arXiv subjects

Lawrence M. Krauss

Publications and source records attributed to Lawrence M. Krauss.

At least 109 records · Page 6Linked to original sources

arXiv1997

Gravitational Lensing Signature of Long Cosmic Strings

The gravitational lensing by long, wiggly cosmic strings is shown to produce a large number of lensed images of a background source. In addition to pairs of images on either side of the string, a number of small images outline the string due to small-scale structure on the string. This image pattern could provide a highly distinctive signature of cosmic strings. Since the optical depth for multiple imaging of distant quasar sources by long strings may be comparable to that by galaxies, these image patterns should be clearly observable in the next generation of redshift surveys such as the Sloan Digital Sky Survey.

astro-ph↗

arXiv1996

Quantum Hair, Instantons, and Black Hole Thermodynamics: Some New Results

We present results obtained by a consideration of the non-classical energy momentum tensor associated with Euclidean Instantons outside the event horizon of black holes. We demonstrate here how this allows an analytic estimate to be made of the effect of discrete quantum hair on the temperature of the black hole, in which the role of violations of the weak energy condition associated with instantons is made explicit, and in which the previous results of Coleman, Preskill, and Wilczek are extended. Last, we demonstrate how the existence of a non-classical electric field outside the event horizon of black holes, uncovered by these authors, can be identified with a well-known effect in the Abelian-Higgs model in two dimensions. In this case, there is a one-to-one connection between the discrete charge of a black hole and a topological phase in two dimensions.

hep-th↗

arXiv1996

Dirty Black Holes and Hairy Black Holes

An approach based on considerations of the non-classical energy momentum tensor outside the event horizon of a black hole provides additional physical insight into the nature of discrete quantum hair on black holes and its effect on black hole temperature. Our analysis both extends previous work based on the Euclidean action techniques, and corrects an omission in that work. We also raise several issues related to the effects of instantons on black hole thermodynamics and the relation between these effects and results in two dimensional quantum field theory.

hep-th↗

arXiv1996

Old Galaxies at High Redshift and the Cosmological Constant

In a recent striking discovery, Dunlop {\bf \it et al} observed a galaxy at redshift z=1.55 with an estimated age of 3.5 Gyr. This is incompatible with age estimates for a flat matter dominated universe unless the Hubble constant is less than $ 45 kms^{-1}Mpc^{-1}$. While both an open universe, and a universe with a cosmological constant alleviate this problem, I argue here that this result favors a non-zero cosmological constant, especially when considered in light of other cosmological constraints. In the first place, for the favored range of matter densities, this constraint is more stringent than the globular cluster age constraint, which already favors a non-zero cosmological constant. Moreover, the age-redshift relation for redshifts of order unity implies that the ratio between the age associated with redshift 1.55 and the present age is also generally larger for a cosmological constant dominated universe than for an open universe. In addition, structure formation is generally suppressed in low density cosmologies, arguing against early galaxy formation. The additional constraints imposed by the new observation on the parameter space of $h$ vs $Ω_{matter}$ (where $H= 100 h kms^{-1}Mpc^{-1}$) are derived for both cosmologies. For a cosmological constant dominated universe this constraint is consistent with the range allowed by other cosmological constraints, which also favor a non-zero value.

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arXiv1996

Problems and Challenges for Cosmology involving Massive Neutrinos

I review the challenges and problems facing the standard cosmological model, involving an $Ω=1$ Universe dominated by non-baryonic dark matter, which arise due to: age estimates of the universe, estimates of the baryon fraction of the universe, and structure formation. Certain of these problems are exacerbated, and certain of these are eased, by the inclusion of some component to the energy density of matter from massive neutrinos. I conclude with a comparison of the two favored current cosmological models, involving either a mixture of cold dark matter and hot dark matter, or the inclusion of a cosmological constant.

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arXiv1996

A Precision Age Determination Technique for Globular Clusters

Globular cluster age estimates based on the absolute magnitude of the main sequence turn-off (\mvto) are generally considered to be the most reliable from a theoretical viewpoint. However, the difficulty in determining \mvto\ in observed colour-magnitude diagrams leads to a large error in the derived age. In this paper, we advocate the use of the absolute magnitude of the point which is brighter than the turn-off and 0.05 mag redder (\mvb) as a precision age indicator. It is easy to measure this point on observed colour-magnitude diagrams, leading to small observational error bars. Furthermore, an extensive Monte Carlo calculation indicates that the theoretical uncertainty in \mvb\ is similar to \mvto. As a result, ages derived using \mvb\ are at least a factor of 2 more precise than those derived using \mvto. This technique is applied to the globular cluster M68 and an age of $12.8\pm 0.3\,$Gyr is derived (assuming $\mvrr = 0.20\,\feh + 0.98$), indicating that M68 is a `young' globular cluster. A homogeneous set of globular cluster age estimates with this precision would provide unprecedented insight into the formation of the Galactic halo.

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arXiv1995

A Lower Limit on the Age of the Universe

We report the results of a detailed numerical study designed to estimate both the absolute age and the uncertainty in age (with confidence limits) of the oldest globular clusters. Such an estimate is essential if a comparison with the Hubble age of the universe is to be made to determine the consistency, or lack thereof, of various cosmological models. Utilizing estimates of the uncertainty range (and distribution) in the input parameters of stellar evolution codes we produced 1000 Monte Carlo realizations of stellar isochrones, with which we could fit the ages of the 18 oldest globular clusters. Incorporating the observational uncertainties in the measured color-magnitude diagrams for these systems and the predicted isochrones, we derived a probability distribution for the mean age of these systems. The one-sided 95$\%$ C.L.~ lower bound for this distribution occurs at an age of 12.07 Gyr. This puts interesting constraints on cosmology which we discuss. Further details, including a description of the distributions, covariance matrices, dependence upon individual input parameters, etc.\ will appear in a future article.

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arXiv1995

The Cosmological Constant is Back

A diverse set of observations now compellingly suggest that 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 wanted 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.

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arXiv1995

Big Bad Nucleosynthesis: A Review

Big Bang Nucleosynthesis represents perhaps the first, and still perhaps the most powerful particle-astrophysics connection. As such, it should provide an example for other work in this area. I discuss the current status of standard model BBN predictions and constraints, and then argue that the issue of observational systematic uncertainties is the key feature limiting our ability to constrain theory with observation. Nevertheless, several very important constraints are currently obtainable. For example, assuming maximal systematic uncertainties in $^4$He, D, and $^7$Li we find a conservative upper limit of $Ω_B \le 0.16$ in order for BBN predictions to agree with observations. Equally significant, we find that BBN predictions are inconsistent unless $^4He$ abundance by mass is greater than $23.9\%$, or D +$^3$He estimates are incorrect. By contrast, unless systematic uncertainties are taken into account, the quoted $2σ$ observational upper limit on the primordial $^4He$ fraction is $23.8\%$.

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arXiv1994

Yet Another Paper on Sn1987a: Large Angle Oscillations, and the Electron Neutrino Mass

We supplement \ML methods with a \MC simulation to re-investigate the SN1987A neutrino burst detection by the IMB and Kamiokande experiments. The detector simulations include background in the the latter and ``dead-time'' in the former. We consider simple neutrinosphere cooling models, explored previously in the literature, to explore the case for or against neutrino vacuum mixing and massive neutrinos. In the former case, involving kinematically irrelevant masses, we find that the full range of vacuum mixing angles, $0 \le \svac \le 1$, is permitted, and the \ML mixing angle is $\svac = .45.$ In the latter case we find that the inclusion of ``dead-time'' reduces previous $\mnue$ upper bounds by 10\%, and supplementing the \ML analysis with a \MC goodness-of-fit test results in a further 15\% reduction in the $\mnue$ upper limit. Our 95\% C.L. upper limit for $\mnue$ is 19.6eV, while the best fit value is $\sim$ 0eV.

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arXiv1994

Big Bang Nucleosynthesis Constraints and Light Element Abundance Estimates

To elucidate the significance of the effect of systematic uncertainties in light element abundance estimates on cosmological bounds derivable from Big Bang Nucleosynthesis (BBN) we present tables giving bounds on $Ω_{baryon}$ and $N_ν$ as one changes the limits on primordial \he and $^7$Li. This allows us to derive new relations between these estimates and constraints on $Ω_{baryon}$ and $N_ν$. For example, only if the helium mass fraction, $Y_p \ge .245$ does $^7$Li (or D) presently play a role in placing an upper limit on the baryon density, and only if $Y_p \ge .250$ does $^4$He cease to play a role in bounding $\etaten$. All the elements combine together tend to give a stringent upper bound of $0.16$ on $Ω_{baryon}$. We also find that $Y_p$ must exceed .239 for consistency between theory and observation if D+$^3$He/H is less than $ 10^{-4}$. Updated nuclear reaction rates, an updated neutron half life, Monte Carlo techniques, and correlations between the predicted abundances are incorporated in our analysis. We also discuss the handling of systematic uncertainties in the context of statistical analyses of BBN predictions.

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arXiv1994

Big Bang Nucleosynthesis and Dark Matter: Baryons and Neutrinos

I present a review of Big Bang Nucleosynthesis, concentrating on the statistical analysis of theoretical uncertainties, and on systematic errors in observed abundances. Both have important implications for constraints on the amount of baryonic dark matter and the number of light neutrino species in nature. Without allowing for systematic uncertainties in abundances, we find that homogenous BBN would lead to the constraint $Ω_B \le .07$ and $N_ν \le 3.07$. Even allowing for maximal systematic uncertainties in $^4$He, and $^7$Li, we find $Ω_B \le .163$. For intermediate ranges we provide new analytic fits for the upper limits to $N_ν$ and $Ω_B$ as a function of the helium mass fraction, $Y_p$.

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arXiv1994

Treating Solar Model Uncertainties: A Consistent Statistical Analysis of Solar Neutrino Models and Data

We describe how to consistently incorporate solar model uncertainties, along with experimental errors and correlations, when analyzing solar neutrino data to derive confidence limits on parameter space for proposed solutions of the solar neutrino problem. Our work resolves ambiguities and inconsistencies in the previous literature. As an application of our methods we calculate the masses and mixing angles allowed by the current data for the proposed MSW solution using both Bayesian and frequentist methods, allowing purely for solar model flux variations, to compare with previous work. We consider the effects of including metal diffusion in the solar models and also discuss implications for future experiments.

hep-ph↗

arXiv1994

Implications of Recent Nucleon Spin Structure Measurements for Neutralino Dark Matter Detection

Predicted rates for direct and indirect detection of dark-matter neutralinos depend in general on the spin content of the nucleon. Neutralinos that are predominantly $B$-ino are the likeliest candidates for detection via spin-dependent interactions. Uncertainties in the measured spin content of the nucleon may lead to dramatic uncertainties in the rates for detection of $B$-inos by scattering off of nuclei with unpaired neutrons. Rates for spin-dependent scattering of $B$-inos off of nuclei with unpaired protons are far more robust, as are rates for capture of $B$-inos in the Sun.

hep-ph↗

arXiv1994

Recent Deuterium Observations and Big Bang Nucleosynthesis: A New Paradigm?

A new observation of D in a primordial gas cloud, made using the high resolution spectrograph at the Keck telescope, indicates an abundance $ D/H =(1.9-2.5) \times 10^{-4}$ \cite{SCHR}. Since deuterium is destroyed by stars, and the predicted Big Bang Nucleosynthesis (BBN) abundance falls monotonically with increasing baryon density, deuterium places a reliable upper limit on the baryon density of the universe. Because the new measurement is substationally larger than previous, galactic estimates, it would force a reassessment of BBN predictions--- if it is confirmed. Using a new BBN Monte Carlo code and analysis technique \cite{KK} we derive constraints implied by a lower limit of $D/H =1.9 \times 10^{-4}$. We find $Ω_B \le .0068h^{-2}$, which is definitively incompatible with baryonic halo dark matter. We also explore implications of combining the D measurement with other light element abundances. $^7Li$ provides a lower bound, $Ω_B \ge .004h^{-2}$. Also, the initial $^4He$ mass fraction ($Y_p$) would have to be less than $23.5\%$, assuming 3 light neutrino species---in good agreement with present best fits. Finally, observational upper limits of $Y_p \le 24 \%$ and $^7Li/H \le 2.3 \times 10^{-10}$ would allow the number of neutrinos to be as big as 3.9.

astro-ph↗

arXiv1994

Refined Big Bang Nucleosynthesis Constraints on $Ω_{Baryon}$ and $N_ν$

We include correlations between elemental abundances in a Monte Carlo statistical analysis of BBN predictions, which, along with updated reaction rates and an improved BBN code, lead to tightened constraints on $Ω_{B}$ and $N_ν$. Observational upper limits on the primordial \he and $\rm {D + {^3He}}$ fractions of $24 \%$ (by weight) and $ 10^{-4}$ respectively lead to the limits: $0.015 \le Ω_{B} \le 0.070$, and $N_ν \le 3.04$. The former limit appears to be incompatible with purely baryonic galactic halo dark matter, while the latter puts qualitatively new constraints on neutrinos, and physics beyond the standard model.

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arXiv1993

Cosmic Variance in CMB Anisotropies: From $1^{\circ}$ to COBE

Cosmic Microwave Background (CMB) anisotropies that result from quantum fluctuations during inflation are explored and the impact of their ``cosmic variance'' on the ability to use existing data to probe inflationary models is studied. We calculate the rms temperature fluctuation, and its cosmic variance, for a number of experiments and for models with primordial power spectra which range from $n={1\over 2}$ to $1$. We find: (1) cosmic variance obscures the information which can be extracted, so a comparison of the rms temperature fluctuation on small scales with the COBE result can fix $n$ to only $\approx\pm 0.2$ at best; (2) measurements of the rms fluctuation on $1^{\circ}$ scales may not allow one to unambiguously infer the tensor contribution to the COBE anisotropy; (3) comparison of this contribution with the predictions of inflation are ambiguous if the quadrupole anisotropy alone is utilized. We discuss means for minimizing the uncertainty due to cosmic variance in comparisons between experiments.

astro-ph↗

arXiv1993

The Impact of COBE on Inflation, Dark Matter, and Structure Formation: a Brief Review

The COBE results, to be followed in the near future by other Cosmic Microwave Background measurements, are providing us with a new fundamental tool to probe the formation of large scale structure in the Universe, and in so doing to constrain our models of the early universe and the nature of dark matter. I describe briefly here how to interpret the COBE results data with these in mind, and conclude with a discussion of implications for the future.

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