arXiv subjects

Philip D. Mannheim

Publications and source records attributed to Philip D. Mannheim.

At least 91 records · Page 5Linked to original sources

arXiv2006

Solution to the ghost problem in fourth order derivative theories

We present a solution to the ghost problem in fourth order derivative theories. In particular we study the Pais-Uhlenbeck fourth order oscillator model, a model which serves as a prototype for theories which are based on second plus fourth order derivative actions. Via a Dirac constraint method quantization we construct the appropriate quantum-mechanical Hamiltonian and Hilbert space for the system. We find that while the second-quantized Fock space of the general Pais-Uhlenbeck model does indeed contain the negative norm energy eigenstates which are characteristic of higher derivative theories, in the limit in which we switch off the second order action, such ghost states are found to move off shell, with the spectrum of asymptotic in and out S-matrix states of the pure fourth order theory which results being found to be completely devoid of states with either negative energy or negative norm. We confirm these results by quantizing the Pais-Uhlenbeck theory via path integration and by constructing the associated first-quantized wave mechanics, and show that the disappearance of the would-be ghosts from the energy eigenspectrum in the pure fourth order limit is required by a hidden symmetry that the pure fourth order theory is unexpectedly found to possess. The occurrence of on-shell ghosts is thus seen not to be a shortcoming of pure fourth order theories per se, but rather to be one which only arises when fourth and second order theories are coupled to each other.

hep-th↗

arXiv2006

Completeness of non-normalizable modes

We establish the completeness of some characteristic sets of non-normalizable modes by constructing fully localized square steps out of them, with each such construction expressly displaying the Gibbs phenomenon associated with trying to use a complete basis of modes to fit functions with discontinuous edges. As well as being of interest in and of itself, our study is also of interest to the recently introduced large extra dimension brane-localized gravity program of Randall and Sundrum, since the particular non-normalizable mode bases that we consider (specifically the irregular Bessel functions and the associated Legendre functions of the second kind) are associated with the tensor gravitational fluctuations which occur in those specific brane worlds in which the embedding of a maximally four-symmetric brane in a five-dimensional anti-de Sitter bulk leads to a warp factor which is divergent. Since the brane-world massless four-dimensional graviton has a divergent wave function in these particular cases, its resulting lack of normalizability is thus not seen to be any impediment to its belonging to a complete basis of modes, and consequently its lack of normalizability should not be seen as a criterion for not including it in the spectrum of observable modes. Moreover, because the divergent modes we consider form complete bases, we can even construct propagators out of them in which these modes appear as poles with residues which are expressly finite. Thus even though normalizable modes appear in propagators with residues which are given as their finite normalization constants, non-normalizable modes can just as equally appear in propagators with finite residues too -- it is just that such residues will not be associated with bilinear integrals of the modes.

hep-th↗

arXiv2006

Causality in the brane world

For the Randall-Sundrum brane world where a positive tension Minkowski brane is embedded in AdS5, two candidate propagators have been suggested in the literature, one being based on the normalized mode solutions to the source-free volcano potential fluctuation equation, and the other being the Giddings, Katz and Randall outgoing Hankel function based one. We show that while both of these two propagators have the same pole plus cut singularity structure in the complex energy plane, they behave differently on their respective circles at infinity, as a consequence of which only the Hankel function based propagator proves to be causal, with the normalized mode based one being found to take support outside the AdS5 lightcone. In addition we show that unlike the Hankel function based propagator, the normalized mode based propagator does not correctly implement the junction conditions which hold in the presence of a perturbative source on the brane.

hep-th↗

arXiv2006

Gauge Invariant Treatment of the Energy Carried by a Gravitational Wave

Even though the energy carried by a gravitational wave is not itself gauge invariant, the interaction with a gravitational antenna of the gravitational wave which carries that energy is. It therefore has to be possible to make some statements which involve the energy which are in fact gauge invariant, and it is the objective of this paper to provide them. In order to develop a gauge invariant treatment of the issues involved, we construct a specific action for gravitational fluctuations which is gauge invariant to second perturbative order. Then, via variation of this action, we obtain an energy-momentum tensor for perturbative gravitational fluctuations around a general curved background whose covariant conservation condition is also fully gauge invariant to second order. Contraction of this energy-momentum tensor with a Killing vector of the background conveniently allows us to convert this covariant conservation condition into an ordinary conservation condition which is also gauge invariant through second order. Then, via spatial integration we are able to obtain a relation involving the time derivative of the total energy of the fluctuation and its asymptotic spatial momentum flux which is also completely gauge invariant through second order. It is only in making the simplification of setting the asymptotic momentum flux to zero that one would actually lose manifest gauge invariance, with only invariance under those particular gauge transformations which leave the asymptotic momentum flux zero then remaining. However, if one works in an arbitrary gauge where the asymptotic momentum flux is non-zero, the gravitational wave will then deliver both energy and momentum to a gravitational antenna in a completely gauge invariant manner, no matter how badly behaved at infinity the gauge function might be.

gr-qc↗

arXiv2006

Bounds on Localized Modes in the Crystal Impurity Problem

Using general properties of the crystal site representation normal mode matrix, we provide some very simple bounds on localized modes in simple, body-centered and face-centered cubic crystals with substitutional point defects. We derive a trace condition constraint on the net change in crystal eigenfrequencies caused by the introduction of a defect, with the condition being a completely general one which holds for any combination of central and non-central crystal force-constants and for all-neighbor interactions. Using this condition we show that the sufficient condition for producing localized modes in an arbitrary cubic crystal by a mass change at the defect site is that the defect mass be less than one half of that of the host atom mass which it replaces, and that the sufficient condition for producing localized modes in an arbitrary cubic crystal by force-constant changes alone is that the defect site self force-constant be greater than twice that of the pure crystal self force-constant of the host atom which it replaces.

cond-mat.other↗

arXiv2005

Arbitrary Force-Constant Changes in the Crystal Impurity Problem

In a previous study of the dynamics of crystals with substitutional point defects we had obtained simple and exact expressions for the positions of the perturbed crystal modes and the intensities in them in the case in which both the host-host and the host-impurity force-constants were taken to be central and nearest neighbor. Such expressions required knowledge of only one pure crystal lattice Green's function, the one at the defect site itself. In this paper we extend our previous study to incorporate non-central force-constants as well. We find that the same simple expressions which we had previously obtained in the nearest neighbor central force-constant case also hold without any modification at all in the case of isotropic force-constant changes in crystals whose host-host force-constants are isotropic, and in the particular mixed case in which the fractional changes in the central and isotropic components of the force-constants are equal to each other. In the most general arbitrary nearest neighbor force-constant case we obtain a reasonably compact exact expression for the dynamics of the perturbed modes which only involves a total of two pure crystal lattice Green's functions.

cond-mat.other↗

arXiv2005

Alternatives to Dark Matter and Dark Energy

We review the underpinnings of the standard Newton-Einstein theory of gravity, and identify where it could possibly go wrong. In particular, we discuss the logical independence from each other of the general covariance principle, the equivalence principle and the Einstein equations, and discuss how to constrain the matter energy-momentum tensor which serves as the source of gravity. We identify the a priori assumption of the validity of standard gravity on all distance scales as the root cause of the dark matter and dark energy problems, and discuss how the freedom currently present in gravitational theory can enable us to construct candidate alternatives to the standard theory in which the dark matter and dark energy problems could then be resolved. We identify three generic aspects of these alternate approaches: that it is a universal acceleration scale which determines when a luminous Newtonian expectation is to fail to fit data, that there is a global cosmological effect on local galactic motions which can replace galactic dark matter, and that to solve the cosmological constant problem it is not necessary to quench the cosmological constant itself, but only the amount by which it gravitates.

astro-ph↗

arXiv2005

Dirac Quantization of the Pais-Uhlenbeck Fourth Order Oscillator

As a model, the Pais-Uhlenbeck fourth order oscillator with equation of motion $d^4q/dt^4+(ω_1^2+ω_2^2)d^2q/dt^2 +ω_1^2ω_2^2 q=0$ is a quantum-mechanical prototype of a field theory containing both second and fourth order derivative terms. With its dynamical degrees of freedom obeying constraints due to the presence of higher order time derivatives, the model cannot be quantized canonically. We thus quantize it using the method of Dirac constraints to construct the correct quantum-mechanical Hamiltonian for the system, and find that the Hamiltonian diagonalizes in the positive and negative norm states that are characteristic of higher derivative field theories. However, we also find that the oscillator commutation relations become singular in the $ω_1 \to ω_2$ limit, a limit which corresponds to a prototype of a pure fourth order theory. Thus the particle content of the $ω_1 =ω_2$ theory cannot be inferred from that of the $ω_1 \neq ω_2$ theory; and in fact in the $ω_1 \to ω_2$ limit we find that all of the $ω_1 \neq ω_2$ negative norm states move off shell, with the spectrum of asymptotic in and out states of the equal frequency theory being found to be completely devoid of states with either negative energy or negative norm. As a byproduct of our work we find a Pais-Uhlenbeck analog of the zero energy theorem of Boulware, Horowitz and Strominger, and show how in the equal frequency Pais-Uhlenbeck theory the theorem can be transformed into a positive energy theorem instead.

hep-th↗

arXiv2004

The Work of Behram Kursunoglu

Talk presented at the 2003 Coral Gables conference in honor and appreciation of the work of Professor Behram Kursunoglu, general relativist extraordinaire and founder of the Coral Gables series of conferences, whose untimely death occurred shortly before the 2003 conference.

gr-qc↗

arXiv2003

Options for cosmology at redshifts above one

We show that detailed exploration of the $1 1$ region, one is thus currently unable to ascertain whether the universe is accelerating or decelerating between $z=1$ and $z=2$. To be able to visualize the supernovae and radio galaxy data simultaneously, we present a representation of the radio galaxy data in terms of an equivalent apparent magnitude Hubble diagram. We discuss briefly some implications of the anisotropies in the cosmic microwave background for the conformal theory, and show that in that theory fluctuations which set in at around nucleosynthesis can readily generate the first peak in the anisotropy data.

astro-ph↗

arXiv2003

Measuring velocity of sound with nuclear resonant inelastic x-ray scattering

Nuclear resonant inelastic x-ray scattering is used to measure the projected partial phonon density of states of materials. A relationship is derived between the low-energy part of this frequency distribution function and the sound velocity of materials. Our derivation is valid for harmonic solids with Debye-like low-frequency dynamics. This method of sound velocity determination is applied to elemental, composite, and impurity samples which are representative of a wide variety of both crystalline and noncrystalline materials. Advantages and limitations of this method are elucidated.

cond-mat↗

arXiv2002

Is Cosmic Acceleration Really Recent?

In the standard cosmological paradigm cosmic acceleration is to only be a very recent (viz. $z \leq 1$) phenomenon, with the universe being required to be decelerating at all higher redshifts. We suggest that this particular expectation of the standard model is to be viewed as a quite definitive test not only of the model itself but also of the fine-tuning assumption on which the expectation is based, with the expectation itself actually being readily amenable to testing once the Hubble plot can be extended out to only $z=2$ or so. Moreover, such a modest extension of the Hubble plot will also provide for definitive testing of the non fine-tuned alternate conformal gravity theory, a theory in which the universe is to accelerate both above and below $z=1$.

astro-ph↗

arXiv2001

How Recent is Cosmic Acceleration?

Possibly the most peculiar expectation of the standard fine-tuned cosmological paradigm is that cosmic acceleration is to only be a very recent (z<1) phenomenon, with the universe being required to be decelerating at all higher redshifts. Detailed exploration of the Hubble plot out to z=2 or so will not only provide an absolute test of this expectation but will also allow for testing of conformal gravity, a non fine-tuned alternate cosmological theory which provides equally good fitting to the current z<1 Hubble plot data while requiring the universe to be accelerating at higher z instead. With both standard and conformal gravity being found to be compatible with a very recently reported z=1.7 data point, additional data points will thus be needed to determine whether the universe is accelerating or decelerating above z=1.

astro-ph↗

arXiv2001

Cosmic acceleration as the solution to the cosmological constant problem

In this paper we provide both a diagnosis and resolution of the cosmological constant problem, one in which a large (as opposed to a small) cosmological constant $Λ$ can be made compatible with observation. We trace the origin of the cosmological constant problem to the assumption that the local gravitational Newton constant $G$ (as measured in a Cavendish experiment) sets the scale for global cosmology. And then we show that once this assumption is relaxed, the very same cosmic acceleration which has served to make the cosmological constant problem so very severe can instead then serve to provide us with its potential resolution. In particular, we present an alternate cosmology, one based on conformal gravity, a theory whose effective cosmological $G_{eff}$ not only differs from the Cavendish one by being altogether smaller than it, but, by even being explicitly negative, naturally leads to cosmological repulsion. We show in the conformal theory, that once given only that the sign of $Λ$ is specifically the negative one associated with spontaneous scale symmetry breaking, then, that alone, no matter how big $Λ$ might actually be in magnitude, is sufficient to not only make the actually measurable contribution $8πG_{eff} Λ/3cH^2(t_0)$ of $Λ$ to current era cosmology naturally be of order one today, but to even do so in a way which is fully compatible with the recent high $z$ supernovae cosmology data. Thus to solve the cosmological constant problem we do not need to change or quench the energy content of the universe, but rather we only need change its effect on cosmic evolution.

astro-ph↗

arXiv2001

Constraints on $AdS_5$ Embeddings

We show that the embedding of either a static or a time dependent maximally 3-symmetric brane with non-zero spatial curvature $k$ into a non-compactified $AdS_5$ bulk does not yield exponential suppression of the geometry away from the brane. Implications of this result for brane-localized gravity are discussed.

hep-th↗

arXiv2001

Conformal gravity and a natural solution to the cosmological constant problem

With attempts to quench the cosmological constant $Λ$ having so far failed, we instead investigate what could be done if $Λ$ is not quenched and actually gets to be as big as elementary particle physics suggests. Since the quantity relevant to cosmology is actually $Ω_Λ$, quenching it to its small measured value is equally achievable by quenching not $Λ$ but $G$ instead, with the $G$ relevant to cosmology then being much smaller than that measured in a low energy Cavendish experiment. A gravitational model in which this explicitly takes place, viz. conformal gravity, is presented, with the model being found to provide for a completely natural, non fine tuned accounting of the recent high $z$ accelerating universe supernovae data, no matter how big $Λ$ itself actually gets to be. Thus to solve the cosmological constant problem we do not need to change or quench the energy content of the universe, but rather only its effect on cosmic evolution.

astro-ph↗

arXiv2001

Cosmic acceleration and a natural solution to the cosmological constant problem

We trace the origin of the cosmological constant problem to the assumption that Newton's constant $G$ sets the scale for cosmology. And then we show that once this assumption is relaxed, the very same cosmic acceleration which has served to make the cosmological constant problem so very severe instead then serves to provide us with its potential resolution. We present an alternate cosmology, one based on conformal gravity, and show that once given only that the sign of the vacuum energy density $Λ$ is explicitly the negative one associated with spontaneous breakdown of the scale invariance of the conformal theory (this actually being the choice of sign for $Λ$ which precisely leads to cosmic acceleration in conformal gravity), then that alone, no matter how big $Λ$ might actually be in magnitude, is sufficient to not only make its measurable contribution to current era cosmology naturally be of order one today, but to even do so in a way which is completely compatible with the recent high $z$ supernovae cosmology data. Cosmology can thus live with either a fundamental $G$ or with the large (and even potentially negative) $Λ$ associated with elementary particle physics phase transitions but not with both. Also, we distinguish between the free energy and the internal energy, with it being the former which determines cosmological phase transitions and the latter which is the source of the gravitational field. Then we show that, even if we make the standard ad hoc assumption that $Λ$ actually is quenched in standard gravity, vacuum energy is nonetheless still found to dominate standard cosmology at the time of the phase transition which produced it. However, within conformal gravity no such difficulty is encountered.

gr-qc↗

arXiv2001

Delta function singularities in the Weyl tensor at the brane

In a recent paper Shiromizu, Maeda and Sasaki derived the gravitational equations of motion which would hold on a brane which is embedded in a higher dimensional bulk spacetime, showing that even when the Einstein equations are imposed in the bulk, nonetheless the embedding leads to a modification of the Einstein equations on the brane. In this comment on their work we explicitly identify and evaluate a delta function singularity effect at the brane which they do not appear to have discussed in their paper, an effect which, while actually being of interest in and of itself, nonetheless turns out not to modify their reported results.

hep-th↗