Oscillating scalar fields and the Hubble tension: a resolution with novel signatures

Tristan L. Smith , Vivian Poulin (1) , Mustafa A. Amin

1 LUPM - Laboratoire Univers et Particules de Montpellier

We present a detailed investigation of a subdominant 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 log10(zc)∼3.5, reaching ρEDE(zc)/ρtot∼10% 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 that scales as V(ϕ)∝ϕ2n with 2≲n≲3.4 around the minimum is preferred at the 68% confidence level, and the Planck polarization places additional constraints on the dynamics of perturbations in the scalar field. In particular, the data prefer a potential that flattens at large field displacements. A Markov-chain Monte Carlo analysis of mock data shows that the next-generation CMB observations (i.e., CMB-S4) can unambiguously detect the presence of the EDE at a 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≈2. This instability and ensuing potentially nonlinear, spatially inhomogeneous, dynamics may provide unique signatures of this scenario.

Mots clés: satellite: Planck; stability; nonlinear; resolution; dark energy; sensitivity; scale dependence; Hubble constant; tension; cosmic background radiation: polarization; signature; resonance: parametric; Monte Carlo: Markov chain; approximation: fluid; supernova: luminosity; oscillation: acoustic; field theory: scalar; isocurvature: perturbation; Cosmology

Domaines: Astrophysique [astro-ph]; Relativité Générale et Cosmologie Quantique [gr-qc]; Physique des Hautes Energies - Phénoménologie [hep-ph]

Tristan L. Smith, Vivian Poulin, Mustafa A. Amin. Oscillating scalar fields and the Hubble tension: a resolution with novel signatures. Physical Review D, 2020, 101 (6), pp.063523. ⟨10.1103/PhysRevD.101.063523⟩. ⟨hal-02302882⟩