Astronomers study the expansion history of the Universe by measuring how galaxies trace large-scale structure across cosmic time. Spectroscopic redshift surveys provide the most reliable method for constructing three-dimensional maps of matter distribution at these scales. The Dark Energy Spectroscopic Instrument (DESI) was designed to perform this task. After five years of observations, the DESI collaboration has now completed its baseline survey, producing the largest three-dimensional map of the Universe ever assembled.

The dataset comprises distances to more than 47 million galaxies and quasars, spanning nearly 11 billion years of cosmic evolution. This milestone marks the completion of the project’s original observing plan. However, the scientific analysis of the dataset has only begun. Researchers will use this map to test the expansion history of the Universe and examine whether dark energy behaves as predicted by the standard cosmological model.

A survey expanding beyond its original design goals

DESI began routine science observations in May 2021 on the Mayall 4-meter telescope at Kitt Peak National Observatory in Arizona. The collaboration initially planned to measure redshifts for about 34 million galaxies and quasars. Over the course of five years of observations, the instrument consistently exceeded this target by a substantial margin. By April 2026, DESI had measured more than 47 million galaxies and quasars and collected spectra from over 20 million stars in the Milky Way.

This achievement represents the largest spectroscopic mapping effort of the distant Universe completed so far. Earlier redshift surveys mapped millions of galaxies and helped establish the framework of modern cosmology. DESI extended that work by increasing the sample size several times over. As a result, astronomers can now trace cosmic structure with much greater statistical accuracy.

Equally important, the survey spans almost 11 billion years of cosmic history. This long timeline allows researchers to compare the distribution of galaxies at different stages of cosmic evolution. In addition, DESI mapped roughly two-thirds of the northern sky during its baseline mission, covering close to 14,000 square degrees. Such wide coverage improves the reliability of large-scale structure measurements and reduces uncertainties in cosmological analysis.

Constructing a three-dimensional map using spectroscopic redshifts

DESI builds its map using spectroscopy rather than imaging alone. Spectroscopy measures how much the wavelength of light from a galaxy shifts as the Universe expands. Astronomers refer to this shift as redshift. Because redshift increases with distance, it provides a direct way to determine how far away a galaxy lies.

Once researchers measure redshifts for millions of galaxies, they can reconstruct their spatial distribution in three dimensions. This approach allows them to trace the cosmic web across vast regions of space.

DESI performs these measurements with remarkable efficiency. The instrument uses 5,000 robotic fiber positioners mounted at the telescope’s focal plane. Each fiber moves precisely to capture light from a selected galaxy. During a single exposure, DESI records spectra from thousands of objects simultaneously. The collected light travels through ten spectrographs that separate it into wavelength components and measure the redshift accurately.

Night after night, the telescope repeats this process across different regions of the sky. Gradually, these measurements combine to form a large and detailed map of galaxy distribution across cosmic time.

Measuring the expansion history of the universe

One of DESI’s central goals is to study how the expansion rate of the Universe changed over billions of years. Observations in the late twentieth century showed that cosmic expansion is accelerating. This discovery led to the introduction of dark energy as a dominant component of the Universe.

Today, dark energy appears to account for roughly seventy percent of the total energy density of the cosmos. Despite its importance, its physical nature remains unknown. For this reason, astronomers rely on large galaxy surveys to measure how the expansion evolved.

DESI addresses this problem by studying patterns in the large-scale distribution of galaxies. These patterns preserve signatures of baryon acoustic oscillations that formed in the early Universe. The characteristic spacing associated with these oscillations acts as a standard cosmological ruler.

By measuring this scale at different distances, researchers can determine how the expansion rate changed across cosmic history. Because DESI observes galaxies at many redshifts, it can trace this evolution across a wide range of epochs. Consequently, the survey provides one of the most sensitive tests available for determining whether dark energy behaves as a constant or varies slowly with time.

Extending the map beyond the original survey plan

Although DESI has completed its baseline five-year mapping campaign, observations will continue through 2028 as part of an extended program. During this phase, the collaboration plans to expand sky coverage from roughly 14,000 square degrees to about 17,000 square degrees.

The extended survey will include additional faint galaxies and regions closer to the plane of the Milky Way. It will also improve measurements of nearby dwarf galaxies and stellar streams that help trace the structure of dark matter in the local Universe.

At the same time, researchers continue to analyze the dataset already collected. Major cosmological results based on the full survey are expected over the next several years. These results may clarify whether dark energy changes with time or behaves as predicted by the current cosmological model.

With the completion of its baseline survey, DESI has produced the most detailed three-dimensional map of the Universe available today. This achievement marks an important step toward understanding how cosmic structure formed and how the expansion history of the Universe evolved across billions of years.

Clear skies!

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