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Related Concept Videos

Gravity between Spherical Bodies01:27

Gravity between Spherical Bodies

Newton's law of gravitation describes the gravitational force between any two point masses. However, for extended spherical objects like the Earth, the Moon, and other planets, the law holds with an assumption that masses of spherical objects are concentrated at their respective centers.
This assumption can be proved easily by showing that the expression for gravitational potential energy between a hollow sphere of mass (M) and a point mass (m) is the same as it would be for a pair of extended...
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Gravitation Between Spherically Symmetric Masses

The gravitational potential energy between two spherically symmetric bodies can be calculated from the masses and the distance between the bodies, assuming that the center of mass is concentrated at the respective centers of the bodies.
The Principle of Superposition and the Gravitational Field01:17

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The principle of superposition applies to gravitational forces of objects that are sufficiently far apart. It states that the net gravitational force on a point object is the vector sum of the gravitational forces on it due to various objects. The principle helps calculate the force by listing the individual forces and then vectorially summing them up. However, it should be noted that the principle of superposition is not always apparent. In the presence of a second force, the first force could...
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The center of gravity of a body is an imaginary point where the body's total weight is assumed to be concentrated, and the body is perfectly balanced. The center of the mass of a body is a point at which the whole of the mass of the body appears to be concentrated. If the acceleration due to gravity, g, has the same value at all points on a body, its center of gravity is identical to its center of mass. The center of gravity of homogeneous bodies such as a sphere, cube, or rectangular plate is...
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Although black holes were theoretically postulated in the 1920s, they remained outside the domain of observational astronomy until the 1970s.
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Related Experiment Video

Updated: May 18, 2026

Reduced-gravity Environment Hardware Demonstrations of a Prototype Miniaturized Flow Cytometer and Companion Microfluidic Mixing Technology
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Testing gravity with the stacked phase space around galaxy clusters.

Tsz Yan Lam1, Takahiro Nishimichi, Fabian Schmidt

  • 1Kavli Institute for the Physics and Mathematics of the Universe, University of Tokyo, Chiba, Japan.

Physical Review Letters
|September 26, 2012
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We present a novel method to measure dark matter’s velocity field around galaxy clusters using galaxy redshifts. This technique, combined with weak lensing, offers a direct test of gravity and significantly refines constraints on modified gravity models.

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Area of Science:

  • Cosmology
  • Astrophysics
  • General Relativity

Background:

  • Dark matter's velocity field in galaxy clusters is linked to the mass profile.
  • Weak lensing is a current method to measure mass profiles.

Purpose of the Study:

  • To introduce a new method for measuring the dark matter velocity field (phase space density).
  • To enable direct tests of gravity using galaxy cluster data.
  • To improve constraints on modified gravity theories.

Main Methods:

  • Stacking galaxy redshifts from spectroscopic samples.
  • Combining velocity field measurements with weak lensing data.
  • Utilizing N-body simulations to validate the method.

Main Results:

  • The proposed method measures the velocity field directly.
  • Combined with lensing, it tests gravity on Mpc scales.
  • N-body simulations indicate significant improvements in constraining modified gravity models.

Conclusions:

  • This technique offers a powerful new tool for cosmology.
  • It promises to significantly advance our understanding of dark matter and gravity.
  • Future surveys will benefit greatly from this method, refining cosmological parameter constraints.