Dark energy is a term scientists use to describe the unknown force causing the universe to expand at an accelerating rate. Though its exact nature remains elusive, it is believed to constitute about 70% of the universe's total energy and matter.
Michael Turner, an astrophysicist from the University of Chicago, coined the term "dark energy" in 1998. Despite extensive research, dark energy remains one of physics' most significant mysteries.
"I think dark energy is the most profound mystery in all of science," said Turner. "Until we understand it, we can’t sensibly speculate about the destiny of the universe."
The concept gained traction when two separate teams studying supernovae in 1998 found that distant galaxies were moving away faster over time. This discovery contradicted previous expectations that gravity would slow down cosmic expansion.
Joshua Frieman, a professor at UChicago who co-founded and led the Dark Energy Survey, explained: “Dark energy couldn't have become the dominant thing too early in the universe because matter would not have been able to clump together to form galaxies and stars and planets—and us.”
Several theories attempt to explain dark energy. One leading idea is the cosmological constant theory, suggesting that empty space itself has inherent energy causing cosmic acceleration. Another hypothesis is evolving dark energy or quintessence, proposing an unknown field with effects opposite those of normal matter and energy.
"We don’t know what its fundamental nature is. Is it just the quantum energy of empty space? Is it a scalar field? Is it something else that we haven’t even dreamed of?" Frieman asked.
Despite its mysterious nature, scientists estimate that dark energy makes up between 68% and 71% of all matter and energy in today's universe. Various methods are used to measure this proportion, including tracking supernovae movements and analyzing light from early cosmic history.
Understanding dark energy could significantly impact our knowledge of cosmic evolution and future predictions for the universe's fate. "Each explanation of dark energy would have big implications for our understanding of the cosmos and of the laws of physics that govern it," Turner noted.
Major experiments like DES (Dark Energy Survey) and DESI (Dark Energy Spectroscopic Instrument) continue to study this phenomenon using advanced telescopes located on mountaintops in Chile and Arizona respectively. Future projects such as NASA’s James Webb Space Telescope and Vera C. Rubin Observatory are expected to provide more precise measurements.
“To see the effects of dark energy, we need to go beyond our galaxy,” said Frieman. “You’re talking about observing hundreds of millions of galaxies with very high precision.”
Turner concluded: “This is a very big, very rich mystery, and the story’s going to take a while to find out."