* Astronomy

Blobrana · L

RE: Cosmological Constant

Blobrana · L

Title: Geometrical Origin of the Cosmological Constant
Author: Hemza Azri, A. Bounames
Version v5

We show that the description of the space-time of general relativity as a diagonal four dimensional submanifold immersed in an eight dimensional hypercomplex manifold, in torsionless case, leads to a geometrical origin of the cosmological constant. The cosmological constant appears naturally in the new field equations and its expression is given as the norm of a four-vector U, i.e., Lambda=6g_{{\mu}{\nu}}U^{{\mu}}U^{{\nu} and where U can be determined from the Bianchi identities. Consequently, the cosmological constant is space-time dependent, a Lorentz invariant scalar, and may be positive, negative or null. The resulting energy momentum tensor of the dark energy depends on the cosmological constant and its first derivative with respect to the metric. As an application, we obtain the spherical solution for the field equations. In cosmology, the modified Friedmann equations are proposed and a condition on Lambda for an accelerating universe is deduced. For a particular case of the vector U, we find a decaying cosmological constant Lambda \propto a(t)^{-6{\alpha}.

Read more (13kb, PDF)

Blobrana · L

Title: On the smallness of the cosmological constant
Author: C. D. Froggatt, R. Nevzorov, H. B. Nielsen, A. W. Thomas

In N=1 supergravity the scalar potential of the hidden sector may have degenerate supersymmetric (SUSY) and non-supersymmetric Minkowski vacua. In this case local SUSY in the second supersymmetric Minkowski phase can be broken dynamically. Assuming that such a second phase and the phase associated with the physical vacuum are exactly degenerate, we estimate the value of the cosmological constant. We argue that the observed value of the dark energy density can be reproduced if in the second vacuum local SUSY breaking is induced by gaugino condensation at a scale which is just slightly lower than \Lambda_{QCD} in the physical vacuum. The presence of a third degenerate vacuum, in which local SUSY and electroweak (EW) symmetry are broken near the Planck scale, may lead to small values of the quartic Higgs self--coupling and the corresponding beta function at the Planck scale in the phase in which we live.

Read more (1502kb, PDF)

Blobrana · L

Title: Natural Emergence of Cosmological Constant and Dark Radiation from Stephenson-Kilmister-Yang-Camenzind Theory of Gravity
Authors: Pisin Chen, Keisuke Izumi, Nien-En Tung

We show that the Stephenson-Kilmister-Yang (SKY) equation combined with Camenzind's matter current term naturally provides the cosmological constant and dark radiation as integration constants of the SKYC field equation. To characterize the property of the dark radiation, we develop a method to separate it from the ordinary radiation. We found a special property of Camenzind's matter current, namely that the solution space for radiation in fact belongs to that of the vacuum solution of SKY equation. We also found that his matter current does not obey the conservation condition suggested by Kilmister. Finally, we discuss the possible role of dark radiation emergent from the SKYC theory in recent cosmic-microwave-background observations and its implications to the inflation scenario.

Read more (16kb, PDF)

Blobrana · L

Title: Apparent versus true value of the cosmological constant
Authors: Antonio Enea Romano, Pisin chen

Supernovae observations strongly support the presence of a cosmological constant, but its value, which we will call apparent, is normally determined assuming that the universe can be accurately described by a homogeneous model. Even in the presence of a cosmological constant we cannot exclude nevertheless the presence of a small local inhomogeneity which could affect the apparent value of the cosmological constant. Neglecting the presence of the inhomogeneity can in fact introduce a systematic misinterpretation of cosmological data, leading to the distinction between an apparent and the true value of the cosmological constant. But is such a difference distinguishable? Recently we set out to model the local inhomogeneity with a {\Lambda}LTB solution and computed the relation between the apparent and the true value of the cosmological constant. In this essay we reproduce the essence of our model with the emphasis on its physical implications.

Read more (12kb, PDF)

Blobrana · L

Title: A Model of Vanishing Cosmological Constant
Authors: Aiichi Iwazaki

We propose a model in which there exists a real scalar field q satisfying a condition \dot{q} =MH and its energy density is given by (½)\dot{q}^2+V(q), where H is the Hubble parameter (H=\dot{a}/a) and M is a mass scale characterising the field. We show that the potential V(q) of the field is uniquely determined by the condition. The potential depends on the energy densities of background matters. We find that the vacuum energy of the matters is cancelled by the potential of the field. As a result, the minimum of the total energy density of the matters and the field vanishes and is located at the infinite scale factor a=\infty. This remarkable property results without a supersymmetry. We show that the present tiny dark energy is caused by early inflation, while the energy is comparable to Planck scale before the inflation. As our model is reduced to the \Lambda CDM model in the limit M\to 0, it is a natural generalization of the \Lambda CDM model.

Read more (15kb, PDF)

Blobrana · L

Title: A Stringy Mechanism for A Small Cosmological Constant
Authors: Yoske Sumitomo, S.-H. Henry Tye

Based on the probability distributions of products of random variables, we propose a simple stringy mechanism that prefers the meta-stable vacua with a small cosmological constant. We state some relevant properties of the probability distributions of functions of random variables. We then illustrate the mechanism within the flux compactification models in Type IIB string theory. As a result of the stringy dynamics, we argue that the generic probability distribution for the meta-stable vacua typically peaks with a divergent behaviour at the zero value of the cosmological constant. However, its suppression in the single modulus model studied here is modest.

Read more (365kb, PDF)

Blobrana · L

Title: The Cosmological Constant in the Quantum Multiverse
Authors: Grant Larsen, Yasunori Nomura, H.L.L. Roberts

Recently, a new framework for describing the multiverse has been proposed which is based on the principles of quantum mechanics. The framework allows for well-defined predictions, both regarding global properties of the universe and outcomes of particular experiments, according to a single probability formula. This provides complete unification of the eternally inflating multiverse and many worlds in quantum mechanics. In this paper we elucidate how cosmological parameters can be calculated in this framework, and study the probability distribution for the value of the cosmological constant. We consider both positive and negative values, and find that the observed value is consistent with the calculated distribution at an order of magnitude level. In particular, in contrast to the case of earlier measure proposals, our framework prefers a positive cosmological constant over a negative one. These results depend only moderately on how we model galaxy formation and life evolution therein.

Read more (256kb, PDF)