Redshift

In physics, light behaves like a wave. When the wavelength of electromagnetic radiation increases, it shifts toward the "red" end of the spectrum (even if the light isn't visible). This increase in wavelength corresponds to a decrease in frequency and photon energy. Its opposite is "blueshift," where waves are compressed, increasing their energy and frequency.

The change is measured by a dimensionless value called z. A positive $z$ indicates a redshift (moving away or losing energy), while a negative $z$ indicates a blueshift (approaching or gaining energy). This value is more than just a number; it is a fundamental tool for astronomers to calculate how fast objects are moving and how far away they reside in the deep reaches of space.

Though the visual result is the same, the causes differ wildly. Doppler redshift is the most intuitive; it occurs when a source moves away from the observer, similar to the pitch of a siren dropping as an ambulance passes. Gravitational redshift happens when light "climbs" out of a massive gravitational well, losing energy as it fights against gravity.

The most profound type is Cosmological redshift, which is not caused by objects moving through space, but by space itself expanding. As the universe grows, the light traveling through it is literally stretched along with the fabric of the cosmos. This means that the further away a galaxy is, the more its light has been stretched by the time it reaches Earth.

In the 19th century, Christian Doppler and Hippolyte Fizeau laid the groundwork by applying wave mechanics to sound and light. However, the real breakthrough came in the 1920s. Using spectral data from Vesto Slipher and his own distance measurements, Edwin Hubble discovered a shocking correlation: almost all distant galaxies are moving away from us, and the further they are, the faster they recede.

This discovery, now known as Hubble’s Law, destroyed the long-held belief in a "steady state" universe. It provided the first observational evidence that the universe had a beginning and has been expanding ever since. This transition from "local star tracking" to "cosmological mapping" allowed scientists to treat the universe as a physical object with a traceable biography.

At speeds approaching the velocity of light, classical physics fails to explain redshift accurately. Because of time dilation—where time slows down for objects moving at high speeds—a "Transverse Redshift" can occur. This means an observer will see a redshift even if an object is moving at a right angle to them, rather than directly away.

This nuance was confirmed by the Ives–Stilwell experiment in 1938 and remains a crucial part of Special Relativity. It proves that redshift is a property of the relationship between two measurement locations in spacetime, influenced not just by direction, but by the sheer magnitude of velocity and the warping of time.

Redshift can be so dramatic that it shifts light across entire categories of the electromagnetic spectrum. For example, a high-energy gamma ray could be perceived as a lower-energy X-ray. The most famous instance of this is the Cosmic Microwave Background (CMB).

When the universe was young, it was filled with brilliant radiation at a temperature of 3,000 Kelvin. Over billions of years, the expansion of the universe has redshifted that light so severely that it has stretched from visible glow into invisible microwaves. It now registers at a frigid 3 Kelvin—a ghostly, stretched-out echo of the birth of the cosmos.