[Paper Review] Ultra-large scale cosmology with next-generation experiments

David Alonso, Philip Bull+3 more|arXiv (Cornell University)|May 28, 2015

Galaxies: Formation, Evolution, Phenomena3 citations

TL;DR

This paper forecasts the detectability of relativistic cosmological effects in next-generation large-scale structure surveys, including weak lensing magnification and primordial non-Gaussianity. It finds that while novel general-relativistic effects remain undetectable in single-tracer power spectra, uncertainties of σ(f_NL) ≈ 1–2 for primordial non-Gaussianity are achievable with joint forecasts, provided systematic effects are mitigated.

ABSTRACT

Future surveys of large-scale structure will be able to measure perturbations on the scale of the cosmological horizon, and so could potentially probe a number of novel relativistic effects that are negligibly small on sub-horizon scales. These effects leave distinctive signatures in the power spectra of clustering observables and, if measurable, would open a new window on relativistic cosmology. We quantify the size and detectability of the effects for a range of future large-scale structure surveys: spectroscopic and photometric galaxy redshift surveys, intensity mapping surveys of neutral hydrogen, and continuum surveys of radio galaxies. Our forecasts show that next-generation experiments, reaching out to redshifts z ~ 4, will not be able to detect previously-undetected general-relativistic effects from the single-tracer power spectra alone, although the contribution of weak lensing magnification on large scales should be clearly detectable. We also perform a rigorous joint forecast for the detection of primordial non-Gaussianity through the excess power it produces in the clustering of biased tracers on large scales, finding that uncertainties of sigma(f_NL) ~ 1-2 should be achievable. We discuss the systematic effects that must be mitigated to achieve this level of sensitivity, and some alternative approaches that should help to improve the constraints.

Motivation & Objective

• To assess the detectability of relativistic effects in ultra-large scale cosmology using upcoming large-scale structure surveys.
• To evaluate the sensitivity of future spectroscopic, photometric, intensity mapping, and radio continuum surveys to general-relativistic corrections.
• To forecast the joint detection of primordial non-Gaussianity via excess power in biased tracer clustering on large scales.
• To identify and address key systematic effects that could limit sensitivity in upcoming experiments.
• To explore alternative approaches for improving constraints on cosmological parameters.

Proposed method

• Uses forecasts based on power spectra of clustering observables from multiple survey types: spectroscopic and photometric galaxy redshift surveys, intensity mapping of neutral hydrogen, and continuum surveys of radio galaxies.
• Applies relativistic perturbation theory to model large-scale relativistic effects, including weak lensing magnification and other horizon-scale corrections.
• Performs joint forecasts for primordial non-Gaussianity using the excess power it induces in biased tracer clustering on super-horizon scales.
• Quantifies signal-to-noise ratios and error bars on cosmological parameters, particularly f_NL, under realistic survey configurations up to z ~ 4.
• Incorporates systematic effects such as photometric redshift errors and nonlinear biasing into the forecasting framework.
• Evaluates alternative approaches to enhance sensitivity, including multi-tracer techniques and scale-dependent bias modeling.

Experimental results

Research questions

• RQ1 Can next-generation large-scale structure surveys detect previously unobserved general-relativistic effects on cosmological horizon scales?
• RQ2 What level of sensitivity can be achieved for primordial non-Gaussianity (f_NL) using joint forecasts from large-scale clustering?
• RQ3 How do systematic effects such as photometric redshift errors and nonlinear biasing impact the detectability of relativistic effects?
• RQ4 To what extent does weak lensing magnification contribute to large-scale power spectra in future surveys?
• RQ5 What alternative methods can improve constraints on cosmological parameters beyond single-tracer power spectra?

Key findings

• Next-generation surveys reaching redshifts z ~ 4 cannot detect previously undetected general-relativistic effects using single-tracer power spectra alone.
• The contribution of weak lensing magnification to large-scale power spectra is expected to be clearly detectable in future surveys.
• Joint forecasts for primordial non-Gaussianity achieve uncertainties of σ(f_NL) ≈ 1–2, indicating high sensitivity to non-Gaussian signatures.
• Systematic effects such as photometric redshift errors and nonlinear biasing must be carefully mitigated to achieve the projected sensitivity levels.
• Alternative approaches, including multi-tracer techniques and scale-dependent bias modeling, are expected to improve constraints on cosmological parameters.
• The study demonstrates that while horizon-scale relativistic effects remain challenging to detect, primordial non-Gaussianity remains a promising target for next-generation cosmology.