Universe Acceleration: Why Nearby Measurements Matter So Much
New data on millions of galaxies hinted that the Universe's expansion might be slowing down right now. But that's odd: we're used to thinking that dark energy is pushing everything apart faster and faster. It turned out to be a case of "short-sightedness" in the new observations—they don't look closely enough at our neighborhood, like a weather forecast from the distant past. So, is the Universe really braking?
The universe is expanding, and about a quarter century ago, Adam Riess and his colleagues showed that this expansion is speeding up. The cause of the accelerating expansion of the universe is tied to mysterious dark energy, though its exact nature remains unknown. To track this acceleration, astronomers build giant maps of galaxy positions using spectroscopy—a method that reveals how fast each galaxy is moving away from us through its redshift. Two major surveys, SDSS and DESI, measured millions of such redshifts and compared them to the imprint of sound waves frozen in the early universe and preserved in the cosmic microwave background. Georges Lemaître laid the theoretical groundwork for this analysis back in the 1920s, and Edwin Hubble was the first to observe galaxies receding from each other.
The reason for this discrepancy is simple, like watching a departing train: to tell if it’s accelerating now, you need to measure its speed right at the platform, not a kilometer away from the station. SDSS “looked” at the closest galaxies to us (with a redshift of about 0.15), while DESI looked at slightly more distant ones (0.295). When scientists excluded the nearest objects from SDSS data, its result also became uncertain—just like DESI’s. And conversely, adding observations of nearby supernovae immediately restored confidence in acceleration.
The bottom line is simple: to reliably learn what dark energy is doing today, astronomers need more observations from the closest possible distances. Future surveys, such as Euclid, will fill this gap and help finally unravel the acceleration of the universe.
🎯 If we only had DESI data, we’d think the universe first slowed down and then accelerated again—but that’s an illusion caused by missing nearby measurements.