arXiv subjects

Stephon H. S. Alexander

Publications and source records attributed to Stephon H. S. Alexander.

14 recordsLinked to original sources

arXiv2016

Inflationary Birefringence and Baryogenesis

A decade ago, the first leptogenesis model based on inflation was proposed, where the complex phase of the inflaton field carries lepton number. If the inflaton field is an axion, it can couple to gravitational waves and gauge fields via. Chern-Simons invariants. Due to these couplings, birefringent gravitational and gauge primordial perturbations are created during inflation to generate a lepton asymmetry, establishing a possible connection between non-vanishing TB-parity violating polarization cross-correlations and leptogenesis. We also discuss the prospect for a subset of these models can directly source circular (V-mode) polarization in the CMB.

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arXiv2014

Leptogenesis from Gravity Waves in Models of Inflation

We present a new mechanism for creating the observed cosmic matter-antimatter asymmetry which satisfies all three Sakharov conditions from one common thread, gravitational waves. We generate lepton number through the gravitational anomaly in the lepton number current. The source term comes from elliptically polarizated gravity waves that are produced during inflation if the inflaton field contains a CP-odd component. In simple inflationary scenarios, the generated matter asymmetry is very small. We describe some special conditions in which our mechanism can give a matter asymmetry of realistic size.

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arXiv2013

The Sterile Neutrino Field and Late Time Acceleration

We show that when the neutrino minimal standard model ($νMSM$) with a light sterile neutrino has a long range interaction between the neutrino vacuum charge density and a dark U(1) gauge potential, late time acceleration is dynamically realized. Moreover, the relation between the scale of dark energy, the weak and Planck masses derived by Arkani-Hamed et. al \cite{ArkaniHamed:2000tc} that addresses the coincidence problem arises naturally in this model. Assuming that the cosmological constant is zero by some as yet known mechanism, the only fine tuning in our model is the mass of the sterile neutrino. We show that perturbations of the vector fields are oscillatory and hence, there are no instabilities.

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arXiv2009

A Theory of Neutrino Oscillations and Late Time Acceleration

We provide a microphysical model that connects neutrino oscillations to dark energy, which has predictions of Lorentz and CPT violating neutrino oscillations. We argue that the DE is a BCS condensate of flavored neutrinos. As neutrinos propigate in their own condensate they naturally have oscillations proportional to the DE energy density. All that is assumed in this model is a covariant coupling of neutrinos to gravity and a finite number density of neutrinos in the cosmic rest frame; this situation yields an attractive channel for the formation of a spin zero neutrino condensate leading to late time acceleration self consistently. Moreover, the vacuum oscillation has two predictions: (1) CPT violating oscillations proportional to the DE density (2) A connection between the evolution of the equation of state of the DE condensate and the neutrino oscillations. These predictions can be probed independently with future Supernovae and Neutrino Telescopes.

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arXiv2009

Superconducting loop quantum gravity and the cosmological constant

We argue that the cosmological constant is exponentially suppressed in a candidate ground state of loop quantum gravity as a nonperturbative effect of a holographic Fermi-liquid theory living on a two-dimensional spacetime. Ashtekar connection components, corresponding to degenerate gravitational configurations breaking large gauge invariance and CP symmetry, behave as composite fermions that condense as in Bardeen-Cooper-Schrieffer theory of superconductivity. Cooper pairs admit a description as wormholes on a de Sitter boundary.

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arXiv2008

Quantum gravity as a Fermi liquid

We present a reformulation of loop quantum gravity with a cosmological constant and no matter as a Fermi-liquid theory. When the topological sector is deformed and large gauge symmetry is broken, we show that the Chern-Simons state reduces to Jacobson's degenerate sector describing 1+1 dimensional propagating fermions with nonlocal interactions. The Hamiltonian admits a dual description which we realize in the simple BCS model of superconductivity. On one hand, Cooper pairs are interpreted as wormhole correlations at the de Sitter horizon; their number yields the de Sitter entropy. On the other hand, BCS is mapped into a deformed conformal field theory reproducing the structure of quantum spin networks. When area measurements are performed, Cooper-pair insertions are activated on those edges of the spin network intersecting the given area, thus providing a description of quantum measurements in terms of excitations of a Fermi sea to superconducting levels. The cosmological constant problem is naturally addressed as a nonperturbative mass-gap effect of the true Fermi-liquid vacuum.

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arXiv2007

Isogravity: Toward an Electroweak and Gravitational Unification

We present a model that unites the electroweak interaction with general relativity without specifying a space-time metric. This is made possible by embedding the kinetic terms for gravity and electroweak theory using one $\SL$ connection variable. The gauge theory is specified without relying on a space-time metric. We show that once a symmetry breaking mechanism is implemented that selects a global time-like direction, the electroweak theory and general relativity emerges with their associated massless degrees of freedom; the spin 1 vector boson and the spin 2 graviton.

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arXiv2006

Gravity Induced Chiral Condensate Formation and the Cosmological Constant

It is well known that the covariant coupling of fermionic matter to gravity induces a four-fermion interaction. The presence of this term in a homogenous and isotropic space-time results in a BCS-like Hamiltonian and the formation of a chiral condensate with a mass gap. We calculate the gap ($Δ$) via a mean-field approximation for minimally coupled fermionic fields in a FRW background and find that it depends on the scale factor. The calculation also yields a correction to the bare cosmological constant ($Λ_0$), and a non-zero vev for $<ψ^\dagψ>$ which then behaves as a scalar field. Hence we conjecture that the presence of fermionic matter in gravity provides a natural mechanism for relaxation of the $Λ_0$ and explains the existence of a scalar field from (almost) first principles.

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arXiv2006

Is Cosmic Parity Violation Responsible for the Anomalies in the WMAP Data?

This research demonstrates that parity violation in general relativity can simultaneously explain the observed loss in power and alignment at a preferred axis ('Axis of Evil')in the low multipole moments of the WMAP data. This observational possibility also provides an experimental window for an inflationary leptogenesis mechanism arising from large-scale parity violation. A velocity dependent potential is induced from gravitational backreaction, which modifies the primordial scalar angular power spectrum of density perturbations in the CMB. This modification suppresses power of odd parity multipoles on large scales which can be associated with the scale of a massive right-handed neutrino.

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arXiv2005

Can the string scale be related to the cosmic baryon asymmetry?

In a previous work, a mechanism was presented by which baryon asymmetry can be generated during inflation from elliptically polarized gravitons. Nonetheless, the mechanism only generated a realistic baryon asymmetry under special circumstances which requires an enhancement of the lepton number from an unspecified GUT. In this note we provide a stringy embedding of this mechanism through the Green-Schwarz mechanism, demonstrating that if the model-independent axion is the source of the gravitational waves responsible for the lepton asymmetry, one can observationally constrain the string scale and coupling.

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arXiv2004

Non-commutative geometry as a realization of varying speed of light cosmology

We examine the cosmological implications of space-time non-commutativity, discovering yet another realization of the varying speed of light model. Our starting point is the well-known fact that non-commutativity leads to deformed dispersion relations, relating energy and momentum, implying a frequency dependent speed of light. A Hot Big Bang Universe therefore experiences a higher speed of light as it gets hotter. We study the statistical physics of this "deformed radiation", recovering standard results at low temperatures, but a number of novelties at high temperatures: a deformed Planck's spectrum, a temperature dependent equation of state $w=p/ρ$ (ranging from 1/3 to infinity), a new Stephan-Boltzmann law, and a new entropy relation. These new photon properties closely mimic those of phonons in crystals, hardly a surprising analogy. They combine to solve the horizon and flatness problems, explaining also the large entropy of the Universe. We also show how one would find a direct imprint of non-commutativity in the spectrum of a cosmic graviton background, should it ever be detected.

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arXiv2002

Brane Gas Cosmology, M-theory and Little String Theory

We generalize the Brane Gas Cosmological Scenario to M-theory degrees of freedom, namely $M5$ and $M2$ branes. Without brane intersections, the Brandenberger Vafa(BV) arguments applied to M-theory degrees of freedom generically predict a large 6 dimensional spacetime. We show that intersections of $M5$ and $M2$ branes can instead lead to a large 4 dimensional spacetime. One dimensional intersections in 11D is related to (2,0) little strings (LST) on NS5 branes in type IIA. The gas regime of membranes in M-theory corresponds to the thermodynamics of LST obtained from holography. We propose a mechanism whereby LST living on the worldvolume of NS5 (M5)-branes wrapping a five dimensional torus, annihilate most efficiently in 3+1 dimensions leading to a large 3+1 dimensional spacetime. We also show that this picture is consistent with the gas approximation in M-theory.

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arXiv2001

Inflation from D-\bar{D} brane annihilation

We demonstrate that the initial conditions for inflation are met when D5 and \bar{D}5 branes annihilate. This scenario uses Sen's conjecture that a co-dimension two vortex forms on the worldvolume of the annihilated 5-brane system. Analogous to a "Big Bang", when the five branes annihilate, a vortex localized on a 3-brane forms and its false vacuum energy generates an inflationy space-time. We also provide a natural mechanism for ending inflation via the motion of the vortex in the bulk due to its extrinsic curvature. We also suggest a consistent way to end inflation and localize matter on our space-time.

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arXiv2000

On The Varying Speed of Light in a Brane-Induced FRW Universe

We investigate a string/M theoretic realization of the varying speed of light scenario. We consider a 3+1 dimensional probe-brane universe in the background of a black hole in the bulk formed by a stack of branes, in the spirit of Kiritsis (hep-th/9906206). We generalize the dynamics of the system at hand by including rotation and Hubble damping of the bulk space-time and show that this may lead to a mechanism to stabilize the brane-universe and hence fix the speed of light at late times.

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