Compare · The Dark Universe

Coupled Quintessence vs Chaplygin Gas

Dark Energy Candidates · within family

	Coupled Quintessence Frontier	Chaplygin Gas Rejected
Proposed	2000	2001 / 2002
Key figures	Luca Amendola	Alexander Kamenshchik, Ugo Moschella, Vincent Pasquier, Maria da Conceicao Bento, Orfeu Bertolami, Anjan Ananda Sen
In one sentence	Coupled quintessence links the dark energy field directly to matter, so the two exchange energy, dark matter in many versions feels a fifth force, and dark energy stays present through the matter era.	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	During matter domination the field keeps a small, nearly constant share of the total energy density, unlike uncoupled quintessence, which quickly becomes negligible. The [[cosmic microwave background]] spectrum tilts at large angles, and its acoustic peaks shift position and change height. In Amendola's original model, matter clumps less on scales of about 8 Mpc/h today than in uncoupled models; the size and sign of the effect depend on the coupling chosen. Dark matter particles attract each other more strongly than gravity alone predicts, through a fifth force set by the coupling.	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	Already in 2000, the data limited the dimensionless coupling to at most 0.1, so any effect on cosmic history must be small. Pettorino notes that the Planck plus Hubble Space Telescope preference comes mostly from the Hubble tension and may reflect systematics in one data set. When Planck is combined with BAO, supernova, and lensing data, interacting dark energy is fully consistent with a [[cosmological constant]] and shows no compelling coupling (Di Valentino et al. 2017).	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 evidence problem: hints of a nonzero coupling come mostly from the Hubble tension, and they weaken when more data sets are combined.	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.
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Coupled Quintessence

2000 · Frontier

Chaplygin Gas

2001 / 2002 · Rejected

Proposed

2000

2001 / 2002

Key figures

Luca Amendola

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

In one sentence

Coupled quintessence links the dark energy field directly to matter, so the two exchange energy, dark matter in many versions feels a fifth force, and dark energy stays present through the matter era.

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

During matter domination the field keeps a small, nearly constant share of the total energy density, unlike uncoupled quintessence, which quickly becomes negligible.
The [[cosmic microwave background]] spectrum tilts at large angles, and its acoustic peaks shift position and change height.
In Amendola's original model, matter clumps less on scales of about 8 Mpc/h today than in uncoupled models; the size and sign of the effect depend on the coupling chosen.
Dark matter particles attract each other more strongly than gravity alone predicts, through a fifth force set by the coupling.

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

Already in 2000, the data limited the dimensionless coupling to at most 0.1, so any effect on cosmic history must be small.
Pettorino notes that the Planck plus Hubble Space Telescope preference comes mostly from the Hubble tension and may reflect systematics in one data set.
When Planck is combined with BAO, supernova, and lensing data, interacting dark energy is fully consistent with a [[cosmological constant]] and shows no compelling coupling (Di Valentino et al. 2017).

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 evidence problem: hints of a nonzero coupling come mostly from the Hubble tension, and they weaken when more data sets are combined.

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