Unified dark fluid with null sound speed as an alternative to phantom dark energy

Abstract

Recent BAO measurements from DESI, when combined with CMB and supernovae data, suggest evolving dark energy and in particular point to a possible phantom regime, with an equation of state parameter $w<-1$. We explore an alternative phenomenological way to model dark matter and dark energy based on a unified dark fluid (UDF). By construction, our model reproduces the same background expansion history as DESI's best-fit using the CPL parametrization, but assumes a vanishing rest-frame sound speed and no anisotropic stress. This simple prescription ensures a consistent and physical treatment of perturbations and, in our case, the use of a unified dark sector avoids phantom behaviour. We model CMB, LSS, and redshift-space distortion observables, and find mostly small differences with CPL, suggesting that while stage IV CMB and galaxy surveys will be able to test these models, achieving a decisive distinction between them may prove challenging on linear scales. At the non-linear level, we study spherical collapse in the UDF and show that within this framework, structure formation proceeds very similarly to standard scenarios. Using Planck, DESI BAO DR2, and DES Y5 supernovae data, we demonstrate that this simple UDF model fits current observations nearly as well as CPL, while treating perturbations consistently. Because most cosmological observations are not sensitive to how the dark sector is split, the unified framework can also approximate the phenomenology of interacting dark energy-dark matter scenarios or evolving dark matter, making it a general way to model the data, at least as long as the dark components have a vanishing sound speed, which is the most distinctive feature of our analysis. Our results highlight that a unified dark fluid with evolving equation of state and null sound speed is sufficient to pass current constraints without invoking a phantom component.

arXiv comment28 pages, 10 figures

References in corpus (69)

• Efficient Computation of CMB anisotropies in closed FRW models
• A Phantom Menace? Cosmological consequences of a dark energy component with super-negative equation of state
• Exploring the Expansion History of the Universe
• Accelerating Universes with Scaling Dark Matter
• A Dynamical Solution to the Problem of a Small Cosmological Constant and Late-time Cosmic Acceleration
• k-Inflation
• The Simons Observatory: Science goals and forecasts
• Dark Energy Constraints from the Cosmic Age and Supernova
• Can the dark energy equation-of-state parameter w be less than -1?
• Planck 2018 results. V. CMB power spectra and likelihoods
• Information criteria for astrophysical model selection
• Cobaya: Code for Bayesian Analysis of hierarchical physical models
• Can dark energy evolve to the Phantom?
• Purely kinetic k-essence as unified dark matter
• Euclid. I. Overview of the Euclid mission
• DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints
• CMB power spectrum parameter degeneracies in the era of precision cosmology
• Structure Formation with Generalized Dark Matter
• A Parameterized Post-Friedmann Framework for Modified Gravity
• HMcode-2020: Improved modelling of non-linear cosmological power spectra with baryonic feedback
• Probing dark energy perturbations: the dark energy equation of state and speed of sound as measured by WMAP
• Crossing the Phantom Divide: Dark Energy Internal Degrees of Freedom
• Testing cosmological structure formation using redshift-space distortions
• Constraints on the generalized Chaplygin gas from supernovae observations
• Large Scale Cosmic Microwave Background Anisotropies and Dark Energy
• The Effective Theory of Quintessence: the w<-1 Side Unveiled
• Crossing the Phantom Divide with Parameterized Post-Friedmann Dark Energy
• CMB power spectra and cosmological parameters from Planck PR4 with CamSpec
• Measuring the Speed of Sound of Quintessence
• DESI 2024: Reconstructing Dark Energy using Crossing Statistics with DESI DR1 BAO data
• Robust Preference for Dynamical Dark Energy in DESI BAO and SN Measurements
• Structure formation in the presence of dark energy perturbations
• Interacting Dark Energy after DESI Baryon Acoustic Oscillation measurements
• The dark degeneracy: On the number and nature of dark components
• Spherical collapse in quintessence models with zero speed of sound
• The dark degeneracy and interacting cosmic components
• DESI DR2 Results I: Baryon Acoustic Oscillations from the Lyman Alpha Forest
• Robustness of dark energy phenomenology across different parameterizations
• Euclid Preparation IV. Impact of undetected galaxies on weak-lensing shear measurements
• The Weak Energy Condition and the Expansion History of the Universe
• Cosmological structure formation with clustering quintessence
• Cosmological Measurements from Angular Power Spectra Analysis of BOSS DR12 Tomography
• Quantifying the $S_8$ tension and evidence for interacting dark energy from redshift-space distortion measurements
• Dark degeneracy I: Dynamical or interacting dark energy?
• Spherical collapse of dark energy with an arbitrary sound speed
• On the Degeneracy Inherent in Observational Determination of the Dark Energy Equation of State
• Using dark energy to suppress power at small scales
• An effective description of dark matter and dark energy in the mildly non-linear regime
• Constraining the dark fluid
• On the implementation of the spherical collapse model for dark energy models
• Skewness as a test for Quartessence
• DESI results: Hint towards coupled dark matter and dark energy
• Limits on Clustering and Smooth Quintessence from the EFTofLSS
• Clustering dark energy and halo abundances
• On dark degeneracy and interacting models
• Non-linear dark energy clustering
• Cosmographic Degeneracy
• A short review on clustering dark energy
• Evolving Dark Energy or Evolving Dark Matter?
• Clustering dark energy imprints on cosmological observables of the gravitational field
• The nonlinear power spectrum in clustering quintessence cosmologies
• Cold dark energy constraints from the abundance of galaxy clusters
• New Scalar Field Quartessence
• Bayesian analysis of a Unified Dark Matter model with transition: can it alleviate the $H_0$ tension?
• Constraining f(R) gravity with cross-correlation of galaxies and cosmic microwave background lensing
• Dissipative Unified Dark Fluid Model
• A PAge-like Unified Dark Fluid Model
• Spherical collapse of non-top-hat profiles in the presence of dark energy with arbitrary sound speed
• Improving CMB constraints on early Universe physics with LSS: A multi-probe forecast including cross-covariance

Cited by in corpus (3)

• Late-time Background Constraints on Linear and Non-linear Interacting Dark Energy after DESI DR2
• Dark Acoustic Oscillations as an Early-Universe Explanation of the DESI Anomaly
• Physical Implications and Updated Observational Constraints of the PAge-like Unified Dark Fluid Model