273 research outputs found
Stability of multifield cosmological solutions in the presence of a fluid
We explore the stability properties of multifield solutions in the presence of a perfect fluid, as appropriate to assisted quintessence scenarios. We show that the stability condition for multiple fields phi(i) in identical potentials V-i is simply d(2)V(i)/d phi(2)(i) > 0, exactly as in the absence of a fluid. A possible new instability associated with the fluid is shown not to arise in situations of cosmological interest
Predictions in multifield inflation
Models of inflation with more than one active field are an important class where it is not fully understood how to compute predictions. This problem can be understood in
terms of two characteristics of these models: the sensitivity to initial conditions and the superhorizon evolution of the primordial density perturbation Ī¶. This thesis seeks to make significant progress in understanding how to overcome these two issues.
To track the superhorizon evolution of Ī¶ in general requires numerical techniques. By extending the transport method first proposed by Mulryne, Seery and Wesley, here, a
computationally efficient and highly versatile method for computing the statistics of Ī¶ is developed. The increased efficiency and versatility allows models that were previously unaccessible to be studied.
Utilising this new capability two models are explored. A new toy model of inflation in the Landscape and a 6-field D-brane model of inflation first proposed by Agarwal,
Bean, McAllister, and Xu. The nature of these models allows for a statistical analysis of inflationary realisations to be performed. We conclude that the fundamental ability to
constrain models of this kind is determined by the scale of features in the potential. We also show the D-brane model is under considerable pressure from current observations of
the spectral index and may be ruled out by future observations.
Finally, I show that there exists a class of models for which the probability distribution of observables may be computed analytically. I show the peak of the density function is largely dominated by the geometry of the potential and comparatively insensitive to the distribution of initial conditions. I argue that this characteristic should be expected in a broader range of models and for such models, it is possible to make robust predictions
Multifield consequences for D-brane inflation
We analyse the multifield behaviour in D-brane inflation when contributions from the bulk are taken into account. For this purpose, we study a large number of realisations of the potential; we find the nature of the inflationary trajectory to be very consistent despite the complex construction. Inflation is always canonical and occurs in the vicinity of an inflection point. Extending the transport method to non-slow-roll and to calculate the running, we obtain distributions for observables. The spectral index is typically blue and the running positive, putting the model under moderate pressure from WMAP7 constraints. The local f_NL and tensor-to-scalar ratio are typically unobservably small, though we find approximately 0.5% of realisations to give observably large local f_NL. Approximating the potential as sum-separable, we are able to give fully analytic explanations for the trends in observed behaviour. Finally we find the model suffers from the persistence of isocurvature perturbations, which can be expected to cause further evolution of adiabatic perturbations after inflation. We argue this is a typical problem for models of multifield inflation involving inflection points and renders models of this type technically unpredictive without a description of reheating
Chaotic inflation with kinetic alignment of axion fields
N-flation is a radiatively stable scenario for chaotic inflation in which the displacements of Nā«1 axions with decay constants f1ā¤ā¦ā¤fN<MP lead to a super-Planckian effective displacement equal to the Pythagorean sum fPy of the fi. We show that mixing in the axion kinetic term generically leads to the phenomenon of kinetic alignment, allowing for effective displacements as large as NāāāfNā„fPy, even if f1,ā¦,fNā1 are arbitrarily small. At the level of kinematics, the necessary alignment occurs with very high probability, because of eigenvector delocalization. We present conditions under which inflation can take place along an aligned direction. Our construction sharply reduces the challenge of realizing N-flation in string theory
Designing and testing inflationary models with Bayesian networks
Even simple inflationary scenarios have many free parameters. Beyond the
variables appearing in the inflationary action, these include dynamical initial
conditions, the number of fields, and couplings to other sectors. These
quantities are often ignored but cosmological observables can depend on the
unknown parameters. We use Bayesian networks to account for a large set of
inflationary parameters, deriving generative models for the primordial spectra
that are conditioned on a hierarchical set of prior probabilities describing
the initial conditions, reheating physics, and other free parameters. We use
--quadratic inflation as an illustrative example, finding that the number
of -folds between horizon exit for the pivot scale and the end of
inflation is typically the most important parameter, even when the number of
fields, their masses and initial conditions are unknown, along with possible
conditional dependencies between these parameters.Comment: 24 pages, 9 figures, 1 table; discussion update
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