Engineering topics

Turner, Michael S.

Publications and source records attributed to Turner, Michael S..

At least 19 records

NASA NTRS1994

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 - 10MeV 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 'tau CDM' are larger than standard CDM. Experiments at e(sup +/-) collides may be able to probe such a mass range.

Dodelson, Scott↗

NASA NTRS1994

Second-order reconstruction of the inflationary potential

To first order in the deviation from scale invariance the inflationary potential and its first two derivatives can be expressed in terms of the spectral indices of the scalar and tensor perturbations, n and n(sub T), and their contributions to the variance of the quadrupole CBR temperature anisotropy, S and T. In addition, there is a 'consistency relation' between these quantities: n(sub T) = (-1/ 7)(T/S). We derive the second-order expressions for the inflationary potential and its first two derivatives and the first-order expression for its third derivative, in terms, of n, n(sub T), S, T, and dn/d ln gamma. We also obtain the second-order consistency relation, n(sub T) = (-1/7)(T/S)(1 + 0.11(T/S) + 0.15(n-1)). As an example we consider the exponential potential, the only known case where exact analytic solutions for the perturbation spectra exist. We reconstruct the potential via Taylor expansion (with coefficients calculated at both first and second order), and introduce the Pade approximate as a greatly improved alternative.

Liddle, Andrew R.↗

NASA NTRS1993

Extension of the Parker bound on the flux of magnetic monopoles

The Parker bound on the flux of magnetic monopoles is extended and strengthened. A new limit on the survival and growth of a small galactic seed field is obtained which rules out the possibility that monopoles much lighter than 10 exp 17 GeV/sq c can provide the closure density of the universe.

Adams, Fred C.↗

NASA NTRS1993

Inflation at the electroweak scale

We present a model for slow-rollover inflation where the vacuum energy that drives inflation is of the order of G(F) exp -2; unlike most models, the conversion of vacuum energy to radiation ('reheating') is moderately efficient. The scalar field responsible for inflation is a standard-model singlet, develops a vacuum expectation value of 4 x 10 exp 6 GeV, has a mass of about 1 GeV, and can play a role in electroweak phenomena. We also discuss models where the energy scale of inflation is somewhat larger, but still well below the unification scale.

Knox, Lloyd↗

NASA NTRS1993

Limits on neutrino radiative decay from SN1987A

We calculate limits on the properties of neutrinos using data from gamma ray detectors on the Pioneer Venus Orbiter and Solar Max Mission satellites. A massive neutrino decaying in flight from the supernova would produce gamma rays detectable by these instruments. The lack of such a signal allows us to constrain the mass, radiative lifetime, and branching ratio to photons of a massive neutrino species produced in the supernova.

Jaffe, Andrew H.↗

NASA NTRS1992

Gravitational waves from first-order cosmological phase transitions

A first-order cosmological phase transition that proceeds through the nucleation and collision of true-vacuum bubbles is a potent source of gravitational radiation. Possibilities for such include first-order inflation, grand-unified-theory-symmetry breaking, and electroweak-symmetry breaking. We have calculated gravity-wave production from the collision of two scalar-field vacuum bubbles, and, using an approximation based upon these results, from the collision of 20 to 30 vacuum bubbles. We present estimates of the relic background of gravitational waves produced by a first-order phase transition.

Kosowsky, Arthur↗

NASA NTRS1992

Cosmic microwave background probes models of inflation

Inflation creates both scalar (density) and tensor (gravity wave) metric perturbations. We find that the tensor-mode contribution to the cosmic microwave background anisotropy on large-angular scales can only exceed that of the scalar mode in models where the spectrum of perturbations deviates significantly from scale invariance. If the tensor mode dominates at large-angular scales, then the value of DeltaT/T predicted on 1 deg is less than if the scalar mode dominates, and, for cold-dark-matter models, bias factors greater than 1 can be made consistent with Cosmic Background Explorer (COBE) DMR results.

Davis, Richard L.↗

NASA NTRS1992

Massive Dirac neutrinos and SN 1987A

The wrong-helicity states of a Dirac neutrino can provide an important cooling mechanism for young neutron stars. Based on numerical models of the early cooling of the neutron star associated with SN 1987A which self-consistently incorporate wrong-helicity neutrino emission, it is argued that a Dirac neutrino of mass greater than 30 keV (25 keV if it is degenerate) leads to shortening of the neutrino burst that is inconsistent with the Irvine-Michigan-Brookhaven and Kamiokande II data. If pions are as abundant as nucleons in the cores of neutron stars, the present limit improves to 15 keV.

Burrows, Adam↗

NASA NTRS1992

Constraints to the decays of Dirac neutrinos from SN 1987A

The decay mode of a keV-mass Dirac neutrino is addressed with reference to the nineteen neutrino events associated with SN 1987A that were detected by the Kamiokande II and Irvine-Brookhaven-Michigan detectors. A complementary constraint is presented which is based upon a distinctive signal associated with the decay of wrong-helicity neutrinos that was not seen: high-energy (50 MeV and higher) neutrino events. The absence of such events excludes the decay of wrong-helicity neutrinos into proper-helicity neutrinos for a Dirac neutrino of mass between 1 and 300 keV. The constraint also rules out models of the 17-keV neutrino.

Dodelson, Scott↗

NASA NTRS1992

First-order inflation

I discuss the most recent model of inflation. In first-order inflation the inflationary epoch is associated with a first-order phase transition, with the most likely candidate being GUT symmetry breaking. The transition from the false-vacuum inflationary phase to the true-vacuum radiation-dominated phase proceeds through the nucleation and percolation of true-vacuum bubbles. The first successful and simplest model of first-order inflation, extended inflation, is discussed in some detail: evolution of the cosmic-scale factor, reheating, density perturbations, and the production of gravitational waves both from quantum fluctuations and bubble collisions. Particular attention is paid to the most critical issue in any model of first-order inflation: the requirements on the nucleation rate to ensure a graceful transition from the inflationary phase to the radiation-dominated phase.

Turner, Michael S.↗

NASA NTRS1991

Constraints from primordial nucleosynthesis on the mass of the tau neutrino

It is shown that primordial nucleosynthesis excludes a tau-neutrino mass from 0.3 to 25 MeV (Dirac) and 0.5 to 25 MeV (Majorana) provided that its lifetime is not less than about 1 sec, and from 0.3 to 30 MeV (Dirac) and 0.5 to 32 MeV (Majorana) for a lifetime of not less than about 1000 sec. A modest improvement in the laboratory mass limit - from 35 to 25 MeV - would imply that the tau-neutrino mass must be less than 0.5 MeV (provided the lifetime is not less than about 1 sec).

Kolb, Edward W.↗

NASA NTRS1991

Astrophysics and cosmology confront the 17-keV neutrino

A host of astrophysical and cosmological arguments severely constrain the properties of a 17 keV Dirac neutrino. Such a neutrino must have interactions beyond those of the standard electroweak theory to reduce its cosmic abundance (through decay or annihilation) by a factor of two hundred. A predicament arises because the additional helicity states of the neutrino necessary to construct a Dirac mass must have interactions strong enough to evade the astrophysical bound from SN 1987A, but weak enough to avoid violating the bound from primordial nucleosynthesis.

Kolb, Edward W.↗

NASA NTRS1991

Dirac neutrinos and SN 1987A

Previous work has shown that the cooling of SN 1987A excludes a Dirac-neutrino mass greater than theta(20 keV) for nu(sub e), nu(sub mu), or nu(sub tau). The emission of wrong-helicity, Dirac neutrinos from SN 1987A, is re-examined. It is concluded that the effect of a Dirac neutrino on the cooling of SN 1987A has been underestimated due to neutrino degeneracy and additional emission processes. The limit that follows from the cooling of SN 1987A is believed to be greater (probably much greater) than 10 keV. This result is significant in light of the recent evidence for a 17 keV mass eigenstate that mixes with the electron neutrino.

Turner, Michael S.↗

NASA NTRS1991

Telescope search for a 3-eV to 8-eV axion

Axions of mass 3-8 eV should have a cosmological abundance of about 50/cu cm and reside in rich clusters of galaxies. Their decays to two photons will produce a line at a wavelength of about 3100-8300 A. This effort has searched unsuccessfully for such a feature in the tergalactic light of three rich clusters, closing this 'window', and leaving open only the window from 10 to the -6th to 0.001 eV. This implies that if the axion exists, it likely comprises the dark matter. The present flux limits are of relevance to other relics whose decays produce monoenergetic photons.

Bershady, Matthew A.↗

NASA NTRS1991

Dark matter in the universe

What is the quantity and composition of material in the Universe? This is one of the most fundamental questions we can ask about the Universe, and its answer bears on a number of important issues including the formation of structure in the Universe, and the ultimate fate and the earliest history of the Universe. Moreover, answering this question could lead to the discovery of new particles, as well as shedding light on the nature of the fundamental interactions. At present, only a partial answer is at hand. Most of the radiation in the Universe does not give off detectable radiation; it is dark. The dark matter associated with bright galaxies contributes somewhere between 10 and 30 percent of the critical density; baryonic matter contributes between 1.1 and 12 percent of the critical. The case for the spatially flat, Einstein-de Sitter model is supported by three compelling theoretical arguments - structure formation, the temporal Copernican principle, and inflation - and by some observational data. If Omega is indeed unity, or even just significantly greater than 0.1, then there is a strong case for a Universe comprised of nonbaryonic matter. There are three well motivated particle dark matter candidates: an axion of mass 10 (exp -6) eV to 10 (exp -4) eV; a neutrino of mass 10 GeV to about 3 TeV; or a neutrino of mass 20 eV to 90 eV. All three possibilities can be tested by experiments that are either planned or are underway.

Turner, Michael S.↗

NASA NTRS1991

A proposed search for dark-matter axions in the 0.6-16 micro-eV range

A proposed experiment is described to search for dark matter axions in the mass range 0.6 to 16 micro-eV. The method is based on the Primakoff conversion of axions into monochromatic microwave photons inside a tunable microwave cavity in a large volume high field magnet, as described by Sikivie. This proposal capitalizes on the availability of two Axicell magnets from the decommissioned Mirror Fusion Test Facility (MFTF-B) fusion machine at LLNL. Assuming a local dark matter density in axions of rho = 0.3 GeV/cu cm, the axion would be found or ruled out at the 97 pct. c.l. in the above mass range in 48 months.

Vanbibber, Karl↗

NASA NTRS1991

Astrophysics and cosmology confront the 17 keV neutrino

Kolb, Edward W.↗

NASA NTRS1991

Microwave distortions from collapsing domain-wall bubbles

It has been suggested that large-scale structure can be seeded by a postrecombination phase transition that produces soft domain walls. It is found that oscillating domain-wall bubbles produce a distinctive signature on the microwave sky; hot and cold spots with amplitude characterized by G(sigma)/H0 (sigma is the surface tension of the wall). These fluctuations are non-Gaussian and offer a powerful probe of such models.

Turner, Michael S.↗