Graceful Exit Research Articles

Attraction towards an inflection point inflation

Many models of high-energy physics possess metastable vacua. It is conceivable that the Universe can get trapped in such a false vacuum, irrespective of its origin and prior history, at an earlier stage during its evolution. The ensuing false vacuum inflation results in a cold and empty universe and has a generic graceful exit problem. We show that an inflection point inflation along the flat directions of the minimal supersymmetric standard model (MSSM) can resolve this graceful exit problem by inflating the bubble, which nucleates out of a false vacuum. The important point is that the initial condition for an MSSM inflation can be naturally realized, due to an attractor behavior toward the inflection point. We investigate these issues in detail and also present an example where metastable vacua, hence the false vacuum inflation, can happen within the MSSM.

MTSS

Out-of-memory errors are a serious source of unreliability in most embedded systems. Applications run out of main memory because of the frequent difficulty of estimating the memory requirement before deployment, either because it depends on input data, or because certain language features prevent estimation. The typical lack of disks and virtual memory in embedded systems has a serious consequence when an out-of-memory error occurs. Without swap space, the system crashes if its memory footprint exceeds the available memory by even 1 byte. This work improves reliability for multitasking embedded systems by proposing MTSS, a multitask stack sharing technique. If a task attempts to overflow the bounds of its allocated stack space, MTSS grows its stack into the stack memory space allocated for other tasks. This technique can avoid the out-of-memory error if the extra space recovered is sufficient to complete execution. Experiments show that MTSS is able to recover an average of 54% of the stack space allocated to the overflowing task in the free space of other tasks. In addition, unlike conventional systems, MTSS detects memory overflows, allowing the possibility of remedial action or a graceful exit if the recovered space is not enough. Alternatively, MTSS can be used for decreasing the required physical memory of an embedded system by reducing the initial memory allocated to each of the tasks and recovering the deficit by sharing stack with other tasks. The overheads of MTSS are low: the runtime and energy overheads are 3.1% and 3.2%, on average. These are tolerable given that reliability is the most important concern in virtually all systems, ahead of other concerns, such as runtime and energy.

Graceful exit via polymerization of pre-big-bang cosmology

We consider a phenomenological modification of the pre-big-bang scenario using ideas from the resolution of curvature singularities in loop quantum cosmology. We show that nonperturbative loop modifications to the dynamics, arising from the underlying polymer representation, can resolve the graceful exit problem. The curvature and the dilaton energy stay finite at all times, in both the string and Einstein frames. In the string frame, the dilaton tends to a constant value at late times after the bounce.

Lovelock inflation and the number of large dimensions

We discuss an inflationary scenario based on Lovelock terms. These higher order curvature terms can lead to inflation when there are more than three spatial dimensions. Inflation will end if the extra dimensions are stabilised, so that at most three dimensions are free to expand. This relates graceful exit to the number of large dimensions.

Inflationary nonsingular quantum cosmological model

A stiff matter-dominated universe modeled by a free massless scalar field minimally coupled to gravity in a Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) geometry is quantized. Generalized complex-width Gaussian superpositions of the solutions of the Wheeler-DeWitt equation are constructed and the Bohm-de Broglie interpretation of quantum cosmology is applied. A planar dynamical system is found in which a diversity of quantum Bohmian trajectories are obtained and discussed. One class of solutions represents nonsingular inflationary models starting at infinity past from flat space-time with Planckian size spacelike hypersurfaces, which inflates without inflaton but due to a quantum cosmological effect, until it makes an analytical graceful exit from this inflationary epoch to a decelerated classical stiff matter expansion phase.

Graceful exit from a stringy landscape via MSSM inflation

The cosmological evolution of the string landscape is expected to consist of multiple stages of old inflation with a large cosmological constant, each of which ends by tunneling. Old inflation has a well-known graceful exit problem as the observable universe becomes empty, devoid of any entropy. Simultaneously, in the quest for reheating the right degrees of freedom, it is important that the final stage of inflation reheat the standard model sector. It is known that inflation can occur naturally along a flat direction of the minimal supersymmetric standard model (MSSM), solving the reheating problem, but the initial conditions require a large degree of fine-tuning. In this paper, we study how inflation of a MSSM flat direction can be embedded into the string theory landscape to solve both the graceful exit problem of old inflation and the fine-tuning problem of MSSM inflation, elaborating on ideas of Bousso and Polchinski. The fluctuations of the MSSM flat direction during old inflation create regions with initial conditions favorable for eternal inflation, which also allows the cosmological constant to continue to relax. This final phase of inflation also provides all the usual benefits of MSSM inflation, including straightforward reheating into standard model degrees of freedom.

Fine-tuning free paradigm of two-measures theory:k-essence, absence of initial singularity of the curvature, and inflation with graceful exit to the zero cosmological constant state

The dilaton-gravity sector of the two-measures field theory (TMT) is explored in detail in the context of spatially flat Friedman-Robertson-Walker (FRW) cosmology. The model possesses scale invariance which is spontaneously broken due to the intrinsic features of the TMT dynamics. The dilaton $\ensuremath{\phi}$ dependence of the effective Lagrangian appears only as a result of the spontaneous breakdown of the scale invariance. If no fine-tuning is made, the effective $\ensuremath{\phi}$-Lagrangian $p(\ensuremath{\phi},X)$ depends quadratically upon the kinetic term $X$. Hence TMT represents an explicit example of the effective $k$-essence resulting from first principles without any exotic term in the underlying action intended for obtaining this result. Depending of the choice of regions in the parameter space (but without fine-tuning), TMT exhibits different possible outputs for cosmological dynamics: (a) Absence of initial singularity of the curvature while its time derivative is singular. This is a sort of sudden singularities studied by Barrow on purely kinematic grounds. (b) Power law inflation in the subsequent stage of evolution. Depending on the region in the parameter space the inflation ends with a graceful exit either into the state with zero cosmological constant (CC) or into the state driven by both a small CC and the field $\ensuremath{\phi}$ with a quintessencelike potential. (c) Possibility of resolution of the old CC problem. From the point of view of TMT, it becomes clear why the old CC problem cannot be solved (without fine-tuning) in conventional field theories. (d) TMT enables two ways for achieving small CC without fine-tuning of dimensionful parameters: either by a seesaw type mechanism or due to a correspondence principle between TMT and conventional field theories (i.e. theories with only the measure of integration $\sqrt{\ensuremath{-}g}$ in the action). (e) There is a wide range of the parameters such that in the late time universe: the equation of state $w=p/\ensuremath{\rho}<\ensuremath{-}1$; $w$ asymptotically (as $t\ensuremath{\rightarrow}\ensuremath{\infty}$) approaches $\ensuremath{-}1$ from below; $\ensuremath{\rho}$ approaches a constant, the smallness of which does not require fine-tuning of dimensionful parameters.

Predictions of dynamically emerging brane inflation models

We confront the recent proposal of emerging brane inflation with $\mathrm{WMAP}3+\mathrm{SDSS}$, finding a scalar spectral index of ${n}_{s}={0.9659}_{\ensuremath{-}0.0052}^{+0.0049}$ in excellent agreement with observations. The proposal incorporates a preceding phase of isotropic, nonaccelerated expansion in all dimensions, providing suitable initial conditions for inflation. Additional observational constraints on the parameters of the model provide an estimate of the string scale. A graceful exit to inflation and stabilization of extra dimensions is achieved via a string gas. The resulting preheating phase shows some novel features due to a redshifting potential, comparable to effects due to the expansion of the universe itself. However, the model at hand suffers from either a potential overproduction of relics after inflation or insufficient stabilization at late times.

Curvaton reheating mechanism in inflation on warped Dvali–Gabadadze–Porrati brane

Abstract An impressed feature of inflation on warped Dvali–Gabadadze–Porrati (DGP) brane is that the inflationary phase exits spontaneously for a scalar inflaton field with exponential potential, which presents a graceful exit mechanism for the inflation. But its reheating mechanism leaves open. We investigate the curvaton reheating in inflation on warped DGP brane model. The reheating may occur in effectively 5-dimensional or 4-dimensional stage. We study the permitted parameter space of the curvaton field in detail. We demonstrate how the inflation model of the warped DGP brane is improved by the curvaton mechanism.

PHYSICAL CONSEQUENCES OF THE HOLOGRAPHIC PRINCIPLE

The would-be theory of quantum gravity, from which one should deduce the holographic principle, is not available yet. Here it is shown that the classical gravitational interaction is well inside the set of potentials allowed by the holographic principle. The role which such a principle could have in lowering the value of the cosmological constant computed in QFT to the observed one is clarified. It is also argued that, according to the Holographic Principle, the matter-antimatter asymmetry could behave like a seed for the inflationary phase of the universe. This scheme is supported by a good prediction of the number of e-folds. It seems that such a mechanism can only work in the presence of a Hagedorn-like phase transition. The issue of the "graceful exit" can be also naturally accounted for.

“Graceful” old inflation

We show that inflation in a false vacuum becomes viable in the presence of a spectator scalar field nonminimally coupled to gravity. The field is unstable in this background; it grows exponentially and slows down the pure de Sitter phase itself, allowing then fast tunneling to a true vacuum. We compute the constraint from graceful exit through bubble nucleation and the spectrum of cosmological perturbations.

How to make a graceful exit: Lessons learned from 2 cases

How to make a graceful exit: Lessons learned from 2 cases

Interaction between Lipid Monolayers and Poloxamer 188: An X-Ray Reflectivity and Diffraction Study

The mechanism by which poloxamer 188 (P188) seals a damaged cell membrane is examined using the lipid monolayer as a model system. X-ray reflectivity and grazing-incidence x-ray diffraction results show that at low nominal lipid density, P188, by physically occupying the available area and phase separating from the lipids, forces the lipid molecules to pack tightly and restore the barrier function of the membrane. Upon compression to bilayer equivalent pressure, P188 is squeezed out from the lipid monolayer, allowing a graceful exit of P188 when the membrane integrity is restored.

The holographic principle and the early universe

A scenario is proposed in which the matter–antimatter asymmetry behaves like a seed for the inflationary phase of the universe. The mechanism which makes this scenario plausible is the holographic principle: this scheme is supported by a good prediction of the number of e-folds. It seems that such a mechanism can only work in the presence of a Hagedorn-like phase transition. The issue of the “graceful exit” can be also naturally accounted for.

Moduli stabilization and inflation using wrapped branes

We demonstrate that a gas of wrapped branes in the early Universe can help resolve the cosmological Dine-Seiberg/Brustein-Steinhardt overshoot problem in the context of moduli stabilization with steep potentials in string theory. Starting from this mechanism, we propose a cosmological model with a natural setting in the context of an early phase dominated by brane and string gases. The Universe inflates at early times due to the presence of a wrapped two brane (domain wall) gas and all moduli are stabilized. A natural graceful exit from the inflationary regime is achieved. However, the basic model suffers from a generalized domain wall/reheating problem and cannot generate a scale invariant spectrum of fluctuations without additional physics. Several suggestions are presented to address these issues.

B-inflation

We propose a novel model of inflation based on a large class of covariant effective actionscontaining only the second derivatives of a scalar field. The initial conditions leading to theinflationary solutions in this model are rather generic. The graceful exit from theinflationary regime is natural once the first-order derivative terms are included.

Coupled inflation and brane gases

We study an effective four-dimensional theory with an action with two scalar fields minimally coupled to gravity, and with a matter action which couples to the two scalar fields via an overall field-dependent coefficient in the action. Such a theory could arise from a dimensional reduction of supergravity coupled to a gas of branes winding the compactified dimensions. We show the existence of solutions corresponding to power-law inflation. The graceful exit from inflation can be obtained by postulating the decay of the branes, as would occur if the branes are unstable in the vacuum and stabilized at high densities by plasma effects. This construction provides an avenue for connecting string gas cosmology and the late-time universe.

Inflation in string-inspired cosmology and suppression of CMB low multipoles

We construct a string-inspired nonsingular cosmological scenario in which an inflaton field is driven up the potential before the graceful exit by employing a low-energy string effective action with an orbifold compactification. This sets up an initial condition for the inflaton to lead to a sufficient number of e-foldings and to generate a nearly scale-invariant primordial density perturbation during slow-roll inflation. Our scenario provides an interesting possibility to explain a suppressed power spectrum at low multipoles due to the presence of the modulus-driven phase prior to slow-roll inflation and thus can leave a strong imprint of extra dimensions in observed CMB anisotropies.

Dynamical relaxation of the cosmological constant and matter creation in theUniverse

In this paper we discuss the issues of the graceful exit from inflation and of matter creationin the context of a recent scenario in which the back-reaction of long-wavelengthcosmological perturbations induces a negative contribution to the cosmological constantand leads to a dynamical relaxation of the bare cosmological constant. The initially largecosmological constant gives rise to primordial inflation, during which cosmologicalperturbations are stretched beyond the Hubble radius. The cumulative effectof the long-wavelength fluctuations back-reacts on the background geometry ina form which corresponds to the addition of a negative effective cosmologicalconstant to the energy–momentum tensor. In the absence of an effective scalarfield driving inflation, whose decay can reheat the Universe, the challenge is tofind a mechanism which produces matter at the end of the relaxation process. Inthis paper, we point out that the decay of a condensate representing the orderparameter for a ‘flat’ direction in the field theory moduli space can naturallyprovide a matter generation mechanism. The order parameter is displaced from itsvacuum value by thermal or quantum fluctuations, it is frozen until the Hubbleconstant drops to a sufficiently low value, and then begins to oscillate about itsground state. During the period of oscillation it can decay into standard modelparticles similar to how the inflaton decays in scalar-field-driven models of inflation.

Old Inflation in String Theory

We propose a stringy version of the old inflation scenario which does not require any slow-roll inflaton potential and is based on a specific example of string compatification with warped metric. Our set-up admits the presence of anti-D3-branes in the deep infrared region of the metric and a false vacuum state with positive vacuum energy density. The latter is responsible for the accelerated period of inflation. The false vacuum exists only if the number of anti-D3-branes is smaller than a critical number and the graceful exit from inflation is attained if a number of anti-D3-branes travels from the ultraviolet towards the infrared region. The cosmological curvature perturbation is generated through the curvaton mechanism.