85,756 research outputs found
Upper limits on neutrino masses from the 2dFGRS and WMAP: the role of priors
Solar, atmospheric, and reactor neutrino experiments have confirmed neutrino
oscillations, implying that neutrinos have non-zero mass, but without pinning
down their absolute masses. While it is established that the effect of
neutrinos on the evolution of cosmic structure is small, the upper limits
derived from large-scale structure data could help significantly to constrain
the absolute scale of the neutrino masses. In a recent paper the 2dF Galaxy
Redshift Survey (2dFGRS) team provided an upper limit m_nu,tot < 2.2 eV, i.e.
approximately 0.7 eV for each of the three neutrino flavours, or phrased in
terms of their contributioin to the matter density, Omega_nu/Omega_m < 0.16.
Here we discuss this analysis in greater detail, considering issues of assumed
'priors' like the matter density Omega_m and the bias of the galaxy
distribution with respect the dark matter distribution. As the suppression of
the power spectrum depends on the ratio Omega_nu/Omega_m, we find that the
out-of- fashion Mixed Dark Matter Model, with Omega_nu=0.2, Omega_m=1 and no
cosmological constant, fits the 2dFGRS power spectrum and the CMB data
reasonably well, but only for a Hubble constant H_0<50 km/s/Mpc. As a
consequence, excluding low values of the Hubble constant, e.g. with the HST Key
Project is important in order to get a strong constraint on the neutrino
masses. We also comment on the improved limit by the WMAP team, and point out
that the main neutrino signature comes from the 2dFGRS and the Lyman alpha
forest.Comment: 24 pages, 12 figures Minor changes to matched version published in
JCA
Helioseismic analysis of the hydrogen partition function in the solar interior
The difference in the adiabatic gradient gamma_1 between inverted solar data
and solar models is analyzed. To obtain deeper insight into the issues of
plasma physics, the so-called ``intrinsic'' difference in gamma_1 is extracted,
that is, the difference due to the change in the equation of state alone. Our
method uses reference models based on two equations of state currently used in
solar modeling, the Mihalas-Hummer-Dappen (MHD) equation of state, and the OPAL
equation of state (developed at Livermore). Solar oscillation frequencies from
the SOI/MDI instrument on board the SOHO spacecraft during its first 144 days
in operation are used. Our results confirm the existence of a subtle effect of
the excited states in hydrogen that was previously studied only theoretically
(Nayfonov & Dappen 1998). The effect stems from internal partition function of
hydrogen, as used in the MHD equation of state. Although it is a pure-hydrogen
effect, it takes place in somewhat deeper layers of the Sun, where more than
90% of hydrogen is ionized, and where the second ionization zone of helium is
located. Therefore, the effect will have to be taken into account in reliable
helioseismic determinations of the astrophysically relevant helium-abundance of
the solar convection zone.Comment: 30 pages, 4 figures, 1 table. Revised version submitted to Ap
Faculty Publications and Creative Works 1998
One of the ways in which we recognize our faculty at the University of New Mexico is through Faculty Publications & Creative Works. An annual publication, it highlights our faculty\u27s scholarly and creative activities and achievements and serves as a compendium of UNM faculty efforts during the 1998 calendar year. Faculty Publications & Creative Works strives to illustrate the depth and breadth of research activities performed throughout our University\u27s laboratories, studios and classrooms. We believe that the communication of individual research is a significant method of sharing concepts and thoughts and ultimately inspiring the birth of new ideas. In support of this, UNM faculty during 1998 produced over 2,457 works, including 1,990 scholarly papers and articles, 69 books, 98 book chapters, 119 reviews, 165 creative works and 16 patents. We are proud of the accomplishments of our faculty which are in part reflected in this book, which illustrates the diversity of intellectual pursuits in support of research and education at the University of New Mexico. Nasir Ahmed, Ph.D. Interim Associate Provost for Research and Dean of Graduate Studie
Faculty Publications and Creative Works 2003
Faculty Publications & Creative Works is an annual compendium of scholarly and creative activities of University of New Mexico faculty during the noted calendar year. It serves to illustrate the robust and active intellectual pursuits conducted by the faculty in support of teaching and research at UNM
The non-linear correlation function and the shapes of virialized halos
The correlation function xi(r) of matter in the non-linear regime is assumed
to be determined by the density profiles rho(r) and the mass distribution n(M)
of virialized halos. The Press--Schechter approach is used to compute n(M), and
the stable clustering hypothesis is used to determine the density profiles of
these Press--Schechter halos. Thus, the shape and amplitude of xi(r) on small
scales is related to the initial power spectrum of density fluctuations.
The case of clustering from scale-free initial conditions is treated in
detail. If n is the slope of the initial power spectrum of density
fluctuations, then stable clustering requires that xi(r)\propto r^{-gamma},
where gamma is a known function of n. If halo--halo correlations can be
neglected, then rho(r)\propto r^{-epsilon}, where epsilon = (gamma+3)/2 =
3(4+n)/(5+n). For all values of n of current interest, this slope is steeper
than the value 3(3+n)/(4+n) that was obtained by Hoffman & Shaham in their
treatment of the shapes of the outer regions of collapsed halos. Our main
result is a prediction for the amplitude of the non-linear correlation
function. The predicted amplitude and its dependence on n are in good
quantitative agreement with N-body simulations of self-similar clustering.
If stable clustering is a good approximation only inside the half-mass radii
of Press--Schechter halos, then the density contrast required for the onset of
stable clustering can be estimated. This density contrast is in the range
~300-600 and increases with the initial slope n, in agreement with estimates
from N-body simulations.Comment: 8 pages, uuencoded, gzipped, postscript, submitted to M
The distribution of pairwise peculiar velocities in the nonlinear regime
The distribution of pairwise, relative peculiar velocities, , on
small nonlinear scales, , is derived from the Press--Schechter approach.
This derivation assumes that Press--Schechter clumps are virialized and
isothermal. The virialized assumption requires that the circular velocity, , where denotes the mass of the clump. The isothermal
assumption means that the circular velocity is independent of radius. Further,
it is assumed that the velocity distribution within a clump is Maxwellian, that
the pairwise relative velocity distribution is isotropic, and that on nonlinear
scales clump-clump motions are unimportant when calculating the distribution of
velocity differences. Comparison with -body simulations shows that, on small
nonlinear scales, all these assumptions are accurate. For most power spectra of
interest, the resulting line of sight, pairwise, relative velocity
distribution, , is well approximated by an exponential, rather
than a Gaussian distribution. This simple Press--Schechter model is also able
to provide a natural explanation for the observed, non-Maxwellian shape of
, the distribution of peculiar velocities.Comment: (MNRAS, in press) 16 pages, uuencode
Numerical solutions of nonlinear elliptic problem using combined-block iterative methods
This thesis is concerned with iterative and monotone methods for numerical
solutions of nonlinear elliptic boundary value problems. The methods we study here
are called block iterative methods, which solve the nonlinear elliptic problems in twodimensional
domain in R2 or higher dimensional domain in Rn. In these methods the
nonlinear boundary value problem is discretized by the finite difference method. Two
iteration processes, block Jacobi and block Gauss-Seidel monotone iterations, are
investigated for computation of solutions of finite difference system using either an
upper solution or a lower solution as the initial iteration. The numerical examples are
presented for both linear and nonlinear problems, and for both block and pointwise
methods. The numerical results are compared and discussed
- …