Compare · The Dark Universe

Chaplygin Gas vs K-essence

Dark Energy Candidates · within family

	Chaplygin Gas Rejected	K-essence Frontier
Proposed	2001 / 2002	2000
Key figures	Alexander Kamenshchik, Ugo Moschella, Vincent Pasquier, Maria da Conceicao Bento, Orfeu Bertolami, Anjan Ananda Sen	Cristian Armendariz-Picon, Viatcheslav Mukhanov, Paul Steinhardt
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.	K-essence is a dark energy field driven by an unusual kinetic energy, which in its attractor models switches on when matter starts to dominate and overtakes matter at about the present epoch.
Predictions	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.	The present epoch is the earliest time dark energy can start to dominate while staying consistent with [[Big Bang nucleosynthesis]] limits. Depending on the model, the acceleration either continues forever or lasts only for a finite time. Dark energy fluctuations travel at their own sound speed, near zero in some models, which [[cosmic microwave background]] maps can detect.
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.	Bonvin, Caprini, and Durrer showed in 2006 that k-essence solving the timing puzzle needs faster-than-light fluctuations at some stage. They argue this breaks causality, so k-essence cannot emerge from a consistent high-energy theory. There is no single preferred model. The founding papers give guidelines for building examples, so predictions differ between constructions.
Key unresolved problem	The structure problem: a single fluid that drives the acceleration distorts the growth of cosmic structure, and only the version identical to ΛCDM fits the data.	The causality problem: in the class of k-essence models covered by the 2006 no-go theorem, a field that explains dark energy's timing must at some stage send fluctuations faster than light, and physicists disagree whether that breaks causality.
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Chaplygin Gas

2001 / 2002 · Rejected

K-essence

2000 · Frontier

Proposed

2001 / 2002

2000

Key figures

Alexander Kamenshchik, Ugo Moschella, Vincent Pasquier, Maria da Conceicao Bento, Orfeu Bertolami, Anjan Ananda Sen

Cristian Armendariz-Picon, Viatcheslav Mukhanov, Paul Steinhardt

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.

K-essence is a dark energy field driven by an unusual kinetic energy, which in its attractor models switches on when matter starts to dominate and overtakes matter at about the present epoch.

Predictions

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.

The present epoch is the earliest time dark energy can start to dominate while staying consistent with [[Big Bang nucleosynthesis]] limits.
Depending on the model, the acceleration either continues forever or lasts only for a finite time.
Dark energy fluctuations travel at their own sound speed, near zero in some models, which [[cosmic microwave background]] maps can detect.

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.

Bonvin, Caprini, and Durrer showed in 2006 that k-essence solving the timing puzzle needs faster-than-light fluctuations at some stage. They argue this breaks causality, so k-essence cannot emerge from a consistent high-energy theory.
There is no single preferred model. The founding papers give guidelines for building examples, so predictions differ between constructions.

Key unresolved problem

The structure problem: a single fluid that drives the acceleration distorts the growth of cosmic structure, and only the version identical to ΛCDM fits the data.

The causality problem: in the class of k-essence models covered by the 2006 no-go theorem, a field that explains dark energy's timing must at some stage send fluctuations faster than light, and physicists disagree whether that breaks causality.

Reader vote

No votes yet