Red-shift and the expanding universe

A galaxy's overall colour cannot tell astronomers exactly how far its spectral lines have shifted. They separate its light into a spectrum and identify lines produced when atoms absorb or emit particular wavelengths.

Each identified line has a laboratory reference wavelength. If the matching line in the galaxy's light has a longer wavelength, it is red-shifted. Checking several lines helps astronomers recognise the pattern and avoid confusing one line with another. The galaxy need not look red to the eye.

There are different ways to produce red-shift. A source moving away through space can change the wavelengths an observer receives. Expansion of space can also stretch light during its journey. This second effect is cosmological red-shift.

Measurements across many distant galaxies show an average relationship: larger distances go with larger red-shifts. Expansion explains this pattern. Nearby galaxies also have individual motions, so some approach us instead of following the overall trend.

Early telescopes showed fuzzy patches whose distances were unknown. Some were objects inside the Milky Way. Others were separate galaxies far beyond it. Astronomers debated the possibilities, and suggestions of distant star systems existed long before decisive measurements.

In the 1920s, Edwin Hubble used variable stars to estimate distances to some of the patches. Their brightness changes helped reveal how far away they were. The distances placed them beyond the Milky Way, establishing that other galaxies existed.

Vesto Slipher had already measured red-shifts in many of these objects during the 1910s. A shift measurement alone did not supply the missing galaxy distances. Once astronomers had both quantities, they could compare them.

Hubble found that more distant galaxies tended to recede faster. The relationship supported the expanding-universe model also developed by Georges Lemaître. Finding other galaxies did not cause their red-shifts. Distance measurements made it possible to identify a relationship between distance and recession.

Expansion tells us that the universe was denser in the past. By itself, that conclusion does not tell us how hot the earlier universe was. Evidence for an early hot state must also be explained.

The hot Big Bang model describes a universe that expanded and cooled from an early hot, dense state. Radiation from that early universe remains detectable. Expansion has stretched its wavelengths, and astronomers now observe it as the cosmic microwave background across the sky.

This background and the galaxy distance-red-shift pattern test different predictions. The galaxy pattern supports expanding space. The microwave background supports an early hot state. A model that accounts for both has more support than an explanation of only one observation.

Scientists still check measurements and compare possible explanations. A single galaxy's red-shift could come from its own motion. Many galaxies are needed to establish the wider pattern. Agreement between different observations strengthens the hot Big Bang model without making further tests unnecessary.