Redshift

Redshift refers to the shift toward longer wavelengths of light emitted by an object that ismoving away from the observer. In astronomy, it is one of the most powerful tools for measuring distances, velocities, and the expansion of the universe.

Understanding Redshift

Imagine an ambulance moving away from you: the sound of its siren seems lower-pitched than when it’s stationary, because the sound waves are stretched. Light behaves in a similar way. When a light source moves away, the light waves it emits are stretched. Longer wavelengths correspond to red in the visible spectrum—hence the name “redshift.” Conversely, an object that is approaching produces a blueshift, a shift toward the blue.

Three Types of Redshift

In astronomy, there are three possible causes. The Doppler redshift is related to the source’s motion through space. The gravitational redshift occurs when light travels through a strong gravitational field— it loses energy and shifts toward the red. Finally, cosmological redshift is caused by the expansion of the universe itself: light travels through an expanding space, stretching its wavelengths along the way.

An essential cosmological tool

Redshift is denoted by z. The farther away a galaxy is, the higher its redshift. A galaxy at z=1 sends us light whose wavelengths have doubled since it was emitted. The most distant quasars reach values of z greater than 7, which means we see them as they were less than a billion years after the Big Bang. It was thanks to redshift that Edwin Hubble discovered the expansion of the universe in 1929.

Did you know?

The cosmic microwave background, an echo of the Big Bang, has a redshift of z = 1100. The light it sends us was emitted 380,000 years after the Big Bang, and its wavelengthswavelengths have increased by a factor of 1,100 since then.

Frequently Asked Questions

Does redshift mean that galaxies are moving through space?

Not exactly. Cosmological redshift is not the movement of galaxies through space, but the expansion of space itself. The distinction is subtle but important in cosmology.

Can redshift be seen with the naked eye?

No. Redshift is measured using spectroscopy, by analyzing an object’s light spectrum and comparing the absorption or àemission lines to their reference positions in the laboratory.

What is the relationship between redshift and distance?

For nearby galaxies, redshift is approximately proportional to distance, according to Hubble’s law. For very distant objects, the relationship becomes more complex and depends on the cosmological model used.