Stephon Alexander
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View article: Can weak-gravity, causality-violation arguments constrain modified gravity?
Can weak-gravity, causality-violation arguments constrain modified gravity? Open
We investigate limitations of causality arguments from flat-spacetime amplitudes, based on the eikonal limit of gravitational scattering, to place constraints on modified gravity. We show that causality constraints are only valid in the we…
View article: Quantum excitation transfer in an artificial photosynthetic light-harvesting system
Quantum excitation transfer in an artificial photosynthetic light-harvesting system Open
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 excit…
View article: Can weak-gravity, causality-violation arguments constrain modified gravity?
Can weak-gravity, causality-violation arguments constrain modified gravity? Open
We investigate limitations of causality arguments from flat-spacetime amplitudes, based on the eikonal limit of gravitational scattering, to place constraints on modified gravity. We show that causality constraints are only valid in the we…
View article: The Cosmological Constant from a Quantum Gravitational $θ$-Vacua and the Gravitational Hall Effect
The Cosmological Constant from a Quantum Gravitational $θ$-Vacua and the Gravitational Hall Effect Open
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 a…
View article: Kinetically Coupled Dark Matter Condensates
Kinetically Coupled Dark Matter Condensates Open
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 $-$ pseudoscalar…
View article: Birefringence in fermion-attenuated gravitational wave power spectrum
Birefringence in fermion-attenuated gravitational wave power spectrum Open
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…
View article: Higgs Inflation and the Electroweak Gauge Sector
Higgs Inflation and the Electroweak Gauge Sector Open
We introduce a method that allows the Higgs to be the inflaton. The Higgs is considered as a pseudo‐Nambu‐Goldstone (pNG) boson of a global coset symmetry , which is spontaneously breaks at an energy scale . A suitable Chern−Simons (CS) in…
View article: Vacuum Amplification of Chiral Gravitational Waves and the Stochastic Gravitational Wave Background
Vacuum Amplification of Chiral Gravitational Waves and the Stochastic Gravitational Wave Background Open
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 it…
View article: de Sitter State in Heterotic String Theory
de Sitter State in Heterotic String Theory Open
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 suc…
View article: A path integral formula of quantum gravity emergent from entangled local structures
A path integral formula of quantum gravity emergent from entangled local structures Open
A bstract 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 act…
View article: Nontriviality of dynamical Chern-Simons gravity and the standard model
Nontriviality of dynamical Chern-Simons gravity and the standard model Open
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 gravity persists when …
View article: Field theory axiverse
Field theory axiverse Open
Axion and axionlike particles 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 ax…
View article: A Dark Matter Fermionic Quantum Fluid from Standard Model Dynamics
A Dark Matter Fermionic Quantum Fluid from Standard Model Dynamics Open
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 sector…
View article: The Field Theory Axiverse
The Field Theory Axiverse Open
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 …
View article: A realist interpretation of unitarity in quantum gravity
A realist interpretation of unitarity in quantum gravity Open
Unitarity is a difficult concept to implement in canonical quantum gravity because of state non-normalisability and the problem of time. We take a realist approach based on pilot-wave theory to address this issue in the Ashtekar formulatio…
View article: The Non-triviality of Dynamical Chern-Simons Gravity and the Standard Model
The Non-triviality of Dynamical Chern-Simons Gravity and the Standard Model Open
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 formulat…
View article: Spacetime geometry of acoustics and electromagnetism
Spacetime geometry of acoustics and electromagnetism Open
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. Aco…
View article: The Kalb-Ramond field and Gravitational Parity Violation
The Kalb-Ramond field and Gravitational Parity Violation Open
Gravitational wave physics can probe theories that extend beyond General Relativity. Motivated by the attention on the Kalb-Ramond field as a dark matter candidate, in this work, we study a parity violating dimension four operator which co…
View article: UV completions of Chern-Simons gravity
UV completions of Chern-Simons gravity Open
Dynamical Chern-Simons (dCS) gravity is a four-dimensional parity-violating extension of general relativity. The standard mechanism to obtain this extension predicts negligible observational effects due to a large decay constant $f$ close …
View article: Parity-violating trispectrum from Chern-Simons gravity
Parity-violating trispectrum from Chern-Simons gravity Open
A potential source for parity violation in the Universe is inflation. The simplest inflationary models have two fields: the inflaton and graviton, and the lowest-order parity-violating coupling between them is dynamical Chern-Simons (dCS) …
View article: A Realist Interpretation of Unitarity in Quantum Gravity
A Realist Interpretation of Unitarity in Quantum Gravity Open
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 formulatio…
View article: What can galaxy shapes tell us about physics beyond the standard model?
What can galaxy shapes tell us about physics beyond the standard model? Open
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 …
View article: Domain Adaptation for Simulation-based Dark Matter Searches with Strong Gravitational Lensing
Domain Adaptation for Simulation-based Dark Matter Searches with Strong Gravitational Lensing Open
The identity of dark matter has remained surprisingly elusive. While terrestrial experiments may be able to nail down a model, an alternative method is to identify dark matter based on astrophysical or cosmological signatures. A particular…
View article: An inner product for 4D quantum gravity and the Chern–Simons–Kodama state
An inner product for 4D quantum gravity and the Chern–Simons–Kodama state Open
We demonstrate that reality conditions for the Ashtekar connection imply a non-trivial measure for the inner product of gravitational states in the polarization where the Ashtekar connection is diagonal, and we express the measure as the d…
View article: Higgs Inflation and the Electroweak Gauge Sector
Higgs Inflation and the Electroweak Gauge Sector Open
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 …
View article: Spacetime geometry of acoustics and electromagnetism
Spacetime geometry of acoustics and electromagnetism Open
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. Aco…
View article: The $π$-axion and $π$-axiverse of dark QCD
The $π$-axion and $π$-axiverse of dark QCD Open
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,…
View article: A path integral formula of quantum gravity emergent from entangled local structures
A path integral formula of quantum gravity emergent from entangled local structures Open
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. Inspe…
View article: de Sitter State in Heterotic String Theory
de Sitter State in Heterotic String Theory Open
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 suc…