How DESI works

The idea in one cycle

Everything DESI does repeats one loop:

pick 5,000 galaxies → collect each one’s light separately → split that light into its colours → measure the redshift → place the galaxy in a 3D map.

Do that for millions of objects across the sky and across cosmic time, and the map that emerges carries the fingerprint of how the universe has expanded.

Choosing what to look at

DESI does not sweep the sky blindly. It starts from wide survey images that already exist and selects the most useful targets by their brightness, shape, and colour. The main catches are:

• bright, relatively nearby galaxies;
• luminous red galaxies — massive and more distant;
• emission-line galaxies, whose spectra carry sharp, easy-to-recognise features;
• quasars, brilliant beacons used to probe the most distant universe and the gas that lies between us and them.

Colours give a rough guess of an object’s type and distance. DESI then makes the precise measurement with its spectrographs.

Five thousand robots and fibres

DESI sits on the Nicholas U. Mayall 4-metre Telescope at Kitt Peak, in Arizona. A set of large lenses focuses the light onto a curved focal plane about 0.8 metres across, covering a patch of sky roughly 3 degrees wide — an enormous field for a spectrograph.

That focal plane is split into ten wedge-shaped sections, and each holds 500 tiny robots: 5,000 robotic positioners in total. Every robot carries the tip of a thin optical fibre. Before each exposure, software works out where each target will land on the focal plane and drives every fibre to its target — like placing 5,000 microscopic straws over 5,000 points of light at once. Neighbouring fibres must avoid colliding, so not every target in a crowded patch can be caught in one go; the sky is revisited with different assignments.

From light to spectrum

During the exposure, each fibre collects the light of the one object it was aimed at. The light then travels down fibre bundles to ten spectrographs, each fed by 500 fibres. Each spectrograph spreads the light across three wavelength bands, from the near-ultraviolet to the near-infrared, roughly 360 to 980 nanometres.

The result is not a photograph but a spectrum for every object: a curve of brightness against wavelength, with peaks and dips where chemical elements, stars, and gas emit or absorb light.

From spectrum to redshift

Those spectral lines have wavelengths measured in the laboratory. In a distant galaxy they arrive stretched to longer wavelengths, because the expansion of the universe stretched the light on its way to us. The amount of stretch is the redshift, z:

z = (observed wavelength − emitted wavelength) / emitted wavelength.

If a line that should sit at 500 nm is seen at 750 nm, its redshift is (750 − 500) / 500 = 0.5. A larger cosmological redshift generally means older light, from further away. DESI’s pipeline calibrates each spectrum, identifies the object, and reports a redshift together with how reliable that redshift is.

From redshifts to a 3D map

For each galaxy DESI knows two angles on the sky and, from the redshift, roughly how far along the line of sight it lies. Stack millions of these together and the cosmic web appears: filaments of galaxies, dense clusters, sheet-like walls, and great near-empty voids.

There is a subtlety here. A redshift is not automatically an exact distance — turning one into the other requires knowing how the universe expanded. And that expansion history is precisely what DESI is trying to measure. The map and the measurement are the same problem seen from two sides.

How a map becomes a measurement of dark energy

DESI reads dark energy mainly from two statistical features of the map.

The first is the baryon acoustic oscillation, or BAO — a standard ruler frozen into the early universe. Before there were stars, pressure waves ran through the hot plasma of matter and light; when the universe cooled and the waves stopped, they left a slightly preferred separation between galaxies, a comoving scale of about 490 million light-years — a distance quoted with the universe’s expansion factored out, so it can be compared across epochs. Because that true length is known from early-universe physics, DESI can measure how large the ruler appears at different redshifts — across the sky and along the line of sight — and chart how fast the universe expanded at each epoch. If dark energy has changed over cosmic time, it shows up as a change in that expansion history.

The second is redshift-space distortions, or RSD. Galaxies do not only ride the cosmic expansion; they also fall toward clusters and filaments. Those extra motions shift redshifts slightly and statistically squash the cosmic web along the line of sight. How strong the effect is tells you how fast structure is growing under gravity — which accelerated expansion tends to slow. Comparing the two lets cosmologists test gravity and dark-energy models against each other.

So dark energy is never seen directly. It is inferred from how the expansion rate changes over time, measured against the acoustic ruler and the growth of structure.

What DESI does not do

DESI does not detect particles of dark energy, photograph an invisible substance, or range-find each galaxy’s distance directly. What it measures, very well, is spectra, redshifts, the statistical distribution of galaxies, and how that distribution changes with cosmic epoch. Dark energy is the physical explanation put on trial when those measurements are compared with cosmological models — not something the instrument records.

In one sentence

DESI reads the bright barcodes of millions of galaxies, builds a three-dimensional map of the universe from their redshifts, and looks in the pattern of that map for traces of how cosmic expansion has changed over time.