Compare · The Dark Universe

Galileon Gravity vs DGP Braneworld Gravity

Large-Distance Modified Gravity · within family

	Galileon Gravity Rejected	DGP Braneworld Gravity Rejected
Proposed	2009	2000
Key figures	Alberto Nicolis, Riccardo Rattazzi, Enrico Trincherini	Gia Dvali, Gregory Gabadadze, Massimo Porrati
In one sentence	Gravity gains an extra field, taken from the DGP braneworld, that can accelerate the expansion while switching itself off near stars and planets.	Our universe is a brane in a five-dimensional space, and at distances near the size of the observable universe gravity behaves five-dimensionally; one solution accelerates the expansion but is unstable.
Predictions	Late-time acceleration driven by the Galileon field, with no [[cosmological constant]] In the cubic version, gravitational potentials deepen at late times, which flips the sign of the correlation between [[cosmic microwave background]] temperature and nearby galaxies In the quartic and quintic versions, gravitational waves travel at a speed different from light at low redshift The extra force is screened inside the solar system and other gravitationally bound systems	Below a crossover distance, gravity on the brane follows the ordinary four-dimensional inverse-square law On the self-accelerating branch, the expansion accelerates at late times without a [[cosmological constant]] Perturbations that leak into the [[bulk]] change how structure grows, adding extra [[cosmic microwave background]] anisotropy on the largest angular scales Near massive bodies, an extra mode that bends the brane is screened by the Vainshtein effect, restoring the predictions of [[general relativity]]
Where it breaks	Once coupled to curved spacetime, the covariant Galileon keeps the Galileon interactions but breaks the exact galilean symmetry that defined the original model. Fits need a non-zero neutrino mass to match the data reasonably, around 0.5 eV in Renk and colleagues' analysis. Peirone and colleagues (2018) compared Bayesian evidence with ΛCDM and found covariant Galileon models strongly disfavoured by cosmological data.	Luty, Porrati and Rattazzi (2003) found that with the crossover near the Hubble length, the theory becomes strongly coupled below about 1000 km. The self-accelerating branch contains a ghost, a mode with negative energy that makes spacetime unstable. The ghost-free normal branch only accelerates with brane tension or a cosmological constant, which brings dark energy back.
Key unresolved problem	The speed-of-gravity problem: the quartic and quintic terms that let the Galileon fit cosmological data also change the speed of gravitational waves, and the cubic version alone fails the ISW test.	The ghost problem: the branch that accelerates without dark energy carries a negative-energy mode, and the ghost-free branch needs a cosmological constant to accelerate at all.
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Galileon Gravity

2009 · Rejected

DGP Braneworld Gravity

2000 · Rejected

Proposed

2009

2000

Key figures

Alberto Nicolis, Riccardo Rattazzi, Enrico Trincherini

Gia Dvali, Gregory Gabadadze, Massimo Porrati

In one sentence

Gravity gains an extra field, taken from the DGP braneworld, that can accelerate the expansion while switching itself off near stars and planets.

Our universe is a brane in a five-dimensional space, and at distances near the size of the observable universe gravity behaves five-dimensionally; one solution accelerates the expansion but is unstable.

Predictions

Late-time acceleration driven by the Galileon field, with no [[cosmological constant]]
In the cubic version, gravitational potentials deepen at late times, which flips the sign of the correlation between [[cosmic microwave background]] temperature and nearby galaxies
In the quartic and quintic versions, gravitational waves travel at a speed different from light at low redshift
The extra force is screened inside the solar system and other gravitationally bound systems

Below a crossover distance, gravity on the brane follows the ordinary four-dimensional inverse-square law
On the self-accelerating branch, the expansion accelerates at late times without a [[cosmological constant]]
Perturbations that leak into the [[bulk]] change how structure grows, adding extra [[cosmic microwave background]] anisotropy on the largest angular scales
Near massive bodies, an extra mode that bends the brane is screened by the Vainshtein effect, restoring the predictions of [[general relativity]]

Where it breaks

Once coupled to curved spacetime, the covariant Galileon keeps the Galileon interactions but breaks the exact galilean symmetry that defined the original model.
Fits need a non-zero neutrino mass to match the data reasonably, around 0.5 eV in Renk and colleagues' analysis.
Peirone and colleagues (2018) compared Bayesian evidence with ΛCDM and found covariant Galileon models strongly disfavoured by cosmological data.

Luty, Porrati and Rattazzi (2003) found that with the crossover near the Hubble length, the theory becomes strongly coupled below about 1000 km.
The self-accelerating branch contains a ghost, a mode with negative energy that makes spacetime unstable.
The ghost-free normal branch only accelerates with brane tension or a cosmological constant, which brings dark energy back.

Key unresolved problem

The speed-of-gravity problem: the quartic and quintic terms that let the Galileon fit cosmological data also change the speed of gravitational waves, and the cubic version alone fails the ISW test.

The ghost problem: the branch that accelerates without dark energy carries a negative-energy mode, and the ghost-free branch needs a cosmological constant to accelerate at all.

Reader vote

No votes yet