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| Chaplygin gas | |
|---|---|
| Name | Chaplygin gas |
| Equation of state | p = -A/ρ |
| Introduced | 1904 |
| Named after | Sergey Chaplygin |
Chaplygin gas is a theoretical exotic fluid defined by an inverse relationship between pressure and energy density, first introduced in classical aerodynamics and later adopted in cosmology and theoretical physics. The concept links work by Sergey Chaplygin with modern developments involving Vladimir Fock, Lev Landau, Andrei Sakharov, Alexander Friedmann, and researchers studying dark energy, dark matter, inflation, cosmic microwave background, and large-scale structure. Used as a unified candidate for dark matter and dark energy and as an effective description in string theory and brane cosmology, the model has inspired investigations across general relativity, quantum field theory, scalar field theory, k-essence, and modified gravity.
The Chaplygin gas model originates from work by Sergey Chaplygin on aerodynamics and was reformulated in cosmology by authors including Alexander Kamenshchik, Ugo Moschella, Vasco Pasquier, and Nicolás Bilic to address accelerating universe observations associated with Supernova Cosmology Project, High-Z Supernova Search Team, Wilkinson Microwave Anisotropy Probe, and Planck (spacecraft). It posits an exotic equation of state connecting pressure and density and has been compared against alternatives such as ΛCDM model, quintessence, phantom energy, Chaplygin-inspired models, and unified dark sector proposals advanced at conferences like the International Astronomical Union meetings.
The canonical Chaplygin gas equation of state is p = -A/ρ, where A is a positive constant; this relation was generalized into p = -A/ρ^α for 0 < α ≤ 1 in studies by Marco Bento, Osvaldo Bertolami, Nuno Pereira, and others building on Friedmann equations and Robertson–Walker metric solutions. Cosmological dynamics use the Friedmann–Lemaître–Robertson–Walker framework with conservation equations derived from Einstein field equations and energy–momentum tensors similar to those employed by Einstein–Hilbert action formulations, leading to analytic interpolation between matter-dominated behavior matching dust-dominated era solutions and de Sitter-like acceleration akin to de Sitter space. The generalized model parameters are constrained through likelihood analyses utilizing datasets from Type Ia supernovae, baryon acoustic oscillations, cosmic microwave background, and Hubble Space Telescope measurements influenced by methods developed in Bayesian statistics and Markov chain Monte Carlo techniques.
Interpretations connect the Chaplygin gas to effective scalar field descriptions via a Born–Infeld type Lagrangian related to work by Max Born and Leopold Infeld, and to tachyon condensates studied by Ashoke Sen in string theory. The model provides an effective fluid description in brane-world cosmology scenarios influenced by Randall–Sundrum model and D-brane dynamics, and has been invoked in phenomenological studies of the cosmic coincidence problem, comparisons with cold dark matter profiles like Navarro–Frenk–White profile, and tests against data from Sloan Digital Sky Survey and Dark Energy Survey. Applications extend to modeling late-time acceleration phenomena discussed at symposia hosted by Perimeter Institute and published in journals associated with American Physical Society and Institute of Physics.
Cosmological analyses contrast Chaplygin gas scenarios with the concordance ΛCDM model using observations from Planck (spacecraft), WMAP, Supernova Cosmology Project, Baryon Oscillation Spectroscopic Survey, and gravitational lensing surveys including Hubble Space Telescope programs and Subaru Telescope campaigns. Parameter estimation efforts by groups at institutions such as Princeton University, Harvard University, Cambridge University, and Instituto Superior Técnico employ statistical tools from COSMOMC and techniques advanced in Planck Collaboration papers to bound A and α, confronting predictions with constraints from the integrated Sachs–Wolfe effect, galaxy clustering, and matter power spectrum measurements. Many results indicate tensions with pure Chaplygin models relative to ΛCDM fits cited by collaborations like SDSS Collaboration and DES Collaboration.
Analyses of linear perturbations apply techniques from Cosmological perturbation theory developed by Yakov Zel'dovich and Rainer Sachs to assess sound speed, Jeans instability, and growth of density contrasts in Chaplygin cosmologies. Studies by researchers at Universidade de São Paulo, SISSA, Cambridge University, and Institute of Theoretical Physics examine adiabatic and nonadiabatic modes, showing that large sound speeds in some parameter ranges suppress small-scale structure formation compared to expectations from cold dark matter simulations like those of the Millennium Simulation and analytic work by James Peebles. Constraints derive from comparisons with galaxy surveys including 2dF Galaxy Redshift Survey and SDSS analyses and from halo formation studies referencing Press–Schechter formalism.
Generalizations include the generalized Chaplygin gas (GCG) with p = -A/ρ^α, the modified Chaplygin gas combining barotropic and Chaplygin terms explored by Sayan Kar and collaborators, variable A(t) models, and connections to k-essence and tachyonic models inspired by Ashoke Sen and Gabriele Veneziano. Related proposals include bulk viscous fluids investigated by groups at Kavli Institute for Theoretical Physics and varying-speed-of-light frameworks discussed by proponents of alternatives to ΛCDM model. Comparative studies place Chaplygin-based models alongside f(R) gravity, Brans–Dicke theory, and scalar–tensor theories developed by Carl Brans and Robert H. Dicke.
The original mathematical relation appeared in work by Sergey Chaplygin on aerodynamics in the early 20th century, later reintroduced in cosmology by Alexander Kamenshchik, Ugo Moschella, and Vitali Pasquier in the early 2000s. Key contributions include parameter studies by Marco Bento, Osvaldo Bertolami, Nuno Pereira and observational confrontations by teams associated with Planck Collaboration, WMAP, SDSS Collaboration, and DES Collaboration, while theoretical links to string theory and brane cosmology were developed by researchers influenced by Edward Witten, Joseph Polchinski, and Leonard Susskind. Ongoing debates involve groups at CERN, Perimeter Institute, ICTP, and major universities evaluating the model versus competing paradigms such as the ΛCDM model, quintessence, and modified gravity approaches.