arXiv subjects

Lawrence M. Krauss

Publications and source records attributed to Lawrence M. Krauss.

At least 91 records · Page 5Linked to original sources

arXiv2000

Angular Signatures for Galactic Halo WIMP Scattering in Direct Detectors: Prospects and Challenges

Angular Sensitivity can provide a key additional tool which might allow unambiguous separation of a signal due to Galactic halo WIMPs from other possible backgrounds in direct detectors. We provide a formalism which allows a calculation of the expected angular distribution of events in terrestrial detectors with angular sensitivity for any incident distribution of Galactic halo dark matter. This can be used as an input when studying the sensitivity of specific detectors to halo WIMPs. We utilize this formalism to examine the expected signature for WIMP dark matter using a variety of existing analytic halo models in order to explore how uncertainty in the Galactic halo distribution impact on the the event rates that may be required to separate a possible WIMP signal from other terrestrial backgrounds. We find that as few as 30 events might be required to disentangle the signal from backgrounds if the WIMP distribution resembles an isothermal sphere distribution. On the other hand, for certain halo distributions, even detectors with fine scale resolution may require in excess of a 100-400 events to distinguish a WIMP signal from backgrounds using angular sensitivity. We also note that for finite thresholds the different energy dependence of spin-dependent scattering cross sections may require a greater number of events to discern a WIMP signal than for spin independent interactions. Finally, we briefly describe ongoing studies aimed at developing strategies to better exploit angular signatures, and the use of N-body simulations to better model the expected halo distribution in predicting the expected signature for direct WIMP detectors.

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arXiv1999

Implications of a New Solar System Population of Neutralinos on Indirect Detection Rates

Recently, a new Solar System population of weakly interacting massive particle (WIMP) dark matter has been proposed to exist. We investigate the implications of this population on indirect signals in neutrino telescopes (due to WIMP annihilations in the Earth) for the case when the WIMP is the lightest neutralino of the MSSM, the minimal supersymmetric extension of the standard model. The velocity distribution and capture rate of this new population is evaluated and the flux of neutrino-induced muons from the center of the Earth in neutrino telescopes is calculated. The strength of the signal is very sensitive to the velocity distribution of the new population. We analytically estimate this distribution using the approximate conservation of the component of the WIMP angular momentum orthogonal to the ecliptic plane. The non-linear problem of combining a fixed capture rate from the standard galactic WIMP population with one rising linearly with time from the new population to obtain the present-day annihilation rate in the Earth is also solved analytically. We show that the effects of the new population can be crucial for masses below around 150 GeV, where enhancements of the predicted muon flux from the center of the Earth by up to a factor of 100 compared to previously published estimates occur. As a result of the new WIMP population, the next generation of neutrino telescopes should be able to probe a much larger region of parameter space in the mass range 60-130 GeV.

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arXiv1999

Gravitational lensing statistics and constraints on the cosmological constant revisited

We re-analyze constraints on the cosmological constant that can be obtained by examining the statistics of strong gravitational lensing of distant quasars by intervening galaxies, focusing on uncertainties in galaxy models (including velocity dispersion, luminosity functions, core radii and magnification bias effects) and on the parameters of the galaxy distribution and luminosity functions. In the process we derive new results on magnification biasing for galaxy lenses with non-zero core radii, and on how to infer the proper velocity dispersions appropriate for use in lensing statistics. We argue that the existing data do not disfavor a large cosmological constant. In fact, for a set of reasonable parameter choices, using the results of 5 optical quasar lensing surveys we find that a maximum likelihood analysis favors a value of $\om$ in the range $ \approx 0.25$-0.55 in a flat universe. An open cosmology is not favored by the same statistical analysis. Systematic uncertainties are likely to be dominant, however, as these results are sensitive to uncertainties in our understanding of galaxy luminosity functions, and dark matter velocity dispersions, as well as the choice of lensing survey, and to a lesser extent the existence of core radii. Further observational work will be required before it is possible to definitively distinguish between cosmological models on the basis of gravitational lensing statistics.

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arXiv1999

The Age of Globular Clusters

I review here recent developments which have affected our understanding of both the absolute age of globular clusters and the uncertainties in this age estimate, and comment on the implications for cosmological models. This present estimate is in agreement with the range long advocated by David Schramm. The major uncertainty in determining ages of globular clusers based upon the absolute magnitude of the main sequence turn-off remains the uncertainty in the distance to these clusters. Estimates of these distances have recently been upwardly revised due to Hipparcos parallax measurements, if one calibrates luminosities of main sequence stars. However, it is important to realize that at the present time, different distance measures are in disagreement. A recent estimate is that the oldest clusters are $11.5 \pm 1.3$ Gyr, implying a one-sided 95% confidence level lower limit of 9.5 Gyr, if statistical parallax distance measures are not incorporated. Incorporating more recent measures, including Hipparcos based statistical parallax measures, raises the mean predicted age to $12.8 \pm 1$ Gyr, with a 95 % confidence range of 10-17 Gyr. I conclude by discussing possible improvements which may allow a more precise age distribution in the near future.

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arXiv1999

Directional Sensitivity, WIMP Detection, and the Galactic Halo

Distinguishing the signals due to scattering of WIMP dark matter off of nuclear targets from those due to background noise is a major challenge. The Earth's motion relative to the galactic halo should produce halo-dependent seasonal modulation in the event rate, but it also should produce an angular signal that is both far stronger and less ambiguous. Distinct patterns in the recoil spectrum can reflect the details of the galactic halo. We derive a new formalism to calculate angular event rates, and present the predicted angular signal for a variety of halo models and calculate the number of events needed to distinguish a dark matter signal from an isotropic background.

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arXiv1999

Geometry and Destiny

The recognition that the cosmological constant may be non-zero forces us to re-evaluate standard notions about the connection between geometry and the fate of our Universe. An open Universe can recollapse, and a closed Universe can expand forever. As a corollary, we point out that there is no set of cosmological observations we can perform that will unambiguously allow us to determine what the ultimate destiny of the Universe will be.

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arXiv1999

Baryogenesis Below The Electroweak Scale

We propose a new alternative for baryogenesis which resolves a number of the problems associated with GUT and electroweak scenarios, and which may allow baryogenesis even in modest extensions of the standard model. If the universe never reheats above the electroweak scale following inflation, GUT baryon production does not occur and at the same time thermal sphalerons, gravitinos and monopoles are not produced in abundance. Nevertheless, non-thermal production of sphaleron configurations via preheating could generate the observed baryon asymmetry of the universe.

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arXiv1999

Life, The Universe, and Nothing: Life and Death in an Ever-Expanding Universe

Current evidence suggests that the cosmological constant is not zero, or that we live in an open universe. We examine the implications for the future under these assumptions, and find that they are striking. If the Universe is cosmological constant-dominated, our ability to probe the evolution of large scale structure will decrease with time ---presently observable distant sources will disappear on a time-scale comparable to the period of stellar burning. Moreover, while the Universe might expand forever, the integrated conscious lifetime of any civilization will be finite, although it can be astronomically long. We find that this latter result is far more general. In the absence of possible exotic and uncertain strong gravitational effects, the total information recoverable by any civilization over the entire history of our universe is finite, and assuming that consciousness has a physical computational basis, life cannot be eternal.

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arXiv1998

A New WIMP Population in the Solar System and New Signals for Dark-Matter Detectors

We describe in detail how perturbations due to the planets can cause a sub-population of WIMPs captured by scattering in surface layers of the Sun to evolve to have orbits which no longer intersect the Sun. We argue that such WIMPs, if their orbit has a semi-major axis less than 1/2 of Jupiter's, can persist in the solar system for cosmological timescales. This leads to a new, previously unanticipated WIMP population intersecting the Earth's orbit. The WIMP-nucleon cross sections required for this population to be significant are precisely those in the range predicted for SUSY dark matter, lying near the present limits obtained by direct underground dark matter searches using cyrogenic detectors. Thus, if a WIMP signal is observed in the next generation of detectors, a potentially measurable signal due to this new population must exist. This signal, lying in the keV range for Germanium detectors, would be complementary to that of galactic halo WIMPs. A comparison of event rates, anisotropies, and annual modulations would not only yield additional confirmation that any claimed signal is indeed WIMP-based, but would also allow one to gain information on the nature of the underlying dark matter model.

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arXiv1998

A New Solar System Population of WIMP Dark Matter

Perturbations due to the planets combined with the non-Coulomb nature of the gravitational potential in the Sun imply that WIMPs that are gravitationally captured by scattering in surface layers of the Sun can evolve into orbits that no longer intersect the Sun. For orbits with a semi-major axis $ < 1/2$ of Jupiter's orbit, such WIMPs can persist in the solar system for $ > 10^9$ years, leading to a previously unanticipated population intersecting Earth's orbit. For WIMPs detectable in the next generation of detectors, this population can provide a complementary signal, in the keV range, to that of galactic halo dark matter.

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arXiv1998

High Temperature Symmetry Breaking, SUSY Flat Directions, and the Monopole Problem

Supersymmetric flat directions allow a generic counterexample to the phenomenon of symmetry restoration at high temperatures. We show that (exponentially) large VEVs can be developed along these directions through temperature-induced `dimensional transmutation'. A minimum with broken gauge charges (e.g. electric charge) can exist at arbitrarily high temperatures. In such a scenario magnetic monopoles are never formed, independent of whether inflation occurs before or after the GUT phase transition, or whether it occurs at all.

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arXiv1998

"Just So" Neutrino Oscillations Are Back

Recent evidence for oscillations of atmospheric neutrinos at Super-Kamiokande suggest, in the simplest see-saw interpretation, neutrino masses such that `just so' vacuum oscillations can explain the solar neutrino deficit. Super-K solar neutrino data provide preliminary support for this interpretation. We describe how the just-so signal---an energy dependent seasonal variation of the event rate, might be detected within the coming years and provide general arguments constraining the sign of the variation. The expected variation at radiochemical detectors may be below present sensitivity, but a significant modulation in the $^7$Be signal could shed light on the physics of the solar core---including a direct measure of the solar core temperature.

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arXiv1998

Gravitational Lensing and Dark Structures

We examine whether a cosmologically significant distribution of dark galaxy group or cluster-sized objects can have an optical depth for multiple imaging of distant background sources which is comparable to that from known galaxies while at the same time producing angular splittings of the same order of magnitude. Our purpose is to explore whether such objects could realistically account for some of the observed lenses. Modeling such systems as isothermal spheres with core radii, and assuming a Schechter-type distribution function, we find that independent of the cosmology (open, flat matter dominated, or flat cosmological constant dominated) an allowed parameter range exists which is comparable in velocity dispersion to that for known compact groups of galaxies, although the preferred core radii are somewhat smaller than that normally assumed for compact groups. Dark cluster-sized objects, on the other hand, cannot reproduce the observed lensing characteristics. If the one known Dark cluster were a good representative of such a distribution, most such objects would not produce multiple images. We also present a result for the angular splitting due to an isothermal sphere lens with non-zero core radius, extending earlier work of Hinshaw and Krauss (1987). Our results are expressed as contour plots for fixed lensing probabilities, and angular splittings.

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arXiv1998

New Signatures and Sources for the Detection of WIMP Dark Matter in the Solar System

I first outline new results on the angular modulation of WIMP dark matter scattering on targets in terrestrial laboratories, based on our uncertainties of the WIMP halo distribution, I then outline an exciting new result which indicates that for the high end of allowed SUSY WIMP scattering cross sections there exists a new distribution of WIMP dark matter in our solar system which could produce a dramatically different signal from halo WIMP dark matter in terrestrial detectors.

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arXiv1998

A New Cosmological Paradigm: the Cosmological Constant and Dark Matter

The Standard Cosmological Model of the 1980's is no more. I describe the definitive evidence that the density of matter is insufficient to result in a flat universe, as well as the mounting evidence that the cosmological constant is not zero. I finally discuss the implications of these results for particle physics and direct searches for non-baryonic dark matter, and demonstrate that the new news is good news.

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arXiv1997

The Age Of Globular Clusters In Light Of Hipparcos: Resolving the Age Problem?

We review five independent techniques which are used to set the distance scale to globular clusters, including subdwarf main sequence fitting utilizing the recent Hipparcos parallax catalogue. These data together all indicate that globular clusters are farther away than previously believed, implying a reduction in age estimates. This new distance scale estimate is combined with a detailed numerical Monte Carlo study designed to assess the uncertainty associated with the theoretical age-turnoff luminosity relationship in order to estimate both the absolute age and uncertainty in age of the oldest globular clusters. Our best estimate for the mean age of the oldest globular clusters is now $11.5\pm 1.3 $Gyr, with a one-sided, 95% confidence level lower limit of 9.5 Gyr. This represents a systematic shift of over 2 $σ$ compared to our earlier estimate, due completely to the new distance scale---which we emphasize is not just due to the Hipparcos data. This now provides a lower limit on the age of the universe which is consistent with either an open universe, or a flat, matter dominated universe (the latter requiring $H_0 \le 67 \kmsmpc$). Our new study also explicitly quantifies how remaining uncertainties in the distance scale and stellar evolution models translate into uncertainties in the derived globular cluster ages. Simple formulae are provided which can be used to update our age estimate as improved determinations for various quantities become available.

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arXiv1997

Hybrid Inflation and Particle Physics

The prototype hybrid SUSY SU(5) inflation models, while well motivated from particle physics, and while allowing an acceptable inflationary phase with little or no fine tuning, are shown to have two fundamental phenomenological problems. (1) They inevitably result in the wrong vacuum after inflation is over; and (2) they do not solve the monopole problem. In order to get around the first problem the level of complexity of these models must be increased. One can also avoid the second problem in this way. We also demonstate another possibility by proposing a new general mechanism to avoid the monopole problem with, or without inflation.

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arXiv1997

The End of the Age Problem, And The Case For A Cosmological Constant Revisited

The lower limit on the age of the universe derived from globular cluster dating techniques, which previously strongly motivated a non-zero cosmological constant, has now been dramatically reduced, allowing consistency for a flat matter dominated universe with a Hubble Constant, $H_0 \le 66 km s^{-1} Mpc^{-1}$. The case for an open universe versus a flat universe with non-zero cosmological constant is reanalyzed in this context, incorporating not only the new age data, but also updates on baryon abundance constraints, and large scale structure arguments. For the first time, the allowed parameter space for the density of non-relativistic matter appears larger for an open universe than for a flat universe with cosmological constant, while a flat universe with zero cosmological constant remains strongly disfavored. Several other preliminary observations suggest a non-zero cosmological constant, but a definitive determination awaits refined measurements of $q_0$, and small scale anisotropies of the Cosmic Microwave background. I argue that fundamental theoretical arguments favor a non-zero cosmological constant over an open universe. However, if either case is confirmed, the challenges posed for fundamental particle physics will be great.

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