  We develop a novel framework for describing quantum fluctuations in field
theory, with a focus on cosmological applications. Our method uniquely
circumvents the use of operator/Hilbert-space formalism, instead relying on a
systematic treatment of classical variables, quantum fluctuations, and an
effective Hamiltonian. Our framework not only aligns with standard formalisms
in flat and de Sitter spacetimes, which assumes no backreaction, demonstrated
through the $\varphi^3$-model, but also adeptly handles time-dependent
backreaction in more general cases. The uncertainty principle and spatial
symmetry emerge as critical tools for selecting initial conditions and
understanding effective potentials. We discover that modes inside the Hubble
horizon \emph{do not} necessarily feel an initial Minkowski vacuum, as is
commonly assumed. Our findings offer fresh insights into the early universe's
quantum fluctuations and potential explanations to large-scale CMB anomalies.
