The universe is accelerating, and we have no idea why.
In short: For decades, scientists assumed gravity would eventually act as a cosmic brake, slowing down the expansion of the universe. They were wrong. In the late 1990s, observations revealed a shocking truth: the universe isn't just expanding; it is speeding up, driven by a mysterious force that makes up the vast majority of our cosmos.
The discovery of dark energy fundamentally upended our understanding of physics. In the late 1990s, two independent teams used the Hubble Space Telescope to observe Type 1A supernovae. Because these explosions have a consistent luminosity, they act as 'standard candles' to measure distance. The data revealed that distant galaxies were moving away at increasing speeds. This suggested that an enormous, mysterious energy is pushing against gravity to drive this acceleration.
What exactly is this force? Scientists currently lack a definitive answer, leading them to use 'dark energy' as a catchall term. One leading theory is the 'cosmological constant,' which suggests that the energy of empty space itself causes the acceleration. Another possibility is 'quintessence,' a previously unknown field with properties opposite to normal matter. It is also possible that our current understanding of gravity is simply incomplete.
The scale of dark energy is staggering. It is estimated to comprise between 68% and 71% of the universe today—more than 10 times the amount of all visible matter. Some theories suggest it acts as a form of 'anti-gravity' emanating from the fabric of space. As space expands, more of this force comes into existence, creating a positive feedback loop that could eventually overwhelm all gravity, potentially tearing apart galaxies, solar systems, and even atoms over trillions of years.
This mystery was only made possible by foundational work in the early 20th century. In 1912, Henrietta Swan Leavitt discovered a relationship between the period and luminosity of Cepheid variables, allowing for precise distance measurements. Later, Vesto Slipher used a spectrograph to observe 'redshift'—the stretching of light wavelengths as objects move away—providing the first evidence of how quickly galaxies were receding.
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Sources
- Summary by Intellect · source: What is Dark Energy? simply explained
- Also: What is Dark Energy? Inside Our Accelerating, Expanding Universe - NASA Science
- Also: Dark energy, explained
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