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Philip D. Mannheim

Publications and source records attributed to Philip D. Mannheim.

At least 109 records · Page 6Linked to original sources
arXiv2000

Dynamical Localization of Gravity

We show that the thin wall limit of the thick domain wall associated with a sine-Gordon soliton in a single non-compactified patch of 5-dimensional spacetime explicitly yields the Randall-Sundrum localized gravity two patch brane, with its discrete $Z_2$ symmetry arising from the discrete symmetry of the potential, and with the thin Minkowski brane $Λ_5+κ^2_5Λ^2_b/6=0$ relation between bulk and brane cosmological constants arising naturally without any need for fine tuning. Additionally we show that for an embedded thin de Sitter brane, localization of gravity is again possible provided the 5-space is compactified, with the now non-zero net cosmological constant $Λ_5+κ^2_5Λ^2_b/6$ on the brane being found to vary inversely with the compactification radius.

hep-th↗
arXiv2000

Constraints on Brane-Localized Gravity

In this paper we explore some general aspects of the embeddings associated with brane-localized gravity. In particular we show that the consistency of such embeddings can require (or impose) very specific relations between all the involved bulk and brane matter source parameters. We specifically explore the embeddings of 3-branes with non-zero spatial 3-curvature $k$ into 5-dimensional spacetime bulks, and show that for such embeddings, a 5-dimensional bulk cosmological constant is not able to produce the exponential suppression of the geometry thought necessary to localize gravity to the brane.

hep-th↗
arXiv2000

The equivalence principle in classical mechanics and quantum mechanics

We discuss our understanding of the equivalence principle in both classical mechanics and quantum mechanics. We show that not only does the equivalence principle hold for the trajectories of quantum particles in a background gravitational field, but also that it is only because of this that the equivalence principle is even to be expected to hold for classical particles at all.

gr-qc↗
arXiv2000

Fourth Order Theories Without Ghosts

Using the Dirac constraint method we show that the pure fourth-order Pais-Uhlenbeck oscillator model is free of observable negative norm states. Even though such ghosts do appear when the fourth order theory is coupled to a second order one, the limit in which the second order action is switched off is found to be a highly singular one in which these states move off shell. Given this result, construction of a fully unitary, renormalizable, gravitational theory based on a purely fourth order action in 4 dimensions now appears feasible.

hep-th↗
arXiv2000

Attractive and Repulsive Gravity

We discuss the circumstances under which gravity might be repulsive rather than attractive. In particular we show why our standard solar system distance scale gravitational intuition need not be a reliable guide to the behavior of gravitational phenomena on altogether larger distance scales such as cosmological, and argue that in fact gravity actually gets to act repulsively on such distance scales. With such repulsion a variety of current cosmological problems (the flatness, horizon, dark matter, universe age, cosmic acceleration and cosmological constant problems) are then all naturally resolved.

gr-qc↗
arXiv1998

Curvature, Galactic Dynamics and Cosmic Repulsion

We show that a cosmological negative spatial curvature can account for both a recently identified phenomenological imprint of the global Hubble flow on galactic rotation curves and for the recently detected cosmic repulsion and cosmic acceleration. Even though such a negative curvature (a curvature which a galactic rest frame observer recognizes as a universal linear potential which gives rise to a universal acceleration) is only achievable in standard gravity by extreme fine tuning, nonetheless, it is naturally realizable in conformal gravity, a currently viable, fully covariant, candidate alternate gravitational theory.

astro-ph↗
arXiv1998

How We Got Into The Dark Matter Fix And How We Can Get Out

The recent discovery of cosmic repulsion represents a major challenge to standard gravity, adding an apparent missing energy problem to its still not yet adequately resolved missing mass one, while simultaneously requiring the current universe to be a very special, fine-tuned one. We suggest that the resolution of these difficulties lies not in the existence of more and more exotic gravitational sources, but rather in the lack of reliability of the extrapolation of standard gravity from its solar system origins to the altogether larger distance scales associated with cosmology. We suggest a very different such extrapolation, namely that associated with conformal gravity, to find that all of these issues are then readily and naturally resolved.

astro-ph↗
arXiv1998

Implications of Cosmic Repulsion for Gravitational Theory

In this paper we present a general, model independent analysis of a recently detected apparent cosmic repulsion, and discuss its potential implications for gravitational theory. In particular, we show that a negatively spatially curved universe acts like a diverging refractive medium, to thus naturally cause galaxies to accelerate away from each other. Additionally, we show that it is possible for a cosmic acceleration to only be temporary, with some accelerating universes actually being able to subsequently recontract.

astro-ph↗
arXiv1998

Curvature and Cosmic Repulsion

Some recent supernovae studies have extended the distance versus velocity Hubble plot to very high redshift, and have revealed the apparent presence of a cosmic repulsion. We show that such a repulsion has a natural origin within conformal gravity, a fully covariant candidate alternative to standard gravity, and that it arises from the gravitational field itself via the negative spatial curvature which conformal cosmology is required to possess. In this same cosmology the flatness, horizon and cosmological constant problems are all resolved, and no need is found for dark matter.

astro-ph↗
arXiv1997

Dark Matter: a Challenge to Standard Gravity or a Warning?

We suggest that the conventional need for overwhelming amounts of astrophysical dark matter should be regarded as a warning to standard gravity rather than as merely a challenge to it, and show that the systematics of galactic rotation curve data can just as readily point in the direction of the equally covariant conformal gravity alternative. In particular we identify an apparent imprint of the Hubble flow on those data, something which while quite natural to conformal gravity is not at all anticipated in the standard gravitational paradigm.

astro-ph↗
arXiv1996

Classical Underpinnings of Gravitationally Induced Quantum Interference

We show that the gravitational modification of the phase of a neutron beam (the COW experiment) has a classical origin, being due to the time delay which classical particles experience in traversing a background gravitational field. Similarly, we show that classical light waves also undergo a phase shift in traversing a gravitational field. We show that the COW experiment respects the equivalence principle even in the presence of quantum mechanics.

gr-qc↗
arXiv1996

Local and global gravity

Our long experience with Newtonian potentials has inured us to the view that gravity only produces local effects. In this paper we challenge this quite deeply ingrained notion and explicitly identify some intrinsically global gravitational effects. In particular we show that the global cosmological Hubble flow can actually modify the motions of stars and gas within individual galaxies, and even do so in a way which can apparently eliminate the need for galactic dark matter. Also we show that a classical light wave acquires an observable, global, path dependent phase in traversing a gravitational field. Both of these effects serve to underscore the intrinsic difference between non-relativistic and relativistic gravity.

gr-qc↗
arXiv1996

Are galactic rotation curves really flat?

In this paper we identify a new regularity in the systematics of galactic rotation curves, namely we find that at the last detected points in galaxies of widely varying luminosity, the centripetal acceleration is found to have the completely universal form $v^2/R=c^2(γ_0+γ^{*}N^{*})/2$ where $γ_0$ and $γ^{*}$ are new universal constants and $N^{*}$ is the amount of visible matter in each galaxy. This regularity points to a role for the linear potentials associated with conformal gravity, with the galaxy independent $γ_0$ term being found to be generated not from within individual galaxies at all, but rather to be of cosmological origin being due to the global Hubble flow of a necessarily spatially open Universe of 3-space scalar curvature $k=-(γ_0/2)^2=-2.3 \times 10^{-60}$cm$^{-2}$.

astro-ph↗
arXiv1996

Linear potentials and galactic rotation curves - detailed fitting

We continue our study of the astrophysical implications of the linear potential $V(r)=-βc^2/r+γc^2 r/2$ associated with fundamental gravitational sources in the conformal invariant fourth order theory of gravity which has recently been advanced by Mannheim and Kazanas as a candidate alternative to the standard second order Einstein theory. We provide fitting to the rotation curves of an extensive and diverse set of 11 spiral galaxies whose data are regarded as being particularly reliable. Without the assumption of the existence of any dark matter the model is found to fit the shapes of the rotation curves extremely well, but with a pattern of normalizations which proves to be very instructive.

astro-ph↗
arXiv1996

Conformal cosmology and the age of the universe

We show that within the cosmology associated with conformal gravity the age of the universe is given as $1/H_0$, to thus overcome the current cosmological age crisis. We show that while the parameter $Ω_{mat} = ρ_{mat} / ρ_C$ takes on all values between zero and infinity in conformal gravity, nonetheless it is of order one (but not identically equal to one) for half a Hubble time to thus naturally explain its current closeness to one without fine tuning. We show that the cosmological constant is naturally of order the energy density $ρ_{mat}$ of ordinary matter again without fine tuning. We compare and contrast conformal cosmology with that of the standard Friedmann cosmology.

astro-ph↗
arXiv1995

Cosmology and galactic rotation curves

We explore the possibility that the entire departure of galactic rotational velocities from their luminous Newtonian expectation be cosmological in origin, and show that within the framework of conformal gravity (but not Einstein gravity apparently) every static observer sees the overall Hubble flow as a local universal linear potential which is able to account for available data without any need for dark matter. We find that the Universe is necessarily an open one with 3-space scalar curvature given by $k = -3.5\times 10^{-60}$cm$^{-2}$.

astro-ph↗
arXiv1995

Linear Potentials in the Cores of Clusters of Galaxies

We make a first application of the linear gravitational potentials of the conformal gravity theory to the distance scale associated with clusters of galaxies, with the theory being found to give a reasonable value for the mean velocity of the virialized core of the typical Coma cluster, with no need to invoke the existence of dark matter.

astro-ph↗