Need For Dark Energy Research Articles

Cosmic parallax as a probe of late time anisotropic expansion

Cosmic parallax is the change of angular separation between a pair of sources at cosmological distances induced by an anisotropic expansion. An accurate astrometric experiment like Gaia could observe or put constraints on cosmic parallax. Examples of anisotropic cosmological models are Lemaitre-Tolman-Bondi void models for off-center observers (introduced to explain the observed acceleration without the need for dark energy) and Bianchi metrics. If dark energy has an anisotropic equation of state, as suggested recently, then a substantial anisotropy could arise at $z\ensuremath{\lesssim}1$ and escape the stringent constraints from the cosmic microwave background. In this paper we show that such models could be constrained by the Gaia satellite or by an upgraded future mission.

Dark torsion as the cosmic speed-up

It is shown that the recently detected acceleration of the Uuniverse can be understood by considering a modification of the teleparallel equivalent of general relativity, with no need of dark energy. The solution also exhibits phases dominated by matter and radiation as expected in the standard cosmological evolution. We perform a joint analysis with measurements of the most recent type Ia supernovae, baryon acoustic oscillation peak, and estimates of the cosmic microwave background shift parameter data to constrain the only new parameter this theory has.

An accelerating cosmology without dark energy

The negative pressure accompanying gravitationally-induced particlecreation can lead to a cold dark matter (CDM) dominated, accelerating Universe (Lima et al. 1996 [1]) without requiring the presence of dark energy or a cosmological constant. In a recent study, Lima et al. 2008 [2] (LSS) demonstrated that particle creation driven cosmological models are capable of accounting for the SNIa observations [3] of the recent transition from a decelerating to an accelerating Universe, without the need for Dark Energy. Here we consider a class of such models where the particle creation rate is assumed to be of the form Γ = βH+γH0, where H is the Hubble parameter and H0 is its present value. The evolution of such models is tested at low redshift by the latest SNe Ia data provided by the Union compilation [4] and at high redshift using the value of zeq, the redshift of the epoch of matter — radiation equality, inferred from the WMAP constraints on the early Integrated Sachs-Wolfe (ISW) effect [5]. Since the contributions of baryons and radiation were ignored in the work of LSS, we include them in our study of this class of models. The parameters of these more realistic models with continuous creation of CDM are constrained at widely-separated epochs (zeq ≈ 3000 and z ≈ 0) in the evolution of the Universe. The comparison of the parameter values, {β, γ}, determined at these different epochs reveals a tension between the values favored by the high redshift CMB constraint on zeq from the ISW and those which follow from the low redshift SNIa data, posing a potential challenge to this class of models. While for β = 0 this conflict is only at ≲ 2σ, it worsens as β increases from zero.

Testing the copernican principle via cosmological observations

Observations of distances to Type-Iasupernovae can be explained by cosmological models that include either a gigaparsec-scale void, or a cosmic flow, without the need for Dark Energy. Instead of invoking dark energy, theseinhomogeneous models instead violate the Copernican Principle.we show that current cosmological observations (Supernovae, Baryon Acoustic Oscillations and estimates of the Hubble parameters based on the age of the oldest stars) are not able to rule out inhomogeneous anti-Copernican models.The next generation of surveys for baryonic acoustic oscillationswill be sufficiently precise to either validatethe Copernican Principle or determine the existence of a local Gpc scale inhomogeneity.

Pressure gradients, shell-crossing singularities and acoustic oscillations – application to inhomogeneous cosmological models

Abstract Inhomogeneous cosmological models have recently become a very interesting alternative to standard cosmology. This is because these models are able to fit cosmological observations without the need for dark energy. However, due to inhomogeneity and pressureless matter content, these models can suffer from shell-crossing singularities. These singularities occur when two shells of dust collide with each other leading to infinite values of the density. In this Letter, we show that if inhomogeneous pressure is included then these singularities can be prevented from occurring over the period of structure formation. Thus, a simple incorporation of a gradient of pressure allows for more comprehensive studies of inhomogeneous cosmological models and their application to cosmology.

Latest supernovae constraints on [formula omitted] cosmologies

A class of modified gravity, known as f(R)-gravity, has presently been applied to Cosmology as a realistic alternative to dark energy. In this Letter we use the most recent Type-Ia Supernova (SNe Ia) data, the so-called Union sample of 307 SNe Ia, to place bounds on a theory of the form f(R)=R−β/Rn within the Palatini approach. Given the complementarity of SNe Ia data with other cosmological observables, a joint analysis with measurements of baryon acoustic oscillation peak and estimates of the CMB shift parameter is also performed. We show that, for the allowed intervals of n, Ωmo, and β, models based on f(R)=R−β/Rn gravity in the Palatini approach can produce the sequence of radiation-dominated, matter-dominated, and accelerating periods without need of dark energy.

Cosmological constraints from the Hubble parameter onf(R) cosmologies

Modified f(R) gravity in the Palatini approach has been recently applied in cosmology as a realistic alternative todark energy. In this regard, a number of authors have searched for observational constraints on severalf(R) gravity functional forms using mainly data from type Ia supernovae, cosmic microwavebackground (CMB) radiation and large scale structure. In this paper, considering ahomogeneous and isotropic flat universe, we use determinations of the Hubble functionH(z), which are based on a differential age method, to place bounds on the free parameters of thef(R) = R−β/Rn functional form.We also combine the H(z) data with constraints from baryon acoustic oscillations and CMB measurements, obtaining ranges of valuesfor n and β in agreement with other independent analyses. We find that, for some intervals ofn andβ, modelsbased on f(R) = R−β/Rn gravity in the Palatini approach, unlike the metric formalism, can produce a sequence ofradiation-dominated, matter-dominated, and accelerating periods without the need for darkenergy.

Geometry for the accelerating universe

The Lorentzian spacetime metric is refined to an area metric which naturally emerges as a generalized geometry in quantum string and gauge theory. Employing the area metric curvature scalar, the Einstein-Hilbert gravitational action is reinterpreted as dynamics for an area metric. The area metric cosmology of the radiation-dominated early universe does not depart from general relativity, enabling successful nucleosynthesis. But intriguingly, without the need for dark energy or fine-tuning, area metric cosmology explains the observed small acceleration of the late universe.

The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-mass white dwarf star

The accelerating expansion of the Universe, and the need for dark energy, were inferred from observations of type Ia supernovae. There is a consensus that type Ia supernovae are thermonuclear explosions that destroy carbon-oxygen white dwarf stars that have accreted matter from a companion star, although the nature of this companion remains uncertain. These supernovae are thought to be reliable distance indicators because they have a standard amount of fuel and a uniform trigger: they are predicted to explode when the mass of the white dwarf nears the Chandrasekhar mass of 1.4 solar masses (M(o)). Here we show that the high-redshift supernova SNLS-03D3bb has an exceptionally high luminosity and low kinetic energy that both imply a super-Chandrasekhar-mass progenitor. Super-Chandrasekhar-mass supernovae should occur preferentially in a young stellar population, so this may provide an explanation for the observed trend that overluminous type Ia supernovae occur only in 'young' environments. As this supernova does not obey the relations that allow type Ia supernovae to be calibrated as standard candles, and as no counterparts have been found at low redshift, future cosmology studies will have to consider possible contamination from such events.

Inhomogeneous alternative to dark energy?

Recently, there have been suggestions that the apparent accelerated expansion of the universe is not caused by repulsive gravitation due to dark energy, but is rather a result of inhomogeneities in the distribution of matter. In this work, we investigate the behavior of a dust-dominated inhomogeneous Lema\^{\i}tre-Tolman-Bondi universe model, and confront it with various astrophysical observations. We find that such a model can easily explain the observed luminosity distance-redshift relation of supernovae without the need for dark energy, when the inhomogeneity is in the form of an underdense bubble centered near the observer. With the additional assumption that the universe outside the bubble is approximately described by a homogeneous Einstein-de Sitter model, we find that the position of the first peak in the cosmic microwave background (CMB) power spectrum can be made to match the WMAP observations. Whether or not it is possible to reproduce the entire CMB angular power spectrum in an inhomogeneous model without dark energy is still an open question.

Need of dark energy for dynamical compactification of extra dimensions on the brane

We consider a six-dimensional braneworld model and we study the cosmological evolution of a (4+1) brane-universe. Introducing matter on the brane we show that the scale factor of the physical three-dimensional brane-universe is related to the scale factor of the fourth dimension on the brane, and the suppression of the extra dimension compared to the three dimensions requires the presence of dark energy.

Need of dark energy for dynamical compactification of extra dimensions on the brane

We consider a six-dimensional braneworld model and we study the cosmological evolution of a (4+1) brane-universe. Introducing matter on the brane we show that the scale factor of the physical three-dimensional brane-universe is related to the scale factor of the fourth dimension on the brane, and the suppression of the extra dimension compared to the three dimensions requires the presence of dark energy.

Problems with the Current Cosmological Paradigm

We note that despite the apparent support for the ΛCDM model from the acoustic peaks of the CMB power spectrum and the SNIa Hubble diagram, the standard cosmological model continues to face several fundamental problems. First, the model continues to depend wholly on two pieces of undiscovered physics, namely dark energy and cold dark matter. Then, the implied dark energy density is so small that it is unstable to quantum correction and its size is fine-tuned to the almost impossible level of one part in ≈ 10102; it is also difficult to explain the coincidence between the dark energy, dark matter and baryon densities at the present day. Moreover, any model with a positive Λ also creates fundamental difficulties for superstring theories of quantum gravity. We also review the significant number of astrophysical observations which are now in contradiction with the ΛCDM model. on the grounds that the SNIa Hubble diagram is prone to evolutionary corrections and also that the CMB power spectrum may be contaminated by the effects of foreground ionised gas, we argue that the existence of such systematics could still allow more satisfactory, alternative, models to appear. We suggest that if H0 ≲ 50 kms--1Mpc--1 then a simpler, inflationary model with Ωbaryon = 1 might be allowed with no need for dark energy or cold dark matter. We note that the clear scale error between HST Cepheid and Tully-Fisher galaxy distances and also potential metallicity dependencies for both the Cepheid P-L relation and the SNIa Hubble diagram may mean that such a low value of H0 cannot yet be ruled out.

Modified Gravity with ln R Terms and Cosmic Acceleration

The modified gravity with $\ln R$ or $R^{-n} (\ln R)^m$ terms which grow at small curvature is discussed. It is shown that such a model which has well-defined newtonian limit may eliminate the need for dark energy and may provide the current cosmic acceleration. It is demonstrated that $R^2$ terms are important not only for early time inflation but also to avoid the instabilities and the linear growth of the gravitational force. It is very interesting that the condition of no linear growth for gravitational force coincides with the one for scalar mass in the equivalent scalar-tensor theory to be very large. Thus, modified gravity with $R^2$ term seems to be viable classical theory.

THE MINIMAL CURVATURE OF THE UNIVERSE IN MODIFIED GRAVITY AND CONFORMAL ANOMALY RESOLUTION OF THE INSTABILITIES

We discuss the modified gravity which may produce the current cosmic acceleration of the universe and eliminate the need for dark energy. It is shown that such models where the action quickly grows with the decrease of the curvature define the FRW universe with the minimal curvature. Infinite time is required to reach the minimal curvature during the universe evolution. It is demonstrated that quantum effects of conformal fields may strongly suppress the instabilities discovered in modified gravity. We also briefly speculate on the modification of gravity combined with the presence of the cosmological constant dark energy.

Modified gravity with negative and positive powers of curvature: Unification of inflation and cosmic acceleration

A modified gravity, which eliminates the need for dark energy and which seems to be stable, is considered. The terms with positive powers of curvature support the inflationary epoch while the terms with negative powers of curvature serve as effective dark energy, supporting current cosmic acceleration. The equivalent scalar-tensor gravity may be compatible with the simplest solar system experiments.

Where new gravitational physics comes from: M-theory?

It is suggested [astro-ph/0306438] that current cosmic acceleration arises due to modification of general relativity by the terms with negative powers of curvature. We argue that time-dependent (hyperbolic) compactifications of (4+n)-dimensional gravity (string/M-theory) lead to the effective 4d gravity which naturally contains such terms. The same may be achieved in the braneworld by the proper choice of the boundary action. Hence, such a model which seems to eliminate the need for dark energy may have the origin in M-theory.

Cosmological effects of radion oscillations

We show that the redshift of pressureless matter density due to the expansion of the universe generically induces small oscillations in the stabilized radius of extra dimensions (the radion field). The frequency of these oscillations is proportional to the mass of the radion and can have interesting cosmological consequences. For very low radion masses $m_b$ ($m_b\sim10-100 H_0\simeq10^{-32} eV$) these low frequency oscillations lead to oscillations in the expansion rate of the universe. The occurrence of acceleration periods could naturally lead to a resolution of the coincidence problem, without need of dark energy. Even though this scenario for low radion mass is consistent with several observational tests it has difficulty to meet fifth force constraints. If viewed as an effective Brans-Dicke theory it predicts $\omega=-1+\frac{1}{D}$ ($D$ is the number of extra dimensions), while experiments on scales larger than $1mm$ imply $\omega>2500$. By deriving the generalized Newtonian potential corresponding to a massive toroidally compact radion we demonstrate that Newtonian gravity is modified only on scales smaller than $m_b^{-1}$. Thus, these constraints do not apply for $m_b>10^{-3} eV$ (high frequency oscillations) corresponding to scales less than the current experiments ($0.3mm$). Even though these high frequency oscillations can not resolve the coincidence problem they provide a natural mechanism for dark matter generation. This type of dark matter has many similarities with the axion.