1,721,027 research outputs found
Simulations for 21 cm radiation lensing at EoR redshifts
Full text linkWe introduce simulations aimed at assessing how well weak gravitational lensing of 21cm radiation from the Epoch of Reionization (z ̃ 8) can be measured by a Square Kilometre Array (SKA)-like radio telescope. A simulation pipeline has been implemented to study the performance of lensing reconstruction techniques. We show how well the lensing signal can be reconstructed using the 3D quadratic lensing estimator in Fourier space assuming different survey strategies. The numerical code introduced in this work is capable of dealing with issues that cannot be treated analytically such as the discreteness of visibility measurements and the inclusion of a realistic model for the antennas distribution. This paves the way for future numerical studies implementing more realistic re-ionization models, foreground subtraction schemes, and testing the performance of lensing estimators that take into account the non-Gaussian distribution of HI after re-ionization. If multiple frequency channels covering z ̃ 7-11.6 are combined, Phase 1 of SKA-Low should be able to obtain good quality images of the lensing potential with a total resolution of ̃1.6 arcmin. The SKA-Low Phase 2 should be capable of providing images with high fidelity even using data from z ̃ 7.7 to 8.3. We perform tests aimed at evaluating the numerical implementation of the mapping reconstruction. We also discuss the possibility of measuring an accurate lensing power spectrum. Combining data from z ̃ 7 to 11.6 using the SKA2-Low telescope model, we find constraints comparable to sample variance in the range L < 1000, even for survey areas as small as 25 deg2
Modelling non-linear effects of dark energy
Full text linkWe investigate the capabilities of perturbation theory in capturing non-linear effects of dark energy. We test constant and evolving w models, as well as models involving momentum exchange between dark energy and dark matter. Specifically, we compare perturbative predictions at 1-loop level against N-body results for four non-standard equations of state as well as varying degrees of momentum exchange between dark energy and dark matter. The interaction is modelled phenomenologically using a time dependent drag term in the Euler equation. We make comparisons at the level of the matter power spectrum and the redshift space monopole and quadrupole. The multipoles are modelled using the Taruya, Nishimichi and Saito (TNS) redshift space spectrum. We find perturbation theory does very well in capturing non-linear effects coming from dark sector interaction. We isolate and quantify the 1-loop contribution coming from the interaction and from the non-standard equation of state. We find the interaction parameter ξ amplifies scale dependent signatures in the range of scales considered. Non-standard equations of state also give scale dependent signatures within this same regime. In redshift space the match with N-body is improved at smaller scales by the addition of the TNS free parameter σv. To quantify the importance of modelling the interaction, we create mock data sets for varying values of ξ using perturbation theory. This data is given errors typical of Stage IV surveys. We then perform a likelihood analysis using the first two multipoles on these sets and a ξ=0 modelling, ignoring the interaction. We find the fiducial growth parameter f is generally recovered even for very large values of ξ both at z=0.5 and z=1. The ξ=0 modelling is most biased in its estimation of f for the phantom w=-1.1 case
Interferometric H i intensity mapping: perturbation theory predictions and foreground removal effects
Full text linkWe provide perturbation theory predictions for the H i intensity mapping power spectrum multipoles using the Effective Field Theory of Large Scale Structure, which should allow us to exploit mildly non-linear scales. Assuming survey specifications typical of proposed interferometric H i intensity mapping experiments like Canadian Hydrogen Observatory and Radio transient Detector and PUMA, and realistic ranges of validity for the perturbation theory modelling, we run mock full shape Markov chain Monte Carlo (MCMC) analyses at z = 0.5, and compare with Stage-IV optical galaxy surveys. We include the impact of 21cm foreground removal using simulations-based prescriptions, and quantify the effects on the precision and accuracy of the parameter estimation. We vary 11 parameters in total: three cosmological parameters, seven bias and counter terms parameters, and the H i brightness temperature. Amongst them, the four parameters of interest are: the cold dark matter density, ωc, the Hubble parameter, h, the primordial amplitude of the power spectrum, As, and the linear H i bias, b1. For the best-case scenario, we obtain unbiased constraints on all parameters wit
Cosmic strings with junctions : dynamics and cosmological implications
No full textCosmic strings are linear concentrations of energy that may have been formed after cosmological phase transitions in the early universe. Cosmic superstrings are analogous objects arising in string theory, and in particular in models of brane inflation. The latter possess two particular features, which differentiate them from the ordinary cosmic strings: a reduced intercommuting probability, and the ability to form junctions. This thesis is concerned with the dynamics and cosmological implications of cosmic strings and superstrings with junctions.
In Chapter1, we give a brief introduction to the standard Big Bang model and t he inflationary paradigm. W e also discuss cosmic string formation after the spontaneous breaking of an Abelian U (I) gauge symmetry in the early Universe. In Chapter 2, we present an overview of cosmic string dynamics using the Nambu-Goto method. We discuss the properties of individual cosmic string segments and loops, as well as network evolution in an expanding Universe. We also introduce cosmic superstrings, and review the Nambu-Goto approach to study the evolution of junctions and the kinematic constraints that govern their formation. We conclude with the study of junctions in an expanding spacetime and present an exact solution for a closed loop of three strings and two junctions in a de Sitter Universe.
In Chapter3, we compare the two different approaches developed to study the dynamics of strings with junctions. We first extensively study the dynamics and stability of a cosmic string loop with junctions using the modified Nambu-Goto approach. Comparing our results with a field theory model that permits junctions we find very good agreement. The Nambu-Goto method is once again confirmed to be a good approximation for studying cosmic string configurations.
In Chapter4, we review the observational signatures of cosmic strings. More specifically, we concentrate on their gravitational effects, discussing results and constraints from lensing, gravitational radiation, CMB and pulsar timing. We also present recent results of the case of cosmic (super)-strings with junctions.
Chapter5 is concerned with the cosmological implications of cosmic superstring networks. W e first study the scaling patterns of such networks for different values of the string coupling g8, and different charges (p, q) on the strings. We then focus on their CMB signatures and derive upper bounds for the fundamental tension MuF using CMB and pulsar timing constraints. The difference between the scaling behaviour of the networks at high and low values of g, is imprinted as a movement of the position of the peak in the B –mode spectrum. Together with the constraints on GMuF from CMB and pulsar timing, this allows for the exciting possibility to constrain the value of the string coupling g8 using CMB data. We conclude in Chapter 6
Cosmic strings with junctions : dynamics and cosmological implications
Full text linkCosmic strings are linear concentrations of energy that may have been formed after cosmological phase transitions in the early universe. Cosmic superstrings are analogous objects arising in string theory, and in particular in models of brane inflation. The latter possess two particular features, which differentiate them from the ordinary cosmic strings: a reduced intercommuting probability, and the ability to form junctions. This thesis is concerned with the dynamics and cosmological implications of cosmic strings and superstrings with junctions. In Chapter1, we give a brief introduction to the standard Big Bang model and t he inflationary paradigm. W e also discuss cosmic string formation after the spontaneous breaking of an Abelian U (I) gauge symmetry in the early Universe. In Chapter 2, we present an overview of cosmic string dynamics using the Nambu-Goto method. We discuss the properties of individual cosmic string segments and loops, as well as network evolution in an expanding Universe. We also introduce cosmic superstrings, and review the Nambu-Goto approach to study the evolution of junctions and the kinematic constraints that govern their formation. We conclude with the study of junctions in an expanding spacetime and present an exact solution for a closed loop of three strings and two junctions in a de Sitter Universe. In Chapter3, we compare the two different approaches developed to study the dynamics of strings with junctions. We first extensively study the dynamics and stability of a cosmic string loop with junctions using the modified Nambu-Goto approach. Comparing our results with a field theory model that permits junctions we find very good agreement. The Nambu-Goto method is once again confirmed to be a good approximation for studying cosmic string configurations. In Chapter4, we review the observational signatures of cosmic strings. More specifically, we concentrate on their gravitational effects, discussing results and constraints from lensing, gravitational radiation, CMB and pulsar timing. We also present recent results of the case of cosmic (super)-strings with junctions. Chapter5 is concerned with the cosmological implications of cosmic superstring networks. W e first study the scaling patterns of such networks for different values of the string coupling g8, and different charges (p, q) on the strings. We then focus on their CMB signatures and derive upper bounds for the fundamental tension MuF using CMB and pulsar timing constraints. The difference between the scaling behaviour of the networks at high and low values of g, is imprinted as a movement of the position of the peak in the B –mode spectrum. Together with the constraints on GMuF from CMB and pulsar timing, this allows for the exciting possibility to constrain the value of the string coupling g8 using CMB data. We conclude in Chapter 6.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Momentum transfer models of interacting dark energy
Full text linkWe consider two models of interacting dark energy, both of which interact
only through momentum exchange. One is a phenomenological one-parameter
extension to CDM, and the other is a coupled quintessence model described by
a Lagrangian formalism. Using a variety of high and low redshift data sets, we
perform a global fitting of cosmological parameters and compare to
CDM, uncoupled quintessence, and CDM. We find that the models are
competitive with CDM, even obtaining a better fit when certain data
sets are included.Comment: 22 pages, 10 figures, 3 tables; matches published version in JCA
Probing dark energy and modified gravity with galaxy clustering and weak lensing
No full textThe new generation of cosmological surveys, called Stage-IV surveys, provides measurements
of unprecedented volume and quality. This, in turn, leads to increased statistical
precision. Now our knowledge of the Universe is restricted by the efficiency of our
methods and accuracy of our models, rather than by our measurements (assuming that
observational systematics are fully understood). Therefore, our goal is to develop and
test methods and models for analysing the data in order to reliably extract as much
information as possible. We do so by extending standard modelling techniques into
the nonlinear regime. We apply the nonlinear techniques to non-standard cosmological
models, such as evolving and interacting dark energy models, modified gravity theories,
and massive neutrinos. These alternative models attempt to resolve tensions between
the cosmological parameters extracted from the early-time (high-redshift) and late-time
(low-redshift) Universe, which arise in the paradigm of the standard cosmology, ΛCDM.
We refer to the measurements with which we study the laws and constituents of our
Universe as cosmological probes. In this thesis we work with two cosmological probes of
the late-time Universe: galaxy clustering and weak lensing. The redshifts of galaxies
can be precisely measured with spectroscopic instruments. From the distribution of
galaxies in redshift space we can build and then model summary statistics. Here we
focus on the power spectrum and bispectrum multipoles as our main galaxy clustering
observables. Regarding weak gravitational lensing, this phenomenon corresponds to the
coherent distortion in the observed shapes of distant galaxies due to the bending of light
by the Large Scale Structure. The degree of distortion, or shear, depends on the amount
and concentration of matter along the propagation of light. Based on the measured galaxy
shape distribution from photometric images, we again can build various statistics. In this
thesis we focus on the cosmic shear power spectrum as our main weak lensing observable.
The goal of Chapter 1 and Chapter 2 is to provide the background information required
to grasp the findings of the latter chapters. Chapter 1 familiarises the reader with the
main cosmological and theoretical concepts that are necessary to understand the modelling
of our main observables. We motivate our interest in the extensions of the standard
cosmological model, as well as demonstrate explicitly what changes they introduce at
the level of the observables. Chapter 2 establishes the notion of probability in two
frameworks, Bayesian and Frequentist, that allows us to assess the accuracy of a model
given the data. In the same chapter we also describe the mechanisms of the probability
distribution sampling, i.e., the mechanisms of geometrically exploring the regions of the
model’s parameter space that describe the data best.
In Chapter 3, we analyse the redshift space power spectrum and bispectrum multipoles
of dark matter halos from a large set of simulations. Interacting dark energy, in particular
the “Dark Scattering” model in which dark energy and dark matter are coupled by pure
momentum exchange, is the main focus of this chapter. We find a substantial improvement
of constraints on the dark energy parameters when the bispectrum multipoles are
combined with the power spectrum multipoles. In Chapter 4, using the same perturbative
framework, we analyse the power spectrum multipoles from a Stage-III spectroscopic
survey, BOSS DR12. Now we focus on constraining a generalised parameterisation of
modified gravity via the growth index and massive neutrinos. Albeit without a crucial
statistical significance, we find values of the growth index which deviate from its ΛCDM
limit and lead to the suppression of structure growth at late-times. Such behaviour might
resolve the so-called S8 tension. In a nutshell, this cosmological tension corresponds to the
contradiction between the high- and low-redshift measurements of the matter fluctuation
amplitude in the standard cosmological model. Additionally, in both chapters we observe
strong projection effects in the parameter space, which are pedagogically introduced in
Chapter 2. We explore the projection effects in the context of spectroscopic surveys and
propose possible solutions.
In Chapter 5, we develop a model-independent approach with a generalised screening
function to model the nonlinear matter power spectrum for many dark energy and
modified gravity models. Screening is a mechanism that allows for a theory to recover the
classical gravitational interactions at small scales and in dense regions, such as our Solar
System. This time we work in the halo-based reaction framework. Then, in Chapter 6,
we apply this approach in the modelling of the cosmic shear power spectra. When
tested on mock Stage-IV cosmic shear data, we discover promising hints of detecting
the screening transition. We also explore a strong degeneracy between baryonic physics
and the screening transition, as well as the impact of massive neutrinos.
In the final chapter, Chapter 7, we summarise the studies presented in this thesis
and list our next steps in probing and constraining extended cosmologies with galaxy
clustering and weak lensing
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