Search Results (25 total)

While observations indicate that the predominant source of cosmic inhomogeneities are adiabatic perturbations, there are a variety of candidates to provide auxiliary trace effects, including inflation-generated primordial tensors and cosmic defects which both produce B-mode cosmic microwave background polarization. We investigate whether future experiments may suffer confusion as to the true origin of such effects, focusing on the ability of Planck to distinguish tensors from cosmic strings, and show that there is no significant degeneracy.

We study how the uncertainty in the cosmological parameters impacts on the detection of topological signals, focussing on three cubic torus universes and using three tests: the information content, the S/N statistic, and the Bayesian evidence. We find, within the concordance cosmological model, that 3D torus universes with a size of ∼29  Gpc³ or larger cannot be detected. For the toroidal models that can be detected, the detection significance is primarily influenced by Ω_Λ, which enters both in the noise amplitude due to the Integrated Sachs-Wolfe effect and in the size of the causal horizon which limits the accessible fundamental domain. On large angular scales ℓ<40, only Ω_Λ significantly alters the detection for all three estimators considered here.

We perform a multiparameter likelihood analysis to compare measurements of the cosmic microwave background (CMB) power spectra with predictions from models involving cosmic strings. Adding strings to the standard case of a primordial spectrum with power-law tilt n_s, we find a 2σ detection of strings: f₁₀=0.11±0.05, where f₁₀ is the fractional contribution made by strings in the temperature power spectrum (at ℓ=10). CMB data give moderate preference to the model n_s=1 with cosmic strings over the standard zero-strings model with variable tilt. When additional non-CMB data are incorporated, the two models become on a par. With variable n_s and these extra data, we find that f₁₀<0.11, which corresponds to G_μ<0.7×10^-6 (where μ is the string tension and G is the gravitational constant).

We present the first calculation of the possible (local) cosmic string contribution to the cosmic microwave background polarization spectra from simulations of a string network (rather than a stochastic collection of unconnected string segments). We use field-theory simulations of the Abelian Higgs model to represent local U(1) strings, including their radiative decay and microphysics. Relative to previous estimates, our calculations show a shift in power to larger angular scales, making the chance of a future cosmic string detection from the B-mode polarization slightly greater. We explore a future ground-based polarization detector, taking the CLOVER project as our example. In the null hypothesis (that cosmic strings make a zero contribution) we find that CLOVER should limit the string tension μ to Gμ<0.12×10^-6 (where G is the gravitational constant), above which it is likely that a detection would be possible.

Observed data are often contaminated by undiscovered interlopers, leading to biased parameter estimation. Here we present BEAMS (Bayesian estimation applied to multiple species) which significantly improves on the standard maximum likelihood approach in the case where the probability for each data point being “pure” is known. We discuss the application of BEAMS to future type-Ia supernovae (SNIa) surveys, such as LSST, which are projected to deliver over a million supernovae light curves without spectra. The multiband light curves for each candidate will provide a probability of being Ia (pure) but the full sample will be significantly contaminated with other types of supernovae and transients. Given a sample of N supernovae with mean probability, ⟨P⟩, of being Ia, BEAMS delivers parameter constraints equal to N⟨P⟩ spectroscopically confirmed SNIa. In addition BEAMS can be simultaneously used to tease apart different families of data and to recover properties of the underlying distributions of those families (e.g. the type-Ibc and II distributions). Hence BEAMS provides a unified classification and parameter estimation methodology which may be useful in a diverse range of problems such as photometric redshift estimation or, indeed, any parameter estimation problem where contamination is an issue.

There is now strong observational evidence that the expansion of the Universe is accelerating. The standard explanation invokes an unknown “dark energy” component. But such scenarios are faced with serious theoretical problems, which has led to increased interest in models where instead general relativity is modified in a way that leads to the observed accelerated expansion. The question then arises whether the two scenarios can be distinguished. Here we show that this may not be so easy, demonstrating explicitly that a generalized dark energy model can match the growth rate of the Dvali-Gabadadze-Porrati model and reproduce the 3+1 dimensional metric perturbations. Cosmological observations are then unable to distinguish the two cases.

We present the first field-theoretic calculations of the contribution made by cosmic strings to the temperature power spectrum of the cosmic microwave background (CMB). Unlike previous work, in which strings were modeled as idealized one-dimensional objects, we evolve the simplest example of an underlying field theory containing local U(1) strings, the Abelian Higgs model. Limitations imposed by finite computational volumes are overcome using the scaling property of string networks and a further extrapolation related to the lessening of the string width in comoving coordinates. The strings and their decay products, which are automatically included in the field theory approach, source metric perturbations via their energy-momentum tensor, the unequal-time correlation functions of which are used as input into the CMB calculation phase. These calculations involve the use of a modified version of CMBEASY, with results provided over the full range of relevant scales. We find that the string tension μ required to normalize to the WMAP 3-year data at multipole ℓ=10 is Gμ=[2.04±0.06(stat.)±0.12(sys.)]×10^-6, where we have quoted statistical and systematic errors separately, and G is Newton’s constant. This is a factor 2–3 higher than values in current circulation.

We consider fluid perturbations close to the “phantom divide” characterized by p=-ρ and discuss the conditions under which divergencies in the perturbations can be avoided. We find that the behavior of the perturbations depends crucially on the prescription for the pressure perturbation δp. The pressure perturbation is usually defined using the dark energy rest-frame, but we show that this frame becomes unphysical at the divide. If the pressure perturbation is kept finite in any other frame, then the phantom divide can be crossed. Our findings are important for generalized fluid dark energy used in data analysis (since current cosmological data sets indicate that the dark energy is characterized by p≈-ρ so that p<-ρ cannot be excluded) as well as for any models crossing the phantom divide, like some modified gravity, coupled dark energy, and braneworld models. We also illustrate the results by an explicit calculation for the “Quintom” case with two scalar fields.

We describe a high-pressure x-ray diffraction (XRD) study of the compressibility of several samples of ZnS nanoparticles. The nanoparticles were synthesized with a range of sizes and surface chemical treatments in order to identify the factors that determine nanoparticle compressibility. Refinement of the XRD data revealed that all ZnS nanoparticles in the nominally cubic (sphalerite) phase exhibited a previously unobserved structural distortion under ambient conditions that exhibited, in addition, a dependence on pressure. Our results show that the compressibility of ZnS nanoparticles increases substantially as the particle size decreases, and we propose an interpretation based upon the available mechanisms of structural compliance in nanoscale vs bulk materials.

We introduce a statistical measure of the effective model complexity, called the Bayesian complexity. We demonstrate that the Bayesian complexity can be used to assess how many effective parameters a set of data can support and that it is a useful complement to the model likelihood (the evidence) in model selection questions. We apply this approach to recent measurements of cosmic microwave background anisotropies combined with the Hubble Space Telescope measurement of the Hubble parameter. Using mildly noninformative priors, we show how the 3-year WMAP data improves on the first-year data by being able to measure both the spectral index and the reionization epoch at the same time. We also find that a nonzero curvature is strongly disfavored. We conclude that although current data could constrain at least seven effective parameters, only six of them are required in a scheme based on the ΛCDM concordance cosmology.

We show that the dipole of the luminosity distance is a useful observational tool which allows us to determine the Hubble parameter as a function of redshift H(z). We determine the number of supernovae needed to achieve a given precision for H(z) and to distinguish between different models for dark energy. We analyze a sample of nearby supernovae and find a dipole consistent with the cosmic microwave background at a significance of more than 2σ.

We consider several ways to test for topology directly in harmonic space by comparing the measured a_ℓm with the expected correlation matrices. Two tests are of a frequentist nature while we compute the Bayesian evidence as the third test. Using correlation matrices for cubic and slab-space tori, we study how these tests behave as a function of the minimal scale probed and as a function of the size of the Universe. We also apply them to different first-year Wilkinson microwave anisotropy probe CMB maps and confirm that the Universe is compatible with being infinitely big for the cases considered. We argue that there is an information theoretical limit (given by the Kullback-Leibler divergence) on the size of the topologies that can be detected.

Detecting dark energy dynamics is the main quest of current dark energy research. Addressing the issue demands a fully consistent analysis of cosmic microwave background, large-scale structure and SN-Ia data with multiparameter freedom valid for all redshifts. Here we undertake a ten parameter analysis of general dark energy confronted with the first year Wilkinson Microwave Anisotropy Probe, 2dF galaxy survey and latest SN-Ia data. Despite the huge freedom in dark energy dynamics there are no new degeneracies with standard cosmic parameters apart from a mild degeneracy between reionization and the redshift of acceleration, both of which effectively suppress small scale power. Breaking this degeneracy will help significantly in detecting dynamics, if it exists. Our best-fit model to the data has significant late-time evolution at z<1.5. Phantom models are also considered and we find that the best-fit crosses w=-1 which, if confirmed, would be a clear signal for radically new physics. Treatment of such rapidly varying models requires careful integration of the dark energy density usually not implemented in standard codes, leading to crucial errors of up to 5%. Nevertheless cosmic variance means that standard Λ cold dark matter models are still a very good fit to the data and evidence for dynamics is currently very weak. Independent tests of reionization or the epoch of acceleration (e.g., integrated Sachs-Wolfe–large scale structure correlations) or reduction of cosmic variance at large scales (e.g., cluster polarization at high redshift) may prove key in the hunt for dynamics.

We use the cosmic microwave background angular power spectra to place upper limits on the degree to which global defects may have aided cosmic structure formation. We explore this under the inflationary paradigm, but with the addition of textures resulting from the breaking of a global O(4) symmetry during the early stages of the Universe. As a measure of their contribution, we use the fraction of the temperature power spectrum that is attributed to the defects at a multipole of 10. However, we find a parameter degeneracy enabling a fit to the first-year WMAP data to be made even with a significant defect fraction. This degeneracy involves the baryon fraction and the Hubble constant, plus the normalization and tilt of the primordial power spectrum. Hence, constraints on these cosmological parameters are weakened. Combining the WMAP data with a constraint on the physical baryon fraction from big bang nucleosynthesis calculations and high-redshift deuterium abundance limits the extent of the degeneracy and gives an upper bound on the defect fraction of 0.13 (95% confidence).

By combining the recent WMAP measurements of the cosmic microwave background anisotropies and the results of the recent luminosity distance measurements to type-Ia supernovae, we find that the normalization of the matter power spectrum on cluster scales, σ₈, can be used to discriminate between dynamical models of dark energy (quintessence models) and a conventional cosmological constant model (ΛCDM).

In cosmology, distances based on standard candles (e.g., supernovae) and standard rulers (e.g., baryon oscillations) agree as long as three conditions are met: (1) photon number is conserved, (2) gravity is described by a metric theory with (3) photons traveling on unique null geodesics. This is the content of distance duality (the reciprocity relation) which can be violated by exotic physics. Here we analyze the implications of the latest cosmological data sets for distance duality. While broadly in agreement and confirming acceleration we find a 2-sigma violation caused by excess brightening of SNIa at z>0.5, perhaps due to lensing magnification bias. This brightening has been interpreted as evidence for a late-time transition in the dark energy but because it is not seen in the d_A data we argue against such an interpretation. Our results do, however, rule out significant SNIa evolution and extinction: the “replenishing” gray-dust model with no cosmic acceleration is excluded at more than 4-sigma despite this being the best fit to SNIa data alone, thereby illustrating the power of distance duality even with current data sets.

Imagine a scenario in which the dark energy forms via the condensation of dark matter at some low redshift. The Compton wavelength therefore changes from small to very large at the transition, unlike quintessence or metamorphosis. We study cosmic microwave background (CMB), large scale structure, supernova and radio galaxy constraints on condensation by performing a four parameter likelihood analysis over the Hubble constant and the three parameters associated with Q, the condensate field: Ω_Q, w_f and z_t (energy density and equation of state today, and redshift of transition). Condensation roughly interpolates between ΛCDM (for large z_t) and SCDM (low z_t) and provides a slightly better fit to the data than ΛCDM. We confirm that there is no degeneracy in the CMB between H and z_t and discuss the implications of late-time transitions for the Lyman-α forest. Finally we discuss the nonlinear phase of both condensation and metamorphosis, which is much more interesting than in standard quintessence models.

The high-pressure behavior of both β- and α-Ba(OH)₂ was investigated by in situ powder synchrotron x-ray diffraction. Pressures up to 13 GPa were generated using a diamond-anvil cell. At least one phase transition was detected in β-Ba(OH)₂ [P2₁/n; a=9.3396(2), b=7.8550(2), c=6.7267(2) Å, β=95.607(2)° at 0.95 GPa], indicated by the appearance of an additional low-angle peak. The high-pressure β₂ phase [P2₁/c; a=11.6632(8), b=7.6684(5), c=10.7785(7) Å, β=108.881(5)° at 5.40 GPa] is characterized by a doubling of the unit-cell volume. Bulk moduli were determined for β-Ba(OH)₂ with K₀=40(1) GPa (K^′ fixed to 6) and for β₂-Ba(OH)₂ with K₀=60(4) GPa (K^′ fixed to 6). There are indications for two other phase transitions between 8 and 10 GPa. Compressing α-Ba(OH)₂ produces new diffraction patterns, which cannot be interpreted unambiguously. Upon pressure release down to 0.3 GPa, the β₂ phase is recovered.

We determine the detailed thermodynamic behavior of vortices in the O(2) scalar model in two dimensions (2D) and of global monopoles in the O(3) model in 3D. We construct numerical techniques, based on cluster decomposition algorithms, to analyze the point defect configurations. We find that these criteria produce results for the Kosterlitz-Thouless temperature in agreement with a topological transition between a polarizable insulator and a conductor, at which free topological charges appear in the system. For global monopoles we find no pair unbinding transition. Instead a transition to a dense state where pairs are no longer distinguishable occurs at T<T_c, without leading to long-range disorder. We produce both extensive numerical evidence of this behavior as well as a semianalytic treatment of the partition function for defects. General expectations for N=D>3 are drawn, based on the observed behavior.

During recent years it has become clear that global O(N) defects and U(1) cosmic strings do not lead to the pronounced first acoustic peak in the power spectrum of anisotropies of the cosmic microwave background (CMB) which has recently been observed to high accuracy. Inflationary models cannot easily accommodate the low second peak indicated by the data. Here we construct causal scaling seed models which reproduce the first and second peak. Future, more precise CMB anisotropy and polarization experiments will however be able to distinguish them from the ordinary adiabatic models.

We compute the skewness of the matter distribution arising from nonlinear evolution and from non-Gaussian initial perturbations. We apply our result to a very generic class of models with non-Gaussian initial conditions and we estimate analytically the ratio between the skewness due to nonlinear clustering and the part due to the intrinsic non-Gaussianity of the models. We finally extend our estimates to higher moments.

We investigate the global texture model of structure formation in cosmogonies with a nonzero cosmological constant for different values of the Hubble parameter. We find that the absence of significant acoustic peaks and little power on large scales are robust predictions of these models. However, from a careful comparison with data we conclude that at present we cannot safely reject the model on the grounds of present CMB data. Exclusion by means of galaxy correlation data requires assumptions on biasing and statistics. New, very stringent constraints come from peculiar velocities. Investigating the large-N limit, we argue that our main conclusions apply to all global O(N) models of structure formation.

In this work we present a partially new method to analyze fluctuations which are induced by causal scaling seeds. We show that the power spectra due to these kinds of seed perturbations are determined by five analytic functions, which we determine numerically for a special example. We put forward the view that, even if recent work disfavors the models with cosmic strings and global O(4) texture, causal scaling seed perturbations merit a more thorough and general analysis, which we initiate in this paper.

We compute cosmic microwave background angular power spectra for scaling seed models of structure formation. A generic parametrization of the energy momentum tensor of the seeds is employed. We concentrate on two regions of parameter space inspired by global topological defects: O(4) texture models and the large- N limit of O(N) models. We use χ² fitting to compare these models to recent flat-band power measurements of the cosmic microwave background. Only scalar perturbations are considered.

We present an analysis of cosmic microwave background (CMB) anisotropies induced by global scalar fields in the large N limit. In this limit, the CMB anisotropy spectrum can be determined without cumbersome 3D simulations. We determine the source functions and their unequal time correlation functions and show that they are quite similar to the corresponding functions in the texture model. This leads us to the conclusion that the large N limit provides a “cheap approximation” to the texture model of structure formation.