Largest ever map of universe captures 47 million galaxies and quasars

A five-year survey of the sky that has captured more than 47 million galaxies and quasars is now complete, enabling researchers to put the finishing touches on the most detailed map of the universe ever made. The 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 originally expected its survey to gather data on 34 million galaxies and quasars, but DESI surprised researchers with its efficiency. Because of the vast distances involved, some of these extremely faint galaxies have been observed from just 100 or 200 photons.

David Schlegel at 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 almost 10.

“We’ve actually been on this curve now for my whole career where, every 10 years, we’re making 10-times-larger maps,” he says. “You can ask the question, at what point have you mapped every observable galaxy within 10 billion light years… and if we stayed on the curve, we would do that by 2061.”

The main survey is now complete, but the data will take another year to analyse before it is made available to researchers. The project will continue to collect data for at least another two and a half years, and Schlegel says there are hopes that DESI can be upgraded and kept running well into the 2030s. “This is still the leading instrument like it in the world,” he says.

DESI’s map now covers 14,000 square degrees of the sky, but the team hopes to expand this to 17,000 square degrees. The full sky has over 41,000 square degrees, but much of that is hard to observe because of relatively close and bright objects, such as our own galaxy, the Milky Way.

The data will allow scientists to compare how galaxies were distributed in the distant past and today. This could lead to insights into the power of dark energy, which makes up about 70 per cent of the universe. An earlier dataset from DESI in 2024 suggested that, rather than remaining constant as expected, dark energy is weakening over time.

If dark energy is indeed weakening, it would have profound implications for the standard model of cosmology, known as lambda-CDM. The full set of DESI data will allow that phenomenon to be investigated further.

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

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

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