Snowmass2021 Cosmic Frontier White Paper: Cosmology and Fundamental Physics from the three-dimensional Large Scale Structure

Abstract

Advances in experimental techniques make it possible to map the high redshift Universe in three dimensions at high fidelity in the near future. This will increase the observed volume by many-fold, while providing unprecedented access to very large scales, which hold key information about primordial physics. Recently developed theoretical techniques, together with the smaller size of non-linearities at high redshift, allow the reconstruction of an order of magnitude more "primordial modes", and should improve our understanding of the early Universe through measurements of primordial non-Gaussianity and features in the primordial power spectrum. In addition to probing the first epoch of accelerated expansion, such measurements can probe the Dark Energy density in the dark matter domination era, tightly constraining broad classes of dynamical Dark Energy models. The shape of the matter power spectrum itself has the potential to detect sub-percent fractional amounts of Early Dark Energy to z ∼ 10 5 , probing Dark Energy all the way to when the Universe was only a few years old. The precision of these measurements, combined with CMB observations, also has the promise of greatly improving our constraints on the effective number of relativistic species, the masses of neutrinos, the amount of spatial curvature and the gravitational slip. Studies of linear or quasi-linear large-scale structure with redshift surveys and the CMB currently provide our tightest constraints on cosmology and fundamental physics. Pushing the redshift and volume frontier will provide guaranteed, significant improvements in the state-of-the-art in a manner that is easy to forecast and optimize.

Snowmass2021 Cosmic Frontier White Paper: Enabling Flagship Dark Energy Experiments to Reach their Full Potential

“…The optimal solution for wide-field spectroscopy spanning the full LSST footprint would be a highly-multiplexed, wide field-of-view instrument in the South, ideally on an aperture at least as large as LSST's; unfortunately no such capability exists in the US OIR system. Such a facility could improve LSST cosmology while simultaneously enabling new constraints of dark energy, as described in other Snowmass white papers [138,139]. The MegaMapper, MSE, and SpecTel projects all would be well-suited for this role.…”

The MegaMapper: A Stage-5 Spectroscopic Instrument Concept for the Study of Inflation and Dark Energy

“…The mechanisms driving the accelerated expansion of the Universe in its very first moments (Inflation) and at late times (Dark Energy) represent some of the most important open problems in fundamental physics, and have been the subject of several of the Snowmass Community Science White Papers [e.g., [1][2][3][4][5]. We refer readers to those papers, and the questions therein, which have informed the development of the MegaMapper concept.…”

Can we actually constrain $f_{\rm NL}$ using the scale-dependent bias effect? An illustration of the impact of galaxy bias uncertainties using the BOSS DR12 galaxy power spectrum

“…The current tightest bounds on f nl come from the analysis of three-point statistics of the cosmic microwave background (CMB) by the Planck satellite, which constrain f nl = −0.9 ± 5.1 (1σ) [9]. The next improvements over this bound are expected to come from analyses of the late-time spatial distribution of galaxies [10][11][12][13][14][15][16][17][18][19][20][21][22][23], and it has been claimed that future galaxy surveys have in principle the potential to probe f nl with order unity precision, σ fnl ∼ 1. Reaching for the σ fnl = 1 mark has since become a major science goal in galaxy survey analyses, as even if this happens without a detection of f nl (i.e.…”