Is MOND Valid in Describing Universal Expansion Beyond 10^26 Metres?

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Discussion Overview

The discussion centers on the validity of Modified Newtonian Dynamics (MOND) in describing universal expansion beyond distances of 10^26 metres. Participants explore the implications of MOND on gravitational forces and the behavior of galaxies in the context of cosmic expansion, touching on both theoretical and conceptual aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant calculates the gravitational acceleration at a radius of 10^26 metres and suggests that if MOND is valid, the acceleration of universal expansion should be slower than expected at these distances.
  • Another participant questions the nature of MOND, seeking clarification on its principles and implications.
  • A participant notes that the deceleration due to gravity at 10^26 metres is comparable to the acceleration attributed to dark energy, proposing a speculative model involving gravitational force carriers with dual spin states that could lead to gravitational repulsion.
  • Further clarification on MOND is requested, with one participant referring to a "numerology version" of the theory.
  • A participant explains MOND as a modification of Newton's law to account for the constant velocities of stars in galaxies, which do not decrease with distance from the galactic center as expected.
  • Another participant proposes an alternative explanation for high orbital velocities, suggesting that they may result from external pressure rather than internal gravitational attraction, raising questions about the stability of the universe and its effects on existing structures.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and interpretation of MOND, with some proposing speculative models and others seeking clarification. No consensus is reached on the validity of MOND in the context of universal expansion or the nature of gravitational forces at large scales.

Contextual Notes

Participants express uncertainty regarding the assumptions underlying MOND and its implications for cosmic expansion. The discussion includes speculative ideas that are not universally accepted or agreed upon.

kurious
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The gravitational force acting on a particle of mass m, on the
surface of a sphere of radius 10^26 metres and with a mass of 10^52
kg is given by
G x10^52 m / (10^26) ^ 2

The acceleration is given by G x10^52 / (10^26) ^ 2 = 10^ - 11 m/ s^2

This is the magnitude of acceleration at which modified Newtonian dynamics
becomes a mathematically accurate description of the velocities of stars in spiral galaxies.MOND describes a gravitational force that is stronger than
the usual Newtonian expectation.As the universe expands beyond 10^26 metres,
if MOND is valid as a force law,then we should expect the acceleration of the expansion of the universe at distances greater than 10^26 metres,to be slower than expected.Does anyone agree with this?
 
Space news on Phys.org
Please clarify what this MOND is.
 
The decceleration due to gravity at 10^26 metres is about 10^-11 m/s^2
using the Newtonian calculation.
This is of a similar order of magnitude to the acceleration
of supernovae due to dark energy at this distance (10^-10 m/s^2).
It is as though gravity has changed signs.This may be a trivial point or it may not.What if the gravitational force carrier comes in particles with two spin states (associated with mass and not charge),one spin positive and the other, of the same magnitude, but negative sign,which exist in a field (could be gravity's own field if force carriers self-interact like gluons), and the high energy spin state is becoming the low energy state, and the low energy state is causing gravitational repulsion (and will cause more repulsion as time goes on and more low energy states form).In a galaxy such as ours, anomlously high star velocities could then be caused by there being fewer low energy repulsive states than expected, and with all the
radiation that a galaxy contains, most low energy spin states could be promoted to high energy states by the radiation.
 
ArmoSkater87 said:
Please clarify what this MOND is.
The numerology version.
 
Modified Newtonian dynamics.
Basically the Newtonian law F =ma can be modified to give a stronger gravitational
force so that anomalously high orbiting velocities of stars in galaxies can be explained.
The velocities should get smaller with increasing distance from the galactic centre
but they don't - they stay constant.
 
What if the source of this high perimeter orbiting velocity is due to external pressure, rather than internal attraction?
We accept that space is expanding, but if the universe is stable, then the only option for the resulting pressure would be to affect existing structures.
 

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