Q0

q0 is a shorthand label used across different disciplines to denote a starting point, initial condition, or key reference parameter. Because it appears in varied contexts, its meaning is field-dependent, and it is typically defined within a specific theory or model.

In automata theory and computer science, q0 commonly denotes the initial state of a finite automaton or state machine. It is the state from which any input processing begins. Diagrams often show an incoming transition arrow pointing to q0, signaling the starting point before any input is read. In deterministic or nondeterministic automata, the transitions guide movement from q0 to other states based on input symbols.

In cosmology, q0 represents the deceleration parameter of the universe at the present time. Defined as q0 = q(t0) = - (a(t0) a''(t0)) / (a'(t0))^2, where a(t) is the scale factor and dots denote time derivatives, q0 measures whether cosmic expansion is slowing down or speeding up. A negative q0 indicates accelerating expansion, while a positive q0 would indicate deceleration. Observational data from supernovae, the cosmic microwave background, and large-scale structure place q0 in a negative range, roughly around -0.5 to -0.6 in the standard Lambda-CDM model.

Other uses of q0 may appear in different fields as a generic symbol for an initial parameter or reference quantity. Because q0 is not unique to a single domain, users should consult the relevant context or definitions to interpret its meaning correctly. Related concepts include the initial or start state, scale factor and Hubble parameter in cosmology, and the interpretation of q0 in models of cosmic expansion.