Initial Condition of the Inflationary Universe and Its Imprint on the Cosmic Microwave Background
There is an apparent power deficit relative to the Lambda-CDM prediction of the CMB spectrum at large scales, which, though not yet statistically significant, persists from WMAP to Planck data. We first present a simple toy model corresponding to a network of frustrated topological defects of domain walls or cosmic strings that exist previous to the standard slow-roll inflationary era of the universe. Those features are phenomenologically modeled by a Chaplygin gas that can interpolate between a network of frustrated topological defects and a de Sitter-like or a power-law inflationary era. We show that these scenarios can alleviate the quadrupole anomaly of the CMB spectrum, based on the approximate initial conditions for the long-wavelength perturbations. We then go further to show that the large-scale spectrum at the end of inflation reflects the super-horizon spectrum of the initial state of the inflaton field. By studying the curvature perturbations of a scalar field in the FLRW universe parameterized by the equation of state parameter w, we find that the large-scale spectrum is suppressed if the universe begins with the adiabatic vacuum in a superinflation (w < -1) or positive-pressure (w > 0) era. In the latter case, there is however no causal mechanism to establish the initial adiabatic vacuum. To search for a more realistic initial condition of the inflationary universe, we consider the Hartle-Hawking no-boundary wave function, which is a solution to the Wheeler-DeWitt equation, as the initial condition of the universe. We find that the power suppression can be the consequence of a massive inflaton, whose initial vacuum is the Euclidean instanton in a compact manifold. We calculate the primordial power spectrum of the perturbations and show that, as long as the scalar field is moderately massive, the power spectrum is suppressed at the long-wavelength scales.