MILGROMIAN DYNAMICS - SCI
& TECH
News: With bad news from Cassini, is
dark matter’s main rival theory dead?
What's in the news?
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One of the biggest mysteries in astrophysics today
is that the forces in galaxies do not seem to add up.
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Galaxies rotate much faster than predicted by applying Newton’s law of
gravity to their visible matter, despite those laws
working well everywhere in the Solar System.
Key takeaways:
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The main postulate of MOND is that gravity starts
behaving differently to what Newton expected when it becomes very weak.
MOND Theory:
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Israeli Physicist Mordehai Milgrom initiated a new
research program in cosmology, called MOND (for MODified Newtonian Dynamics) or
Milgromian dynamics, in 1983.
Idea of the Theory:
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Milgrom proposed a set of postulates in three
papers describing how Newton’s laws of
gravity and motion should be changed in regimes of very low acceleration.
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Milgrom’s postulates were designed to explain the
asymptotic flatness of galaxy rotation curves without the necessity of
postulating the existence of “dark matter”.
Aim:
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MOND seeks to replace
Newtonian dynamics and general relativity to account for the ubiquitous
mass discrepancies in the Universe without invoking dark matter, an inherent
part of the Newtonian Theory of Standard Dynamics.
Limitations of the MOND
Theory:
1. Cassini Mission:
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Due to a quirk of MOND, the gravity from the rest
of our galaxy should have caused Saturn’s orbit to deviate from the Newtonian
expectation subtly, but Cassini did not
find any anomaly of the kind expected in MOND.
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The timing of radio pulses between Earth and
Cassini was tested, which allowed for the precise tracking of Saturn’s orbit.
2. Findings of the
Study:
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The study found out that no matter how the
calculations are tweaked, given how MOND would have to work to fit with models
for galaxy rotation, it cannot fit the
Cassini radio tracking results.
3. Test Provided by Wide
Binary Stars:
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MOND predicted that two stars that orbit a shared
centre several thousand AU apart should orbit around each other 20% faster than
expected with Newton’s laws.
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But a detailed study rules out this prediction,
with the chance of the MOND theory prevailing being the same as a fair coin
landing heads up 190 times in a row.
4. Failure to Explain
Small Bodies in the Distant Outer Solar System:
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Comets coming in from there have a much narrower energy distribution than Mond
predicts.
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These bodies also have orbits that are usually only
slightly inclined to the plane that all the planets orbit close to. Mond would
cause the inclinations to be much larger.
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MOND theory also fails on scales larger than
galaxies and is unable to explain the motions within galaxy clusters.
5. Gravity Anomaly:
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MOND cannot provide enough gravity either, at least
in the central regions of galaxy clusters. However, on their outskirts, MOND
provides too much gravity.
Significance of MOND
Theory:
1. Dynamics of
Individual Galaxies:
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MOND predicts quite accurately the observed
dynamics of individual galaxies of all
types (from dwarf to giant spirals, ellipticals, dwarf spheroidals, etc.),
and of galaxy groups, based only on the
distribution of visible matter (and no dark matter).
2. General Laws of
Galactic Dynamics:
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MOND’s basic tenets predicted the general laws of
galactic dynamics (with some additional, plausible, non-MOND-specific
requirements) are well obeyed by the data, with an appearing in these laws in
different, independent roles.
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MOND has unearthed a number of unsuspected laws of
galactic dynamics, predicting them a priori, and leading to their subsequent
tests and verification with data of ever increasing quality.
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One of these phenomenological laws is the baryonic
Tully-Fisher relation, which is underlain by the MOND mass-asymptotic-speed
relation (MASR)
3. New Tools:
● MOND, as a set of new laws, affords new tools for astronomical measurements, such as of masses and distances of far-away objects in ways not afforded by standard dynamics.