{
  "id": "1908.06995",
  "version": "v1",
  "published": "2019-08-19T18:00:08.000Z",
  "updated": "2019-08-19T18:00:08.000Z",
  "title": "Oscillating scalar fields and the Hubble tension: a resolution with novel signatures",
  "authors": [
    "Tristan L. Smith",
    "Vivian Poulin",
    "Mustafa A. Amin"
  ],
  "comment": "26 pages, 22 figures, comments welcome",
  "categories": [
    "astro-ph.CO",
    "gr-qc",
    "hep-ph"
  ],
  "abstract": "We present a detailed investigation of a sub-dominant oscillating scalar field ('early dark energy', EDE) in the context of resolving the Hubble tension. Consistent with earlier work, but without relying on fluid approximations, we find that a scalar field frozen due to Hubble friction until ${\\rm log}_{10}(z_c)\\sim3.5$, reaching $\\rho_{\\rm EDE}(z_c)/\\rho_{\\rm tot}\\sim10$%, and diluting faster than matter afterwards can bring cosmic microwave background (CMB), baryonic acoustic oscillations, supernovae luminosity distances, and the late-time estimate of the Hubble constant from the SH0ES collaboration into agreement. A scalar field potential which scales as $V(\\phi) \\propto \\phi^{2n}$ with $2\\lesssim n\\lesssim 3.4$ around the minimum is preferred at the 68% confidence level, and the {\\em Planck} polarization places additional constraints on the dynamics of perturbations in the scalar field. In particular, the data prefers a potential which flattens at large field displacements. An MCMC analysis of mock data shows that the next-generation CMB observations (i.e., CMB-S4) can unambiguously detect the presence of the EDE at very high significance. This projected sensitivity to the EDE dynamics is mainly driven by improved measurements of the $E$-mode polarization. We also explore new observational signatures of EDE scalar field dynamics: (i) We find that depending on the strength of the tensor-to-scalar ratio, the presence of the EDE might imply the existence of isocurvature perturbations in the CMB. (ii) We show that a strikingly rapid, scale-dependent growth of EDE field perturbations can result from parametric resonance driven by the anharmonic oscillating field for $n\\approx 2$. This instability and ensuing potentially nonlinear, spatially inhomogenoues, dynamics may provide unique signatures of this scenario.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2019-08-19T18:00:08.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "hubble tension",
      "novel signatures",
      "ede scalar field dynamics",
      "resolution",
      "polarization places additional constraints"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 26,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}