Description Of Inflation Research Articles

Inflation with non-canonical scalar fields revisited

We revisit inflation with non-canonical scalar fields by applying deformed-steepness exponential potentials. We show that the resulting scenario can lead to inflationary observables, and in particular to scalar spectral index and tensor-to-scalar ratio, in remarkable agreement with observations. Additionally, a significant advantage of the scenario is that the required parameter values, such as the non-canonicality exponent and scale, as well as the potential exponent and scale, do not need to acquire unnatural values and hence can accept a theoretical justification. Hence, we obtain a significant improvement with respect to alternative schemes, and we present distinct correlations between the model parameters that better fit the data, which can be tested in future probes. This combination of observational efficiency and theoretical justification makes the scenario at hand a good candidate for the description of inflation.

Global Portraits of Nonminimal Teleparallel Inflation

We construct global phase portraits of inflationary dynamics in teleparallel gravity models with a scalar field nonminimally coupled to torsion scalar. The adopted set of variables can clearly distinguish between different asymptotic states as fixed points, including the kinetic and inflationary regimes. The key role in the description of inflation is played by the heteroclinic orbits that run from the asymptotic saddle points to the late time attractor point and are approximated by nonminimal slow roll conditions. To seek the asymptotic fixed points, we outline a heuristic method in terms of the “effective potential” and “effective mass”, which can be applied for any nonminimally coupled theories. As particular examples, we study positive quadratic nonminimal couplings with quadratic and quartic potentials and note how the portraits differ qualitatively from the known scalar-curvature counterparts. For quadratic models, inflation can only occur at small nonminimal coupling to torsion, as for larger coupling, the asymptotic de Sitter saddle point disappears from the physical phase space. Teleparallel models with quartic potentials are not viable for inflation at all, since for small nonminimal coupling, the asymptotic saddle point exhibits weaker than exponential expansion, and for larger coupling, it also disappears.

Flattening the inflaton potential beyond minimal gravity

We review the status of the Starobinsky-like models for inflation beyond minimal gravity and discuss the unitarity problem due to the presence of a large non-minimal gravity coupling. We show that the induced gravity models allow for a self-consistent description of inflation and discuss the implications of the inflaton couplings to the Higgs field in the Standard Model.

Frame covariant nonminimal multifield inflation

We introduce a frame-covariant formalism for inflation of scalar-curvature theories by adopting a differential geometric approach which treats the scalar fields as coordinates living on a field-space manifold. This ensures that our description of inflation is both conformally and reparameterization covariant. Our formulation gives rise to extensions of the usual Hubble and potential slow-roll parameters to generalized fully frame-covariant forms, which allow us to provide manifestly frame-invariant predictions for cosmological observables, such as the tensor-to-scalar ratio r, the spectral indices nR and nT, their runnings αR and αT, the non-Gaussianity parameter fNL, and the isocurvature fraction βiso. We examine the role of the field space curvature in the generation and transfer of isocurvature modes, and we investigate the effect of boundary conditions for the scalar fields at the end of inflation on the observable inflationary quantities. We explore the stability of the trajectories with respect to the boundary conditions by using a suitable sensitivity parameter. To illustrate our approach, we first analyze a simple minimal two-field scenario before studying a more realistic nonminimal model inspired by Higgs inflation. We find that isocurvature effects are greatly enhanced in the latter scenario and must be taken into account for certain values in the parameter space such that the model is properly normalized to the observed scalar power spectrum PR. Finally, we outline how our frame-covariant approach may be extended beyond the tree-level approximation through the Vilkovisky–De Witt formalism, which we generalize to take into account conformal transformations, thereby leading to a fully frame-invariant effective action at the one-loop level.

String cosmology — Large-field inflation in string theory

This is a short review of string cosmology. We wish to connect string-scale physics as closely as possible to observables accessible to current or near-future experiments. Our possible best hope to do so is a description of inflation in string theory. The energy scale of inflation can be as high as that of Grand Unification (GUT). If this is the case, this is the closest we can possibly get in energy scales to string-scale physics. Hence, GUT-scale inflation may be our best candidate phenomenon to preserve traces of string-scale dynamics. Our chance to look for such traces is the primordial gravitational wave, or tensor mode signal produced during inflation. For GUT-scale inflation this is strong enough to be potentially visible as a B-mode polarization of the cosmic microwave background (CMB). Moreover, a GUT-scale inflation model has a trans-Planckian excursion of the inflaton scalar field during the observable amount of inflation. Such large-field models of inflation have a clear need for symmetry protection against quantum corrections. This makes them ideal candidates for a description in a candidate fundamental theory like string theory. At the same time the need of large-field inflation models for UV completion makes them particularly susceptible to preserve imprints of their string-scale dynamics in the inflationary observables, the spectral index ns and the fractional tensor mode power r. Hence, we will focus this review on axion monodromy inflation as a mechanism of large-field inflation in string theory.

The Exchange Rate Dimension of Inflation Targeting: Target Levels and Currency Volatility

AbstractThe surprising volatility of floating exchange rates have puzzled macroeconomists and challenged policy makers since the seventies. This is no less true in South Arica where the Rand's volatility is a longstanding policy and business challenge. This paper extends the literature on nominal and institutional factors associated with currency volatility. Rose's description of inflation as “Bretton Woods in reverse” is the departure point and is read with Berganza and Broto's recent demonstration in a time series study that inflation targeting emerging market economies have experience higher exchange rate volatility. Meanwhile Bleaney and Tian have shown the cross‐sectional connection between the level of inflation and exchange rate volatility. We build on Bleaney and Tian's cross‐sectional approach to investigate the association between the level at which inflation targeting countries target inflation and exchange rate volatility over the long run. Crucially, we control for the average level of inflation and distinguish between inflation targeting countries that target high and low levels of inflation, in order to investigate whether the choice of the level of the inflation target (an institutional feature) is associated with greater exchange rate volatility.

Modelling Inflation Using Markov Switching Models: Case of Poland, 1992-2005

We investigate inflation in Poland in the period of economic transition by examining the potential application of Markov Switching Models to model the inflation generating process in Poland. The time horizon of analysis was limited to the period between March 1992 and October 2005 defined as the process of disinflation, i.e. the process of continued decrease in inflation rates following the economic transition period in early 1990s which was accompanied by a high level of inflation. According to the Ball-Friedman hypothesis, variation of inflation during periods of high inflation can be unstable. Indeed, the results show that non-linear models significantly improve the description of inflation generating process in Poland. Apart from univariate Markov Models, we also use a model that incorporates inflation expectations measured by Future Inflation Indicator (FII). We find that the model, where lagged values of FII are included as exogenous variables is significantly better in modelling inflation than simple univariate Markov Model.

Inflation in the czech republic in the mid-nineties (a comparison of alternative explanations)

The aim of the present study is, firstly, to organize existing theoretical approaches to the explanation of inflation in the Czech Republic. Secondly to investigate to what extent the rate of inflation in the Czech Republic was determined by the growth of the money stock and to what extent by the development of wages and productivity of labor. Chapter 1 gives a general description of inflation in the Czech Republic. Chapter 2 offers a list of its existing theoretical explanations. Chapter 3 contains a brief summary of the previous empirical research. Chapters 4, 5 and 6 contain our empirical model, its interpretation, and recommendations for economic policy and for further research.