Thermal-Expansion Coefficient and Universality near the Superfluid Transition ofHe4under Pressure

High-resolution measurements of the thermal-expansion coefficient of pressurized ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ ($5 \mathrm{bar}<~P<~29 \mathrm{bar}$) near $T_{\lambda }$ ($2\times {10}^{-5\mathrm{}}\lesssim \mathrm{}\left|t\right|\mathrm{}\lesssim 0.07$; $t\equiv \frac{T}{T_{\lambda }}-1\mathrm{}$) by a new technique yielded a universal (pressure-independent) critical exponent $\alpha ={\alpha }^{\prime }=-0.026\mathrm{}\pm 0.004$, a universal amplitude ratio $\frac{A}{A^{\prime }}=1.11\mathrm{}\pm 0.02$ for the leading singularity, and a universal ratio $\frac{D}{D^{\prime }}=1.27\mathrm{}\pm 0.2$ for the amplitudes of the confluent singularity. These results agree with renormalization-group theory, but are in part contrary to previous experiments.