The
DGP model is a model of gravity proposed by
Gia DvaliGeorgi Dvali is a professor of physics at LMU Munich and a director at the Max Planck Institute for Physics. He received his Ph.D. in high energy physics and cosmology from Tbilisi State University, Georgia in 1992...
,
Gregory GabadadzeGregory Gabadadze is a Georgian theoretical physicist. He is currently member of the Department of Physics at New York University. He is known for the Dvali-Gabadadze-Porrati multidimensional braneworld model of gravity, also known as DGP model....
, and
Massimo PorratiMassimo Porrati is a professor of physics, and a member of the Center for Cosmology and Particle Physics, at New York University. He graduated from the Scuola Normale Superiore di Pisa, Italy with a "Diploma di Scienze" degree in 1985. Later he worked as a postdoctoral fellow at UCLA and UC...
in 2000. The model is popular among some
model buildersIn particle physics, the term model building refers to a construction of new quantum field theories beyond the Standard Model that have certain features making them attractive theoretically or for possible observations in the near future...
, but has resisted being embedded into
string theoryString theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...
.
The model assumes the existence of a 4+1 dimensional
Minkowski spaceIn physics and mathematics, Minkowski space or Minkowski spacetime is the mathematical setting in which Einstein's theory of special relativity is most conveniently formulated...
, within which ordinary 3+1 dimensional Minkowski space is embedded. The model assumes an
actionIn physics, action is an attribute of the dynamics of a physical system. It is a mathematical functional which takes the trajectory, also called path or history, of the system as its argument and has a real number as its result. Action has the dimension of energy × time, and its unit is...
consisting of two terms: One term is the usual
Einstein-Hilbert actionThe Einstein–Hilbert action in general relativity is the action that yields the Einstein's field equations through the principle of least action...
, which involves only the 4-D
spacetimeIn physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...
dimensions. The other term is the equivalent of the Einstein-Hilbert action, as extended to all 5 dimensions. The 4-D term dominates at short distances, and the 5-D term dominates at long distances.
The model was proposed in part in order to reproduce the cosmic acceleration of
dark energyIn physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...
without any need for a small but non-zero
vacuum energyVacuum energy is an underlying background energy that exists in space even when the space is devoid of matter . The concept of vacuum energy has been deduced from the concept of virtual particles, which is itself derived from the energy-time uncertainty principle...
density. But critics argue that this branch of the theory is unstable. However, the theory remains interesting because of Dvali's claim that the unusual structure of the graviton
propagatorIn quantum mechanics and quantum field theory, the propagator gives the probability amplitude for a particle to travel from one place to another in a given time, or to travel with a certain energy and momentum. Propagators are used to represent the contribution of virtual particles on the internal...
makes non-perturbative effects important in a seemingly linear regime, such as the solar system. Because there is no four-dimensional, linearized
effective theoryIn physics, an effective field theory is, as any effective theory, an approximate theory, that includes appropriate degrees of freedom to describe physical phenomena occurring at a chosen length scale, while ignoring substructure and degrees of freedom at shorter distances .-The renormalization...
that reproduces the DGP model for weak-field gravity the theory avoids the vDVZ discontinuity that otherwise plagues attempts to write down a theory of massive gravity.
In 2008, Fang
et al. argued that recent cosmological observations (including measurements of
baryon acoustic oscillationsIn cosmology, baryon acoustic oscillations refers to an overdensity or clustering of baryonic matter at certain length scales due to acoustic waves which propagated in the early universe. In the same way that supernova experiments provide a "standard candle" for astronomical observations, BAO...
by the
Sloan Digital Sky SurveyThe Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...
, and measurements of the cosmic microwave background and type 1a supernovae) is in direct conflict with the DGP cosmology unless a
cosmological constantIn physical cosmology, the cosmological constant was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe...
or some other form of dark energy is added. However, this negates the appeal of the DGP cosmology, which accelerates without needing to add dark energy.