The universe is expanding faster and faster, but we have no idea why
In short: In the late 1990s, astronomers expected to find that gravity was slowing the universe's expansion. Instead, they discovered the opposite: space is stretching at an accelerating rate. This revelation introduced dark energy, a mysterious force that makes up nearly 70% of the cosmos yet remains fundamentally unknown.
The discovery of cosmic acceleration relies on Type Ia supernovae, which serve as 'standard candles.' By measuring the brightness and redshift of these distant explosions, researchers like Adam Riess and Brian Schmidt—who shared the 2011 Nobel Prize for this work—determined that distant objects are receding faster than they would in a universe governed solely by gravity. This led to the current consensus that dark energy acts like a form of repulsive gravity, possessing negative pressure that stretches spacetime. These white dwarf star explosions are extremely bright. This allows for precise distance measurements across cosmic history.
Several theories attempt to explain this phenomenon. One is the 'cosmological constant,' where empty space contains a constant energy density called vacuum energy. Quantum mechanics suggests that vacuum fluctuations create energy, a fact supported by the Casimir effect. However, this model faces a massive discrepancy: the predicted vacuum energy is about 10¹²⁰ times larger than what is actually observed, making it arguably the worst prediction in physics. This constant energy density is represented by the Greek letter lambda. It is a core component of Einstein's equations.
Alternatively, dark energy might be a dynamic field called quintessence. Unlike a constant, quintessence could correspond to weakly interacting particles that change strength over time. If this field strengthens, it could eventually lead to a 'Big Rip,' where galaxies, atoms, and spacetime itself are torn apart. While some speculative studies have suggested a correlation between supermassive black holes and dark energy, the primary mystery of what drives this acceleration remains one of the most profound open questions in modern science. This field might have particle-like excitations. Recent research also explores a possible link to how black holes grow over billions of years.
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Sources
- Summary by Intellect · source: The Universe Is Accelerating...and No One Knows Why
- Also: The universe is still speeding up, but nobody knows why
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