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Search Results: 1 - 10 of 192516 matches for " Glenn D. Starkman "
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Modifying Gravity: You Can't Always Get What You Want
Glenn D. Starkman
Physics , 2012, DOI: 10.1098/rsta.2011.0292
Abstract: The combination of GR and the Standard Model disagrees with numerous observations on scales from our Solar System up. In the concordance model of cosmology, these contradictions are removed or alleviated by the introduction of three completely independent new components of stress-energy -- the inflaton, dark matter, and dark energy. Each of these in its turn is meant to have (or to currently) dominate the dynamics of the universe. There is still no non-gravitational evidence for any of these dark sectors; nor for the required extensions of the standard model. An alternative is to imagine that GR itself must be modified. Certain coincidences of scale even suggest that one might expect not to have to make three independent. Because they must address the most different types of data, attempts to replace dark matter with modified gravity are the most controversial. A phenomenological model (or family of models), Modified Newtonian Dynamics, has, over the last few years seen several covariant realizations. We discuss a number of challenges that any model that seeks to replace dark matter with modified gravity must face: the loss of Birkhoff's Theorem, and the calculational simplifications it implies; the failure to explain clusters, whether static or interacting, and the consequent need to introduce dark matter of some form, whether hot dark matter neutrinos, or dark fields that arise in new sectors of the modified gravity theory; the intrusion of cosmological expansion into the modified force law, that arises precisely because of the coincidence in scale between the centripetal acceleration at which Newtonian gravity fails in galaxies, and the cosmic acceleration. We conclude with the observation that, although modified gravity may indeed manage to replace dark matter, it is likely to do so by becoming or incorporating, a dark matter theory itself.
Galactic Cosmic Strings as Sources of Primary Antiprotons
Glenn D. Starkman,Tanmay Vachaspati
Physics , 1996, DOI: 10.1103/PhysRevD.53.R6711
Abstract: A possible signature of a class of superconducting cosmic strings trapped in the Milky Way plasma is the emission of low energy antiprotons due to baryon number violating processes on the string. We find the terrestrial flux and the energy spectrum of such antiprotons. Current observational bounds on the flux of low energy antiprotons place a {\it lower} bound on the string tension which is comparable to that given by the electroweak scale.
Squeezing MOND into a Cosmological Scenario
Arthur Lue,Glenn D. Starkman
Physics , 2003, DOI: 10.1103/PhysRevLett.92.131102
Abstract: Explaining the effects of dark matter using modified gravitational dynamics (MOND) has for decades been both an intriguing and controversial possibility. By insisting that the gravitational interaction that accounts for the Newtonian force also drives cosmic expansion, one may kinematically identify which cosmologies are compatible with MOND, without explicit reference to the underlying theory so long as the theory obeys Birkhoff's law. Using this technique, we are able to self-consistently compute a number of quantities of cosmological interest. We find that the critical acceleration a_0 must have a slight source-mass dependence (a_0 ~ M^(1/3)) and that MOND cosmologies are naturally compatible with observed late-time expansion history and the contemporary cosmic acceleration. However, cosmologies that can produce enough density perturbations to account for structure formation are contrived and fine-tuned. Even then, they may be marginally ruled out by evidence of early (z \~ 20) reionization.
Neutrino Masses and Mixing with General Mass Matrices
Glenn D. Starkman,Dejan Stojkovic
Physics , 1999, DOI: 10.1016/S0370-2693(00)00397-X
Abstract: We consider the most general neutrino masses and mixings including Dirac mass terms, M_D, as well as Majorana masses, M_R and M_L. Neither the Majorana nor the Dirac mass matrices are expected to be diagonal in the eigenbasis of weak interactions, and so the resulting eigenstates of the Hamiltonian are admixtures of $\SU(2)_L$ singlet and doublet fields of different ``generations.'' We show that for three generations each of doublet and singlet neutrinos, diagonalization of the Hamiltonian to obtain the propagating eigenstates in the general case requires diagonalization of a $12\times12$ Hermitian matrix, rather than the traditional $6\times6$ symmetric mass matrix. The symmetries of the $12\times12$ matrix {\em are} such that it has 6 pairs of real eigenvalues. Although the standard "see-saw" mechanism remains valid, and indeed the eigenvalues obtained are identical to the standard ones, the correct description of diagonalization and mixing is more complicated. The analogs of the CKM matrix for the light and the heavy neutrinos are nonunitary, enriching the opportunities for CP violation in the full neutrino sector.
The Angular Scale of Topologically-Induced Flat Spots in the Cosmic Microwave Background Radiation
David Olson,Glenn D. Starkman
Physics , 2000, DOI: 10.1088/0264-9381/17/16/301
Abstract: The notion that the topology of the universe need not be that of the universal covering space of its geometry has recently received renewed attention. Generic signatures of cosmological topology have been sought, both in the distribution of objects in the universe, and especially in the temperature fluctuations of the cosmic microwave background radiation (CMBR). One signature identified in the horn topology but hypothesized to be generic is featureless regions or flat spots in the CMBR sky. We show that typical observation points within the cusped 3-manifold m003 from the Snappea census have flat spots with an angular scale of about five degrees for $\Omega_0$=0.3. We expect that this holds for other small volume cusped manifolds with this $\Omega_0$ value.
Galaxy-CMB Lensing
Scott Dodelson,Glenn D. Starkman
Physics , 2003,
Abstract: A long-standing problem in astrophysics is to measure the mass associated with galaxies. Gravitational lensing provides one of the cleanest ways to make this measurement. To date, the most powerful lensing probes of galactic mass have been multiply imaged QSO's (strong lensing of a background point source) and galaxy-galaxy lensing (weak deformation of many background galaxies). Here we point out that the mass associated with galaxies also lenses the Cosmic Microwave Background (CMB) and this effect is potentially detectable in small scale experiments. The signal is small (roughly a few tenths of a microKelvin) but has a characteristic shape and extends out well beyond the visible region of the galaxy.
How a brane cosmological constant can trick us into thinking that w < -1
Arthur Lue,Glenn D. Starkman
Physics , 2004, DOI: 10.1103/PhysRevD.70.101501
Abstract: Observations exploring the contemporary cosmic acceleration have sparked interest in dark energy models possessing equations of state with w < -1. We review how the cosmic expansion history of a Dvali-Gabadadze-Porrati (DGP) braneworld model with a standard brane cosmological constant can mimic that of ordinary 4-dimensional gravity with w < -1 "phantom" dark energy for observationally relevant redshifts. We reinterpret the effective phantom nature of the dark energy as arising from dynamical-screening of the brane cosmological constant in DGP. This unusual variety of expansion history is thus possible without violating the null-energy condition, without ghosts and without any big rip, in a model which seems no more contrived than most evolving dark energy models. We indicate ways by which one may observationally test this effective w < -1 possibility, and differentiate it from ``ordinary'' phantom dark-energy.
Cosmic Deconstructionism
Krauss, Lawrence M.;Starkman, Glenn D.
High Energy Physics - Phenomenology , 2007,
Abstract: Dark Matter that is composed of WIMP remnants of incomplete particle-antiparticle annihilation in the early universe experiences ongoing annihilation in gravitationally bound large scale structure. This annihilation will have important consequences in the perhaps distant cosmic future, as the annihilation time-scale becomes comparable to the age of the universe. Much of large scale structure, from galaxy satellites to galaxy clusters will disappear.
Dark energy, the colored anti-de Sitter vacuum, and LHC phenomenology
Stojkovic, Dejan;Starkman, Glenn D.;Matsuo, Reijiro
High Energy Physics - Phenomenology , 2007, DOI: 10.1103/PhysRevD.77.063006
Abstract: We study the possibility that the current accelerated expansion of the universe is driven by the vacuum energy density of a colored scalar field which is responsible for a phase transition in which the gauge SU(3)_c symmetry breaks. We show that if we are stuck in a SU(3)_c - preserving false vacuum, then SU(3)_c symmetry breaking can be accommodated without violating any experimental QCD bounds or bounds from cosmological observations. Moreover, unless there is an unnatural fine-tuning beyond the usual cosmological constant fine-tuning, the true vacuum state of the universe is anti-de Sitter. The model can likely be tested at the LHC. A possible (though not necessary) consequence of the model is the existence of fractionally charged massive hadrons. The model can be embedded in supersymmetric theories where massive colored scalar fields appear naturally.
The Big Occulting Steerable Satellite (BOSS)
Craig J. Copi,Glenn D. Starkman
Physics , 1999, DOI: 10.1086/308525
Abstract: Natural (such as lunar) occultations have long been used to study sources on small angular scales, while coronographs have been used to study high contrast sources. We propose launching the Big Occulting Steerable Satellite (BOSS), a large steerable occulting satellite to combine both of these techniques. BOSS will have several advantages over standard occulting bodies. BOSS would block all but about 4e-5 of the light at 1 micron in the region of interest around the star for planet detections. Because the occultation occurs outside the telescope, scattering inside the telescope does not degrade this performance. BOSS could be combined with a space telescope at the Earth-Sun L2 point to yield very long integration times, in excess of 3000 seconds. If placed in Earth orbit, integration times of 160--1600 seconds can be achieved from most major telescope sites for objects in over 90% of the sky. Applications for BOSS include direct imaging of planets around nearby stars. Planets separated by as little as 0.1--0.25 arcseconds from the star they orbit could be seen down to a relative intensity as little as 1e-9 around a magnitude 8 (or brighter) star. Other applications include ultra-high resolution imaging of compound sources, such as microlensed stars and quasars, down to a resolution as little as 0.1 milliarcseconds.
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