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J. Magueijo

Publications and source records attributed to J. Magueijo.

16 recordsLinked to original sources
arXiv2010

Non-adiabatic primordial fluctuations

We consider general mixtures of isocurvature and adiabatic cosmological perturbations. With a minimal assumption set consisting of the linearized Einstein equations and a primordial perfect fluid we derive the second-order action and its curvature variables. We also allow for varying equation of state and speed of sound profiles. The derivation is therefore carried out at the same level of generality that has been achieved for adiabatic modes before. As a result we find a new conserved super-horizon quantity and relate it to the adiabatically conserved curvature perturbation. Finally we demonstrate how the formalism can be applied by considering a Chaplygin gas-like primordial matter model, finding two scale-invariant solutions for structure formation.

astro-ph.CO↗
arXiv2010

Primordial fluctuations without scalar fields

We revisit the question of whether fluctuations in hydrodynamical, adiabatical matter could explain the observed structures in our Universe. We consider matter with variable equation of state $w=p_0/\ep_0$ and a concomitant (under the adiabatic assumption) density dependent speed of sound, $c_s$. We find a limited range of possibilities for a set up when modes start inside the Hubble radius, then leaving it and freezing out. For expanding Universes, power-law $w(\ep_0)$ models are ruled out (except when $c_s^2\propto w \ll 1$, requiring post-stretching the seeded fluctuations); but sharper profiles in $c_s$ do solve the horizon problem. Among these, a phase transition in $c_s$ is notable for leading to scale-invariant fluctuations if the initial conditions are thermal. For contracting Universes all power-law $w(\ep_0)$ solve the horizon problem, but only one leads to scale-invariance: $w\propto \ep_0^2$ and $c_s\propto \ep_0$. This model bypasses a number of problems with single scalar field cyclic models (for which $w$ is large but constant).

astro-ph.CO↗
arXiv2005

Lorentz invariance for mixed neutrinos

We show that a proper field theoretical treatment of mixed (Dirac) neutrinos leads to non-trivial dispersion relations for the flavor states. We analyze such a situation in the framework of the non-linear relativity schemes recently proposed by Magueijo and Smolin. We finally examine the experimental implications of our theoretical proposals by considering the spectrum and the end-point of beta decay in tritium.

hep-ph↗
arXiv2002

Variations of Alpha in Space and Time

We study inhomogeneous cosmological variations in the fine structure 'constant', $α,$ in Friedmann universes. Inhomogeneous motions of the scalar field driving changes in $α$ display spatial oscillations that decrease in amplitude with increasing time. The inhomogeneous evolution quickly approaches that found for exact Friedmann universes. We prove a theorem to show that oscillations of $α$ in time (or redshift) cannot occur in Friedmann universes in the BSBM theories considered here.

astro-ph↗
arXiv2002

Estimate of the Cosmological Bispectrum from the MAXIMA-1 Cosmic Microwave Background Map

We use the measurement of the cosmic microwave background taken during the MAXIMA-1 flight to estimate the bispectrum of cosmological perturbations. We propose an estimator for the bispectrum that is appropriate in the flat sky approximation, apply it to the MAXIMA-1 data and evaluate errors using bootstrap methods. We compare the estimated value with what would be expected if the sky signal were Gaussian and find that it is indeed consistent, with a $χ^2$ per degree of freedom of approximately unity. This measurement places constraints on models of inflation.

astro-ph↗
arXiv2002

A Cosmological Tale of Two Varying Constants

We formulate a simple extension of general relativity which incorporates space-time variations in the Newtonian gravitation 'constant', $G$, and the fine structure 'constant', $α$, which generalises Brans-Dicke theory and our theory of varying $α.$ We determine the behaviour of Friedmann universes in this theory. In the radiation and dust-dominated eras $αG$ approaches a constant value and the rate of variation of $α$ is equal to the magnitude of the rate of variation in $G$. The expansion dynamics of the universe are dominated by the variation of $G$ but the variation of $G$ has significant effects upon the time variation of $α.$ Time variations in $α$ are extinguished by the domination of the expansion by spatial curvature or quintessence fields, as in the case with no $G$ variation.

astro-ph↗
arXiv2002

Anthropic Reasons for Non-Zero Flatness and Lambda

In some cosmological theories with varying constants there are anthropic reasons why the expansion of the universe must not be too {\it close} to flatness or the cosmological constant too close to zero. Using exact theories which incorporate time-variations in $α$ and in $G$ we show how the presence of negative spatial curvature and a positive cosmological constant play an essential role in bringing to an end variations in the scalar fields driving time change in these 'constants' during any dust-dominated era of a universe's expansion. In spatially flat universes with $Λ=0$ the fine structure constant grows to a value which makes the existence of atoms impossible.

astro-ph↗
arXiv2001

A Simple Varying-alpha Cosmology

We investigate the cosmological consequences of a simple theory in which the electric charge $e$ is allowed to vary. The theory is locally gauge and Lorentz invariant, and satisfies general covariance. We find that in this theory the fine structure 'constant', $α$, remains almost constant in the radiation era, undergoes small increase in the matter era, but approaches a constant value when the universe starts accelerating because of the presence of a positive cosmological constant. This model satisfies geonuclear, nucleosynthesis, and CMB constraints on time-variation in $α$, while fitting simultaneously the observed accelerating universe and the recent high-redshift evidence for small $α$ variations in quasar spectra. The model also places specific restrictions on the nature of the dark matter. Further tests, involving stellar spectra and the Eötvös experiment, are proposed.

astro-ph↗
arXiv2001

The Behaviour of Varying-Alpha Cosmologies

We determine the behaviour of a time-varying fine structure 'constant' $α(t)$ during the early and late phases of universes dominated by the kinetic energy of changing $α(t)$, radiation, dust, curvature, and lambda, respectively. We show that after leaving an initial vacuum-dominated phase during which $α$ increases, $α$ remains constant in universes like our own during the radiation era, and then increases slowly, proportional to a logarithm of cosmic time, during the dust era. If the universe becomes dominated by negative curvature or a positive cosmological constant then $α$ tends rapidly to a constant value. The effect of an early period of de Sitter or power-law inflation is to drive $α$ to a constant value. Various cosmological consequences of these results are discussed with reference to recent observational studies of the value of $α$ from quasar absorption spectra and to the existence of life in expanding universes.

astro-ph↗
arXiv2001

Nielsen-Olesen vortex in varying-alpha theories

We consider soliton solutions to Bekenstein's theory, for which the fine structure constant $α=e^2/(4π\hbar c)$ is allowed to vary due to the presence of a dielectric field pervading the vacuum. More specifically we investigate the effects of a varying $α$ upon a complex scalar field with a U(1) electromagnetic gauge symmetry subject to spontaneous symmetry breaking. We find vortex solutions to this theory, similar to the Nielsen-Olesen vortex. Near the vortex core the electric charge is typically much larger than far away from the string, lending these strings a superconducting flavour. In general the dielectric field coats the usual local string with a global string envelope. We discuss the cosmological implications of networks of such strings, with particular emphasis on their ability to generate inhomogeneous recombination scenarios. We also consider the possibility of the dielectric being a charged free field. Even though the vacuum of such a field is trivial, we find that the dielectric arranges itself in the shape of a local string, with a quantized magnetic flux at the core -- presumably borrowing these topological features from the underlying Nielsen-Olesen vortex.

hep-ph↗
arXiv2000

The Complete Bispectrum of COBE-DMR Four Year Maps

We extend a previous bispectrum analysis of the COBE-DMR 4 year maps, allowing for the presence of correlations between all possible angular scales. We find that the non-Gaussian signal found earlier for bispectrum components including adjacent modes does not extend to triplets of modes with larger separations. Indeed for all separations $Δ\ell >1$ we find that the COBE-DMR data is very Gaussian. The implication seems to be that the previously detected non-Gaussian scale-scale correlation falls off very quickly with mode separation.

astro-ph↗
arXiv1999

Photographing the wave function of the Universe

We show that density fluctuations in standard inflationary scenarios may take the most general non-Gaussian distribution if the wave function of the Universe is not in the ground state. We adopt the Schrödinger picture to find a remarkable similarity between the most general inflaton wavefunction and the Edgeworth expansion used in probability theory. Hence we arrive at an explicit relation between the cumulants of the density fluctuations and the amplitudes or occupation numbers of the various energy eigenstates. For incoherent superpositions only even cumulants may be non-zero, but coherent superpositions may generate non-zero odd cumulants as well. Within this framework measurements of cumulants in Galaxy surveys directly map the wavefunction of the Universe.

astro-ph↗
arXiv1999

A Bayesian estimate of the skewness of the Cosmic Microwave Background

We propose a formalism for estimating the skewness and angular power spectrum of a general Cosmic Microwave Background data set. We use the Edgeworth Expansion to define a non-Gaussian likelihood function that takes into account the anisotropic nature of the noise and the incompleteness of the sky coverage. The formalism is then applied to estimate the skewness of the publicly available 4 year Cosmic Background Explorer (COBE) Differential Microwave Radiometer data. We find that the data is consistent with a Gaussian skewness, and with isotropy. Inclusion of non Gaussian degrees of freedom has essentially no effect on estimates of the power spectrum, if each $C_\ell$ is regarded as a separate parameter or if the angular power spectrum is parametrized in terms of an amplitude (Q) and spectral index (n). Fixing the value of the angular power spectrum at its maxiumum likelihood estimate, the best fit skewness is $S=6.5\pm6.0\times10^4(\muK)^3$; marginalizing over Q the estimate of the skewness is $S=6.5\pm8.4\times10^4(\muK)^3$ and marginalizing over n one has $S=6.5\pm8.5\times10^4(\muK)^3$.

astro-ph↗
arXiv1999

The 4 Year COBE DMR data is non-Gaussian

I review our recent claim that there is evidence of non-Gaussianity in the 4 Year COBE DMR data. I describe the statistic we apply, the result we obtain and make a detailed list of the systematics we have analysed. I finish with a qualitative understanding of what it might be and its implications.

astro-ph↗
arXiv1998

Evidence for non-Gaussianity in the CMB

In a recent Letter we have shown how COBE-DMR maps may be used to disprove Gaussianity at a high confidence level. In this report we digress on a few issues closely related to this Letter. We present the general formalism for surveying non-Gaussianity employed. We present a few more tests for systematics. We wonder about the theoretical implications of our result.

astro-ph↗