Justin Khoury

TL;DR: In this paper, the authors present an alternative explanation which allows scalar fields to evolve cosmologically while having couplings to matter of order unity in the solar system, where the mass of the fields depends on the local matter density.

TL;DR: In this article, the utility of precise cosmic microwave background (CMB) polarization measurements as probes of the physics of inflation was summarized, and the potential for using CMB measurements to differentiate various inflationary mechanisms was discussed.

TL;DR: The cosmology of a Galileon scalar-tensor theory, obtained by covariantizing the decoupling lagrangian of the Dvali-Gabadadze-Poratti (DGP) model, is studied in this paper.

Abstract: We propose a novel theory of dark matter (DM) superfluidity that matches the successes of the $\mathrm{\ensuremath{\Lambda}}$ cold dark matter ($\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$) model on cosmological scales while simultaneously reproducing the modified Newtonian dynamics (MOND) phenomenology on galactic scales. The DM and MOND components have a common origin, representing different phases of a single underlying substance. DM consists of axionlike particles with mass of order eV and strong self-interactions. The condensate has a polytropic equation of state $P\ensuremath{\sim}{\ensuremath{\rho}}^{3}$ giving rise to a superfluid core within galaxies. Instead of behaving as individual collisionless particles, the DM superfluid is more aptly described as collective excitations. Superfluid phonons, in particular, are assumed to be governed by a MOND-like effective action and mediate a MONDian acceleration between baryonic matter particles. Our framework naturally distinguishes between galaxies (where MOND is successful) and galaxy clusters (where MOND is not); due to the higher velocity dispersion in clusters, and correspondingly higher temperature, the DM in clusters is either in a mixture of superfluid and the normal phase or fully in the normal phase. The rich and well-studied physics of superfluidity leads to a number of observational signatures: an array of low-density vortices in galaxies; merger dynamics that depend on the infall velocity vs phonon sound speed; distinct mass peaks in bulletlike cluster mergers, corresponding to superfluid and normal components; and interference patters in supercritical mergers. Remarkably, the superfluid phonon effective theory is strikingly similar to that of the unitary Fermi gas, which has attracted much excitement in the cold atom community in recent years. The critical temperature for DM superfluidity is of order mK, comparable to known cold atom Bose--Einstein condensates. Identifying a precise cold atom analog would give important insights on the microphysical interactions underlying DM superfluidity. Tantalizingly, it might open the possibility of simulating the properties and dynamics of galaxies in laboratory experiments.

TL;DR: In this paper, the authors derived the cosmological expansion history in the presence of a symmetron field, tracking the evolution through the inflationary, radiation-and matter-dominated epochs, using a combination of analytical approximations and numerical integration.