Compare · The Dark Universe

K-essence vs Chaplygin Gas

Dark Energy Candidates · within family

	K-essence Frontier	Chaplygin Gas Rejected
Proposed	2000	2001 / 2002
Key figures	Cristian Armendariz-Picon, Viatcheslav Mukhanov, Paul Steinhardt	Alexander Kamenshchik, Ugo Moschella, Vincent Pasquier, Maria da Conceicao Bento, Orfeu Bertolami, Anjan Ananda Sen
In one sentence	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.	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.
Predictions	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.	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.
Where it breaks	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.	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.
Key unresolved problem	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.	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.
Reader vote	No votes yet	No votes yet

K-essence

2000 · Frontier

Chaplygin Gas

2001 / 2002 · Rejected

Proposed

2000

2001 / 2002

Key figures

Cristian Armendariz-Picon, Viatcheslav Mukhanov, Paul Steinhardt

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

In one sentence

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.

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.

Predictions

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.

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.

Where it breaks

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.

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.

Key unresolved problem

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.

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.

Reader vote

No votes yet