Redshift

Redshift is a fascinating phenomenon in physics and astronomy. It describes how the wavelength of light or other electromagnetic radiation gets longer, while its frequency gets lower. This happens when the source of the light is moving away from us, or because of the expansion of the universe, or even due to strong gravity. The opposite effect, where the wavelength gets shorter and the frequency higher, is called blueshift.

In astronomy, scientists measure redshift to learn about distant objects and the structure of the universe. For example, the light from the Big Bang, which started as very high-energy radiation, has redshifted over time to become the cold cosmic microwave background we detect today. Redshift helps us understand how fast objects are moving and how far away they are.

Redshift is not just important for space studies. It is also used in everyday technologies like Doppler radar and radar guns, which measure the speed of cars or weather systems. Even gravitational waves, ripples in space-time, experience redshift just like light does when they travel through the universe.

History

The idea of redshift began in the 1800s when scientists studied waves and motion. Austrian mathematician Christian Doppler explained in 1842 that waves, like sound or light, change when their source moves. Dutch scientist Christophorus Buys Ballot tested this with sound waves in 1845.

Later, scientists used this idea for light from stars. In 1929, astronomer Edwin Hubble discovered that the farther away a galaxy is, the faster it seems to move away from us. This became known as Hubble's law. This helped scientists understand that the universe is expanding.

Physical origins

Redshifts happen when the wavelength of light gets longer, which means the light’s frequency gets lower. This can occur in three main ways: because of movement, the expansion of the universe, and gravity.

When a light source moves away from us, we see a redshift. This is called the Doppler effect. When the universe expands, light from faraway galaxies also appears redshifted. This is called cosmological redshift. Finally, gravity can cause redshift too. When light escapes from a strong gravitational field, it loses energy and its wavelength stretches. This is known as gravitational redshift.

Redshift summary

Redshift type	Geometry	Formulae
Relativistic Doppler	Minkowski space (flat spacetime)	For motion completely in the radial or line-of-sight direction: 1 + z = γ ( 1 + v ∥ c ) = 1 + v ∥ c 1 − v ∥ c {\displaystyle 1+z=\gamma \left(1+{\frac {v_{\parallel }}{c}}\right)={\sqrt {\frac {1+{\frac {v_{\parallel }}{c}}}{1-{\frac {v_{\parallel }}{c}}}}}} z ≈ v ∥ c {\displaystyle z\approx {\frac {v_{\parallel }}{c}}} for small v ∥ {\displaystyle v_{\parallel }} For motion completely in the transverse direction: 1 + z = 1 1 − v ⊥ 2 c 2 {\displaystyle 1+z={\frac {1}{\sqrt {1-{\frac {v_{\perp }^{2}}{c^{2}}}}}}} z ≈ 1 2 ( v ⊥ c ) 2 {\displaystyle z\approx {\frac {1}{2}}\left({\frac {v_{\perp }}{c}}\right)^{2}} for small v ⊥ {\displaystyle v_{\perp }}
Cosmological redshift	FLRW spacetime (expanding Big Bang universe)	1 + z = a n o w a t h e n {\displaystyle 1+z={\frac {a_{\mathrm {now} }}{a_{\mathrm {then} }}}} Hubble's law: z ≈ H 0 D c {\displaystyle z\approx {\frac {H_{0}D}{c}}} for D ≪ c H 0 {\displaystyle D\ll {\frac {c}{H_{0}}}}
Gravitational redshift	Any stationary spacetime	1 + z = g t t ( receiver ) g t t ( source ) {\displaystyle 1+z={\sqrt {\frac {g_{tt}({\text{receiver}})}{g_{tt}({\text{source}})}}}} For the Schwarzschild geometry: 1 + z = 1 − r S r receiver 1 − r S r source = 1 − 2 G M c 2 r receiver 1 − 2 G M c 2 r source {\displaystyle 1+z={\sqrt {\frac {1-{\frac {r_{S}}{r_{\text{receiver}}}}}{1-{\frac {r_{S}}{r_{\text{source}}}}}}}={\sqrt {\frac {1-{\frac {2GM}{c^{2}r_{\text{receiver}}}}}{1-{\frac {2GM}{c^{2}r_{\text{source}}}}}}}} z ≈ 1 2 ( r S r source − r S r receiver ) {\displaystyle z\approx {\frac {1}{2}}\left({\frac {r_{S}}{r_{\text{source}}}}-{\frac {r_{S}}{r_{\text{receiver}}}}\right)} for r ≫ r S {\displaystyle r\gg r_{S}} In terms of escape velocity: z ≈ 1 2 ( v e c ) source 2 − 1 2 ( v e c ) receiver 2 {\displaystyle z\approx {\frac {1}{2}}\left({\frac {v_{\text{e}}}{c}}\right)_{\text{source}}^{2}-{\frac {1}{2}}\left({\frac {v_{\text{e}}}{c}}\right)_{\text{receiver}}^{2}} for v e ≪ c {\displaystyle v_{\text{e}}\ll c}

Measurement

Using a telescope and a spectrometer, scientists can measure how the light from stars changes with frequency. They compare this to the light we know from gases like hydrogen studied in labs on Earth. By looking at features like absorption lines or emission lines in the starlight, they can see if the light has shifted.

Redshift and blueshift are described using a number called z. This number shows how much the wavelength of light has changed. The Doppler effect causes blueshifts when objects move away from us, making the light shift to lower energies. Gravitational redshifts happen when light escapes from strong gravitational fields.

Calculation of redshift, z {\displaystyle z}

Based on wavelength	Based on frequency
z = λ o b s v − λ e m i t λ e m i t {\displaystyle z={\frac {\lambda _{\mathrm {obsv} }-\lambda _{\mathrm {emit} }}{\lambda _{\mathrm {emit} }}}}	z = f e m i t − f o b s v f o b s v {\displaystyle z={\frac {f_{\mathrm {emit} }-f_{\mathrm {obsv} }}{f_{\mathrm {obsv} }}}}
1 + z = λ o b s v λ e m i t {\displaystyle 1+z={\frac {\lambda _{\mathrm {obsv} }}{\lambda _{\mathrm {emit} }}}}	1 + z = f e m i t f o b s v {\displaystyle 1+z={\frac {f_{\mathrm {emit} }}{f_{\mathrm {obsv} }}}}

Observations in astronomy

Astronomers measure redshift by looking at the unique patterns of light, called spectra, from stars and galaxies. These patterns act like fingerprints, and when they shift toward red light, it tells us about the object's movement. This shift happens because of three main reasons: the object moving away from us, gravity pulling on the light, or the expansion of the universe itself.

Studying these redshifts helps scientists understand how stars move, how fast galaxies spin, and even how the universe is expanding. By measuring how much the light has shifted, astronomers can figure out distances to faraway objects and learn about the history of the universe.

Effects from physical optics or radiative transfer

When light interacts with matter or other light, its wavelength and frequency can change. This happens through processes like scattering from tiny particles or changes in how light travels through different materials. These changes are often called "reddening" instead of "redshift" because the light's color shifts but its spectral lines don't move to new positions.

Scattering can make light appear redder, especially when many low-energy photons are present. This is why the sky looks blue during the day and the Sun appears red at sunrise or sunset. Unlike true redshift, reddening also makes the light dimmer and can distort its appearance.

Blueshift

"Blueshift" redirects here. For the term as used in photochemistry, see hypsochromic shift. For the political phenomenon, see blue shift (politics). For other uses of "blueshift" or "blue shift", see Blueshift (disambiguation).

The opposite of a redshift is a blueshift. A blueshift is any decrease in wavelength (increase in energy), with a corresponding increase in frequency, of an electromagnetic wave. In visible light, this shifts a color towards the blue end of the spectrum.

Doppler blueshift happens when a source moves toward an observer. This can be seen in objects like the Andromeda Galaxy, which is moving toward our own Milky Way galaxy. Stars and galaxies spinning in certain ways can also show blueshifts.

Gravitational blueshift occurs when light falls into a strong gravity area, making it more energetic. This effect was confirmed in experiments and helps explain patterns in the cosmic microwave background. Some far-away active galaxies show unexpected blueshifts in their light emissions.