Taking the Temperature of the Universe's Most Extreme States

Temperature measurement at the extremes of pressure and density has been a decades-long challenge. In the laboratory, such conditions exist for less than a billionth of a second inside micron-scale optically opaque targets, demanding a new class of intense ultrafast penetrating diagnostics. I will discuss millielectronvolt-resolution inelastic X-ray scattering at X-ray free-electron lasers [1], which extracts ion temperature directly from Doppler broadening in a backscattering geometry, providing a model-independent ion thermometer. As a first demonstration, laser-excited gold foils exhibit extreme superheating that surpasses the predicted entropy catastrophe limit [2]. This platform enables direct time-resolved investigation of electron-ion equilibration, a long-standing open question in warm dense matter, revealing significantly enhanced coupling compared to weakly excited gold and placing new constraints on non-equilibrium energy transfer in dense plasmas [3]. Together, these measurements open a window into thermodynamic parameters, energy flow, and transport coefficients previously beyond reach.

References

[1] E. E. McBride et al., "Setup for meV-resolution inelastic X-ray scattering measurements at the Matter in Extreme Conditions endstation of the Linac Coherent Light Source," Rev. Sci. Instrum. 89, 10F104 (2018).
[2] T. G. White et al., "Superheating gold beyond the predicted entropy catastrophe threshold," Nature 643, 950–954 (2025).
[3] T. Griffin et al., "Electron-ion equilibration in superheated gold," Nat. Commun. (under review).

Biography

Thomas White is an Associate Professor and Clemons-Magee Endowed Professor in Physics at the University of Nevada, Reno. He earned an MS in physics from the University of Bath (2010) and a PhD from the University of Oxford (2015), where his dissertation received the Culham Thesis Prize in plasma physics. Before joining UNR in 2017, he held postdoctoral positions at Imperial College London and the University of Oxford. His research probes matter under extreme conditions using high-power lasers and X-ray free-electron lasers, combining experiments with first-principles simulations. He serves as Vice Chair of the High Energy Density Science Association (HEDSA).