Non-Gaussianity in LSS

Standard inflation in ΛCDM predicts that primordial density perturbations are nearly Gaussian, and while small non-Gaussianities can be generated by nonlinear gravitational evolution, the primordial f_NL parameter is expected to be negligibly small. Observations of large-scale structure, however, reveal non-Gaussian signatures in the galaxy distribution at levels that are difficult to reconcile with a purely Gaussian primordial spectrum evolved through standard gravitational instability. SCT offers a fundamentally different origin for these non-Gaussianities: the collision between two spacetime pockets is inherently a non-Gaussian event. The collision front sweeps the overlap volume and deposits kinetic energy with a spatial profile determined by the pocket geometries and impact parameter, imprinting non-Gaussian correlations in the initial thermalized density field that no inflationary model needs to produce.

The angular momentum inherited from the collision impact parameter further breaks the statistical symmetry of the density field, generating bispectrum configurations aligned with the collision axis. In SCT, the primordial non-Gaussianity is not a free parameter to be tuned but a direct consequence of the collision geometry — its amplitude and shape are fixed by the impact parameter and relative velocity of the colliding pockets. This predicts a specific squeezed-limit bispectrum shape correlated with the collision axis direction, distinct from both local and equilateral primordial non-Gaussianity templates used in standard analyses. The non-Gaussian signature observed in large-scale structure therefore encodes fossil information about the collision geometry, offering a potential reconstruction of the original event parameters.