AcademyParticles and Cosmology
Academy
Open Questions
Level 1 - Physics topic page in Particles and Cosmology.
Principle
Modern particle physics and cosmology have successful models, but observations still point beyond them: dark matter, dark energy, neutrino masses, matter-antimatter asymmetry, and quantum gravity.
Notation
\(\Omega_m\)
matter density fraction
1
\(\Omega_\Lambda\)
dark-energy density fraction
1
\(\nu\)
neutrino
\(m_\nu\)
neutrino mass
eV/c^{2}
\(G\)
gravitational constant
\(\mathrm{N\,m^{2}\,kg^{-2}}\)
\(H_0\)
Hubble constant
s^{-1}, km s^{-1} Mpc^{-1}
Method
Derivation 1: Dark matter is inferred gravitationally
If visible matter cannot supply the gravitational field needed for observed motion, additional non-luminous mass is inferred.
Circular motion
\[\frac{v^2}{r}=\frac{GM(r)}{r^2}\]
Enclosed mass
\[M(r)=\frac{v^2r}{G}\]
Derivation 2: Dark energy is inferred from expansion
Accelerated expansion requires an energy component with different gravitational behaviour from matter.
Matter plus dark energy
\[\Omega_m+\Omega_\Lambda\approx1\]
Expansion scale
\[v=H_0d\]
Derivation 3: Neutrino oscillations imply mass
Changing neutrino flavour during propagation means neutrino flavour states are not fixed mass states.
Oscillation evidence
\[\nu_e\leftrightarrow\nu_\mu\leftrightarrow\nu_\tau\]
Mass implication
\[m_\nu\ne0\]
Rules
Enclosed mass estimate
\[M(r)=\frac{v^2r}{G}\]
Hubble law
\[v=H_0d\]
Neutrino mass implication
\[m_\nu\ne0\]
Examples
Question
Why does a flat galaxy rotation curve suggest unseen matter?
Answer
If \(v\) remains large far from the centre,
\[M(r)=v^2r/G\]
keeps increasing beyond the visible matter.Checks
- Open questions should be tied to observations.
- Dark matter is not just ordinary dim matter in the current model.
- Dark energy is inferred from cosmic expansion data.
- Neutrino oscillations are evidence beyond the simplest original Standard Model.