scholarly journals INFLATION FROM IIB SUPERSTRINGS WITH FLUXES

2009 ◽  
Vol 24 (13) ◽  
pp. 2431-2452
Author(s):  
ERANDY RAMÍREZ ◽  
TONATIUH MATOS ◽  
ISRAEL QUIROS
Keyword(s):  
Friedmann Equation ◽  
Field Equations ◽  
Superstring Theory ◽  
Early Epoch ◽  
Cosmological Context ◽  
Hybrid Inflation ◽  
Inflation Scenario

We study the conditions needed to have an early epoch of inflationary expansion with a potential coming from type IIB superstring theory with fluxes involving two modulus fields. The phenomenology of this potential is different from the usual hybrid inflation scenario and we analyze the possibility that the system of field equations undergoes a period of inflation in three different regimes with the dynamics modified by a Randall–Sundrum term in the Friedmann equation. We find that the system can produce inflation and, due to the modification of the dynamics, a period of accelerated contraction can follow or precede this inflationary stage, depending on the sign of one of the parameters of the potential. We discuss the viability of this model in a cosmological context.

Braneworld mimetic f(R) gravity

2019 ◽  
Vol 16 (03) ◽  
pp. 1950042 ◽  
Author(s):  
Kourosh Nozari ◽  
Naser Sadeghnezhad
Keyword(s):  
Scalar Field ◽  
Recent Work ◽  
Friedmann Equation ◽  
Equation Of Motion ◽  
Einstein’S Field Equations ◽  
Field Equations ◽  
Einstein's Field Equations ◽  
Bulk Scalar Field ◽  
Codazzi Equations ◽  
Mimetic Gravity

Following our recent work on braneworld mimetic gravity, in this paper, we study an extension of braneworld mimetic gravity to the case that the gravitational sector on the brane is modified in the spirit of [Formula: see text] theories. We assume the physical 5D bulk metric in the Randall–Sundrum II braneworld scenario consists of a 5D scalar field (which mimics the dark sectors on the brane) and an auxiliary 5D metric. We find the 5D Einstein’s field equations and the 5D equation of motion of the bulk scalar field in this setup. By using the Gauss–Codazzi equations, we obtain the induced Einstein’s field equations on the brane. Finally, by adopting the FRW background, we find the Friedmann equation on the brane in this [Formula: see text] mimetic braneworld setup.

HOW DID THE UNIVERSE BEGIN?

1998 ◽  
Vol 07 (05) ◽  
pp. 727-735 ◽  
Author(s):  
M. D. POLLOCK
Keyword(s):  
Time Interval ◽  
Field Equations ◽  
Superstring Theory ◽  
Higher Derivative ◽  
Planck Time ◽  
Contracting Phase ◽  
Three Space ◽  
Previous Phase ◽  
The Universe ◽  
Radius Function

The question of the initial configuration of the Universe — did the expanding Friedmann space-time ds2 = dt2 - a2(t)dx2 tend to a singularity when extrapolated back in time, or was there a turning point, indicating a previous phase of contraction? — is re-examined in the context of the heterotic superstring theory of Gross et al. If the adiabatic index tends to the value γ = 1, then the higher-derivative terms ℛ2 in the Lagrangian L dominate the Einstein–Hilbert term R/16πG in the time interval t p ≲ t ≲ 4t p , during which the action is S ≈ 25ℏ, guaranteeing the approximate validity of the classical field equations (if the compactification process is ignored), where [Formula: see text] is the Newton gravitational constant and t p is the Planck time. Under these conditions, Ruzmaĭkina and Ruzmaĭkina have shown, for a flat three-space with K = 0, that the initial singularity can only be avoided at all if there is a spin-zero tachyon, a conclusion modified by Barrow and Ottewill if K = ±1. We have previously shown, however, that the theory is tachyon-free, and have argued that K has to vanish for the existence of a well-defined, quantum-mechanical ground state. Also, if there is no inflation, the radius function is always much too large for the terms in K to exert any effect, a(t) ≳ 5 × 1029t p . While if γ = 2, then ℛ2 never dominates R/16πG. Accordingly, we conjecture that the Universe did not bounce, irrespective of the value of γ, the absence of a prior contracting phase thus being an aspect of causality.

scholarly journals Rastall gravity extension of the standard $$\Lambda $$CDM model: theoretical features and observational constraints

2020 ◽  
Vol 80 (11) ◽  
Author(s):  
Özgür Akarsu ◽  
Nihan Katırcı ◽  
Suresh Kumar ◽  
Rafael C. Nunes ◽  
Burcu Öztürk ◽  
...  
Keyword(s):  
General Relativity ◽  
Cold Dark Matter ◽  
Friedmann Equation ◽  
Momentum Conservation ◽  
Density Parameter ◽  
Acoustic Oscillations ◽  
Field Equations ◽  
Accurate Analysis ◽  
Material Sources ◽  
Effective Source

AbstractWe present a detailed investigation of the Rastall gravity extension of the standard $$\Lambda $$ Λ CDM model. We review the model for two simultaneous modifications of different nature in the Friedmann equation due to the Rastall gravity: the new contributions of the material (actual) sources (considered as effective source) and the altered evolution of the material sources. We discuss the role/behavior of these modifications with regard to some low redshift tensions, including the so-called $$H_0$$ H 0 tension, prevailing within the standard $$\Lambda $$ Λ CDM. We constrain the model at the level of linear perturbations, and obtain the first constraints through a robust and accurate analysis using the latest full Planck cosmic microwave background (CMB) data, with and without including baryon acoustic oscillations (BAO) data. We find that the Rastall parameter $$\epsilon $$ ϵ (null for general relativity) is consistent with zero at 68% CL (with a tendency towards positive values, $$-0.0001< \epsilon < 0.0007$$ - 0.0001 < ϵ < 0.0007 (CMB+BAO) at 68% CL), which in turn implies no significant statistical evidence for deviation from general relativity, and also a precision of $$\mathcal {O}(10^{-4})$$ O ( 10 - 4 ) for the coefficient $$-1/2$$ - 1 / 2 of the term $$g_{\mu \nu }R$$ g μ ν R in the Einstein field equations of general relativity (guaranteeing the local energy-momentum conservation). We explore the consequences led by the Rastall gravity on the cosmological parameters in the light of the observational analyses. It turns out that the effective source, with a present-day density parameter $$\Omega _\mathrm{X0}=-0.0010\pm 0.0013$$ Ω X 0 = - 0.0010 ± 0.0013 (CMB+BAO, 68% CL), dynamically screens the usual vacuum energy at high redshifts, but this mechanism barely works due to the opposition by the altered evolution of cold dark matter. Consequently, two simultaneous modifications of different nature in the Friedmann equation by the Rastall gravity act against each other, and do not help to considerably relax the low redshift tensions, including the so-called $$H_0$$ H 0 tension. Our results may offer a guide for the research community that studies the Rastall gravity in various aspects of gravitation and cosmology.

scholarly journals A generalization of unimodular gravity with vacuum-matter energy exchange

2017 ◽  
Vol 26 (02) ◽  
pp. 1750006 ◽  
Author(s):  
F. R. Klinkhamer
Keyword(s):  
Energy Exchange ◽  
Minkowski Spacetime ◽  
Momentum Conservation ◽  
Mass Scale ◽  
Effective Theory ◽  
Material System ◽  
Field Equations ◽  
Cosmological Context ◽  
The Impact ◽  
Cold Matter

An effective theory of gravity in the infrared is proposed, which involves the determinant of the metric relative to the determinant of a prior metric taken to be that of Minkowski spacetime. This effective theory can be interpreted as a generalization of unimodular gravity. In a cosmological context with ultrarelativistic or cold matter, the resulting field equations have only one solution, empty Minkowski spacetime (selected by the prior metric of the theory). The introduction of energy exchange between vacuum and matter gives rise to nonstatic cosmic solutions. It is found that Minkowski spacetime (from the prior metric) appears as an attractor of the dynamic equations. A further result is that energy–momentum conservation of any localized material system is violated in a nonconstant gravitational background. The impact for experiment appears, however, negligible if the vacuum-energy mass-scale is of order meV.

scholarly journals Brane Gases on K3 and Calabi–Yau Manifolds

2003 ◽  
Vol 18 (23) ◽  
pp. 4295-4314 ◽  
Author(s):  
Damien A. Easson
Keyword(s):  
String Theory ◽  
Particle Physics ◽  
Thermal Equilibrium ◽  
Initial Singularity ◽  
Superstring Theory ◽  
Initial State ◽  
Four Dimensions ◽  
Cosmological Context ◽  
Spatial Dimensions ◽  
The Universe

We initiate the study of Brane Gas Cosmology (BGC) on manifolds with nontrivial holonomy. Such compactifications are required within the context of superstring theory in order to make connections with realistic particle physics. We study the dynamics of brane gases constructed from various string theories on background spaces having a K3 submanifold. The K3 compactifications provide a stepping stone for generalizing the model to the case of a full Calabi–Yau threefold. Duality symmetries are discussed within a cosmological context. Using a duality, we arrive at an N=2 theory in four dimensions compactified on a Calabi–Yau manifold with SU(3) holonomy. We argue that the Brane Gas model compactified on such spaces maintains the successes of the trivial toroidal compactification while greatly enhancing its connection to particle physics. The initial state of the universe is taken to be a small, hot and dense gas of p-branes near thermal equilibrium. The universe has no initial singularity and the dynamics of string winding modes allow three spatial dimensions to grow large, providing a possible solution to the dimensionality problem of string theory.

A cyclic non-singular universe from Gauss–Bonnet and superstring corrections

2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Rami Ahmad El-Nabulsi
Keyword(s):  
Scalar Field ◽  
Time Derivative ◽  
Low Energy ◽  
Exponential Potential ◽  
Field Equations ◽  
Superstring Theory ◽  
Higher Dimensional ◽  
Free Parameters ◽  
Higher Order Corrections ◽  
The Universe

Abstract In this study, we have constructed a viable cosmological model characterized by the presence of the Gauss–Bonnet four-dimensional invariant, higher-order corrections to the low energy effective action motivated from heterotic superstring theory and a general exponential potential comparable to those obtained in higher dimensional supergravities. The field equations were studied by assuming a particular relation between the Hubble parameter and the time derivative of the scalar field. It was observed that, for specific relations between the free parameters in the theory, the universe is cyclic, expands and contracts alternately without singularity with an equation of state oscillating around −1. The model is found to fit the recent astrophysical data.

scholarly journals ANOMALY INDUCED EFFECTIVE ACTION COMBINED WITH GENERALIZED BRANS–DICKE THEORY

2006 ◽  
Vol 15 (11) ◽  
pp. 1969-1975
Author(s):  
IVER BREVIK
Keyword(s):  
Scalar Field ◽  
Effective Action ◽  
Energy Exchange ◽  
Friedmann Equation ◽  
Conformal Anomaly ◽  
Field Equations ◽  
Yang Mills ◽  
Classical Gravity ◽  
Scalar Field Equations ◽  
Matter Fields

We consider the anomaly induced effective action in [Formula: see text] super Yang–Mills theory in interaction with the Brans–Dicke (BD) field. The generalization of the BD theory so as to permit an energy exchange between the scalar field and ordinary matter fields, was recently worked out by T. Clifton and J. D. Barrow [Phys. Rev. D73, 104022 (2006)]. We derive the scalar field equations for the dilaton field, and the BD field, and discuss the Friedmann equation in the general case. The present paper is a continuation of an investigation few years ago dealing with the case of conformal anomaly plus ordinary classical gravity [I. Brevik and S. D. Odintsov, Phys. Lett. B455, 104 (1999)].

scholarly journals WARM INFLATION MODEL BUILDING

2009 ◽  
Vol 24 (12) ◽  
pp. 2207-2240 ◽  
Author(s):  
MAR BASTERO-GIL ◽  
ARJUN BERERA
Keyword(s):  
First Principles ◽  
Model Building ◽  
First Principles Calculations ◽  
Small Component ◽  
Warm Inflation ◽  
Dissipative Dynamics ◽  
Cosmological Observations ◽  
Friction Term ◽  
Hybrid Inflation ◽  
Inflation Scenario

We review the main aspects of the warm inflation scenario, focusing on the inflationary dynamics and the predictions related to the primordial spectrum of perturbations, to be compared with the recent cosmological observations. We study in detail three different classes of inflationary models, chaotic, hybrid models and hilltop models, and discuss their embedding into supersymmetric models and the consequences for model building of the warm inflationary dynamics based on first principles calculations. Due to the extra friction term introduced in the inflaton background evolution generated by the dissipative dynamics, inflation can take place generically for smaller values of the field, and larger values of couplings and masses. When the dissipative dynamics dominates over the expansion, in the so-called strong dissipative regime, inflation proceeds with sub-Planckian inflaton values. Models can be naturally embedded into a supergravity framework, with SUGRA corrections suppressed by the Planck mass now under control, for a larger class of Kähler potentials. In particular, this provides a simpler solution to the "eta" problem in supersymmetric hybrid inflation, without restricting the Kähler potentials compatible with inflation. For chaotic models dissipation leads to a smaller prediction for the tensor-to-scalar ratio and a less tilted spectrum when compared to the cold inflation scenario. We find in particular that a small component of dissipation renders the quartic model now consistent with the current CMB data.

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