arXiv subjects

Michael S. Turner

Publications and source records attributed to Michael S. Turner.

At least 109 records · Page 6Linked to original sources

arXiv1995

Gravitational Microlensing and the Galactic Halo

By means of extensive galactic modeling we study the implications of the more than eighty microlensing events that have now been observed for the composition of the dark halo of the Galaxy, as well as for other properties of the Galaxy. We take the Galaxy to be comprised of luminous and dark disk components, a bulge, and a dark halo consisting of both MACHOs and cold dark matter with each component being described by several observationally motivated parameters. We pare down an initial model space of millions of galactic models to viable models, those which are consistent with the observational data, including rotation curve, local projected mass density, and microlensing rates toward the LMC and bulge. On the basis of a conservative, minimal set of observational constraints an all-MACHO halo cannot yet be excluded, although in most viable models of the Galaxy the halo MACHO fraction is between 0\% and 30\%, consistent with expectations for a universe whose primary component is cold dark matter. An all-MACHO halo is required to be light, and when data on the local escape velocity and satellite-galaxy proper motions, which probe the extent of the dark halo, are taken into account, models which have a high MACHO mass fraction are ruled out. We also explore the possibility that there are no MACHOs in the halo. Finally, we point out several important tests that could definitively exclude an all-MACHO e.g., optical depth for microlensing which is less than $1.5\times 10^{-7}$ toward the LMC or greater than $3\times 10^{-6}$ toward the bulge.

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arXiv1995

Assessing Big-Bang Nucleosynthesis

Systematic uncertainties in the light-element abundances and their evolution make a rigorous statistical assessment difficult. However, using Bayesian methods we show that the following statement is robust: the predicted and measured abundances are consistent with 95\% credibility only if the baryon-to-photon ratio is between $2\times 10^{-10}$ and $6.5\times 10^{-10}$ and the number of light neutrino species is less than 3.9. Our analysis suggests that the $^4$He abundance may have been systematically underestimated.

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arXiv1995

CBR Anisotropy and the Running of the Scalar Spectral Index

Accurate ($\lesssim 1\% $) predictions for the anisotropy of the Cosmic Background Radiation (CBR) are essential for using future high-resolution ($\lesssim 1^\circ$) CBR maps to test cosmological models. In many inflationary models the variation (``running'') of the spectral index of the spectrum of density perturbations is a significant effect and leads to changes of around 1\% to 10\% in the CBR power spectrum. We propose a general method for taking running into account which uses the derivative of the spectral index ($dn/d\ln k$). Conversely, high-resolution CBR maps may be able to determine $dn/d\ln k$, giving important information about the inflationary potential.

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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

The Hot Big Bang and Beyond

The hot big-bang cosmology provides a reliable accounting of the Universe from about $10^{-2}\sec$ after the bang until the present, as well as a robust framework for speculating back to times as early as $10^{-43}\sec$. Cosmology faces a number of important challenges; foremost among them are determining the quantity and composition of matter in the Universe and developing a detailed and coherent picture of how structure (galaxies, clusters of galaxies, superclusters, voids, great walls, and so on) developed. At present there is a working hypothesis---cold dark matter---which is based upon inflation and which, if correct, would extend the big bang model back to $10^{-32}\sec$ and cast important light on the unification of the forces. Many experiments and observations, from CBR anisotropy experiments to Hubble Space Telescope observations to experiments at Fermilab and CERN, are now putting the cold dark matter theory to the test. At present it appears that the theory is viable only if the Hubble constant is smaller than current measurements indicate (around $30\kms\Mpc^{-1}$), or if the theory is modified slightly, e.g., by the addition of a cosmological constant, a small admixture of hot dark matter ($5\eV$ ``worth of neutrinos''), more relativistic particles, or a tilted spectrum of density perturbations.

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arXiv1994

Microlensing and Halo Cold Dark Matter

We discuss the implications of the more than 50 microlensing events seen by the EROS, MACHO, and OGLE collaborations for the composition of the halo of our galaxy. The event rates indicate that the halo mass fraction in MACHO's is less than 30\%, consistent with expectations for a universe whose primary component is cold dark matter. We caution that the uncertainties are such that a larger MACHO fraction cannot yet be excluded.

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arXiv1994

MeV Tau Neutrino: Astrophysical and Cosmological Constraints and Mischief

Terrestrial and ``Heavenly'' experiments severely constrain the mass and lifetime of an MeV tau neutrino. Irrespective of decay mode, for $τ_ν\ga 300\sec$ the mass of the tau neutrino must be either approximately $30\MeV$ or less than $0.4\MeV$ (Majorana), $15\keV$ (Dirac). If the dominant decay mode includes electromagnetic daughter products, the mass must be less than $0.4\MeV$ (Majorana or Dirac) provided $τ_ν\ga 2.5\times 10^{-12}\sec$, $15\keV$ (Dirac) provided $τ_ν\ga 10^{-6}\sec (m_ν/\MeV )$. A tau neutrino of mass between $1\MeV$ and $30\MeV$ can have a host of interesting astrophysical and cosmological consequences: relaxing the big-bang nucleosynthesis bound to the baryon density and the number of neutrino species, allowing big-bang nucleosynthesis to accommodate a low (less than 22\%) $^4$He mass fraction or high (greater than $10^{-4}$) deuterium abundance, improving significantly the agreement between the cold dark matter theory of structure formation and observations, and helping to explain how type II supernovae explode. Exploring the MeV mass range not only probes fundamental particle physics, but also interesting astrophysical and cosmological scenarios.

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arXiv1994

Detectability of Tensor Perturbations Through CBR Anisotropy (final published version)

Detection of the tensor perturbations predicted in inflationary models is important for testing inflation as well as for reconstructing the inflationary potential. We show that because of cosmic variance the tensor contribution to the square of the CBR quadrupole anisotropy must be greater than about 20\% of the scalar contribution to ensure a statistically significant detection of tensor perturbations. This sensitivity could be achieved by full-sky measurements on angular scales of $3^{\circ}$ and $0.5^\circ$.

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arXiv1994

Big-Bang Nucleosynthesis and the Baryon Density of the Universe

Big-bang nucleosynthesis is one of the cornerstones of the standard cosmology. For almost thirty years its predictions have been used to test the big-bang model to within a fraction of a second of the bang. The concordance that exists between the predicted and observed abundances of D, $^3$He, $^4$He and $^7$Li provides important confirmation of the standard cosmology and leads to the most accurate determination of the baryon density, between $1.7 \times 10^{-31}\gcmm3$ and $4.1\times 10^{-31}\gcmm3$ (corresponding to between about 1\% and 14\% of critical density). This measurement of the density of ordinary matter is crucial to almost every aspect of cosmology and is pivotal to the establishment of two dark-matter problems: (i) most of the baryons are dark, and (ii) if the total mass density is greater than about 14\% of the critical density as many determinations now indicate, the bulk of the dark matter must be ``nonbaryonic,'' comprised of elementary particles left from the earliest moments. We critically review the present status of primordial nucleosynthesis and discuss future prospects.

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arXiv1994

Is a Massive Tau Neutrino Just What Cold Dark Matter Needs?

The cold dark matter (CDM) scenario for structure formation in the Universe is very attractive and has many successes; however, when its spectrum of density perturbations is normalized to the COBE anisotropy measurement the level of inhomogeneity predicted on small scales is too large. This can be remedied by a tau neutrino of mass $1\MeV - 10\MeV$ and lifetime $0.1\sec - 100\sec$ whose decay products include electron neutrinos because it allows the total energy density in relativistic particles to be doubled without interfering with nucleosynthesis. The anisotropies predicted on the degree scale for ``$τ$CDM'' are larger than standard CDM. Experiments at $e^\pm$ colliders may be able to probe such a mass range.

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arXiv1994

Dynamical Solutions to the Horizon and Flatness Problems

We discuss in some detail the requirements on an early-Universe model that solves the horizon and flatness problems during the epoch of classical cosmology ($t\ge t_i\gg 10^{-43}\sec$). We show that a dynamical resolution of the horizon problem requires superluminal expansion (or very close to it) and that a truly satisfactory resolution of the flatness problem requires entropy production. This implies that a proposed class of adiabatic models in which the Planck mass varies by many orders of magnitude cannot fully resolve the flatness problem. Furthermore, we show that, subject to minimal assumptions, such models cannot solve the horizon problem either. Because superluminal expansion and entropy production are the two generic features of inflationary models, our results suggest that inflation, or something very similar, may be the only dynamical solution to the horizon and flatness problems.

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arXiv1993

Halo Cold Dark Matter and Microlensing

There is good evidence that most of the baryons in the Universe are dark and some evidence that most of the matter in the Universe is nonbaryonic with cold dark matter (cdm) being a promising possibility. We discuss expectations for the abundance of baryons and cdm in the halo of our galaxy and locally. We show that in plausible cdm models the local density of cdm is at least $10^{-25}\gcmm3$. We also discuss what one can learn about the the local cdm density from microlensing of stars in the LMC by dark stars in the halo and, based upon a suite of reasonable two-component halo models, conclude that microlensing is not a sensitive probe of the local cdm density.

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arXiv1993

Gravitational Radiation from First-Order Phase Transitions

We consider the stochastic background of gravity waves produced by first-order cosmological phase transitions from two types of sources: colliding bubbles and hydrodynamic turbulence. First we discuss the fluid mechanics of relativistic spherical combustion. We then numerically collide many bubbles expanding at a velocity $v$ and calculate the resulting spectrum of gravitational radiation in the linearized gravity approximation. Our results are expressed as simple functions of the mean bubble separation, the bubble expansion velocity, the latent heat, and the efficiency of converting latent heat to kinetic energy of the bubble walls. We also estimate the gravity waves produced by a Kolmogoroff spectrum of turbulence and find that the characteristic amplitude of gravity waves produced is comparable to that from bubble collisions. Finally, we apply these results to the electroweak transition. Using the one-loop effective potential for the minimal electroweak model, the characteristic amplitude of gravity waves produced is $h\simeq 1.5\times 10^{-27}$ at a characteristic frequency of $4.1\times 10^{-3} \,\rm Hz$ corresponding to $Ω\sim10^{-22}$ in gravity waves, far too small for detection. Gravity waves from more strongly first-order phase transitions, including the electroweak transition in non-minimal models, have better prospects for detection, though probably not by LIGO.

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arXiv1993

The Meaning of Eros/Macho

Most of the mass density in the Universe---and in the halo of our own galaxy---exists in the form of dark matter. Overall, the contribution of luminous matter (in stars) to the mass density of the Universe is less than 1\%; primordial nucleosynthesis indicates that baryons contribute between 1\% and 10\% of the critical density ($0.01h^{-2}\la Ω_B\la 0.02h^{-2}$; $h=$ the Hubble constant in units of $100\kms\Mpc^{-1}$); and other evidence indicates that the total mass density is at least 10\% of critical density, and likely much greater. If the universal density is as low as 10\% of the critical density there may be but one kind of dark matter. More likely, the universal density is greater than 10\%, and there are two kinds of dark matter, and thus two dark matter problems: In what form does the baryonic dark matter exist? and In what form does the nonbaryonic dark matter exist? The MACHO and EROS collaborations have presented evidence for the microlensing of stars in the LMC by $10^{-1\pm 1}M_\odot$ dark objects in the halo of our own galaxy and may well have solved {\it one} of the dark matter puzzles by identifying the form of the baryonic dark matter.

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arXiv1993

Effect of Finite Nucleon Mass on Primordial Nucleosynthesis

We have modified the standard code for primordial nucleosynthesis to include the effect of finite nucleon mass on the weak-interaction rates as calculated by Seckel \cite{seckel}. We find a small, systematic increase in the $^4$He yield, $ΔY \simeq 0.0057\,Y$, which is insensitive to the value of the baryon-to-photon ratio $η$ and slightly larger than Seckel's estimate. The fractional changes in the abundances of D, $^3$He, and $^7$Li range from 0.08\% to 3\% for $10^{-11}\le η\le 10^{-8}$.

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arXiv1993

Tensor Perturbations in Inflationary Models as a Probe of Cosmology

In principle, the tensor metric (gravity-wave) perturbations that arise in inflationary models can, beyond probing the underlying inflationary model, provide information about the Universe: ionization history, presence of a cosmological constant, and epoch of matter-radiation equality. Because tensor perturbations give rise to anisotropy of the cosmic background radiation (CBR) solely through the Sachs-Wolfe effect we are able to calculate analytically the variance of the multipole moments of this part of the CBR temperature anisotropy. In so doing, we carefully take account of the contribution of tensor perturbations that entered the Hubble radius during both the matter-dominated and radiation-dominated epoch by means of a transfer function. The striking feature in the spectrum of multipole amplitudes is a dramatic fall off for $l\ga \sqrt{1+\zl}$, where $\zl$ is the red shift of the last-scattering surface. ...

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arXiv1993

Inflation After COBE: Lectures on Inflationary Cosmology

In these lectures I review the standard hot big-bang cosmology, emphasizing its successes, its shortcomings, and its major challenge---a detailed understanding of the formation of structure in the Universe. I then discuss the motivations for---and the fundamentals of---inflationary cosmology, particularly emphasizing the quantum origin of metric (density and gravity-wave) perturbations. Inflation addresses the shortcomings of the standard cosmology and provides the ``initial data'' for structure formation. I conclude by addressing the implications of inflation for structure formation, evaluating the various cold dark matter models in the light of the recent detection of temperature anisotropies in the cosmic background radiation by COBE. In the near term, the study of structure formation offers a powerful probe of inflation, as well as specific inflationary models.

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arXiv1992

Gravitational Radiation from Colliding Vacuum Bubbles: Envelope Approximation to Many-Bubble Collisions

We introduce an approximation to calculate the gravitational radiation produced by the collision of true-vacuum bubbles that is simple enough to allow the simulation of a phase transition by the collision of hundreds of bubbles. This ``envelope approximation'' neglects the complicated ``overlap'' regions of colliding bubbles and follows only the evolution of the bubble walls. The approximation accurately reproduces previous results for the gravitational radiation from the collision of two scalar-field vacuum bubbles. Using a bubble nucleation rate given by $Γ= Γ_0 e^{βt}$, we simulate a phase transition by colliding 20 to 200 bubbles; the fraction of vacuum energy released into gravity waves is $E_{\rm GW}/E_{\rm vac} = 0.06(H/β)^2$ and the peak of the spectrum occurs at $ω_{\rm max}=1.6β$ ($H^2=8πGρ/3$ is the Hubble constant associated with the false-vacuum phase). The spectrum is very similar to that in the two-bubble case, except that the efficiency of gravity-wave generation is about five times higher, presumably due to the fact that a given bubble collides with many others. Finally, we consider two further ``statistical'' approximations, where the gravitational radiation is computed as an incoherent sum over individual bubbles weighted by the distribution of bubble sizes. These approximations provide reasonable estimates of the gravitational-wave spectrum with far less computation.

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