The invisible force making up 70% of our universe
In short: For decades, scientists assumed gravity would eventually slow down the expansion of the cosmos. They were wrong. In the late 1990s, we discovered a mysterious force pushing everything apart at an accelerating rate. It is the most dominant yet unknown element in existence.
The universe began with the Big Bang approximately 13.8 billion years ago. Initially, gravity acted as a cosmic brake, slowing down the rapid expansion that followed the first fraction of a second. However, about five billion years after the Big Bang, this trend reversed. Instead of slowing down, the expansion began to speed up, driven by an unknown force now identified as dark energy.
Dark energy is not something we have directly seen or measured; it is a 'catchall' term for whatever is causing this acceleration. Modern projections indicate that roughly 68% to 71% of today's universal matter and energy consists of this substance. While its exact nature remains one of physics' greatest mysteries, scientists have proposed several leading theories to explain it.
One primary hypothesis is the 'cosmological constant,' which suggests that empty space itself possesses energy that drives cosmic acceleration. Another theory, known as 'quintessence,' proposes a previously unknown field that behaves oppositely to matter and normal energy. A third possibility is that our fundamental understanding of gravity is simply incorrect.
The discovery of this phenomenon in the late 1990s was made possible by centuries of foundational work. It traces back to Henrietta Swan Leavitt's 1912 study of Cepheid variables, which allowed for distance measurements in far-off galaxies, and Vesto Slipher's early observations of galactic redshift—the stretching of light as objects move away from us.
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Sources
- Summary by Intellect · source: Dark Energy
- Also: What is Dark Energy? Inside Our Accelerating, Expanding Universe - NASA Science
- Also: Dark energy, explained
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