Dark sector interactions in the $w\to -1$ limit: velocity locking in pure momentum exchange models

Abstract

Models of interacting dark energy (DE) and dark matter (DM) involving pure momentum exchange are a promising avenue for resolving cosmological tensions. However, the behaviour of these interactions in the theoretically challenging limit where the DE equation of state, $w$, approaches $-1$ is not fully understood. We demonstrate that a generic feature of these models is a $w$-dependent velocity-locking mechanism, which systematically shifts the onset of matter power spectrum suppression to smaller scales as $w\to -1$. The suppression magnitude depends on the difference in fluid velocities. In this limit, however, the interaction's drag dominates over the DE pressure support and causes the DE velocity to track that of the DM fluid at larger scales. This mechanism provides a physical explanation for the weaker constraints found in the literature when $w\approx -1$ in models where the interaction strength does not explicitly depend on $w$. We also demonstrate that the common approximation of neglecting DE perturbations (${\delta }_{\mathrm{DE}}={\theta }_{\mathrm{DE}}=0$) fails in this limit. By artificially increasing the velocity difference between the fluids, this simplification incorrectly removes the $w$-dependent velocity-locking mechanism and erases the shift in power spectrum suppression to smaller scales. This leads to an overestimation of the constraining power of cosmological data on the interaction strength.

12 pages, 9 figures, plus references

References in corpus (30)

• A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team
• Dark Energy Survey Year 3 Results: Cosmological Constraints from Galaxy Clustering and Weak Lensing
• KiDS-1000 Cosmology: Cosmic shear constraints and comparison between two point statistics
• DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints
• Cosmology Intertwined III: $f{\sigma }_8$ and $S_8$
• Large-scale instability in interacting dark energy and dark matter fluids
• Indications of a late-time interaction in the dark sector
• The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics
• KiDS-Legacy: Cosmological constraints from cosmic shear with the complete Kilo-Degree Survey
• The Parameterized Post-Friedmannian Framework for Interacting Dark Energy Theories
• On the large-scale instability in interacting dark energy and dark matter fluids
• Nonlinear interacting cosmological models after Planck 2018 legacy release and the $H_0$ tension
• Cosmology with the EFTofLSS and BOSS: dark energy constraints and a note on priors
• Inhomogeneous vacuum energy
• Scattering of Dark Matter and Dark Energy
• Probing elastic interactions in the dark sector and the role of $S_8$
• Observational constraints on dark matter-dark energy scattering cross section
• Understanding the suppression of structure formation from dark matter$\unicode{x2013}$dark energy momentum coupling
• Scaling solutions and weak gravity in dark energy with energy and momentum couplings
• Simulating Momentum Exchange in the Dark Sector
• KiDS-Legacy: Consistency of cosmic shear measurements and joint cosmological constraints with external probes
• Latest evidence for a late time vacuum -- geodesic CDM interaction
• On the dangers of using the growth equation on large scales in the Newtonian gauge
• J-PAS: Forecasts for dark matter - dark energy elastic couplings
• Velocity-dependent interacting dark energy and dark matter with a Lagrangian description of perfect fluids
• Interacting dark energy from the joint analysis of the power spectrum and bispectrum multipoles with the EFTofLSS
• Two per cent measurement of $H_0$ from Cepheids alone
• The interacting vacuum and tensions: a comparison of theoretical models
• Forecasts for interacting dark energy with time-dependent momentum exchange
• Perturbation level interacting dark energy model and its consequence on late-time cosmological parameters

Cited by in corpus (1)

• Revisiting observational constraints on coupled exponential quintessence with energy and momentum transfers: degeneracy with massive neutrinos