Optimizing future experimental probes of inflation

Abstract

The discovery of many novel realizations of the inflationary universe paradigm has led to a degeneracy problem: many different inflationary Lagrangians generate the same perturbation spectra. Resolving this problem requires the future discovery of additional observables, beyond the scalar adiabatic and tensor two-point functions on CMB scales. One important source of degeneracy arises in models where the density perturbation is generated by a non-inflationary degree of freedom, for example, through curvatons or modulated reheating. We consider the curvaton scenario as representative of this class, and analyze the degeneracy with single field, canonical inflation that results if the curvaton goes undetected by future observations. We perform Monte Carlo potential reconstructions in the absence of distinguishing observables, such as non-Gaussiantities or isocurvature modes. The resulting degeneracy is considerable and the improved measurements of spectral parameters from future probes like CMBPol, offer little to better the situation. Given a degeneracy-breaking observation, the observables must still be inverted to obtain the inflationary potential, with different observations resulting in reconstructions of varying quality. We find that a future detection of isocurvature modes or a precision measurement of the tensor spectral index will enable the most successful reconstructions in the presence of curvatons.

The degeneracy problem in non-canonical inflation

“…It must be stressed that a determination of r still serves to reliably constrain the energy scale of inflation, even amongst models described by more general Lagrangians. And, we will see in the case of DBI inflation, as we did with curvatons in [4], that a measurement of both r and n T will prove to be quite advantageous to the reconstruction effort [3]. However, the results in Figure 1 indicate that improvements in a measurement of r on its own offer little corresponding improvement in reconstruction results.…”

“…This 'worst case scenario' makes it impossible to reconstruct the inflationary potential or categorize the inflationary model according to the standard zoology scheme. This is of course only the 'tip of the iceberg', as many other k-inflation models and (even other paradigms such as the curvaton as discussed in [4]) exacerbate the problem. We then demonstrate the ability of additional observables such as non-Gaussianity, B-mode polarization and direct detection of primordial gravitational waves to both break the degeneracy and improve the potential reconstruction program.…”

“…(2.8) and (2.10) to write down the first few derivatives of the potential, 20) where Γ = γ/(γ + 1). An important distinction should be made between the DBI reconstruction and that of the curvaton carried out in the companion analysis [4]: the curvaton does not modify the inflationary dynamics, it serves only as a means of generating the primordial density perturbation. In other words, the same inflationary Lagrangian will generate different spectra depending on whether a curvaton is present or not -a degeneracy with single field models develops if we are unable to detect its presence.…”

Features and new physical scales in primordial observables: Theory and observation

“…Although one can invoke some selective criteria of 'inference to best theory' [9] to take the current set of observations as confirmation of the inflationary paradigm, it needn't be too much of a stretch to probe the available observations for further evidence before declaring a particular class of inflationary models as the true description of the very early universe. Furthermore, given the large degeneracies that exist between the predictions of different models [10][11][12] (even drastically different cosmologies [13]) at the level of the primordial power spectrum, one might ask, are there any other handles on the data that increase our ability to discriminate between these, or even test the very inflationary paradigm itself?…”

How Much Can We Learn about the Physics of Inflation?

“…Here I focus on what comes next: what can we learn about this new physics from the next generation or two of CMB polarization experiments? Although this topic has been addressed before [10][11][12][13][14][15], we are now (probably) in a new era, in which we know the amplitude of the signal so are standing on firmer ground.…”