Astronomy
Why the Universe's Expansion Is Accelerating
Quick fact
The discovery that the universe's expansion is accelerating won the 2011 Nobel Prize in Physics. It implies that about 70% of the universe's energy is made of something completely unknown—dark energy.
Why this is interesting
You've probably heard that the universe is expanding, but what if I told you it's not just expanding—it's speeding up? Astronomers expected gravity to slow the expansion, but the opposite is happening, and the cause remains one of the biggest mysteries in science.
Read the full explanation
Understanding Why the Universe's Expansion Is Accelerating
Imagine throwing a ball upward. You expect gravity to pull it back down, so it slows. For the universe, every galaxy feels the gravitational pull of every other galaxy, so astronomers expected the expansion to slow down over time. But in the 1990s, two teams were using Type Ia supernovae (exploding white dwarfs) as 'standard candles' to measure distances to distant galaxies. They found that these supernovae were dimmer than predicted, meaning they were farther away than expected. This could only be true if the expansion of the universe was speeding up in the past, implying that some unknown force is pushing galaxies apart. This force is now called dark energy.
A deeper explanation
The mechanism behind the acceleration is not fully understood, but the leading explanation is the cosmological constant—a property of empty space itself. According to Einstein's general relativity, empty space has an inherent energy, often called dark energy, that exerts negative pressure, working against gravity. This negative pressure drives the accelerated expansion. Observations of the cosmic microwave background and large-scale structure support this model, known as the ΛCDM model, where dark energy makes up ~70% of the universe. Why such an energy exists is unresolved, tying to quantum field theory and the cosmological constant problem. This acceleration determines the ultimate fate: a universe that expands forever, cooling into a 'Big Freeze', unless dark energy changes over time.