331 research outputs found
Limits on Cosmological Variation of Strong Interaction and Quark Masses from Big Bang Nucleosynthesis, Cosmic, Laboratory and Oklo Data
Recent data on cosmological variation of the electromagnetic fine structure
constant from distant quasar (QSO) absorption spectra have inspired a more
general discussion of possible variation of other constants. We discuss
variation of strong scale and quark masses. We derive the limits on their
relative change from (i) primordial Big-Bang Nucleosynthesis (BBN); (ii)
Oklo natural nuclear reactor, (iii) quasar absorption spectra, and (iv)
laboratory measurements of hyperfine intervals.Comment: 10 pages 2 figurs: second version have several references added and
some new comment
Towards a sensitive search for variation of the fine structure constant using radio-frequency E1 transitions in atomic dysprosium
It has been proposed that the radio-frequency electric-dipole (E1) transition
between two nearly degenerate opposite-parity states in atomic dysprosium
should be highly sensitive to possible temporal variation of the fine structure
constant () [V. A. Dzuba, V. V. Flambaum, and J. K. Webb, Phys. Rev. A
{\bf 59}, 230 (1999)]. We analyze here an experimental realization of the
proposed search in progress in our laboratory, which involves monitoring the E1
transition frequency over a period of time using direct frequency counting
techniques. We estimate that a statistical sensitivity of |\adota| \sim
10^{-18}/yr may be achieved and discuss possible systematic effects that may
limit such a measurement.Comment: 8 pages, 7 figure
Current-induced highly dissipative domains in high Tc thin films
We have investigated the resistive response of high Tc thin films submitted
to a high density of current. For this purpose, current pulses were applied
into bridges made of Nd(1.15)Ba(1.85)Cu3O7 and Bi2Sr2CaCu2O8. By recording the
time dependent voltage, we observe that at a certain critical current j*, a
highly dissipative domain develops somewhere along the bridge. The successive
formation of these domains produces stepped I-V characteristics. We present
evidences that these domains are not regions with a temperature above Tc, as
for hot spots. In fact this phenomenon appears to be analog to the nucleation
of phase-slip centers observed in conventional superconductors near Tc, but
here in contrast they appear in a wide temperature range. Under some
conditions, these domains will propagate and destroy the superconductivity
within the whole sample. We have measured the temperature dependence of j* and
found a similar behavior in the two investigated compounds. This temperature
dependence is just the one expected for the depairing current, but the
amplitude is about 100 times smaller.Comment: 9 pages, 9 figures, Revtex, to appear in Phys. Rev.
Charge conservation and time-varying speed of light
It has been recently claimed that cosmologies with time dependent speed of
light might solve some of the problems of the standard cosmological scenario,
as well as inflationary scenarios. In this letter we show that most of these
models, when analyzed in a consistent way, lead to large violations of charge
conservation. Thus, they are severly constrained by experiment, including those
where is a power of the scale factor and those whose source term is the
trace of the energy-momentum tensor. In addition, early Universe scenarios with
a sudden change of related to baryogenesis are discarded.Comment: 4 page
Machine learning-based investigation of the association between CMEs and filaments
YesIn this work we study the association between eruptive filaments/prominences and coronal mass ejections (CMEs) using machine learning-based algorithms that analyse the solar data available between January 1996 and December 2001. The Support Vector Machine (SVM) learning algorithm is used for the purpose of knowledge extraction from the association results. The aim is to identify patterns of associations that can be represented using SVM learning rules for the subsequent use in near real-time and reliable CME prediction systems. Timing and location data in the NGDC filament catalogue and the SOHO/LASCO CME catalogue are processed to associate filaments with CMEs. In the previous studies which classified CMEs into gradual and impulsive CMEs, the associations were refined based on CME speed and acceleration. Then the associated pairs were refined manually to increase the accuracy of the training dataset. In the current study, a data- mining system has been created to process and associate filament and CME data, which are arranged in numerical training vectors. Then the data are fed to SVMs to extract the embedded knowledge and provide the learning rules that could have the potential, in the future, to provide automated predictions of CMEs. The features representing the event time (average of the start and end times), duration, type and extent of the filaments are extracted from all the associated and not-associated filaments and converted to a numerical format that is suitable for SVM use. Several validation and verification methods are used on the extracted dataset to determine if CMEs can be predicted solely and efficiently based on the associated filaments. More than 14000 experiments are carried out to optimise the SVM and determine the input features that provide the best performance
Further evidence for a variable fine-structure constant from Keck/HIRES QSO absorption spectra
[Abridged] We previously presented evidence for a varying fine-structure
constant, alpha, in two independent samples of Keck/HIRES QSO spectra. Here we
present a detailed many-multiplet analysis of a third Keck/HIRES sample
containing 78 absorption systems. We also re-analyse the previous samples,
providing a total of 128 absorption systems over the redshift range
0.2<z_abs<3.7. All three samples separately yield consistent, significant
values of da/a. The analyses of low- and high-z systems rely on different
ions/transitions with very different dependencies on alpha, yet they also give
consistent results. We identify additional random errors in 22 high-z systems
characterized by transitions with a large dynamic range in apparent optical
depth. Increasing the statistical errors on da/a for these systems gives our
fiducial result, a weighted mean da/a=(-0.543+/-0.116)x10^-5, representing
4.7-sigma evidence for a smaller weighted mean alpha in the absorption clouds.
Assuming that da/a=0 at z_abs=0, the data marginally prefer a linear increase
in alpha with time: dota/a=(6.40+/-1.35)x10^-16 yr^-1. The two-point
correlation function for alpha is consistent with zero over 0.2-13 Gpc comoving
scales and the angular distribution of da/a shows no significant dipolar
anisotropy. We therefore have no evidence for spatial variations in da/a. We
extend our previous searches for possible systematic errors, identifying
atmospheric dispersion and isotopic structure effects as potentially the most
significant. However, overall, known systematic errors do not explain the
results. Future many-multiplet analyses of QSO spectra from different
telescopes and spectrographs will provide a now crucial check on our Keck/HIRES
results.Comment: 31 pages, 25 figures (29 EPS files), 8 tables. Accepted by MNRAS.
Colour versions of Figs. 6, 8 & 10 and text version of Table 3 available at
http://www.ast.cam.ac.uk/~mim/pub.htm
Violations of the equivalence principle in a dilaton-runaway scenario
We explore a version of the cosmological dilaton-fixing and decoupling
mechanism in which the dilaton-dependence of the low-energy effective action is
extremized for infinitely large values of the bare string coupling . We study the efficiency with which the dilaton runs away
towards its ``fixed point'' at infinity during a primordial inflationary stage,
and thereby approximately decouples from matter. The residual dilaton couplings
are found to be related to the amplitude of the density fluctuations generated
during inflation. For the simplest inflationary potential, , the residual dilaton couplings are shown to predict
violations of the universality of gravitational acceleration near the level. This suggests that a modest improvement in the
precision of equivalence principle tests might be able to detect the effect of
such a runaway dilaton. Under some assumptions about the coupling of the
dilaton to dark matter and/or dark energy, the expected time-variation of
natural ``constants'' (in particular of the fine-structure constant) might also
be large enough to be within reach of improved experimental or observational
data.Comment: 32 pages, 1 figure, minor change
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