Cosmological Distance Measurements Research Articles

Effect of inhomogeneities on high precision measurements of cosmological distances

We study effects of inhomogeneities on distance measures in an exact relativistic Swiss-cheese model of the Universe, focusing on the distance modulus. The model has $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ background dynamics, and the ``holes'' are nonsymmetric structures described by the Szekeres metric. The Szekeres exact solution of Einstein's equations, which is inhomogeneous and anisotropic, allows us to capture potentially relevant effects on light propagation due to nontrivial evolution of structures in an exact framework. Light beams traversing a single Szekeres structure in different ways can experience either magnification or demagnification, depending on the particular path. Consistent with expectations, we find a shift in the distance modulus $\ensuremath{\mu}$ to distant sources due to demagnification when the light beam travels primarily through the void regions of our model. Conversely, beams are magnified when they propagate mainly through the overdense regions of the structures, and we explore a small additional effect due to time evolution of the structures. We then study the probability distributions of $\mathrm{\ensuremath{\Delta}}\ensuremath{\mu}={\ensuremath{\mu}}_{\mathrm{\ensuremath{\Lambda}}\text{CDM}}\ensuremath{-}{\ensuremath{\mu}}_{\mathrm{SC}}$ for sources at different redshifts in various Swiss-cheese constructions, where the light beams travel through a large number of randomly oriented Szekeres holes with random impact parameters. We find for $\mathrm{\ensuremath{\Delta}}\ensuremath{\mu}$ the dispersions $0.004\ensuremath{\le}{\ensuremath{\sigma}}_{\mathrm{\ensuremath{\Delta}}\ensuremath{\mu}}\ensuremath{\le}0.008$ mag for sources with redshifts $1.0\ensuremath{\le}z\ensuremath{\le}1.5$, which are smaller than the intrinsic dispersion of, for example, magnitudes of type Ia supernovae. The shapes of the distributions we obtain for our Swiss-cheese constructions are peculiar in the sense that they are not consistently skewed toward the demagnification side, as they are in analyses of lensing in cosmological simulations. Depending on the source redshift, the distributions for our models can be skewed to either the demagnification or the magnification side, reflecting a limitation of these constructions. This could be the result of requiring the continuity of Einstein's equations throughout the overall spacetime patchwork, which imposes the condition that compensating overdense shells must accompany the underdense void regions in the holes. The possibility to explore other uses of these constructions that could circumvent this limitation and lead to different statistics remains open.

First experimental constraints on the disformally coupled Galileon model

The Galileon model is a modified gravity model that can explain the late-time accelerated expansion of the Universe. In a previous work, we derived experimental constraints on the Galileon model with no explicit coupling to matter and showed that this model agrees with the most recent cosmological data. In the context of braneworld constructions or massive gravity, the Galileon model exhibits a disformal coupling to matter, which we study in this paper. After comparing our constraints on the uncoupled model with recent studies, we extend the analysis framework to the disformally coupled Galileon model and derive the first experimental constraints on that coupling, using precise measurements of cosmological distances and the growth rate of cosmic structures. In the uncoupled case, with updated data, we still observe a low tension between the constraints set by growth data and those from distances. In the disformally coupled Galileon model, we obtain better agreement with data and favour a non-zero disformal coupling to matter at the $2.5\sigma$ level. This gives an interesting hint of the possible braneworld origin of Galileon theory.

Reconstructing the lensing mass in the Universe from photometric catalogue data

High precision cosmological distance measurements towards individual objects such as time delay gravitational lenses or type Ia supernovae are affected by weak lensing perturbations by galaxies and groups along the line of sight. In time delay gravitational lenses, "external convergence," kappa, can dominate the uncertainty in the inferred distances and hence cosmological parameters. In this paper we attempt to reconstruct kappa, due to line of sight structure, using a simple halo model. We use mock catalogues from the Millennium Simulation, and calibrate and compare our reconstructed P(kappa) to ray-traced kappa "truth" values; taking into account realistic observational uncertainties. We find that the reconstruction of kappa provides an improvement in precision of ~50% over galaxy number counts. We find that the lowest-kappa lines of sight have the best constrained P(kappa). In anticipation of large future samples of lenses, we find that selecting the third of the systems with the highest precision kappa estimates gives a sample of unbiased time delay distance measurements with just ~1% uncertainty due to line of sight external convergence effects. Photometric data are sufficient to pre-select the best-constrained lines of sight, and can be done before investment in light-curve monitoring. Conversely, we show that selecting lines of sight with high external shear could, with the reconstruction model presented, induce biases of up to 1% in time delay distance. We find that a major potential source of systematic error is uncertainty in the high mass end of the stellar mass-halo mass relation; this could introduce ~2% biases on the time-delay distance if completely ignored. We suggest areas for the improvement of this general analysis framework (including more sophisticated treatment of high mass structures) that should allow yet more accurate cosmological inferences to be made.

EVIDENCE FOR A CORRELATION BETWEEN THE Si II λ4000 WIDTH AND TYPE Ia SUPERNOVA COLOR

We study the pseudo equivalent width of the Si II 4000 feature of Type Ia supernovae in the redshift range 0.0024 < z < 0.634. We find that this spectral indicator correlates with the lightcurve color excess (SALT2 c) as well as previously defined spectroscopic subclasses (Branch types) and the evolution of the Si II 6150 velocity, i.e., the so called velocity gradient. Based on our study of 55 objects from different surveys, we find indications that the Si II 4000 spectral indicator could provide important information to improve cosmological distance measurements with Type Ia supernovae.

Constraints on cosmic opacity and beyond the standard model physics from cosmological distance measurements

We update constraints on cosmic opacity by combining recent SN Type Ia data with the latest measurements of the Hubble expansion at redshifts between 0 and 2. The new constraint on the parameter ϵ parametrising deviations from the luminosity-angular diameter distance relation (dL = dA(1+z)2+ϵ), is ϵ = −0.04−0.07+0.08 (2-σ). For the redshift range between 0.2 and 0.35 this corresponds to an opacity Δτ < 0.012 (95% C.L.), a factor of 2 stronger than the previous constraint. Various models of beyond the standard model physics that predict violation of photon number conservation contribute to the opacity and can be equally constrained. In this paper we put new limits on axion-like particles, including chameleons, and mini-charged particles.

Near-term measurements with 21 cm intensity mapping: Neutral hydrogen fraction and BAO atz&lt;2

It is shown that 21 cm intensity mapping could be used in the near term to make cosmologically useful measurements. Large scale structure could be detected using existing radio telescopes, or using prototypes for dedicated redshift survey telescopes. This would provide a measure of the mean neutral hydrogen density, using redshift space distortions to break the degeneracy with the linear bias. We find that with only 200 hours of observing time on the Green Bank Telescope, the neutral hydrogen density could be measured to 25% precision at redshift 0.54<z<1.09. This compares favourably to current measurements, uses independent techniques, and would settle the controversy over an important parameter which impacts galaxy formation studies. In addition, a 4000 hour survey would allow for the detection of baryon acoustic oscillations, giving a cosmological distance measure at 3.5% precision. These observation time requirements could be greatly reduced with the construction of multiple pixel receivers. Similar results are possible using prototypes for dedicated cylindrical telescopes on month time scales, or SKA pathfinder aperture arrays on day time scales. Such measurements promise to improve our understanding of these quantities while beating a path for future generations of hydrogen surveys.

The IR-completion of gravity: what happens at Hubble scales?

We have recently proposed an ‘ultra-strong’ version of the equivalence principle (EP) that is not satisfied by standard semiclassical gravity. In the theory that we are conjecturing, the vacuum expectation value of the (bare) energy momentum tensor is exactly the same as in flat space: quartically divergent with the cut-off and with no spacetime-dependent (subleading) terms. The presence of such terms seems in fact related to some known difficulties, such as the black hole information loss and the cosmological constant problem. Since the terms that we want to get rid of are subleading in the high-momentum expansion, we attempt to explore the conjectured theory by ‘infrared (IR)-completing’ general relativity (GR). We consider a scalar field in a flat Friedman–Robertson–Walker (FRW) universe and isolate the first IR-correction to its Fourier modes operators that kills the quadratic (next to leading) time-dependent divergence of the stress energy tensor vacuum expectation value (VEV). Analogously to other modifications of field operators that have been proposed in the literature (typically in the UV), the present approach seems to suggest a breakdown (here, in the IR, at large distances) of the metric manifold description. We show that corrections to GR are in fact very tiny, become effective at distances comparable to the inverse curvature and do not contain any adjustable parameter. Finally, we derive some cosmological implications. By studying the consistency of the canonical commutation relations, we infer a correction to the distance between two co-moving observers, which grows as the scale factor only when small compared to the Hubble length, but gets relevant corrections otherwise. The corrections to cosmological distance measures are also calculable and, for a spatially flat matter dominated Universe, go in the direction of an effective positive acceleration.

COSMOLOGICAL DISTANCE MEASURE CONSTRAINTS ON THE DECELERATION PARAMETER

Constraints on a parametrized deceleration parameter, q(a) = q0+ q1(1 - a), are investigated using cosmological observations of type Ia supernovae (SN Ia), baryon acoustic oscillations (BAOs) and cosmic microwave background (CMB) and observational Hubble data (OHD) which correspond to the cosmological distance measure. When a high redshift dataset of CMB is added in the SN + BAO + OHD case, a stronger constraint is obtained than for the lower redshift case, SN + BAO + OHD. The evolutions of the deceleration and Hubble parameters with respect to redshift z are reconstructed from cosmic observations. With cosmic observations as constraints, it is found that a smaller current deceleration parameter and a larger transition redshift are obtained in the SN + BAO + OHD case than in the SN + BAO + OHD + CMB case. In the SN + BAO + OHD + CMB case, slightly larger Hubble parameter values will be obtained than in the SN + BAO + OHD case.

The Hubble constant and dark energy from cosmological distance measures

We study how the determination of the Hubble constant from cosmological distancemeasures is affected by models of dark energy and vice versa. For this purpose, constraintson the Hubble constant and dark energy are investigated using the cosmologicalobservations of cosmic microwave background, baryon acoustic oscillations and type Iasupernovae. When one investigates dark energy, the Hubble constant is often a nuisanceparameter; thus it is usually marginalized over. On the other hand, when one focuses onthe Hubble constant, simple dark energy models such as a cosmological constant and aconstant equation of state are usually assumed. Since we do not know the nature ofdark energy yet, it is interesting to investigate the Hubble constant assumingsome types of dark energy and see to what extent the constraint on the Hubbleconstant is affected by the assumption concerning dark energy. We show thatthe constraint on the Hubble constant is not affected much by the assumptionfor dark energy. We furthermore show that this holds true even if we removethe assumption that the universe is flat. We also discuss how the prior on theHubble constant affects the constraints on dark energy and/or the curvature of theuniverse.

Cosmological models in scalar tensor theories of gravity and observations: a class of general solutions

Aims. We study cosmological models in scalar tensor theories of gravity with power-law potentials as models of an accelerating universe. Methods. We consider cosmological models in scalar tensor theories of gravity that describe an accelerating universe and study a family of inverse power-law potentials, for which exact solutions of the Einstein equations are known. We also compare theoretical predictions of our models with observations. For this we use the following data: the publicly available catalogs of type Ia supernovae and high redshift gamma ray bursts, the parameters of large-scale structure determined by the 2-degree Field Galaxy Redshift Survey (2dFGRS), and measurements of cosmological distances based on the Sunyaev-Zel’dovich effect, among others. Results. We present a class of cosmological models that describe the evolution of a homogeneous and isotropic universe filled with dust-like matter and a scalar field that is non minimally-coupled to gravity. We show that this class of models depends on three parameters: V0 – the amplitude of the scalar field potential, � H0 – the present value of the Hubble constant, and a real parameter s that determines the overall evolution of the universe. It turns out that these models have a very interesting feature naturally producing an epoch of accelerated expansion. We fix the values of these parameters by comparing predictions of our model with observational data. It turns out that our model is compatible with the presently available observational data.

Improving Cosmological Distance Measurements by Reconstruction of the Baryon Acoustic Peak

The baryon acoustic oscillations are a promising route to the precision measure of the cosmological distance scale and hence the measurement of the time evolution of dark energy. We show that the non-linear degradation of the acoustic signature in the correlations of low-redshift galaxies is a correctable process. By suitable reconstruction of the linear density field, one can sharpen the acoustic peak in the correlation function or, equivalently, restore the higher harmonics of the oscillations in the power spectrum. With this, one can achieve better measurements of the acoustic scale for a given survey volume. Reconstruction is particularly effective at low redshift, where the non-linearities are worse but where the dark energy density is highest. At z=0.3, we find that one can reduce the sample variance error bar on the acoustic scale by at least a factor of 2 and in principle by nearly a factor of 4. We discuss the significant implications our results have for the design of galaxy surveys aimed at measuring the distance scale through the acoustic peak.

Large scale structure off(R)gravity

We study the evolution of linear cosmological perturbations in $f(R)$ models of accelerated expansion in the physical frame where the gravitational dynamics are fourth order and the matter is minimally coupled. These models predict a rich and testable set of linear phenomena. For each expansion history, fixed empirically by cosmological distance measures, there exists two branches of $f(R)$ solutions that are parametrized by $B\ensuremath{\propto}{d}^{2}f/d{R}^{2}$. For $B&lt;0$, which include most of the models previously considered, there is a short-timescale instability at high curvature that spoils agreement with high redshift cosmological observables. For the stable $B&gt;0$ branch, $f(R)$ models can reduce the large-angle CMB anisotropy, alter the shape of the linear matter power spectrum, and qualitatively change the correlations between the CMB and galaxy surveys. All of these phenomena are accessible with current and future data and provide stringent tests of general relativity on cosmological scales.

Differential Density Statistics of the Galaxy Distribution and the Luminosity Function

This paper uses data obtained from the galaxy luminosity function (LF) to calculate two types of radial number density statistics of the galaxy distribution as discussed in Ribeiro, namely, the differential density γ and the integral differential density γ*. By applying the theory advanced by Ribeiro & Stoeger, which connects the relativistic cosmology number counts with the astronomically derived LF, the differential number counts dN/dz are extracted from the LF and used to calculate both γ and γ* with various cosmological distance definitions, namely, area distance, luminosity distance, galaxy area distance, and redshift distance. LF data are taken from the CNOC2 galaxy redshift survey, and γ and γ* are calculated for two cosmological models: Einstein-de Sitter and an Ω = 0.3, Ω = 0.7 standard cosmology. The results confirm the strong dependency of both statistics on the distance definition, as predicted in Ribeiro, as well as showing that plots of γ and γ* against the luminosity and redshift distances indicate that the CNOC2 galaxy distribution follows a power-law pattern for redshifts higher than 0.1. These findings support Ribeiro's theoretical proposition that using different cosmological distance measures in statistical analyses of galaxy surveys can lead to significant ambiguity in drawing conclusions about the behavior of the observed large-scale distribution of galaxies.

Accelerating universe in scalar tensor models – comparison of theoretical predictions with observations

We consider scalar tensor theories of gravity assuming that the scalar field is non minimally coupled with gravity. We use this theory to study evolution of a flat homogeneous and isotropic universe. In this case the dynamical equations can be derived form a point like Lagrangian. We study the general properties of dynamics of this system and show that for a wide range of initial conditions such models lead in a natural way to an accelerated phase of expansion of the universe. Assuming that the point like Lagrangian admits a Noether symmetry we are able to explicitly solve the dynamical equations. We study one particular model and show that its predictions are compatible with observational data, namely the publicly available data on type Ia supernovae, the parameters of large scale structure determined by the 2-degree Field Galaxy Redshift Survey (2dFGRS), the measurements of cosmological distances with the Sunyaev-Zel'dovich effect and the rate of growth of density perturbations}{It turns out that this model have a very interesting feature of producing in a natural way an epoch of accelerated expansion. With an appropriate choice of parameters our model is fully compatible with several observed characteristics of the universe

Near-infrared surface brightness fluctuations and optical colours of Magellanic star clusters

This work continues our efforts to calibrate model surface brightness fluctuation luminosities for the study of unresolved stellar populations, through a comparison with the data of Magellanic Cloud star clusters. We present here the relation between absolute Ks-band fluctuation magnitude and (V−I) integrated colour, using data from the Two-Micron All-Sky Survey (2MASS) and the Deep Near-Infrared Southern Sky Survey (DENIS), and from the literature. We compare the star cluster sample with the sample of early-type galaxies and spiral bulges studied by Liu et al. We find that intermediate-age to old star clusters lie along a linear correlation with the same slope, within the errors, of that defined by the galaxies in the versus (V−I) diagram. While the calibration by Liu et al. was determined in the colour range 1.05 < (V−IC)0 < 1.25, ours holds in the interval . This implies, according to Bruzual-Charlot and Mouhcine-Lançon models, that the star clusters and the latest star formation bursts in the galaxies and bulges constitute an age sequence. At the same time, a slight offset between the galaxies and the star clusters [the latter are ~0.7 mag fainter than the former at a given value of (V−I)], caused by the difference in metallicity of roughly a factor of 2, confirms that the versus (V−I) plane may contribute to break the age-metallicity degeneracy in intermediate-age and old stellar populations. The confrontation between models and galaxy data also suggests that galaxies with Ks fluctuation magnitudes that are brighter than predicted, given their (V−I) colour, might be explained in part by longer lifetimes of thermally pulsing asymptotic giant branch stars. A preliminary comparison between the H 2MASS data of the Magellanic star clusters and the sample of 47 early-type galaxies and spiral bulges observed by Jensen et al. through the F160WHubble Space Telescope filter leads to the same basic conclusions: galaxies and star clusters lie along correlations with the same slope, and there is a slight offset between the star cluster sample and the galaxies, caused by their different metallicities. Magellanic star clusters are single populations, while galaxies are composite stellar systems; moreover, the objects analysed live in different environments. Therefore, our findings mean that the relationship between fluctuation magnitudes in the near-infrared, and (V−I) might be a fairly robust tool for the study of stellar population ages and metallicities, could provide additional constraints on star formation histories, and aid in the calibration of near-infrared surface brightness fluctuations for cosmological distance measurements.

Detection of the Baryon Acoustic Peak in the Large‐Scale Correlation Function of SDSS Luminous Red Galaxies

We present the large-scale correlation function measured from a spectroscopic sample of 46,748 luminous red galaxies from the Sloan Digital Sky Survey. The survey region covers 0.72 h^{-3} Gpc^3 over 3816 square degrees and 0.16<z<0.47, making it the best sample yet for the study of large-scale structure. We find a well-detected peak in the correlation function at 100h^{-1} Mpc separation that is an excellent match to the predicted shape and location of the imprint of the recombination-epoch acoustic oscillations on the low-redshift clustering of matter. This detection demonstrates the linear growth of structure by gravitational instability between z=1000 and the present and confirms a firm prediction of the standard cosmological theory. The acoustic peak provides a standard ruler by which we can measure the ratio of the distances to z=0.35 and z=1089 to 4% fractional accuracy and the absolute distance to z=0.35 to 5% accuracy. From the overall shape of the correlation function, we measure the matter density Omega_mh^2 to 8% and find agreement with the value from cosmic microwave background (CMB) anisotropies. Independent of the constraints provided by the CMB acoustic scale, we find Omega_m = 0.273 +- 0.025 + 0.123 (1+w_0) + 0.137 Omega_K. Including the CMB acoustic scale, we find that the spatial curvature is Omega_K=-0.010+-0.009 if the dark energy is a cosmological constant. More generally, our results provide a measurement of cosmological distance, and hence an argument for dark energy, based on a geometric method with the same simple physics as the microwave background anisotropies. The standard cosmological model convincingly passes these new and robust tests of its fundamental properties.

Constraining the quintessence equation of state with SnIa data and CMB peaks

Quintessence has been introduced as an alternative to the cosmological constant scenario to account for the current acceleration of the universe. This new dark energy component allows values of the equation of state parameter ${w}_{Q}^{0}&gt;~\ensuremath{-}1$ and in principle measurements of cosmological distances to type Ia supernovae can be used to distinguish between these two types of models. Assuming a flat universe, we use the supernovae data and measurements of the position of the acoustic peaks in the cosmic microwave background spectra to constrain a rather general class of quintessence potentials, including inverse power law models and recently proposed supergravity inspired potentials. In particular we use a likelihood analysis, marginalizing over the dark energy density ${\ensuremath{\Omega}}_{Q},$ the physical baryon density ${\ensuremath{\Omega}}_{b}{h}^{2}$ and the scalar spectral index n, to constrain the slopes of our quintessence potential. Considering only the first Doppler peak the best fit in our range of models gives ${w}_{Q}^{0}\ensuremath{\sim}\ensuremath{-}0.8.$ However, including the SnIa data and the three peaks, we find an upper limit on the present value of the equation of state parameter, $\ensuremath{-}1&lt;~{w}_{Q}^{0}&lt;~\ensuremath{-}0.93$ at $2\ensuremath{\sigma},$ a result that appears to rule out a class of recently proposed potentials.