arXiv subjects

Neil Turok

Publications and source records attributed to Neil Turok.

At least 91 records · Page 5Linked to original sources

arXiv2003

Cosmological Perturbations in a Big Crunch/Big Bang Space-time

A prescription is developed for matching general relativistic perturbations across singularities of the type encountered in the ekpyrotic and cyclic scenarios i.e. a collision between orbifold planes. We show that there exists a gauge in which the evolution of perturbations is locally identical to that in a model space-time (compactified Milne mod Z_2) where the matching of modes across the singularity can be treated using a prescription previously introduced by two of us. Using this approach, we show that long wavelength, scale-invariant, growing-mode perturbations in the incoming state pass through the collision and become scale-invariant growing-mode perturbations in the expanding hot big bang phase.

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arXiv2003

Conditions for Generating Scale-Invariant Density Perturbations

We analyze the general conditions on the equation of state $w$ required for quantum fluctuations of a scalar field to produce a scale-invariant spectrum of density perturbations, including models which (in the four dimensional effective description) bounce from a contracting to an expanding phase. We show that there are only two robust cases: $w\approx -1$ (inflation) and $w \gg 1$ (the ekpyrotic/cyclic scenario). All other cases, including the $w \approx 0$ case considered by some authors, require extreme fine-tuning of initial conditions and/or the effective potential. For the ekpyrotic/cyclic ($w \gg 1$) case, we also analyze the small deviations from scale invariance.

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arXiv2002

Cosmological Perturbations Through a General Relativistic Bounce

The ekpyrotic and cyclic universe scenarios have revived the idea that the density perturbations apparent in today's universe could have been generated in a `pre-singularity' epoch before the big bang. These scenarios provide explicit mechanisms whereby a scale invariant spectrum of adiabatic perturbations may be generated without the need for cosmic inflation, albeit in a phase preceding the hot big bang singularity. A key question they face is whether there exists a unique prescription for following perturbations through the bounce, an issue which is not yet definitively settled. This goal of this paper is more modest, namely to study a bouncing Universe model in which neither General Relativity nor the Weak Energy Condition is violated. We show that a perturbation which is pure growing mode before the bounce does not match to a pure decaying mode perturbation after the bounce. Analytical estimates of when the comoving curvature perturbation varies around the bounce are given. It is found that in general it is necessary to evaluate the evolution of the perturbation through the bounce in detail rather than using matching conditions.

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arXiv2002

Is Vacuum Decay Significant in Ekpyrotic and Cyclic Models?

It has recently been argued that bubble nucleation in ekpyrotic and cyclic cosmological scenarios can lead to unacceptable inhomogeneities unless certain constraints are satisfied. In this paper we show that this is not the case. We find that bubble nucleation is completely negligible in realistic models.

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arXiv2002

A Cyclic Model of the Universe

We propose a cosmological model in which the universe undergoes an endless sequence of cosmic epochs each beginning with a `bang' and ending in a `crunch.' The temperature and density are finite at each transition from crunch to bang. Instead of having an inflationary epoch, each cycle includes a period of slow accelerated expansion (as recently observed) followed by slow contraction. The combination produces the homogeneity, flatness, density fluctuations and energy needed to begin the next cycle.

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arXiv2002

Quantum Fields in a Big Crunch/Big Bang Spacetime

We consider quantum field theory on a spacetime representing the Big Crunch/Big Bang transition postulated in the ekpyrotic or cyclic cosmologies. We show via several independent methods that an essentially unique matching rule holds connecting the incoming state, in which a single extra dimension shrinks to zero, to the outgoing state in which it re-expands at the same rate. For free fields in our construction there is no particle production from the incoming adiabatic vacuum. When interactions are included the total particle production for fixed external momentum is finite at tree level. We discuss a formal correspondence between our construction and quantum field theory on de Sitter spacetime.

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arXiv2002

Quantization of the massless minimally coupled scalar field and the dS/CFT correspondence

We consider the quantization of the massless minimally coupled scalar field in de Sitter spacetime. The no-boundary Euclidean prescription naturally picks out the de Sitter invariant vacuum state of Kirsten and Garriga. We extend Strominger's dS/CFT correspondence to this case which allows us to interpret the massless field in terms of a Euclidean CFT. The extension is non-trivial and requires careful treatment of the zero mode. Since the graviton is massless, this work may also be considered a step towards a theory of gravity in de Sitter space.

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arXiv2002

The Cyclic Universe: An Informal Introduction

The Cyclic Model is a radical, new cosmological scenario which proposes that the Universe undergoes an endless sequence of epochs which begin with a `big bang' and end in a `big crunch.' When the Universe bounces from contraction to re-expansion, the temperature and density remain finite. The model does not include a period of rapid inflation, yet it reproduces all of the successful predictions of standard big bang and inflationary cosmology. We point out numerous novel elements that have not been used previously which may open the door to further alternative cosmologies. Although the model is motivated by M-theory, branes and extra-dimensions, here we show that the scenario can be described almost entirely in terms of conventional 4d field theory and 4d cosmology.

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arXiv2002

Cosmic Evolution in a Cyclic Universe

Based on concepts drawn from the ekpyrotic scenario and M-theory, we elaborate our recent proposal of a cyclic model of the Universe. In this model, the Universe undergoes an endless sequence of cosmic epochs which begin with the Universe expanding from a `big bang' and end with the Universe contracting to a `big crunch.' Matching from `big crunch' to `big bang' is performed according to the prescription recently proposed with Khoury, Ovrut and Seiberg. The expansion part of the cycle includes a period of radiation and matter domination followed by an extended period of cosmic acceleration at low energies. The cosmic acceleration is crucial in establishing the flat and vacuous initial conditions required for ekpyrosis and for removing the entropy, black holes, and other debris produced in the preceding cycle. By restoring the Universe to the same vacuum state before each big crunch, the acceleration insures that the cycle can repeat and that the cyclic solution is an attractor.

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arXiv2002

Density Perturbations in the Ekpyrotic Scenario

We study the generation of density perturbations in the ekpyrotic scenario for the early universe, including gravitational backreaction. We expose interesting subtleties that apply to both inflationary and ekpyrotic models. Our analysis includes a detailed proposal of how the perturbations generated in a contracting phase may be matched across a `bounce' to those in an expanding hot big bang phase. For the physical conditions relevant to the ekpyrotic scenario, we re-obtain our earlier result of a nearly scale-invariant spectrum of energy density perturbations. We find that the perturbation amplitude is typically small, as desired to match observation.

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arXiv2002

From Big Crunch to Big Bang

We consider conditions under which a universe contracting towards a big crunch can make a transition to an expanding big bang universe. A promising example is 11-dimensional M-theory in which the eleventh dimension collapses, bounces, and re-expands. At the bounce, the model can reduce to a weakly coupled heterotic string theory and, we conjecture, it may be possible to follow the transition from contraction to expansion. The possibility opens the door to new classes of cosmological models. For example, we discuss how it suggests a major simplification and modification of the recently proposed ekpyrotic scenario.

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arXiv2001

Detecting magnetic CMB polarization on an incomplete sky

The full sky cosmic microwave background polarization field can be decomposed into `electric' and `magnetic' components. Working in harmonic space we construct magnetic variables that can be measured from observations over only a portion of the sky. Our construction is exact for azimuthally symmetric patches, but should continue to perform well for arbitrary patches. For isotropic, uncorrelated noise the variables have a very simple diagonal noise correlation, and further analysis using them should be no harder than analysing the temperature field. We estimate the tensor mode amplitude that could be detected by the Planck satellite and discuss the sensitivity of future experiments in the presence of a contaminating weak lensing signal.

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arXiv2001

Analysis of CMB polarization on an incomplete sky

The full sky cosmic microwave background polarization field can be decomposed into 'electric' and 'magnetic' components. Working in harmonic space we construct window functions that allow clean separation of the electric and magnetic modes from observations over only a portion of the sky. Our construction is exact for azimuthally symmetric patches, but should continue to perform well for arbitrary patches. From the window functions we obtain variables that allow for robust estimation of the magnetic component without risk of contamination from the probably much larger electric signal. For isotropic, uncorrelated noise the variables have a very simple diagonal noise correlation, and further analysis using them should be no harder than analysing the temperature field. For an azimuthally-symmetric patch, such as that obtained from survey missions when the galactic region is removed, the exactly-separated variables are fast to compute allowing us to estimate the magnetic signal that could be detected by the Planck satellite in the absence of non-galactic foregrounds. We also discuss the sensitivity of future experiments to tensor modes in the presence of a magnetic signal generated by weak lensing, and give lossless methods for analysing the electric polarization field in the case that the magnetic component is negligible.

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arXiv2001

Closed Universes from Cosmological Instantons

Current observational data is consistent with the universe being slightly closed. We investigate families of singular and non-singular closed instantons that could describe the beginning of a closed inflationary universe. We calculate the scalar and tensor perturbations that would be generated from singular instantons and compute the corresponding CMB power spectrum in a universe with cosmological parameters like our own. We investigate spatially homogeneous modes of the instantons, finding unstable modes which render the instantons sub-dominant contributions in the path integral. We show that a suitable condition may be imposed on singular closed instantons, constraining their instabilities. With this constraint these instantons can provide a suitable model of the early universe, and predict CMB power spectra in close agreement with the predictions of slow-roll inflation.

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arXiv2001

The Ekpyrotic Universe: Colliding Branes and the Origin of the Hot Big Bang

We propose a cosmological scenario in which the hot big bang universe is produced by the collision of a brane in the bulk space with a bounding orbifold plane, beginning from an otherwise cold, vacuous, static universe. The model addresses the cosmological horizon, flatness and monopole problems and generates a nearly scale-invariant spectrum of density perturbations without invoking superluminal expansion (inflation). The scenario relies, instead, on physical phenomena that arise naturally in theories based on extra dimensions and branes. As an example, we present our scenario predominantly within the context of heterotic M-theory. A prediction that distinguishes this scenario from standard inflationary cosmology is a strongly blue gravitational wave spectrum, which has consequences for microwave background polarization experiments and gravitational wave detectors.

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arXiv2001

Halo Formation in Warm Dark Matter Models

Discrepancies have emerged between the predictions of standard cold dark matter (CDM) theory and observations of clustering on sub-galactic scales. Warm dark matter (WDM) is a simple modification of CDM in which the dark matter particles have initial velocities due either to their having decoupled as thermal relics, or having been formed via non-equilibrium decay. We investigate the nonlinear gravitational clustering of WDM with a high resolution N-body code, and identify a number of distinctive observational signatures. Relative to CDM, halo concentrations and core densities are lowered, core radii are increased, and large halos emerge with far fewer low mass satellites. The number of small halos is suppressed, and those present are formed by `top down' fragmentation of caustics, as part of a `cosmic web' connecting massive halos. Few small halos form outside this web. If we identify small halos with dwarf galaxies, their number, spatial distribution, and formation epoch appear in better agreement with the observations for WDM than they are for CDM.

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arXiv2001

A Brief Comment on "The Pyrotechnic Universe"

We respond to the criticisms by Kallosh, Kofman and Linde concerning our proposal of the ekpyrotic universe scenario. We point out a number of errors in their considerations and argue that, at this stage, the ekpyrotic model is a possible alternative to inflationary cosmology as a description of the very early universe.

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arXiv2001

Visible Branes with Negative Tension in Heterotic M-Theory

It is shown that there exist large classes of BPS vacua in heterotic M-theory which have negative tension on the visible orbifold plane, positive tension on the hidden plane and positive tension, physical five-branes in the bulk space. Explicit examples of such vacua are presented. Furthermore, it is demonstrated that the ratio, beta/|alpha|, of the bulk five-brane tension to the visible plane tension can, for several large classes of such vacua, be made arbitrarily small. Hence, it is straightforward to find vacua with the properties required in the examples of the Ekpyrotic theory of cosmology - a visible brane with negative tension and beta/|alpha| small. This contradicts recent claims in the literature.

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