Podcast
Questions and Answers
According to Modified Newtonian Dynamics (MOND), below a certain acceleration threshold, what is the relationship between gravity and distance from a mass?
According to Modified Newtonian Dynamics (MOND), below a certain acceleration threshold, what is the relationship between gravity and distance from a mass?
- Gravity is independent of distance.
- Gravity is inversely proportional to the square of the distance.
- Gravity becomes linearly dependent on distance. (correct)
- Gravity is exponentially related to distance.
Why is the solar system not an ideal environment to test the validity of Newtonian dynamics at accelerations typical of galaxies?
Why is the solar system not an ideal environment to test the validity of Newtonian dynamics at accelerations typical of galaxies?
- The solar system's accelerations are too high. (correct)
- The solar system contains too much dark matter.
- The solar system's dynamics are governed by MOND.
- The solar system is too far away for accurate measurements.
According to the content, what is a minimum achievement of MOND's success in explaining galaxy properties?
According to the content, what is a minimum achievement of MOND's success in explaining galaxy properties?
- It proves the existence of non-baryonic dark matter.
- It accurately predicts the distribution of dark matter halos.
- It exactly describes the functional form of the force in galaxies. (correct)
- It eliminates the need for dark matter entirely.
What is the Tully-Fisher relation, and how does MOND explain it?
What is the Tully-Fisher relation, and how does MOND explain it?
Why does Newtonian dynamics predict different Tully-Fisher relations for high and low surface brightness galaxies, and how does this compare to observations?
Why does Newtonian dynamics predict different Tully-Fisher relations for high and low surface brightness galaxies, and how does this compare to observations?
How does MOND explain the fact that elliptical galaxies follow a 'fundamental plane' in the space defined by luminosity, size, and velocity dispersion?
How does MOND explain the fact that elliptical galaxies follow a 'fundamental plane' in the space defined by luminosity, size, and velocity dispersion?
According to MOND, what causes elliptical galaxies to deviate from the virial relation?
According to MOND, what causes elliptical galaxies to deviate from the virial relation?
Why should no mass discrepancy be found in Ultra-Compact Dwarf galaxies (UCDs) according to MOND?
Why should no mass discrepancy be found in Ultra-Compact Dwarf galaxies (UCDs) according to MOND?
What is the status of MOND's agreement with observations of galaxy clusters?
What is the status of MOND's agreement with observations of galaxy clusters?
What does the content suggest is needed in galaxy clusters, even in the context of MOND, due to strong gravitational lensing?
What does the content suggest is needed in galaxy clusters, even in the context of MOND, due to strong gravitational lensing?
Why does strong gravitational lensing not constitute a test for MOND?
Why does strong gravitational lensing not constitute a test for MOND?
What characterizes the deflection of light at large impact parameters from a point mass, according to MOND?
What characterizes the deflection of light at large impact parameters from a point mass, according to MOND?
According to the content, what do cosmic shear results typically indicate about galaxies, when analyzed without MOND?
According to the content, what do cosmic shear results typically indicate about galaxies, when analyzed without MOND?
What does MOND imply about the relation between black hole mass and bulge luminosity ($M_{BH}$ and $L_{Bulge}$)?
What does MOND imply about the relation between black hole mass and bulge luminosity ($M_{BH}$ and $L_{Bulge}$)?
Why is it challenging to reconcile the $M_{BH}$-$\sigma$ and $M_{BH}$-$L_{Bulge}$ relations in the presence of dark matter?
Why is it challenging to reconcile the $M_{BH}$-$\sigma$ and $M_{BH}$-$L_{Bulge}$ relations in the presence of dark matter?
How does MOND relate to globular clusters?
How does MOND relate to globular clusters?
What does total acceleration state about whether MOND effects are relevant for a given object?
What does total acceleration state about whether MOND effects are relevant for a given object?
According to the content, what is the author's opinion on open clusters?
According to the content, what is the author's opinion on open clusters?
In what area have astronomers decided to stick with dark matter?
In what area have astronomers decided to stick with dark matter?
What is a key area of focus in the study of double stars, globular clusters, and other small structures regarding dark matter?
What is a key area of focus in the study of double stars, globular clusters, and other small structures regarding dark matter?
Flashcards
What is MOND?
What is MOND?
A modification of Newtonian dynamics proposed by M. Milgrom as an alternative to non-baryonic dark matter.
What happens below ao in MOND?
What happens below ao in MOND?
MOND posits a breakdown of Newton's law of gravity (or inertia) below ao, after which the dependence with distance becomes linear.
When are mass discrepancies observed?
When are mass discrepancies observed?
mass discrepancies in stellar systems are observed when and only when the acceleration of gravity falls below a fix valued a₀ = 1.2 × 10⁻⁸ cm s⁻².
MOND acceleration formula
MOND acceleration formula
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The Tully-Fisher relation.
The Tully-Fisher relation.
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Faber-Jackson relation
Faber-Jackson relation
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Ultra-compact dwarfs (UCD)
Ultra-compact dwarfs (UCD)
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Strong lensing and MOND
Strong lensing and MOND
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Galaxy clusters
Galaxy clusters
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Study Notes
Overview of Dark Matter and MOND
- In 1937, astronomer Zwicky proposed the universe contains dark matter after measuring the velocity dispersion of the Coma cluster of galaxies.
- After a century of research, it's understood dark matter is non-baryonic and interacts with normal matter only through gravitation.
- Dark matter effects are seen in stellar systems only when internal gravity acceleration drops below a fixed value of 1.2×10-8 cm s-2.
- The amount of dark matter varies significantly between objects, which contrasts with the idea of a fixed acceleration threshold.
- The systematic discrepancy suggests a possible breakdown of gravity laws at weak field limits, rather than the effects of dark matter.
- Modified Newtonian Dynamics (MOND) is a successful alternative that modifies gravity laws, avoiding the need for dark matter.
- MOND posits Newton's law of gravity breaks down below a certain acceleration (a0), after which dependence with distance becomes linear.
- MOND has resisted falsification attempts and explains properties of numerous objects without non-baryonic dark matter.
- MOND suggests something important about gravity in the weak field limit.
Introduction to MOND
- MOND was proposed in 1983 by M. Milgrom as an alternative to non-baryonic dark matter.
- MOND suggests gravity or inertia doesn't follow Newtonian dynamics for accelerations smaller than 1.2×10-8 cm s-2
- Below this acceleration threshold, gravity's behavior shifts to where a = √aNa0, and aN is the usual Newtonian acceleration.
- The transition from Newtonian to MOND regimes isn't detected within the solar system due to the sun's strong gravitational field.
- There's no way to validate Newtonian dynamics at galaxy-typical acceleration regimes within the solar system
- The validity of Newtonian dynamics below a0 is unconfirmed, and its applicability to galaxies is not guaranteed.
- The MOND idea explains properties of galaxies without needing non-baryonic dark matter
- MOND explains the functional form of force in galaxies, and any galaxy or structure formation theory must reproduce MOND phenomenology.
MOND Basics
- MOND acceleration of gravity is related to Newtonian acceleration by aN = aµ( a /a0 )
- A new constant of physics, a0 = 1.2 × 10-8 cm s-2, is introduced
- The interpolation function µ(a/a0) behaves as µ=1 for strong fields (a>>a0), retrieving the Newtonian expression, and µ=a/a0 for weak fields (a<<a0).
- The weak acceleration limit of gravity is a = √GMa0 / r, with a 1/r dependence on distance.
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