Abstract
We present a novel theory of a unified dark sector, where late-time cosmic acceleration emerges from the dark matter superfluid framework. The system is described by a superfluid mixture consisting of two distinguishable states with a small energy gap, such as the ground state and an excited state of dark matter. Given their contact in the superfluid, interaction between those states can happen, converting one state into the other. This long range interaction within the superfluid couples the two superfluid phonon species through a cosine potential motivated by Josephson/Rabi interactions. As a consequence of this potential, a new dynamics of late-time accelerated expansion emerges in this system, without the need of dark energy, coming from a universe containing only this two-state DM superfluid. Because the superfluid species are non-relativistic, their sound speeds remain suitably small throughout the evolution. We calculate the expansion history and growth of linear perturbations, and compare the results to ΛCDM cosmology. For the fiducial parameters studied here, the predicted expansion and growth function are close to those of ΛCDM, but the difference in the predicted growth rate is significant at late times. The present theory nicely complements the recent proposal of dark matter superfluidity to explain the empirical success of MOdified Newtonian Dynamics (MOND) on galactic scales, thus offering a unified framework for dark matter, dark energy, and MOND phenomenology.
Highlights
The Λ Cold Dark Matter (ΛCDM) model is currently the concordance model of cosmology
We present a novel theory of a unified dark sector, where late-time cosmic acceleration emerges from the dark matter superfluid framework
The present theory nicely complements the recent proposal of dark matter superfluidity to explain the empirical success of MOdified Newtonian Dynamics (MOND) on galactic scales, offering a unified framework for dark matter, dark energy, and MOND phenomenology
Summary
The Λ Cold Dark Matter (ΛCDM) model is currently the concordance model of cosmology. The model relies on two distinct dark components: dark energy (DE), in the form of a cosmological constant; and dark matter (DM), described as a cold pressureless fluid. Milgrom’s law states that th√e total gravitational acceleration a is approximately aN in the regime aN a0, and approaches the geometric mean aNa0 whenever aN a0, where aN is the usual Newtonian gravitational field generated from the observed distribution of baryonic matter alone, and the transition acceleration scale is a0 ∼ 10−8cm/s2 This empirical law was proposed as an alternative to DM known as MOdified Newtonian Dynamics (MOND). (See [40] for a recent proposal for obtaining the MDAR from particle interactions between DM and baryons.) For a particular choice of the superfluid equation of state, the resulting phonon effective Lagrangian is similar to the MOND scalar field theory [41] This phonon effective theory is strikingly similar to that of the Unitary Fermi Gas (e.g., [42]), which has generated much excitement in the cold atom community in recent years. In recent work [43], the finite-temperature equation of state of DM superfluids was computed using a self-consistent mean-field approximation
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