arXiv subjects

Andrei Gruzinov

Publications and source records attributed to Andrei Gruzinov.

At least 73 records · Page 4Linked to original sources

arXiv2006

Force-Free Electrodynamics of Pulsars

We show that FFE (Force-Free Electrodynamics) describes pulsar magnetospheres -- even when radiation and particle wind power are comparable to the spin-down power. At the same time, we show that FFE is insufficient for calculating pulsar magnetospheres. This is because FFE admits a large family of stationary solutions with different currents flowing in the closed line region. To choose the actual solution one needs a model of current generation which goes beyond pure FFE. We calculate several FFE magnetospheres for the aligned rotator. The Poynting power of these solutions fills the range $c^{-3}μ^2Ω^4~< ~L~ <~ 0.67 c^{-3}μ^2Ω^4(c/ΩR_s)^2$, for angular velocity $Ω$ and magnetic field which is a pure dipole $μ$ on the surface of the star of radius $R_s$. Anomalous braking indices of young pulsars might be explained by currents flowing in the closed-line region.

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arXiv2005

Pulsar Magnetosphere: Variation Priciple, Singularities, Estimate of Power

We formulate variation principle for force-free magnetosphere of an inclined pulsar: ${\cal E} +{\bf Ω}\cdot {\bf M}$ ($\cal E$, ${\bf M}$ are electromagnetic energy and angular momentum, ${\bf Ω}$ is the angular velocity of a star) is stationary under isotopological variations of magnetic field and arbitrary variations of electric field. The variation principle gives the reason for existence and proves local stability of current singular layers along magnetic separatrices. Magnetic field lines of inclined pulsar magnetosphere lie on magnetic surfaces, and do have magnetic separatrices. In the framework of the isotopological variation principle, inclined magnetospheres are expected to be simple deformations of the axisymmetric pulsar magnetosphere. A singular line should exist on the light cylinder, where inner separatrix terminates and outer separatrix emanates. The electromagnetic field should have an inverse square root singularity near the singular line inside the inner magnetic separatrix. Large distance asymptotic solution is calculated, and used to estimate the pulsar power, $L\approx c^{-3}μ^2Ω^4$ for spin-dipole inclinations $\lesssim 30^{\circ}$ .

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arXiv2005

Power of a Turbulent Pulsar

We speculate that stationary force-free pulsar magnetospheres are screw unstable, and the spin-down power of real pulsars is carried by turbulent Poynting jets. The turbulent jet entrains poloidal flux as it propagates away from the star. Due to poloidal flux entrainment, poloidal field inside the jet decreases at a slower rate than the dipole's $r^{-3}$, and the pulsar power increases accordingly: $L\sim (μ^2Ω^4/c^3)(ΩR/c)^{-α}$, where $μ$ is the magnetic dipole, $Ω$ is the frequency, $R$ is the neutron star radius, and $α\lesssim 1$ is an index which depends on spin-dipole angle. Our speculation is of interest because it seems to provide the only possible explanation (without fine-tuning) of observed pulsar braking indices.

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arXiv2004

The Power of Axisymmetric Pulsar

Stationary force-free magnetosphere of an axisymmetric pulsar is shown to have a separatrix inclination angle of 77.3$^\circ$. The electromagnetic field has an $R^{-1/2}$ singularity inside the separatrix near the light cylinder. A numerical simulation of the magnetosphere which crudely reproduces these properties is presented. The numerical results are used to estimate the power of an axisymmetric pulsar: $L=(1\pm 0.1)μ^2Ω^4/c^3$. A need for a better numerical simulation is pointed out.

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arXiv2004

Consistency relation for single scalar inflation

Single scalar field inflation with a generic, non-quadratic in derivatives, field Lagrangian is considered. It is shown that non-Gaussianity of curvature perturbations is characterized by two dimensionless amplitudes. One of these amplitudes can be expressed in terms of the usual inflationary observables -- the scalar power, the tensor power, and the tensor index. This consistency relation provides an observational test for the single scalar inflation.

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arXiv2004

Dark Energy with w>-4/3

Acceleration of the universe might be driven by a continuous elastic medium -- elastic dark energy (Bucher and Spergel 1999). Elastic dark energy can stably support equations of state with pressure to energy ratio w > -4/3. Stable expansion with w<-1 combined with an assumption of ``reheating of elastic energy'' leads to exotic possibilities such as Expanding Cyclic Universe -- an ever-expanding universe with periodically repeating inflationary epochs.

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arXiv2004

Elastic Inflation

Inflation of a universe filled by an elastic continuous medium is considered. Elastic inflation, while capable of describing current observations, is qualitatively different from standard inflationary models. The scalar and tensor modes keep evolving after crossing the horizon. Due to this superhorizon evolution, the amplitude of the tensor mode (inflationary gravitational wave) is not simply proportional to the energy density of the universe at the time of horizon crossing. The spectral index of tensor modes can be positive.

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arXiv2004

Three Form Inflation

It is noted that inflation driven by a three form with arbitrary self coupling satisfies a consistency relation $n_S-1=2n_T$, between spectral indices of scalar and tensor cosmological perturbations. The standard consistency relation for the ratio of the amplitudes of perturbations is only valid for quadratic self coupling of the three form.

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arXiv2004

Post-Inflationary Curvature Perturbations

Curvature perturbations generated after the end of inflation are estimated for a generic multi-field inflationary potential. The curvature perturbations are generated by conversion of inflationary isocurvature perturbations, similar to curvaton and inhomogeneous reheating scenarios. During inflation, the trajectory along which the fields are rolling down will fluctuate. When inflation ends, the fields enter the oscillatory phase along different directions in different parts of the universe. Different oscillatory trajectories can have different effective equations of state and reheating rates, so that different parts of the universe will evolve differently. This translates the isocurvature perturbations into the curvature perturbations. If all light fields make similar contributions to the potential energy, the postinflationary curvature perturbations are much smaller than the standard inflationary curvature perturbations. But if different light fields make very different contributions to the potential energy, the postinflationary curvature perturbations can exceed the inflationary perturbations. The characteristic feature of postinflationary perturbations is non-Gaussianity $\sim \pm 1$.

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arXiv2003

Graviton Mass or Cosmological Constant?

To describe a massive graviton in 4D Minkowski space-time one introduces a quadratic term in the Lagrangian. This term, however, can lead to a readjustment or instability of the background instead of describing a massive graviton on flat space. We show that for all local Lorentz-invariant mass terms Minkowski space is unstable. We start with the Pauli-Fierz (PF) term that is the only local mass term with no ghosts in the linearized approximation. We show that nonlinear completions of the PF Lagrangian give rise to instability of Minkowski space. We continue with the mass terms that are not of a PF type. Although these models are known to have ghosts in the linearized approximations, nonlinear interactions can lead to background change due to which the ghosts are eliminated. In the latter case, however, the graviton perturbations on the new background are not massive. We argue that a consistent theory of a massive graviton on flat space can be formulated in theories with extra dimensions. They require an infinite number of fields or non-local description from a 4D point of view.

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arXiv2003

Non-Gaussianity of Large-Scale Cosmological Perturbations

Perturbations in a universe with matter and cosmological constant are computed in terms of the primordial scalar perturbation up to second order. The calculation is easy -- a generalization of the spherical dust solution. Our expression might allow accurate measurements of the primordial non-Gaussianity.

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arXiv2003

Photon Mass Bound Destroyed by Vortices

The Particle Data Group gives an upper bound on the photon mass $m < 2 \times 10^{-16}$eV from a laboratory experiment and lists, but does not adopt, an astronomical bound $m < 3 \times 10^{-27}$eV, both of which are based on the plausible assumption of large galactic vector-potential. We argue that the interpretations of these experiments should be changed, which alters significantly the bounds on $m$. If $m$ arises from a Higgs effect, both limits are invalid because the Proca vector-potential of the galactic magnetic field may be neutralized by vortices giving a large-scale magnetic field that is effectively Maxwellian. In this regime, experiments sensitive to the Proca potential do not yield a useful bound on $m$. As a by-product, the non-zero photon mass from Higgs effect predicts generation of a primordial magnetic field in the early universe. If, on the other hand, the galactic magnetic field is in the Proca regime, the very existence of the observed large-scale magnetic field gives $m^{-1}\gtrsim 1$kpc, or $m\lesssim 10^{-26}$eV.

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arXiv2003

Cosmological Perturbations From Inhomogeneous Reheating, Freeze-Out, and Mass Domination

We generalize a recently proposed mechanism for the origin of primordial metric perturbations in inflationary models. Quantum fluctuations of light scalar fields during inflation give rise to super-horizon fluctuations of masses and reaction rates of various particles. Reheating, freeze-out, and matter-domination processes become inhomogeneous and generate super-horizon metric perturbations. We also calculate the degree of non-Gaussianity $f_{nl}$ for this new model of cosmological perturbations. The precise value of $f_{nl}$ depends on the specific models, but $|f_{nl}|\sim$few is a natural lower bound for our mechanisms. This is much larger than the currently assumed theoretical value $f_{nl}\sim tilt \lesssim 0.05$, and is thought to be observable. In a particularly attractive model of inhomogeneous mass-domination, the non-Gaussianity of perturbations generated by our mechanism is simply $f_{nl}=5$, irrespective of the detailed structure of the underlying field theory.

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arXiv2003

A new mechanism for generating density perturbations from inflation

We propose a new mechanism to generate density perturbations in inflationary models. Spatial fluctuations in the decay rate of the inflaton field to ordinary matter lead to fluctuations in the reheating temperature. We argue that in most realistic models of inflation the coupling of the inflaton to normal matter is determined by the vacuum expectation values of fields in the theory. If those fields are light during inflation (this is a generic situation in the minimal models of supersymmetric inflation) they will fluctuate leading to density perturbations through the proposed mechanism. We show that these fluctuations could easily dominate over the ones generated through the standard mechanism. The new scenario has several consequences for inflation model building and observations. The proposed mechanism allows to generate the observed level of density perturbations with a much lower scale of inflation and thus generically predicts a smaller level of gravitational waves. The relation between the slope of the spectrum of the produced density perturbations and the potential of the inflaton field is different from the standard relations obtained in the context of slow roll inflation. Because the field responsible for the fluctuations is not the inflaton, it can have significantly larger self couplings and thus density perturbations could be non-Gaussian. The non-Gaussianity can be large enough to be detectable by CMB and Large Scale Structure observations.

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arXiv2003

Self-similarity of the Third Type in the Strong Explosion Problem

Propagation of a blast wave due to strong explosion in the center of a power-law-density ($ρ\propto r^{-α}$) spherically symmetric atmosphere is studied. For adiabatic index of 5/3, the solution was known to be self-similar, (of type I) for $α<3$, self-similar (of type II) for $α>3.26$, and unknown in between. We find a self-similar solution for $3<α<3.26$, and give a (tentative) numerical proof that this solution is indeed an asymptotic of the strong explosion. This self-similar solution is neither of type I (dimensional analysis does not work), nor of type II (the index of the solution is known without solving an eigenvalue problem).

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arXiv2002

The Accelerated Universe and the Moon

Cosmologically motivated theories that explain small acceleration rate of the Universe via modification of gravity at very large, horizon or super-horizon distances, can be tested by precision gravitational measurements at much shorter scales, such as the Earth-Moon distance. Contrary to the naive expectation the predicted corrections to the Einsteinian metric near gravitating sources are so significant that fall within sensitivity of the proposed Lunar Ranging experiments. The key reason for such corrections is the van Dam-Veltman-Zakharov discontinuity present in linearized versions of all such theories, and its subsequent absence at the non-linear level ala Vainshtein.

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arXiv2002

A new solution to the solar neutrino deficit

We reexamine the transition magnetic moment solution to the solar neutrino problem. We argue that the absence of large time variations in the Super-Kamiokande rate provides strong evidence against spin-flavor flip in the solar convective zone. Spin-flavor flip could, however, occur in the primordial magnetic field in the radiative zone. We compute the longest lived toroidal mode for this field and show that spin-flavor flip in the radiative zone can account for all available solar data.

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arXiv2002

Bounds on the Magnetic Fields in the Radiative Zone of the Sun

We discuss bounds on the strength of the magnetic fields that could be buried in the radiative zone of the Sun. The field profiles and decay times are computed for all axisymmetric toroidal Ohmic decay eigenmodes with lifetimes exceeding the age of the Sun. The measurements of the solar oblateness yield a bound <~ 7 MG on the strength of the field. A comparable bound is expected to come from the analysis of the splitting of the solar oscillation frequencies. The theoretical analysis of the double diffusive instability also yields a similar bound. The oblateness measurements at their present level of sensitivity are therefore not expected to measure a toroidal field contribution.

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