1,515 research outputs found
Dark spinor inflation -- theory primer and dynamics
Inflation driven by a single dark spinor field is discussed. We define the
notion of a dark spinor field and derive the cosmological field equations for
such a matter source. The conditions for inflation are determined and an
exactly solvable model is presented. We find the power spectrum of the quantum
fluctuation of this field and compare the results with scalar field inflation.Comment: 13 pages; typo in Eq. (12) corrected, minor improvement
Physics of dark energy particles
We consider the astrophysical and cosmological implications of the existence
of a minimum density and mass due to the presence of the cosmological constant.
If there is a minimum length in nature, then there is an absolute minimum mass
corresponding to a hypothetical particle with radius of the order of the Planck
length. On the other hand, quantum mechanical considerations suggest a
different minimum mass. These particles associated with the dark energy can be
interpreted as the ``quanta'' of the cosmological constant. We study the
possibility that these particles can form stable stellar-type configurations
through gravitational condensation, and their Jeans and Chandrasekhar masses
are estimated. From the requirement of the energetic stability of the minimum
density configuration on a macroscopic scale one obtains a mass of the order of
10^55 g, of the same order of magnitude as the mass of the universe. This mass
can also be interpreted as the Jeans mass of the dark energy fluid. Furthermore
we present a representation of the cosmological constant and of the total mass
of the universe in terms of `classical' fundamental constants.Comment: 10 pages, no figures; typos corrected, 4 references added; 1
reference added; reference added; entirely revised version, contains new
parts, now 14 page
On the relation between mass of pion, fundamental physical constants and cosmological parameters
In this article we reconsider the old mysterious relation, advocated by Dirac
and Weinberg, between the mass of the pion, the fundamental physical constants,
and the Hubble parameter. By introducing the cosmological density parameters,
we show how the corresponding equation may be written in a form that is
invariant with respect to the expansion of the Universe and without invoking a
varying gravitational "constant", as was originaly proposed by Dirac. It is
suggest that, through this relation, Nature gives a hint that virtual pions
dominante the "content" of the quantum vacuum
Einstein static universes are unstable in generic f(R) models
We study Einstein static universes in the context of generic f(R) models. It
is shown that Einstein static solutions exist for a wide variety of modified
gravity models sourced by a barotropic perfect fluid with equation of state
w=p/rho, but these solutions are always unstable to either homogeneous or
inhomogeneous perturbations. Our general results are in agreement with specific
models investigated in that past. We also discuss how our techniques can be
applied to other scenarios in f(R) gravity.Comment: 7 pages, 2 figures. Minor corrections. Minor changes and references
added to match version accepted by Phys. Rev.
Stability of the Einstein static universe in f(R) gravity
We analyze the stability of the Einstein static universe by considering
homogeneous scalar perturbations in the context of f(R) modified theories of
gravity. By considering specific forms of f(R), the stability regions of the
solutions are parameterized by a linear equation of state parameter w=p/rho.
Contrary to classical general relativity, it is found that in f(R) gravity a
stable Einstein cosmos with a positive cosmological constant does indeed exist.
Thus, we are lead to conclude that, in principle, modifications in f(R) gravity
stabilize solutions which are unstable in general relativity.Comment: 7 pages, 2 figures, 2 tables; references adde
A new two-sphere singularity in general relativity
The Florides solution, proposed as an alternative to the interior
Schwarzschild solution, represents a static and spherically symmetric geometry
with vanishing radial stresses. It is regular at the center, and is matched to
an exterior Schwarzschild solution. The specific case of a constant energy
density has been interpreted as the field inside an Einstein cluster. In this
work, we are interested in analyzing the geometry throughout the permitted
range of the radial coordinate without matching it to the Schwarzschild
exterior spacetime at some constant radius hypersurface. We find an interesting
picture, namely, the solution represents a three-sphere, whose equatorial
two-sphere is singular, in the sense that the curvature invariants and the
tangential pressure diverge. As far as we know, such singularities have not
been discussed before. In the presence of a large negative cosmological
constant (anti-de Sitter) the singularity is removed.Comment: 17 pages, 3 figure
Minimum mass-radius ratio for charged gravitational objects
We rigorously prove that for compact charged general relativistic objects
there is a lower bound for the mass-radius ratio. This result follows from the
same Buchdahl type inequality for charged objects, which has been extensively
used for the proof of the existence of an upper bound for the mass-radius
ratio. The effect of the vacuum energy (a cosmological constant) on the minimum
mass is also taken into account. Several bounds on the total charge, mass and
the vacuum energy for compact charged objects are obtained from the study of
the Ricci scalar invariants. The total energy (including the gravitational one)
and the stability of the objects with minimum mass-radius ratio is also
considered, leading to a representation of the mass and radius of the charged
objects with minimum mass-radius ratio in terms of the charge and vacuum energy
only.Comment: 19 pages, accepted by GRG, references corrected and adde
Debye relaxation and 250 K anomaly in glass forming monohydroxy alcohols
A previous dielectric, near-infrared (NIR), and nuclear magnetic resonance
study on the hydrogen-bonded liquid 2-ethyl-1-hexanol [C. Gainaru et al., Phys.
Rev. Lett. 107, 118304 (2011)] revealed anomalous behavior in various static
quantities near 250 K. To check whether corresponding observations can be made
for other monohydroxy alcohols as well, these experimental methods were applied
to such substances with 5, 6, 7, 8, and 10 carbon atoms in their molecular
backbone. All studied liquids exhibit a change of behavior near 250 K which is
tentatively ascribed to effects of hydrogen bond cooperativity. By analyzing
the NIR band intensities, a linear cluster size is derived that agrees with
estimates from dielectric spectroscopy. All studied alcohols, except
4-methyl-3-heptanol, display a dominant Debye-like peak. Furthermore, neat
2-ethyl-1-butanol exhibits a well resolved structural relaxation in its
dielectric loss spectrum which so far has only been observed for diluted
monohydroxy alcohols.Comment: 39 pages including 12 figure
NMR Study of the New Magnetic Superconductor CaK(Fe$0.951Ni0.049)4As4: Microscopic Coexistence of Hedgehog Spin-vortex Crystal and Superconductivity
Coexistence of a new-type antiferromagnetic (AFM) state, the so-called
hedgehog spin-vortex crystal (SVC), and superconductivity (SC) is evidenced by
As nuclear magnetic resonance study on single-crystalline
CaK(FeNi)As. The hedgehog SVC order is clearly
demonstrated by the direct observation of the internal magnetic induction along
the axis at the As1 site (close to K) and a zero net internal magnetic
induction at the As2 site (close to Ca) below an AFM ordering temperature
52 K. The nuclear spin-lattice relaxation rate 1/ shows
a distinct decrease below 10 K, providing also unambiguous
evidence for the microscopic coexistence. Furthermore, based on the analysis of
the 1/ data, the hedgehog SVC-type spin correlations are found to be
enhanced below 150 K in the paramagnetic state. These results
indicate the hedgehog SVC-type spin correlations play an important role for the
appearance of SC in the new magnetic superconductor.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. B rapid
communicatio
Extra force in modified theories of gravity
The equation of motion for test particles in modified theories of
gravity is derived. By considering an explicit coupling between an arbitrary
function of the scalar curvature, , and the Lagrangian density of matter, it
is shown that an extra force arises. This extra force is orthogonal to the
four-velocity and the corresponding acceleration law is obtained in the weak
field limit. Connections with MOND and with the Pioneer anomaly are further
discussed.Comment: Revtex4 file, 5 pages. Version to appear in Physical Review
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