When we look at the night sky, stars and galaxies appear as mere points of light. Yet that light carries detailed information about the temperature, chemical composition and motion of its source. When starlight is passed through a prism or a spectrograph, thin dark or bright lines appear superimposed on the continuous, rainbow-like band of colour. These patterns, called spectral lines, act like a fingerprint for each chemical element: hydrogen, calcium or iron always produce lines at the same wavelengths. The clue that led astronomers to realise the universe is expanding was that these lines appear shifted from the positions measured in the laboratory.
What Is Redshift?
Redshift is the effect in which the wavelengths of light from a source appear longer than they should be, shifted toward the red end of the spectrum. A stretched wavelength also means a lower frequency and less energy. In the opposite case, when wavelengths become shorter and shift toward the blue end, the effect is called blueshift.
The size of the shift is usually expressed by a dimensionless number denoted "z", the fractional difference between the observed wavelength and the rest wavelength. For example, if the hydrogen lines in a galaxy's spectrum all appear about 10 percent longer in wavelength, the value of z is roughly 0.1. The larger z is, the farther away and the earlier in cosmic history the light originates; the most distant galaxies observed today have z values above 10.
Doppler Shift Versus Cosmological Redshift
Redshift is often compared to the siren of an ambulance, which sounds higher pitched as it approaches and lower pitched as it recedes. This Doppler effect, which applies to sound waves, also works for light: light from a star moving toward us shifts to the blue, while light from a star moving away shifts to the red. A Doppler shift arises from the relative motion of source and observer through space and is used to measure the velocities of nearby stars.
The shift seen in distant galaxies, however, is a different phenomenon. As the European Space Agency (ESA) emphasises, this "expansion redshift" is caused not by galaxies moving to some location within space, but by the expansion of space itself between two galaxies. As the space the light travels through stretches in the background, the wavelength of the light stretches with it. For this reason a redshift appears even when two objects are entirely at rest relative to space, as long as the distance between them is growing. Cosmological redshift is therefore not exactly the same as the classical Doppler effect.
Hubble's Law: The Link Between Distance and Velocity
In 1912 Henrietta Swan Leavitt discovered a relationship between the pulsation period and the true brightness of Cepheid variable stars, opening the way to measuring cosmic distances. In the 1910s Vesto Slipher showed that the spectra of objects then called "spiral nebulae" were mostly shifted toward the red. In 1927 the Belgian astronomer and priest Georges Lemaître derived a solution for an expanding universe from the equations of general relativity and predicted that galaxies should recede at speeds proportional to their distance.
In 1929 Edwin Hubble established this relationship observationally from measurements made at Mount Wilson Observatory: the farther away a galaxy was, the faster it was receding. The relation is written simply as v = H0 x d, where v is the galaxy's recession velocity, d its distance and H0 the constant of proportionality known as the Hubble constant. Today the law is called the Hubble-Lemaître law in recognition of both scientists' contributions.
What Does the Expansion of the Universe Mean?
The fact that every galaxy appears to be moving away from us does not mean the Milky Way sits at the centre of the universe. ESA's raisin-bread analogy explains this well: as the dough rises in the oven, every raisin moves away from every other, and whichever raisin you sit on you will see all of your neighbours receding. What expands in the universe is not the void through which galaxies move, but the fabric of space itself.
When this expansion is "rewound", it leads back to a beginning in which all matter and energy were gathered in a far hotter, denser and smaller state. This conclusion, which Lemaître recognised, forms the basis of the Big Bang model. The value of the Hubble constant also gives a rough estimate of the age of the universe; current measurements indicate an age of about 13.8 billion years.
The Hubble Constant and the "Hubble Tension"
The Hubble constant is usually given in units of kilometres per second per megaparsec (km/s/Mpc): two points one megaparsec apart (about 3.26 million light-years) move apart at that speed. The problem is that the two different ways of measuring the constant do not agree.
The "cosmic distance ladder" method, based on the nearby universe, uses Cepheid stars and Type Ia supernovae rung by rung to reach a value of about 73 km/s/Mpc. Data from the Planck satellite, which studies the cosmic microwave background, the fossil light of the early universe, yield about 67.5 km/s/Mpc when combined with the standard cosmological model. As NASA has explained, the difference between these two results is a persistent discrepancy with only a one-in-a-million chance of being a statistical fluke. This conflict, called the Hubble tension, may point either to systematic errors not yet found in the measurements or to new physics beyond what is currently known; it is one of the liveliest debates in cosmology today.
What Else Does Redshift Tell Us?
Redshift does not only prove expansion; it also serves as a kind of time machine. Light from a galaxy with a high z value set out billions of years ago, so studying it means looking at the youth of the universe. Infrared observatories such as the James Webb Space Telescope were designed precisely to capture the light of very distant, highly redshifted objects.
Redshift measurements also underpin efforts to map galaxies in three dimensions, to trace the distribution of dark matter, and to study the "dark energy" that has revealed the expansion of the universe to be accelerating over the last several billion years. A story that began a century ago with a small shift in dark lines now stands at the centre of the effort to understand the past and future of the cosmos.
Sources
- NASA StarChild - "Redshift and Hubble's Law": https://starchild.gsfc.nasa.gov/docs/StarChild/questions/redshift.html
- European Space Agency (ESA) - "What is 'red shift'?": https://www.esa.int/Science_Exploration/Space_Science/What_is_red_shift
- NASA Science - "Hubble Reaches New Milestone in Mystery of Universe's Expansion Rate": https://science.nasa.gov/missions/hubble/hubble-reaches-new-milestone-in-mystery-of-universes-expansion-rate/