2,513 research outputs found
Non-Gravitating Scalar Field in the FRW Background
We study interacting scalar field theory non-minimally coupled to gravity in
the FRW background. We show that for a specific choice of interaction terms,
the energy-momentum tensor of the scalar field vanishes, and as a result the
scalar field does not gravitate. The naive space dependent solution to
equations of motion gives rise to singular field profile. We carefully analyze
the energy-momentum tensor for such a solution and show that the singularity of
the solution gives a subtle contribution to the energy-momentum tensor. The
space dependent solution therefore is not non-gravitating. Our conclusion is
applicable to other space-time dependent non-gravitating solutions as well. We
study hybrid inflation scenario in this model when purely time dependent
non-gravitating field is coupled to another scalar field.Comment: 7 Pages, 2 figures, RevTeX4, v2:added a section on regularized
energy-momentum tensor, references and conclusions modifie
No Evidence Supporting Flare Driven High-Frequency Global Oscillations
The underlying physics that generates the excitations in the global
low-frequency, < 5.3 mHz, solar acoustic power spectrum is a well known process
that is attributed to solar convection; However, a definitive explanation as to
what causes excitations in the high-frequency regime, > 5.3 mHz, has yet to be
found. Karoff and Kjeldsen (Astrophys. J. 678, 73-76, 2008) concluded that
there is a correlation between solar flares and the global high-frequency solar
acoustic waves. We have used the Global Oscillations Network Group (GONG)
helioseismic data in an attempt to verify Karoff and Kjeldsen (2008) results as
well as compare the post-flare acoustic power spectrum to the pre-flare
acoustic power spectrum for 31 solar flares. Among the 31 flares analyzed, we
observe that a decrease in acoustic power after the solar flare is just as
likely as an increase. Furthermore, while we do observe variations in acoustic
power that are most likely associated with the usual p-modes associated with
solar convection, these variations do not show any significant temporal
association with flares. We find no evidence that consistently supports flare
driven high-frequency waves.Comment: 20 pages, 9 figures, Accepted for publication in Solar Physic
Discounting in Games across Time Scales
We introduce two-level discounted games played by two players on a
perfect-information stochastic game graph. The upper level game is a discounted
game and the lower level game is an undiscounted reachability game. Two-level
games model hierarchical and sequential decision making under uncertainty
across different time scales. We show the existence of pure memoryless optimal
strategies for both players and an ordered field property for such games. We
show that if there is only one player (Markov decision processes), then the
values can be computed in polynomial time. It follows that whether the value of
a player is equal to a given rational constant in two-level discounted games
can be decided in NP intersected coNP. We also give an alternate strategy
improvement algorithm to compute the value
Effect of Interband Transitions on the c axis Penetration Depth of Layered Superconductors
The electromagnetic response of a system with two planes per unit cell
involves, in addition to the usual intraband contribution, an added interband
term. These transitions affect the temperature dependence and the magnitude of
the zero temperature c-axis penetration depth. When the interplane hopping is
sufficiently small, the interband transitions dominate the low temperature
behaviour of the penetration depth which then does not reflect the linear
temperature dependence of the intraband term and in comparison becomes quite
flat even for a d-wave gap. It is in this regime that the pseudogap was found
in our previous normal state calculations of the c-axis conductivity, and the
effects are connected.Comment: 8 pages, 5 figure
Properties of Higher-Order Phase Transitions
Experimental evidence for the existence of strictly higher-order phase
transitions (of order three or above in the Ehrenfest sense) is tenuous at
best. However, there is no known physical reason why such transitions should
not exist in nature. Here, higher-order transitions characterized by both
discontinuities and divergences are analysed through the medium of partition
function zeros. Properties of the distributions of zeros are derived, certain
scaling relations are recovered, and new ones are presented.Comment: 12 pages. To be published in Nuclear Physics
The force within: Recommendations via gravitational attraction between items
Recommendation systems rely on various definitions of similarities. These definitions while having numerous design factors in different domains help identify and recommend relevant content. For example, similarity between users, or items, are measured based on, but not limited to, explicit feedback such as ratings, thumbs up; or/and implicit feedback such as clicks, views etc; or/and based on composition of item such as tags, metadata etc. In this paper, we explore a similarity model while very intuitive to find similar items using a very common natural law of attraction between bodies, that is gravitational law. We show how the two attributes, relative mass and distance between the bodies, of gravitation law can be interpreted for an effective personalized recommendations; in both spatial and non-spatial domains. Finally, we illustrate the use of distance and mass in a non-spatial domain and we exhibit the accuracy in recommendations against popular baselines
Fermionic Molecular Dynamics for nuclear dynamics and thermodynamics
A new Fermionic Molecular Dynamics (FMD) model based on a Skyrme functional
is proposed in this paper. After introducing the basic formalism, some first
applications to nuclear structure and nuclear thermodynamics are presentedComment: 5 pages, Proceedings of the French-Japanese Symposium, September
2008. To be published in Int. J. of Mod. Phys.
Multiwavelength Study on Solar and Interplanetary Origins of the Strongest Geomagnetic Storm of Solar Cycle 23
We study the solar sources of an intense geomagnetic storm of solar cycle 23
that occurred on 20 November 2003, based on ground- and space-based
multiwavelength observations. The coronal mass ejections (CMEs) responsible for
the above geomagnetic storm originated from the super-active region NOAA 10501.
We investigate the H-alpha observations of the flare events made with a 15 cm
solar tower telescope at ARIES, Nainital, India. The propagation
characteristics of the CMEs have been derived from the three-dimensional images
of the solar wind (i.e., density and speed) obtained from the interplanetary
scintillation data, supplemented with other ground- and space-based
measurements. The TRACE, SXI and H-alpha observations revealed two successive
ejections (of speeds ~350 and ~100 km/s), originating from the same filament
channel, which were associated with two high speed CMEs (~1223 and ~1660 km/s,
respectively). These two ejections generated propagating fast shock waves
(i.e., fast drifting type II radio bursts) in the corona. The interaction of
these CMEs along the Sun-Earth line has led to the severity of the storm.
According to our investigation, the interplanetary medium consisted of two
merging magnetic clouds (MCs) that preserved their identity during their
propagation. These magnetic clouds made the interplanetary magnetic field (IMF)
southward for a long time, which reconnected with the geomagnetic field,
resulting the super-storm (Dst_peak=-472 nT) on the Earth.Comment: 24 pages, 16 figures, Accepted for publication in Solar Physic
Momentum noise in a quantum point contact
Ballistic electrons flowing through a constriction can transfer momentum to
the lattice and excite a vibration of a free-standing conductor. We show (both
numerically and analytically) that the electromechanical noise power P does not
vanish on the plateaus of quantized conductance -- in contrast to the current
noise. The dependence of on the constriction width can be oscillatory or
stepwise, depending on the geometry. The stepwise increase amounts to an
approximate quantization of momentum noise.Comment: 4 pages including 4 figure
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