Markku Oksanen

Modified F(R) Hořava–Lifshitz gravity: a way to accelerating FRW cosmology

We propose a general approach for the construction of modified gravity which is invariant under foliation-preserving diffeomorphisms. Special attention is paid to the formulation of a modified F(R) Hořava–Lifshitz gravity (FRHL), whose Hamiltonian structure is studied. It is demonstrated that the spatially flat FRW equations of FRHL are consistent with the constraint equations. The analysis of de Sitter solutions for several versions of FRHL indicates that the unification of the early-time inflation with the late-time acceleration is possible. It is shown that a special choice of parameters for FRHL leads to the same spatially flat FRW equations as in the case of the traditional F(R)-gravity. Finally, an essentially most general modified Hořava–Lifshitz gravity is proposed, motivated by its fully diffeomorphism-invariant counterpart, with the restriction that the action does not contain derivatives higher than the second order with respect to the time coordinate.

Mimetic dark matter, ghost instability and a mimetic tensor-vector-scalar gravity

A bstractRecently modified gravitational theories which mimic the behaviour of dark matter, the so-called “Mimetic Dark Matter”, have been proposed. We study the consistency of such theories with respect to the absence of ghost instability and propose a new tensor-vector-scalar theory of gravity, which is a generalization of the previous models of mimetic dark matter with additional desirable features. The original model proposed by Chamseddine and Mukhanov [JHEP 11 (2013) 135] is concluded to describe a regular pressureless dust, presuming that we consider only those configurations where the energy density of the mimetic dust remains positive under time evolution. For certain type of configurations the theory can become unstable. Both alternative modified theories of gravity, which are based on a vector field (tensor-vector theory) or a vector field and a scalar field (tensor-vector-scalar theory), are free of ghost instabilities.

Photoacoustic evaluation of elasticity and integrity of pharmaceutical tablets

A nondestructive method based on pulse photoacoustics was applied for evaluation of elasticity and integrity of pharmaceutical tablets. Variations in porosity, density and sodium chloride content of microcrystalline cellulose tablets were found to be related to parameters extracted from the through-transmitted ultrasonic wave forms. By using the amplitudes and ultrasonic velocities of these wave forms, it was possible to obtain values of a transverse to longitudinal amplitude ratio, and also elastic parameters, such as Youngs and shear moduli, for the tablets. Poissons ratio was calculated from the elastic moduli as well as from the amplitudes. An exponential relationship between tablet porosity and the attenuation of longitudinal wave form was noticed. The transverse to longitudinal amplitude ratio and the amplitudinal Poissons ratio were indicative of structural variations, e.g., changes in the porosity and the sodium chloride content of tablets. Youngs and shear moduli of microcrystalline cellulose tablets were found to follow similar porosity trends to those in previously published beam bending and twisting studies, although the absolute values and the values extrapolated to zero porosity were slightly smaller. The Poissons ratio calculated from the experimental Youngs and shear modulus values was also in agreement with earlier studies, but the values extrapolated to zero porosity differed significantly. The method is a promising tool for evaluating the elastic properties of tableting materials and the structural variations in tablets.

How unimodular gravity theories differ from general relativity at quantum level

We investigate path integral quantization of two versions of unimodular gravity. First a fully diffeomorphism-invariant theory is analyzed, which does not include a unimodular condition on the metric, while still being equivalent to other unimodular gravity theories at the classical level. The path integral has the same form as in general relativity (GR), except that the cosmological constant is an unspecified value of a variable, and it thus is unrelated to any coupling constant. When the state of the universe is a superposition of vacuum states, the path integral is extended to include an integral over the cosmological constant. Second, we analyze the standard unimodular theory of gravity, where the metric determinant is fixed by a constraint. Its path integral differs from the one of GR in two ways: the metric of spacetime satisfies the unimodular condition only in average over space, and both the Hamiltonian constraint and the associated gauge condition have zero average over space. Finally, the canonical relation between the given unimodular theories of gravity is established.

On gravity as an entropic force

Abstract We consider E. Verlindeʼs proposal that gravity is an entropic force — we shall call this theory entropic gravity (EG) — and reanalyze a recent claim that this theory is in contradiction with the observation of the gravitationally-bound ground state of neutrons in the GRANIT experiment. We find that EG does not necessarily contradict the existence of gravitationally-bound quantum states of neutrons in the Earthʼs gravitational field, since EG is equivalent to Newtonian gravity in this case. However, certain transitions between the gravitationally-bound quantum states of neutrons, in particular spontaneous decays of excited states, which can hopefully be observed in future experiments, cannot be explained in the framework of EG, unless essential ingredients are introduced into it. Otherwise, a quantized description of gravity will be required.

Hamiltonian analysis of curvature-squared gravity with or without conformal invariance

Analyzujeme gravitacni teorie se cleny vyssich řadů v křivosti, kde specialnim připadem je Weylovska gravitace. Urcime lokalni podminky, ktere urcuji pocet fyzikalnich stupňů volnosti.

Prediction of the temporal shape of an ultrasonic pulse in a photoacoustic sensing application

Abstract Temporal shapes of low power photoacoustic sound sources were predicted and measured. The applicability of a relatively low-power laser in generating detectable ultrasound for a sensing application was confirmed.

Photoacoustic breakdown sound source in air

Abstract Laser induced, breakdown-generated sound pulses in air were studied in the zero to 200 kHz frequency range. Theoretical predictions, based on a theory originally developed for macroscopic blast waves, were used to predict peak sound pressure, power spectra and pulse peak-to-peak time. The predictions were found to agree with the measured data within the accuracy of the measured and predicted parameters for laser pulse energy in the 100 mJ range.

Hamiltonian analysis of non-projectable modified F(R) Hořava–Lifshitz gravity

Abstract We study a version of the recently proposed modified F ( R ) Hořava–Lifshitz gravity that abandons the projectability condition of the lapse variable. We discovered that the projectable version of this theory has a consistent Hamiltonian structure, and that the theory has interesting cosmological solutions which can describe the eras of accelerated expansion of the universe in a unified manner. The usual Hořava–Lifshitz gravity is a special case of our theory. Hamiltonian analysis of the non-projectable theory, however, shows that this theory has serious problems. These problems are compared with those found in the original Hořava–Lifshitz gravity. A general observation on the structure of the Poisson bracket of Hamiltonian constraints in all theories of the Hořava–Lifshitz type is made: in the resulting tertiary constraint the highest order spatial derivative of the lapse N is always of uneven order. Since the vanishing of the lapse ( N = 0 ) is required by the preservation of the Hamiltonian constraints under time evolution, we conclude that the non-projectable version of the theory is physically inconsistent.

Gauging the twisted Poincare symmetry as noncommutative theory of gravitation

Einsteins theory of general relativity was formulated as a gauge theory of Lorentz symmetry by Utiyama in 1956, while the Einstein-Cartan gravitational theory was formulated by Kibble in 1961 as the gauge theory of Poincare transformations. In this framework, we propose a formulation of the gravitational theory on canonical noncommutative space-time by covariantly gauging the twisted Poincare symmetry, in order to fulfil the requirement of covariance under the general coordinate transformations, an essential ingredient of the theory of general relativity. It appears that the twisted Poincare symmetry cannot be gauged by generalizing the Abelian twist to a covariant non-Abelian twist, nor by introducing a more general covariant twist element. The advantages of such a formulation as well as the related problems are discussed and possible ways out are outlined.