Abstract
According to common lore, Equations of State of field theories with gravity duals tend to be soft, with speeds of sound either below or around the conformal value of {upupsilon}_s=1/sqrt{3} . This has important consequences in particular for the physics of compact stars, where the detection of two solar mass neutron stars has been shown to require very stiff equations of state. In this paper, we show that no speed limit exists for holographic models at finite density, explicitly constructing examples where the speed of sound becomes arbitrarily close to that of light. This opens up the possibility of building hybrid stars that contain quark matter obeying a holographic equation of state in their cores.
Highlights
Ranges from a relatively dilute gas of nuclei immersed in a sea of electrons in the crust of the star to dense nuclear and superdense neutron matter deep inside the star, expected to reach at least a few times the nuclear saturation density, ns ≈ 0.16/fm3, in the cores of the most massive stars
With the deconfinement transition of Quantum Chromodynamics (QCD) expected to take place around these densities, it is at the moment still unclear, whether quark matter should be present inside the stars or not
It is clear that approaches based on weak coupling expansions in the quark matter phase, such as perturbative QCD [6,7,8,9], cannot be used to describe the transition region, and the standard approaches for the description of this regime typically include model calculations and interpolations between the low- and high-density regimes [11]
Summary
We will use holographic models as a tool to study the EoS of strongly coupled gauge theories at finite density and temperature, we will be more interested in low temperatures. The models will be chosen in such a way that the theory is well defined in the UV, in the sense that there is a fixed point at asymptotically large energies. If the theory was conformal, the EoS would be fixed by symmetry; here, this will be avoided by introducing a relevant deformation of the UV fixed point that breaks conformal invariance explicitly. We will consider two cases in parallel: a top-down model with a well defined string theory construction, and a family of phenomenological bottom-up models that allow a wider analysis while keeping the main ingredients of the top-down model
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