Explain briefly 3 theories that provide explanations for Dark Matter If it's not worth answering, LEAVE IT!
Answers
□Dark Matter:
Dark matter can refer to any substance which interacts predominantly via gravity with visible matter (e.g., stars and planets). ✍
Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard baryonic matter, such as protons or neutrons.✍
□Theory:
Most dark matter is thought to be non-baryonic in nature;
it may be composed of some as-yet undiscovered subatomic particles. The primary candidate for dark matter is some new kind of elementary particle that has not yet been discovered, in particular, weakly interacting massive particles (WIMPs).✍
Answer:
Dark Matter Theory :-
The existence of dark matter can be traced back to the pioneering discoveries of Fritz Zwicky and Jan Oort that the motion of galaxies in the Coma cluster, and of nearby stars in our own Galaxy, do not follow the expected motion based on Newton's law of gravity and the observed visible masses. Since then a host of experimental data from precise measurements of the cosmic microwave background, of gravitational lensing of galaxy clusters, and of the rotational speeds of stars and galaxies, provide strong, internally consistent, evidence for the existence of dark matter as a particle. But we still do not have any direct evidence for dark matter, let alone have some idea as to its physical properties or its non-gravitational interactions with the Standard Model particles.
Newtonian Dynamics:-
Modified Newtonian dynamics (MOND) is an empirically motivated modification of Newtonian gravity or inertia suggested by Milgrom as an alternative to cosmic dark matter. The basic idea is that at accelerations below a0 ~ 10^{-8} cm/s^2 ~ cH0/6 the effective gravitational attraction approaches sqrt{gN*a0} where gN is the usual Newtonian acceleration. This simple algorithm yields flat rotation curves for spiral galaxies and a mass-rotation velocity relation of the form M ~ V^4 that forms the basis for the observed luminosity-rotation velocity relation-- the Tully-Fisher law. We review the phenomenological success of MOND on scales ranging from dwarf spheroidal galaxies to superclusters, and demonstrate that the evidence for dark matter can be equally well interpreted as evidence for MOND. We discuss the possible physical basis for an acceleration-based modification of Newtonian dynamics as well as the extension of MOND to cosmology and structure formation.
Entropic Theory:-
The theory claims to be consistent with both the macro-level observations of Newtonian gravity as well as Einstein's theory of general relativity and its gravitational distortion of spacetime. Importantly, the theory also explains (without invoking the existence of dark matter and its accompanying math featuring new free parameters that are tweaked to obtain the desired outcome) why galactic rotation curves differ from the profile expected with visible matter.
The theory of entropic gravity posits that what has been interpreted as unobserved dark matter is the product of quantum effects that can be regarded as a form of positive dark energy that lifts the vacuum energy of space from its ground state value. A central tenet of the theory is that the positive dark energy leads to a thermal-volume law contribution to entropy that overtakes the area law of anti-de Sitter space precisely at the cosmological horizon.
The theory has been controversial within the physics community but has sparked research and experiments to test its validity.