Largest ever map of universe captures 47 million galaxies and quasars

A five-year survey of the sky that captured more than 47 million galaxies and quasars is now complete, allowing researchers to put the finishing touches on the most detailed map of the universe ever made. This data could help solve the mystery of an apparent weakening of dark energy, which threatens to upend our standard model of the cosmos.

The Dark Energy Spectroscopic Instrument (DESI) at Kitt Peak National Observatory in Arizona has been scanning the sky since 2021. Researchers initially expected its survey to collect data on 34 million galaxies and quasars, but DESI surprised researchers with its effectiveness. Because of the large distances involved, some of these extremely faint galaxies have been observed from just 100 or 200 photons.

David Schlegel of the Lawrence Berkeley National Laboratory in California says our previous maps of the cosmos include a total of 5 million galaxies, so the DESI data increases our knowledge of the universe by a factor of nearly 10.

“We’ve followed this curve throughout my career: every ten years we create cards 10 times larger,” he says. “You may ask yourself: When did you map every observable galaxy within 10 billion light years…and if we stayed on the curve, we would do it by 2061.”

The main survey has now been completed, but the data will take another year to analyze before it is made available to researchers. The project will continue to collect data for at least two and a half years, and Schlegel says there is hope that DESI can be upgraded and continue operating into the 2030s. “It’s still the leading instrument of its type in the world,” he says.

DESI’s map now covers 14,000 square degrees of the sky, but the team hopes to expand that to 17,000 square degrees. The entire sky measures more than 41,000 square degrees, but much of this area is difficult to observe due to the presence of relatively nearby, bright objects, such as our own galaxy, the Milky Way.

The data will allow scientists to compare the distribution of galaxies in the distant past and today. This could lead to a better understanding of the power of dark energy, which makes up around 70% of the universe. An earlier DESI data set from 2024 suggested that, rather than remaining constant as expected, dark energy weakens over time.

If dark energy were indeed weakening, it would have profound implications for the Standard Model of cosmology, known as lambda-CDM. The complete DESI data set will allow this phenomenon to be studied in more depth.

Ofer Lahav, of University College London, says having access to the latest DESI map would have seemed like science fiction at the start of his career. “When I was a PhD student at Cambridge 40 years ago, we had a sample of thousands of galaxies. The community was hungry for data,” he says. “I think my students [today] may have the opposite problem; have been inundated with data, and it is very difficult to analyze it.

With so much data, there will be scientific breakthroughs about the nature of the universe, Lahav says, but we’ve also likely detected unusual one-off cosmological incidents that lead to exciting research.

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