Sunyaev–Zeldovich effecteffect
In 1969, Soviet astrophysicists Rashid Sunyaev and Yakov Zeldovich predicted that hot intergalactic gas in galaxy clusters should leave an imprint on the cosmic microwave background. Plasma electrons, accelerated to enormous speeds, transfer energy to photons via inverse Compton scattering. The effect is like a 'fog' of electrons that slightly tints the background light. This allows us to see galaxy clusters at any distance, since the surface brightness of the effect does not depend on distance.
How it works
Astronomers observe this distortion and use it to determine the gas pressure in the cluster, its mass, and its distance, independent of redshift.
💡 The Sunyaev-Zeldovich effect was one of the first methods to confirm the existence of dark energy, when it was used to measure distances to distant clusters and compare them with the expansion model.
y = \frac{\sigma_T}{m_e c^2} \int n_e k_B T_e \, dl
y — dimensionless Comptonization parameter, σ_T ≈ 6.652×10⁻²⁹ m² — Thomson scattering cross-section of an electron, m_e ≈ 9.109×10⁻³¹ kg — electron mass, c ≈ 3×10⁸ m/s — speed of light, n_e — electron concentration (m⁻³), k_B ≈ 1.381×10⁻²³ J/K — Boltzmann constant, T_e — electron temperature (K), dl — line-of-sight length element (m).
\Delta T_\text{CMB} = T_\text{CMB} \, y \left( x \frac{e^x+1}{e^x-1} - 4 \right)
ΔT_CMB — change in CMB temperature (K), T_CMB ≈ 2.725 K — average CMB temperature, y — Compton parameter, x = hν/(k_B T_CMB) — dimensionless frequency, h ≈ 6.626×10⁻³⁴ J·s — Planck constant.