Testing MOND with Cassini: Challenges to Dark Matter's Rival Theory

What’s in today’s article?

• Why in News?
• What is Standard Model of Particle Physics?
• What is Dark Matter?
• MOND Theory: as an alternative to the dark matter

Why in News?

The findings of Cassini mission, which orbited Saturn from 2004 to 2017, provided an opportunity to test Milgromian dynamics (MOND) - an alternative theory to the dark matter.

What is Standard Model of Particle Physics?

• About
  • The Standard Model of particle physics is like a blueprint that explains what everything in the universe is made of and how these things interact.
• Basic Building Blocks
  • Particles: Everything around us is made up of tiny particles. The Standard Model identifies the fundamental particles, which are the smallest building blocks of matter.
  • Quarks and Leptons: There are two main families of particles:
  • Quarks: These combine to form protons and neutrons, which are found in the nucleus of an atom.
  • Leptons: The most famous lepton is the electron, which orbits the nucleus of an atom.
• Forces
  • Fundamental Forces: The Standard Model also explains how particles interact through four fundamental forces:
  • Electromagnetic Force: This force is responsible for electricity, magnetism, and light. It’s carried by particles called photons.
  • Weak Nuclear Force: This force is responsible for some types of radioactive decay and is carried by particles called W and Z bosons.
  • Strong Nuclear Force: This force holds the nucleus of an atom together, binding protons and neutrons. It’s carried by particles called gluons.
  • Gravity: While gravity is a fundamental force, it's not fully explained by the Standard Model. It's described separately by the theory of general relativity.
• Interactions
  • How They Work: Particles interact with each other by exchanging force-carrying particles. For example, when two electrons repel each other, they exchange photons.
• Higgs Boson
  • Mass: The Standard Model also includes the Higgs boson, a particle that gives other particles their mass through the Higgs field.
• Challenges
  • The Standard Model is incredibly successful in explaining the known particles and their interactions, but it doesn't include everything.
  • For example, the Higgs boson gives mass to quarks, charged leptons (like electrons), and the W and Z bosons. 
  • However, it is not yet known whether the Higgs boson also gives mass to neutrinos – ghostly particles that interact very rarely with other matter in the universe. 
  • Also, physicists understand that about 95 percent of the universe is not made of ordinary matter as we know it. 
  • Instead, much of the universe consists of dark matter and dark energy that do not fit into the Standard Model.

What is Dark Matter?

• Like ordinary matter, dark matter takes up space and holds mass. But it doesn’t reflect, absorb, or radiate light – at least not enough for us to detect yet.
• While scientists have measured that dark matter makes up about 27% of the cosmos, they’re not sure what it is.
  • Scientists believe there is something called dark matter that forms a huge, web-like structure throughout the universe. 
  • This structure acts like a gravitational framework, pulling most of the regular matter in the cosmos towards it. 
  • They’ve determined that dark matter isn’t composed of known particles of matter. The search for what makes up dark matter continues.

MOND Theory: as an alternative to the dark matter

• Background: Origin of MOND Theory
  • One of astrophysics' biggest mysteries is why galaxies rotate faster than Newton’s law of gravity predicts based on their visible matter. 
  • To explain this, the concept of dark matter was proposed. 
  • However, dark matter has never been observed directly, and it doesn't fit within the Standard Model of particle physics.
  • In order address this anomaly, an alternative theory known as Milgromian dynamics (MOND) was proposed by Israeli physicist Mordehai Milgrom in 1982.
• MOND Theory
  • MOND theory suggests that Newton's laws break down under very weak gravity, as at the edges of galaxies. 
  • MOND has had some success in predicting galaxy rotation without dark matter, but many of these successes can also be explained by dark matter while preserving Newton’s laws.
  • MOND affects gravity at low accelerations, not specific distances. 
  • Therefore, while MOND effects typically appear several thousand light years from a galaxy, they could become significant at much shorter distances, such as in the outer Solar System.
• MOND Theory cahllenged 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.
• MOND is no longer seen as a viable alternative
  • Despite issues with the standard dark matter model, such as the universe’s expansion rate and cosmic structures, MOND is no longer seen as a viable alternative. 
  • Dark matter remains the prevailing explanation, though its nature might differ from current models, or gravity could be stronger on very large scales.

Q.1. What is Astronomical Unit (AU)?

An Astronomical Unit (AU) is a unit of measurement used to describe distances within our solar system. One Astronomical Unit is defined as the average distance from the Earth to the Sun. It is approximately 149,597,870.7 kilometers (about 92,955,807.3 miles).

Q.2. What is Cassini-Huygens?

Cassini-Huygens was a collaborative mission between NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) designed to study Saturn, its rings, and its moons.

Source: With bad news from Cassini, is dark matter’s main rival theory dead? | NASA | Office of Science