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| phantom energy | |
|---|---|
| Name | phantom energy |
| Type | hypothetical cosmological component |
| First proposed | 2002 |
| Key people | Robert R. Caldwell, Sean M. Carroll, Martin J. Rees, Andrei Linde, Paul J. Steinhardt |
| Related | dark energy, cosmological constant, quintessence, Big Rip |
| Governing equation | equation of state p = wρ with w < -1 |
phantom energy is a hypothetical form of dark energy postulated to have an equation of state with parameter w < -1, where w relates pressure p to energy density ρ via p = wρ. It was introduced in the early 2000s in discussions of accelerated expansion following observations by teams led by Adam Riess and Saul Perlmutter and subsequent theoretical work by Robert R. Caldwell and others. Phantom energy, if real, would drive super-accelerated cosmic expansion with distinct consequences for the Friedmann equations, cosmic fate scenarios like the Big Rip, and the behavior of cosmological perturbations.
Phantom energy belongs to speculative proposals addressing the accelerated expansion discovered through measurements of Type Ia supernovae by the High-Z Supernova Search Team and the Supernova Cosmology Project. It contrasts with a positive cosmological constant (Λ) and dynamic quintessence models by violating the dominant energy condition used in classical general relativity and some formulations of quantum field theory. Prominent proponents and critics include Sean M. Carroll, Robert R. Caldwell, Paul Steinhardt, and Andrei Linde, who debated its theoretical plausibility and observational signatures in papers and conferences influenced by results from Wilkinson Microwave Anisotropy Probe and Planck.
Phantom energy arises in extensions of Friedmann–Lemaître–Robertson–Walker cosmology when the equation-of-state parameter w falls below −1. This regime can be modeled by scalar fields with negative kinetic terms, sometimes called ghost fields, a trick invoked in some variants of string theory and in effective descriptions inspired by brane cosmology and k-essence. Attempts to embed phantom behavior into frameworks associated with supergravity, loop quantum gravity, or M-theory face difficulties related to energy conditions championed in theorems by Stephen Hawking and Roger Penrose. Debate around consistency has involved researchers such as Nick Kaiser and Juan Maldacena in the context of energy bounds and holographic arguments tied to AdS/CFT correspondence.
If phantom energy dominated the universe, the expansion rate would accelerate such that the scale factor could diverge in finite proper time, producing a Big Rip singularity discussed by Robert R. Caldwell and colleagues. Intermediate signatures include alterations to the growth of large-scale structure traced by surveys like Sloan Digital Sky Survey and Dark Energy Survey, changes in the integrated Sachs–Wolfe effect measured by Planck and WMAP, and distinctive evolution of baryon acoustic oscillations probed by projects such as BOSS and eBOSS. The fate of bound systems — galaxies, solar systems, and atomic structures — was analyzed in scenarios by authors citing tidal disruption timescales tied to the phantom equation of state.
Mathematical descriptions use modified Friedmann equations with ρ + p < 0, leading to super-exponential solutions for the scale factor a(t). Common parameterizations include constant-w models and time-dependent forms like w(a) or w(z) used in analyses by teams associated with Supernova Legacy Survey and Pan-STARRS. Field-theoretic realizations employ Lagrangians with negative kinetic terms or noncanonical kinetic functions as in k-essence; alternative constructions introduce interaction terms between phantom components and dark matter inspired by models from V. Sahni and A. Starobinsky. Formal work often references stability criteria derived from perturbation theory in the context of the Cosmic Microwave Background anisotropy calculations performed by groups linked to CAMB and CLASS codes.
Observationally, constraints on w arise from combined probes: Type Ia supernovae luminosity distances, Cosmic Microwave Background angular power spectra from Planck and Wilkinson Microwave Anisotropy Probe, baryon acoustic oscillations from Baryon Oscillation Spectroscopic Survey, and large-scale structure measurements by 2dFGRS and Sloan Digital Sky Survey. Current best-fit analyses by consortia including Planck Collaboration and teams led by Adam Riess generally favor w ≈ −1 with uncertainties that allow mild phantom regions but do not require w < −1. Dedicated studies by Supernova Cosmology Project and High-Z Supernova Search Team set limits that increasingly disfavor extreme phantom behavior.
Phantom models often violate energy conditions, generating classical instabilities and quantum pathologies such as vacuum decay, ghost-induced particle production, and negative-norm states. Attempts at curing these problems include higher-derivative operators, nonlocal actions inspired by string field theory, and UV completions invoking supersymmetry or modified gravity frameworks like f(R) gravity and Horndeski theory. Critics point to arguments by Juan Maldacena and others showing tension with unitary evolution or positivity constraints derived in scattering-amplitude analyses leveraged by researchers at institutions such as CERN and Perimeter Institute.
Alternatives addressing late-time acceleration without phantom behavior include a true cosmological constant (Λ), dynamic quintessence fields, k-essence, interacting dark matter–dark energy models, and modified gravity proposals such as f(R) gravity, DGP model, and Horndeski theory. Related speculative concepts invoked in literature encompass the Big Rip, Little Rip, and transient phantom crossings in parametrizations by groups led by Eric Linder and Robert Scherrer, as well as cyclic and ekpyrotic ideas promoted by Paul Steinhardt and collaborators.