Milgromian dynamics or MOND

07.06.2024

Milgromian dynamics or MOND

Why in the news?                                                                                                                                                                                                                                     

Recent research challenges MOND, a theory alternative to dark matter, with tests like the Cassini mission and observations of wide binary stars strongly favouring traditional Newtonian gravity and dark matter.

About MOND:

• Galaxies rotate much faster than predicted by applying Newton’s law of gravity to their visible matter.
• To explain this, some additional gravity is needed. This is why the idea of dark matter was first proposed.
• Milgromian dynamics, also known as Modified Newtonian Dynamics (MOND), is an alternative theory to explain the dynamics of galaxies without invoking dark matter.
• It was proposed by Mordehai Milgrom in 1983.
• It suggests that Newtonian physics breaks down at very low accelerations (as seen at the edges of galaxies) altering gravitational behaviour without the need for dark matter.

Why has this theory failed?

• MOND predicts noticeable gravitational effects at small scales, but tests involving Saturn's orbit during the Cassini mission showed no anomalies, supporting Newtonian gravity over MOND in the solar system.
• MOND theory has also faced failures in its inability to explain galaxy clusters, wide binary stars and cosmic microwave background radiation, as well as its lack of a relativistic framework.

MOND Theory challenged by Cassini mission

• The Cassini mission, which orbited Saturn from 2004 to 2017, provided an opportunity to test MOND.
• Saturn orbits the Sun at 10 AU, and MOND predicts subtle deviations in Saturn's orbit due to the galaxy's gravity.
• Cassini measured the Earth-Saturn distance using radio pulses, but found no anomalies expected by MOND.
• Newton’s laws still hold for Saturn, putting the MOND theory in trouble.

Further evidence against MOND

• Further evidence against MOND comes from wide binary stars.
• A study found that MOND's prediction of faster orbits was incorrect, with a probability of being right equivalent to flipping heads 190 times in a row.
• Additionally, MOND fails to explain the narrow energy distribution and orbital inclinations of comets in the outer Solar System.
• Newtonian gravity is preferred over MOND for distances below a light year.
• MOND also fails at larger scales, like galaxy clusters, where it provides too little gravity centrally and too much at the outskirts. Newtonian gravity with dark matter fits the data better.

What is dark matter?

• It is a hypothetical form of matter that does not interact with light or the electromagnetic field, but its presence is inferred by its gravitational effects on visible matter.
• It emits no light or energy and thus cannot be detected by conventional sensors and detectors.

                                                            Source: The Hindu

Ques :- The term “Milgromian dynamics” best describes which one of the following?

A.It is a theory related to forming planetary systems from protoplanetary disks

B.It describes the behavior of subatomic particles in high-energy physics

C.It is an alternative theory to explain the dynamics of galaxies without invoking dark matter

D.It explains the thermodynamic properties of black holes in the context of string theory

Answer C