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Stephon Alexander

Publications and source records attributed to Stephon Alexander.

At least 37 records · Page 2Linked to original sources
arXiv2025

A Dark Matter Fermionic Quantum Fluid from Standard Model Dynamics

We present a model of dark matter as a superconducting fluid of Cooper pairs of right handed neutrinos or of vector-like quarks. The superconducting dark matter is induced by attractive channels in the Standard Model Higgs and color sectors of the Standard Model, respectively. We show that, for each case, the solution to the gap equation provides viable dark matter candidates for suitable chemical potential values. The mechanism yields an ultra-light neutrino condensate with a mass of $m_{\rm DM} \sim 10^{-19} \text{eV}$ or a vector-like quark condensate with wide range of possible masses. Both cosmological and particle physics constraints on the model lead to a connection between the number of effective relativistic species $N_{\rm eff}$, and the chemical potential and CMB temperature at the time of fermion creation. We also find a relation between the superconducting fermion and baryon densities, with implications for the coincidence between the dark matter and baryon densities in standard cosmology. Given the natural $\text{eV}$ scale of neutrinos, this mechanism may have implications for the Hubble tension.

hep-ph↗
arXiv2025

The Cosmological Constant from a Quantum Gravitational $θ$-Vacua and the Gravitational Hall Effect

We provide a new perspective on the cosmological constant by exploring the background-independent Wheeler-DeWitt quantization of general relativity. The Chern-Simons-Kodama state of quantum gravity, a generalization of the Hartle-Hawking and Vilenkin states, has a striking structural similarity to the topological field theory of the quantum Hall effect. As a result, we study the gravitational topological $θ$-sectors in analogy to Yang-Mills theory. We find that the cosmological constant $Λ$ is intimately linked to the $θ$-parameter by $θ=12π^2/(Λ\ell^2_{\rm Pl}) \mod 2π$ due to the fact that Chern-Simons-Kodama state must live in a particular $θ$-sector. This result is shown in the canonical, non-perturbative formalism. Furthermore, we explain how the physics of the Hamiltonian constraint is analogous to the quantum Hall effect, with the cosmological constant playing the role of a quantum gravitational Hall resistivity. These relations suggest that $Λ$ is topologically protected against perturbative graviton loop corrections, analogous to the robustness of quantized Hall conductance against disorder in a metal.

gr-qc↗
arXiv2025

Kinetically Coupled Dark Matter Condensates

Dark matter consisting of ultralight bosons can form a macroscopic Bose-Einstein condensate with distinctive observational signatures. While this possibility has been extensively studied for axions and axion-like particles $-$ pseudoscalars with masses protected by shift symmetry $-$ realistic models from string theory and other higher-dimensional theories predict more complex structures. Here we investigate a two-field generalization where an axion couples to a moduli field through its kinetic term, representing the phase and radial modes of a complex scalar field. We demonstrate that when this system forms a gravitationally bound Bose-Einstein condensate, the kinetic coupling produces dramatic modifications to cosmological evolution compared to the canonical single-field case. Most notably, the axion Jeans scale becomes dynamically dependent on the moduli field's evolution, fundamentally altering structure formation. By mapping existing observational constraints from canonical axion models to our two-field scenario, we identify regions of parameter space that are already excluded by current observations. In particular, consistency with observations requires that the moduli field must take on small field values, $χ/M_{\rm pl} \ll 1$, throughout most of cosmic history for this class of axions to remain a viable description of all dark matter.

astro-ph.CO↗
arXiv2025

Vacuum Amplification of Chiral Gravitational Waves and the Stochastic Gravitational Wave Background

We investigate cosmological vacuum amplification of gravitational waves in dynamical Chern-Simons gravity. We develop a comprehensive framework to compute graviton production induced by the parity violating Pontryagin coupling and study its imprint on the stochastic gravitational wave background energy power spectrum. We explore gravitational vacuum amplification in four concrete scenarios for the evolution of the Chern-Simons pseudoscalar. We show that a parity-violating contribution dominates over an initially flat spectrum when the velocity of the pseudoscalar quickly interpolates between two asymptotically constant values or when it is nonvanishing and constant through a finite period of time. This is also the case when we parametrize the pseudoscalar evolution by a perfect fluid with radiation- and dust-like equations of state for large enough values of its energy density. The resulting spectra are compared with the sensitivity curves of current and future gravitational wave observational searches.

gr-qc↗
arXiv2025

Quantum Excitation Transfer in an Artificial Photosynthetic Light-Harvesting System

We analytically derive transfer probabilities and efficiencies for an artificial light-harvesting photosynthetic system, which consists of a ring coupled to a central acceptor. For an incident photon pair, we find near-perfect single excitation transfer efficiency with negligible double excitation transfer in the weak coupling regime. In the strong coupling regime, single excitation transfer efficiency was greater than 90%, while the double excitation efficiency was approximately 50%. We have found that the three main factors which determine high transfer efficiencies are large acceptor probabilities, long acceptor decay times, and strong photon-ring coupling. A possible implementation of the theoretical framework to bio-inspired solar energy devices is also discussed.

physics.chem-ph↗
arXiv2025

Perturbations in pseudo-Nambu-Goldstone Higgs Inflation

Pseudo-Nambu-Goldstone (pNG) Higgs Inflation is a novel approach to relate the Higgs boson and its interaction with Electroweak gauge bosons with cosmic inflation, with the potential of solving both the fine-tuning issues in the Higgs mass and inflationary potentials. In this work, we present a linear perturbation analysis of the minimal implementation of pNG Higgs inflation using the symmetry coset SU($5$)/SO($5$). Similar to Chromo-natural inflation, this model exhibits a period of instability in the tensor modes that exponentially enhance left-handed gravitational waves. Thus, large Chern-Simons couplings $β\gtrsim 6 \times 10^8$ and decay constants $f\gtrsim1\times10^{18}~\mathrm{GeV}$ are required to suppress the tensor-to-scalar ratio $r$ to be compatible with the cosmic microwave background (CMB) measurement. These large couplings also cause an overproduction of the scalar modes, making the minimal construction of pNG Higgs inflation disfavored by CMB observations. However, this tension could potentially be relieved by considering multi-field inflation. The pNG Higgs construction naturally contains multiple scalar fields via the interplay of spontaneously broken global and gauge symmetries. The rich structure enables a broad range of multi-field inflation, and we conclude by briefly discussing this possibility and future work.

astro-ph.CO↗
arXiv2025

Birefringence in fermion-attenuated gravitational wave power spectrum

Within the framework of Chern-Simons gravity, a theory that dynamically violates parity, we analyze the power spectrum of gravitational waves in light of the damping effect due to the free streaming relativistic neutrinos and dark fermions. The power spectrum is expressed terms of right- and left-handed polarizations, and the evolution of the gravitational waves is studied numerically. Birefringence is explicitly shown in the power spectrum, though the difference in the amplitudes is small. Specific features of peaks and dips appear gravitational wave power spectrum mirroring chiral gravitational wave mediated parametric resonance during reheating. Our result represents a useful tool to test Chern-Simons gravity and enables to constrain mechanisms of inflation and reheating related to this theory. We predict a falsifiable pattern of observable peaks and dips in the chiral independent gravitational power spectrum, eventually observable in next space-borne gravitational interferometers, including LISA, Taiji and Tianqin.

gr-qc↗
arXiv2024

de Sitter State in Heterotic String Theory

Recent no-go theorems have ruled out four-dimensional classical de Sitter vacua in heterotic string theory. On the other hand, the absence of a well-defined Wilsonian effective action and other related phenomena also appear to rule out such time-dependent vacua with de Sitter isometries, even in the presence of quantum corrections. In this note, we argue that a four-dimensional de Sitter space can still exist in SO(32) heterotic string theory as a Glauber-Sudarshan state, i.e. as a coherent state, over a supersymmetric Minkowski background, albeit within a finite temporal domain. Borel resummation and resurgence play a crucial role in constructing such a state in the Hilbert space of heterotic theory governed entirely by the IR degrees of freedom.

hep-th↗
arXiv2024

Spacetime geometry of acoustics and electromagnetism

Both acoustics and electromagnetism represent measurable fields in terms of dynamical potential fields. Electromagnetic force-fields form a spacetime bivector that is represented by a dynamical energy-momentum 4-vector potential field. Acoustic pressure and velocity fields form an energy-momentum density 4-vector field that is represented by a dynamical action scalar potential field. Surprisingly, standard field theory analyses of spin angular momentum based on these traditional potential representations contradict recent experiments, which motivates a careful reassessment of both theories. We analyze extensions of both theories that use the full geometric structure of spacetime to respect essential symmetries enforced by vacuum wave propagation. The resulting extensions are geometrically complete and phase-invariant (i.e., dual-symmetric) formulations that span all five grades of spacetime, with dynamical potentials and measurable fields spanning complementary grades that are related by a spacetime vector derivative (i.e., the quantum Dirac operator). These complete representations correct the equations of motion, energy-momentum tensors, forces experienced by probes, Lagrangian densities, and allowed gauge freedoms, while making manifest the deep structural connections to relativistic quantum field theories. Finally, we discuss the implications of these corrections to experimental tests.

math-ph↗
arXiv2024

A path integral formula of quantum gravity emergent from entangled local structures

We couple to group field theory (GFT) a scalar field that encodes the entanglement between manifold sites. The scalar field provides a relational clock that enables the derivation of the Hamiltonian of the system from the GFT action. Inspecting the Hamiltonian, we show that a theory of emergent gravity arises, and that this can be recast according to the Ashtekar's formulation of general relativity. The evolution of the GFT observables is regulated by the Shroedinger equation generated by the Hamiltonian. This is achieved by imposing a renormalization group (RG) flow that corresponds to a simplified Ricci flow. As a consequence of the quantization procedure, the Hamiltonian is recovered to be non-Hermitian, and can be related to the complex action formalism, in which the initial conditions and the related future evolution of the systems are dictated by the imaginary part of the action.

hep-th↗
arXiv2024

A Realist Interpretation of Unitarity in Quantum Gravity

Unitarity is a difficult concept to implement in canonical quantum gravity because of state non-normalizability and the problem of time. We take a realist approach based on pilot-wave theory to address this issue in the Ashtekar formulation of the Wheeler-DeWitt equation. We use the postulate of a definite configuration in the theory to define a global time for the gravitational-fermionic system recently discussed in (Phys. Rev. D 106.10 (2022): 106012), by parameterizing a variation of a Weyl-spinor that depends on the Kodama state. The total Hamiltonian constraint yields a time-dependent Schrodinger equation, without semi-classical approximations, which we use to derive a local continuity equation over the configuration space. We implement the reality conditions at the level of the guidance equation, and obtain a real spin-connection, extrinsic curvature and triad along the system trajectory. We obtain quantum corrections to deSitter spacetime from the guidance equation. The non-normalizable Kodama state is naturally factored out of the full quantum state in the conserved current density, opening the possibility for quantum-mechanical unitarity. We also give a pilot-wave generalisation of the notion of unitarity applicable to non-normalizable states, and show the existence of equilibrium density for our system. Lastly, we find unitary states in mini-superspace by finding an approximate solution to the Hamiltonian constraint.

gr-qc↗
arXiv2024

The Field Theory Axiverse

Axion and axion-like particles (ALPs) are a prominent candidate for physics beyond the Standard Model, and can play an important role in cosmology, serving as dark matter or dark energy, or both, drawing motivation in part from the string theory axiverse. Axion-like particles (ALPs) can also arise as composite degrees of freedom following chiral symmetry breaking in a dark confining gauge theory, analogous to the Standard Model (SM) pion. A dark sector with arbitrary $N_f$ flavors of dark quarks leads to $N_f^2-1$ axion-like states, effectively a field theory axiverse (or '$π$-axiverse'). A portal to the visible sector can be achieved through the standard kinetic mixing between the dark photon and SM photon, generating millicharges for the dark quarks and consequently couplings, both parity-even and parity-odd, between the SM and the dark pions. This scenario has been studied for the $N_f=2$ case and more recently for a dark Standard Model with $N_f=6$. In this work, we study the spectrum of this field theory axiverse for an arbitrary number of flavors, and apply this to the example $N_f=10$. We calculate the couplings to the SM photon analogous to the conventional axion-photon coupling, including the $N_f$ and $N_c$ dependence, and compute the present and future constraints on the $N_f=10$, $N_c=3$, $π$-axiverse. We elucidate the accompanying 'bary-verse' of superheavy dark baryons, namely an ensemble of charged and neutral dark baryons with a mass set by the dark pion decay constant.

hep-ph↗
arXiv2024

The Non-triviality of Dynamical Chern-Simons Gravity and the Standard Model

Given the growing interest in gravitational-wave and cosmological parity-violating effects in dynamical Chern-Simons (dCS) gravity, it is crucial to investigate whether the scalar-gravitational Pontryagin term in dCS persists when formulated in the context of the $\text{U(1)}_{\text{B}-\text{L}}$ anomaly in the Standard Model (SM). In particular, it has been argued that dCS gravity can be reduced to Einstein gravity after ''rotating away'' the gravitational-Pontryagin coupling into the phase of the Weinberg operator $\unicode{x2013}$ analogous to the rotation of the axion zero-mode into the quark mass matrix. We find that dCS is nontrivial if the scalar field $ϕ$ has significant space-time dependence from dynamics. We provide a comprehensive consideration of the dCS classical and quantum symmetries relevant for embedding a dCS sector in the SM. We find that, because of the B-L chiral gravitational anomaly, the scalar-Pontryagin term cannot be absorbed by a field redefinition. Assuming a minimal extension of the SM, we also find that a coupling of the dCS scalar with right-handed neutrinos induces both the scalar-Pontryagin coupling and an axion-like phase in the dimension-five Weinberg operator. We comment on the issue of gauging the $\text{U(1)}_{\text{B}-\text{L}}$, the observational effects with these two operators present for upcoming experiments, and the origin of dCS gravity in string theory.

hep-th↗
arXiv2024

What can galaxy shapes tell us about physics beyond the standard model?

The shapes of galaxies trace scalar physics in the late-Universe through the large-scale gravitational potential. Are they also sensitive to higher-spin physics? We present a general study into the observational consequences of vector and tensor modes in the early and late Universe, through the statistics of cosmic shear and its higher-order generalization, flexion. Higher-spin contributions arise from both gravitational lensing and intrinsic alignments, and we give the leading-order correlators for each (some of which have been previously derived), in addition to their flat-sky limits. In particular, we find non-trivial sourcing of shear $EB$ and $BB$ spectra, depending on the parity properties of the source. We consider two sources of vector and tensor modes: scale-invariant primordial fluctuations and cosmic strings, forecasting the detectability of each for upcoming surveys. Shear is found to be a powerful probe of cosmic strings, primarily through the continual sourcing of vector modes; flexion adds little to the constraining power except on very small scales ($\ell\gtrsim 1000$), though it could be an intriguing probe of as-yet-unknown rank-three tensors or halo-scale physics. Such probes could be used to constrain new physics proposed to explain recent pulsar timing array observations.

astro-ph.CO↗
arXiv2023

The $π$-axion and $π$-axiverse of dark QCD

Axions and axion-like particles (ALPs) are a prominent dark matter candidate, drawing motivation in part from the axiverse of string theory. Axion-like particles can also arise as composite degrees of freedom of a dark sector, for example, as dark pions in dark Quantum Chromo-Dynamics. In a dark Standard Model (SM) wherein all 6 quark flavors are light while the photon is massive, one finds a rich low-energy spectrum of stable and ultralight particles, in the form of neutral and charged dark scalars, and complex neutral scalars analogous to the SM kaon, with mass splittings determined by the mass and charge of the dark quarks. The model finds a natural portal to the visible sector via kinetic coupling of the dark and visible photons, and consequent millicharges for dark matter. The dark matter can be a mixture of all these ultralight bosonic degrees of freedom, and exhibit both parity-even and parity-odd interactions, making the theory testable at a wide variety of experiments. In context of dark QCD with $N_f$ flavors of light quarks, this scenario predicts $N_f^2-1$ ultralight axion-like particles -- effectively an axiverse from dark QCD. This '$π$-axiverse' is consistent with but makes no recourse to string theory, and is complementary to the conventional string theory axiverse.

hep-ph↗
arXiv2023

Higgs Inflation and the Electroweak Gauge Sector

We introduce a new method that allows for the Higgs to be the inflaton. That is, we let the Higgs be a pseudo-Nambu-Goldstone (pNG) boson of a global coset symmetry $G/H$ that spontaneously breaks at an energy scale $\sim 4πf$ and give it a suitable $SU(2) \subset G$ Chern-Simons interaction, with $β$ the dimensionless Chern-Simons coupling strength and $f$ an $SU(2)$ decay constant. As a result, slow-roll inflation occurs via $SU(2)$-induced friction down a steep sinusoidal potential. In order to obey electroweak $SU(2)_{\rm L}\times U(1)_Y$ symmetry, the lowest-order Chern-Simons interaction is required to be quadratic in the Higgs with coupling strength $\propto β^2/f^2$. Higher-order interaction terms keep the full Lagrangian nearly invariant under the approximate pNG shift symmetry. Employing the simplest symmetry coset $SU(5)/SO(5)$, $N$ $e$-folds of inflation occur when $N \approx 60 \left(g/0.64\right)^2\left[β/\left(3\times 10^6\right)\right]^{8/3}\left[f/\left(5\times 10^{11}\ {\rm GeV}\right)\right]^{2/3}$, with $g$ the weak isospin gauge coupling constant. Small values of the decay constant, $f \lesssim 5 \times 10^{11} {\rm GeV}$, which are needed to address the Higgs hierarchy problem, are ruled out by electric dipole measurements and so successfully explaining inflation requires large $β$. We discuss possible methods to achieve such large couplings and other alternative Higgs inflation scenarios outside the standard modified-gravity framework.

hep-ph↗
arXiv2023

Inflation and Electroweak Symmetry Breaking

We present a model of Cosmological Electroweak Symmetry Breaking (CEWSB), where a Higgs-like field and a cosmological background of weak boson gauge fields interact with gravity to realize the epoch of cosmic inflation, which is then followed by a Higgs resonance preheating. As a result, the scale of electroweak symmetry breaking is linked with the end of inflation. The theory is equipped with a shift symmetry that can protect the Higgs mass, and it has close semblance to natural inflation and its variants. As the Higgs field's amplitude decays at the end of inflation, its mass emerges. The model has a built in Higgs self-resonance preheating mechanism which leads to the possible emergence of the cosmic microwave background (CMB) due to resonant Higgs, quark and lepton production after inflation. We provide a pathway to implement a similar mechanism with the realistic Higgs-doublet of the standard electroweak theory and discuss phenomenological considerations.

hep-ph↗