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We compute the dynamical friction on a small perturber moving through an inviscid fluid, i.e., a superfluid. Crucially, we account for the tachyonic gravitational mass for sound waves, reminiscent of the Jeans instability of the fluid, which results in non-zero dynamical friction even for subsonic velocities. Moreover, we illustrate that the standard leading order effective theory in the derivative expansion is in general inadequate for analysing supersonic processes. We show this in two ways: (i) with a fluid treatment, where we solve the linearized hydrodynamical equations coupled to Newtonian gravity; and (ii) with a quasiparticle description, where we study the energy dissipation of a moving perturber due to phonon radiation. Ordinarily a subsonic perturber moving through a superfluid is kinematically prohibited from losing energy, however the Jeans instability modifies the dispersion relation of the fluid which can result in a small but non-vanishing dynamical friction force. We also analyse the soft phonon bremsstrahlung by a subsonic perturber scattered off an external field.
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We study dynamical friction in interacting relativistic systems with arbitrary mean free paths and medium constituent masses. Our novel framework recovers the known limits of ideal gas and ideal fluid when the mean free path goes to infinity or zero, respectively, and allows for a smooth interpolation between these limits. We find that in an infinite system the drag force can be expressed as a sum of ideal-gas-like and ideal-fluid-like contributions leading to a finite friction even at subsonic velocities. This simple picture receives corrections in any finite system and the corrections become especially significant for a projectile moving at a velocity $v$ close to the speed of sound $vapprox c_s$. These corrections smoothen the ideal fluid discontinuity around the speed of sound and render the drag force a continuous function of velocity. We show that these corrections can be computed to a good approximation within effective theory of viscous fluid dynamics.
We introduce a novel method to circumvent Weinbergs no-go theorem for self-tuning the cosmological vacuum energy: a Lorentz-violating finite-temperature superfluid can counter the effects of an arbitrarily large cosmological constant. Fluctuations of the superfluid result in the graviton acquiring a Lorentz-violating mass and we identify a unique class of theories that are pathology free, phenomenologically viable, and do not suffer from instantaneous modes. This new and hitherto unidentified phase of massive gravity propagates the same degrees of freedom as general relativity with an additional Lorentz-violating scalar that is introduced by higher-derivative operators in a UV insensitive manner. The superfluid is therefore a consistent infrared modification of gravity. We demonstrate how the superfluid can degravitate a cosmological constant and discuss its phenomenology.
We present an in-depth exploration of the phenomenon of dynamical friction in a universe where the dark matter is composed entirely of so-called Fuzzy Dark Matter (FDM), ultralight bosons of mass $msimmathcal{O}(10^{-22}),$eV. We review the classical treatment of dynamical friction before presenting analytic results in the case of FDM for point masses, extended mass distributions, and FDM backgrounds with finite velocity dispersion. We then test these results against a large suite of fully non-linear simulations that allow us to assess the regime of applicability of the analytic results. We apply these results to a variety of astrophysical problems of interest, including infalling satellites in a galactic dark matter background, and determine that emph{(1)}~for FDM masses $mgtrsim 10^{-21}, {rm eV}, c^{-2}$, the timing problem of the Fornax dwarf spheroidals globular clusters is no longer solved and emph{(2)}~the effects of FDM on the process of dynamical friction for satellites of total mass $M$ and relative velocity $v_{rm rel}$ should require detailed numerical simulations for $left(M/10^9~M_{odot}right) left(m/10^{-22}~{rm eV}right)left(100~{rm km}~{rm s}^{-1}/v_{rm rel}right) sim 1$, parameters which would lie outside the validated range of applicability of any currently developed analytic theory, due to transient wave structures in the time-dependent regime.
We consider the gravitational force exerted on a point-like perturber of mass $M$ travelling within a uniform gaseous, opaque medium at constant velocity $V$. The perturber irradiates the surrounding gas with luminosity $L$. The diffusion of the heat released is modelled with a uniform thermal diffusivity $chi$. Using linear perturbation theory, we show that the force exerted by the perturbed gas on the perturber differs from the force without radiation (or standard dynamical friction). Hot, underdense gas trails the mass, which gives rise to a new force component, the heating force, with direction $+V$, thus opposed to the standard dynamical friction. In the limit of low Mach numbers, the heating force has expression $F_mathrm{heat}=gamma(gamma-1)GML/(2chi c_s^2)$, $c_s$ being the sound speed and $gamma$ the ratio of specific heats. In the limit of large Mach numbers, $F_mathrm{heat}=(gamma-1)GML/(chi V^2)f(r_mathrm{min}V/4chi)$, where $f$ is a function that diverges logarithmically as $r_mathrm{min}$ tends to zero. Remarkably, the force in the low Mach number limit does not depend on the velocity. The equilibrium speed, when it exists, is set by the cancellation of the standard dynamical friction and heating force. In the low Mach number limit, it scales with the luminosity to mass ratio of the perturber. Using the above results suggests that Mars- to Earth-sized planetary embryos heated by accretion in a gaseous protoplanetary disc should have eccentricities and inclinations that amount to a sizeable fraction of the discs aspect ratio, for conditions thought to prevail at a few astronomical units.
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