Abstract
New early dark energy (NEDE) makes the cosmic microwave background consistent with a higher value of the Hubble constant inferred from supernovae observations. It is an improvement over the old early dark energy model (EDE) because it explains naturally the decay of the extra energy component in terms of a vacuum first-order phase transition that is triggered by a subdominant scalar field at zero temperature. With hot NEDE, we introduce a new mechanism to trigger the phase transition. It relies on thermal corrections that subside as a subdominant radiation fluid in a dark gauge sector cools. We explore the phenomenology of hot NEDE and identify the strong supercooled regime as the scenario favored by phenomenology. In a second step, we propose different microscopic embeddings of hot NEDE. This includes the (non-)Abelian dark matter model, which has the potential to also resolve the LSS tension through interactions with the dark radiation fluid. We also address the coincidence problem generically present in EDE models by relating NEDE to the mass generation of neutrinos via the inverse seesaw mechanism. We finally propose a more complete dark sector model, which embeds the NEDE field in a larger symmetry group and discuss the possibility that the hot NEDE field is central for spontaneously breaking lepton number symmetry.
Highlights
The Hubble tension is a well-known discrepancy between the value of the Hubble constant today, H0, measured directly using supernovae (SNe) observations and the lower value indirectly inferred from measurements of the cosmic microwave background (CMB) [1] and baryonic acoustic oscillations (BAO) [2] when assuming the ΛCDM standard model of cosmology
We derived the condition under which this transition is a strong first-order phase transition with α ≫ 1 and capable of accommodating a sizable fraction of New early dark energy (NEDE) required for resolving the Hubble tension
After computing the effective potential valid for low temperatures and a large gauge boson mass, we found that γ 1⁄4 λ=ð4πg4NEDEÞ ≲ 1 is the preferred region in parameter space
Summary
The Hubble tension is a well-known discrepancy between the value of the Hubble constant today, H0, measured directly using supernovae (SNe) observations and the lower value indirectly inferred from measurements of the cosmic microwave background (CMB) [1] and baryonic acoustic oscillations (BAO) [2] when assuming the ΛCDM standard model of cosmology (for recent reviews see [3–5]). We will express different phenomenological parameters such as the fraction of NEDE at decay time fNEDE, the temperature of the dark sector Td, the inverse duration βand strength α of the phase transition, and the wall-thickness parameter δeff in terms of gNEDE as well as the field’s vacuum mass scale μ and quartic coupling parameter λ This in turn singles out γ 1⁄4 λ=ð4πg4NEDEÞ ≲ 1 as the most promising parameter region for hot NEDE, which includes the strong supercooled regime where the energy difference between the false and true vacuum dominates over the dark radiation (DR) fluid (in agreement with the discussion in [38] as well as [68] in an inflationary context). When the NEDE phase transition takes place at the eV temperature scale, the sterile neutrinos acquire their (super) eV Majorana mass, which, in turn, completes the inverse seesaw mass matrix while breaking lepton number spontaneously This model naturally incorporates different fermionic and bosonic decay channels for the bubble wall condensate, which can realize the proposed mixed DM scenario needed for a successful NEDE phenomenology. This point will be made more prominently in our companion paper [81]
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