The conjecture that the ancient globular clusters (GCs) formed at the center of their own dark matter halos was first proposed by Peebles (1984), and has recently been revived to explain the puzzling abundance patterns observed within many GCs. In this Letter we demonstrate that the outer stellar density profile of isolated GCs is very sensitive to the presence of an extended dark halo. The GCs NGC 2419, located at 90 kpc from the center of our Galaxy, and MGC1, located at ~200 kpc from the center of M31, are ideal laboratories for testing the scenario that GCs formed at the centers of massive dark halos. Comparing analytic models to observations of these GCs, we conclude that these GCs cannot be embedded within dark halos with a virial mass greater than 10^6 Msun, or, equivalently, the dark matter halo mass-to-stellar mass ratio must be Mdm/M_*<1. If these GCs have indeed orbited within weak tidal fields throughout their lifetimes, then these limits imply that these GCs did not form within their own dark halos. Recent observations of an extended stellar halo in the GC NGC 1851 are also interpreted in the context of our analytic models. Implications of these results for the formation of GCs are briefly discussed.
Saturday, October 30, 2010
arXiv: 29 October 2010
Evidence Against Dark Matter Halos Surrounding the Globular Clusters MGC1 and NGC 2419
Wednesday, October 27, 2010
arXiv: 28 October 2010
Cosmology of Chameleons with Power-Law Couplings
In chameleon field theories a scalar field can couple to matter with gravitational strength and still evade local gravity constraints due to a combination of self-interactions and the couplings to matter. Originally, these theories were proposed with a constant coupling to matter, however, the chameleon mechanism also extends to the case where the coupling becomes field-dependent. We study the cosmology of chameleon models with power-law couplings and power-law potentials. It is found that these generalized chameleons, when viable, have a background expansion very close to LCDM, but can in some special cases enhance the growth of the linear perturbations at low redshifts. For the models we consider it is found that this region of the parameter space is ruled out by local gravity constraints. Imposing a coupling to dark matter only, the local constraints are avoided, and it is possible to have observable signatures on the linear matter perturbations.
Detecting and distinguishing topological defects in future data from the CMBPol satellite
The proposed CMBPol mission will be able to detect the imprint of topological defects on the cosmic microwave background (CMB) provided the contribution is sufficiently strong. We quantify the detection threshold for cosmic strings and for textures, and analyse the satellite's ability to distinguish between these different types of defects. We also assess the level of danger of misidentification of a defect signature as from the wrong defect type or as an effect of primordial gravitational waves. A 0.002 fractional contribution of cosmic strings to the CMB temperature spectrum at multipole ten, and similarly a 0.001 fractional contribution of textures, can be detected and correctly identified at the 3{\sigma} level. We also confirm that a tensor contribution of r = 0.0018 can be detected at over 3{\sigma}, in agreement with the CMBpol mission concept study. These results are supported by a model selection analysis.
Constraining Modified Gravity with Euclid
Authors: Matteo Martinelli, Erminia Calabrese, Francesco De Bernardis, Alessandro Melchiorri, Luca Pagano, Roberto Scaramella
Future proposed satellite missions as Euclid can offer the opportunity to test general relativity on cosmic scales through mapping of the galaxy weak lensing signal. In this paper we forecast the ability of these experiments to constrain modified gravity scenarios as those predicted by scalar-tensor and $f(R)$ theories. We found that Euclid will improve constraints expected from the PLANCK satellite on these modified gravity models by two orders of magnitude. We discuss parameter degeneracies and the possible biases introduced by modified gravity.
Future CMB Constraints on Early, Cold, or Stressed Dark Energy
We investigate future constraints on early dark energy (EDE) achievable by the Planck and CMBPol experiments, including cosmic microwave background (CMB) lensing. For the dark energy, we include the possibility of clustering through a sound speed c_s^2 <1 (cold dark energy) and anisotropic stresses parameterized with a viscosity parameter c_vis^2. We discuss the degeneracies between cosmological parameters and EDE parameters. In particular we show that the presence of anisotropic stresses in EDE models can substantially undermine the determination of the EDE sound speed parameter c_s^2. The constraints on EDE primordial energy density are however unaffected. We also calculate the future CMB constraints on neutrino masses and find that they are weakened by a factor of 2 when allowing for the presence of EDE, and highly biased if it is incorrectly ignored.
Using Dark Matter Haloes to Learn about Cosmic Acceleration: A New Proposal for a Universal Mass Function
Structure formation provides a strong test of any cosmic acceleration model because a successful dark energy model must not inhibit {\black or overpredict} the development of observed large-scale structures. Traditional approaches to studies of structure formation in the presence of dark energy or a modified gravity implement a modified Press-Schechter formalism, which relates the linear overdensities to the abundance of dark matter haloes it at the same time. We critically examine the universality of the Press-Schechter formalism for different cosmologies, and show that the halo abundance is best correlated with spherical linear overdensity at 94% of collapse (or observation) time. We then extend this argument to ellipsoidal collapse (which decreases the fractional time of best correlation for small haloes, and show that our results agree with deviations from modified Press-Schechter formalism seen in simulated mass functions. This provides a novel universal prescription to measure linear density evolution, based on current and future observations of cluster (or dark matter) halo mass function. In particular, even observations of cluster abundance in a single epoch will constrain the entire history of linear growth of cosmological of perturbations.
Tuesday, October 26, 2010
arXiv: 27 October 2010
Measuring primordial non-Gaussianity through weak lensing peak counts
We explore the possibility of detecting primordial non-Gaussianity of the local type using weak lensing peak counts. We measure the peak abundance in sets of simulated weak lensing maps corresponding to three models f_NL=(0, -100, 100). Using survey specifications similar to those of EUCLID and without assuming any knowledge of the lens and source redshifts, we find the peak functions of the non-Gaussian models with f_NL=+/-100 to differ by up to 15% from the Gaussian peak function at the high-mass end. For the assumed survey parameters, the probability of fitting an f_NL=0 peak function to the f_NL=+/-100 peak functions is less than 0.1%. Assuming the other cosmological parameters known, f_NL can be measured with an error sigma(f_NL)~13. It is therefore possible that future weak lensing surveys like EUCLID may detect primordial non-Gaussianity from the abundance of peak counts.
On A Cosmological Invariant as an Observational Probe in the Early Universe
k-essence scalar field models are usually taken to have lagrangians of the form ${\mathcal L}=-V(\phi)F(X)$ with $F$ some general function of $X=\nabla_{\mu}\phi\nabla^{\mu}\phi$. Under certain conditions this lagrangian in the context of the early universe can take the form of that of an oscillator with time dependent frequency. The Ermakov invariant for a time dependent oscillator in a cosmological scenario then leads to an invariant quadratic form involving the Hubble parameter and the logarithm of the scale factor. In principle, this invariant can lead to further observational probes for the early universe. Moreover, if such an invariant can be observationally verified then the presence of dark energy will also be indirectly confirmed.
Anisotropic Inflation from Charged Scalar Fields
We consider models of inflation with $U(1)$ gauge fields and charged scalar fields including symmetry breaking potential, chaotic inflation and hybrid inflation. We show that there exist attractor solutions where the anisotropies produced during inflation becomes comparable to the slow-roll parameters. In the models where the inflaton field is a charged scalar field the gauge field becomes highly oscillatory at the end of inflation ending inflation quickly. Furthermore, in charged hybrid inflation the onset of waterfall phase transition at the end of inflation is affected significantly by the evolution of the background gauge field. Rapid oscillations of the gauge field and its coupling to inflaton can have interesting effects on preheating and non-Gaussianities.
Monday, October 25, 2010
arXiv: 26 October 2010
The Nature of Damped Lyman Alpha Systems and Their Hosts in the Standard Cold Dark Matter Universe
Authors: Renyue Cen
Using adaptive mesh-refinement cosmological hydrodynamic simulations with a physically motivated supernova feedback prescription we show that the standard cold dark matter model can account for extant observed properties of damped Lyman alpha systems (DLAs). We then examine the properties of DLA host galaxies. We find: (1) While DLA hosts roughly trace the overall population of galaxies at all redshifts, they are always gas rich. (2) The history of DLA evolution reflects primarily the evolution of the underlying cosmic density, galaxy size and galaxy interactions. With higher density and more interactions at high redshift DLAs are larger in both absolute terms and in relative terms with respect to virial radii of halos. (3) The variety of DLAs at high redshift is richer with a large contribution coming from galactic filaments, created through close galaxy interactions. The portion of gaseous disks of galaxies where most stars reside makes relatively small contribution to DLA incidence at z=3-4. (4) The vast majority of DLAs arise in halos of mass M_h=10^10-10^12 Msun at z=1.6-4. At z=3-4, 20-30% of DLA hosts are Lyman Break Galaxies (LBGs). (5) Galactic winds play an indispensable role in shaping the kinematic properties of DLAs. Specifically, the high velocity width DLAs are a mixture of those arising in high mass, high velocity dispersion halos and those arising in smaller mass systems where cold gas clouds are entrained to high velocities by galactic winds. (6) In agreement with observations, we see a weak but noticeable evolution in DLA metallicity. The metallicity distribution centers at [Z/H]=-1.5 to -1 at z=3-4, with the peak moving to [Z/H]=-0.75 at z=1.6 and [Z/H]=-0.5 by z=0. (7) The star formation rate of DLA hosts is concentrated in the range 0.3-30Msun/yr at z=3-4, gradually shifting lower to peak at ~0.5-1 Msun/yr by z=0.
Atomic Precision Tests and Light Scalar Couplings
We calculate the shift in the atomic energy levels induced by the presence of a scalar field which couples to matter and photons. We find that a combination of atomic measurements can be used to probe both these couplings independently. A new and stringent bound on the matter coupling springs from the precise measurement of the 1s to 2s energy level difference in the hydrogen atom, while the coupling to photons is essentially constrained by the Lamb shift. Combining these constraints with current particle physics bounds we find that the contribution of a scalar field to the recently claimed discrepancy in the proton radius measured using electronic and muonic atoms is negligible.
Sunday, October 24, 2010
arXiv: 25 October 2010
Unifying Einstein and Palatini gravities
We consider a novel class of $f(\R)$ gravity theories where the connection is related to the conformally scaled metric $\hat g_{\mu\nu}=C(\R)g_{\mu\nu}$ with a scaling that depends on the scalar curvature $\R$ only. We call them C-theories and show that the Einstein and Palatini gravities can be obtained as special limits. In addition, C-theories include completely new physically distinct gravity theories even when $f(\R)=\R$. With nonlinear $f(\R)$, C-theories interpolate and extrapolate the Einstein and Palatini cases and may avoid some of their conceptual and observational problems. We further show that C-theories have a scalar-tensor formulation, which in some special cases reduces to simple Brans-Dicke-type gravity. If matter fields couple to the connection, the conservation laws in C-theories are modified. The stability of perturbations about flat space is determined by a simple condition on the lagrangian.
Searching for Galactic Hidden Gas through interstellar scintillation: Results from a test with the NTT-SOFI detector
Aims: Stars twinkle because their light propagates through the atmosphere. The same phenomenon is expected at longer time scale when the light of remote stars crosses an interstellar molecular cloud, but it has never been observed at optical wavelength. In a favorable case, the light of a background star can be subject to stochastic fluctuations of order of a few percent at a characteristic time scale of a few minutes. Our ultimate aim is to discover or exclude such scintillation effects, in order to estimate the contribution of molecular hydrogen to the Galactic baryonic hidden mass. This feasibility study is a pathfinder towards an observational strategy to search for scintillation, probing sensitivity of future surveys and estimating the background level. Methods: Scintillation induced by molecular gas in visible dark nebulae as well as by hypothetical halo clumpuscules of cool molecular hydrogen ($\mathrm{H_2-He}$) has been searched for during two nights. We have taken long series of 10s infrared exposures with the ESO-NTT telescope toward stellar populations located behind visible nebulae and toward the SMC. We therefore searched for stars exhibiting stochastic flux variations similar to the ones expected from the scintillation effect. According to our simulations of the scintillation process, this search should allow one to detect (stochastic) transverse gradients of column density in cool Galactic molecular clouds of order of $\sim 3\times 10^{5}\,\mathrm{g/cm^2/10\,000\,km}$. Results: We found one light-curve which is compatible with a strong scintillation effect through a turbulent structure characterized by a diffusion radius $R_{diff}<100\, km$ in B68 nebula. Complementary observations are needed to clarify the status of such candidate, and no firm conclusion can be established from this single observation. We can also infer limits on the existence of turbulent dense cores (of number density $n>10^9\, cm^{-3}$) within the dark nebulae. As no candidate is found towards the Small Magellanic Cloud, we are also able to establish upper limits on the contribution of gas clumpuscules to the Galactic halo mass. Conclusions: The limits set by this test do not seriously constrain the known models, but we show that the short time-scale monitoring for a few $10^6 star\times hour$ in the visible band with a $>4$ meter telescope and a fast readout camera should allow one to quantify the contribution of turbulent molecular gas to the Galactic halo. The LSST (Large Synoptic Survey Telescope) is perfectly suitable for this search.
Friday, October 22, 2010
arXiv: 21 October 2010
The peculiar velocity field: constraining the tilt of the Universe
A large bulk flow, which is in tension with the Lambda Cold Dark Matter cosmological model, has been observed \cite{Watkins08,Feldman09}. In this letter, we provide a physical explanation for this very large bulk flow, based on the assumption that the cosmic microwave background (CMB) rest frame does not coincide with the matter rest frame, resulting in a "tilted Universe". We propose a model that takes into account the relative velocity of CMB frame with respect to to the matter rest frame (hereafter tilted velocity), and use Type Ia Supernovae (SN), ENEAR, SFI++, SMAC, and COMPOSITE galaxy catalogues to constrain this tilted velocity.
We find that: (1) the magnitude of the tilted velocity $u$ is around 400 km/s, and its direction is close to what is found by \cite{Watkins08}; for SN, SMAC and COMPOSITE catalogues, $u=0$ is excluded at the two to three sigma level; (2) the constraints on the magnitude of the tilted velocity can result in the constraints on the duration of inflation, due to the fact that inflation can neither be too long (no dipole effect) nor too short (very large dipole effect); (3) under certain assumptions, the constraints on the tilted velocity requires that inflation lasts at least 6 e-folds longer than that required to solve the horizon problem. This opens a new window for testing inflation and the models of the early Universe from observations of large scale structure.
We find that: (1) the magnitude of the tilted velocity $u$ is around 400 km/s, and its direction is close to what is found by \cite{Watkins08}; for SN, SMAC and COMPOSITE catalogues, $u=0$ is excluded at the two to three sigma level; (2) the constraints on the magnitude of the tilted velocity can result in the constraints on the duration of inflation, due to the fact that inflation can neither be too long (no dipole effect) nor too short (very large dipole effect); (3) under certain assumptions, the constraints on the tilted velocity requires that inflation lasts at least 6 e-folds longer than that required to solve the horizon problem. This opens a new window for testing inflation and the models of the early Universe from observations of large scale structure.
A Testable Solution of the Cosmological Constant and Coincidence Problems
We present a new solution to the cosmological constant (CC) and coincidence problems in which the observed value of the CC, Lambda, is linked to other observable properties of the universe. This is achieved by promoting the CC from a parameter which must to specified, to a field which can take many possible values. The observed value of Lambda = 1/(9.3 Gyrs)^2 (~ 10^(-120) in Planck units) is determined by a new constraint equation which follows from the application of a causally restricted variation principle. When applied to our visible universe, the model makes a testable prediction for the dimensionless spatial curvature of Omega_K0 = -0.0056 (s_b/0.5); where s_b ~ 1/2 is a QCD parameter. Requiring that a classical history exist, our model determines the probability of observing a given Lambda. The observed CC value, which we successfully predict, is typical within our model even before the effects of anthropic selection are included. When anthropic selection effects are accounted for, we find that the observed coincidence between t_Lambda = Lambda^(-1/2) and the age of the universe, t_{U}, is a typical occurrence in our model. In contrast to multiverse explanations of the CC problems, our solution is independent of the choice of a prior weighting of different Lambda-values and does not rely on anthropic selection effects. Our model includes no unnatural small parameters and does not require the introduction of new dynamical scalar fields or modifications to general relativity, and it can be tested by astronomical observations in the near future.
Wednesday, October 20, 2010
arXiv: 20 October 2010
Linear growth of matter density perturbations in $f(R,\GB)$ theories
We derive the equation of matter density perturbations on sub-horizon scales around a flat Friedmann-Lema\^\i tre-Robertson-Walker background for the general Lagrangian density $f(R,\GB)$ that is a function of a Ricci scalar $R$ and a Gauss-Bonnet term $\GB$. We find that the effective gravitational constant generically scales as distance squared at small distances. The effect of this diminishing of the gravitational constant might be important in the gravitational dynamics of cosmic objects such as galaxies, which can be in principle tested by observations. We also provide the general expressions for the effective anisotropic stress, which is useful to constrain modified gravity models from observations of large-scale structure and weak lensing. We also find that there is a special class of theories which evade this unusual behaviour and that the condition to belong to this special class is exactly the same as the one for not having super-luminal modes with propagation speed proportional to their wavenumber.
Chameleon dark energy models with characteristic signatures
Authors: Radouane Gannouji, Bruno Moraes, David F. Mota, David Polarski, Shinji Tsujikawa, Hans A. Winther
In chameleon dark energy models, local gravity constraints tend to rule out parameters in which observable cosmological signatures can be found. We study viable chameleon potentials consistent with a number of recent observational and experimental bounds. A novel chameleon field potential, motivated by f(R) gravity, is constructed where observable cosmological signatures are present both at the background evolution and in the growth-rate of the perturbations. We study the evolution of matter density perturbations on low redshifts for this potential and show that the growth index today gamma_0 can have significant dispersion on scales relevant for large scale structures. The values of gamma_0 can be even smaller than 0.2 with large variations of gamma on very low redshifts for the model parameters constrained by local gravity tests. This gives a possibility to clearly distinguish these chameleon models from the Lambda-Cold-Dark-Matter model in future high-precision observations.
Gravitational Lensing
Authors: Matthias Bartelmann (Zentrum fuer Astronomie der Universitaet Heidelberg, Institut fuer Theoretische Astrophysik)
Gravitational lensing has developed into one of the most powerful tools for the analysis of the dark universe. This review summarises the theory of gravitational lensing, its main current applications and representative results achieved so far. It has two parts. In the first, starting from the equation of geodesic deviation, the equations of thin and extended gravitational lensing are derived. In the second, gravitational lensing by stars and planets, galaxies, galaxy clusters and large-scale structures is discussed and summarised.
On the Effects of Coupled Scalar Fields on Structure Formation
A coupling between a scalar field (representing the dark energy) and dark matter could produce rich phenomena in cosmology. It affects cosmic structure formation mainly through the fifth force, a velocity-dependent force that acts parallel to particle's direction of motion and proportional to its speed, an effective rescaling of the particle masses, and a modified background expansion rate. In many cases these effects entangle and it is difficult to see which is the dominant one. Here we perform N-body simulations to study their qualitative behaviour and relative importance in affecting the key structure formation observables, for a model with exponential scalar field coupling. We find that the fifth force, a prominent example of the scalar-coupling effects, is far less important than the rescaling of particle mass or the modified expansion rate. In particular, the rescaling of particle masses is shown to be the key factor leading to less concentration of particles in halos than in LCDM, a pattern which is also observed in previous independent coupled scalar field simulations
Self-Similar Solutions of Triaxial Dark Matter Halos
We investigate the collapse and internal structure of dark matter halos. We consider halo formation from initially scale-free perturbations, for which gravitational collapse is self-similar. Fillmore and Goldreich (1984) and Bertschinger (1985) solved the one dimensional (i.e. spherically symmetric) case. We generalize their results by formulating the three dimensional self-similar equations. We solve the equations numerically and analyze the similarity solutions in detail, focusing on the internal density profiles of the collapsed halos. By decomposing the total density into subprofiles of particles that collapse coevally, we identify two effects as the main determinants of the internal density structure of halos: adiabatic contraction and the shape of a subprofile shortly after collapse; the latter largely reflects the triaxiality of the subprofile. We develop a simple model that describes the results of our 3D simulations. In a companion paper, we apply this model to more realistic cosmological fluctuations, and thereby explain the origin of the nearly universal (NFW-like) density profiles found in N-body simulations
A generalized local ansatz and its effect on halo bias
Motivated by the properties of early universe scenarios that produce observationally large local non-Gaussianity, we perform N-body simulations with non-Gaussian initial conditions from a generalized local ansatz. The bispectra are schematically of the local shape, but with scale-dependent amplitude. We find that in such cases the size of the non-Gaussian correction to the bias of small and large mass objects depends on the amplitude of non-Gaussianity roughly on the scale of the object. In addition, some forms of the generalized bispectrum alter the scale dependence of the non-Gaussian term in the bias by a fractional power of k. These features may allow significant observational constraints on the particle physics origin of any observed local non-Gaussianity, distinguishing between scenarios where a single field or multiple fields contribute to the curvature fluctuations. While analytic predictions for the non-Gaussian bias agree qualitatively with the simulations, we find numerically a stronger observational signal than expected. This suggests that a more precise understanding of halo formation is needed to fully explain the consequences of primordial non-Gaussianity
Do we know the mass of a black hole? Mass of some cosmological black hole models
Using a cosmological black hole model proposed recently, we have calculated the quasi-local mass of a collapsing structure within a cosmological setting due to different definitions put forward in the last decades to see how similar or different they are. It has been shown that the mass within the horizon follows the familiar Brown-York behavior. It increases, however, outside the horizon again after a short decrease, in contrast to the Schwarzschild case. Further away, near the void, outside the collapsed region, and where the density reaches the background minimum, all the mass definitions roughly coincide. They differ, however, substantially far from it. Generically, we are faced with three different Brown-York mass maxima: near the horizon, around the void between the overdensity region and the background, and another at cosmological distances corresponding to the cosmological horizon. While the latter two maxima are always present, the horizon mass maxima is absent before the onset of the central singularity.
Tuesday, October 19, 2010
arXiv: 19 October 2010
General relativistic effects on non-linear matter power spectrum
Non-linear nature of Einstein equation introduces genuine relativistic higher order corrections to the usual Newtonian fluid equations describing the evolution of cosmological matter perturbations. We study the effect of such novel non-linearities on the next-to-leading order matter power spectrum for the case of pressureless, irrotational fluid in a flat Friedmann background. We find that pure general relativistic corrections are negligibly small over all scales. Our result guarantees that one can safely use Newtonian cosmology even in non-linear regimes.
Large scale structure simulations of inhomogeneous LTB void models
We perform numerical simulations of large scale structure evolution in an inhomogeneous Lemaitre-Tolman-Bondi (LTB) model of the Universe. We follow the gravitational collapse of a large underdense region (a void) in an otherwise flat matter-dominated Einstein-deSitter model. We observe how the (background) density contrast at the centre of the void grows to be of order one, and show that the density and velocity profiles follow the exact non-linear LTB solution to the full Einstein equations for all but the most extreme voids. This result seems to contradict previous claims that fully relativistic codes are needed to properly handle the non-linear evolution of large scale structures, and that local Newtonian dynamics with an explicit expansion term is not adequate. We also find that the (local) matter density contrast grows with the scale factor in a way analogous to that of an open universe with a value of the matter density Omage_M(r) corresponding to the appropriate location within the void.
Scale Invariance as a Solution to the Cosmological Constant Problem
We show that scale invariance may provide a solution to the fine tuning problem of the cosmological constant. We construct a generalization of the standard model of particle physics which displays exact quantum scale invariance. The action is invariant under global scale transformations in arbitrary dimensions. We introduce two additional scalar fields, beside the Higgs field. The scale symmetry is broken spontaneously in the matter sector of the theory. In the gravitational sector it is broken cosmologically. The scaling symmetry forbids the presence of cosmological constant in the action. Hence the contribution to the cosmological constant is identically zero from the matter sector within the full quantum theory. However the gravitational sector does lead to a non-zero cosmological constant after cosmological symmetry breaking. The value of the cosmological constant can be fitted to the observed value by an appropriate choice of the scalar self coupling parameter. No fine tuning is required at loop orders since the matter sector gives zero contribution to the cosmological constant.
arXiv: 18 October 2010
Topology of large scale structure as test of modified gravity
The genus of the iso-density contours is a robust measure of the topology of large-scale structure, and relatively insensitive to galaxies biasing and redshift-space distortions. We show that the growth of density fluctuations is scale-dependent even in the linear regime in some modified gravity theories, which opens a possibility of testing the theories observationally. We propose to use the genus of the iso-density contours, an intrinsic measure of the topology of large-scale structure, as a statistic to be used in such tests. In Einstein's general theory of relativity density fluctuations are growing at the same rate on all scales in the linear regime and the topology of large-scale structure is conserved in time in comoving space because structures are growing homologously. In this theory we expect the genus-smoothing scale relation is time-independent. However, in modified gravity models where structures grow with different rates on different scales, the genus-smoothing scale relation should change in time and this can be used to test for the gravity models on large scales. We studied the case of the f(R) theory, DGP braneworld theory as well as the parameterized post-Friedmann (PPF) models. We also forecast how the modified gravity models can be constrained with optical/IR or 21cm surveys in the near future.
Charge, domain walls and dark energy
Authors: Jonathan A. Pearson
One idea to explain the mysterious dark energy which appears to pervade the Universe is that it is due to a network of domain walls which has frozen into some kind of static configuration, akin to a soap film. Such models predict an equation of state with w=P/rho=-2/3 and can be represented in cosmological perturbation theory by an elastic medium with rigidity and a relativistic sound speed. An important question is whether such a network can be created from random initial conditions. We consider various models which allow the formation of domain walls, and present results from an extensive set of numerical investigations. The idea is to give a mechanism which prevents the natural propensity of domain walls to collapse and lose energy, almost to the point where a domain wall network freezes in. We show that when domain walls couple to a field with a conserved charge, there is a parameter range for which charge condenses onto the domain walls, providing a resistive force to the otherwise natural collapse of the walls
Friday, October 15, 2010
arXiv: 15 October 2010
Cores and Cusps in Warm Dark Matter Halos
The apparent presence of large core radii in Low Surface Brightness galaxies has been claimed as evidence in favor of warm dark matter. Here we show that WDM halos do not have cores that are large fractions of the halo size: typically, r_core/r_200 < 0.001. This suggests an astrophysical origin for the large cores observed in these galaxies, as has been argued by other authors.
Modelling the shapes of the largest gravitationally bound objects
We combine the physics of the ellipsoidal collapse model with the excursion set theory to study the shapes of dark matter halos. In particular, we develop an analytic approximation to the nonlinear evolution that is more accurate than the Zeldovich approximation; we introduce a planar representation of halo axis ratios, which allows a concise and intuitive description of the dynamics of collapsing regions and allows one to relate the final shape of a halo to its initial shape; we provide simple physical explanations for some empirical fitting formulae obtained from numerical studies. Comparison with simulations is challenging, as there is no agreement about how to define a non-spherical gravitationally bound object. Nevertheless, we find that our model matches the conditional minor-to-intermediate axis ratio distribution rather well, although it disagrees with the numerical results in reproducing the minor-to-major axis ratio distribution. In particular, the mass dependence of the minor-to-major axis distribution appears to be the opposite to what is found in many previous numerical studies, where low-mass halos are preferentially more spherical than high-mass halos. In our model, the high-mass halos are predicted to be more spherical, consistent with results based on a more recent and elaborate halo finding algorithm, and with observations of the mass dependence of the shapes of early-type galaxies. We suggest that some of the disagreement with some previous numerical studies may be alleviated if we consider only isolated halos.
Thursday, October 14, 2010
arXive: 14 October 2010
Dark Matter Halos: The Dynamical Basis of Effective Empirical Models
Authors: A. Lapi (1,2), A. Cavaliere (1) (1-Univ. "Tor Vergata", Roma, Italy, 2-SISSA/ISAS, Trieste, Italy)
We investigate the dynamical basis of the classic empirical models (specifically, Sersic-Einasto and generalized NFW) that are widely used to describe the distributions of collisionless matter in galaxies. We submit that such a basis is provided by our \alpha-profiles, shown to constitute solutions of the Jeans dynamical equilibrium with physical boundary conditions. We show how to set the parameters of the empirical in terms of the dynamical models; we find the empirical models, and specifically Sersic-Einasto, to constitute a simple and close approximation to the dynamical models. Finally, we discuss how these provide an useful baseline for assessing the impact of the small-scale dynamics that may modulate the density slope in the central galaxy regions.
The Origin of Dark Matter Halo Profiles
A longstanding puzzle of fundamental importance in modern cosmology has been the origin of the nearly universal density profiles of dark matter halos found in N-body simulations -- the so-called NFW profile. We show how this behavior may be understood, simply, by applying adiabatic contraction to peaks of Gaussian random fields. We argue that dynamical friction acts to reduce enormously the effect of random scatter in the properties of initial peaks, providing a key simplification. We compare our model predictions with results of the ultra-high resolution Via Lactea-II N-body simulation, and find superb agreement. We show how our model may be used to predict the distribution of halo properties like concentration. Our results suggest that many of the basic properties of halo structure may be understood using extremely simple physics.
Particle cosmology
Authors: A. Riotto (CERN)
In these lectures the present status of the so-called standard cosmological model, based on the hot Big Bang theory and the inflationary paradigm is reviewed. Special emphasis is given to the origin of the cosmological perturbations we see today under the form of the cosmic microwave background anisotropies and the large scale structure and to the dark matter and dark energy puzzles.
Tuesday, October 12, 2010
arXiv: 13 October 2010
Cluster number counts in quintessence models
Even though the abundance and evolution of clusters have been used to study the cosmological parameters including the properties of dark energy owing to their pure dependence on the geometry of the Universe and the power spectrum, it is necessary to pay particular attention to the effects of dark energy on the analysis. We obtain the explicit dark energy dependent {\it rms} linear mass fluctuation $\sigma_8$ which is consistent with the CMB normalization with less than $2$ % errors for general constant dark energy equation of state, $\oQ$. Thus, we do not have any degeneracy between $\sigma_8$ and the matter energy density contrast $\Omo$. When we use the correct value of the critical density threshold $\delta_{c} = 1.58$ obtained recently \cite{09090826, 09100126} into the cluster number density $n$ calculation in the Press-Schechter (PS) formalism, $n$ increases as compared to the one obtained by using $\delta_{c} = 1.69$ by about $60$, $80$, and $110$ % at $z = 0$, $0.5$, and $1$, respectively. Thus, PS formalism predicts the cluster number consistent with both simulation and observed data at the high mass region. We also introduce the improved coefficients of Sheth-Tormen (ST) formalism, which is consistent with the recently suggested mass function \cite{10052239}. We found that changing $\oQ$ by $\Delta \oQ = -0.1$ from $\oQ = -1.0$ causes the changing of the comoving numbers of high mass clusters of $M = 10^{16} h^{-1} M_{\odot}$ by about $20$ and $40$ % at $z = 0$ and $1$, respectively.
Induced Gravity and the Attractor Dynamics of Dark Energy/Dark Matter
Attractor solutions that give dynamical reasons for dark energy to act like the cosmological constant, or behavior close to it, are interesting possibilities to explain cosmic acceleration. Coupling the scalar field to matter or to gravity enlarges the dynamical behavior; we consider both couplings together, which can ameliorate some problems for each individually. Such theories have also been proposed in a Higgs-like fashion to induce gravity and unify dark energy and dark matter origins. We explore restrictions on such theories due to their dynamical behavior compared to observations of the cosmic expansion. Quartic potentials in particular have viable stability properties and asymptotically approach general relativity.
Monday, October 11, 2010
arXiv; 12 October 2010
Cosmic Acceleration and the Helicity-0 Graviton
Authors: Claudia de Rham, Gregory Gabadadze, Lavinia Heisenberg, David Pirtskhalava
http://arxiv.org/abs/1010.1780v1
http://arxiv.org/abs/1010.1780v1
We explore cosmology in the decoupling limit of a non-linear covariant extension of Fierz-Pauli massive gravity obtained recently in arXiv:1007.0443. In this limit the theory is a scalar-tensor model of a unique form defined by symmetries. We find that it admits a self-accelerated solution, with the Hubble parameter set by the graviton mass. The negative pressure causing the acceleration is due to a condensate of the helicity-0 component of the massive graviton, and the background evolution, in the approximation used, is indistinguishable from the \Lambda CDM model. Fluctuations about the self-accelerated background are stable for a certain range of parameters involved. Most surprisingly, the fluctuation of the helicity-0 field above its background decouples from an arbitrary source in the linearized theory.
We also show how massive gravity can remarkably screen an arbitrarily large cosmological constant in the decoupling limit, while evading issues with ghosts. The obtained static solution is stable against small perturbations, suggesting that the degravitation of the vacuum energy is possible in the full theory. Interestingly, however, this mechanism postpones the Vainshtein effect to shorter distance scales. Hence, fifth force measurements severely constrain the value of the cosmological constant that can be neutralized, making this scheme phenomenologically not viable for solving the old cosmological constant problem. We briefly speculate on a possible way out of this issue.
We also show how massive gravity can remarkably screen an arbitrarily large cosmological constant in the decoupling limit, while evading issues with ghosts. The obtained static solution is stable against small perturbations, suggesting that the degravitation of the vacuum energy is possible in the full theory. Interestingly, however, this mechanism postpones the Vainshtein effect to shorter distance scales. Hence, fifth force measurements severely constrain the value of the cosmological constant that can be neutralized, making this scheme phenomenologically not viable for solving the old cosmological constant problem. We briefly speculate on a possible way out of this issue.
arXiv: 11 October 2010
Cosmography of f(R) - brane cosmology
Cosmography is a useful tool to constrain cosmological models, in particular dark energy models. In the case of modified theories of gravity, where the equations of motion are generally quite complicated, cosmography can contribute to select realistic models without imposing arbitrary choices a priori. Indeed, its reliability is based on the assumptions that the universe is homogeneous and isotropic on large scale and luminosity distance can be "tracked" by the derivative series of the scale factor a(t). We apply this approach to induced gravity brane-world models where an f(R)-term is present in the brane effective action. The virtue of the model is to self-accelerate the normal and healthy DGP branch once the f(R)-term deviates from the Hilbert-Einstein action. We show that the model, coming from a fundamental theory, is consistent with the LCDM scenario at low redshift. We finally estimate the cosmographic parameters fitting the Union2 Type Ia Supernovae (SNeIa) dataset and the distance priors from Baryon Acoustic Oscillations (BAO) and then provide constraints on the present day values of f(R) and its second and third derivatives.
Friday, October 8, 2010
arXiv: 8 October 2010
Minimally Parametric Power Spectrum Reconstruction from the Lyman-alpha Forest
Authors: Simeon Bird (IoA/Cambridge), Hiranya V. Peiris (UCL), Matteo Viel (INAF-OATS/Trieste), Licia Verde (ICC/Barcelona)
http://arxiv.org/abs/1010.1519v1
http://arxiv.org/abs/1010.1519v1
Current results from the Lyman alpha forest assume that the primordial power spectrum of density perturbations follows a simple power law form. We present the first analysis of Lyman alpha data to study the effect of relaxing this strong assumption on primordial and astrophysical constraints. We perform a large suite of numerical simulations, using them to calibrate a minimally parametric framework for describing the power spectrum. Combined with cross-validation, a statistical technique which prevents over-fitting of the data, this framework allows us to reconstruct the power spectrum shape without strong prior assumptions. We find no evidence for deviation from scale-invariance; our analysis also shows that current Lyman alpha data do not have sufficient statistical power to robustly probe the shape of the power spectrum at these scales. In contrast, the ongoing Baryon Oscillation Sky Survey (BOSS) will be able to do so with high precision. Furthermore, this near-future data will be able to break degeneracies between the power spectrum shape and astrophysical parameters
Understanding the faint red galaxy population using large-scale clustering measurements from SDSS DR7
We use data from the SDSS to investigate the evolution of the large-scale galaxy bias as a function of luminosity for red galaxies. We carefully consider correlation functions of galaxies selected from both photometric and spectroscopic data, and cross-correlations between them, to obtain multiple measurements of the large-scale bias. We find, for our most robust analyses, a strong increase in bias with luminosity for the most luminous galaxies, an intermediate regime where bias does not evolve strongly over a range of two magnitudes in galaxy luminosity, and no evidence for an upturn in bias for fainter red galaxies. Previous work has found an increase in bias to low luminosities that has been widely interpreted as being caused by an increase in the satellite fraction. We can recover such an upturn in bias to faint luminosities if we push our measurements to small scales, and include galaxy clustering measurements along the line-of-sight, where we expect non-linear effects to be the strongest. The results that we expect to be most robust suggest that central galaxies dominate the observed low luminosity population of red galaxies rather than satellite galaxies in more massive haloes.
Needatool: A Needlet Analysis Tool for Cosmological Data Processing
We introduce NeedATool (Needlet Analysis Tool), a software for data analysis based on needlets, a wavelet rendition which is powerful for the analysis of fields defined on a sphere. Needlets have been applied successfully to the treatment of astrophysical and cosmological observations, and in particular to the analysis of cosmic microwave background (CMB) data. Usually, such analyses are performed in real space as well as in its dual domain, the harmonic one. Both spaces have advantages and disadvantages: for example, in pixel space it is easier to deal with partial sky coverage and experimental noise; in harmonic domain, beam treatment and comparison with theoretical predictions are more effective. During the last decade, however, wavelets have emerged as a useful tool for CMB data analysis, since they allow to combine most of the advantages of the two spaces, one of the main reasons being their sharp localisation. In this paper, we outline the analytical properties of needlets and discuss the main features of the numerical code, which should be a valuable addition to the CMB analyst's toolbox.
Thursday, October 7, 2010
arXiv: 7 October 2010
f(T) gravity and local Lorentz invariance
We show that in theories of generalised teleparallel gravity, whose Lagrangians are algebraic functions of the usual teleparallel Lagrangian, the action and the field equations are not invariant under local Lorentz transformations. We also argue that these theories appear to have extra degrees of freedom with respect to general relativity. Both of these facts appear to have been overlooked but are crucial for assessing the viability of these theories as alternative explanations for the acceleration of the universe.
Statistical Classification Techniques for Photometric Supernova Typing
Authors: James Newling, Melvin Varughese, Bruce Bassett, Heather Campbell, Renée Hlozek, Martin Kunz, Hubert Lampeitl, Bryony Martin, Robert Nichol, David Parkinson, Mathew Smith
Future photometric supernova surveys will produce vastly more candidates than can be followed up spectroscopically, highlighting the need for effective classification methods based on lightcurves alone. Here we introduce boosting and kernel density estimation techniques which have minimal astrophysical input, and compare their performance on 20,000 simulated Dark Energy Survey lightcurves. We demonstrate that these methods are comparable to the best template fitting methods currently used, and in particular do not require the redshift of the host galaxy or candidate. However both methods require a training sample that is representative of the full population, so typical spectroscopic supernova subsamples will lead to poor performance. To enable the full potential of such blind methods, we recommend that representative training samples should be used and so specific attention should be given to their creation in the design phase of future photometric surveys.
The Accelerating Universe
Authors: Dragan Huterer (University of Michigan)
In this article we review the discovery of the accelerating universe using type Ia supernovae. We then outline ways in which dark energy - component that causes the acceleration - is phenomenologically described. We finally describe principal cosmological techniques to measure large-scale properties of dark energy. This chapter complements other articles in this book that describe theoretical understanding (or lack thereof) of the cause for the accelerating universe.
Wednesday, October 6, 2010
arXiv: 6 October 2010
Distance and time constraints on accelerating cosmological models
The absence of guidance from fundamental physics about the mechanism behind cosmic acceleration has given rise to a number of alternative cosmological scenarios. These are based either on modifications of general relativistic gravitation theory on large scales or on the existence of new fields in Nature. In this paper we investigate the observational viability of some accelerating cosmological models in light of current measurements of lookback time as a function of redshift from passively evolving galaxies and recent estimates of the product of the cosmic microwave background acoustic scale and the baryonic acoustic oscillation peak scale. By using information-criteria model selection, we select the best-fit models and rank the alternative scenarios. We show that some of these models may provide a better fit to the data than does the current standard cosmological constant dominated ($\Lambda$CDM) model.
Rotation Speed of the First Stars
We estimate the rotation speed of Population III (Pop III) stars within a minihalo at z ~ 20 using a smoothed particle hydrodynamics (SPH) simulation, beginning from cosmological initial conditions. We follow the evolution of the primordial gas up to densities of 10^12 cm^-3. Representing the growing hydrostatic cores with accreting sink particles, we measure the velocities and angular momenta of all particles that fall onto these protostellar regions. This allows us to record the angular momentum of the sinks and estimate the rotational velocity of the Pop III stars expected to form within them. The rotation rate has important implications for the evolution of the star, the fate encountered at the end of its life, and the potential for triggering a gamma-ray burst (GRB). We find that there is sufficient angular momentum to yield rapidly rotating stars (> 1000 km s^-1, or near break-up speeds). This indicates that Pop III stars likely experienced strong rotational mixing, impacting their structure and nucleosynthetic yields. A subset of them was also likely to result in hypernova explosions, and possibly GRBs.
Laboratory constraints on chameleon dark energy and power-law fields
Authors: Jason H. Steffen (1), Amol Upadhye (2), Al Baumbaugh (1), Aaron S. Chou (1), Peter O. Mazur (1), Ray Tomlin (1), Amanda Weltman (3), William Wester
We report results from the GammeV Chameleon Afterglow Search---a search for chameleon particles created via photon/chameleon oscillations within a magnetic field. This experiment is sensitive to a wide class of chameleon power-law models and dark energy models not previously explored. These results exclude five orders of magnitude in the coupling of chameleons to photons covering a range of four orders of magnitude in chameleon effective mass and, for individual chameleon models, exclude between 4 and 12 orders of magnitude in chameleon couplings to matter.
Constraining the expansion rate of the Universe using low-redshift ellipticals as cosmic chronometers
We present a new methodology to determine the expansion history of the Universe analyzing the spectral properties of early type galaxies (ETG). We found that for these galaxies the 4000\AA break is a spectral feature that correlates with the relative ages of ETGs. In this paper we describe the method, explore its robustness using theoretical synthetic stellar population models, and apply it using a SDSS sample of $\sim$14 000 ETGs. Our motivation to look for a new technique has been to minimise the dependence of the cosmic chronometer method on systematic errors. In particular, as a test of our method, we derive the value of the Hubble constant $H_0 = 72.3 \pm 2.8$ (68% confidence), which is not only fully compatible with the value derived from the Hubble key project, but also with a comparable error budget. Using the SDSS, we also derive, assuming w=constant, a value for the dark energy equation of state parameter $w = -0.8 \pm 0.2$. Given the fact that the SDSS ETG sample only reaches $z \sim 0.3$, this result shows the potential of the method. In future papers we will present results using the high-redshift universe, to yield a determination of H(z) up to $z \sim 1$.
Combining cluster observables and stacked weak lensing to probe dark energy: Self-calibration of systematic uncertainties
We develop a new method of combining cluster observables (number counts and cluster-cluster correlation functions) and stacked weak lensing signals of background galaxy shapes, both of which are available in a wide-field optical imaging survey. Assuming that the clusters have secure redshift estimates, we show that the joint experiment enables a self-calibration of important systematic errors including the source redshift uncertainty and the cluster mass-observable relation, by adopting a single population of background source galaxies for the lensing analysis. It allows us to use the relative strengths of stacked lensing signals at different cluster redshifts for calibrating the source redshift uncertainty, which in turn leads to accurate measurements of the mean cluster mass in each bin. In addition, our formulation of stacked lensing signals in Fourier space simplifies the Fisher matrix calculations, as well as the marginalization over the cluster off-centering effect, the most significant uncertainty in stacked lensing. We show that upcoming wide-field surveys yield stringent constraints on cosmological parameters including dark energy parameters, without any priors on nuisance parameters that model systematic uncertainties. Specifically, the stacked lensing information improves the dark energy FoM by a factor of 4, compared to that from the cluster observables alone. The primordial non-Gaussianity parameter can also be constrained with a level of f_NL~10. In this method, the mean source redshift is well calibrated to an accuracy of 0.1 in redshift, and the mean cluster mass in each bin to 5-10% accuracies, which demonstrates the success of the self-calibration of systematic uncertainties from the joint experiment. (Abridged)
Metric compatibility and torsionlessness in the Einstein-Palatini formulation of general relativity
It has been shown recently that Einstein's general relativity equations are obtained within the Einstein-Palatini formalism with no restrictions whatsoever on the connection. We rederive and extend this result for the vielbein Einstein-Palatini formulation and in the process discover the underlying gauge symmetry and the corresponding gauge condition. So far as we know this had remained unnoticed in the previous considerations of this problem. We obtain in a very simple way Einstein's condition on the connection to yield the Levi Civita connection and we show that it is merely the gauge condition of the $U(1)^d$ gauge symmetry. We perform a canonical analysis and show by counting the degrees of freedom that the additional degrees introduced in the Palatini formalism are exactly acounted for by the $U(1)^d$ gauge symmetry. Finally we also include the fermions and note that this gauge symmetry still holds good implying metricity with torsion.
Inflationary Perturbations in Palatini Generalised Gravity
We examine the generation of primordial perturbations during an inflationary epoch in generalised theories of gravity when the equations of motion are derived using the Palatini variational principle. Both f(R) and Scalar-Tensor theories are considered and we compare our results with those obtained under the conventional metric formalism. Non-linear generalisations of the action lead to different theories under the two variational choices and we obtain distinct results for scalar and tensor spectral indices and their ratio. We find the following general result; inflation driven solely by f(R) modifications alone do not result in suitable curvature perturbations whilst Scalar-Tensor theories generate nearly scalar invariant curvature perturbations but no tensor modes.
Monday, October 4, 2010
arXiv: 5 October 2010
Observational constraints on Galileon cosmology
Authors: Savvas Nesseris, Antonio De Felice, Shinji Tsujikawa
http://arxiv.org/abs/1010.0407v1
http://arxiv.org/abs/1010.0407v1
We study the cosmology of a covariant Galileon field with five covariant Lagrangians and confront this theory with the most recent cosmological probes: the type Ia supernovae data (Constitution and Union2 sets), cosmic microwave background (WMAP7) and the baryon acoustic oscillations (SDSS7). In the Galileon cosmology with a late-time de Sitter attractor, there is a tracker that attracts solutions with different initial conditions to a common trajectory. Including the cosmic curvature K, we place observational constraints on two distinct cases: (i) the tracker, and (ii) the generic solutions to the equations of motion. We find that the tracker solution can be consistent with the individual observational data, but it is disfavored by the combined data analysis. The generic solutions fare quite well when a non-zero curvature parameter is taken into account, but the Akaike and Bayesian information criteria show that they are not particularly favored over the LCDM model.
The CMB Bispectrum, Trispectrum, non-Gaussianity, and the Cramer-Rao Bound
Minimum-variance estimators for the parameter fnl that quantifies local-model non-Gaussianity can be constructed from the cosmic microwave background (CMB) bispectrum (three-point function) and also from the trispectrum (four-point function). Some have suggested that a comparison between the estimates for the values of fnl from the bispectrum and trispectrum allow a consistency test for the model. But others argue that the saturation of the Cramer-Rao bound by the bispectrum estimator implies that no further information on fnl can be obtained from the trispectrum. Here we elaborate the nature of the correlation between the bispectrum and trispectrum estimators for fnl. We show that the two estimators become statistically independent in the limit of large number of CMB pixels and thus that the trispectrum estimator does indeed provide additional information on fnl beyond that obtained from the bispectrum. We explain how this conclusion is consistent with the Cramer-Rao bound. Our discussion of the Cramer-Rao bound may be of interest to those doing Fisher-matrix parameter-estimation forecasts or data analysis in other areas of physics as well.
arXiv: 4 October 2010
Post-Decadal White Paper: A Dual-Satellite Dark-Energy/Microlensing NASA-ESA Mission
Authors: Andrew Gould
A confluence of scientific, financial, and political factors imply that launching two simpler, more narrowly defined dark-energy/microlensing satellites will lead to faster, cheaper, better (and more secure) science than the present EUCLID and WFIRST designs. The two satellites, one led by ESA and the other by NASA, would be explicitly designed to perform complementary functions of a single, dual-satellite dark-energy/microlensing ``mission''. One would be a purely optical wide-field camera, with large format and small pixels, optimized for weak-lensing, which because of its simple design, could be launched by ESA on relatively short timescales. The second would be a purely infrared satellite with marginally-sampled or under-sampled pixels, launched by NASA. Because of budget constraints, this would be launched several years later. The two would complement one another in 3 dark energy experiments (weak lensing, baryon oscillations, supernovae) and also in microlensing planet searches. Signed international agreements would guarantee the later NASA launch, and on this basis equal access of both US and European scientists to both data sets.
Testing Modified Gravity with Gravitational Wave Astronomy
The emergent area of gravitational wave astronomy promises to provide revolutionary discoveries in the areas of astrophysics, cosmology, and fundamental physics. One of the most exciting possibilities is to use gravitational-wave observations to test alternative theories of gravity. In this contribution we describe how to use observations of extreme-mass-ratio inspirals by the future Laser Interferometer Space Antenna to test a particular class of theories: Chern-Simons modified gravity.
Friday, October 1, 2010
arXiv: 1 October 2010
Cosmology of the selfaccelerating third order Galileon
In this paper we start from the original formulation of the galileon model with the original choice for couplings to gravity. Within this framework we find that there is still a subset of possible Lagrangians that give selfaccelerating solutions with stable spherically symmetric solutions. This is a certain constrained subset of the third order galileon which has not been explored before. We develop and explore the background cosmological evolution of this model drawing intuition from other even more restricted galileon models. The numerical results confirm the presence of selfacceleration, but also reveals a possible instability with respect to galileon perturbations.
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