arXiv subjects

Martin Bucher

Publications and source records attributed to Martin Bucher.

47 records · Page 3Linked to original sources

arXiv2001

Cosmological Perturbations Generated in the Colliding Bubble Braneworld Universe

We compute the cosmological perturbations generated in the colliding bubble braneworld universe in which bubbles filled with five-dimensional anti-de Sitter space (AdS5)expanding within a five dimensional de Sitter space (dS5) or Minkowski space (M5) collide to form a (3+1) dimensional local brane on which the cosmology is virtually identical to that of the Randall-Sundrum model. The perturbation calculation presented here is valid to linear order but treats the fluctuations of the expanding bubbles as (3+1) dimensional fields localized on the bubble wall. We find that for bubbles expanding in dS5 the dominant contribution to the power spectrum is `red' but very small except in certain cases where the fifth dimension is not large or the bubbles have expanded to far beyond the dS5 apparent horizon length. This paper supersedes a previous version titled "Exactly Scale-Invariant Cosmological Perturbations From a Colliding Bubble Braneworld Universe" in which we erroneously claimed that a scale-invariant spectrum results for the case of bubbles expanding in M5. This present paper corrects the errors of the previous version and extends the analysis to the more interesting and general case of bubbles expanding in dS5.

hep-th↗

arXiv2000

Primordial Isocurvature Perturbations: Testing the Adiabaticity of the CMB Anisotropy

Prospects for testing the adiabaticity of the primordial cosmological perturbations using MAP and PLANCK are evaluated. The most general cosmological perturbation in a universe with just baryons, photons, neutrinos, and a cold dark matter (CDM) component is described. In addition to the familiar adiabatic mode, there are four nonsingular isocurvature modes: a baryon isocurvature mode, a CDM isocurvature mode, a neutrino density isocurvature mode, and a neutrino velocity isocurvature mode. The most general perturbation is described by a 5x5, positive-definite, symmetric, matrix-valued function of wave number whose off-diagonal elements represent the correlations between the above mentioned modes. We found that when three modes and their correlations are admitted, the fractional uncertainties in the cosmological parameters and amplitudes of the isocurvature modes and their correlations become of order unity. These degeneracies, however, can be broken with the polarization information provided by PLANCK, reducing the uncertainties to below the ten percent level. Polarization is thus crucial to testing the adiabaticity of the primordial fluctuations.

astro-ph↗

arXiv2000

Characterising the Primordial Cosmic Perturbations

The most general homogeneous and isotropic statistical ensemble of linear scalar perturbations which are regular at early times, in a universe with only photons, baryons, neutrinos, and a cold dark matter (CDM) component, is described by a 5x5 symmetric matrix-valued generalization of the power spectrum. This description is complete if the perturbations are Gaussian, and even in the non-Gaussian case describes all observables quadratic in the small perturbations. The matrix valued power spectrum describes the auto- and cross-correlations of the adiabatic, baryon isocurvature, CDM isocurvature, neutrino density isocurvature, and neutrino velocity isocurvature modes. In this paper we examine the prospects for constraining or discovering isocurvature modes using forthcoming MAP and PLANCK measurements of the cosmic microwave background (CMB) anisotropy. We also consider the degradation in estimates of the cosmological parameters resulting from the inclusion of these modes. In the case of MAP measurements of the temperature alone, the degradation is drastic. When isocurvature modes are admitted, uncertainties in the amplitudes of the mode auto-- and cross--correlations, and in the cosmological parameters, become of order one. With the inclusion of polarisation (at an optimistic sensitivity) the situation improves for the cosmological parameters but the isocurvature modes are still only weakly constrained. Measurements with PLANCK's estimated errors are far more constraining, especially so with the inclusion of polarisation. If PLANCK operates as planned the amplitudes of isocurvature modes will be constrained to less than ten per cent of the adiabatic mode and simultaneously key cosmological parameters will be estimated to a few per cent or better.

astro-ph↗

arXiv1999

Domain Wall Dominated Universes

We consider a cosmogony with a dark matter component consisting of a network of frustrated domain walls. Such a network provides a solid dark matter component with $p=-(2/3)ρ$ that remains unclustered on small scales and with $Ω_{\rm dw}\approx 0.7$ can reconcile a spatially flat universe with the many observations indicating $Ω_{\rm m}\approx 0.3.$ Because of its large negative pressure, this component can explain the recent observations indicating an accelerating universe without recourse to a non-vanishing cosmological constant. We explore the viability of this proposal and prospects for distinguishing it from other kinds of proposed dark matter with significant negative pressure.

astro-ph↗

arXiv1999

Is the Dark Matter a Solid?

A smooth unclustered dark matter component with negative presure could reconcile a flat universe with the many observations that find a density in ordinary, clustered matter well below the critical density and also explain the recent high redshift supernova data suggesting that the expansion of the universe is now accelerating. For a perfect fluid negative presure leads to instabilities that are most severe on the shortest scales. However, if instead the dark matter is a solid, with an elastic resistance to pure shear deformations, an equation of state with negative presure can avoid these short wavelength instabilities. Such a solid may arise as the result of different kinds of microphysics. Two possible candidates for a solid dark matter component are a frustrated network of non-Abelian cosmic strings or a frustrated network of domain walls. If these networks settle down to an equilibrium configuration that gets carried along and stretched by the Hubble flow, equations of state result with $w=-1/3$ and $w=-2/3,$ respectively. One expects the sound speeds for the solid dark matter component to comprise an appreciable fraction of the speed of light. Therefore, the solid dark matter does not cluster, expect on the very largest scales, accessible only through observing the large-angle CMB anisotropy. In this paper we develop a generally-covariant, continuum description for the dynamics of a solid dark matter component. We derive the evolution equations for the cosmological perturbations in a flat universe with CDM+(solid) and compute the resulting large-angle CMB anisotropy. The formalism presented here applies to any generalized dark matter with negative pressure and a non-dissipative resistance to shear.

astro-ph↗

arXiv1996

Non-Gaussian Isocurvature Perturbations From Goldstone Modes Generated During Inflation

We investigate non-Gaussian isocurvature perturbations generated by the evolution of Goldstone modes during inflation. If a global symmetry is broken ``before'' inflation, the resulting Goldstone modes are disordered during inflation in a precise and predictable way. After inflation these Goldstone modes order themselves in a self-similar way, much as Goldstone modes in field ordering scenarios based on the Kibble mechanism. For $(H_{inf}^2/M_{pl}^2)\sim 10^{-6},$ through their gravitational interaction these Goldstone modes generate density perturbations of approximately the right magnitude to explain the cosmic microwave background (CMB) anisotropy and seed the structure seen in the universe today. We point out that for the pattern of symmetry breaking in which a global $U(1)$ is completely broken, the inflationary evolution of the Goldstone field may be treated as that of a massless scalar field. Unlike the more commonly discussed case in which a global $U(1)$ is completely broken in a cosmological phase transition, in the inflationary case the production of defects can be made exponentially small, so that Goldstone field evolution is completely linear. In such a model non-Gaussian perturbations result because to lowest order density perturbations are sourced by products of Gaussian fields. Consequently, in this non-Gaussian model N-point correlations may be calculated by evaluating Feynman diagrams. We explore the issue of phase dispersion and conclude that this non-Gaussian model predicts Doppler peaks in the CMB anisotropy.

astro-ph↗

arXiv1995

Open Inflation with Arbitrary False Vacuum Mass

We calculate the power spectrum of adiabatic density perturbations in an open inflationary model in which inflation occurs in two stages. First an epoch of old inflation creates a large, smooth universe, solving the horizon and homogeneity problems. Then an open universe emerges through the nucleation of a single bubble, with constant density hypersurfaces inside the bubble having constant negative spatial curvature. An epoch of `slow roll' inflation, shortened to give $Ω_0<1$ today, occurs within the bubble, which contains our entire observable universe. In this paper we compute the resulting density perturbations in the same `new thin wall' approximation used in a previous paper, but for an arbitrary positive mass of the inflaton field in the false vacuum.

hep-ph↗

arXiv1993

Embedded Defects

We give a prescription for embedding classical solutions and, in particular, topological defects in field theories which are invariant under symmetry groups that are not necessarily simple. After providing examples of embedded defects in field theories based on simple groups, we consider the electroweak model and show that it contains the $Z$ string and a one parameter family of strings called the $W(α)$ string. It is argued that, although the members of this family are gauge equivalent when considered in isolation, each member should be considered distinct when multi-string solutions are considered. We then turn to the issue of stability of embedded defects and demonstrate the instability of a large class of such solutions in the absence of bound states or condensates. The $Z$ string is shown to be unstable when the Weinberg angle ($θ_w$) is $π/4$ for all values of the Higgs mass. The $W$ strings are also shown to be unstable for a large range of parameters. Embedded monopoles suffer from the Brandt-Neri-Coleman instability. A simple physical understanding of this instability is provided in terms of the phenomenon of W-condensation. Finally, we connect the electroweak string solutions to the sphaleron: ``twisted'' loops of W string and finite segments of W and Z strings collapse into the sphaleron configuration, at least, for small values of $θ_w$.

hep-th↗

arXiv1993

Skyrmions and Semilocal Strings in Cosmology

It has been pointed out that cosmic string solutions can exist in gauge field theories with broken symmetry even when $π_1(G/H)$ is trivial. The stability of such semilocal defects is not guaranteed by topology and depends on dynamical considerations. In the literature it has been tacitly assumed that if stable, such strings would form in the Early Universe in a manner analogous to the formation of a network of more robust topologically-stable strings. In this paper we find that except for unnaturally small values of the correlation length, a network of semilocal strings does not form. Instead, delocalized skyrmionic string configurations, which expand with the Hubble flow, dominate.

hep-ph↗

arXiv1991

Topological Approach to Alice Electrodynamics

We analyze the unlocalized ``Cheshire charge'' carried by ``Alice strings.'' The magnetic charge on a string loop is carefully defined, and the transfer of magnetic charge from a monopole to a string loop is analyzed using global topological methods. A semiclassical theory of electric charge transfer is also described.

hep-th↗

arXiv1991

On Detecting Discrete Cheshire Charge

We analyze the charges carried by loops of string in models with non-abelian local discrete symmetry. The charge on a loop has no localized source, but can be detected by means of the Aharonov--Bohm interaction of the loop with another string. We describe the process of charge detection, and the transfer of charge between point particles and string loops, in terms of gauge--invariant correlation functions.

hep-th↗