfields: Artificial Intelligence, Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/fc63290b0dbb4453aeb1f2f20717652d15a2ed24

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Citations in the corpus

Generalized Chaplygin gas as a unified scenario of dark matter/energy: Observational constraints (2004, background): “…are a huge number of candidates for DE in the literature, such as a cosmological constantΛ (Carroll et al. 1992; Krauss and Turner 1995; Zhu 1998; Sahni 2002; Padmanabhan 2003), the so-called “X- Send offprint requests to: Zong-Hong Zhu matter” (Turner and White 1997; Zhu, Fujimoto and Tatsumi…”
Is there supernova evidence for dark energy metamorphosis (2003, background): “An interesting example of an evolving DE model in which w0 < −1 at present whereas w(z) > −1 at earlier times is provided by the Braneworld models (called BRANE1) examined in Sahni & Shtanov (2003) which have been shown to agree very well with current SN observations (Alam & Sahni 2002).”
Exploring the expanding Universe and dark energy using the statefinder diagnostic (2003, background): “These models do not have horizons and therefore help to reconcile an accelerating universe with the demands of the string/M-theory (Sahni 2002) (as well as any theory which requires dark energy to decay in the future and transform into matter with w > −1/3).”