Free Astronomy Magazine July-August 2024

JULY-AUGUST 2024 A slice of the 3D map of galaxies collected in the first year of the Dark Energy Spectroscopic Instrument (DESI) Survey with annotations identifying key features in the map. Earth is at the tip, with the furthest galaxies plotted at dis- tances of 11 billion light-years. Each point represents one galaxy. This version of the DESI map includes 600,000 galaxies — less than 0.1% of the survey’s full vol- ume. [DESI Collaboration/NOIRLab/NSF/AURA/R. Proctor] billion years in the past, when the Universe was only a quarter of its current age, using a feature of the large-scale structure of the Universe called Baryon Acoustic Oscillations (BAO). BAO are the leftover imprint of pressure waves that permeated the early Universe when it was nothing but a hot, dense soup of subatomic particles. As the Universe expanded and cooled the waves stagnated, freezing the ripples in place and seeding future galaxies in the dense areas. This pattern, re- sembling the rippling surface of a pond after a handful of pebbles is tossed in, can be seen in DESI’s de- tailed map, which shows strands of galaxies clustered together, sepa- rated by voids where there are fewer objects. At a certain distance, the BAO pattern becomes too faint to detect using typical galaxies. So instead astronomers look at the ‘shadow’ of the pattern as it’s back- lit by extremely distant, bright ga- lactic cores known as quasars. As the quasars’ light travels across the cosmos it gets absorbed by inter- galactic clouds of gas, allowing as- tronomers to map the pockets of dense matter. To implement this technique, researchers used 450,000 quasars — the largest set ever col- lected for this type of study. With DESI’s unique ability to map millions of objects both near and far, the BAO pattern can be used as a cosmic ruler. By mapping nearby galaxies and distant quasars, as- tronomers can measure the spread of the ripples across several periods of cosmic history to see how dark energy has stretched the scale over time. “We’re incredibly proud of the data, which have produced world-leading cosmology results,” said Michael Levi, DESI director and LBNL scientist. “So far we’re seeing basic agreement with our best model of the Universe, but we’re also seeing some potentially inter- esting differences that could indi- cate dark energy is evolving with time.”