http://arxiv.org/abs/2210.16459
In [1], a most general higher curvature non-local gravity action admitting $R^2$-like inflationary solution predicting scalar spectral index $n_s(N)\approx 1-\frac{2}{N}$, where $N$ is the number of e-folds before the end of inflation, $N\gg 1$, any value of the tensor-to-scalar ratio $r(N)<0.036$ and tensor tilt $n_t(N)$ violating the $r= -8n_t$ condition was obtained. In this paper, we compute scalar primordial non-Gaussianities (PNGs) in this theory and effectively demonstrate that higher curvature non-local terms lead to new shapes of reduced bispectrum $f_{\rm NL}\left( k_1,\,k_2,\,k_3 \right)$ mimicking several classes of scalar field models of inflation known in the literature. We obtain $\vert f_{\rm NL}\vert \sim O(1-10)$ in the equilateral, orthogonal, and squeezed limits and the running of PNGs measured by the quantity $\vert\frac{d\ln f_{\rm NL}}{d\ln k}\vert\lesssim 1$. We project these results in the scope of future CMB, Large Scale Structure observations to probe the nature of quantum gravity. Furthermore, we demonstrate that $R^2$-like inflation in non-local modification of gravity brings a paradigm shift in our understanding of early Universe cosmology through non-trivial predictions which go beyond the current status of effective field theories (EFTs) of single field, quasi-single field, and multiple field inflation. In summary, through our generalized non-local $R^2$-like inflation, we obtain for the first time a robust geometric framework of inflation that can explain any detection of observable quantities related to scalar PNGs.
A. Koshelev, K. Kumar and A. Starobinsky
Tue, 1 Nov 22
80/100
Comments: 30 pages, 7 figures
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