Top PDF DARK MATTER AND VARIABLE COSMOLOGICAL CONSTANT

DARK MATTER AND VARIABLE COSMOLOGICAL CONSTANT

DARK MATTER AND VARIABLE COSMOLOGICAL CONSTANT

percentage consists of baryonic matter [2]. Evidences for the existence of an unseen ‘dark’ component in the energy density of the universe comes from several independent observations at different length scales, rotations of peripheral stars in the galaxies, cosmic microwave background anisotropies universe large scale structure, gravitational field in cluster of galaxies etc. The baryonic content is well known, both from element abundances produced in primordial nucleosynthesis roughly 100 seconds after the Big Bang and the measurement of anisotropies in the CMB. The evidence for the existence of Dark Matter is overwhelming and comes from a wide variety of astrophysical measurements.
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Cosmological dynamics of tachyonic teleparallel dark energy

Cosmological dynamics of tachyonic teleparallel dark energy

The fixed points I.a, I.d, and I.e are the same points that were found for teleparallel dark energy in Refs. [22–24]. The scaling solutions I.b and I.c are new solutions that are not present in teleparallel dark energy. Such as in tele- parallel dark energy, in tachyonic teleparallel dark energy the Universe is attracted for the dark-energy-dominated de Sitter solution I.d or I.e. However, unlike the former sce- nario, in tachyonic teleparallel dark energy the Universe may present a phase MDE, that is, the scaling solution I.b or I.c, in which it has some portions of the energy density of  in the matter dominated era. This type of phase MDE is also common in coupled dark energy in GR (see Refs. [7,9,10]). But since the scaling solutions I.b and I.c both require 1=2u 2
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Braz. J. Phys.  vol.38 número1

Braz. J. Phys. vol.38 número1

In modern cosmological theories, a dynamic cosmological term Λ(t) remains a focal point of interest as it solves the cos- mological constant problem in a natural way. There are sig- nificant observational evidence for the detection of Einstein’s cosmological constant, Λ or a component of material content of the universe that varies slowly with time and space to act like Λ. A wide range of observations now compellingly sug- gest that the universe possesses a non-zero cosmological term [41]. In the context of quantum field theory, a cosmological term corresponds to the energy density of vacuum. The birth of the universe has been attributed to an excited vacuum fluc- tuation triggering off an inflationary expansion followed by the super-cooling. The release of locked up vacuum energy re- sults in subsequent reheating. The cosmological term, which is measure of the energy of empty space, provides a repulsive force opposing the gravitational pull between the galaxies. If the cosmological term exists, the energy it represents counts as mass because mass and energy are equivalent. If the cos- mological term is large enough, its energy plus the matter in the universe could lead to inflation. Unlike standard inflation, a universe with a cosmological term would expand faster with time because of the push from the cosmological term [42]. Some of the recent discussions on the cosmological constant “problem” and on cosmology with a time-varying cosmologi- cal constant by Ratra and Peebles [43], Dolgov [44] and Sahni and Starobinsky [45] point out that in the absence of any in- teraction with matter or radiation, the cosmological constant remains a “constant”. However, in the presence of interac- tions with matter or radiation, a solution of Einstein equations and the assumed equation of covariant conservation of stress- energy with a time-varying Λ can be found. This entails that energy has to be conserved by a decrease in the energy den- sity of the vacuum component followed by a corresponding increase in the energy density of matter or radiation (see also Weinberg [46], Carroll, Press and Turner [47], Peebles [48], Padmanabhan [49] and Pradhan et al. [50] ).
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A unified model of inflation, dark matter, neutrino masses and baryogenesis

A unified model of inflation, dark matter, neutrino masses and baryogenesis

In this work, we have developed an unification model where we describe, using the same field, an inflationary evolution and the dark matter content of the Universe, due to an incom- plete decay of the inflaton. At the same time, this incomplete decay introduces right-handed neutrinos that allow the generation of light neutrino masses and also the generation of the observed baryon asymmetry in the Universe through a thermal Leptogenesis evolution. We started with a review of the introductory topics for this thesis. First, we gave a descrip- tion of the basic features of neutrino physics including the seesaw mechanism, a description of neutrino mass and mixing matrices and neutrino oscillations. Moving to cosmology, we introduced the Standard Cosmological paradigm and then the inflationary evolution. In the latter, after describing the basic dynamics, we have detailed how the experimental observ- ables can be related to the metric and matter fluctuations and to the slow-roll parameters. We then discussed Dark Matter basic notions and reproduced the standard WIMP’s abundance calculation. Finally, we discussed the three Sakharov Conditions [2] to then introduce some models of Baryogenesis.
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Complex scalar dark matter in a B-L model

Complex scalar dark matter in a B-L model

In the second chapter, we have studied the relic abundance calculation when one assumes thermal equilibrium at the early Universe. We have spent most of the time working on the understanding of the methods involved in this calculation and on how to implemment them inside numerical calculations. The following subjects have been studied or reviewed: Basics of GR, Λ-CDM Cosmological Model, Thermodynamics of the Early Universe, arriving in the end at the Boltzmann equation (its properties have been analyzed carefully) and how it generates the current relic abundance for a generic particle. In the following chapter, an introduction to direct and indirect detection experiments was provided, where different techniques were exposed and discussed. Also, the necessary theoretical background on direct detection was provided because the final chapter would have to deal with the limits coming from these experiments and comparison to theory becomes necessary.
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Unified description of dark energy and dark matter within the generalized hybrid metric-palatini theory of gravity

Unified description of dark energy and dark matter within the generalized hybrid metric-palatini theory of gravity

However, notwithstanding this success, theories of gravity going beyond General Relativity have been receiving increased attention for a variety of reasons, among which the search for an alternative explanation to the observed late-time accelerated expansion of the Universe stands out [4,5]. The standard—and simplest—explanation for this accelerated expansion, provided by the concordance (ΛCDM) model, resorts to a cosmological constant, but it turns out that the theoretical value of this constant, resulting from quantum field theory calculations, is overwhelmingly different from the one required by observations. This is the well known cosmological-constant problem [6,7], which, hopefully, could be avoided within the framework of a modified theory of gravity.
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Cosmic microwave background and large-scale structure constraints on a simple quintessential inflation model

Cosmic microwave background and large-scale structure constraints on a simple quintessential inflation model

This model was implemented in a hybrid inflation con- text by Masso´ and Zsembinszki [10]. A model similar to ours, in which the modulus field  is responsible for inflation and the phase ’ produces dark matter, was studied in Ref. [11]. In models in which the axionlike field is the dark matter, a high energy scale for the axion decay constant f is also necessary, in order to suppress isocurva- ture fluctuations to acceptable levels. In our case, however, the axion field only becomes dynamical at such late times that the bounds from isocurvature fluctuations do not apply [12].
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Astrophysical and cosmological doomsdays

Astrophysical and cosmological doomsdays

It was investigated in subsection 4.1 whether a composition of specific IR and UV effects could alter a big rip singularity setting (cf. Ref. [42]). More concretely, it was employed a simple model: A DGP brane model, with phantom matter on the brane and a GB term in the bulk action. The DGP brane configuration has relevant IR effects, whereas the GB component is important at high energies; phantom matter (with a constant equation of state) in a standard FLRW model is known to induce the emergence of a big rip singularity [9]. This analysis indicates that the big rip can be replaced by a smoother singularity, named a sudden future singularity [13], through some intertwining between late time dynamics and high energy effects. Subsequently, it was determined values of the redshift and the future cosmic time, where the brane would reach the sudden singularity. These results can be contrasted with those for the big rip occurrence in a FLRW setting (e.g., see Ref. [11]). Notice that, these conclusions are based on a rather particular result, that was extracted from a specific model. Subsequent research work would assist in clarifying some remaining issues. For example, further studies of how other singularities can be appeased or removed by means of the herein combined IR and UV effects have been done in Ref. [215]. On the other hand, it might be interesting to consider a modified Einstein-Hilbert action on the brane, which in addition could alleviate the ghost problem present on the self-accelerating DGP model by self-accelerating the normal DGP branch [216], and see if some of the dark energy singularities can be removed or at least appeased in this setup.
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Braz. J. Phys.  vol.34 número1A

Braz. J. Phys. vol.34 número1A

This short review focused on some alternative candi- dates to dark energy. This ubiquitous component plus the dark matter are responsible for nearly 95% of the matter- energy content filling the Universe. However, different from dark matter, the extra dark (energy) component is intrinsi- cally relativistic and its negative pressure is required by the present accelerating stage of the Universe. Its tiny density and weak interaction presumably preclude the possibility of identification in the terrestrial laboratory. Unfortunately, even considering that we are in the golden age of empirical cosmology, the existing data are still unable to discriminate among the different dark energy candidates, thereby signal- ing that we need better observations in order to test the basic predictions. In particular, this means that the determination of cosmological parameters will continue to be a central goal in the near future. The fundamental aim is to shed some light on the nature of the dark energy.
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Advanced Instrumentation for Superheated Liquid Detectors in Dark Matter Searches

Advanced Instrumentation for Superheated Liquid Detectors in Dark Matter Searches

Following each measurement, an initial data set was formed by passing the data files through the pulse validation routine which tagged signal events if their amplitude exceeded the noise level of the detector by 2 mV. All signals with less than five pulse spikes above threshold were rejected, since they do not form a pulse and are electromagnetic noise. The signal waveform, decay time constant and spectral density structure of the remaining single events were next inspected individually. As said before, a particle-induced nucleation event possesses a characteristic frequency response, with a time span of a few milliseconds, a decay constant of 5-40 ms, and a primary harmonic between 0.45-0.75 kHz; this response has been shown to differ significantly from gel-associated acoustic and local acoustic backgrounds.
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Environmental Assessment of a Diesel Engine Under Variable Stroke Length and Constant Compression Ratio

Environmental Assessment of a Diesel Engine Under Variable Stroke Length and Constant Compression Ratio

Abstract: In the light of the energy crisis and the stringent environmental regulations, diesel engines are offering good hope for automotive vehicles. However, lot of work is needed to reduce the diesel exhaust emissions and give the way for full utilization of the diesel fuel’s excellent characteristics. This paper presents a theoretical study on the effect of variable stroke length technique on the emissions of a four-stroke, water-cooled direct injections diesel engine with the help of experimentally verified computer software designed mainly for diesel engines. The emission levels were studied over the speed range (1000 rpm to 3000 rpm) and stroke lengths (120 mm to 200 rpm) and were compared with those of the original engine design. The simulation results clearly indicate the advantages and utility of variable stroke technique in the reduction of the exhaust emission levels. A reduction of about 10% to 75% was achieved for specific particulate matter over the entire speed range and bore-to-stroke ratio studied. Further, a reduction of about 10% to 59% was achieved for the same range. As for carbon dioxide, a reduction of 0% to 37% was achieved. On the other hand, a less percent change was achieved for the case of nitrogen dioxide and nitrogen oxides as indicated by the results. This study clearly shows the advantage of VSE over fixed stroke engines. This study showed that the variable stroke technique proved a good way to curb the diesel exhaust emissions and hence helped making these engines more environmentally friendly.
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Braz. J. Phys.  vol.35 número4B

Braz. J. Phys. vol.35 número4B

It is very common to find numerical studies of dark energy and dark matter. Among these, one can find the interesting proposal of unifying dark matter and dark energy with the use of a single component with an ‘exotic’ equation of state. However, there is a certain shortage of analyses involving exact analytic models following this proposal. Therefore, here are presented examples of simple exact cosmological models which can reproduce some of the desired properties of an unified dark matter/energy fluid.

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Dynamical 3-Space: Alternative Explanation of the "Dark Matter Ring"

Dynamical 3-Space: Alternative Explanation of the "Dark Matter Ring"

Jee et al. [1] claim that the analysis of gravitational lens- ing data from the HST observations of the galaxy cluster CL 0024+17 demonstrates the existence of a “dark matter ring”. While the lensing is clearly evident, as an observable phe- nomenon, it does not follow that this must be caused by some undetected form of matter, namely the putative “dark matter”. Here we show that the lensing can be given an alternative ex- planation that does not involve “dark matter”. This explana- tion comes from the new dynamics of 3-space [2, 3, 4, 5, 6]. This dynamics involves two constant G and — the fine structure constant. This dynamics has explained the bore hole anomaly, spiral galaxy flat rotation speeds, the masses of black holes in spherical galaxies, gravitational light bend- ing and lensing, all without invoking “dark matter”. The 3- space dynamics also has a Hubble expanding 3-space solution that explains the supernova redshift data without the need for “dark energy” [8]. The issue is that the Newtonian theory of gravity [9], which was based upon observations of planetary motion in the solar system, missed a key dynamical effect that is not manifest in this system. The consequences of this fail- ure has been the invoking of the fix-ups of “dark matteranddark energy”. What is missing is the 3-space self-interaction effect. Experimental and observational data has shown that the coupling constant for this self-interaction is the fine struc- ture constant,  1/137, to within measurement errors. It is shown here that this 3-space self-interaction effect gives a direct explanation for the reported ring-like gravitational lens- ing effect.
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Cubic Galileon theory in the Effective Field theory formalism: a cosmological study

Cubic Galileon theory in the Effective Field theory formalism: a cosmological study

No entanto, um dos maiores desafios tanto para a RG como para o Modelo Padr˜ ao da F´ısica de Part´ıculas viria a surgir em 1998, quando dados provenientes do estudo de supernovas de tipo Ia mostraram que o Universo est´ a a expandir de forma acelerada. O problema reside no facto de nenhum tipo de mat´ eria ou energia conhecida ou detetada at´ e ao momento ser capaz de explicar esta acelera¸ c˜ ao. Dentro do formalismo da RG, a explica¸ c˜ ao mais simples para o fen´ omeno em quest˜ ao passa pela introdu¸ c˜ ao da constante cosmol´ ogica, Λ, para descrever uma componente de mat´ eria/energia caracterizada por uma press˜ ao negativa constante. Sabe-se ainda que esta componente de natureza desconhecida, genericamente considerada como uma forma de Energia Escura (EE), precisa de ser a mais abundante no Universo de modo a ajustar os dados observacionais. Adicionalmente, uma grande quantidade e variedade de observa¸ c˜ oes apoia ainda a existˆ encia de um tipo de mat´ eria que aparenta n˜ ao absorver nem emitir radia¸ c˜ ao eletromagn´ etica. Esta componente, consequentemente designada de Mat´ eria Escura (ME), ´ e ainda restringida observacionalmente a ser n˜ ao relativista e de natureza n˜ ao bari´ onica. Part´ıculas de ME que se movem lentamente em compara¸ c˜ ao com a luz s˜ ao designadas por Mat´ eria Escura Fria (Cold Dark Matter - CDM).
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The Newtonian and MOND dynamical models of NGC 5128: Investigation of the dark matter contribution

The Newtonian and MOND dynamical models of NGC 5128: Investigation of the dark matter contribution

MOND based on the theoretical modification of the Newtonian theory, is expected to provide successful fits for observed galaxies without DM. One of the biggest successes of MOND is no doubt its ability to fit spiral galaxies without DM (for a review, see e.g. Sanders and McGaugh 2002). However, when ETGs are modeled using MOND, the results are mixed and the existence of additional, dark, component cannot be excluded in numerous cases (see below, for more details). Also, it is well known that MOND has problems at the scales of clusters of galaxies (see, e.g. Sanders 2003) where additional dark mass ap- pears to be necessary. The well-known case of the Bullet Cluster (1E 0657-558) which consists of two colliding clusters of galaxies, was considered as the best example in favor of DM (Bradaˇc et al. 2006). Recently, MOND was also used in various numeri- cal simulations: see, e.g. the work of Angus et al. (2014) where the cosmological particle-mesh N-body code was used to investigate the feasibility of struc- ture formation in a framework involving MOND and light sterile neutrinos, which means that an addi- tional, hypothetical dark particle is needed. It is worth noting that an another interesting line of in- vestigation of MOND is that based on the study of shell galaxies and promising results have been ob- tained (B´ılek et al. 2015) which may lead to further tests of the DM/MOND dichotomy in the future. Therefore, as will be shown below, since the number of early-type galaxies studied using MOND is still small, it is important to study as many interesting objects as possible, and NGC 5128 being the nearest large elliptical galaxy is certainly worth investigat- ing.
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Consequences of dark matter-dark energy interaction on cosmological parameters derived from type Ia supernova data

Consequences of dark matter-dark energy interaction on cosmological parameters derived from type Ia supernova data

However, a direct comparison of our result for  0 DM with the CMB results is not possible, since the latter has been obtained in the context of uncoupled models. It is therefore important to see whether there are upper limits to  0 DM which are independent of the cosmological model. An upper limit to  0 DM which does not depend on the back- ground cosmology can be obtained from the galaxy cluster dynamics. However the current data yield very weak con- straints: Ref. [35] gives  0 m  0:30 0:17 0:07 so that even

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Dark Radiation and Localization of Gravity on the Brane

Dark Radiation and Localization of Gravity on the Brane

We discuss the dynamics of a spherically symmetric dark radiation vaccum in the Randall-Sundrum brane world scenario. Under certain natural assump- tions we show that the Einstein equations on the brane form a closed system. For a de Sitter brane we determine exact dynamical and inhomogeneous solu- tions which depend on the brane cosmological constant, on the dark radiation tidal charge and on its initial configuration. We define the conditions leading to singular or globally regular solutions. We also analyse the localization of gravity near the brane and show that a phase transition to a regime where gravity propagates away from the brane may occur at short distances during the collapse of positive dark energy density.
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Dark Radiation Dynamics on the Brane

Dark Radiation Dynamics on the Brane

We investigate the dynamics of a spherically symmetric vaccum on a Randall and Sundrum 3-brane world. Under certain natural conditions, the effective Einstein equations on the brane form a closed system for spherically symmetric dark radiation. We determine exact dynamical and inhomogeneous solutions, which are shown to depend on the brane cosmological constant, on the dark radiation tidal charge and on its initial energy configuration. We identify the conditions defining these solutions as singular or as globally regular. Finally, we discuss the confinement of gravity to the vicinity of the brane and show that a phase transition to a regime where gravity is not bound to the brane may occur at short distances during the collapse of positive dark energy density on a realistic de Sitter brane.
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Inhomogeneous Dark Radiation Dynamics on a de Sitter Brane

Inhomogeneous Dark Radiation Dynamics on a de Sitter Brane

Assuming spherical symmetry we analyse the dynamics of an inhomoge- neous dark radiation vaccum on a Randall and Sundrum 3-brane world. Under certain natural conditions we show that the effective Einstein equations on the brane form a closed system. On a de Sitter brane and for negative dark energy density we determine exact dynamical and inhomogeneous solutions which de- pend on the brane cosmological constant, on the dark radiation tidal charge and on its initial configuration. We also identify the conditions leading to the formation of a singularity or of regular bounces inside the dark radiation vaccum.
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Electrical Conductivity of Metals: A New Look at this Subject

Electrical Conductivity of Metals: A New Look at this Subject

Highly purified water is a bad electrical conductor. However, the addition of small amounts of sodium chloride (NaCl) to this liquid, can increase its electrical conductivity in a sub- stantial way. At the ambient temperature (295K), the wa- ter’s dielectric constant of 80, permits the Na+ and Cl- ions to move freely through the liquid and this feature can ac- count for the change in its conductive behavior. It seems that the concentration of free charge carriers has the most relevant role in determining the electrical conductivity of the substances. But what to say about electrical conductivity in metals? Isolated metallic atoms have their inner electrons belonging to closed shells and hence tightly bound to their corresponding atomic nucleus. However the electrons of the outer most shell are weakly bonded to its respective nucleus. When arranged in a crystal lattice structure, the bond weak- ness of these outer electrons is enhanced due to the interac- tions among neighbor atoms of the lattice, so that the elec- trons of conduction are free to travel through the whole crys- tal. Resistance to their motion is due to the thermal vibrations (phonons) and defects provoked by the presence of impurities and lattice dislocations. In a perfect crystal at zero absolute temperature, these free electrons can be described by using the quantum mechanical formalism of the Bloch waves [1,2]. The concentration of free electrons plays an important role in the description of the electrical conductivity in metals.
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