421 research outputs found
Bouncing solutions in Rastall's theory with a barotropic fluid
Rastall's theory is a modification of Einstein's theory of gravity where the
covariant divergence of the stress-energy tensor is no more vanishing, but
proportional to the gradient of the Ricci scalar. The motivation of this theory
is to investigate a possible non-minimal coupling of the matter fields to
geometry which, being proportional to the curvature scalar, may represent an
effective description of quantum gravity effects. Non-conservation of the
stress-energy tensor, via Bianchi identities, implies new field equations which
have been recently used in a cosmological context, leading to some interesting
results. In this paper we adopt Rastall's theory to reproduce some features of
the effective Friedmann's equation emerging from loop quantum cosmology. We
determine a class of bouncing cosmological solutions and comment about the
possibility of employing these models as effective descriptions of the full
quantum theory.Comment: Latex file, 14 pages, 1 figure in eps format. Typos corrected, one
reference added. Published versio
High precision spectra at large redshift for dynamical DE cosmologies
The next generation mass probes will investigate DE nature by measuring
non-linear power spectra at various z, and comparing them with high precision
simulations. Producing a complete set of them, taking into account baryon
physics and for any DE state equation w(z), would really be numerically
expensive. Regularities reducing such duty are essential. This paper presents
further n-body tests of a relation we found, linking models with DE state
parameter w(z) to const.-w models, and also tests the relation in hydro
simulations.Comment: PASCOS 2010, the 16th International Symposium on Particles, Strings
and Cosmology, Valencia (Spain), July 19th - 23rd, 201
On general features of warm dark matter with reduced relativistic gas
Reduced Relativistic Gas (RRG) is a useful approach to describe the warm dark
matter (WDM) or the warmness of baryonic matter in the approximation when the
interaction between the particles is irrelevant. The use of Maxwell
distribution leads to the complicated equation of state of the J\"{u}ttner
model of relativistic ideal gas. The RRG enables one to reproduce the same
physical situation but in a much simpler form. For this reason RRG can be a
useful tool for the theories with some sort of a "new Physics". On the other
hand, even without the qualitatively new physical implementations, the RRG can
be useful to describe the general features of WDM in a model-independent way.
In this sense one can see, in particular, to which extent the cosmological
manifestations of WDM may be dependent on its Particle Physics background. In
the present work RRG is used as a complementary approach to derive the main
observational exponents for the WDM in a model-independent way. The only
assumption concerns a non-negligible velocity for dark matter particles
which is parameterized by the warmness parameter . The relatively high
values of ( ) erase the radiation (photons and
neutrinos) dominated epoch and cause an early warm matter domination after
inflation. Furthermore, RRG approach enables one to quantify the lack of power
in linear matter spectrum at small scales and in particular, reproduces the
relative transfer function commonly used in context of WDM with accuracy of
. A warmness with (equivalent to ) does not alter significantly the CMB power spectrum and is in
agreement with the background observational tests.Comment: 15 pages, 8 figures. Essential improvements in style and presentatio
3D printed small-scale acoustic metamaterials based on Helmholtz resonators with tuned overtones
Acoustic metamaterials have been extensively studied in recent decades due to their ability to control acoustic waves. In this paper, we present a prototype of a small-scale acoustic metamaterial based on Helmholtz resonators fabricated with additive manufacturing technology. The results confirm that 3D printed small-scale metamaterials can break the mass law by creating band gaps where the sound is deeply attenuated. We have also introduced a modification of the resonators whereby overtones are exploited and tuned in order to broaden the band gap. The output of this research could be used to provide passive filtering for transducers, to improve noise cancelling headphones, as well as in other smart acoustic sensors and IoT audio applications
Stereolabile and Configurationally Stable Atropisomers of Hindered Aryl Carbinols.
CAN 143:132926 AN 2005:441860 CAPLUS (COPYRIGHT (C) 2008 ACS ON SCIFINDER (R))
ABSTRACT
CARBINOLS OF THE AR-C(OH)R2 TYPE, AR BEING O-ISOPROPYLPHENYL, EXIST AS STEREOLABILE SYN-CLINAL (S.C.) AND ANTI-PERIPLANAR (AP) ATROPISOMERS WHEN R = ME, ET, I-PR. IN THE LATTER COMPD., THE MAJOR ATROPISOMER ALSO COMPRISES TWO ENANTIOMERIC FORMS THAT INTERCHANGE WITH A BARRIER OF 6.4 KCAL MOL-1. X-RAY DIFFRACTION, NOE EXPTS., AND AB INITIO CALCNS. INDICATE THAT THE S.C.-ATROPISOMER IS THE MORE STABLE FORM WHEN R = ME, I-PR, T-BU BUT IS THE LESS STABLE ONE WHEN R = ET. NMR SPECTRA AT VARIABLE TEMP. ALLOWED THE DETN. OF THE BARRIERS FOR THE INTERCONVERSION OF THE S.C.- INTO THE AP-ATROPISOMERS (ΔG.THERMOD. = 7.6, 8.8, AND 13.5 KCAL MOL-1 FOR ME, ET, I-PR, RESP.). WHEN R IS A TERT-BU GROUP, THE TWO ATROPISOMERS ARE CONFIGURATIONALLY STABLE: THE AP-ATROPISOMER IS OBTAINED AS THE KINETIC CONTROLLED COMPD., WHICH CAN BE TRANSFORMED INTO THE THERMODYNAMICALLY MORE STABLE S.C.-..
Structure, Conformation, and Stereodynamics of the Atropisomers of Highly Hindered Benzyl Ethers.
CAN 145:145171 AN 2006:476210 CAPLUS (COPYRIGHT (C) 2008 ACS ON SCIFINDER (R))
ABSTRACT
LOW-TEMP. AND NOE NMR SPECTRA OF FOUR OF THE TITLE COMPDS. INDICATE THAT THEY ADOPT A SYNCLINAL (S.C.) CONFORMATION, IN AGREEMENT WITH THE PREDICTION OF AB INITIO COMPUTATIONS. IN THE CASE OF THE MOST-HINDERED DERIV. (COMPD. 4), THE CONFORMATION IS SYN-PERIPLANAR (SP), AS IS ALSO SHOWN BY X-RAY DIFFRACTION. SUCH STEREOLABILE SP- OR S.C.-ATROPISOMERS EXIST AS TWO CONFORMATIONAL ENANTIOMERS: THE CORRESPONDING ENANTIOMERIZATION BARRIERS, COVERING THE RANGE 6.6 TO 9.7 KCAL MOL-1, COULD BE MEASURED FOR ALL THE EXAMD. COMPDS. IN TWO CASES (COMPDS. 3 AND 5), THE MINOR ANTIPERIPLANAR (AP) ATROPISOMER HAS BEEN ALSO OBSD., AND THE S.C. TO AP INTERCONVERSION BARRIER MEASURED (11.7 AND 11.9 KCAL MOL-1, RESP.). IN ADDN., RESTRICTED ROTATION OF THE ISO-PR AND TERT-BU SUBSTITUENTS HAS BEEN DETECTED, AND THE CORRESPONDING BARRIERS HAVE BEEN DETD
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