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Immediately after sunset, astronomers and coyotes at the Kitt Peak National Observatory start their nightly routine once they escape the sun's heat and light. Located in the Arizona-Sonoran Desert on the Tohono Oʼodham Nation, more than twenty telescopes start their calibrations with metallic stretching sounds. On the taller mountain, the biggest of these telescopes hosts a four-meter mirror that has to bear a wild wind that never eases up. This is where the Dark Energy Spectroscopic Instrument, also known as DESI, is challenging our knowledge of the Universe’s expansion.
The Universe’s expansion is speeding up
In 1998 astronomers found from observations of supernovae, the colossal explosions of giant stars, that the expansion of the Universe is accelerating. This discovery taught us that distant galaxies are moving away from each other faster and faster as the Universe evolves. The known laws of physics could not explain this accelerated expansion, so astronomers decided to use the term dark energy to refer to it. As the new millennium started, new technologies pushed astronomical observations forward, allowing us to make more precise measurements and start to understand the properties of dark energy in more detail.
Nowadays, cosmologists use millions of galaxies, some of them observed when the universe was 11 billion years younger, to see if this expansion actually changes with time... and that might be the case! In order to get this information, collecting this kind of data requires decades of observations with an incredibly powerful telescope. As no single scientist is able to build it by themselves and analyze its large amounts of data, we group and create collaborations.
The DESI Collaboration is revealing more mysteries about dark energy in only its first year of observations
DESI is such a collaboration. The DESI instrument, built at the Mayall Telescope at Kitt Peak National Observatory, uses 5,000 optical fibers that capture light collected from a four-meter mirror. These tiny tubes, similar to those that carry high-speed internet connections, transport the light to a spectrograph, a machine that splits the light into the different frequencies that comprise it. The resulting pattern, a spectrum, gives DESI scientists (like me) valuable information about what the galaxy is made up of and how far away it is. It is not for nothing that astronomers say that a spectrum is worth a thousand pictures!
Gathering the information from these 5,000 fibers for several years enables us to build up a 3D map of the Universe containing tens of millions of galaxies, some of which are up to 11 billion years old. These galaxies allow us to study the historical effect of dark energy on the expansion of the Universe. In particular, we use Baryonic Acoustic Oscillations (BAO), which are geometrical features in the form of bubbles in the distribution of galaxies (see the video below).
Sound waves from the nascent universe, called baryon acoustic oscillations (BAOs), left their imprint on the cosmos by influencing galaxy distribution. Researchers have explored this imprint back to when the universe was three billion years old, or roughly 20% of its current age of 13.8 billion years. (Credit: NASA/JPL/USRA.)
The BAO carries the imprint of an early Universe, which has expanded and cooled since the Big Bang. Measuring the bubbles at different times helps us to see if there are any substantial changes throughout the history of our cosmos. In April of 2024, the DESI collaboration used its first year of data to measure the BAO, leading to one of the most precise measurements of the cosmos. DESI found that the properties of dark energy have changed in the last 5 billion years (check our official press release here!). Soon, early in 2025, we will release the data to the world and we expect other scientists will also use it for their science goals.
What does an early career scientist like me do in DESI?
Measuring the BAO comes with plenty of challenges. One, for example, is that the signal gets smeared with time: galaxies not only move as the Universe expands, but they can also feel the gravity from neighboring galaxies, adding another layer of complexity to their motion. We can account for this effect by figuring out the galaxies' original, unperturbed positions and moving them back through a technique called BAO reconstruction. For my research, I use these displacements to calculate the galaxies' approximate velocities and learn about how they move on top of the Universe's expansion. Then I study their interaction with remnant light from the early Big Bang. I am planning to publish my results by the end of 2024!
But before going through the publication and peer review processes, we must check our results: many things could affect our science, from one galaxy overlapping another one in the sky to a short circuit in our telescope. Something I love about DESI is that early career scientists can get the chance to support observations both in person and remotely, to learn about these kinds of errors. But when we're at the top of the mountain instead of working remotely, it's an unforgettable experience: isolated from the rest of the world, we've stepped away from the hectic and stressful environment we are usually exposed to.
Besides doing great science, DESI scientists are also encouraged to do scientific outreach. We even have an official ambassador called BaoBan! BaoBan is a coyote who assists us with science communications. “Bao” is reminiscent of baryon acoustic oscillations (BAO), while “Ban” is the word for coyote in the official language used by the Tohono O’odham Nation. If you are lucky enough, you could hear BaoBan in the middle of the night at the top of the mountain at Kitt Peak National Observatory while looking at the data from millions of galaxies.
The author is deeply grateful for permission to undertake observations of the night sky within the Tohono O’odham Nation homeland at Kitt Peak National Observatory.
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