85 research outputs found
Power Suppression at Large Scales in String Inflation
We study a possible origin of the anomalous suppression of the power spectrum
at large angular scales in the cosmic microwave background within the framework
of explicit string inflationary models where inflation is driven by a closed
string modulus parameterizing the size of the extra dimensions. In this class
of models the apparent power loss at large scales is caused by the background
dynamics which involves a sharp transition from a fast-roll power law phase to
a period of Starobinsky-like slow-roll inflation. An interesting feature of
this class of string inflationary models is that the number of e-foldings of
inflation is inversely proportional to the string coupling to a positive power.
Therefore once the string coupling is tuned to small values in order to trust
string perturbation theory, enough e-foldings of inflation are automatically
obtained without the need of extra tuning. Moreover, in the less tuned cases
the sharp transition responsible for the power loss takes place just before the
last 50-60 e-foldings of inflation. We illustrate these general claims in the
case of Fibre Inflation where we study the strength of this transition in terms
of the attractor dynamics, finding that it induces a pivot from a blue to a
redshifted power spectrum which can explain the apparent large scale power
loss. We compute the effects of this pivot for example cases and demonstrate
how magnitude and duration of this effect depend on model parameters.Comment: 30 pages, 6 figure
Attractors, Universality and Inflation
Studies of the initial conditions for inflation have conflicting predictions
from exponential suppression to inevitability. At the level of phase space,
this conflict arises from the competing intuitions of CPT invariance and
thermodynamics. After reviewing this conflict, we enlarge the ensemble beyond
phase space to include scalar potential data. We show how this leads to an
important contribution from inflection point inflation, enhancing the
likelihood of inflation to an inverse cubic power law. In the process, we
emphasize the attractor dynamics of the gravity-scalar system and the existence
of universality classes from inflection point inflation. Finally, we comment on
the predictivity of inflation in light of these results.Comment: 32 pages, 8 figures. Typos and figures fixe
Just enough inflation: power spectrum modifications at large scales
We show that models of `just enough' inflation, where the slow-roll evolution
lasted only e-foldings, feature modifications of the CMB power spectrum
at large angular scales. We perform a systematic and model-independent analysis
of any possible non-slow-roll background evolution prior to the final stage of
slow-roll inflation. We find a high degree of universality since most common
backgrounds like fast-roll evolution, matter or radiation-dominance give rise
to a power loss at large angular scales and a peak together with an oscillatory
behaviour at scales around the value of the Hubble parameter at the beginning
of slow-roll inflation. Depending on the value of the equation of state
parameter, different pre-inflationary epochs lead instead to an enhancement of
power at low-, and so seem disfavoured by recent observational hints for
a lack of CMB power at . We also comment on the importance of
initial conditions and the possibility to have multiple pre-inflationary
stages.Comment: 31 pages, 13 figure
Hints of Universality from Inflection Point Inflation
This work aims to understand how cosmic inflation embeds into larger models of particle physics and string theory. Our work operates within a weakened version of the Landscape paradigm, wherein it is assumed that the set of possible Lagrangians is vast enough to admit the notion of a generic model. By focusing on slow-roll inflation, we examine the roles of both the scalar potential and the space of couplings which determine its precise form. In particular, we focus on the structural properties of the scalar potential, and find a surprising result: inflection point inflation emerges as an important —and under certain assumptions, dominant — possibility in the context of generic scalar potentials.
We begin by a systematic coarse graining over the set of possible inflection point inflation models using V.I. Arnold’s ADE classification of singularities. Similar to du Val’s pioneering work on surface singularities, these determine structural classes for inflection point inflation which depened on a distinct number of control parameters. We consider both single and multifield inflation, and show how the various structural classes embed within each other. We also show how such control parameters influence the larger physical models in to which inflation is embedded. These techniques are then applied to both MSSM inflation and KKLT-type models of string cosmology.
In the former case, we find that the scale of inflation can be entirely encoded within the super- potential of supersymmetric quantum field theories. We show how this relieves the fine-tuning required in such models by upwards of twelve orders of magnitude. Moreover, unnatural tuning between SUSY breaking and SUSY preserving sectors is eliminated without the explicit need for any hidden sector dynamics. In the later case, we discuss how structural stability vastly generalizes — and addresses — the Kallosh-Linde problem. Implications for the spectrum of SUSY breaking soft terms are then discussed, with an emphasis on how they may assist in constraining the inflationary scalar potential.
We then pivot to a general discussion of the FLRW-scalar phase space, and show how inflection points induce caustics — or dynamical fixed points — amongst the space of possible trajectories. These fixed points are then used to argue that for uninformative priors on the space of couplings, the likelihood of inflection point inflation scales with the inverse cube of the number of e-foldings. We point out the geometric origin for the known ambiguity in the Liouville measure, and demonstrate of inflection point inflation ameliorates this problem.
Finally we investigate the effect of the fixed point structure on the spectrum of density perturbations. We show how an anomaly in the Cosmic Mircowave Background data — low power at large scales — can be explained as a by product of the fixed point dynamics
Constructing Flat Inflationary Potentials in Supersymmetry
We show that in supersymmetry one can obtain inflationary potentials in the
observable sector that are sufficiently flat at sub-Planckian field values.
Structure of the supersymmetric scalar potential along a flat direction
combined with the existence of higher order terms in an effective field theory
expansion allows one to find scales below the effective field theory cut off
where two or a higher number of the potential derivatives may vanish. As an
explicit example, we demonstrate that inflection point inflation within a broad
range of scales O(TeV) << H < 3 x 10^9 GeV can be accommodated within weak
scale supersymmetry. The fine tuning of model parameters needed for successful
inflation is considerably improved in this scenario.Comment: 4 pages, 1 figur
A reliability analysis method using binary decision diagrams in phased mission planning
The use of autonomous systems is becoming increasingly common in many fields. A significant example of this is the ambition to deploy UAVs (unmanned aerial vehicles) for both civil and military applications. In order for autonomous systems such as these to operate effectively they must be capable of making decisions regarding the appropriate future course of their mission responding to changes in circumstance in as short a time as possible. The systems will typically perform phased missions and, due to the uncertain nature of the environments in which the systems operate, the mission objectives may be subject to change at short notice. The ability to evaluate the different possible mission configurations is crucial in making the right decision about the mission tasks that should be performed in order to give the highest possible probability of mission success.
Since Binary Decision Diagrams (BDD) may be quickly and accurately quantified to give measures of the system reliability it is anticipated that they are the most appropriate analysis tools to form the basis of a reliability-based prognostics methodology. This paper presents a new Binary Decision Diagram based approach for phased mission analysis, which seeks to take advantage of the proven fast analysis characteristics of the BDD and enhance it in ways which are suited to the demands of a decision making capability for autonomous systems. The BDD approach presented allows BDDs representing the failure causes in the different phases of a mission to be constructed quickly by treating component failures in different phases of the mission as separate variables. This allows flexibility when building mission phase failure BDDs since a global variable ordering scheme is not required. An alternative representation of component states in time intervals allows the dependencies to be efficiently dealt with during the quantification process. Nodes in the BDD can represent components with any number of failure modes or factors external to the system that could affect its behaviour, such as the weather. Path simplification rules and quantification rules are developed that allow the calculation of phase failure probabilities for this new BDD approach.
The proposed method provides a phased mission analysis technique that allows the rapid construction of reliability models for phased missions and, with the use of BDDs, rapid quantification
Catastrophic inflation
We study inflection point inflation using Singularity Theory, which relates
degenerate critical points of functions to their local behavior. This approach
illuminates universal features of small-field models and gives analytic control
over parametrized families of scalar potentials near inflationary solutions.
The behavior of the scalar potential is tied to the number of physical input
parameters, which determines a set of universality classes. Within these
classes, we obtain universal scaling relations for density perturbations and
the scale of inflation. In specific models, we show that the scale of
supersymmetry breaking also possesses scaling behavior. We illustrate this
general structure with a specific example: the Racetrack Inflation model in
type IIB string theory, with the inflaton being the real part of the Kahler
modulus, and the input parameters being flux dependent quantities that appear
in the 4D, N=1 superpotential.Comment: 20 pages, 6 figures; v2: general discussion reworked for clarity,
minor terminology changes throughout. accepted to PR
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