# Standard model Higgs field and hidden sector cosmology

## TL;DR

This paper explores the relationship between the Higgs field during inflation, primordial gravitational waves, and dark matter isocurvature perturbations, showing that certain cosmological scenarios are incompatible with future observations.

## Contribution

It derives a relation between the tensor-to-scalar ratio and dark matter isocurvature perturbations considering the Higgs field's behavior during inflation in various cosmological histories.

## Key findings

- Future detection of dark matter isocurvature will rule out simple decay scenarios after inflation.
- The Higgs field generally does not threaten models with decoupled inflaton and dark matter sectors.
- Kination-dominated epochs can lead to sizeable dark matter isocurvature sourced by the Higgs.

## Abstract

We consider scenarios where the inflaton field decays dominantly to a hidden dark matter (DM) sector. By studying the typical behavior of the Standard Model (SM) Higgs field during inflation, we derive a relation between the primordial tensor-to-scalar ratio $r$ and amplitude of the residual DM isocurvature perturbations $\beta$ which is typically generated if the DM is thermally decoupled from the SM sector. We consider different expansion histories and find that if the Universe was radiation- or matter-dominated after inflation, a future discovery of primordial DM isocurvature will rule out all simple scenarios of this type because generating observable $\beta$ from the Higgs is not possible without violating the bounds on $r$. Seen another way, the Higgs field is generically not a threat to models where both the inflaton and DM reside in a decoupled sector. However, this is not necessarily the case for an early kination-dominated epoch, as then the Higgs can source sizeable $\beta$. We also discuss why the Higgs cannot source the observed curvature perturbation at large scales in any of the above cases but how the field can still be the dominant source of curvature perturbations at small scales.