Current Affairs 6th June 2024 for UPSC Prelims Exam

Milgromian Dynamics/Modified Newtonian Dynamics (MOND)

Context

• Astrophysicists face a puzzling mystery: galaxies rotate much faster than predicted by Newton’s laws of gravity.
• To explain this discrepancy, the concept of dark matter, an invisible substance, was proposed. However, despite decades of searching, dark matter remains elusive.
• An alternative explanation, Modified Newtonian Dynamics (MOND), suggests that Newton’s laws break down at low accelerations.
• Recent research, however, revealed significant challenges to the MOND theory.

Milgromian Dynamics/Modified Newtonian Dynamics (MOND) Theory

• It is a theoretical framework proposed by physicist Mordehai Milgrom in 1983 as an alternative to the standard Newtonian dynamics and general relativity.
• MOND suggests that Newton’s laws of gravity do not hold at very low accelerations, below a certain threshold.
• According to Newtonian mechanics, the force of gravity between two masses decreases with the square of the distance between them.
• However, observations of galaxy rotation speeds show that stars in the outer regions of galaxies move at similar speeds to those near the centre, contrary to what would be expected if only visible matter were exerting gravitational force.
• Predictive Success:
  • MOND has been successful in explaining the flat rotation curves of many spiral galaxies, where the speed of stars at varying distances from the galactic centre remains approximately constant rather than decreasing as expected in a purely Newtonian framework.
  • It also provides a good fit for the dynamics of some dwarf spheroidal galaxies without requiring additional unseen mass.

Challenges to MOND on Small Scales

• Cassini Mission: The Cassini spacecraft, which orbited Saturn, provided data to test MOND by measuring deviations in Saturn’s orbit due to galactic gravitational effects predicted by MOND.
  • MOND predicts subtle deviations in Saturn’s orbit at 10 AU from the Sun due to the gravitational influence of the galaxy.
  • No such deviations were found in Saturn’s orbit as predicted by MOND. This strongly supports the validity of Newton’s laws in the outer Solar System.
• Wide Binary Stars: The orbital speeds of wide binary stars, pairs of stars orbiting each other at large distances, do not exhibit the 20% faster orbits predicted by MOND. This casts further doubt on the theory’s accuracy on small scales.
• Outer Solar System Bodies: Observations of comets and other small bodies in the outer Solar System show distributions of energy and orbital inclinations that do not match MOND’s predictions.

Challenges to MOND on Large Scales

Galaxy Clusters: MOND struggles to explain the motions within galaxy clusters.

• While it fails to provide enough gravity in the central regions, it overestimates gravity in the outskirts.
• Dark matter, with five times the mass of normal matter, offers a better fit to the observed data.

Conclusions

• MOND, while successful in explaining galaxy rotation without dark matter, faces significant challenges on both small and large scales.
• Evidence from the Cassini mission, wide binary stars, and outer Solar System bodies strongly favours Newtonian gravity over MOND on scales smaller than a light year.
• The behaviour of galaxy clusters further weakens MOND’s credibility, highlighting the need for dark matter or modified gravity on very large scales.
• The standard dark matter model, while not perfect, remains the most plausible explanation for the observed discrepancies in galactic rotation and cluster dynamics.
• Future research may reveal the true nature of dark matter or uncover new gravitational phenomena on large scales, but MOND, as currently formulated, seems to be an unlikely alternative.

Examples, Case Studies and Data

• India’s Climate Finance Needs and Challenges (GS 3): India’s climate finance needs are substantial, with estimates suggesting a requirement of $160-288 billion (Rs 12.6-22.63 trillion) annually until 2030 to meet its nationally determined contributions for climate action.
  • In 2023, India received less than $8 billion in climate finance from official external sources, primarily from multilateral development banks.
  • Fiscal Constraints: There is limited fiscal space available, with the debt-to-GDP ratio increasing from 70.4% in the fiscal year 2018-19 to 82% in 2023-24, largely due to extensive pandemic-related expenditures.
    • The tax-to-GDP ratio has remained relatively static, fluctuating between 16% and 17.5% over the past twelve years, showing no significant improvement.
  • Need for Alternative Financing Solutions: Given the fiscal constraints, there is a pressing need to explore alternative options for raising resources to fulfill climate finance requirements.

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