We just took the biggest selfie ever: 47 million galaxies and a dark energy twist

Adam Spencer
2 days ago
5 min read

DESI's 5,000 "eyes" have mapped 47 million galaxies, building the biggest 3D map ever – and it may be hinting dark energy is changing.

The belnded Northern Star Trail above Kitt Peak. — (Credit: KPNO/NOIRLab/NSF/AURA/B. Tafreshi.)

(First published on my Substack where you can get #NerdNews, marvellous maths and general geekery.)

The mountain that mapped the universe.

If you tried to draw a map of everything in the universe, where would you even start?

A team on a lonely mountain in Arizona has just had a crack at it – and their effort is so good it might be about to mess with our favourite theory of how the cosmos works.

That mountain is Kitt Peak, 2,100 metres up in the Sonoran Desert, far enough from city lights that the sky wheels overhead clearly enough to photograph the rotation of the Earth itself. The white dome you can just make out is the Mayall Telescope. DESI lives inside it.

This awesome photo shows star trails over the Mayall 4-metre telescope at Kitt Peak National Observatory, Arizona — DESI's home. The streaks are stars; the bullseye in the centre is the celestial pole.

Meet DESI: a galaxy census taker.

Perched on Kitt Peak, the Dark Energy Spectroscopic Instrument – DESI to its friends – has spent the last five years turning a four-metre telescope into the ultimate galaxy census taker. Night after night, DESI has swivelled to new patches of sky and pointed 5,000 tiny robotic "eyes" at 5,000 different smudges of light at once. Each eye grabs a galaxy or quasar and splits its light into a barcode-like spectrum, from which you can tell how far away it is and what it's made of.

Each of those eyes is a robot about the size of a lipstick tube, nudging the end of a hair-thin fibre into place so it can drink the light of one galaxy while its 4,999 mates do the same next door.

Each observation takes about 20 minutes, before DESI rotates and drinks up another 5,000 bits of the universe.

On clear nights it can sample more than 100,000 corners of the cosmos. Do that a few hundred times over five years and you end up with the incredible 47 million galaxies and quasars DESI has mapped.

The original plan was ambitious enough: measure the distances to around 34 million galaxies and quasars. DESI overshot. The finished survey has logged more than 47 million. That is, by a comfortable margin, the most detailed three-dimensional map of the universe we've ever made.

"DESI is 20 times better than any previous facility at mapping the universe in 3D." — Will Percival, DESI co-spokesperson, University of Waterloo / Perimeter Institute.

Pretty fly for a night sky.

(Credit: Claire Lamman/DESI collaboration.)

This grainy innocuous image is what it's all about. DESI's "butterfly" view. Left: two wedge-shaped slices of the survey – each dot a galaxy or quasar DESI has measured, with colour tracking distance and therefore look-back time. Right: a zoomed-in visualisation of the cosmic web – the filaments and voids where galaxies cluster. The ripples and clumps hiding in patterns like these are what cosmologists use to weigh dark energy.

Why cosmologists worship big maps.

This map is more than just an awesomely cool picture. It's a fossil record of how the universe has been expanding for billions of years. Galaxies clump and spread in patterns that carry the imprint of gravity pulling things together and dark energy pushing them apart. The standard model of cosmology – with the catchy name lambda-CDM – treats dark energy as a constant cosmic background pressure, gently shoving galaxies apart forever. DESI is the sharpest tool we've built to test whether that story is actually true.

Is dark energy losing its grip?

Here's the theory, minus the equations. In the early universe, sound waves sloshed through the hot plasma like ripples in a crowded pool. When things cooled and atoms formed, those ripples froze in place as a faint preference for galaxies to be separated by a particular distance – a "standard ruler" baked into the large-scale structure of the cosmos.

Measure that ruler at different distances and you can read off how dark energy has behaved over time.

DESI's 47 million-object map lets you do that with frightening precision.

Earlier data already hinted that dark energy might not be behaving like a constant. The full five-year survey is the chance to check whether that was a statistical fluke, a hidden systematic error, or the first sign that the universe is trying to tell us something new.

"We're in the business of letting the universe tell us how it works, and maybe the universe is telling us it's more complicated than we thought it was." — Andrei Cuceu, Lawrence Berkeley National Laboratory.

If DESI confirms dark energy's push really is evolving, then lambda-CDM isn't the whole story. The ultimate fate of the universe – endless acceleration, gentle easing, or something stranger – would be back up for grabs.

It is a remarkably demanding series of calculations. Yes the DESI map is a hardware triumph, but turning spectra into science is an exercise in statistical paranoia. It involves checking and rechecking calibration, worrying about selection biases, arguing over tiny shifts in the best-fit parameters.

A shifting map.

We've gone from pinprick samples to a 3D atlas of tens of millions of galaxies. And instead of quietly confirming our neat little model, the universe might be hinting we need to redraw the legend. We've built the sharpest cosmic map in history – and it's just possible the road itself is changing under our feet.

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