Compare · The Dark Universe

DGP Braneworld Gravity vs Galileon Gravity

Large-Distance Modified Gravity · within family

	DGP Braneworld Gravity Rejected	Galileon Gravity Rejected
Proposed	2000	2009
Key figures	Gia Dvali, Gregory Gabadadze, Massimo Porrati	Alberto Nicolis, Riccardo Rattazzi, Enrico Trincherini
In one sentence	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.	Gravity gains an extra field, taken from the DGP braneworld, that can accelerate the expansion while switching itself off near stars and planets.
Predictions	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]]	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
Where it breaks	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.	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.
Key unresolved problem	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.	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.
Reader vote	No votes yet	No votes yet

DGP Braneworld Gravity

2000 · Rejected

Galileon Gravity

2009 · Rejected

Proposed

2000

2009

Key figures

Gia Dvali, Gregory Gabadadze, Massimo Porrati

Alberto Nicolis, Riccardo Rattazzi, Enrico Trincherini

In one sentence

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.

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

Predictions

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]]

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

Where it breaks

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.

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.

Key unresolved problem

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.

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.

Reader vote

No votes yet