207 research outputs found
Quintessence Models and the Cosmological Evolution of alpha
The cosmological evolution of a quintessence-like scalar field, phi, coupled
to matter and gauge fields leads to effective modifications of the coupling
constants and particle masses over time. We analyze a class of models where the
scalar field potential V(phi) and the couplings to matter B(phi) admit common
extremum in phi, as in the Damour-Polyakov ansatz. We find that even for the
simplest choices of potentials and B(phi), the observational constraints on
delta alpha/alpha coming from quasar absorption spectra, the Oklo phenomenon
and Big Bang nucleosynthesis provide complementary constraints on the
parameters of the model. We show the evolutionary history of these models in
some detail and describe the effects of a varying mass for dark matter.Comment: 26 pages, 20 eps figure
Neutron Moderation in the Oklo Natural Reactor and the Time Variation of alpha
In the analysis of the Oklo (gabon) natural reactor to test for a possible
time variation of the fine structure constant alpha, a Maxwell-Boltzmann low
energy neutron spectrum was assumed. We present here an analysis where a more
realistic spectrum is employed and show that the most recent isotopic analysis
of samples implies a non-zero change in alpha, over the last two billion years
since the reactor was operating, of \Delta\alpha/\alpha\geq 4.5\times 10^{-8}
(6\sigma confidence). Issues regarding the interpretation of the shifts of the
low energy neutron resonances are discussed.Comment: 7 pages, 4 figures; version 2 included reference to Flambaum/Shuryak
work and corrects error in abstract version three corrects a few points and
adds discussion on hydrogen and impurity concentration
Re/Os constraint on the time-variability of the fine-structure constant
We argue that the accuracy by which the isochron parameters of the decay
are determined by dating iron meteorites may
not directly constrain the possible time-dependence of the decay rate and hence
of the fine-structure constant . From this point of view, some of the
attempts to analyze the Oklo constraint and the results of the QSO absorption
lines are re-examined.Comment: 7 pages, 3 figures; v2, revised top sentence on p.
Declarative symbolic pure-logic model checking
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.Includes bibliographical references (p. 173-181).Model checking, a technique for findings errors in systems, involves building a formal model that describes possible system behaviors and correctness conditions, and using a tool to search for model behaviors violating correctness properties. Existing model checkers are well-suited for analyzing control-intensive algorithms (e.g. network protocols with simple node state). Many important analyses, however, fall outside the capabilities of existing model checkers. Examples include checking algorithms with complex state, distributed algorithms over all network topologies, and highly declarative models. This thesis addresses the problem of building an efficient model checker that overcomes these limitations. The work builds on Alloy, a relational modeling language. Previous work has defined the language and shown that it can be analyzed by translation to SAT. The primary contributions of this thesis include: a modeling paradigm for describing complex structures in Alloy; significant improvements in scalability of the analyzer; and improvements in usability of the analyzer via addition of a debugger for over constraints. Together, these changes make model-checking practical for important new classes of analyses. While the work was done in the context of Alloy, some techniques generalize to other verification tools.by Ilya A. Shlyakhter.S.M
Constraining Theories with Temporal Variation of Fine Structure Constant
It is well-known that theories in Einstein frame is conformally
equivalent to quintessence models in which the scalar field minimally couples
with gravity. If there exists a matter system in Jordan frame, then it
interacts with the scalar field in Einstein frame due to the conformal
transformations. This interaction, in general, may lead to changes of
fundamental constants. Here we will consider possible time variation of fine
structure constant in a general theory. We will use observational bounds
on these variations and argue that it provides a criterion for constraining
models.Comment: 10 pages, 6 figures. To appear in Physics Letters B
On The Complexity and Completeness of Static Constraints for Breaking Row and Column Symmetry
We consider a common type of symmetry where we have a matrix of decision
variables with interchangeable rows and columns. A simple and efficient method
to deal with such row and column symmetry is to post symmetry breaking
constraints like DOUBLELEX and SNAKELEX. We provide a number of positive and
negative results on posting such symmetry breaking constraints. On the positive
side, we prove that we can compute in polynomial time a unique representative
of an equivalence class in a matrix model with row and column symmetry if the
number of rows (or of columns) is bounded and in a number of other special
cases. On the negative side, we show that whilst DOUBLELEX and SNAKELEX are
often effective in practice, they can leave a large number of symmetric
solutions in the worst case. In addition, we prove that propagating DOUBLELEX
completely is NP-hard. Finally we consider how to break row, column and value
symmetry, correcting a result in the literature about the safeness of combining
different symmetry breaking constraints. We end with the first experimental
study on how much symmetry is left by DOUBLELEX and SNAKELEX on some benchmark
problems.Comment: To appear in the Proceedings of the 16th International Conference on
Principles and Practice of Constraint Programming (CP 2010
The Oklo bound on the time variation of the fine-structure constant revisited
It has been pointed out by Shlyakhter that data from the natural fission
reactors which operated about two billion years ago at Oklo (Gabon) had the
potential of providing an extremely tight bound on the variability of the
fine-structure constant alpha. We revisit the derivation of such a bound by:
(i) reanalyzing a large selection of published rare-earth data from Oklo, (ii)
critically taking into account the very large uncertainty of the temperature at
which the reactors operated, and (iii) connecting in a new way (using isotope
shift measurements) the Oklo-derived constraint on a possible shift of thermal
neutron-capture resonances with a bound on the time variation of alpha. Our
final (95% C.L.) results are:
-0.9 \times 10^{-7} <(alpha^{Oklo} - alpha^{now})/alpha <1.2\times 10^{-7}
and -6.7 \times 10^{-17} {yr}^{-1} < {\dot alpha}^{averaged}/alpha
<5.0\times10^{-17} {yr}^{-1}$.Comment: 23 pages, Latex, submitted to Nucl.Phys.
Possible time-variability of the fine-structure constant expected from the accelerating universe
We present a theoretical calculation on the time-variability of the
fine-structure constant to fit the result of the recent precise analysis of the
measurement of the QSO absorption lines. We find the parameters and initial
values of the scalar-tensor theory to be determined much more accurately than
fitting the accelerating universe itself, but leading not to easy detections of
the effect on the equation of state of the dark energy in the earlier epochs.Comment: 9 pages, 9 figures. Revised interpretation of the theoretical fit;
more recent reference
- …