The energy scale of inflation: is the hunt for the primordial B-mode a waste of time?

TL;DR

This work examines whether primordial B-mode polarization from inflation is detectable given a lensing foreground floor and EFT-based expectations of a small tensor amplitude. It contrasts the traditional EFT viewpoint with a flow-equation framework that tracks slow-roll parameters without specifying a microphysical inflaton, employing Monte Carlo reconstruction to map model space to observables such as the tensor-to-scalar ratio $r$ and the scalar spectral index $n$. The analysis shows that while EFT arguments favor a small $r$, a substantial subset of dynamically viable models yield observable tensor signals, potentially within Planck-like reach ($r\sim 0.01$). The main contribution is to argue that the B-mode search remains scientifically valuable, as it can distinguish between conventional EFT-based inflation and more exotic dynamics, with significant implications for the energy scale of inflation and early-universe physics.

Abstract

Recent theoretical results indicate that the detection of primordial gravity waves from inflation may be a hopeless task. First, foregrounds from lensing put a strict lower limit on the detectability of the B-mode polarization signal in the Cosmic Microwave Background, the ``smoking gun'' for tensor (gravity wave) fluctuations. Meanwhile, widely accepted theoretical arguments indicate that the amplitude of gravity waves produced in inflation will be below this limit. I argue that failure is not inevitable, and that the effort to detect the primordial signal in the B-mode, whether it succeeds or fails, will yield crucial information about the nature of inflation.

Figures (7)

• Figure 1: Angular power spectra observed by WMAP. The top panel shows the temperature anisotropy, and the bottom panel shows the temperature-polarization cross correlation spectrum. (Figure courtesy of the WMAP science team.) Especially important for the confirmation of acausal physics typical of inflation is the measured T/E anticorrelation around $\ell = 100$.
• Figure 2: Models generated by Monte Carlo plotted in the $(n,r)$ plane. The solid line is the power-law inflation fixed point $n = 1 - 2 r / (10 - r)$.
• Figure 3: n vs log r. The horizontal line is the lensing limit of Knox and Song. There is a substantial population of models with a large enough tensor/scalar ratio to be detectable above the lensing foreground.
• Figure 4: Errors from current CMB data on the $(r,n)$ plane. The contours represent $1\sigma$, $2\sigma$, and $3\sigma$ errors. The points are models generated by Monte Carlo, categorized into small-field (red, circles), large-field (green, triangles), and hybrid (blue, crosses). No class of models is yet ruled out by the data.
• Figure 5: Errors from current CMB data on the $(n, dn/d\log{k})$ plane. Contours represent $1\sigma$, $2\sigma$, and $3\sigma$ errors, with the models plotted as in Fig. \ref{['nrWMAP']}. Note that the region with negative running is well populated by models, especially of the hybrid class, indicating that the WMAP best fit is easily accomodated by simple inflationary models.