One strange fluid was meant to be both dark matter and dark energy.
Chaplygin Gas
One exotic fluid that acts like dark matter early on and like a cosmological constant later, a unification that structure-formation data ruled out.
Placeholder for a 3D visualisation of Dark Energy Candidates. The interactive scene will land in Phase 3. Roughly 69% of the universe's energy density drives accelerating expansion. Plain ΛCDM models it as a cosmological constant with equation of state w fixed at -1. This family asks what dark energy is as a physical substance, not how to parameterize it. The leading physical candidate is quintessence, a dynamical scalar field with w greater than -1 that evolves over cosmic time; phantom energy is its mirror image with w less than -1. Other candidates change the field's kinetic energy (k-essence), couple it to matter (coupled quintessence), tie its density to the cosmic horizon (holographic dark energy), or merge it with dark matter into one fluid (the Chaplygin gas, ruled out in its unified form). DESI's 2024 hint of evolving dark energy (w0 greater than -1, wa less than 0) is exactly the signature a physical field would leave, which is why these models are an active frontier rather than a settled question.
§1 · The claim, in one sentence
The Chaplygin gas was proposed as a single exotic fluid that behaves like matter early on and like a cosmological constant later, replacing both dark matter and dark energy.
Why this was rejected
- ▸Sandvik, Tegmark, Zaldarriaga, and Waga (2004) showed that unified dark matter models produce oscillations or exponential blowup in the matter power spectrum, inconsistent with observation. In their analysis of the unified generalized Chaplygin gas, 99.999% of the previously allowed parameter space was excluded, leaving essentially only the ΛCDM limit. The failure lies in how perturbations grow, not in the expansion history.
- ▸Amendola, Finelli, Burigana, and Carturan (2003) ruled out the original Chaplygin gas (alpha = 1) as dark energy at more than the 99.99% confidence level, using WMAP and type Ia supernova data.
§2 · Why it might be true
Kamenshchik, Moschella, and Pasquier proposed in 2001 that the universe is filled with an exotic fluid called a Chaplygin gas. Its pressure is negative, and it weakens as the fluid becomes denser.
That one rule gave the gas two regimes. While the universe was dense, the fluid behaved like dark matter. As expansion thinned it out, the fluid behaved more and more like a cosmological constant.
Bento, Bertolami, and Sen generalized the rule in 2002 with an extra exponent, motivated by the dynamics of a generalized brane. Its appeal was that a single component could replace both dark matter and dark energy.
The unified version failed once its effect on the growth of cosmic structure was computed. It survives only in a limit that cannot be told apart from ΛCDM.
The family stance
Dark energy may not be a constant. If its equation of state differs from -1, or changes with time, a cosmological constant cannot be the whole story and a dynamical component, most naturally a scalar field, is required. The case that the universe accelerates is overwhelming; the case for any one physical model of what drives it is not.
§2.5 · Evidence
- The 2001 model's expansion history agreed with the observations of cosmic acceleration available at the time.
- The generalized equation of state emerges from the dynamics of a generalized d-brane, giving the fluid a motivation from higher-dimensional physics.
- Bilic, Tupper, and Viollier extended the model to large perturbations in 2002 and placed the unification in a geometric setting reminiscent of M-theory.
§3 · What you'd need to test it
- A single fluid that behaves like non-relativistic matter at early times and like a cosmological constant at late times, with an intermediate phase between.
- An effective cosmological constant whose value increases over time, as the 2001 paper states.
- No separate cold dark matter, since the same fluid must both seed structure and drive the acceleration.
- Oscillations or exponential growth in the matter power spectrum, the signature that later ruled the unified model out.
§4 · Where it breaks
- With dark matter kept separate, WMAP and supernova data force the generalized gas close to a cosmological constant, with its exponent below 0.2 (Amendola et al. 2003).
- As a unified model of dark matter and dark energy, only the limit that matches ΛCDM survives the data, so the surviving version makes no distinct prediction.
Go deeper
The original Chaplygin gas has equation of state p = -A/rho with A positive. Bento, Bertolami, and Sen (2002) generalized it to p = -A/rho^alpha with 0 < alpha <= 1, derived from a generalized d-brane in a (d+1, 1) spacetime. The fluid evolves from a phase dominated by non-relativistic matter to one dominated by a cosmological constant, through an intermediate period with effective equation of state p = alpha rho.
The decisive test came from perturbations. Sandvik et al. (2004), in a paper titled 'The end of unified dark matter?', found the predicted oscillations or blowup in the matter power spectrum inconsistent with observation. Earlier, Beca et al. (2003) had argued that analyses of unified models must include baryons separately, which they found lets the model behave differently from ΛCDM.
The Chaplygin gas connects to two other variants. Chimento (2004) found k-essence models that generate families of extended tachyon fields and Chaplygin gases (see the K-essence sibling). Kamenshchik, Moschella, and Pasquier, with Gorini, later studied a tachyon-field cosmology whose evolution can end in a Big Brake, a singularity with infinite deceleration. The Big Brake variant in the Exotic Future Singularities family (Chapter 7) covers it.
▸§5 · Who built it, and when(8 sources, 8 established)
- EstablishedKamenshchik et al. (2001). 'An Alternative to quintessence.' Phys. Lett. B 511, 265
- EstablishedBento et al. (2002). 'Generalized Chaplygin gas, accelerated expansion and dark energy matter unification.' Phys. Rev. D 66, 043507
- EstablishedBilic et al. (2002). 'Unification of dark matter and dark energy: The Inhomogeneous Chaplygin gas.' Phys. Lett. B 535, 17
- EstablishedAmendola et al. (2003). 'WMAP and the generalized Chaplygin gas.' JCAP 07, 005
- EstablishedBeca et al. (2003). 'The Role of baryons in unified dark matter models.' Phys. Rev. D 67, 101301
- EstablishedSandvik et al. (2004). 'The end of unified dark matter?' Phys. Rev. D 69, 123524
- EstablishedChimento (2004). 'Extended tachyon field, Chaplygin gas and solvable k-essence cosmologies.' Phys. Rev. D 69, 123517
- EstablishedGorini et al. (2004). 'Tachyons, scalar fields and cosmology.' Phys. Rev. D 69, 123512359 citations
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