Fluctuations and effective temperatures in coarsening

We study dynamic fluctuations in non-disordered finite dimensional ferromagnetic systems quenched to the critical point and the low-temperature phase. We investigate the fluctuations of two two-time quantities, called $\chi$ and $C$, the averages of which %$<\chi >$, $<C>$ yield the self linear response and correlation function. We introduce a restricted average of the $\chi$’s, summing over all configurations with a given value of $C$. We find that the restricted average $<\chi {>}_C$ obeys a scaling form, and that the slope of the scaling function approaches the universal value $X_{\mathrm{\infty }}$ of the limiting effective temperature in the long-time limit and for $C\to 0$. Our results tend to confirm the expectation that time-reparametrization invariance is not realized in coarsening systems at criticality. Finally, we discuss possible experimental tests of our proposal.