**The size of our causal universe**

# Parallel Session: COSMOLOGY - LARGE SCALE STRUCTURE THEORY (G)

## Location: Guildhall Portsmouth

#### G1 - Monday 14:00-15:40 (Matteo Viel, Chris Clarkson)

**14:00-14:20**

**Enrique Gaztanaga (INSTITUTO DE CIENCIAS DEL ESPACIO (IEEC/CSIC))**

A Universe with finite age also has a finite causal scale χ§, so the metric can not be homogeneous for χ > χ§, as it is usually assumed. To account for this, we propose a new causal boundary condition, that can be fulfil by fixing the cosmological constant Λ (a free parameter for gravity). The resulting Universe is inhomogeneous, with possible variation of cosmological parameters on scales χ ≃ χ§. The size of χ§ depends on the details of inflation, but regardless of its size, the boundary condition forces Λ/8πG to cancel the contribution of a constant vacuum energy (ρvac) to the measured ρΛ ≡Λ/8πG+ρvac. To reproduce the observed ρΛ ≃ 2ρm today with χ§ → ∞ we then need a universe filled with evolving dark energy (DE) with pressure pDE > −ρDE and a fine tuned value of ρDE ≃ 2ρm today. This seems very odd, but there is another solution to this puzzle. We can have a finite value of χ§ ≃ 3c/H0 without the need of DE. This scale corresponds to half the sky at z ∼ 1 and 60deg at z ∼ 1000, which is consistent with the anomalous lack of correlations observed in the CMB.

**Multipoles of the relativistic galaxy bispectrum**

**14:20-14:40**

**Eline Maaike De Weerd (Physics & Astronomy, Queen Mary, University of London)**

The bispectrum will play an important role in future galaxy surveys. On large scales it is a key probe for measuring primordial non-Gaussianity which can help discriminate between different inflationary models and other theories of the early universe. On these scales a variety of relativistic effects come into play once the galaxy number-count fluctuation is projected onto our past lightcone. One can expand the relativistic bispectrum in terms of spherical harmonics; unique to this expansion is that only the relativistic effects induce odd multipoles, and hence it offers a clear signature of relativistic effects in large scale structure. We analyse the full set of non-vanishing multipoles, give an analytic formula to calculate these, and compute the amplitudes of non-zero multipoles for both an Euclid-like galaxy survey, and for HI intensity mapping/SKA.

**Perturbation theory in universes with nonlinear density contrasts**

**14:40-15:00**

**Christopher Gallagher (Physics and Astronomy, Queen Mary University of London)**

Two-parameter perturbation theory is a scheme tailor-made to consistently include nonlinear density contrasts on small scales < 100 Mpc, whilst retaining a traditional approach to cosmological perturbations in the long-wavelength universe. We study the solutions that arise from this theory in a spatially-flat dust-filled cosmology, and what these imply for the matter bispectrum induced by gravity. This is achieved by using Newtonian perturbation theory to model the gravitational fields of nonlinear structures in the quasi-linear regime, and then using the resulting solutions as source terms for the cosmological equations. We find that our approach results in the leading-order part of the cosmological gravitational potentials being identical to those that result from standard cosmological perturbation theory at second-order, while the matter bispectrum itself yields some differences. This demonstrates that our approach is sufficient to capture most leading-order relativistic effects, but within a framework that is far easier to generalize. We expect this latter property to be particularly useful for calculating leading-order relativistic corrections to the matter power spectrum, as well as for deriving predictions for relativistic effects in alternative theories of gravity.

**Cosmological trajectories approach to modelling large-scale structure**

**15:00-15:20**

**Fran Lane (Institute for Astronomy, University of Edinburgh)**

Our current perturbation theory techniques for predicting the power spectrum break down when we enter the non-linear regime (k~0.1 h/Mpc). Simulations allow us to probe this regime, however one must be run for every cosmology and gravity model one wishes to constrain. A perturbation theory technique that could push further into the non-linear scales than current methods and match the accuracy of a simulation would be more economical. I will present the cosmological trajectories approach to modelling large-scale structure (Lane et al. 2019, in prep). This technique can be described as performing a simulation on paper and could match current predictions of the non-linear power spectrum from simulations. I will introduce our method and demonstrate how it can be related to previous work in this field. I will then show how an analytic expression for the power spectrum can be obtained and use this to compare the cosmological trajectories approach to standard perturbation theory and simulations.

**Modeling non-linear gravity in a cosmological setting**

**15:20-15:40**

**James Mertens (Physics and Astronomy, York University / Perimeter Institute)**

Modern cosmology has been built upon a theoretical pillar: our Universe appears to be homogeneous and isotropic on large scales. We can, and do, construct approximate models based upon this assumption, and it has been demonstrated that such models describe the evolution of cosmological spacetimes with a surprising degree of accuracy. Yet, issues begin to emerge when considering these models in the context of a nonlinear treatment of general relativity. I will describe new questions that arise within a full, general relativistic treatment of cosmology, recent progress modeling our Universe in a fully general relativistic context, and how a full general relativistic treatment can impact the way in which we interpret observations of our Universe.

#### G2 - Monday 16:10-17:50 (Matteo Viel, Chris Clarkson)

**Imprints of GR effects in the odd-multipoles of the galaxy correlation functions**

**16:10-16:30**

**Obinna Umeh (Institute of Cosmology and Gravitation, University of Portsmouth, UK)**

The odd multipoles of galaxy correlation functions will play an important role in future galaxy surveys. The odd multipoles are generated by radial displacement of the galaxy position due to redshift-space distortions. They vanish exactly in the Newtonian limit under a distance observer approximation, however, in General relativity within standard Cosmology, we show that future surveys can detect these odd multipoles in the power spectrum (two-point correlation function) from a joint analysis of the SKA HI intensity mapping and Euclid-like H-alpha galaxy survey. Furthermore, we show that for each tracer, future surveys can potentially detect the odd multipoles in the bispectrum (three-point correlation function).

**Relativistic effects in the QSO — Lyman-alpha cross-correlation**

**16:30-16:50**

**Vid Iršič (Kavli Institute for Cosmology, University of Cambridge)**

As the observations move out to larger scales the validity of a simple Newtonian description starts to come into question. Recent years have seen a lot of effort put into understanding, and possibly measuring, the signal of relativistic effects and weak gravitational lensing in the 2-point statistics of the galaxy clustering. Such studies have been extended to the Lyman-alpha forest tracer of underlying matter at unique redshift range (z = 2 − 5). The main application of these calculations is the Quasar – Lyman-alpha cross-correlation function. The results indicate that the signal of the relativistic effects can be as large as 30% at BAO scale, which is much larger than anticipated and mainly due to large differences in the density bias factors of our tracers. The relativistic effects are dominated by the Doppler contribution. Computing the signal-to-noise ratio for BOSS, eBOSS and DESI surveys shows that DESI will be able to detect the Doppler contribution in a LLS survey for the first time, with a S/N > 7. By using a Fisher matrix approach, we conclude that if the Doppler contribution is neglected in the data analysis, this will determine shifts in the derived parameters by a non-negligible amount for the upcoming surveys.

**Computation of the Relativistic Galaxy Bispectrum**

**16:50-17:10**

**Sam Lawrence (ICG)**

The next generation of galaxy surveys will hold a wealth of knowledge that will need to be unlocked using the correct analysis. One particular area of interest is the relativistic galaxy bispectrum: the lowest order statistic sensitive to non-Gaussianity using the spatial distribution of large scale structure. If we are to correctly use this tool on cosmological scales (which the new surveys will probe), then general relativistic corrections need to be well understood, since they may have a significant contribution to the 3-point correlation function. Although much of this has been acknowledged theoretically, the computation of the relativistic galaxy bispectrum is yet to be performed; an exercise which could prove fruitful in understanding early universe cosmology.

**Averaging observables in the LSS surveys: a new rigorous approach**

**17:10-17:30**

**Giuseppe Fanizza (IA, University of Lisbon)**

In this talk I will provide a systematic and rigorous classification of all the possible choices for averaging observables in cosmology. New results will be shown with respect to the recent literature and their impact on the bias that they can induce in the estimation of statistical properties, such as mean value and dispersion, will be discussed. The connection between all the presented theoretical prescriptions and the observations will be discussed: in this regards it will be shown how the main differences significantly appear for higher redshift sources.

#### G3 - Tuesday 14:00-15:40 (Matteo Viel, Chris Clarkson)

**21cm Intensity Mapping: Prospects and Challenges**

**14:00-14:20**

**Steve Cunnington (Queen Mary University of London)**

Mapping the unresolved 21cm emission from neutral hydrogen within galaxies using low angular resolutions has the potential to be an excellent method for mapping large-scale structure. The lack of angular precision is deemed acceptable for the purposes of probing large-scale structure since many of the scales of interest are still accessible. Furthermore, the excellent constraints on redshift provide an alternative to more conventional optical galaxy redshift surveys which either suffer from poorly constrained redshift (in photometric surveys) or low number densities (in spectroscopic surveys). If certain systematics can be controlled such as 21cm foregrounds, which I will discuss, intensity mapping can be an excellent tool for future precision cosmology.

**Toward a Robust Inference Method for the Joint Analysis of Power Spectrum and Bispectrum**

**14:20-14:40**

**Andrea Oddo (Astroparticle Physics, SISSA)**

The forthcoming generation of galaxy redshift surveys will sample the large-scale structure of the Universe over unprecedented volumes with high-density tracers. This advancement will make robust measurements of two-point and three-point clustering statistics possible. In preparation for this improvement, I investigate how several methodological choices can influence inferences about galaxy bias, shot noise, and cosmological parameters based on the power spectrum and the bispectrum. After measuring the real-space power spectrum and bispectrum of dark-matter haloes from 300 N-body simulations, I fit a series of theoretical models based on standard perturbation theory, including a tree-level prediction for the bispectrum and a one-loop prediction for the power spectrum, to the numerical data. The fitting procedure is performed by the means of Monte Carlo Markov-Chains simulations. Moreover, I show how the inclusion of the bispectrum can improve constraints on the parameters inferred from the power spectrum alone.

**Non-linear matter power spectra predictions for arbitrary cosmologies**

**14:40-15:00**

**Benjamin Giblin (Institute for Astronomy, University of Edinburgh)**

Cosmic emulators, trained on numerical simulations to predict observables, have proven invaluable in calibrating models on small-scales and improving cosmological constraints. However, to date, emulators have been trained to predict only a small number of modest extensions to vanilla LCDM cosmology, which strongly limits our capacity to test the standard model. This is a significant drawback given recent tensions between high- and low-redshift cosmological surveys.

We present a solution to this problem, having developed an emulator which, combined with the ‘halo model response’ approach from Cataneo et al. (2019), facilitates computationally inexpensive non-linear matter power spectra predictions in arbitrary cosmological models, including LCDM, with accuracies at the per cent level. We find that one requires only ~500 standard dark-matter-only LCDM simulations, with modified initial conditions, on which to train our emulator. This methodology represents a significant step forward in practically constraining dark energy and modified gravity paradigms.

**ReAct and HyPk: unified, general and non-linear codes for lensing and clustering**

**15:00-15:20**

**Benjamin Bose (Physics, University of Geneva)**

I will present two codes with great relevancy for this era of cosmology.

ReAct: Hyper fast, highly accurate calculations of the non-linear matter power spectrum in general cosmologies and gravitational theories. The code is implemented in a widely used MCMC code, ready for cosmological analyses of gravitational lensing.

HyPk: Accurate, calculations of the quasi-non-linear matter power spectrum multipoles in redshift space using input from ReAct, based on a simple proposed prescription.

Improvements to this second code, namely implementation of tracer bias and speed optimisations, will provide an ideal framework for the joint analyses of lensing and clustering for upcoming surveys such as Euclid.

**The Weak Lensing Bispectrum Induced By Gravity**

**15:20-15:40**

**Dipak Munshi (MSSL, University College London)**

Authors: D. Munshi, T. Namikawa, T. D. Kitching, J. D. McEwen,

R. Takahashi, F. R. Bouchet, A. Taruya, B. Bose

Recent studies have demonstrated that {em secondary} non-Gaussianity induced by gravity will be detected with a high

signal-to-noise (S/N) by future and even by on-going weak lensing surveys. One way to characterise such non-Gaussianity

is through the detection of a non-zero three-point correlation function of the lensing convergence field, or of its harmonic transform, the bispectrum. A recent study analysed the properties of the squeezed configuration of the bispectrum, when two modes are much larger than the third one. We extend this work by estimating the amplitude of the (reduced) bispectrum in four generic configurations, i.e., {em squeezed, equilateral, isosceles} and {em folded}, and for four different source redshifts $z_s=0.5,1.0,1.5,2.0$, by using an ensemble of all-sky high-resolution simulations. We compared these results against theoretical predictions. We find that, while the theoretical expectations based on widely used fitting functions can predict the general trends of the reduced bispectra, a more accurate theoretical modelling will be required to analyse the next generation of all-sky weak lensing surveys. The disagreement is particularly pronounced in the squeezed limit. Impact of modification of gravity on weak lensing bispectrum and its detectability are discussed.

#### G4 - Tuesday 16:10-17:50 (Matteo Viel, Chris Clarkson)

**Cosmological implications of large spectroscopic galaxy surveys**

**16:10-16:30**

**Gongbo Zhao (NAOC)**

I will present the recent progress of data analysis and cosmological implications of large redshift surveys, including BOSS, eBOSS and DESI. Specifically, I will focus on the following points: (1) a new method to measure RSD from the reconstructed galaxy sample; (2) an efficient method of BAO and RSD based on a PCA analysis; (3) latest result of the eBOSS survey

**Consistency relations for large-scale structure in modified gravity**

**16:30-16:50**

**Marco Crisostomi (University of Paris-Saclay)**

In LCDM the consistency relations for large-scale structure relate higher order correlation functions in the squeezed limit. Using the effective field theory of dark energy, I will show when and why these relations hold in modified gravity, and will present the observational consequences of their violation for the matter bispectrum.

**Fractality and Homogeneity from Multiple Tracers**

**16:50-17:10**

**Pierros Ntelis (Aix-Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France)**

In the framework of extended test of Gravity and the Concordance Cosmological Model I will summarise existing work done to ameliorate cosmology through the fractal behaviour of cosmic objects. Firstly, fractal characteristics of galaxies and quasars up to redshift z<2.2 will be presented. I will explain how these characteristics can improve measurements of cosmological parameters. Then, a first attempt will be given work can be extended to Black Hole and Neutron Mergers at large scale.