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In physics, **gravity** is the name given to a force that attracts masses
together
generally according to the product of their masses and the reciprocal of the
square of the intervening distance (**law of gravitation**).
It was theorized by Isaac Newton
who observed the force drawing objects toward Earth and that drawing
planets toward the Sun and moons toward planets
could all be explained
by a single law, according to his estimates of feasible masses of the
Sun, planets, and moons. Newton posited it as universal, i.e.,
that in other situations masses would affect each other according to
the same law, its effect between everyday objects being negligible
because of their relatively tiny mass.

Albert Einstein recast the theory as space itself being sucked into
each mass (general relativity, GR), calibrating his formula to virtually match
Newton's excepting extreme circumstances,
but with some consequences, such as electromagnetic radiation passing near a massive
object being affected just as a passing object would be.
The term **Newtonian gravity** is used when it is necessary to
distinguish it from GR.

These theories are phenomenally successful: for example, their reliability and precision has made space navigation as we know it possible, and the theorized bending of light has been observed. Yet they have failed to explain some observations:

- Stars in galaxies do not orbit the galaxy in accordance to gravitational theory, given the apparent masses of the galaxies' constituent stars and clouds.
- Gravity would make the universe accelerate inwardly (or given its current expansion, would make that expansion decelerate), but observations suggest it is doing the opposite.

Scientists have sufficient faith in gravitational theory that they cite it to assert galaxies must include matter that has yet to be seen (dark matter), and that there must be an as-yet-unexplained outward force at work in the universe (dark energy). Alternately, attempts have been made to further refine gravitational theory to explain these observations (modified Newtonian dynamics and DGP gravity).

accretion

accretion disk

advection

Alcock-Paczyński effect

atmosphere

atmospheric tide

baryon acoustic oscillations (BAO)

binary star

binding energy

Birkhoff's theorem

black hole (BH)

Castro

cold dark matter (CDM)

Chandrasekhar limit

computational astrophysics

conic section

corotation torque

cosmic string

cosmological constant (Λ)

Coulomb's law

critical density (ρ

curvature

Darcy velocity field

dark flow

dark matter

dark matter annihilation

dark matter halo

DGP gravity

Doppler shift

Effelsberg 100-m Radio Telescope

Einstein-de Sitter model

electron degeneracy

electroweak

escape velocity (V

evaporation

extra-solar planet

star formation feedback

field

free-fall time

galactic halo

galactic tide

galaxy cluster (CL)

geodesic

gravitational instability (GI)

giant molecular cloud (GMC)

general relativity (GR)

gravitational collapse

gravitational constant

gravitational instability model

gravitational lensing

gravitational potential (Φ)

gravity sounding

gravity wave

gravitational wave (GW)

GW detection (GW)

Hale Telescope

helium rain

hypermassive neutron star (HMNS)

homologous collapse

hydrodynamic equations

hydrostatic equilibrium

internal gravity wave

inverse square law

isothermal core

J

Jeans length

Jeans parameter (λ)

JUICE

Juno

Kepler's laws

Kelvin-Helmholtz mechanism

Lagrangian point

Lambda-CDM model (ΛCDM)

Lane-Emden equation

libration

liquid mirror telescope

long-period comet

mass

maximum iron fraction

Maxwell-Boltzmann distribution

mirror support cell

mixing length theory

modified Newtonian dynamics (MOND)

multi-messenger astronomy

multipole expansion

N-body problem

N-body simulation

neutron star

orbital resonance

orbital speed

pointing error (PE)

planet

planetary differentiation

planetesimal

potential energy (PE)

power law

precession

Psyche

redshift (z)

reduced mass

relativity

Richardson number (Ri)

Roche limit

Solar and Heliospheric Observatory (SOHO)

spiral density wave

stellar cluster

stellar core

stellar dynamics

stellar evolution

stellar mass determination

stellar structure

strong-field gravity

strong force

surface gravity

symmetry breaking

tidal capture

tidal force

tide

time dilation

timescale (t)

time standard

Titan

Theory of Everything (TOE)

Toomre Q parameter (Q)

topological defect

virial parameter

virial theorem

wavefront error (WFE)

wormhole