Mass Density Parameter Research Articles

Inhomogeneous big-bang model, revived, and evolution of the light elements in cosmic rays

Abstract Inhomogeneous big-bang model for primordial nucleosynthesis allows higher universal mass density parameter of baryons than the standard homogeneous model does, which is marginally consistent with recent astronomical suggestion that some kind of dark matter is made of baryons. Observation of the enhanced heavy elements in metal weak dwarfs is proposed to test the inhomogeneous model. The studies of radioactive nuclear reactions help predict the theoretical abundances more precisely.

COBE DMR-normalized open inflation cold dark matter cosmogony

view Abstract Citations (66) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS COBE DMR--normalized Open Inflation, Cold Dark Matter Cosmology Gorski, Krzysztof M. ; Ratra, Bharat ; Sugiyama, Naoshi ; Banday, Anthony J. Abstract Fourier analysis on a cut sky of the two-year COBE DMR 53 and 90 GHz sky maps (in galactic coordinates) is used to determine the normalisation of an open inflation model based on the cold dark matter scenario. The normalised model is compared to measures of large-scale structure in the universe. Although the DMR data alone does not provide sufficient discriminative power to prefer a particular value of the mass density parameter, the open model appears to be reasonably consistent with observations when Omega_0 ~ 0.3 - 0.4 and merits further study. Publication: The Astrophysical Journal Pub Date: May 1995 DOI: 10.1086/187861 arXiv: arXiv:astro-ph/9502034 Bibcode: 1995ApJ...444L..65G Keywords: Background Radiation; Cosmology; Dark Matter; Density Distribution; Galactic Evolution; Universe; Anisotropy; Mass Distribution; Mathematical Models; Maximum Likelihood Estimates; Microwaves; Power Spectra; Astrophysics; COSMOLOGY: COSMIC MICROWAVE BACKGROUND; COSMOLOGY: OBSERVATIONS; COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE; GALAXIES: FORMATION; Astrophysics E-Print: uuencoded file containing 6 postscript files (11 pgs text, 4 figures, 1 table) full text sources arXiv | ADS |

On the photoionization of the intergalactic medium by quasars at high redshift

view Abstract Citations (141) References (71) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Photoionization of the Intergalactic Medium by Quasars at High Redshift Meiksin, Avery ; Madau, Piero Abstract We discuss the reionization of the intergalactic medium (IGM) by quasars at high redshift. We compute the integrated UV background from observed QSOs, taking into account the hydrogen opacity associated with intervening Lyα clouds and Lyman limit systems. We note that the published data appear to indicate a significant underdensity of absorption systems in the Lyα forest with column densities N_HI_ > 10^15^ cm^-2^. This deficit, if real, would result in a reduction of the opacity of the universe at high redshift by a factor of 1.5-3 relative to previous estimates. The QSO contribution over the range z = 3-5 to the metagalactic flux at 912 A may then be as large as 6[(1+z)/4.5]^0.5^ X 10^-22^ ergs cm^-2^ s^-1^ Hz^-1^ sr^-1^ for q_0_= 0, and 3 x 10^-22^ ergs cm^-2^ s^-1^ Hz^-1^ sr^-1^ for q_0_ = 0.5. We show that this ionizing flux is consistent with current 1 σ upper limits to the Gunn-Peterson optical depth τ_GP_ at z ~ 3 if the density of the diffuse component of the IGM satisfies {OMEGA}_D_h_50_^2^ <~ 0.02. Models based on QSO photoionization can generate a value of τ_GP_ < 0.1 at z >~ 3.8 only in the case of a highly clumped medium. We argue that the Lyα forest may contain a large fraction of the baryons in the universe and estimate the corresponding mass density parameter to be in the range 0.002 < {OMEGA}_Lyα_h_50_ < 0.05. We also set constraints on an alternative scenario in which the "intercloud" medium is collisionally ionized, and QSOs photoionize the Lyα forest alone. We conclude that, within the uncertainties, the observed QSOs can provide the required number of ionizing photons at early epochs in several of the models examined. If quasars turn on suddenly, the universe will be completely photoionized within {DELTA}z <~ 0.5 of the turn-on redshift. If instead, QSOs turn on gradually, the associated H II regions may not have fully percolated until z ~ 5-5.5. We argue that a new class of absorption systems, arising from intervening patches of neutral unprocessed material which have not yet been engulfed by an H II region, could be observed in the spectra of QSOs just beyond this breakthrough epoch. We estimate their typical H I column densities, sizes, and velocity widths to be 10^18^-10^20^ cm^-2^, 0.1-1 Mpc, and 100-1000 km s^-1^. The patches are expected to persist as distinct absorption features over an extended redshift interval of width {DELTA}z ~ 0.5-1. Eventually, the increase of the equivalent widths of the lines with redshift results in their collective formation of a Gunn-Peterson absorption trough shortward of the Lyα emission line. We find that, for most successful models, the patches appear when τ_GP_ is still less than unity. Because of the difficulty of measuring such small optical depths at high z, due to the presence of the Lyα forest, neutral patches along the line of sight may provide the first unambiguous detection of a diffuse, primordial IGM. The appearance of these systems would be a sensitive probe of the epoch of reionization. We also discuss the radio signatures of the lines. Although our emphasis is on the photoionization of the IGM by QSOs, the formalism we develop, including the effects of UV attenuation by intervening absorption systems, as well as many of our results, applies to any discrete source of photoionizing radiation of known number density, intensity, and spectrum. Publication: The Astrophysical Journal Pub Date: July 1993 DOI: 10.1086/172898 Bibcode: 1993ApJ...412...34M Keywords: Intergalactic Media; Photoionization; Quasars; Red Shift; Astronomical Models; Computational Astrophysics; H Ii Regions; Ultraviolet Radiation; Astrophysics; COSMOLOGY: OBSERVATIONS; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: QUASARS: GENERAL full text sources ADS | data products NED (9) SIMBAD (4)

Variation of Effective Mass and Level Density Parameter with Nuclear Temperature in Semi-infinite Nuclear Matter

The temperature dependence of the self-energy of a particle is studied in semiinfinite nuclear matter. Based on the finite temperature Green's function Matsubara formalism and applying it to the single particle-states in 208Pb, the calculated results show that the characteristic enhancement of the effective mass at near the Fermi energy disappears at temperature T=3 MeV, and correspondingly, as a consequence the level-density parameter should decrease in agreement with recent empirical data.

Previrialization

view Abstract Citations (62) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Previrialization Peebles, P. J. E. Abstract The relation between the virialized mass concentrations produced in a hierarchical gravitational instability picture and the amplitude of the primeval mass fluctuations is estimated under the Ansatz that one can choose an epoch a = a_x_ that marks the transition between the growth of small departures from homogeneity and the final production of a virialized cluster, and such that at a = a_x_ the mass distribution is usefully approximated as loosely concentrated but discrete protoclusters. The orbits that lead to this configuration are computed by minimizing the action, and the departures of these orbits from homogeneous expansion at high redshifts are used to find the primeval mass fluctuation needed to produce the postulated distribution at a = a_x_. This procedure has the advantage that the primeval density fluctuation that produced the protocluster can be reliably measured and the evolution followed in a way that suppresses artificial two- body relaxation. In the computations presented here, the test protocluster at a = a_x_ is modeled by 50 particles randomly placed in a sphere at mean density 3 times the background. When this test system is isolated, the orbits trace back to primeval density contrast consistent with the spherical model. In the presence of neighboring protoclusters, one sees the effects of previrialization: matter moves in a decidedly nonradial way, increasing the primeval contrast and reducing the collapse factor after the system stops expanding. The protoclusters virialize at half mass density β ~ 30 times the background, and the primeval mass contrast extrapolated to the epoch of virialization in linear perturbation theory is α ~ 8, about 5 times the prediction of the spherical model. It is not yet possible to check whether this is an artifact of the model: it is conceivable that the protoclusters in the model develop out of highly special initial conditions. However, since the nonradial motions leading to previrialization are what one would expect to see in a growing clumpy distribution, the model result seems to be well worth considering, and, if true, would be important for theories of galaxy formation. For example, it would imply that if the dark halo of a giant spiral galaxy formed out of dissipationless matter, the mass would have been assembled at redshift 1 + z ~ 7{OMEGA}^-1/2^, where {OMEGA} is the mass density parameter. Publication: The Astrophysical Journal Pub Date: December 1990 DOI: 10.1086/169456 Bibcode: 1990ApJ...365...27P Keywords: Astronomical Models; Dark Matter; Galactic Clusters; Gravitational Collapse; Mass Distribution; Virial Theorem; Many Body Problem; Perturbation Theory; Spiral Galaxies; Astrophysics; COSMOLOGY; DARK MATTER; GALAXIES: CLUSTERING; GALAXIES: FORMATION; NUMERICAL METHODS full text sources ADS |

Making the massless string massive

We investigate the possibility of associating a finite rest mass density with the relativistic string at the classical level. We choose a modification of the massless Lagrangian which reduces in the non-relativistic limit to the usual non-relativistic string. After studying some simple classical motions of this system, we quantize the massive string in a similar way to the massless case. The Regge trajectories are linear as in the massless case, but their intercepts are shifted downward from unity by an amount proportional to the square of the rest mass density parameter. Our naive quantization procedure is covariant only in 25 space-time dimensions. However, we are able to use the extra degree of freedom implicit in the mass density to modify the Lorentz generators so that covariance is maintained for any dimension less than 25. We envisage hadronic interactions to be introduced through the breaking and joining of strings, as in the massless case, and we discuss the problems associated with the construction of covariant amplitudes when D < 25.