The universe is being pushed apart by a ghost force
In short: For decades, scientists assumed gravity would eventually slow down the expansion of our cosmos. They were wrong. Something invisible and mysterious—dark energy—is currently winning the tug-of-war against gravity, driving galaxies away from each other at an ever-increasing speed.
Dark energy is a catchall term for the mysterious force making up approximately 68% to 70% of the universe. Despite its dominance, it remains one of the most profound mysteries in physics because no one has ever directly seen or measured it. It was only identified in the late 1990s when observations of supernovae revealed that distant galaxies are moving away from us at an accelerating rate rather than slowing down as expected. The term was officially coined by Michael Turner in 1998. It represents a force that challenges our current grasp of fundamental physics.
The history of this discovery relies on foundational work from the early 20th century. In 1912, Henrietta Swan Leavitt established a relationship between the period and luminosity of Cepheid variables, allowing astronomers to measure distances to far-off galaxies. Around that same time, Vesto Slipher used a spectrograph to observe 'redshift'—the stretching of light waves as objects move away. These tools eventually allowed 1990s researchers to confirm that the universe began accelerating its expansion about nine billion years after the Big Bang. Leavitt's work specifically utilized stars whose brightness fluctuates with regularity. Slipher used a relatively new device called a spectrograph to split light into colors.
Physicists currently propose several competing theories to explain this behavior. One leading concept is the 'cosmological constant,' which suggests that empty space itself possesses energy. Another theory, known as 'quintessence' or evolving dark energy, proposes a previously unknown field with properties opposite to matter. A third possibility is that our fundamental understanding of gravity is simply incorrect. Until we solve this enigma, we cannot accurately predict the ultimate destiny of the universe. The expansion began speeding up again about five billion years ago. This acceleration occurs despite gravity attempting to pull galaxies together.
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Sources
- Summary by Intellect · source: Dark Energy Explained | Cosmology 101 Episode 8
- Also: What is Dark Energy? Inside Our Accelerating, Expanding Universe - NASA Science
- Also: Dark energy, explained
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