Primordial non-Gaussianity in the large-scale structure of the Universe

  • Matteo Tellarini

Student thesis: Doctoral Thesis


Primordial fluctuations are expected to be produced in the very early Universe, sourcing the anisotropies in the cosmic microwave background and seeding the formation of structures. In this thesis we study the effect of density perturbations produced during inflation on the large-scale galaxy bispectrum.

We start by reviewing the basic concepts of modern cosmology and introducing the tools used in this research: Newtonian perturbation theory, statistics of random fields, the mass function of collapsed halos and the halo bias model.We then briefly describe how models of inflation source local-type non-Gaussian distributed primordial density perturbations.

We apply these tools to justify the bivariate model for the halo density in the presence of primordial non-Gaussianity and derive some known results, like the scale-dependent halo bias. The aim is to show that the statistics of large-scale structure can be used to probe local-type non-Gaussianity of the primordial density field, complementary to existing constraints from the cosmic microwave background.

Parametrising the amount of primordial non-Gaussianity with the leading-order non-linear parameter fNL and the next-order one, gNL, we will investigate how galaxy and matter bispectra can distinguish between them, despite their effects being nearly degenerate in the power spectra. We determine a connection between the sign of the halo bispectrum on large scales and the parameter gNL and construct a combination of halo and matter bispectra that is sensitive to fNL.

After that, we will focus on local-type non-Gaussianity with fNL only. It is known that the non-linear evolution of the matter density introduces a non-local tidal term in the halo bias model. Furthermore, we will show that the bivariate model in the Lagrangian frame leads to a novel non-local convective term in the Eulerian frame which can lead to non-negligible corrections in the halo bispectra, in particular on large scales or at high redshift.

Finally, we address the problem of modelling redshift space distortions in the galaxy bispectrum, finding novel contributions with the characteristic large scale amplification induced by local-type non-Gaussianity. Therefore, redshift space distortions can potentially lead to a biased measurement of fNL, if not properly accounted for. Moreover, we propose an analytic template for the monopole which can be used to t against data on large scales, extending models used in recent measurements.

We conclude the thesis with some discussion of future developments. Observational constraints will also be discussed, based on idealised forecasts onfNL { the accuracy of the determination of fNL. Our findings suggest that the constraining power of the galaxy bispectrum in current surveys would provide fNL measurements competitive with constraints from the cosmic microwave background and future surveys could improve this further.
Date of AwardMar 2016
Original languageEnglish
Awarding Institution
  • University of Portsmouth
SupervisorDavid Wands (Supervisor), Ashley Jacob Ross (Supervisor), Gianmassimo Tasinato (Supervisor), William James Percival (Supervisor) & David Wands (Supervisor)

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