Scalar–tensor–vector gravity

Scalar–tensor–vector gravity (STVG)^[1] is a modified theory of gravity developed by John Moffat, a researcher at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario. The theory is also often referred to by the acronym MOG (MOdified Gravity).

Overview[]

Scalar–tensor–vector gravity theory,^[2] also known as MOdified Gravity (MOG), is based on an action principle and postulates the existence of a vector field, while elevating the three constants of the theory to scalar fields. In the weak-field approximation, STVG produces a Yukawa-like modification of the gravitational force due to a point source. Intuitively, this result can be described as follows: far from a source gravity is stronger than the Newtonian prediction, but at shorter distances, it is counteracted by a repulsive fifth force due to the vector field.

STVG has been used successfully to explain galaxy rotation curves,^[3] the mass profiles of galaxy clusters,^[4] gravitational lensing in the Bullet Cluster,^[5] and cosmological observations^[6] without the need for dark matter. On a smaller scale, in the Solar System, STVG predicts no observable deviation from general relativity.^[7] The theory may also offer an explanation for the origin of inertia.^[8]

Mathematical details[]

STVG is formulated using the action principle. In the following discussion, a metric signature of ${\displaystyle [+,-,-,-]}$ will be used; the speed of light is set to ${\displaystyle c=1}$, and we are using the following definition for the Ricci tensor:

${\displaystyle R_{\mu \nu }=\partial _{\alpha }\Gamma _{\mu \nu }^{\alpha }-\partial _{\nu }\Gamma _{\mu \alpha }^{\alpha }+\Gamma _{\mu \nu }^{\alpha }\Gamma _{\alpha \beta }^{\beta }-\Gamma _{\mu \beta }^{\alpha }\Gamma _{\alpha \nu }^{\beta }.}$

We begin with the Einstein–Hilbert Lagrangian:

${\displaystyle {\mathcal {L}}_{G}=-{\frac {1}{16\pi G}}(R+2\Lambda ){\sqrt {-g}},}$

where ${\displaystyle R}$ is the trace of the Ricci tensor, ${\displaystyle G}$ is the gravitational constant, ${\displaystyle g}$ is the determinant of the metric tensor ${\displaystyle g_{\mu \nu }}$, while ${\displaystyle \Lambda }$ is the cosmological constant.

We introduce the Maxwell-Proca Lagrangian for the STVG vector field ${\displaystyle \phi _{\mu }}$:

${\displaystyle {\mathcal {L}}_{\phi }=-{\frac {1}{4\pi }}\omega \left[{\frac {1}{4}}B^{\mu \nu }B_{\mu \nu }-{\frac {1}{2}}\mu ^{2}\phi _{\mu }\phi ^{\mu }+V_{\phi }(\phi )\right]{\sqrt {-g}},}$

where ${\displaystyle B_{\mu \nu }=\partial _{\mu }\phi _{\nu }-\partial _{\nu }\phi _{\mu },\mu }$ is the mass of the vector field, ${\displaystyle \omega }$ characterizes the strength of the coupling between the fifth force and matter, and ${\displaystyle V_{\phi }}$ is a self-interaction potential.

The three constants of the theory, ${\displaystyle G,\mu ,}$ and ${\displaystyle \omega ,}$ are promoted to scalar fields by introducing associated kinetic and potential terms in the Lagrangian density:

${\displaystyle {\mathcal {L}}_{S}=-{\frac {1}{G}}\left[{\frac {1}{2}}g^{\mu \nu }\left({\frac {\nabla _{\mu }G\nabla _{\nu }G}{G^{2}}}+{\frac {\nabla _{\mu }\mu \nabla _{\nu }\mu }{\mu ^{2}}}-\nabla _{\mu }\omega \nabla _{\nu }\omega \right)+{\frac {V_{G}(G)}{G^{2}}}+{\frac {V_{\mu }(\mu )}{\mu ^{2}}}+V_{\omega }(\omega )\right]{\sqrt {-g}},}$

where ${\displaystyle \nabla _{\mu }}$ denotes covariant differentiation with respect to the metric ${\displaystyle g_{\mu \nu },}$ while ${\displaystyle V_{G},V_{\mu },}$ and ${\displaystyle V_{\omega }}$ are the self-interaction potentials associated with the scalar fields.

The STVG action integral takes the form

${\displaystyle S=\int {({\mathcal {L}}_{G}+{\mathcal {L}}_{\phi }+{\mathcal {L}}_{S}+{\mathcal {L}}_{M})}~d^{4}x,}$

where ${\displaystyle {\mathcal {L}}_{M}}$ is the ordinary matter Lagrangian density.

Spherically symmetric, static vacuum solution[]

The field equations of STVG can be developed from the action integral using the variational principle. First a test particle Lagrangian is postulated in the form

${\displaystyle {\mathcal {L}}_{\mathrm {TP} }=-m+\alpha \omega q_{5}\phi _{\mu }u^{\mu },}$

where ${\displaystyle m}$ is the test particle mass, ${\displaystyle \alpha }$ is a factor representing the nonlinearity of the theory, ${\displaystyle q_{5}}$ is the test particle's fifth-force charge, and ${\displaystyle u^{\mu }=dx^{\mu }/ds}$ is its four-velocity. Assuming that the fifth-force charge is proportional to mass, i.e., ${\displaystyle q_{5}=\kappa m,}$ the value of ${\displaystyle \kappa ={\sqrt {G_{N}/\omega }}}$ is determined and the following equation of motion is obtained in the spherically symmetric, static gravitational field of a point mass of mass ${\displaystyle M}$:

${\displaystyle {\ddot {r}}=-{\frac {G_{N}M}{r^{2}}}\left[1+\alpha -\alpha (1+\mu r)e^{-\mu r}\right],}$

where ${\displaystyle G_{N}}$ is Newton's constant of gravitation. Further study of the field equations allows a determination of ${\displaystyle \alpha }$ and ${\displaystyle \mu }$ for a point gravitational source of mass ${\displaystyle M}$ in the form^[9]

${\displaystyle \mu ={\frac {D}{\sqrt {M}}},}$

${\displaystyle \alpha ={\frac {G_{\infty }-G_{N}}{G_{N}}}{\frac {M}{({\sqrt {M}}+E)^{2}}},}$

where ${\displaystyle G_{\infty }\simeq 20G_{N}}$ is determined from cosmological observations, while for the constants ${\displaystyle D}$ and ${\displaystyle E}$ galaxy rotation curves yield the following values:

${\displaystyle D\simeq 6250M_{\odot }^{1/2}\mathrm {kpc} ^{-1},}$

${\displaystyle E\simeq 25000M_{\odot }^{1/2},}$

where ${\displaystyle M_{\odot }}$ is the mass of the Sun. These results form the basis of a series of calculations that are used to confront the theory with observation.

Agreement with observations[]

STVG/MOG has been applied successfully to a range of astronomical, astrophysical, and cosmological phenomena.

On the scale of the Solar System, the theory predicts no deviation^[7] from the results of Newton and Einstein. This is also true for star clusters containing no more than a few million solar masses.^{[citation needed]}

The theory accounts for the rotation curves of spiral galaxies,^[3] correctly reproducing the Tully-Fisher law.^[9]

STVG is in good agreement with the mass profiles of galaxy clusters.^[4]

STVG can also account for key cosmological observations, including:^[6]

• The acoustic peaks in the cosmic microwave background radiation;
• The accelerating expansion of the universe that is apparent from type Ia supernova observations;
• The matter power spectrum of the universe that is observed in the form of galaxy-galaxy correlations.

Problems and criticism[]

This section is empty. The article reads as if STVG has explained all observations there are, without postulating dark matter. This is unlikely the case. As any theory, it must have issues, and should have received at least some critique within the physics community. Such an imbalance in the article does the theory a disservice: most uncriticised research is "not even wrong," and STVG is indeed far from it. Wikipedia prefers reliable secondary sources. Since modified-gravity theories are currently far from the theoretical mainstream, primary academic sources also qualify. Currently this section has neither. You can help by . (September 2021)

See also[]

• Modified Newtonian dynamics
• Nonsymmetric gravitational theory
• Tensor–vector–scalar gravity

References[]