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Non-Gaussianity as a Probe of the Physics of the Primordial Universe and the Astrophysics of the Low Redshift Universe

E. Komatsu, N. Afshordi, N. Bartolo, D. Baumann, J. R. Bond, E. I. Buchbinder, C. T. Byrnes, X. Chen, D. J. H. Chung, A. Cooray, P. Creminelli, N. Dalal, O. Dore, R. Easther, A. V. Frolov, K. M. Gorski, M. G. Jackson, J. Khoury, W. H. Kinney, L. Kofman, K. Koyama, L. Leblond, J. -L. Lehners, J. E. Lidsey, M. Liguori, E. A. Lim, A. Linde, D. H. Lyth, J. Maldacena, S. Matarrese, L. McAllister, P. McDonald, S. Mukohyama, B. Ovrut, H. V. Peiris, C. Raeth, A. Riotto, Y. Rodriguez, M. Sasaki, R. Scoccimarro, D. Seery, E. Sefusatti, U. Seljak, L. Senatore, S. Shandera, E. P. S. Shellard, E. Silverstein, A. Slosar, K. M. Smith, A. A. Starobinsky, P. J. Steinhardt, F. Takahashi, M. Tegmark, A. J. Tolley, L. Verde, B. D. Wandelt, D. Wands, S. Weinberg, M. Wyman, A. P. S. Yadav, M. Zaldarriaga

TL;DR

Non-Gaussianity offers a rich, information-dense window into both the primordial Universe and the astrophysics of the low-redshift cosmos, going beyond the power spectrum to higher-order statistics like the bispectrum $B(k_1,k_2,k_3)$ and trispectrum. The paper outlines how violations of key inflationary conditions yield distinctive bispectrum shapes, parameterized by $f_{NL}$, with squeezed configurations capable of ruling out all single-field models if detected. It also emphasizes that late-time non-Gaussianity from non-linear structure formation and baryonic effects provides complementary constraints on galaxy bias, lensing, SZ, and dark energy, while highlighting the need for multi-tracer, multi-probe observational strategies. By advocating coordinated use of CMB, LSS, lensing, Ly-$\alpha$, and 21-cm data—and extending to higher-order statistics—the work argues for achieving sensitivities like $\Delta f_{NL} \lesssim 1$ and for developing theory to exploit configurations beyond the standard shapes, thereby maximizing discovery potential in the coming decade.

Abstract

A new and powerful probe of the origin and evolution of structures in the Universe has emerged and been actively developed over the last decade. In the coming decade, non-Gaussianity, i.e., the study of non-Gaussian contributions to the correlations of cosmological fluctuations, will become an important probe of both the early and the late Universe. Specifically, it will play a leading role in furthering our understanding of two fundamental aspects of cosmology and astrophysics: (i) the physics of the very early universe that created the primordial seeds for large-scale structures, and (ii) the subsequent growth of structures via gravitational instability and gas physics at later times. To date, observations of fluctuations in the Cosmic Microwave Background (CMB) and the Large-Scale Structure of the Universe (LSS) have focused largely on the Gaussian contribution as measured by the two-point correlations (or the power spectrum) of density fluctuations. However, an even greater amount of information is contained in non-Gaussianity and a large discovery space therefore still remains to be explored. Many observational probes can be used to measure non-Gaussianity, including CMB, LSS, gravitational lensing, Lyman-alpha forest, 21-cm fluctuations, and the abundance of rare objects such as clusters of galaxies and high-redshift galaxies. Not only does the study of non-Gaussianity maximize the science return from a plethora of present and future cosmological experiments and observations, but it also carries great potential for important discoveries in the coming decade.

Non-Gaussianity as a Probe of the Physics of the Primordial Universe and the Astrophysics of the Low Redshift Universe

TL;DR

Non-Gaussianity offers a rich, information-dense window into both the primordial Universe and the astrophysics of the low-redshift cosmos, going beyond the power spectrum to higher-order statistics like the bispectrum and trispectrum. The paper outlines how violations of key inflationary conditions yield distinctive bispectrum shapes, parameterized by , with squeezed configurations capable of ruling out all single-field models if detected. It also emphasizes that late-time non-Gaussianity from non-linear structure formation and baryonic effects provides complementary constraints on galaxy bias, lensing, SZ, and dark energy, while highlighting the need for multi-tracer, multi-probe observational strategies. By advocating coordinated use of CMB, LSS, lensing, Ly-, and 21-cm data—and extending to higher-order statistics—the work argues for achieving sensitivities like and for developing theory to exploit configurations beyond the standard shapes, thereby maximizing discovery potential in the coming decade.

Abstract

A new and powerful probe of the origin and evolution of structures in the Universe has emerged and been actively developed over the last decade. In the coming decade, non-Gaussianity, i.e., the study of non-Gaussian contributions to the correlations of cosmological fluctuations, will become an important probe of both the early and the late Universe. Specifically, it will play a leading role in furthering our understanding of two fundamental aspects of cosmology and astrophysics: (i) the physics of the very early universe that created the primordial seeds for large-scale structures, and (ii) the subsequent growth of structures via gravitational instability and gas physics at later times. To date, observations of fluctuations in the Cosmic Microwave Background (CMB) and the Large-Scale Structure of the Universe (LSS) have focused largely on the Gaussian contribution as measured by the two-point correlations (or the power spectrum) of density fluctuations. However, an even greater amount of information is contained in non-Gaussianity and a large discovery space therefore still remains to be explored. Many observational probes can be used to measure non-Gaussianity, including CMB, LSS, gravitational lensing, Lyman-alpha forest, 21-cm fluctuations, and the abundance of rare objects such as clusters of galaxies and high-redshift galaxies. Not only does the study of non-Gaussianity maximize the science return from a plethora of present and future cosmological experiments and observations, but it also carries great potential for important discoveries in the coming decade.

Paper Structure

This paper contains 5 sections, 1 figure.

Table of Contents

  1. Beyond a simple approximation to nature
  2. Non-Gaussianity as a probe of the physics of the primordial Universe
  3. Non-Gaussianity as a probe of the astrophysics of the low-redshift Universe
  4. How to exploit non-Gaussianity in the coming decade
  5. Conclusion

Figures (1)

  • Figure 1: Bispectrum shapes, $B(k_1,k_2,k_3)$, which can be characterized by triangles formed by three wave vectors. The shape (a) has the maximum signal at the squeezed configuration, $k_3\ll k_2\approx k_1$, and can be produced by models of inflation involving multiple fields. The shape (b) has the maximum signal at the equilateral configuration, $k_1=k_2=k_3$, and can be produced by non-canonical kinetic terms of quantum fields. The shape (c) has the maximum signal at the flattened configuration, $k_1\approx 2k_2\approx 2k_3$, and can be produced by non-vacuum initial conditions.