  When a subdominant light scalar field ends slow roll during inflation, but
well after the Hubble exit of the pivot scales, it may determine the
cosmological perturbations. This thesis investigates how such a scalar field,
the spectator, may leave its impact on the Cosmic Microwave Background (CMB)
radiation and be consequently constrained. We first introduce the observables
of the CMB, namely the power spectrum $P_\zeta$, spectral index $n_s$ and its
running $dn_s/d\ln k$, the non-Gaussianities $f_{NL}$, $g_{NL}$ and
$\tau_{NL}$, and the lack of isocurvature and polarization modes. Based on
these studies, we derive the cosmological predictions for the spectator
scenario, revealing its consistency with the CMB for inflection point
potentials, hyperbolic tangent potentials, and those with a sudden phase
transition. In the end, we utilize the spectator scenario to explain the CMB
power asymmetry, with a brief tachyonic fast-roll phase.
