This might shock you: All the stuff you’ve ever seen makes up a paltry 5 percent of the universe. And scientists don’t have a definitive theory to describe the remaining 95 percent.
We think that 95 percent is made of two mysterious substances – dark matter (23 percent) and dark energy (72 percent). Today’s column focuses on the latter.
In 1998 and 1999, two collaborations were measuring how the expansion of the universe was changing. They expected to see that the expansion was slowing, based on this simple fact – gravity attracts. Throw a ball up and it slows and then falls back down. If you had Superman’s arm, you could throw the ball hard enough for it to escape the Earth’s gravity. But even in that case, the ball would slow down as it flew into space.
The expectation was the same with the expansion of the universe. About 14 billion years ago, the universe began with the Big Bang. This explosion caused all of the matter in the universe to fly apart from one another. Early in the history of the universe, the expansion was faster and gravity should have slowed it down in the intervening eons. At the time, the debate was whether the universe would expand to a maximum size and come crashing back together in a Big Crunch or expand forever.
Scientists were then shocked to find that the expansion of the universe wasn’t slowing down. It was speeding up! Something was overcoming the force of ordinary gravity. What could that be?
Early in the days of general relativity, Einstein realized that his formulas said that the universe should be expanding or contracting and he added an extra term to his equations called the cosmological constant to guard against a Big Crunch. However, when Edwin Hubble discovered in the 1920s that the universe was expanding, Einstein abandoned the cosmological constant and called it his biggest blunder.
With the observation of the speeding expansion of the universe, the cosmological constant was revisited. The idea is that the universe has an energy field throughout it with constant density. Cosmologist Michael Turner coined the term “Dark Energy” to describe this energy field.
The unexpected behavior of the universe’s expansion was discovered only about a decade ago. That’s about enough time for the scientific community to overcome the shock and verify the measurement was accurate. Next came designing more sophisticated equipment to better investigate the phenomenon. Soon to be operational, the Dark Energy Survey camera, in conjunction with the Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile, will hopefully begin to catalogue the sky later this year, looking for hints of Dark Energy.
This is an extraordinarily exciting time in the dark energy saga. Over the next years, several facilities will look more carefully at the question of the expansion of the universe, and, hopefully, we will begin to shed some light on the dark.
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