9,040 research outputs found
Exact Cosmological Solutions of Theories via Hojman Symmetry
Nowadays, theory has been one of the leading modified gravity theories
to explain the current accelerated expansion of the universe, without invoking
dark energy. It is of interest to find the exact cosmological solutions of
theories. Besides other methods, symmetry has been proved as a powerful
tool to find exact solutions. On the other hand, symmetry might hint the deep
physical structure of a theory, and hence considering symmetry is also well
motivated. As is well known, Noether symmetry has been extensively used in
physics. Recently, the so-called Hojman symmetry was also considered in the
literature. Hojman symmetry directly deals with the equations of motion, rather
than Lagrangian or Hamiltonian, unlike Noether symmetry. In this work, we
consider Hojman symmetry in theories in both the metric and Palatini
formalisms, and find the corresponding exact cosmological solutions of
theories via Hojman symmetry. There exist some new solutions significantly
different from the ones obtained by using Noether symmetry in theories.
To our knowledge, they also have not been found previously in the literature.
This work confirms that Hojman symmetry can bring new features to cosmology and
gravity theories.Comment: 16 pages, revtex4; v2: discussions added, Nucl. Phys. B in press; v3:
published version. arXiv admin note: text overlap with arXiv:1505.0754
Model-Independent Constraints on Lorentz Invariance Violation via the Cosmographic Approach
Since Lorentz invariance plays an important role in modern physics, it is of
interest to test the possible Lorentz invariance violation (LIV). The time-lag
(the arrival time delay between light curves in different energy bands) of
Gamma-ray bursts (GRBs) has been extensively used to this end. However, to our
best knowledge, one or more particular cosmological models were assumed {\it a
priori} in (almost) all of the relevant works in the literature. So, this makes
the results on LIV in those works model-dependent and hence not so robust in
fact. In the present work, we try to avoid this problem by using a
model-independent approach. We calculate the time delay induced by LIV with the
cosmic expansion history given in terms of cosmography, without assuming any
particular cosmological model. Then, we constrain the possible LIV with the
observational data, and find weak hints for LIV.Comment: 15 pages, 4 figures, 3 tables, revtex4; v2: discussions added, Phys.
Lett. B in pres
New Generalizations of Cosmography Inspired by the Pade Approximant
The current accelerated expansion of the universe has been one of the most
important fields in physics and astronomy since 1998. Many cosmological models
have been proposed in the literature to explain this mysterious phenomenon.
Since the nature and cause of the cosmic acceleration are still unknown,
model-independent approaches to study the evolution of the universe are
welcome. One of the powerful model-independent approaches is the so-called
cosmography. It only relies on the cosmological principle, without postulating
any underlying theoretical model. However, there are several shortcomings in
the usual cosmography. For instance, it is plagued with the problem of
divergence (or an unacceptably large error), and it fails to predict the future
evolution of the universe. In the present work, we try to overcome or at least
alleviate these problems, and we propose two new generalizations of cosmography
inspired by the Pad\'e approximant. One is to directly parameterize the
luminosity distance based on the Pad\'e approximant, while the other is to
generalize cosmography with respect to a so-called -shift
, which is also inspired by the Pad\'e approximant.
Then, we confront them with the observational data with the help of the Markov
chain Monte Carlo (MCMC) code emcee, and find that they work fairly well.Comment: 16 pages, 3 tables, 5 figures, revtex4; v2: discussions added, Eur.
Phys. J. C in press; v3: published versio
Interpretation of the " puzzle" in SM and beyond
The recent measurement on the decay constant of shows a discrepancy
between theory and experiment. We study the leptonic and semileptonic decays of
and simultaneously within the standard model by employing a
lightfront quark model. There is space by tuning phenomenological parameters
which can explain the " puzzle" and do not contradict other
experiments on the semileptonic decays. We also investigate the leptonic decays
of D and with a new physics scenario, unparticle physics. The
unparticle effects induce a constructive interference with the standard model
contribution. The nontrivial phase in unparticle physics could produce direct
CP violation which may distinguish it from other new physics scenarios.Comment: 16 pages, 6 figures, be accepted by PR
The Three-body Force and the Tetraquark Interpretation of Light Scalar Mesons
We study the possible tetraquark interpretation of light scalar meson states
, , , within the framework of the
non-relativistic potential model. The wave functions of tetraquark states are
obtained in a space spanned by multiple Gaussian functions. We find that the
mass spectra of the light scalar mesons can be well accommodated in the
tetraquark picture if we introduce a three-body quark interaction in the quark
model. Using the obtained multiple Gaussian wave functions, the decay constants
of tetraquarks are also calculated within the ``fall apart'' mechanism
Phosphorylation Mechanism of Phosphomevalonate Kinase: Implications for Rational Engineering of Isoprenoid Biosynthetic Pathway Enzymes
The mevalonate pathway is of important
clinical, pharmaceutical,
and biotechnological relevance. However, lack of the understanding
of the phosphorylation mechanism of the kinases in this pathway has
limited rationally engineering the kinases in industry. Here the phosphorylation
reaction mechanism of a representative kinase in the mevalonate pathway,
phosphomevalonate kinase, was studied by using molecular dynamics
and hybrid QM/MM methods. We find that a conserved residue (Ser106)
is reorientated to anchor ATP via a stable H-bond interaction. In
addition, Ser213 located on the α-helix at the catalytic site
is repositioned to further approach the substrate, facilitating the
proton transfer during the phosphorylation. Furthermore, we elucidate
that Lys101 functions to neutralize the negative charge developed
at the β-, γ-bridging oxygen atom of ATP during phosphoryl
transfer. We demonstrate that the dissociative catalytic reaction
occurs via a direct phosphorylation pathway. This is the first study
on the phosphorylation mechanism of a mevalonate pathway kinase. The
elucidation of the catalytic mechanism not only sheds light on the
common catalytic mechanism of the GHMP kinase superfamily but also
provides the structural basis for engineering the mevalonate pathway
kinases to further exploit their applications in the production of
a wide range of fine chemicals such as biofuels or pharmaceuticals
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