Lawrence M. Krauss

The cosmological constant is back

A diverse set of observations now compellingly suggest that the universe possesses a nonzero cosmological constant. In the context of quantum-field theory a cosmological constant corresponds to the energy density of the vacuum, and the favored value for the cosmological constant corresponds to a very tiny vacuum energy density. We discuss future observational tests for a cosmological constant as well as the fundamental theoretical challenges — and opportunities — that this poses for particle physics and for extending our understanding of the evolution of the universe back to the earliest moments.

A Model for neutrino masses and dark matter

Department of PhysicsSyracuse UniversitySyracuse, NY 13244-1130, USA(Dated: February 1, 2008)We propose a model for neutrino masses that simultaneously results in a new dark matter can-didate, the right-handed neutrino. We derive the dark matter abundance in this model, show howthe hierarchy of neutrino masses is obtained, and verify that the model is compatible with existingexperimental results. The model provides an economical method of unifying two seemingly separatepuzzles in contemporary particle physics and cosmology.

Local Discrete Symmetry and Quantum Mechanical Hair

Abstract A charge operator is constructed for a quantum field theory with an abelian discrete gauge symmetry, and a non-local order parameter is formulated that specifies how the gauge symmetry is realized. If the discrete gauge symmetry is manifest, then the charge inside a large region can be detected at the boundary of the region, even in a theory with no massless gauge fields. This long-range effect has no classical analog; it implies that a black hole can in principle carry “quantum-mechanical hair”. If the gauge group is nonabelian, then a charged particle can transfer charge to a loop of cosmic string via the nonabelian Aharonov-Bohmeffect. The string loop can carry charge even though there is no localized source of charge anywhere on the string or in its vicinity. The “total charge” in a closed universe must vanish, but, if the gauge group is nonabelian and the universe is not simply connected, then the “total charge” is not necessarily the same as the sum of all point charges contained in the universe.

New constraints on “INO” masses from cosmology (I). Supersymmetric “inos”

Abstract In this first paper we derive new constraints on gravitino and photino masses in big bang cosmology. First, in the context of induced supersymmetry breaking we calculate explicitly the gravitino decay rate into gauginos, and find that in the absence of significant dilution the gravitino mass must be ⩾5 × 10 4 GeV in order not to affect nucleosynthesis. We also find in this case that constraints in the lightest R-odd particle, the photino, differ significantly from earlier bounds based on analogy with stable heavy neutrino bounds in the standard model, due to out of equilibrium gravitino decay. In order to avoid both these constraints the gravitino distribution must be severely suppressed. If this is due to inflation, it must occur at a scale ≲10 10 −10 11 GeV.

A Lower Limit on the Age of the Universe

A detailed numerical study was designed and conducted to estimate the absolute age and the uncertainty in age (with confidence limits) of the oldest globular clusters in our galaxy, and hence to put a robust lower bound on the age of the universe. Estimates of the uncertainty range and distribution in the input parameters of stellar evolution codes were used to produce 1000 Monte Carlo realizations of stellar isochrones, which were then used to derive ages for the 17 oldest globular clusters. A probability distribution for the mean age of these systems was derived by incorporating the observational uncertainties in the measured color-magnitude diagrams for these systems and the predicted isochrones. The dominant contribution to the width of the distribution (approximately ±5 percent) resulted from the observational uncertainty in RR-Lyrae variable absolute magnitudes. Subdominant contributions came from the choice of the color table used to translate theoretical luminosities and temperatures to observed magnitudes and colors, as well as from theoretical uncertainties in heavy element abundances and mixing length. The one-sided 95 percent confidence limit lower bound for this distribution occurs at an age of 12.07 × 109 years, and the median age for the distribution is 14.56 × 109 years. These age limits, when compared with the Hubble age estimate, put powerful constraints on cosmology.

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 timescale 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 argue 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. Assuming that consciousness has a physical computational basis, and therefore is ultimately governed by quantum mechanics, life cannot be eternal.

Cold dark matter candidates and the solar neutrino problem

Certain currently proposed weakly interacting elementary particles can have a high probability of solar capture if they make up the Galactic halo. Their present abundance in the Sun is here determined by balancing capture rates against annihilation rates. Both particle physics and cosmological considerations impose constraints on scattering and annihilation cross sections. In general, for the candidate particles here discussed (massive neutrinos, supersymmetric scalar neutrinos, and photinos), the inferred solar abundances are too small by three to four orders of magnitude to solve the solar neutrino problem. Extreme fine tuning, marginally possible in the case of the photino, could increase solar abundances to a level where the neutrino signature would be affected. Otherwise, either a particle with a net cosmological asymmetry, or else a new mechanism for strengthening the existing Majorana suppression of s-wave annihilation at very low energies, would seem to be required.

Low energy supergravity and the anomalous magnetic moment of the muon

Abstract The anomalous magnetic moment of the muon ( g − 2) μ imposes constraints on the masses and mixings of spin-zero leptons, gauge fermions, and Higgs fermions in minimal models of low energy supergravity. We demonstrate that there exist only limited values of the parameters in these models that are ruled out by existing limits on ( g − 2) μ .

DIRECTIONAL SENSITIVITY, WIMP DETECTION, AND THE GALACTIC HALO

Abstract 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 Earths 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.

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.