Showing papers on "Particle horizon published in 2007"
TL;DR: The Born-Infeld strategy to smooth theories having divergent solutions is applied to the teleparallel equivalent of general relativity in this article, which leads to second order equations, since the Teleparallel Lagrangian only contains first derivatives of the vierbein.
Abstract: The Born-Infeld strategy to smooth theories having divergent solutions is applied to the teleparallel equivalent of general relativity. Differing from other theories of modified gravity, modified teleparallelism leads to second order equations, since the teleparallel Lagrangian only contains first derivatives of the vierbein. We show that the Born-Infeld-modified teleparallelism solves the particle horizon problem in a spatially flat Friedmann-Robertson-Walker (FRW) universe by providing an initial exponential expansion without resorting to an inflaton field.
934 citations
TL;DR: In this paper, the trispectrum of the primordial curvature perturbation generated by an epoch of slow-roll inion in the early universe has been calculated, and it is shown that the non-Gaussian signature imprinted at horizon crossing is unobservably small, of order lambda NL less than or similar to r/50.
Abstract: We calculate the trispectrum of the primordial curvature perturbation generated by an epoch of slow-roll in. ation in the early universe, and demonstrate that the non-Gaussian signature imprinted at horizon crossing is unobservably small, of order lambda NL less than or similar to r/50, where r < 1 is the tensor-to-scalar ratio. Therefore any primordial non-Gaussianity observed in future microwave background experiments is likely to have been synthesized by gravitational effects on superhorizon scales. We discuss the application of Maldacena's consistency condition to the trispectrum.
221 citations
TL;DR: In this paper, the interplay between cosmological expansion and local attraction in a gravitationally bound system is revisited in various regimes, including weakly gravitating Newtonian systems, and various exact solutions describing a relativistic central object embedded in a Friedmann universe.
Abstract: The interplay between cosmological expansion and local attraction in a gravitationally bound system is revisited in various regimes. First, weakly gravitating Newtonian systems are considered, followed by various exact solutions describing a relativistic central object embedded in a Friedmann universe. It is shown that the ``all or nothing'' behavior recently discovered (i.e., weakly coupled systems are comoving while strongly coupled ones resist the cosmic expansion) is limited to the de Sitter background. New exact solutions are presented which describe black holes perfectly comoving with a generic Friedmann universe. The possibility of violating cosmic censorship for a black hole approaching the big rip is also discussed.
174 citations
TL;DR: In this article, the authors constructed simple inhomogeneous dust-filled universe models in which the speed of the cosmic volume expansion is accelerated for finite periods, by removing spherical domains from the Einstein-de Sitter.
Abstract: If expanding and contracting regions coexist in the universe, the speed of the cosmic volume expansion can be accelerated. We construct simple inhomogeneous dust-filled universe models in which the speed of the cosmic volume expansion is accelerated for finite periods. These models are constructed by removing spherical domains from the Einstein-de Sitter �
122 citations
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TL;DR: In this article, the authors compare two forms of the Robertson-Walker (RW) metric, written in (the traditional) comoving coordinates, and a set of observer-dependent coordinates, first for the well known de Sitter universe containing only dark energy, and then for a newly derived form of the RW metric, for a universe with dark energy and matter.
Abstract: The cosmological principle, promoting the view that the universe is homogeneous and isotropic, is embodied within the mathematical structure of the Robertson-Walker (RW) metric. The equations derived from an application of this metric to the Einstein Field Equations describe the expansion of the universe in terms of comoving coordinates, from which physical distances may be derived using a time-dependent expansion factor. These coordinates, however, do not explicitly reveal properties of the cosmic spacetime manifested in Birkhoff's theorem and its corollary. In this paper, we compare two forms of the metric--written in (the traditional) comoving coordinates, and a set of observer-dependent coordinates--first for the well-known de Sitter universe containing only dark energy, and then for a newly derived form of the RW metric, for a universe with dark energy and matter. We show that Rindler's event horizon--evident in the co-moving system--coincides with what one might call the "curvature horizon" appearing in the observer-dependent frame. The advantage of this dual prescription of the cosmic spacetime is that with the latest WMAP results, we now have a much better determination of the universe's mass-energy content, which permits us to calculate this curvature with unprecedented accuracy. We use it here to demonstrate that our observations have probed the limit beyond which the cosmic curvature prevents any signal from having ever reached us. In the case of de Sitter, where the mass-energy density is a constant, this limit is fixed for all time. For a universe with a changing density, this horizon expands until de Sitter is reached asymptotically, and then it too ceases to change.
99 citations
TL;DR: In this article, the authors investigated the thermodynamic properties of the universe with dark energy and showed that the apparent horizon is the physical horizon in dealing with thermodynamics problems, based on the well known correspondence between the Friedmann equation and the first law of thermodynamics of the visible horizon.
Abstract: We have investigated the thermodynamical properties of the Universe with dark energy Adopting the usual assumption in deriving the constant co-moving entropy density that the physical volume and the temperature are independent, we observed some strange thermodynamical behaviours However, these strange behaviours disappeared if we considered the realistic situation where the physical volume and the temperature of the Universe are related On the basis of the well known correspondence between the Friedmann equation and the first law of thermodynamics of the apparent horizon, we argued that the apparent horizon is the physical horizon in dealing with thermodynamics problems We have concentrated on the volume of the Universe within the apparent horizon and considered that the Universe is in thermal equilibrium with the Hawking temperature on the apparent horizon For dark energy with w ≥ −1, the holographic principle and the generalized second law are always respected
99 citations
TL;DR: In this article, the effects of nonlinearity, galaxy bias and redshift space distortions were investigated and the authors found that a survey of several Gpc 3 galaxies would constrain the sound horizon at z ǫ ∼ Ã 1 to about 1%.
91 citations
TL;DR: In this paper, a full sky search of the CMB, mapped extremely accurately by NASA's WMAP satellite, returned no detection of such matching circles and placed a lower bound on the size of the universe at 24 Gpc.
Abstract: The topology of the Universe can leave an imprint on the cosmic microwave background (CMB) radiation. Clues to the shape of our Universe can be found by searching the CMB for matching circles of temperature patterns. A full sky search of the CMB, mapped extremely accurately by NASA's WMAP satellite, returned no detection of such matching circles and placed a lower bound on the size of the Universe at 24 Gpc. This lower bound can be extended by optimally filtering the WMAP power spectrum. More stringent bounds can be placed on specific candidate topologies by using a combination statistic. We use optimal filtering and the combination statistic to rule out the suggestion that we live in a Poincare dodecahedral space.
85 citations
TL;DR: In this paper, the future event horizon is chosen as the horizon of the flat Friedmann-Lemaitre-Robertson-Walker universe, and the interacting holographic dark energy model is able to explain the phantom divide line crossing.
Abstract: Choosing the future event horizon as the horizon of the flat Friedmann–Lemaitre–Robertson–Walker universe, we show that the interacting holographic dark energy model is able to explain the phantom divide line crossing. We show that if we take the particle event horizon as the horizon of the universe, besides describing the ω = −1 crossing (based on astrophysical data), we are able to determine appropriately the ratio of dark matter to dark energy density at the transition time. In this approach, after the first transition from the quintessence to the phantom phase, there is another transition from the phantom to the quintessence phase which avoids the big rip singularity.
54 citations
TL;DR: In this paper, the authors investigate observational constraints on the curvature of the universe not restricting ourselves to a cosmological constant as dark energy, in particular allowing a dark energy equation of state to evolve with time in several ways.
Abstract: We investigate observational constraints on the curvature of the universe not restricting ourselves to a cosmological constant as dark energy, in particular allowing a dark energy equation of state to evolve with time in several ways. We use type Ia supernovae (SNeIa) data from the latest gold data set which includes 182 SNeIa, along with the cosmic microwave background shift parameter and the baryon acoustic oscillation peak. We show quantitatively that the constraint on the curvature of the universe depends on the dark energy model: some popular parametrizations give constraints closely around the flat universe at 5% level (2σ C.L.) whereas some parametrizations allow the universe to be as open as having Ωk ~ 0.2.
52 citations
TL;DR: In this article, the authors investigate the effect of the bulk content in the general Gauss-Bonnet braneworld on the evolution of the universe and find that the combination of the dark radiation and the matter content plays a crucial role in the universe's evolution.
Abstract: We investigate the effect of the bulk content in the general Gauss-Bonnet braneworld on the evolution of the Universe. We find that the Gauss-Bonnet term and the combination of the dark radiation and the matter content of the bulk play a crucial role in the Universe's evolution. We show that our model can describe the superacceleration of our Universe with the equation of state of the effective dark energy in agreement with observations.
TL;DR: In this paper, the evolution of cosmological perturbations in the cyclic model is analyzed, paying particular attention to their behavior and interplay over multiple cycles, and the ekpyrotic phase, an epoch of gentle contraction with equation of state (EoS) preceding the hot big bang, makes the universe homogeneous, isotropic and flat within any given observer's horizon.
Abstract: We analyze the evolution of cosmological perturbations in the cyclic model, paying particular attention to their behavior and interplay over multiple cycles. Our key results are: (1) galaxies and large scale structure present in one cycle are generated by the quantum fluctuations in the preceding cycle without interference from perturbations or structure generated in earlier cycles and without interfering with structure generated in later cycles; (2) the ekpyrotic phase, an epoch of gentle contraction with equation of state w>>1 preceding the hot big bang, makes the universe homogeneous, isotropic and flat within any given observer's horizon; and (3) although the universe is uniform within each observer's horizon, the structure of the cyclic universe on very large scales is more complex, owing to the effects of superhorizon length perturbations, and cannot be described globally as a Friedmann-Robertson-Walker cosmology. In particular, we show that the ekpyrotic contraction phase is so effective in smoothing, flattening and isotropizing the universe within the horizon that this phase alone suffices to solve the horizon and flatness problems even without an extended period of dark energy domination (a kind of low energy inflation). Instead, the cyclic model rests on a genuinely novel, noninflationary mechanism (ekpyrotic contraction) formore » resolving the classic cosmological conundrums.« less
TL;DR: In this paper, it was shown that as we extrapolate the current CDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our "island universe" will be fundamentally incapable of determining the true nature of the universe.
Abstract: We demonstrate that as we extrapolate the current $\Lambda$CDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our "island universe" will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe. In this sense, the coordinate system appropriate for future observers will perhaps fittingly resemble the static coordinate system in which the de Sitter universe was first presented.
TL;DR: In this article, the authors discuss astronomical and astrophysical evidence, which relate to the principle of zero-total energy of the universe, that imply several relations among the mass M, the radius R and the angular momentum L of a sphere representing a Machian universe.
Abstract: We discuss astronomical and astrophysical evidence, which we relate to the principle of zero-total energy of the Universe, that imply several relations among the mass M, the radius R and the angular momentum L of a “large” sphere representing a Machian Universe. By calculating the angular speed, we find a peculiar centripetal acceleration for the Universe. This is an ubiquituous property that relates one observer to any observable. It turns out that this is exactly the anomalous acceleration observed on the Pioneers spaceships. We have thus shown that this anomaly is to be considered a property of the Machian Universe. We discuss several possible arguments against our proposal.
TL;DR: For the Poincare gauge theory of gravity, the authors explored the possibility of using dynamical scalar torsion to explain the current state of the accelerating universe and showed that with certain suitable sets of chosen parameters, this model can give a (qualitatively) proper description of the current universe without a cosmological constant.
Abstract: For the Poincare gauge theory of gravity we consider the dynamical scalar torsion mode in a cosmological context. We explore in particular the possibility of using dynamical torsion to explain the current state of the accelerating Universe. With certain suitable sets of chosen parameters, this model can give a (qualitatively) proper description of the current universe without a cosmological constant, and the universe described is oscillating with a period of the Hubble time.
TL;DR: In this article, the authors discuss the thermodynamic behavior of the universe by considering three different parametrizations, describing the dark energy component: (1) Model 1: ; (2) Model 2: ω(z) = ω0 + ω2 z; (3) Model 3: ψ(z)) = ψ0 - ω3ln(1 + z).
Abstract: Recent astronomical observations suggest that the bulk of energy in the Universe is repulsive and appears like a dark energy component (which accounts for ~2/3 of the energy content of the Universe) with negative pressure (ω ≡ px/ρx < 0). In this work, we discuss the thermodynamic behavior by considering three different parametrizations, describing the dark energy component: (1) Model 1: ; (2) Model 2: ω(z) = ω0 + ω2 z; (3) Model 3: ω(z) = ω0 - ω3ln(1 + z). It is found that its energy and temperature grow during the evolution of the Universe since work is done on the system. The case of phantom energy (ω < -1), however, seems to be physically meaningless because its entropy is negative. Our analysis also implies that the ultimate fate of the Universe may be considerably modified. Actually, the future of the Universe depends on the kind of parametrization. For Models 1 and 3, the Universe will becoming increasingly hot, while for Model 2 it cools during evolution.
TL;DR: In this paper, the curvaton in an intermediate inflationary universe model is studied, and some interesting constraints on different parameters that appear in the model are found, as well as the curvatures of different parameters.
Abstract: The curvaton in an intermediate inflationary universe model is studied. This study has allowed us to find some interesting constraints on different parameters that appear in the model.
TL;DR: In this paper, the authors investigate a specific class of inhomogeneous models that can be solved analytically, namely the dust dominated Lemaitre-Tolman-Bondi universe models, and show that they do not permit accelerated cosmic expansion.
Abstract: Recently, there have been suggestions that the apparent accelerated expansion of the universe is due not to a cosmological constant, but rather to inhomogeneities in the distribution of matter. In this work, we investigate a specific class of inhomogeneous models that can be solved analytically, namely the dust dominated Lemaitre–Tolman–Bondi universe models. We show that they do not permit accelerated cosmic expansion.
TL;DR: In this paper, it was shown that for a universe with a T' x R 2 spatial section, the residual Newtonian gravitational force provides an anisotropic effect that repels test particles from the cluster in the compact direction, in a way algebraically similar to that of dark energy.
Abstract: Context. Understanding dark energy and measuring the topology of the Universe are two of the biggest open questions in physical cosmology. It was previously shown that multiple connectedness, via the twin paradox of special relativity, provides a novel physical justification for an assumption of the standard FLRW model: it implies a favoured space-time splitting (comoving coordinates). Aims. Could cosmic topology also imply dark energy? Methods. We use a weak field (Newtonian) approximation of gravity and consider the gravitational effect from distant, multiple copies of a large, collapsed (virialised) object today (i.e. a massive galaxy cluster), taking into account the finite propagation speed of gravity, in a flat, multiply connected universe, and assume that due to a prior epoch of fast expansion (e.g. inflation), the gravitational effect of the distant copies is felt locally, from beyond the naively calculated horizon. Results. We find that for a universe with a T' x R 2 spatial section, the residual Newtonian gravitational force (to first order) provides an anisotropic effect that repels test particles from the cluster in the compact direction, in a way algebraically similar to that of dark energy. For a typical test object at comoving distance X from the nearest dense nodes of the cosmic web of density perturbations, the pressure-to-density ratio w of the equation of state in an FLRW universe, is ω ∼ -(X/L) 3 , where L is the size of the fundamental domain, i.e. of the Universe. Clearly, |ω| « 1. For a T 3 spatial section of exactly equal fundamental lengths, the effect cancels to zero. For a T3 spatial section of unequal fundamental lengths, the acceleration effect is anisotropic in the sense that it will tend to equalise the three fundamental lengths. Conclusions. Provided that at least a modest amount of inflation occurred in the early Universe, and given some other conditions, multiple connectedness does generate an effect similar to that of dark energy, but the amplitude of the effect at the present epoch is too small to explain the observed dark energy density and its anisotropy makes it an unrealistic candidate for the observed dark energy.
TL;DR: In this paper, the authors discuss how to reconcile cosmological inflation with cosmic strings and how to find ways so that both can coexist in the early universe, where cosmic strings are predicted to be formed in early universe.
Abstract: Cosmological inflation and topological defects have been considered for a long time, either in disagreement or in competition. On the one hand an inflationary era is required to solve the shortcomings of the hot big bang model, while on the other hand cosmic strings and string-like objects are predicted to be formed in the early universe. Thus, one has to find ways so that both can coexist. I discuss how to reconcile cosmological inflation with cosmic strings.
TL;DR: In this article, it was shown that when we work with coordinate cosmic time, which is not proper time, Robertson-Walker's metric includes a possible rotational state of the universe.
Abstract: We show that when we work with coordinate cosmic time, which is not proper time, Robertson-Walker's metric, includes a possible rotational state of the Universe. An exact formula for the angular speed and the temporal metric coefficient, is found.
TL;DR: In this article, the Baryon Acoustic Oscillations (BAO) was used to constrain the Dvali-Gabadadze-Porrati (DGP) universe.
TL;DR: A novel, string theory-inspired formalism based on a Hamiltonian constraint is obtained, obtaining a conformal mechanical system for the spatially flat four-dimensional Robertson-Walker Universe.
Abstract: Using a novel, string theory-inspired formalism based on a Hamiltonian constraint, we obtain a conformal mechanical system for the spatially flat four-dimensional Robertson-Walker Universe. Depending on parameter choices, this system describes either a relativistic particle in the Robertson-Walker background or metric fluctuations of the Robertson-Walker geometry. Moreover, we derive a tree-level M theory matrix model in this time-dependent background. Imposing the Hamiltonian constraint forces the spacetime geometry to be fuzzy near the big bang, while the classical Robertson-Walker geometry emerges as the Universe expands. From our approach, we also derive the temperature of the Universe interpolating between the radiation and matter dominated eras.
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TL;DR: In this article, the authors discuss how, in a universe restricted to the causal region connected to the observer, General Relativity implies the quantum nature of physical phenomena and directly leads to a string theory scenario, whose dynamics is ruled by a functional that weights all configurations according to their entropy.
Abstract: We discuss how, in a Universe restricted to the causal region connected to the observer, General Relativity implies the quantum nature of physical phenomena and directly leads to a string theory scenario, whose dynamics is ruled by a functional that weights all configurations according to their entropy. The most favoured configurations are those of minimal entropy. Along this class of vacua a four-dimensional space-time is automatically selected; when, at large volume, a description of space-time in terms of classical geometry can be recovered, the entropy-weighted sum reduces to the ordinary Feynman's path integral. What arises is a highly predictive scenario, phenomenologically compatible with the experimental observations and measurements, in which everything is determined in terms of the fundamental constants and the age of the Universe, with no room for freely-adjustable parameters. We discuss how this leads to the known spectrum of particles and interactions. Besides the computation of masses and couplings, CKM matrix elements, cosmological constant, expansion parameters of the Universe etc..., all resulting, within the degree of the approximation we used, in agreement with the experimental observations, we also discuss how this scenario passes the tests provided by cosmology and the constraints imposed by the physics of the primordial Universe.
TL;DR: In this article, it was shown that the cosmological redshift is there simply a relativistic Doppler shift, which is a direct consequence of the special-relativistic phenomenon of time dilation, as well as of the adopted definition of distance in cosmology.
Abstract: In all Friedman models, the cosmological redshift is widely interpreted as a consequence of the general-relativistic phenomenon of expansion of space. Other commonly believed consequences of this phenomenon are superluminal recession velocities of distant galaxies, and the distance to the particle horizon greater than ct (where t is the age of the Universe), in apparent conflict with special relativity. Here, we study a particular Friedman model: empty universe. This model exhibits both cosmological redshift, superluminal velocities and infinite distance to the horizon. However, we show that the cosmological redshift is there simply a relativistic Doppler shift. Moreover, apparently superluminal velocities and ‘acausal’ distance to the horizon are in fact a direct consequence of special-relativistic phenomenon of time dilation, as well as of the adopted definition of distance in cosmology. There is no conflict with special relativity, whatsoever. In
TL;DR: In this article, it was shown that the entanglement entropy between the universe inside our observable horizon and that outside our horizon is sufficient to do this, for the modes which are super-Hubble at the end of inflation.
Abstract: The fluctuations in the inflaton field at the end of inflation which seed the density perturbations are prepared in a pure quantum state. It is generally assumed that some physics causes this pure state to decohere so that it should be treated probabilistically. We show that the entanglement entropy between the universe inside our observable horizon and that outside our horizon is sufficient to do this. For the modes which are super-Hubble at the end of inflation, this entanglement entropy grows with volume inside the horizon, rather than with the horizon's area, and is proportional to the number of e-folds since Hubble crossing.
TL;DR: In this article, the authors developed a new model for the universe based on two key assumptions: first, the inertial energy of the universe is a constant, and second, the total energy of a particle, including the gravitational potential energy produced by the other mass in the universe, is zero.
Abstract: We develop a new model for the Universe based on two key assumptions: first, the inertial energy of the Universe is a constant, and second, the total energy of a particle, the inertial plus the gravitational potential energy produced by the other mass in the Universe, is zero. This model allows the speed of light and the total mass of the Universe to vary as functions of cosmological time, where we assume the gravitational constant to be a constant. By means of these assumptions, the relations between the scale factor and the other parameters are derived. The Einstein equation, by making it compatible with varying-c, is used and the Friedmann equations in this model are obtained. Assuming the matter content of the Universe to be perfect fluids, the model fixes γ to be 2/3. That is, the whole Universe always exhibits a negative pressure. Moreover, the behaviour of the scale factor is the same for any value of the curvature. It is also shown that the Universe began from a big bang with zero initial mass and...
TL;DR: In this article, the authors consider the holographic dark energy in induced gravity by taking the Hubble scale, particle horizon and event horizon as the infrared cutoff, and find that only the event horizon can give accelerating expansion of our universe.
Abstract: Many astrophysics data show that our universe has a critical energy density, and 73% of it is dark energy, which drives the accelerating expansion of the universe. We consider the holographic dark energy in induced gravity by taking the Hubble scale, particle horizon and event horizon as the infrared cutoff. We find that only the event horizon can give accelerating expansion of our universe.
TL;DR: In this article, it was shown that the universe with a cosmic pressure obeying Einstein's field equations, can be inside a white hole, for the flat and open cases, but with the growth of the scale-factor, the condition will be certainly fulfilled for a positive cosmological constant, after some time.
Abstract: Pathria (1972) has shown, for a pressureless closed Universe, that it is inside a black (or white) hole. We show now, that the Universe with a cosmic pressure obeying Einstein’s field equations, can be inside a white-hole. In the closed case, a positive cosmological constant does the job; for the flat and open cases, the condition we find is not verified for the very early Universe, but with the growth of the scale-factor, the condition will be certainly fulfilled for a positive cosmological constant, after some time. We associate the absolute temperature of the Universe, with the temperature of the corresponding white-hole.
TL;DR: In this article, a model of modified Chaplygin gas in VSL theory with variable gravitational constant G was considered and the evolution of the universe started from radiation era to phantom model was shown diagramatically by using statefinder parameters.
Abstract: In this paper, we have considered a model of modified Chaplygin gas in VSL theory with variable gravitational constant G. We have shown that the evolution of the universe starts from radiation era to phantom model. The whole evolution of the universe has been shown diagramatically by using statefinder parameters.