Physics

A New Explanation For Dark Energy Says It's Just Fluctuations In The Vacuum Of Space

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Of all the questions that plague scientists, the reason for the universe's expansion might be the most perplexing. Why distant galaxies aren't just moving away from us, but actually accelerating away from us, has led scientists to invent murky and mysterious substances like dark matter and dark energy—each named specifically for the fact that we don't know what they are. In 2017, physicists from the University of British Columbia published a new explanation for the accelerating expansion of the universe. The most intriguing part about it? It may take the "dark" out of dark energy, explaining what it is once and for all.

Force Of Mystery

When Albert Einstein published his general theory of relativity in 1916, something didn't sit right with him. His theory predicted that the universe must either be expanding or contracting, and he and most scientists at the time believed the universe was static. To "fix" his theory, he came up with a new term he called the cosmological constant as a way to stop the expansion—to put a mathematical lid on it, so to speak. In 1929, however, Edwin Hubble announced that his study of nearby galaxies showed that the universe was indeed expanding, and Einstein later said adding his constant was his "greatest mistake." Modern scientists now recognize that this expansion is actually accelerating.

But physicists didn't throw out the cosmological constant. Instead, they repurposed it. Now, it's generally understood as the "force" that pushes the universe to expand. Put another way, the cosmological constant describes the energy density of empty space, or vacuum energy—and we're not sure what it is, which means we're not sure why the universe's expansion is accelerating. Enter: dark energy, an invisible force of some kind that's making the universe expand.

One theory is that dark energy is actually that aforementioned vacuum energy. The problem there is that quantum field theory (the theory that describes how very tiny particles interact) predicts vacuum energies that are so big that the universe shouldn't even exist. And when you put acceleration into the mix? Forget about it. According to Jesse Empak of LiveScience, "Essentially, the energy density of the universe (how much energy there is per unit volume) should be gigantic, and it clearly isn't."

Can't All The Theories Just Get Along?

To fix this problem, many scientists have toyed with the idea of modifying the theories of general relativity and quantum mechanics. But as PhD student Qingdi Wang, the lead author of the new explanation, told Universe Today, "Instead of trying to modify quantum mechanics or General Relativity, we believe that we should first understand them better. We take the large vacuum energy density predicted by quantum mechanics seriously and just let them gravitate according to General Relativity without modifying either of them." If you can't beat the theories, join 'em.

Wang, his professor William Unruh, and fellow PhD student Zhen Zhu, performed new calculations on vacuum energy, taking into account the massive energies predicted by quantum mechanics. Then they did one more thing: they assumed that, in the very smallest levels of spacetime, there might be wild fluctuations that oscillate between expansion and contraction. As Empak describes it, "The vacuum fluctuations, in Wang's formulation, are like children on a swing pumping their legs. Even though nobody is pushing them, they manage to impart extra energy on the swing, making the swing go up higher than it would otherwise." That would mean that the big effects of vacuum energy cancel each other out, while also leading to a slow but accelerating expansion of the universe.

It's too soon to tell whether this is the solution to the mystery of dark matter, but it's certainly an important development in understanding the nature of our universe.

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