Thread by @DanHooperAstro

(1/6) My Collaborators (Miguel Escudero, @GordanKrnjaic and Mathias Pierre) and I just posted a new paper that I am excited about, "Cosmology With a Very Light L_mu - L_tau Gauge Boson", arxiv.org/pdf/1901.02010…

Let me take a minute to walk you through the main points.
#cosmology

(2/6) We considered a new light particle, a Z' associated with a broken U(1)_mu-tau symmetry, and solved the full set of Boltmann equations to determine how such a particle would impact the energy density in neutrinos and other radiation in the early universe.
#cosmology

(3/6) We found two regions of parameter space that change the expansion history in a way that would help to reconcile the discrepancy between local and cosmological measurements of the Hubble constant. The first of these is the well known region with m_Z' ~ 10-30 MeV.

(4/6) But to my surprise, we also found a second region that can address this issue. This region is a huge PLATEAU of parameter space with a light Z' and a small coupling of g_mu-tau ~10^-9 to 10^-13. Across this region, we find Delta N_eff ~ 0.21.

(5/6) In other words, this model predicts a value of N_eff that can address the observed Hubble tension across a wide range of parameter space. Although plenty of other models can address this problem, this one can do it without any careful tuning of the parameters.

(6/6) The reason that this plateau exists is that for this range of parameters the Z' does not reach equilibrium at early times, but undergoes freeze-in, ultimately reaching equilibrium with the neutrinos, but only AFTER the neutrinos have decoupled from thermal bath.