Alternative initial states for inflation

Inflationary cosmology holds that the large-scale structure of the universe grew from quantum fluctuations seeded in its earliest moments. Standard theory assumes those fluctuations began in a specific quantum state, the Bunch–Davies vacuum, the ground state an oscillator settles into when nothing is exciting it, chosen largely because it is mathematically natural, not because any observation confirms it. The pre-inflationary universe is inaccessible to direct measurement, no complete theory of quantum gravity uniquely enforces this choice, and the assumption can therefore be challenged. This thesis challenges it. The central question is whether coherent states could have been the true starting point of primordial perturbations, and what the consequences would be if they were. The answer emerges from three converging constraints. Ultraviolet consistency , the requirement that the quantum field's energy density remain finite and renormalizable, restricts the amplitude of any coherent displacement to be parametrically small from the outset. A structural degeneracy theorem then shows that, at the level of the observable power spectrum, a coherent displacement is indistinguishable from the ordinary Bunch–Davies vacuum; its traces appear only in higher-order correlators that current data cannot isolate. Most decisively, the expanding inflationary background squeezes every quantum mode exponentially, amplifying vacuum fluctuations by a factor that grows as the universe doubles in size roughly sixty times. This squeezing preserves the ratio of coherent signal to quantum noise as a dynamical invariant, frozen at its initial, parametrically small value so amplified quantum correlations unconditionally drown the coherent contribution before inflation ends. The thesis names this constellation of suppressions the coherent state gap. Whether non-standard inflationary models those with non-canonical kinetic terms, multiple fields, or modified initial conditions, can circumvent this gap is examined in the final chapter. Framed within the Effective Field Theory of inflation and constrained by a classification theorem for the algebra governing inflationary squeezing, the answer is no: no known extension dynamically selects a pure coherent initial state without either violating perturbative unitarity or remaining observationally indistinguishable from the standard vacuum. Environmental decoherence destroys any surviving quantum coherence within roughly ten e-folds after a mode exits the Hubble horizon, regardless of which model is assumed. Coherent states therefore span the full space of admissible Gaussian initial conditions for inflation, yet inflation itself provides no mechanism to select or sustain them. This is not a negative result but a structural one. It maps precisely what future observations must overcome to identify primordial coherence, and what theoretical physics must supply before that identification becomes possible.