arXiv subjects

Glenn D. Starkman

Publications and source records attributed to Glenn D. Starkman.

At least 109 records · Page 6Linked to original sources

arXiv2009

The Virtues of Frugality - Why cosmological observers should release their data slowly

Cosmologists will soon be in a unique position. Observational noise will gradually be replaced by cosmic variance as the dominant source of uncertainty in an increasing number of observations. We reflect on the ramifications for the discovery and verification of new models. If there are features in the full data set that call for a new model, there will be no subsequent observations to test that model's predictions. We give specific examples of the problem by discussing the pitfalls of model discovery by prior adjustment in the context of dark energy models and inflationary theories. We show how the gradual release of data can mitigate this difficulty, allowing anomalies to be identified, and new models to be proposed and tested. We advocate that observers plan for the frugal release of data from future cosmic variance limited observations.

astro-ph.CO↗

arXiv2008

Replacing Anthropy with entropy: Does it work?

Probably not, because there are lots of manifestly unanthropic ways of producing entropy. We demonstrate that the Causal Entropic Principle (CEP), as a replacement for the anthropic principle to explain the properties of the observed universe, suffers from many of the same problems of adopting myopic assumptions in order to predict that various fundamental parameters take approximately the observed values. In particular, we demonstrate that four mechanisms -- black hole production, black hole decay, phase transitions, and dark matter annihilations or decays -- will manifestly change the conclusions of the CEP to predict that we should live in a universe quite different than the one in which we find ourselves.

hep-th↗

arXiv2008

Late Time Decay of the False Vacuum, Measurement, and Quantum Cosmology

The recent suggestion that late time quantum dynamics may be important for resolving cosmological issues associated with our observed universe requires a consideration of several subtle issues associated with quantum cosmology, as we describe here. The resolution of these issues will be important if we are to be able to properly ascribe probability measures associated with eternal inflation, and a string landscape.

gr-qc↗

arXiv2008

Dark energy, the colored anti-de Sitter vacuum, and LHC phenomenology

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.

hep-ph↗

arXiv2007

Difficulties for Compact Composite Object Dark Matter

It has been suggested ``that DM particles are strongly interacting composite macroscopically large objects ... made of well known light quarks (or ... antiquarks)." In doing so it is argued that these compact composite objects (CCOs) are ``natural explanations of many observed data, such as [the] 511 keV line from the bulge of our galaxy" observed by INTEGRAL and the excess of diffuse gamma-rays in the 1-20 MeV band observed by COMPTEL. Here we argue that the atmospheres of positrons that surround CCOs composed of di-antiquark pairs in the favoured Colour-Flavour-Locked superconducting state are sufficiently dense as to stringently limit the penetration of interstellar electrons incident upon them, resulting in an extreme suppression of previously estimated rates of positronium, and hence the flux of 511 keV photons resulting from their decays, and also in the rate of direct electron-positron annihilations, which yield the MeV photons proposed to explain the 1-20 MeV excess. We also demonstrate that even if a fraction of positrons somehow penetrated to the surface of the CCOs, the extremely strong electric fields generated from the bulk antiquark matter would result in the destruction of positronium atoms long before they decay.

astro-ph↗

arXiv2007

Externally Occulted Terrestrial Planet Finder Coronagraph: Simulations and Sensitivities

A multitude of coronagraphic techniques for the space-based direct detection and characterization of exo-solar terrestrial planets are actively being pursued by the astronomical community. Typical coronagraphs have internal shaped focal plane and/or pupil plane occulting masks which block and/or diffract starlight thereby increasing the planet's contrast with respect to its parent star. Past studies have shown that any internal technique is limited by the ability to sense and control amplitude, phase (wavefront) and polarization to exquisite levels - necessitating stressing optical requirements. An alternative and promising technique is to place a starshade, i.e. external occulter, at some distance in front of the telescope. This starshade suppresses most of the starlight before entering the telescope - relaxing optical requirements to that of a more conventional telescope. While an old technique it has been recently been advanced by the recognition that circularly symmetric graded apodizers can be well approximated by shaped binary occulting masks. Indeed optimal shapes have been designed that can achieve smaller inner working angles than conventional coronagraphs and yet have high effective throughput allowing smaller aperture telescopes to achieve the same coronagraphic resolution and similar sensitivity as larger ones. Herein we report on our ongoing modeling, simulation and optimization of external occulters and show sensitivity results with respect to number and shape errors of petals, spectral passband, accuracy of Fresnel propagation, and show results for both filled and segmented aperture telescopes and discuss acquisition and sensing of the occulter's location relative to the telescope.

astro-ph↗

arXiv2007

Solving the cosmic lithium problems with primordial late-decaying particles

We investigate the modifications to predictions for the abundances of light elements from standard Big-Bang nucleosynthesis when exotic late-decaying particles with lifetimes exceeding ~1 sec are prominent in the early Universe. Utilising a model-independent analysis of the properties of these long-lived particles, we identify the parameter space associated with models that are consistent with all observational data and hence resolve the much discussed discrepancies between observations and theoretical predictions for the abundances of Li^7 and Li^6.

astro-ph↗

arXiv2007

Cosmic Deconstructionism

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.

astro-ph↗

arXiv2007

Evaporation of a black hole off of a tense brane

We calculate the gray-body factors for scalar, vector and graviton fields in the background of an exact black hole localized on a tensional 3-brane in a world with two large extra dimensions. Finite brane tension modifies the standard results for the case with of a black hole on a brane with negligible tension. For a black hole of a fixed mass, the power carried away into the bulk diminishes as the tension increases, because the effective Planck constant, and therefore entropy of a fixed mass black hole, increase. In this limit, the semiclassical description of black hole decay becomes more reliable.

hep-th↗

arXiv2006

Why anthropic reasoning cannot predict Lambda

We revisit anthropic arguments purporting to explain the measured value of the cosmological constant. We argue that different ways of assigning probabilities to candidate universes lead to totally different anthropic predictions. As an explicit example, we show that weighting different universes by the total number of possible observations leads to an extremely small probability for observing a value of Lambda equal to or greater than what we now measure. We conclude that anthropic reasoning within the framework of probability as frequency is ill-defined and that in the absence of a fundamental motivation for selecting one weighting scheme over another the anthropic principle cannot be used to explain the value of Lambda, nor, likely, any other physical parameters.

astro-ph↗

arXiv2006

What's the trouble with anthropic reasoning?

Selection effects in cosmology are often invoked to "explain" why some of the fundamental constant of Nature, and in particular the cosmological constant, take on the value they do in our Universe. We briefly review this probabilistic "anthropic reasoning" and we argue that different (equally plausible) ways of assigning probabilities to candidate universes lead to totally different anthropic predictions, presenting an explicit example based on the total number of possible observations observers can carry out. We conclude that in absence of a fundamental motivation for selecting one weighting scheme over another the anthropic principle cannot be used to explain the value of Lambda. .

astro-ph↗

arXiv2006

Production of black holes and their angular momentum distribution in models with split fermions

In models with TeV-scale gravity it is expected that mini black holes will be produced in near-future accelerators. On the other hand, TeV-scale gravity is plagued with many problems like fast proton decay, unacceptably large neutron-antineutron oscillations, flavor changing neutral currents, large mixing between leptons, etc. Most of these problems can be solved if different fermions are localized at different points in the extra dimensions. We study the cross-section for the production of black holes and their angular momentum distribution in these models with "split" fermions. We find that, for a fixed value of the fundamental mass scale, the total production cross section is reduced compared with models where all the fermions are localized at the same point in the extra dimensions. Fermion splitting also implies that the bulk component of the black hole angular momentum must be taken into account in studies of the black hole decay via Hawking radiation.

hep-ph↗

arXiv2006

The Uncorrelated Universe: Statistical Anisotropy and the Vanishing Angular Correlation Function in WMAP Years 1-3

The large-angle (low-ell) correlations of the Cosmic Microwave Background (CMB) as reported by the Wilkinson Microwave Anisotropy Probe (WMAP) after their first year of observations exhibited statistically significant anomalies compared to the predictions of the standard inflationary big-bang model. We suggested then that these implied the presence of a solar system foreground, a systematic correlated with solar system geometry, or both. We re-examine these anomalies for the data from the first three years of WMAP's operation. We show that, despite the identification by the WMAP team of a systematic correlated with the equinoxes and the ecliptic, the anomalies in the first-year Internal Linear Combination (ILC) map persist in the three-year ILC map, in all-but-one case at similar statistical significance. The three-year ILC quadrupole and octopole therefore remain inconsistent with statistical isotropy -- they are correlated with each other (99.6% C.L.), and there are statistically significant correlations with local geometry, especially that of the solar system. The angular two-point correlation function at scales >60 degrees in the regions outside the (kp0) galactic cut, where it is most reliably determined, is approximately zero in all wavebands and is even more discrepant with the best fit LambdaCDM inflationary model than in the first-year data - 99.97% C.L. for the new ILC map. The full-sky ILC map, on the other hand, has a non-vanishing angular two-point correlation function, apparently driven by the region inside the cut, but which does not agree better with LambdaCDM. The role of the newly identified low-ell systematics is more puzzling than reassuring.

astro-ph↗

arXiv2006

Extending the WMAP Bound on the Size of the Universe

Clues to the shape of our Universe can be found by searching the CMB for matching circles of temperature patterns. A full sky search of the CMB, mapped extremely accurately by NASA's WMAP satellite, returned no detection of such matching circles and placed a lower bound on the size of the Universe at 24 Gpc. This lower bound can be extended by optimally filtering the WMAP power spectrum. More stringent bounds can be placed on specific candidate topologies by using a a combination statistic. We use optimal filtering and the combination statistic to rule out the infamous "soccer ball universe'' model.

astro-ph↗

arXiv2006

Information-preserving black holes still do not preserve baryon number and other effective global quantum numbers

It has been claimed recently that the black hole information-loss paradox has been resolved: the evolution of quantum states in the presence of a black hole is unitary and information preserving. We point out that, contrary to some claims in literature, information-preserving black holes still violate baryon number and any other quantum number which follows from an effective (and thus approximate) or anomalous symmetry.

gr-qc↗

arXiv2006

Why black hole production in scattering of cosmic ray neutrinos is generically suppressed

It has been argued that neutrinos originating from ultra-high energy cosmic rays produce black holes deep in the atmosphere in models with TeV-scale quantum gravity. Such black holes would initiate quasi-horizontal showers of particles far above the standard model rate, so that the Auger Observatory would observe hundreds of black hole events. This would provide the first opportunity for experimental study of microscopic black holes. However, any phenomenologically viable model with a low scale of quantum gravity must explain how to preserve protons from rapid decay mediated by virtual black holes. We argue that unless this is accomplished by the gauging of baryon or lepton number, the suppression of proton decay will also suppress quantum gravity mediated lepton-nucleon scattering, and hence black hole production by scattering of ultra-high energy cosmic ray neutrinos in the atmosphere. We demonstrate this explicitly for the split fermion solution to the problem of fast proton decay.

hep-ph↗

arXiv2006

Zero modes on cosmic strings in an external magnetic field

A classical analysis suggests that an external magnetic field can cause trajectories of charge carriers on a superconducting domain wall or cosmic string to bend, thus expelling charge carriers with energy above the mass threshold into the bulk. We study this process by solving the Dirac equation for a fermion of mass $m_f$ and charge $e$, in the background of a domain wall and a magnetic field of strength $B$. We find that the modes of the charge carriers get shifted into the bulk, in agreement with classical expectations. However the dispersion relation for the zero modes changes dramatically -- instead of the usual linear dispersion relation, $ω_k =k$, the new dispersion relation is well fit by $ω\approx m_f tanh(k/k_*)$ where $k_*=m_f$ for a thin wall in the weak field limit, and $k_*=eBw$ for a thick wall of width $w$. This result shows that the energy of the charge carriers on the domain wall remains below the threshold for expulsion even in the presence of an external magnetic field. If charge carriers are expelled due to an additional perturbation, they are most likely to be ejected at the threshold energy $\sim m_f$.

hep-th↗

arXiv2005

Holes in the walls: primordial black holes as a solution to the cosmological domain wall problem

We propose a scenario in which the cosmological domain wall and monopole problems are solved without any fine tuning of the initial conditions or parameters in the Lagrangian of an underlying filed theory. In this scenario domain walls sweep out (unwind) the monopoles from the early universe, then the fast primordial black holes perforate the domain walls, change their topology and destroy them. We find further that the (old vacuum) energy density released from the domain walls could alleviate but not solve the cosmological flatness problem.

hep-ph↗