25,512 research outputs found
A blinded determination of from low-redshift Type Ia supernovae, calibrated by Cepheid variables
Presently a tension exists between values of the Hubble constant
derived from analysis of fluctuations in the Cosmic Microwave Background
by Planck, and local measurements of the expansion using calibrators of type Ia
supernovae (SNe Ia). We perform a blinded reanalysis of Riess et al. 2011 to
measure from low-redshift SNe Ia, calibrated by Cepheid variables and
geometric distances including to NGC 4258. This paper is a demonstration of
techniques to be applied to the Riess et at. 2016 data. Our end-to-end analysis
starts from available CfA3 and LOSS photometry, providing an independent
validation of Riess et al. 2011. We obscure the value of throughout our
analysis and the first stage of the referee process, because calibration of SNe
Ia requires a series of often subtle choices, and the potential for results to
be affected by human bias is significant. Our analysis departs from that of
Riess et al. 2011 by incorporating the covariance matrix method adopted in SNLS
and JLA to quantify SN Ia systematics, and by including a simultaneous fit of
all SN Ia and Cepheid data. We find (stat)
(sys) km s Mpc with a three-galaxy (NGC 4258+LMC+MW) anchor. The
relative uncertainties are 4.3% statistical, 1.1% systematic, and 4.4% total,
larger than in Riess et al. 2011 (3.3% total) and the Efstathiou 2014
reanalysis (3.4% total). Our error budget for is dominated by statistical
errors due to the small size of the supernova sample, whilst the systematic
contribution is dominated by variation in the Cepheid fits, and for the SNe Ia,
uncertainties in the host galaxy mass dependence and Malmquist bias.Comment: 38 pages, 13 figures, 13 tables; accepted for publication in MNRA
Progress in Monte Carlo calculations of Fermi systems: normal liquid 3He
The application of the diffusion Monte Carlo method to a strongly interacting
Fermi system as normal liquid He is explored. We show that the fixed-node
method together with the released-node technique and a systematic method to
analytically improve the nodal surface constitute an efficient strategy to
improve the calculation up to a desired accuracy. This methodology shows
unambiguously that backflow correlations, when properly optimized, are enough
to generate an equation of state of liquid He in excellent agreement with
experimental data from equilibrium up to freezing.Comment: 14 pages, 3 eps figure
An Exact Monte Carlo Method for Continuum Fermion Systems
We offer a new proposal for the Monte Carlo treatment of many-fermion systems
in continuous space. It is based upon Diffusion Monte Carlo with significant
modifications: correlated pairs of random walkers that carry opposite signs;
different functions ``guide'' walkers of different signs; the Gaussians used
for members of a pair are correlated; walkers can cancel so as to conserve
their expected future contributions. We report results for free-fermion systems
and a fermion fluid with 14 He atoms, where it proves stable and correct.
Its computational complexity grows with particle number, but slowly enough to
make interesting physics within reach of contemporary computers.Comment: latex source, 3 separated figures (2 in jpg format, 1 in eps format
Pairing Correlations in the Two-Dimensional Hubbard Model
We present the results of a quantum Monte Carlo study of the extended and
the pairing correlation functions for the two-dimensional Hubbard
model, computed with the constrained-path method. For small lattice sizes and
weak interactions, we find that the pairing correlations are
stronger than the extended pairing correlations and are positive when the
pair separation exceeds several lattice constants. As the system size or the
interaction strength increases, the magnitude of the long-range part of both
correlation functions vanishes.Comment: 4 pages, RevTex, 4 figures included; submitted to Phys. Rev. Let
Effect of confinement potential shape on exchange interaction in coupled quantum dots
Exchange interaction has been studied for electrons in coupled quantum dots
(QD's) by a configuration interaction method using confinement potentials with
different profiles. The confinement potential has been parametrized by a
two-centre power-exponential function, which allows us to investigate various
types of QD's described by either soft or hard potentials of different range.
For the soft (Gaussian) confinement potential the exchange energy decreases
with increasing interdot distance due to the decreasing interdot tunnelling.
For the hard (rectangular-like) confinement potential we have found a
non-monotonic behaviour of the exchange interaction as a function of distance
between the confinement potential centres. In this case, the exchange
interaction energy exhibits a pronounced maximum for the confinement potential
profile which corresponds to the nanostructure composed of the small inner QD
with a deep potential well embedded in the large outer QD with a shallow
potential well. This effect results from the strong localization of electrons
in the inner QD, which leads to the large singlet-triplet splitting.
Implications of this finding for quantum logic operations have been discussed.Comment: 16 pages, including 11 figure
Nanocavities: Optomechanics goes molecular
News and Views.A theoretical framework that interprets Raman scattering as an optomechanical process can be used to understand, and guide, experiments in surface-enhanced Raman spectroscopy.Peer Reviewe
Bilinear Quantum Monte Carlo: Expectations and Energy Differences
We propose a bilinear sampling algorithm in Green's function Monte Carlo for
expectation values of operators that do not commute with the Hamiltonian and
for differences between eigenvalues of different Hamiltonians. The integral
representations of the Schroedinger equations are transformed into two
equations whose solution has the form , where
and are the wavefunctions for the two related systems and
is a kernel chosen to couple and . The Monte Carlo process,
with random walkers on the enlarged configuration space , solves
these equations by generating densities whose asymptotic form is the above
bilinear distribution. With such a distribution, exact Monte Carlo estimators
can be obtained for the expectation values of quantum operators and for energy
differences. We present results of these methods applied to several test
problems, including a model integral equation, and the hydrogen atom.Comment: 27 page
In search of progenitors for supernova-less GRBs 060505 and 060614: re-examination of their afterglows
GRB060505 and GRB060614 are nearby long-duration gamma-ray bursts (LGRBs)
without accompanying supernovae (SNe) down to very strict limits. They thereby
challenge the conventional LGRB-SN connection and naturally give rise to the
question: are there other peculiar features in their afterglows which would
help shed light on their progenitors? To answer this question, we combine new
observational data with published data and investigate the multi-band temporal
and spectral properties of the two afterglows. We find that both afterglows can
be well interpreted within the framework of the jetted standard external shock
wave model, and that the afterglow parameters for both bursts fall well within
the range observed for other LGRBs. Hence, from the properties of the
afterglows there is nothing to suggest that these bursts should have another
progenitor than other LGRBs. Recently, Swift-discovered GRB080503 also has the
spike + tail structure during its prompt gamma-ray emission seemingly similar
to GRB060614. We analyse the prompt emission of this burst and find that this
GRB is actually a hard-spike + hard-tail burst with a spectral lag of
0.80.4 s during its tail emission. Thus, the properties of the prompt
emission of GRB060614 and GRB080503 are clearly different, motivating further
thinking of GRB classification. Finally we note that, whereas the progenitor of
the two SN-less bursts remains uncertain, the core-collapse origin for the
SN-less bursts would be quite certain if a wind-like environment can be
observationally established, e.g, from an optical decay faster than the X-ray
decay in the afterglow's slow cooling phase.Comment: 15 pages, 7 figures, 4 tables, ApJ in press; added Fig. 7 of the
lag-luminosity relatio
Period-doubling bifurcation in strongly anisotropic Bianchi I quantum cosmology
We solve the Wheeler-DeWitt equation for the minisuperspace of a cosmological
model of Bianchi type I with a minimally coupled massive scalar field as
source by generalizing the calculation of Lukash and Schmidt [1]. Contrarily to
other approaches we allow strong anisotropy. Combining analytical and numerical
methods, we apply an adiabatic approximation for , and as new feature we
find a period-doubling bifurcation. This bifurcation takes place near the
cosmological quantum boundary, i.e., the boundary of the quasiclassical region
with oscillating -function where the WKB-approximation is good. The
numerical calculations suggest that such a notion of a ``cosmological quantum
boundary'' is well-defined, because sharply beyond that boundary, the
WKB-approximation is no more applicable at all. This result confirms the
adequateness of the introduction of a cosmological quantum boundary in quantum
cosmology.Comment: Latest update of the paper at
http://www.physik.fu-berlin.de/~mbach/publics.html#
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