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

Understand

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.