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In the recent work Fletcher et al. [Phys. Rev. Lett., v.99, p.145001 (2007)]reported on the novel experimental technique, enabling to measure thetemperature of the expanding ultracold plasmas over a considerable timeinterval (up to 60-70 {\mu}s). It was unexpectedly found that the electrontemperature dropped with time as T_e(t) ~ t^{-\alpha} with \alpha = 1.2 +/- 0.1\approx 1 instead of \alpha = 2, which would be expected for the adiabaticcooling of electrons in the cloud expanding linearly in time. The above-citedauthors supposed that 'the difference is likely due to the significant heatingeffects from 3-body recombination' (i.e. the inelastic processes), but they didnot provide sufficient quantitative estimates supporting such a conclusion. Theaim of the present comment is to mention that the experimentally revealedt^{-1}-dependence can be explained under quite general assumptions by thepurely elastic processes in the ultracold plasma, as it was done a few yearsago in our work [Yu.V. Dumin, J. Low Temp. Phys., v.119, p.377 (2000)].  Therefore, t^{-1}-dependence revealed in the above-cited experiment should beprimarily a manifestation of "virialization" of the electron velocities in theregime of strong electron-ion correlations, while the heat release by inelasticprocesses should be of secondary importance (it might be responsible,particularly, for changing the exponent from -1 to -1.2). This is confirmedalso by our ab initio molecular-dynamic simulations taking into account thestrong electron-ion correlations (i.e. not based on the PIC method, Vlasovapproximation for electrons, etc., which are applicable only to small-angularscattering).

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