The project got off to a flying start when just a few months after its onset the first-ever detection of gravitational waves from a binary neutron-star merger was announced by the LIGO and Virgo collaborations. Due to the preparatory work our group had been performing in the neutron-star-matter equation of state subproject, we were able to immediately apply the new measurement of the tidal deformability of an approx. 1.4 -solar-mass neutron star in the model-independent determination of the neutron-star-matter equation of state. This result was published in Physical Review Letters and became one of the most popular theory papers utilizing the LIGO/Virgo measurement, having gathered over 700 citations to date (February 2023).
Having observed a curious "bending" of the model-independent pressure as a function of density somewhat below the highest densities realized in physical neutron stars, reminiscent of the effect of a phase transition, we decided to immediately tackle the main goal of the DenseMatter project: the physical phase realized deep in the cores of the most massive stable neutron stars. To this end, we developed a new, more versatile way of interpolating the equation of state, and tracked a larger number of physical quantities in the original PRL publication. These results came out in a 2020 Nature Physics article titled "Evidence for quark-matter cores in massive neutron stars" that indeed presented first-ever model-independent evidence for the likely presence of deconfined matter inside the massive stable neutron stars. This paper also proved to be very successful, and has gathered almost 400 citations in less than three years, and the conclusions were further strengthened in a later Physical Review X article in 2022. Finally, we are currently finalizing an article that presents an important generalization of the Nature Physics research, involving the use of Bayesian inference and equations of state obtained using Gaussian processes. Here, we will be able to attach a likelihood estimate for the existence of quark-matter cores, which according to our preliminary results will likely land in the ballpark of 90%.
Due to the shift of attention towards NS physics, our activities on the perturbative-QCD front were slightly delayed, and the completion of the full four-loop pressure is still lacking. We have, however, made significant progress in this direction as well, and e.g. published two PRL articles on the way. In the first of these two papers, we determined a new term of order g^6ln^2(g) in the weak-coupling expansion of the quark-matter pressure, with g denoting the gauge coupling of the theory, while in the latter we derived the entire soft contribution to the quantity at order g^6. In the related but slightly simpler theory of Quantum Electrodynamics, we have also recently determined the so-called mixed contributions to the pressure up to O(g^6), originating from the interplay of the hard and soft momentum scales of the system, and are currently finalizing a similar computation in QCD.
Finally, going slightly beyond the initial plans, we have continued our earlier programme to develop a holographic description of dense QCD matter, especially in the context of neutron stars. Here, our single most important contribution has been to derive the first results for the most important transport coefficients describing cold and dense quark matter. These results were published in PRL in 2020, and the project is currently being pursued further for the bulk viscosity.
