blueshifts

Blueshift is the shortening of wavelengths in electromagnetic radiation, observed when a source moves toward the observer or when photons travel into a stronger gravitational field. It is the opposite of redshift, and in spectroscopy is indicated by a negative redshift value, denoted z.

In practical terms, the observed wavelength λ_obs relates to the emitted wavelength λ_emit by the equation λ_obs = λ_emit (1 + z). Blueshift occurs when z is negative. For low speeds, the Doppler interpretation gives z ≈ -v/c, where v is the velocity of the source toward the observer. At higher (relativistic) speeds, the relation follows 1 + z = sqrt((1 + β)/(1 - β)), with β = v/c, yielding larger blueshifts as v approaches the speed of light toward the observer.

Two primary mechanisms produce blueshift. The Doppler blueshift arises from relative motion: objects moving toward the observer, such as stars in orbital motion, inflowing gas, or other peculiar velocities in galaxies, can imprint blueshifted spectral lines. Gravitational blueshift occurs when photons descend into a stronger gravitational potential, gaining energy and increasing frequency as they climb out of shallower potentials into deeper ones; this effect is notable near massive bodies like white dwarfs, neutron stars, or black holes.

Blueshift is commonly encountered in astronomy on small scales, while the large-scale expansion of the universe yields redshift. Blueshift measurements help determine local kinematics, gravitational effects, and, in some cases, the dynamics of accretion and inflows in galaxies and stellar systems.