PACS 64.60.-i -First pacs description PACS 02.50.-r -Second pacs description PACS 05.45.-a -Third pacs description Abstract -We extend the theory of quasipotentials in dynamical systems by calculating, within a broad class of period-doubling maps, an exact potential for the critical fluctuations of pitchfork bifurcations in the weak noise limit. These far-from-equilibrium fluctuations are described by finitesize mean field theory, placing their static properties in the same universality class as the Ising model on a complete graph. We demonstrate that the effective system size of noisy period-doubling bifurcations exhibits universal scaling behavior along period-doubling routes to chaos.Period-doubling bifurcations have been observed in a wide variety of natural systems spanning many areas of science [1]. Univariate, discrete-time maps are the simplest dynamical systems to exhibit a period-doubling route to chaos [2,3]. Applications range from the population dynamics of species with non-overlapping generations [4,5] to the oscillations of rf-driven Josephson junctions [6,7]. The impact of noise on period-doubling maps has been extensively studied in both ecology [8][9][10][11] and physics [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], including the universal scaling of the Lyapunov exponent along period-doubling routes to chaos [31][32][33][34][35][36]. Connections between such noisy dynamical systems and the universality classes of equilibrium statistical physics has been a subject of great fascination [37][38][39][40][41][42][43][44][45][46][47][48][49]. The theory of quasipotentials, providing a formal link between equilibruim and nonequilibrium physics, has been applied to many systems [50,51]. In particular, the theory of quasipotentials has been used to estimate escape times from the attractors of noisy period-doubling maps [52][53][54] and to estimate the invariant probability distributions of their strange attractors in the chaotic regime [53,55,56].Here, we investigate the invariant probability distributions that characterize critical fluctuations in the pitchfork bifurcations of period-doubling maps far from the chaotic threshold. In the limit of weak noise, we find an exact correspondence between the static behavior of fluctuations at a pitchfork bifurcation and the critical behavior of finite-size mean field theory [57]. This correspondence places pitchfork bifurcations in the same universality class as the Ising model on a complete graph [58][59][60][61][62]. Analytical estimates of critical exponents and amplitudes agree well with the results of numerical simulations. We conclude with evidence of universal scaling behavior in the effective system size of critical fluctuations along period-doubling routes to chaos. p-1 arXiv:1502.04074v3 [cond-mat.stat-mech]