18,221 research outputs found
Model-Independent Constraints on Dark Energy Density from Flux-averaging Analysis of Type Ia Supernova Data
We reconstruct the dark energy density as a free function from
current type Ia supernova (SN Ia) data (Tonry et al. 2003; Barris et al. 2003;
Knop et al. 2003), together with the Cosmic Microwave Background (CMB) shift
parameter from CMB data (WMAP, CBI, and ACBAR), and the large scale structure
(LSS) growth factor from 2dF galaxy survey data. We parametrize as
a continuous function, given by interpolating its amplitudes at equally spaced
values in the redshift range covered by SN Ia data, and a constant at
larger (where is only weakly constrained by CMB data). We
assume a flat universe, and use the Markov Chain Monte Carlo (MCMC) technique
in our analysis. We find that the dark energy density is constant
for 0 \la z \la 0.5 and increases with redshift for 0.5 \la z \la 1 at
68.3% confidence level, but is consistent with a constant at 95% confidence
level. For comparison, we also give constraints on a constant equation of state
for the dark energy.
Flux-averaging of SN Ia data is required to yield cosmological parameter
constraints that are free of the bias induced by weak gravitational lensing
\citep{Wang00b}. We set up a consistent framework for flux-averaging analysis
of SN Ia data, based on \cite{Wang00b}. We find that flux-averaging of SN Ia
data leads to slightly lower and smaller time-variation in
. This suggests that a significant increase in the number of SNe Ia
from deep SN surveys on a dedicated telescope \citep{Wang00a} is needed to
place a robust constraint on the time-dependence of the dark energy density.Comment: Slightly revised in presentation, ApJ accepted. One color figure
shows rho_X(z) reconstructed from dat
The geometrically-averaged density of states as a measure of localization
Motivated by current interest in disordered systems of interacting electrons,
the effectiveness of the geometrically averaged density of states,
, as an order parameter for the Anderson transition is
examined. In the context of finite-size systems we examine complications which
arise from finite energy resolution. Furthermore we demonstrate that even in
infinite systems a decline in with increasing disorder
strength is not uniquely associated with localization.Comment: 8 pages, 8 figures; revised text and figure
Future Type Ia Supernova Data as Tests of Dark Energy from Modified Friedmann Equations
In the Cardassian model, dark energy density arises from modifications to the
Friedmann equation, which becomes H^2 = g(\rhom), where g(\rhom) is a new
function of the energy density. The universe is flat, matter dominated, and
accelerating. The distance redshift relation predictions of generalized
Cardassian models can be very different from generic quintessence models, and
can be differentiated with data from upcoming pencil beam surveys of Type Ia
Supernovae such as SNAP. We have found the interesting result that, once
is known to 10% accuracy, SNAP will be able to determine the sign of
the time dependence of the dark energy density. Knowledge of this sign (which
is related to the weak energy condition) will provide a first discrimination
between various cosmological models that fit the current observational data
(cosmological constant, quintessence, Cardassian expansion). Further, we have
performed Monte Carlo simulations to illustrate how well one can reproduce the
form of the dark energy density with SNAP.
To be concrete we study a class of two parameter (,) generalized
Cardassian models that includes the original Cardassian model (parametrized by
only) as a special case. Examples are given of MP Cardassian models that
fit current supernovae and CMB data, and prospects for differentiating between
MP Cardassian and other models in future data are discussed. We also note that
some Cardassian models can satisfy the weak energy condition even with a
dark energy component that has an effective equation of state .Comment: revised version accepted by Ap
Fiber-diffraction Interferometer using Coherent Fiber Optic Taper
We present a fiber-diffraction interferometer using a coherent fiber optic
taper for optical testing in an uncontrolled environment. We use a coherent
fiber optic taper and a single-mode fiber having thermally-expanded core. Part
of the measurement wave coming from a test target is condensed through a fiber
optic taper and spatially filtered from a single-mode fiber to be reference
wave. Vibration of the cavity between the target and the interferometer probe
is common to both reference and measurement waves, thus the interference fringe
is stabilized in an optical way. Generation of the reference wave is stable
even with the target movement. Focus shift of the input measurement wave is
desensitized by a coherent fiber optic taper
Demonstration of fine pitch FCOB (Flip Chip on Board) assembly based on solder bumps at Fermilab
Bump bonding is a superior assembly alternative compared to conventional wire
bond techniques. It offers a highly reliable connection with greatly reduced
parasitic properties. The Flip Chip on Board (FCOB) procedure is an especially
attractive packaging method for applications requiring a large number of
connections at moderate pitch. This paper reports on the successful
demonstration of FCOB assembly based on solder bumps down to 250um pitch using
a SUESS MA8 flip chip bonder at Fermilab. The assembly procedure will be
described, microscopic cross sections of the connections are shown, and first
measurements on the contact resistance are presented.Comment: 4 pages, 8 figure
Gamma-Ray Burst Environments and Progenitors
Likely progenitors for the GRBs (gamma-ray bursts) are the mergers of compact
objects or the explosions of massive stars. These two cases have distinctive
environments for the GRB afterglow: the compact object explosions occur in the
ISM (interstellar medium) and those of massive stars occur in the preburst
stellar wind. We calculate the expected afterglow for a burst in a Wolf-Rayet
star wind and compare the results to those for constant, interstellar density.
The optical afterglow for the wind case is generally expected to decline more
steeply than in the constant density case, but this effect may be masked by
variations in electron spectral index, and the two cases have the same
evolution in the cooling regime. Observations of the concurrent radio and
optical/X-ray evolution are especially useful for distinguishing between the
two cases. The different rates of decline of the optical and X-ray afterglows
of GRB 990123 suggest constant density interaction for this case. We have
previously found strong evidence for wind interaction in SN 1998bw/GRB 980425
and here present a wind model for GRB 980519. We thus suggest that there are
both wind type GRB afterglows with massive star progenitors and ISM type
afterglows with compact binary star progenitors. The wind type bursts are
likely to be accompanied by a supernova, but not the ISM type.Comment: 11 pages, 1 figure, revised version, ApJ Letters, in pres
Parton distribution functions and nuclear EMC effect in a statistical model
A new and simple statistical approach is performed to calculate the parton
distribution functions (PDFs) of the nucleon in terms of light-front kinematic
variables. Analytic expressions of x-dependent PDFs are obtained in the whole x
region. And thereafter, we treat the temperature T as a parameter of the atomic
number A to explain the nuclear EMC effect in the region . We
give the predictions of PDF ratios, and they are very different from those by
other models, thus experiments aiming at measuring PDF ratios are suggested to
provide a discrimination of different models.Comment: 4 pages, no figure; talk given at the 5th International Conference On
Quarks and Nuclear Physics (QNP09), Sep 2009, Beijing Chin
Nonaxisymmetric Evolution of Magnetically Subcritical Clouds: Bar Growth, Core Elongation, and Binary Formation
We have begun a systematic numerical study of the nonlinear growth of
nonaxisymmetric perturbations during the ambipolar diffusion-driven evolution
of initially magnetically subcritical molecular clouds, with an eye on the
formation of binaries, multiple stellar systems and small clusters. In this
initial study, we focus on the (or bar) mode, which is shown to be
unstable during the dynamic collapse phase of cloud evolution after the central
region has become magnetically supercritical. We find that, despite the
presence of a strong magnetic field, the bar can grow fast enough that for a
modest initial perturbation (at 5% level) a large aspect ratio is obtained
during the isothermal phase of cloud collapse. The highly elongated bar is
expected to fragment into small pieces during the subsequent adiabatic phase.
Our calculations suggest that the strong magnetic fields observed in some
star-forming clouds and envisioned in the standard picture of single star
formation do not necessarily suppress bar growth and fragmentation; on the
contrary, they may actually promote these processes, by allowing the clouds to
have more than one (thermal) Jeans mass to begin with without collapsing
promptly. Nonlinear growth of the bar mode in a direction perpendicular to the
magnetic field, coupled with flattening along field lines, leads to the
formation of supercritical cores that are triaxial in general. It removes a
longstanding objection to the standard scenario of isolated star formation
involving subcritical magnetic field and ambipolar diffusion based on the
likely prolate shape inferred for dense cores. Continuted growth of the bar
mode in already elongated starless cores, such as L1544, may lead to future
binary and multiple star formation.Comment: 5 pages, 2 figures, accepted by ApJ
Carbon Free Boston: Transportation Technical Report
Part of a series of reports that includes:
Carbon Free Boston: Summary Report;
Carbon Free Boston: Social Equity Report;
Carbon Free Boston: Technical Summary;
Carbon Free Boston: Buildings Technical Report;
Carbon Free Boston: Waste Technical Report;
Carbon Free Boston: Energy Technical Report;
Carbon Free Boston: Offsets Technical ReportOVERVIEW:
Transportation connects Boston’s workers, residents and tourists to their livelihoods, health care, education,
recreation, culture, and other aspects of life quality. In cities, transit access is a critical factor determining
upward mobility. Yet many urban transportation systems, including Boston’s, underserve some populations
along one or more of those dimensions. Boston has the opportunity and means to expand mobility access to
all residents, and at the same time reduce GHG emissions from transportation. This requires the
transformation of the automobile-centric system that is fueled predominantly by gasoline and diesel fuel.
The near elimination of fossil fuels—combined with more transit, walking, and biking—will curtail air
pollution and crashes, and dramatically reduce the public health impact of transportation. The City embarks
on this transition from a position of strength. Boston is consistently ranked as one of the most walkable and
bikeable cities in the nation, and one in three commuters already take public transportation.
There are three general strategies to reaching a carbon-neutral transportation system:
• Shift trips out of automobiles to transit, biking, and walking;1
• Reduce automobile trips via land use planning that encourages denser development and affordable
housing in transit-rich neighborhoods;
• Shift most automobiles, trucks, buses, and trains to zero-GHG electricity.
Even with Boston’s strong transit foundation, a carbon-neutral transportation system requires a wholesale
change in Boston’s transportation culture. Success depends on the intelligent adoption of new technologies,
influencing behavior with strong, equitable, and clearly articulated planning and investment, and effective
collaboration with state and regional partners.Published versio
- …