HOW can we make an accurate map of the universe when telescopes deliver only a two-dimensional picture? Measuring how far away cosmic objects are is no trivial affair 鈥� we can鈥檛 move our surveying equipment or lay听a tape measure between galaxies.
For the best part of a century, the answer has been to measure听redshift. In the 1920s,听astronomer Edwin Hubble and others discovered that听light from galaxies beyond the Milky Way is consistently shifted to longer, redder wavelengths.
听 Scroll down for an interactive graphic that shows how redshift can help explain the evolution of the universe
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This was the first inkling that the听universe began in a big bang, and has been expanding ever since. The reason for the shift in colour is that as space-time expands, it stretches the light passing through it.
The more expanding space the light has passed through, the greater the degree of this redshift, so far-off objects appear redder. We also see these objects as they were earlier in听the听universe鈥檚 history because of the听听length of time the light has听been听travelling.
Redshift measurements have听brought surprising discoveries, not least that the universe鈥檚 expansion has apparently begun to accelerate, something attributed to an enigmatic 鈥渄ark energy鈥�. Cosmic maps extending听to even more distant objects听 and covering more of the sky should help听us work out what is going on 鈥� provided our redshift measurements are accurate (see 鈥�Why we can鈥檛 work out where everything is in the universe鈥�).
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The more an object鈥檚 light has shifted to longer, redder wavelengths, the greater the distance through expanding space the light has travelled, and the further back in time we see the object.
Or that apparently far-off entity might be a naturally reddish object, closer in. This is the central conundrum of redshift.
Splitting the difference
Atoms and molecules emit and absorb light at specific wavelengths, creating a barcode-like pattern of bright and dark lines in a galaxy鈥檚 spectrum. A full spectrum lets you assess how each line has shifted, and obtain a consistent value for the redshift. However, large-scale galaxy surveys typically average light intensities over bands of the spectrum. Higher intensities imply lots of bright spectral lines within a band, lower intensities imply more dark lines. This may or may not be enough to work out where the spectral pattern is, and hence the redshift.
The cosmic story
Redshift values are related to the movement of the spectral lines (see 鈥淪plitting the difference鈥�, above) and the expansion of the universe. Accurate redshifts are important to establish what sort of structures existed at what eras in the universe, and thus how factors such as dark energy and invisible dark matter have influenced cosmic evolution. The aim of a new generation of instruments is to refine our measurements and build a more accurate history of the universe. Among these instruments are the Euclid space probe and the Dark Energy Survey and Large Synoptic Survey telescopes.
Redshift measurements reveal how the tussle between matter and dark energy has determined the universe鈥檚 evolution. Explore how this has happened since the big bang in the interactive graphic below. Just tap the circle icons to find out more.
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Explore more in the feature 鈥�Why we can鈥檛 work out where everything is in the universe鈥�
