scholarly journals Out of the swampland with multifield quintessence?

2020 ◽  
Vol 2020 (10) ◽  
Author(s):  
Michele Cicoli ◽  
Giuseppe Dibitetto ◽  
Francisco G. Pedro
Keyword(s):  
Initial Conditions ◽  
Functional Dependence ◽  
Scalar Potential ◽  
Scalar Fields ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Kinetic Coupling ◽  
Current State ◽  
The Stability ◽  
Promising Avenue

Abstract Multifield models with a curved field space have already been shown to be able to provide viable quintessence models for steep potentials that satisfy swampland bounds. The simplest dynamical systems of this type are obtained by coupling Einstein gravity to two scalar fields with a curved field space. In this paper we study the stability properties of the non-trivial fixed points of this dynamical system for a general functional dependence of the kinetic coupling function and the scalar potential. We find the existence of non-geodesic trajectories with a sharp turning rate in field space which can give rise to late-time cosmic acceleration with no need for flat potentials. In particular, we discuss the properties of the phase diagram of the system and the corresponding time evolution when varying the functional dependence of the kinetic coupling. Interestingly, upon properly tuning the initial conditions of the field values, we find trajectories that can describe the current state of the universe. This could represent a promising avenue to build viable quintessence models out of the swampland if they could be consistently embedded in explicit string constructions.

scholarly journals Late-time cosmic acceleration: ABCD of dark energy and modified theories of gravity

2016 ◽  
Vol 25 (12) ◽  
pp. 1630031 ◽  
Author(s):  
M. Sami ◽  
R. Myrzakulov
Keyword(s):  
Dark Energy ◽  
Scalar Fields ◽  
Massive Gravity ◽  
Cosmic Acceleration ◽  
Modified Theories Of Gravity ◽  
Late Time ◽  
Cosmological Constant Problem ◽  
Chameleon Mechanism ◽  
Theories Of Gravity ◽  
Basic Ideas

We briefly review the problems and prospects of the standard lore of dark energy. We have shown that scalar fields, in principle, cannot address the cosmological constant problem. Indeed, a fundamental scalar field is faced with a similar problem dubbed naturalness. In order to keep the discussion pedagogical, aimed at a wider audience, we have avoided technical complications in several places and resorted to heuristic arguments based on physical perceptions. We presented underlying ideas of modified theories based upon chameleon mechanism and Vainshtein screening. We have given a lucid illustration of recently investigated ghost-free nonlinear massive gravity. Again, we have sacrificed rigor and confined to the basic ideas that led to the formulation of the theory. The review ends with a brief discussion on the difficulties of the theory applied to cosmology.

scholarly journals UNIQUENESS OF BUBBLE-FREE SOLUTION IN LINEAR RATIONAL EXPECTATIONS MODELS

2003 ◽  
Vol 7 (2) ◽  
pp. 171-191 ◽  
Author(s):  
Gabriel Desgranges ◽  
Stéphane Gauthier
Keyword(s):  
Rational Expectations ◽  
Initial Conditions ◽  
Free Solution ◽  
State Variables ◽  
Minimal Set ◽  
Current State ◽  
Stable Path ◽  
Market Fundamentals ◽  
The Stability ◽  
Natural Test

One usually identifies bubble solutions to linear rational expectations models by extra components (irrelevant lags) arising in addition to market fundamentals. Although there are still many solutions relying on a minimal set of state variables, i.e., relating in equilibrium the current state of the economic system to as many lags as initial conditions, there is a conventional wisdom that the bubble-free (fundamentals) solution should be unique. This paper examines the existence of endogenous stochastic sunspot fluctuations close to solutions relying on a minimal set of state variables, which provides a natural test for identifying bubble and bubble-free solutions. It turns out that only one solution is locally immune to sunspots, independently of the stability properties of the perfect-foresight dynamics. In the standard saddle-point configuration for these dynamics, this solution corresponds to the so-called saddle stable path.

scholarly journals Quintessential inflation for exponential type potentials: scaling and tracker behavior

2021 ◽  
Vol 81 (2) ◽  
Author(s):  
Llibert Aresté Saló ◽  
Jaume Haro
Keyword(s):  
Exponential Type ◽  
Initial Conditions ◽  
Basin Of Attraction ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Accelerated Evolution ◽  
Scaling Solution ◽  
The One ◽  
Two Parameters ◽  
The Universe

AbstractWe will show that for exponential type potentials of the form $$V(\varphi )\sim e^{-\gamma \varphi ^n/M_{pl}^n}$$ V ( φ ) ∼ e - γ φ n / M pl n , which are used to depict quintessential inflation, the solutions whose initial conditions take place during the slow roll phase in order to describe correctly the inflationary period, do not belong for large values of the parameter n to the basin of attraction of the scaling solution – a solution of the scalar field equation whose energy density scale as the one of the fluid component of the universe during radiation or the matter domination period –, meaning that a late time mechanism to exit this behavior and depict correctly the current cosmic acceleration is not needed. However, in such cases, namely n large enough, these potentials cannot correctly depict the current cosmic acceleration. This is the reason why the potential must be improved introducing another parameter -as the one in the well-known Peebles–Vilenkin quintessential inflation model, which depends on two parameters, one to describe inflation and the other one to correctly depict the present accelerated evolution – able to deal with the late time acceleration of our universe.

scholarly journals Dynamical attractors in contracting spacetimes dominated by kinetically coupled scalar fields

2021 ◽  
Vol 2021 (12) ◽  
pp. 030
Author(s):  
Anna Ijjas ◽  
Frans Pretorius ◽  
Paul J. Steinhardt ◽  
David Garfinkle
Keyword(s):  
Scalar Field ◽  
Initial Conditions ◽  
Scalar Fields ◽  
Density Fluctuations ◽  
Microwave Background ◽  
Scale Invariant ◽  
Linear Evolution ◽  
Kinetic Coupling ◽  
Wide Range ◽  
Non Linear

Abstract We present non-perturbative numerical relativity simulations of slowly contracting spacetimes in which the scalar field driving slow contraction is coupled to a second scalar field through an exponential non-linear σ model-type kinetic interaction. These models are important because they can generate a nearly scale-invariant spectrum of super-Hubble density fluctuations fully consistent with cosmic microwave background observations. We show that the non-linear evolution rapidly approaches a homogeneous, isotropic and flat Friedmann-Robertson-Walker (FRW) geometry for a wide range of inhomogeneous and anisotropic initial conditions. Ultimately, we find, the kinetic coupling causes the evolution to deflect away from flat FRW and towards a novel Kasner-like stationary point, but in general this occurs on time scales that are too long to be observationally relevant.

Coexistence of early-time inflation and late-time acceleration with scalar fields in theory of F(G,T) gravity

2018 ◽  
Vol 27 (08) ◽  
pp. 1850078 ◽  
Author(s):  
A. I. Keskin
Keyword(s):  
Scalar Field ◽  
Source Term ◽  
Friedmann Equation ◽  
Early Time ◽  
Scalar Fields ◽  
Momentum Tensor ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Late Time Acceleration ◽  
The Universe

In this study, we discuss coexistence of the early-time inflation and the late-time acceleration of the universe in the context of the theory of [Formula: see text] gravity with scalar field which is minimally coupled with the gravity, where [Formula: see text] is the gauss bonnet invariant and [Formula: see text] is the trace of energy–momentum tensor (EMT). We reconstruct the Friedmann equation (FE) and then search for the real value of a particular model [Formula: see text], where [Formula: see text] and [Formula: see text] are real constants. A Gauss–Bonnet system (GBS) for viable cosmologies arising from the matter-source term [Formula: see text] and the scalar field, is obtained. We find that the case [Formula: see text] together with [Formula: see text] in the system gives the late-time cosmic acceleration while the source term [Formula: see text] acts as a quintessence type of dark energy. On the other hand, the general entropy expression of the universe is obtained by making use of the first law of thermodynamics (FLT) method. After theoretically analyzing the inflation in the entropy frame, we find a new condition [Formula: see text] with [Formula: see text] in the system. Then, from the observational analysis of inflation, the spectral index parameter and the scalar-tensor ratio are calculated under the new condition. In brief, we obtain a viable cosmological system arising from some modifications such as the scalar field and the source term, which can unify the early inflation and the late-time cosmic acceleration besides the deceleration regions of the universe.

scholarly journals Unified Inflation and Late-Time Accelerated Expansion with Exponential and R2 Corrections in Modified Gravity

Symmetry ◽  
2020 ◽  
Vol 12 (5) ◽  
pp. 794
Author(s):  
Luis Granda
Keyword(s):  
Modified Gravity ◽  
Initial Conditions ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
Late Time ◽  
Time Behavior ◽  
De Sitter ◽  
Cosmological Observations ◽  
Leading Term ◽  
The Stability

Modified gravity models with and exponential function of curvature and R 2 corrections are proposed. At low curvature, the model explains the matter epoch and the late time accelerated expansion while at the inflation epoch the leading term is R 2 . At R → 0 the cosmological constant disappears, giving unified description of inflation and dark energy in pure geometrical context. The models satisfy the stability conditions, pass local tests and are viable in the ( r , m ) -plane, where the trajectories connect the saddle matter dominated critical point ( r = − 1 , m = 0 ) with the late time de Sitter attractor at r = − 2 and 0 < m ≤ 1 . Initial conditions were found, showing that the density parameters evolve in a way consistent with current cosmological observations, predicting late time behavior very close to the Λ CDM with future universe evolving towards the de Sitter attractor.

scholarly journals DARK ENERGY FROM SCALAR FIELD WITH GAUSS–BONNET AND NON-MINIMAL KINETIC COUPLING

2012 ◽  
Vol 27 (04) ◽  
pp. 1250018 ◽  
Author(s):  
L. N. GRANDA
Keyword(s):  
Dark Energy ◽  
Scalar Field ◽  
Power Law ◽  
Dynamical Equation ◽  
Early Time ◽  
Scalar Fields ◽  
Late Time ◽  
Kinetic Coupling ◽  
Energy Behavior ◽  
Phantom Phase

We consider a model of scalar field with non-minimal kinetic couplings to the curvature, and additional coupling to the Gauss–Bonnet four-dimensional invariant. The model presents rich cosmological dynamics and some of its solutions are analyzed. A variety of scalar fields and potentials giving rise to power-law expansion have been found. Two solutions with dynamical equation of state are considered. The first solution unifies early time power-law behavior with late time cosmological constant dominance. The second solution is able to describe a universe in the phantom phase, and depending on the parameters may describe essentially dark energy behavior, or may contain the decelerated and accelerated phases.

scholarly journals SCALAR NONLUMINOUS MATTER IN GALAXIES

2002 ◽  
Vol 17 (04) ◽  
pp. 555-560 ◽  
Author(s):  
BYUNG JOO LEE ◽  
TAE HOON LEE
Keyword(s):  
Rotational Velocity ◽  
Scalar Potential ◽  
Scalar Fields ◽  
Global Symmetry ◽  
Late Time ◽  
Complex Scalar ◽  
Acceleration Of The Universe ◽  
The Universe ◽  
Static Space ◽  
Quintessence Model

As a candidate for dark matter in galaxies, we study an SU(3) triplet of complex scalar fields which are nonminimally coupled to gravity. In the spherically symmetric static space–time where the flat rotational velocity curves of stars in galaxies can be explained, we find simple solutions of scalar fields with SU(3) global symmetry broken to U(1) × U(1), in an exponential scalar potential, which will be useful in a quintessence model of the late-time acceleration of the universe.

scholarly journals Chaplygin-gas solutions of f(R) gravity

2016 ◽  
Vol 13 (10) ◽  
pp. 1650120 ◽  
Author(s):  
Maye Elmardi ◽  
Amare Abebe ◽  
Abiy Tekola
Keyword(s):  
Dark Energy ◽  
Scalar Field ◽  
Early Universe ◽  
Equations Of State ◽  
Initial Conditions ◽  
Exact Formula ◽  
Chaplygin Gas ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Modified Chaplygin Gas

We explore exact [Formula: see text] gravity solutions that mimic Chaplygin-gas inspired [Formula: see text]CDM cosmology. Starting with the original, generalized and modified Chaplygin-gas (MCG) equations of state (EoS), we reconstruct the forms of [Formula: see text] Lagrangians. The resulting solutions are generally quadratic in the Ricci scalar, but have appropriate [Formula: see text]CDM solutions in limiting cases. These solutions, given appropriate initial conditions, can be potential candidates for scalar field-driven early universe expansion (inflation) and dark energy-driven late-time cosmic acceleration.

Tsallis holographic dark energy in Bianchi type-III spacetime with scalar fields

2019 ◽  
Vol 34 (37) ◽  
pp. 1950310 ◽  
Author(s):  
Murat Korunur
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Bianchi Type ◽  
Holographic Dark Energy ◽  
Scalar Fields ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Acceleration Phase ◽  
Eos Parameter ◽  
Type Iii

In this paper, we study one of the new dark energy models named Tsallis holographic dark energy (THDE) model considering the Bianchi type-III spacetime model. Considering deceleration parameter, transition from deceleration to acceleration phase happens at [Formula: see text]. The equation of state (EoS) parameter has been found using the Granda–Oliveros (GO) scale. It is found that for [Formula: see text] values, EoS parameter behaves like the quintessence era; for [Formula: see text], EoS parameter behaves like the phantom dark energy and approaches [Formula: see text]CDM model at late-time cosmic acceleration phase. Also, we reconstructed a correspondence between THDE model and some well-known scalar fields, such as tachyon, quintessence and [Formula: see text]-essence. In addition, we evaluated equation of state parameter, kinetic energy and scalar potential versus time.

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