arXiv subjects

James M. Cline

Publications and source records attributed to James M. Cline.

At least 37 records · Page 2Linked to original sources

arXiv2025

Simple quintessence models in light of DESI-BAO observations

Recent analyses from the DESI collaboration suggest that the dark energy density of the Universe may be decreasing with time, slowing the acceleration of the scale factor $a$. Typically these studies are performed assuming an ansatz for the equation of state $w(a)$. In this work, we instead consider simple models of a scalar quintessence potential with linear and quadratic behavior, which could be more representative of real models than particular parametrizations of $w(a)$. We observe a significant preference for dynamical dark energy when using supernova data from DESY5 along with DESI BAO and Planck data, at the cost of slightly exacerbating the Hubble tension. However, when using supernova data from Pantheon+ or Union3, we find only a mild preference for dynamical dark energy.

astro-ph.CO↗

arXiv2025

Dark photon distortions of NO$ν$A and T2K neutrino oscillations

Dark photons coupling to $L_μ-L_τ$ lepton number difference are a highly studied light dark matter candidate, with potential to be discovered through their impact on terrestrial neutrino oscillation experiments. We re-examine this in the light of claimed tensions between the NO$ν$A and T2K long baseline experiments, also taking into account data from the MINOS experiment. We obtain leading limits on the $L_μ-L_τ$ gauge coupling $g'$ versus dark photon mass $m_{A'}$, and find no statistically significant alleviation of the tension from inclusion of the new physics effect.

hep-ph↗

arXiv2025

Bubble wall velocity for first-order QCD phase transition

Although the QCD phase transition is a crossover in the standard model, nonstandard effects such as a large lepton asymmetry are known to make it first order, with possible applications to gravitational wave production. This process is sensitive to the speed of the bubble walls during the phase transition, which is difficult to compute from first principles. We take advantage of recent progress on wall speed determinations to provide a simple estimate valid in the small supercooling regime which constrains the wall speed to be significantly lower than what has been used in previous literature. This in turn strongly suppresses the production of gravitational waves, to a level that is just out of reach of the most sensitive projected experiment for this signal, $μ$Ares. While our analysis approximates the equation of state using the template model, we demonstrate that our conclusions remain robust when incorporating state-of-the-art QCD equation of state data.

hep-ph↗

arXiv2025

Comment on "Dark Energy from Time Crystals"

It was recently proposed (arxiv:2502.08887) that the time crystal Lagrangian introduced by Shapere and Wilczek in 2012 could be a model of dark energy. I point out that the model has an instability that drives its energy density to negative values, which may render it unsuitable as a model of dark energy.

hep-ph↗

arXiv2024

Axion strings from string axions

A favored scenario for axions to be dark matter is for them to form a cosmic string network that subsequently decays, allowing for a tight link between the axion mass and relic abundance. We discuss an example in which the axion is protected from quantum gravity effects that would spoil its ability to solve the strong CP problem: namely a string theoretic axion arising from gauge symmetry in warped extra dimensions. Axion strings arise following the first-order Randall-Sundrum compactification phase transition, forming at the junctions of three bubbles during percolation. Their tensions are at the low scale associated with the warp factor, and are parametrically smaller than the usual field-theory axion strings, relative to the scale of their decay constant. Simulations of string network formation by this mechanism must be carried out to see whether the axion mass-relic density relation depends on the new parameters in the theory.

hep-ph↗

arXiv2024

Quantum coherence between mass eigenstates of a neutrino cannot be destroyed by its mass-momentum entanglement

In the latest of a series of papers (arXiv:2410.21850), it has been claimed that neutrinos cannot oscillate as a consequence of their mass mixing. I clarify that this arises from the unrealistic assumption that neutrinos will always be in nearly exact eigenstates of energy, and ignoring the spatial dependence of the neutrino wave function. Taking into account the latter, neutrinos will be observed to oscillate in precisely the expected manner.

hep-ph↗

arXiv2024

Comment on "A try for dark energy in quantum field theory: The vacuum energy of neutrino field"

It was recently suggested (arxiv:2410.06604) that the small value of the dark energy of the universe could be explained in terms of the scale of neutrino masses through a simple quantum field theoretic mechanism. I clarify that the quantity computed is only a threshold correction to the vacuum energy at the low $m_ν$ scale, and therefore cannot explain the magnitude of the vacuum energy.

hep-ph↗

arXiv2024

Comment on "Neutrino oscillations originate from virtual excitation of $Z$ bosons" and "Neutrinos produced from $β$ decays of neutrons cannot be in coherent superpositions of different mass eigenstates"

It was recently claimed (arxiv:2407.00954) that neutrino oscillations through conventional mass mixing are forbidden by energy conservation, and that they in fact arise from virtual $Z$ boson exchange. I explain why both claims, and therefore a repetition of these claims in arxiv:2410.03133, are incorrect.

hep-ph↗

arXiv2024

No Pulsar Timing Noise from Brownian Motion of the Sun

It was recently claimed (arXiv:2405.05410 [gr-qc]) that evidence for a stochastic gravitational wave background observed by pulsar timing arrays can be attributed instead to random perturbations of the Sun's motion by transiting asteroids. I show that that this would lead to a large dipole component accompanying a much smaller quadrupolar perturbation of pulsar timing signals, which would not be confused with a gravitational wave signal. Such an anomalous dipole would have been detected and identified as a spurious background by the PTA collaborations, if it existed.

gr-qc↗

arXiv2024

Comment on "Quantum-Mechanical Suppression of Gas Accretion by Primordial Black Holes"

It was recently claimed (arXiv:2409.09081v1 [astro-ph.HE]) that accretion of ordinary matter on black holes of mass $(6\times 10^{14}-4\times10^{19})\,$g would be inhibited by quantum mechanical effects, namely the de Broglie wavelength of the electron being larger than the Schwarzschild radius. However the conclusion is based on considering accretion of a single atom over the age of the Universe. There is no suppression of the accretion rate per atom on such black holes.

astro-ph.HE↗

arXiv2024

Comment on "Excluding Primordial Black Holes as Dark Matter Based on Solar System Ephemeris"

It was recently claimed (arXiv:2408.10799) that solar system ephemeris data can exclude primordial black hole (PBH) dark matter in the mass range $10^{18}$-$10^{22}\,$g. We show that this conclusion is based on an implausible, implicit assumption; namely the uncertainty on the solar system mass within 50 au is as small as the uncertainty on the mass of the sun. Correcting for this error, we find that ephemeris data can only constrain PBH's with mass below $10^{16}\,$g, which is already excluded by constraints on their evaporation via Hawking radiation. Correcting a further error concerning the time-averaged rate of such fluctuations nullifies even this weaker constraint.

hep-ph↗

arXiv2024

Can Baby Universe Absorption Explain Dark Energy?

It has been proposed that the accelerated expansion of the universe can be explained by the merging of our universe with baby universes, resulting in dark energy with a phantom-like equation of state. However, the evidence in favor of it did not include the full set of cosmological observables. Here we examine the implications of this model for both early and late universe cosmology using data from Planck collaboration, DESI 2024 and other experiments. We find that the pure baby universe model gives a poor fit to current data. Extending it to include a contribution from the cosmological constant, we find two allowed regions of parameter space: one close to $Λ$CDM, and another with $Λ<0$ plus the exotic dark energy component. The two regions can be significantly favored over $Λ$CDM, depending on the choice of supernova datasets, and they can ameliorate the Hubble tension to the level of $2σ$, depending on the supernova dataset. The model with $Λ<0$ features an equation of state $w(a)$ with a pole singularity at early times.

astro-ph.CO↗

arXiv2024

Self-interacting dark matter solves the final parsec problem of supermassive black hole mergers

Evidence for a stochastic gravitational wave (GW) background, plausibly originating from the merger of supermassive black holes (SMBHs), is accumulating with observations from pulsar timing arrays. An outstanding question is how inspiraling SMBHs get past the "final parsec" of separation, where they have a tendency to stall before GW emission alone can make the binary coalesce. We argue that dynamical friction from the dark matter (DM) spike surrounding the black holes is sufficient to resolve this puzzle, if the DM has a self-interaction cross section of order $\,$cm$^2$/g. The same effect leads to a softening of the GW spectrum at low frequencies as suggested by the current data. For collisionless cold DM, the friction deposits so much energy that the spike is disrupted and cannot bridge the final parsec, while for self-interacting DM, the isothermal core of the halo can act as a reservoir for the energy liberated from the SMBH orbits. A realistic velocity dependence, such as generated by the exchange of a massive mediator like a dark photon, is favored to give a good fit to the GW spectrum while providing a large enough core. A similar velocity dependence has been advocated for solving the small-scale structure problems of cold DM.

astro-ph.CO↗

arXiv2024

Status of Dark Photons

I review recent developments in the field of dark photons-here taken to be U(1) gauge bosons with mass less than the Z-including both kinetically mixed vectors and those that couple to anomaly-free U(1)'s. Distinctions between Higgs and Stueckelberg masses are highlighted, with discussion of swampland constraints, UV completions, and new experimental search strategies.

hep-ph↗

arXiv2024

AGN constraints on neutrino-dark matter scattering

The IceCube collaboration has identified neutrinos of energy $\sim 10-100$ TeV from the blazar TXS 0506+056 and the active galaxy NGC 1068, which must have traveled through a dense dark matter spike surrounding the supermassive black holes that power the galactic nuclei. We use this to set new constraints on dark matter-neutrino scattering, and interpret the results in terms of a dark photon that couples to baryon minus lepton number.

astro-ph.HE↗

arXiv2024

Phantom Fluid Cosmology -or- Ghosts for Gordon

The quanta of phantom dark energy models are negative energy particles, whose maximum magnitude of energy $|E|$ must be less than a cutoff $Λ\lesssim 20\,$MeV. They are produced by spontaneous decay of the vacuum into phantoms plus normal particles. I review general cosmological constraints that have been derived from the effects of such phantom fluid production, and a possible application: the generation of boosted dark matter or radiation that could be directly detected. Recent excess events from the DAMIC experiment can be well-fit by such processes.

hep-ph↗

arXiv2023

Boosted dark matter from a phantom fluid

It is known that theories of phantom dark energy, considered as quantum fields, predict a continuous production of positive- plus negative-energy particles, from spontaneous decay of the vacuum. We show that this can be a new source of boosted dark matter or radiation, with consequences for direct detection. We set constraints on such models using data from the XENONnT experiment, and we show that recent excess events reported by the DAMIC experiment can be consistently described as coming from dark radiation, produced by vacuum decay, interacting with electrons.

hep-ph↗

arXiv2023

Phantom fluid cosmology: Impact of a phantom hidden sector on cosmological observables

Phantom scalar theories are widely considered in cosmology, but rarely at the quantum level, where they give rise to negative-energy ghost particles. These cause decay of the vacuum into gravitons and photons, violating observational gamma-ray limits unless the ghosts are effective degrees of freedom with a cutoff $Λ$ at the few-MeV scale. We update the constraints on this scale, finding that $Λ\lesssim 19$ MeV. We further explore the possible coupling of ghosts to a light, possibly massless, hidden sector particle, such as a sterile neutrino. Vacuum decays can then cause the dark matter density of the universe to grow at late times. The combined phantom plus dark matter fluid has an effective equation of state $w < -1$, and functions as a new source of dark energy. We derive constraints from cosmological observables on the rate of vacuum decay into such a phantom fluid. We find a mild preference for the ghost model over the standard cosmological one, and a modest amelioration of the Hubble and $S_8$ tensions.

hep-ph↗