Non-vanishing Shift Research Articles

No Evidence of Kinetic Screening in Simulations of Merging Binary Neutron Stars beyond General Relativity.

We have conducted fully relativistic simulations in a class of scalar-tensor theories with derivative self-interactions and screening of local scales. By using high-resolution shock-capturing methods and a nonvanishing shift vector, we have managed to avoid issues plaguing similar attempts in the past. We have first confirmed recent results by ourselves in spherical symmetry, obtained with an approximate approach and pointing at a partial breakdown of the screening in black-hole collapse. Then, we considered the late inspiral and merger of binary neutron stars. We found that screening tends to suppress the (subdominant) dipole scalar emission, but not the (dominant) quadrupole scalar mode. Our results point at quadrupole scalar signals as large as (or even larger than) in Fierz-Jordan-Brans-Dicke theories with the same conformal coupling, for strong-coupling scales in the MeV range that we can simulate.

Ballistic and shift currents in the bulk photovoltaic effect theory

The bulk photovoltaic effect (BPVE) — the generation of electric currents by light in noncentrosymmetric materials in the absence of electric fields and gradients — was intensively investigated at the end of the last century. The outcomes, including all main aspects of this phenomenon, were summarized in reviews and books. A new upsurge of interest in the BPVE occurred recently, resulting in a flood of misleading theoretical and experimental publications centered around the so-called shift current. Numerous top-rated recent publications ignore the basic principles of charge-transport phenomena and the previous results of joint experimental-theoretical studies. Specifically, leading (or substantial) contributions to currents caused by asymmetry of the momentum distributions of electrons and holes are missed. The wide-spread basic relation for the shift current ignores the kinetic processes of relaxation and recombination of photo-excited electrons and leads to nonvanishing shift currents in thermal equilibrium. The goals of this methodological note is to specify and substantiate the benchmarks of the BPVE theory and return studies to the right track in the interest of developing photovoltaic devices.

Towards the renormalization group flow of Hořava gravity in 3+1 dimensions

We compute the renormalization group running of the Newton constant and the parameter $\ensuremath{\lambda}$ in ($3+1$)-dimensional projectable Ho\ifmmode \check{r}\else \v{r}\fi{}ava gravity. We use the background-field method, expanding around configurations with flat spatial metric, but nonvanishing shift. This allows us to reduce the number of interaction vertices and thereby drastically simplify the calculations. The gauge-invariant $\ensuremath{\beta}$-function of $\ensuremath{\lambda}$ has two families of zeros, attractive in the infrared and ultraviolet, respectively. They are candidates for the fixed points of the full renormalization group flow of the theory, once the $\ensuremath{\beta}$-functions for the rest of the couplings are added.

Cosmological perturbations in unimodular gravity

We study cosmological perturbation theory within the framework of unimodular gravity. We show that the Lagrangian constraint on the determinant of the metric required by unimodular gravity leads to an extra constraint on the gauge freedom of the metric perturbations. Although the main equation of motion for the gravitational potential remains the same, the shift variable, which is gauge artifact in General Relativity, cannot be set to zero in unimodular gravity. This non-vanishing shift variable affects the propagation of photons throughout the cosmological evolution and therefore modifies the Sachs-Wolfe relation between the relativistic gravitational potential and the microwave temperature anisotropies. However, for adiabatic fluctuations the difference between the result in General Relativity and unimodular gravity is suppressed on large angular scales. Thus, no strong constraints on the theory can be derived.

Two-Body Orbit Expansion Due to Time-Dependent Relative Acceleration Rate of the Cosmological Scale Factor

By phenomenologically assuming a slow temporal variation of the percent acceleration rate S̈S -1 of the cosmic scale factor S(t), it is shown that the orbit of a local binary undergoes a secular expansion. To first order in the power expansion of S̈S -1 around the present epoch t0, a non-vanishing shift per orbit (Δr) of the two-body relative distance r occurs for eccentric trajectories. A general relativistic expression, which turns out to be cubic in the Hubble parameter H0 at the present epoch, is explicitly calculated for it in the case of matter-dominated epochs with Dark Energy. For a highly eccentric Oort comet orbit with period Pb ≈ 31 Myr, the general relativistic distance shift per orbit turns out to be of the order of (Δr) ≈ 70 km. For the Large Magellanic Cloud, assumed on a bound elliptic orbit around the Milky Way, the shift per orbit is of the order of (Δr) ≈ 2–4 pc. Our result has a general validity since it holds in any cosmological model admitting the Hubble law and a slowly varying S̈S-1(t). More generally, it is valid for an arbitrary Hooke-like extra-acceleration whose “elastic” parameter κ is slowly time-dependent, irrespectively of the physical mechanism which may lead to it. The coefficient κ1 of the first-order term of the power expansion of κ(t) can be preliminarily constrained in a model-independent way down to a κ1 ≲ 2 x 10-13 year-3 level from latest Solar System’s planetary observations. The radial velocities of the double lined spectroscopic binary ALPHA Cen AB yield κ1 ≲ 10-8 year-3.

Testing numerical evolution with the shifted gauge wave

Computational methods are essential to provide waveforms from coalescing black holes, which are expected to produce strong signals for the gravitational wave observatories being developed. Although partial simulations of the coalescence have been reported, scientifically useful waveforms have so far not been delivered. The goal of the AppleswithApples (AwA) Alliance is to design, coordinate and document standardized code tests for comparing numerical relativity codes. The first round of AwA tests has now been completed and the results are being analysed. These initial tests are based upon the periodic boundary conditions designed to isolate performance of the main evolution code. Here we describe and carry out an additional test with periodic boundary conditions which deals with an essential feature of the black hole excision problem, namely a non-vanishing shift. The test is a shifted version of the existing AwA gauge wave test. We show how a shift introduces an exponentially growing instability which violates the constraints of a standard harmonic formulation of Einstein's equations. We analyse the Cauchy problem in a harmonic gauge and discuss particular options for suppressing instabilities in the gauge wave tests. We implement these techniques in a finite difference evolution algorithm and present test results. Although our application here is limited to a model problem, the techniques should benefit the simulation of black holes using harmonic evolution codes.

Neutrinoless double beta decay from singlet neutrinos in extra dimensions

We study the model-building conditions under which a sizable $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$-decay signal to the recently reported level of 0.4 eV is due to Kaluza-Klein singlet neutrinos in theories with large extra dimensions. Our analysis is based on 5-dimensional singlet-neutrino models compactified on an ${S}^{1}{/Z}_{2}$ orbifold, where the standard-model fields are localized on a 3-brane. We show that a successful interpretation of a positive signal within the above minimal 5-dimensional framework would require a non-vanishing shift of the 3-brane from the orbifold fixed points by an amount smaller than the typical scale $(100\mathrm{MeV}{)}^{\ensuremath{-}1}$ characterizing the Fermi nuclear momentum. The resulting 5-dimensional models predict a sizable effective Majorana-neutrino mass that could be several orders of magnitude larger than the light neutrino masses. Most interestingly, the brane-shifted models with only one bulk sterile neutrino also predict novel trigonometric textures leading to mass scenarios with hierarchical active neutrinos and large ${\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{-}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ and ${\ensuremath{\nu}}_{e}\ensuremath{-}{\ensuremath{\nu}}_{\ensuremath{\mu}}$ mixings that can fully explain the current atmospheric and solar neutrino data.

Analytical approach to critical phenomena for dynamical properties of type-I intermittent chaos

A phase transition associated with dynamical properties of a type-I intermittency is analytically studied on the basis of the statistical-mechanical formalism previously proposed. At the saddle-node bifurcation point the continuous phase transition is found to occur at ${\mathit{q}}_{\mathit{c}}$ less than unity, due to the coexistence of different types of strange attractors with different measures. For nonvanishing shift parameter, crossover phenomena are discussed to obtain scaling exponents for the generalized entropies and the singularity spectrum. The exponents including a logarithmic correction are shown to depend on ${\mathit{q}}_{\mathit{c}}$, which means an absence of universality, in contrast to the usual critical phenomena.