Detecting higher spin fields through statistical anisotropy in the CMB and galaxy power spectra

TL;DR

The paper investigates whether higher-spin fields present during inflation can leave detectable statistical anisotropy in the primordial curvature power spectrum. By considering a model where higher-spin fluctuations are effectively massless due to couplings to the inflaton, it shows that the spectrum acquires anisotropic corrections parametrized by $g_{LM}$ up to $L=2s$, with $g_{LM}$ reflecting the background spin orientation. The authors derive how these coefficients imprint on CMB and galaxy power spectra and perform Fisher forecasts for current and future surveys, finding sensitivities down to $\mathcal{O}(10^{-3})$-level under realistic conditions and largely independent of $L$. This suggests that a multi-spin inflationary scenario could be tested with near-future cosmological data, offering a window into high-energy physics during inflation.

Abstract

Primordial inflation may represent the most powerful collider to test high-energy physics models. In this paper we study the impact on the inflationary power spectrum of the comoving curvature perturbation in the specific model where massive higher spin fields are rendered effectively massless during a de Sitter epoch through suitable couplings to the inflaton field. In particular, we show that such fields with spin $s$ induce a distinctive statistical anisotropic signal on the power spectrum, in such a way that not only the usual $g_{2M}$-statistical anisotropy coefficients, but also higher-order ones (i.e., $g_{4M}$, $g_{6M}$, $\cdots$, $g_{(2s-2)M}$ and $g_{(2s) M}$) are nonvanishing. We examine their imprints in the cosmic microwave background and galaxy power spectra. Our Fisher matrix forecasts indicate that the detectability of $g_{LM}$ depends very weakly on $L$: all coefficients could be detected in near future if their magnitudes are bigger than about $10^{-3}$.

Figures (3)

• Figure 1: Expected $1\sigma$ errors on $g_{2M}$ (left panel) and $g_{4M}$ (right panel) computed from temperature alone (purple lines), E-mode polarization alone (yellow lines), and temperature and E-mode polarization jointly (blue lines), by assuming a CMB noiseless CVL-level survey with $f_{\rm sky} = 1$ (solid lines) and a Planck-like one with $f_{\rm sky} = 0.7$ (dashed lines). We here take $\ell_{\rm min} = 2$.
• Figure 2: All nonvanishing BipoSH coefficients for $L=2$ and $4$: $\partial P_{l_1 l_2}^{2M} / \partial g_{2M}$ and $\partial P_{l_1 l_2}^{4M} / \partial g_{4M}$. The black dashed lines describe $P_0$. We here take $b = 2.0$ and $z = 0.5$. The lines for $L = 2$ are fully consistent with those in Ref. Shiraishi:2016wec.
• Figure 3: Expected $1\sigma$ errors $\Delta g_{2M}$ and $\Delta g_{4M}$ in CMASS, PFS and Euclid. For PFS and Euclid, the co-add information from multi-redshift slices is taken into account via Eq. \ref{['eq:Fish_tomography']} with an assumption that different redshift bins are uncorrelated. The results of $\Delta g_{2M}$ are consistent with those in Ref. Shiraishi:2016wec. Here we take $k_{\rm min} = 0.005 h ~{\rm Mpc}^{-1}$