I'm planning to put summaries on this page of the work that I've been doing recently, but for now here are the abstracts and links to the preprints.

New no-go theorems for cosmic acceleration with extra dimensions (short)
Oxidised dark energy (long)

arXiv:0802.2106 (short)
arXiv:0802.3214 (long)

Daniel H. Wesley.

Abstract: We give detailed proofs of several new no-go theorems for producing flat four-dimensional accelerating cosmologies from warped dimensional reduction. These new theorems improve upon previous ones by weakening the energy conditions, by including time-dependent compactifications, and by treating accelerated expansion that is not precisely de Sitter. We show that de Sitter expansion violates the higher-dimensional null energy condition (NEC) if the compactification manifold M is one-dimensional, if its intrinsic Ricci scalar R vanishes everywhere, or if R and the warp function satisfy a simple limit condition. If expansion is not de Sitter, we establish threshold equation-of-state parameters w below which accelerated expansion must be transient. Below the threshold w there are bounds on the number of e-foldings of expansion. If M is one-dimensional or R everywhere vanishing, exceeding the bound implies the NEC is violated. If R does not vanish everywhere on M, exceeding the bound implies the strong energy condition (SEC) is violated. Observationally, the w thresholds indicate that experiments with finite resolution in w can cleanly discriminate between different models which satisfy or violate the relevant energy conditions.

Magnetogenesis from cosmic string loops

Journal of Cosmology and Astroparticle Physics 02 (2008) 001.
arXiv:0708.2901

Diana Battefeld, Thorsten Battefeld, Daniel H. Wesley and Mark Wyman.

Abstract: Large-scale coherent magnetic fields are observed in galaxies and clusters, but their ultimate origin remains a mystery. We reconsider the prospects for primordial magnetogenesis by a cosmic string network. We show that the magnetic flux produced by long strings has been overestimated in the past, and give improved estimates. We also compute the fields created by the loop population, and find that it gives the dominant contribution to the total magnetic field strength on present-day galactic scales. We present numerical results obtained by evolving semi-analytic models of string networks (including both one-scale and velocity-dependent one-scale models) in a $\Lambda$CDM cosmology, including the forces and torques on loops from Hubble redshifting, dynamical friction, and gravitational wave emission. Our predictions include the magnetic field strength as a function of correlation length, as well as the volume covered by magnetic fields. We conclude that string networks could account for magnetic fields on galactic scales, but only if coupled with an efficient dynamo amplification mechanism.

Scale-invariance in expanding and contracting universes from two-field models

Journal of Cosmology and Astroparticle Physics 05 (2007) 006.
arXiv:hep-th/0703101

Andrew J. Tolley and Daniel H. Wesley.

Abstract: We study cosmological perturbations produced by the most general two-derivative actions involving two scalar fields, coupled to Einstein gravity, with an arbitrary field space metric, that admit scaling solutions. For expanding universes, we find that scale-invariant adiabatic perturbation spectra can be produced for any equation of state parameter $w$ that satisfies $-1 \le w < -1/3$, and that all the scaling solutions are attractors. For contracting universes, we show that scale-invariant adiabatic perturbations can be produced continuously as modes leave the horizon for any $w>-1/3$. The corresponding background solutions are unstable, which we argue is a universal feature of contracting models that yield scale-invariant spectra. At no point do we assume slow-roll. The presence of a nontrivial metric on field space is a crucial ingredient in our results.

Observing cosmic string loops with gravitational lensing surveys

Physical Review D76 (2007) 123515.
arXiv:astro-ph/0702648

Katherine J. Mack, Daniel H. Wesley and Lindsay J. King.

Abstract: We show that the existence of cosmic strings can be strongly constrained by the next generation of gravitational lensing surveys at radio frequencies. We focus on cosmic string loops, which simulations suggest would be far more numerous than long (horizon-sized) strings. Using simple models of the loop population and minimal assumptions about the lensing cross section per loop, we estimate the optical depth to lensing and show that extant radio surveys such as CLASS have already ruled out a portion of the cosmic string model parameter space. Future radio interferometers, such as LOFAR and especially SKA, may constrain $G\mu/c^2 < 10^{-9}$ in some regions of parameter space, outperforming current constraints from pulsar timing and the CMB by up to two orders of magnitude. This method relies on direct detections of cosmic strings, and so is less sensitive to the theoretical uncertainties in string network evolution that weaken other constraints.

Kac-Moody Algebras and Controlled Chaos

Classical and Quantum Gravity 24 (2007) F7-F13.
arXiv:hep-th/0610322

Daniel H. Wesley.

Abstract: Compactification can control chaotic Mixmaster behavior in gravitational systems with p-form matter: we consider this in light of the connection between supergravity models and Kac-Moody algebras. We show that different compactifications define "mutations" of the algebras associated with the noncompact theories. We list the algebras obtained in this way, and find novel examples of wall systems determined by Lorentzian (but not hyperbolic) algebras. Cosmological models with a smooth pre-big bang phase require that chaos is absent: we show that compactification alone cannot eliminate chaos in the simplest compactifications of the heterotic string on a Calabi-Yau, or M theory on a manifold of G_2 holonomy.

String pair production in a time-dependent gravitational field

Physical Review D72 (2005) 124009.
arXiv:hep-th/0509151

Andrew J. Tolley and Daniel H. Wesley

Abstract: We study the pair creation of point-particles and strings in a time-dependent, weak gravitational field. We find that, for massive string states, there are surprising and significant differences between the string and point-particle results. Central to our approach is the fact that a weakly curved spacetime can be represented by a coherent state of gravitons, and therefore we employ standard techniques in string perturbation theory. String and point-particle pairs are created through tree-level interactions between the background gravitons. In particular, we focus on the production of excited string states and perform explicit calculations of the production of a set of string states of arbitrary excitation level. The differences between the string and point-particle results may contain important lessons for the pair production of strings in the strong gravitational fields of interest in cosmology and black hole physics.

Controlling chaos through compactification in cosmological models with a collapsing phase

Physical Review D72 (2005) 063413.
arXiv:hep-th/0502108

Daniel H. Wesley, Paul J. Steinhardt, and Neil Turok

Abstract: We consider the effect of compactification of extra dimensions on the onset of classical chaotic "Mixmaster" behavior during cosmic contraction. Assuming a universe that is well-approximated as a four-dimensional Friedmann-Robertson--Walker model (with negligible Kaluza-Klein excitations) when the contraction phase begins, we identify compactifications that allow a smooth contraction and delay the onset of chaos until arbitrarily close the big crunch. These compactifications are defined by the de Rham cohomology (Betti numbers) and Killing vectors of the compactification manifold. We find compactifications that control chaos in vacuum Einstein gravity, as well as in string theories with N = 1 supersymmetry and M-theory. In models where chaos is controlled in this way, the universe can remain homogeneous and flat until it enters the quantum gravity regime. At this point, the classical equations leading to chaotic behavior can no longer be trusted, and quantum effects may allow a smooth approach to the big crunch and transition into a subsequent expanding phase. Our results may be useful for constructing cosmological models with contracting phases, such as the ekpyrotic/cyclic and pre-big bang models.

CMB observations with a compact heterogeneous 150-GHz interferometer in Chile

Astrophysics Journal Supplement 156 (2005) 1-11.
arXiv:astro-ph/0403137

J. W. Fowler et. al

Abstract: We report on the design, first observing season, and analysis of data from a new prototype millimeter-wave interferometer, MINT. MINT consists of four 145 GHz SIS mixers operating in double-sideband mode in a compact heterogeneous configuration. The signal band is subdivided by a monolithic channelizer, after which the correlations between antennas are performed digitally. The typical receiver sensitivity in a 2 GHz band is 1.4 mK sqrt(s). MINT observed the cosmic microwave background (CMB) from the Chilean Altiplano. The site has a median nighttime atmospheric temperature of 9 K at zenith (exclusive of the CMB). Observations of Mars, Jupiter, and a telescope-mounted calibration source establish the system's phase and magnitude stability. MINT is the first CMB-dedicated interferometer to operate above 50 GHz. The same type of system can be used to probe the Sunyaev-Zel'dovich effect in galaxy clusters near the SZ null at 217 GHz. We present an analysis of sideband-separated, digitally sampled data recorded by the array. Based on 215 hours of data taken in late 2001, we set an upper limit on the CMB anisotropy in a band of width Delta ell=700 around ell=1540 of delta T < 105 microK (95% conf). Increased sensitivity can be achieved with more integration time, greater bandwidth, and more elements.

Kasner and mixmaster behavior in universes with equation of state w >= 1

Joel K. Erickson, Daniel H. Wesley, Paul J. Steinhardt, and Neil Turok

Physical Review D69 (2004) 063514.
arXiv:hep-th/0312009

Abstract: We consider cosmological models with a scalar field with equation of state $w\ge 1$ that contract towards a big crunch singularity, as in recent cyclic and ekpyrotic scenarios. We show that chaotic mixmaster oscillations due to anisotropy and curvature are suppressed, and the contraction is described by a homogeneous and isotropic Friedmann equation if $w>1$. We generalize the results to theories where the scalar field couples to p-forms and show that there exists a finite value of $w$, depending on the p-forms, such that chaotic oscillations are suppressed. We show that $Z_2$ orbifold compactification also contributes to suppressing chaotic behavior. In particular, chaos is avoided in contracting heterotic M-theory models if $w>1$ at the crunch.