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Glenn Starkman

Publications and source records attributed to Glenn Starkman.

35 records · Page 2Linked to original sources
arXiv2009

Screening and Antiscreening of the MOND field in Perturbed Spherical Systems

In the context of Modified Newtonian Dynamics (MOND), we study how perturbation of a spherically symmetric system would affect the gravitational field. In particular, we study systems of perturbed and unperturbed spherical shells. For a single perturbed shell, we show that the field inside the shell is much smaller than what would be expected from a naive scaling formula. The strength of the perturbation field within the shell is screened by the spherically symmetric component of the mass, and is reduced as the spherically symmetric component is increased. For a two-shell system, we again show that the perturbed field is screened by the shells, no matter which shell's mass distribution is perturbed. The field within the inner shell is most suppressed when the inner and outer shells coincide. However, for a very light inner shell, the perturbation to the field can be enhanced. The enhancement is typically larger for smaller inner shells, and the perturbed field can be amplified by almost a factor of 2. The relevance to the effect of external fields on galaxy dynamics is discussed.

astro-ph.CO↗
arXiv2009

Birkhoff's theorem fails to save MOND from non-local physics

We investigate the consequences of Birkhoff's theorem in general relativity (GR) and in Modified Newtonian dynamics (MOND). We study, in particular, the system of a finite-mass test particle inside a spherical shell. In both GR and MOND, we find non-vanishing acceleration for that test particle. The direction of the acceleration is such that it pushes the test particle toward the center of the shell. In GR, the acceleration is found analytically in the case of a small test mass with a small displacement from the center of the shell. In MOND, the acceleration is found analytically in the limit of large test mass and small displacement, and a comparison to numerical values is made. Numerical simulations are done for more general cases with parameters that mimic the system of a galaxy in a cluster. In GR, the acceleration is highly suppressed, and physically insignificant. In MOND, on the contrary, the acceleration of the point particle can be a significant fraction of the field just outside of the spherical shell.

astro-ph↗
arXiv2009

Manual of BlackMax, a black-hole event generator with rotation, recoil, split branes, and brane tension

This is the users manual of the black-hole event generator BlackMax, which simulates the experimental signatures of microscopic and Planckian black-hole production and evolution at proton-proton, proton-antiproton and electron-positron colliders in the context of brane world models with low-scale quantum gravity. The generator is based on phenomenologically realistic models free of serious problems that plague low-scale gravity. It includes all of the black-hole gray-body factors known to date and incorporates the effects of black-hole rotation, splitting between the fermions, non-zero brane tension and black-hole recoil due to Hawking radiation (although not all simultaneously).

hep-ph↗
arXiv2008

Gravitational Lenses in Generalized Einstein-Aether theory: the Bullet Cluster

We study the lensing properties of an asymmetric mass distribution and vector field in Generalized Einstein-Aether (GEA) theory. As vector field fluctuations are responsible in GEA for seeding baryonic structure formation, vector field concentrations can exist independently of baryonic matter. Such concentrations would not be expected to be tied to baryonic matter except gravitationally, and so, like dark matter halos, would become separated from baryonic matter in interacting systems such as the Bullet Cluster. These vector field concentrations cause metric deviations that affect weak lensing. Therefore, the distribution of weak lensing deviates from that which would be inferred from the luminous mass distribution, in a way that numerical calculations demonstrate can be consistent with observations. This suggests that MOND-inspired theories can reproduce weak lensing observations, but makes clear the price: the existence of a coherent large-scale fluctuation of a field(s) weakly tied to the baryonic matter, not completely dissimilar to a dark matter halo.

gr-qc↗
arXiv2008

BlackMax: A black-hole event generator with rotation, recoil, split branes and brane tension

We present a comprehensive black-hole event generator, BlackMax, which simulates the experimental signatures of microscopic and Planckian black-hole production and evolution at the LHC in the context of brane world models with low-scale quantum gravity. The generator is based on phenomenologically realistic models free of serious problems that plague low-scale gravity, thus offering more realistic predictions for hadron-hadron colliders. The generator includes all of the black-hole graybody factors known to date and incorporates the effects of black-hole rotation, splitting between the fermions, non-zero brane tension and black-hole recoil due to Hawking radiation (although not all simultaneously). The generator can be interfaced with Herwig and Pythia.

hep-ph↗
arXiv2007

Modified Gravity: living without Birkhoff I. DGP

We consider the consequences of the absence of Birkhoff's theorem in theories of modified gravity. As an example, we calculate the gravitational force on a test particle due to a spherical mass shell in the Dvali-Gabadaze-Porrati model (DGP). We show that unlike in General Relativity, the force depends on the mass distribution. In particular, the gravitational force within a spherical mass shell depends on the geometric structure of the bulk, and is likely non-zero.

gr-qc↗
arXiv2007

Generalized Einstein-Aether theories and the Solar System

It has been shown that generalized Einstein-Aether theories may lead to significant modifications to the non-relativistic limit of the Einstein equations. In this paper we study the effect of a general class of such theories on the Solar System. We consider corrections to the gravitational potential in negative and positive powers of distance from the source. Using measurements of the perihelion shift of Mercury and time delay of radar signals to Cassini, we place constraints on these corrections. We find that a subclass of generalized Einstein-Aether theories are compatible with these constraints.

astro-ph↗
arXiv2007

Anthropics and Myopics: Conditional Probabilities and the Cosmological Constant

We re-examine claims that anthropic arguments provide an explanation for the observed smallness of the cosmological constant, and argue that correlations between the cosmological constant value and the existence of life can be demonstrated only under restrictive assumptions. Causal effects are more subtle to uncover.

hep-th↗
arXiv2003

Differentiating between Modified Gravity and Dark Energy

The nature of the fuel that drives today's cosmic acceleration is an open and tantalizing mystery. We entertain the suggestion that the acceleration is not the manifestation of yet another new ingredient in the cosmic gas tank, but rather a signal of our first real lack of understanding of gravitational physics. By requiring that the underlying gravity theory respects Birkhoff's law, we derive the modified gravitational force-law necessary to generate any given cosmology, without reference to the fundamental theory, revealing modifications of gravity at scales typically much smaller than today's horizon. We discuss how through these modifications, the growth of density perturbations, the late-time integrated Sachs--Wolfe effect, and even solar-system measurements may be sensitive to whether today's cosmic acceleration is generated by dark energy or modified gravitational dynamics, and are subject to imminent observational discrimination. We argue how these conclusions can be more generic, and probably not dependent on the validity of Birkhoff's law.

astro-ph↗
arXiv2003

Parameterization of Dark-Energy Properties: a Principal-Component Approach

Considerable work has been devoted to the question of how to best parameterize the properties of dark energy, in particular its equation of state w. We argue that, in the absence of a compelling model for dark energy, the parameterizations of functions about which we have no prior knowledge, such as w(z), should be determined by the data rather than by our ingrained beliefs or familiar series expansions. We find the complete basis of orthonormal eigenfunctions in which the principal components (weights of w(z)) that are determined most accurately are separated from those determined most poorly. Furthermore, we show that keeping a few of the best-measured modes can be an effective way of obtaining information about w(z).

astro-ph↗
arXiv2002

Gravitational Leakage into Extra Dimensions: Probing Dark Energy Using Local Gravity

The braneworld model of Dvali-Gabadadze-Porrati (DGP) is a theory where gravity is modified at large distances by the arrested leakage of gravitons off our four-dimensional universe. Cosmology in this model has been shown to support both "conventional" and exotic explanations of the dark energy responsible for today's cosmic acceleration. We present new results for the gravitational field of a clustered matter source on the background of an accelerating universe in DGP braneworld gravity, and articulate how these results differ from those of general relativity. In particular, we show that orbits nearby a mass source suffer a universal anomalous precession as large as 5 microarcseconds/year, dependent only on the graviton's effective linewidth and the global geometry of the full, five-dimensional universe. Thus, this theory offers a local gravity correction sensitive to factors that dictate cosmological history.

astro-ph↗
arXiv2002

Zero modes of fermions with general mass matrix

We analyze zero energy solutions of the Dirac equation in the background of a string-like configuration in an extension of the standard model which accommodates the most general fermionic mass matrix for neutrinos. If either the left- or the right-handed Majorana mass vanishes, neutrino and electron zero modes are found to exist. If the harmonic mean of the neutrino Majorana masses is large compared to the Dirac mass, we can prove that normalizable neutrino zero modes cease to exist. This leads to an odd number of fermionic zero modes -- namely, only the electron zero mode -- which we argue implies that the bosonic background is in a topologically distinct sector from the vacuum. This is confirmed by noting that the bosonic sector of the model involves the breaking of a global U(1) symmetry and hence possesses topological global U(1) strings.

hep-th↗
arXiv1999

Observation of Cosmic Acceleration and Determining the Fate of the Universe

Current observations of Type Ia supernovae provide evidence for cosmic acceleration out to a redshift of $z \lsim 1$, leading to the possibility that the universe is entering an inflationary epoch. However, inflation can take place only if vacuum-energy (or other sufficiently slowly redshifting source of energy density) dominates the energy density of a region of physical radius 1/H. We argue that for the best-fit values of $Ω_Λ$ and $Ω_m$ inferred from the supernovae data, one must confirm cosmic acceleration out to at least $z \simeq 1.8$ to infer that the universe is inflating.

astro-ph↗
arXiv1998

Can COBE see the shape of the universe?

In recent years, the large angle COBE--DMR data have been used to place constraints on the size and shape of certain topologically compact models of the universe. Here we show that this approach does not work for generic compact models. In particular, we show that compact hyperbolic models do not suffer the same loss of large angle power seen in flat or spherical models. This follows from applying a topological theorem to show that generic hyperbolic three manifolds support long wavelength fluctuations, and by taking into account the dominant role played by the integrated Sachs-Wolfe effect in a hyperbolic universe.

astro-ph↗
arXiv1998

Circles in the Sky: Finding Topology with the Microwave Background Radiation

If the universe is finite and smaller than the distance to the surface of last scatter, then the signature of the topology of the universe is writ large on the microwave background sky. We show that the microwave background will be identified at the intersections of the surface of last scattering as seen by different ``copies'' of the observer. Since the surface of last scattering is a two-sphere, these intersections will be circles, regardless of the background geometry or topology. We therefore propose a statistic that is sensitive to all small, locally homogeneous topologies. Here, small means that the distance to the surface of last scatter is smaller than the ``topology scale'' of the universe.

astro-ph↗
arXiv1997

Sensitivity of Redshift Distortion Measurements to Cosmological Parameters

The multipole moments of the power spectrum of large scale structure, observed in redshift space, are calculated for a finite sample volume including the effects of both the linear velocity field and geometry. A variance calculation is also performed including the effects of shot noise. The sensitivity with which a survey with the depth and geometry of the Sloan Digital Sky Survey (SDSS) can measure cosmological parameters $Ω_0$ and $b_0$ (the bias) or $λ_0$ (the cosmological constant) and $b_0$ is derived through fitting power spectrum moments to the large scale structure in the linear regime in a way which is independent of the evolution of the galaxy number density. We find that for surveys of the approximate depth of the SDSS no restrictions can be placed on $Ω_0$ at the 99% confidence limit when a fiducial open, $Ω_0 = 0.3 $ model is assumed and bias is unconstrained. At the 95% limit, $Ω_{0} < .85$ is ruled out. Furthermore, for this fiducial model, both flat (cosmological constant) and open models are expected to reasonably fit the data. For flat, cosmological constant models with a fiducial $Ω_{0} = 0.3$, we find that models with $Ω_{0} > 0.48$ are ruled out at the 95% confidence limit regardless of the choice of the bias parameter, and open models cannot fit the data even at the 99% confidence limit.

astro-ph↗