New inflationary probes of axion dark matter

Abstract

If a light axion is present during inflation and becomes part of dark matter afterwards, its quantum fluctuations contribute to dark matter isocurvature. In this article, we introduce a whole new suite of cosmological observables for axion isocurvature, which could help test the presence of axions, as well as its coupling to the inflaton and other heavy spectator fields during inflation such as the radial mode of the Peccei-Quinn field. They include correlated clock signals in the curvature and isocurvature spectra, and mixed cosmological-collider non-Gaussianities involving both curvature and isocurvature fluctuations with shapes and running unconstrained by the current data analyses. Taking into account of the existing strong constraints on axion isocurvature fluctuations from the CMB, these novel signals could still be sizable and potentially observable. In some models, the signals, if observed, could even help us significantly narrow down the range of the inflationary Hubble scale, a crucial parameter difficult to be determined in general, independent of the tensor mode.

BCS in the sky: signatures of inflationary fermion condensation

“…For instance, let us consider the fermion dynamics during inflation. It has been shown that the rolling inflaton can naturally provide an external chemical potential that supports a number of fermions [25][26][27][28][29][30]. Then after turning on a weak attractive interaction, the fermions may pair up and condense, forming a BCS-like state.…”

A cosmological tachyon collider: enhancing the long-short scale coupling

“…It is widely believed that there existed a stage of cosmic inflation during which the universe expanded enormously, with a background geometry well described by a quasi-de Sitter spacetime [1][2][3][4]. The fast expansion of spacetime serves as a natural accelerator for massive fields during inflation, and is capable of producing particles with masses up to the Hubble scale during inflation, which can be as high as H ≤ 5 × 10 13 GeV. These massive particles can interact and decay into massless inflatons and gravitons, sourcing characteristic signals in the primordial non-Gaussianities of curvature and tensor perturbations in the Cosmic Microwave Background (CMB) and Large Scale Structure (LSS).…”

“…These massive particles can interact and decay into massless inflatons and gravitons, sourcing characteristic signals in the primordial non-Gaussianities of curvature and tensor perturbations in the Cosmic Microwave Background (CMB) and Large Scale Structure (LSS). In recent years, this cosmological collider paradigm [5][6][7][8][9] has received much attention and witnessed fast developments across aspects of its theory [10], phenomenology [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29], and observational prospects [30][31][32][33][34][35].…”

Light scalars at the cosmological collider

“…Recent work, however, has shown that the shape function can receive important contributions from isocurvature modes excited by such light scalars[21][22][23][24].…”