Imagine tossing an apple in the air, expecting it to fall, only for it to accelerate upward into the sky. In 1998, astronomers discovered the universe is doing exactly that.
1. Accelerating Expansion (Dark Energy)
For decades, cosmologists assumed gravity was slowly braking the expansion of the cosmos. Instead, observations of distant supernovae revealed that galaxies are actually accelerating apart. This unknown driver, dubbed "dark energy," makes up nearly 70% of the universe, yet its physical nature remains entirely unknown.
2. Ripples in Spacetime (Gravitational Waves)
Einstein predicted that accelerating objects would create invisible ripples in the fabric of spacetime, like a boat moving through water. He thought they would be too faint to ever detect. In 2015, the LIGO observatories detected a signal lasting a fifth of a second—the "chirp" of two black holes colliding over a billion light-years away. For a brief moment, the power radiated by this invisible collision was 50 times greater than the combined light of all stars in the observable universe. The detection was only possible because the physical distance between mirrors on Earth briefly changed by a fraction of the width of a proton.
3. The Boötes Void
The universe is structured like a cosmic web. Galaxies form long filaments, and between those filaments are dark, empty voids. The Boötes Void is a spherical region of space roughly 330 million light-years across. A sphere of space that size anywhere else in the universe typically contains around 10,000 galaxies. In the Boötes Void, astronomers have found only about 60. As astronomer Greg Aldering noted, if the Milky Way had been in the center of the Boötes Void, humans would not have known other galaxies existed until the 1960s.
4. The Deep Fields
In the 1990s, astronomers pointed the Hubble Space Telescope at a tiny, seemingly empty patch of black sky—an area roughly the size of a grain of sand held at arm's length. It seemed like a poor use of telescope time. But after staring at that blank spot for days, the image came back filled with thousands of glowing jewels. They were not stars; they were entire galaxies, billions of light-years away. Recent observations by the James Webb Space Telescope repeated this feat in infrared, confirming that no matter where telescopes look in the darkness, there are trillions of stars and billions of worlds.