Alkistis Pourtsidou

H i intensity mapping: a single dish approach

We discuss the detection of large-scale H I intensity fluctuations using a single dish approach with the ultimate objective of measuring the baryonic acoustic oscillations (BAO) and constraining the properties of dark energy. To characterize the signal we present 3D power spectra, 2D angular power spectra for individual redshift slices and also individual line-of-sight spectra computed using the S 3 simulated H I catalogue which is based on the Millennium Simulation. We consider optimal instrument design and survey strategies for a single dish observation at low and high redshift for a fixed sensitivity. For a survey corresponding to an instrument with T sys = 50 K, 50 feedhorns and 1 year of observations, we find that at low redshift (z ? 0.3), a resolution of ?40 arcmin and a survey of ?5000 deg 2 is close to optimal, whereas at higher redshift (z ? 0.9) a resolution of ?10 arcmin and ?500 deg 2 would be necessary - something which would be difficult to achieve cheaply using a single dish. Continuum foreground emission from the Galaxy and extragalactic radio sources are potentially a problem. In particular, we suggest that it could be that the dominant extragalactic foreground comes from the clustering of very weak sources. We assess its amplitude and discuss ways by which it might be mitigated. We then introduce our concept for a dedicated single dish telescope designed to detect BAO at low redshifts. It involves an underillumintated static ?40 m dish and a ?60 element receiver array held ?90 m above the underilluminated dish. Correlation receivers will be used with each main science beam referenced against an antenna pointing at one of the celestial poles for stability and control of systematics. We make sensitivity estimates for our proposed system and projections for the uncertainties on the power spectrum after 1 year of observations. We find that it is possible to measure the acoustic scale at z ? 0.3 with an accuracy ?2.4 per cent and that w can be measured to an accuracy of 16 per cent.

Models of dark matter coupled to dark energy

We present three distinct types of models of dark energy in the form of a scalar field which is explicitly coupled to dark matter. Our construction draws from the pull-back formalism for fluids and generalises the fluid action to involve couplings to the scalar field. We investigate the cosmology of each class of model both at the background and linearly perturbed level. We choose a potential for the scalar field and a specific coupling function for each class of models and we compute the Cosmic Microwave Background and matter power spectra.

Cosmology with a SKA HI intensity mapping survey

HI intensity mapping (IM) is a novel technique capable of mapping the large-scale structure of the Universe in three dimensions and delivering exquisite constraints on cosmology, by using HI as a biased tracer of the dark matter density field. This is achieved by measuring the intensity of the redshifted 21cm line over the sky in a range of redshifts without the requirement to resolve individual galaxies. In this chapter, we investigate the potential of SKA1 to deliver HI intensity maps over a broad range of frequencies and a substantial fraction of the sky. By pinning down the baryon acoustic oscillation and redshift space distortion features in the matter power spectrum -- thus determining the expansion and growth history of the Universe -- these surveys can provide powerful tests of dark energy models and modifications to General Relativity. They can also be used to probe physics on extremely large scales, where precise measurements of spatial curvature and primordial non-Gaussianity can be used to test inflation; on small scales, by measuring the sum of neutrino masses; and at high redshifts where non-standard evolution models can be probed. We discuss the impact of foregrounds as well as various instrumental and survey design parameters on the achievable constraints. In particular we analyse the feasibility of using the SKA1 autocorrelations to probe the large-scale signal.

Reconciling CMB and structure growth measurements with dark energy interactions

We study a coupled quintessence model with pure momentum exchange and present the effects of such an interaction on the cosmic microwave background (CMB) and matter power spectrum. For a wide range of negative values of the coupling parameter β structure growth is suppressed and the model can reconcile the tension between cosmic microwave background observations and structure growth inferred from cluster counts. We find that this model is as good as ΛCDM for CMB and baryon acoustic oscillation data, while the addition of cluster data makes the model strongly preferred, improving the best-fit χ2 value by more than 16.

Parametrized post-Friedmannian framework for interacting dark energy theories

We present the most general parametrization of models of dark energy in the form of a scalar field which is explicitly coupled to dark matter. We follow and extend the parametrized post-Friedmannian approach, previously applied to modified gravity theories, in order to include interacting dark energy. We demonstrate its use through a number of worked examples and show how the initially large parameter space of free functions can be significantly reduced and constrained to include only a few nonzero coefficients. This paves the way for a model-independent approach to classify and test interacting dark energy theories.

Evolution and stability of cosmic string loops with Y-junctions

Neil Bevis, ∗ Edmund J. Copeland, † Pierre-Yves Martin, 4, ‡ Gustavo Niz, § Alkistis Pourtsidou, ¶ Paul M. Saffin, ∗∗ and D. A. Steer †† Theoretical Physics, Blackett Laboratory, Imperial College, Prince Consort Road, London, SW7 2BZ, United Kingdom School of Physics & Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD, United Kingdom APC, University Paris 7, 10, Rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France Glaizer Group, 32 rue Guy Moquet, 92240 Malakoff, France (Dated: October 9, 2018)

Prospects for clustering and lensing measurements with forthcoming intensity mapping and optical surveys

We explore the potential of using intensity mapping surveys (MeerKAT, SKA) and optical galaxy surveys (DES, LSST) to detect HI clustering and weak gravitational lensing of 21cm emission in auto- and cross-correlation. Our forecasts show that high precision measurements of the clustering and lensing signals can be made in the near future using the intensity mapping technique. Such studies can be used to test the intensity mapping method, and constrain parameters such as the HI density

Constraints on the Fundamental String Coupling from B-Mode Experiments

\Omega_{\rm HI}

Cosmology from EoR/Cosmic Dawn with the SKA

, the HI bias

Testing gravity at large scales with H i intensity mapping

b_{\rm HI}