The idea known as the anthropic principle states that human existence is possible only if fundamental constants such as the speed of light or the strength of gravity are not higher or lower than what is observed.
Scientists who support anthropic reasoning suggest they can understand fundamental properties of the universe by determining what conditions intelligent beings such as humans need to exist. For instance, if gravity was too strong, black holes would form too often and suck up all matter before humans could evolve on Earth, but if gravity was too weak, it could not attract matter to form stars or planets.
According to advocates of anthropic reasoning, in 1987 Nobel laureate Steven Weinberg used the logic to calculate the cosmological constant-the strength of the mysterious force driving the universe apart-with surprising accuracy, well before astronomical observations turned up similar findings. What astronomers have since observed is that the universe's expansion is accelerating, driven by a sort of mysterious force dubbed either dark energy or vacuum energy.
Quantum physics predicts the cosmological constant should be far larger than what is actually seen: roughly 10120 times larger, a number representing 1 with 120 zeroes behind it.
In comparison, Weinberg's original estimate was off just by roughly a hundredfold, and refined versions of this argument claim greater accuracy, suggesting anthropic reasoning could provide answers quantum physics currently cannot.
However, theoretical physicist Glenn Starkman of Case Western Reserve University in Cleveland and cosmologist Roberto Trotta at Oxford University in Englandtake issue with how anthropic reasoning predicts the cosmological constant.
Anthropic reasoning calculates the cosmological constant by first assuming that humanity needs a galaxy to exist. Galaxies form when gravity pulls matter together. This continues to happen until dark energy or vacuum energy, begins a tug of war against gravity that gravity inevitably loses. Some current predictions hold that the universe will eventually come apart so completely that planets will explode and even molecules will be ripped apart.
The number of galaxies seen in the universe can exist only if vacuum energy is weak enough to allow gravity to win for a time. Scientists know the strength of gravity, so by measuring the number of galaxies, anthropic reasoning can tell us the maximum size of the cosmological constant-that is, the maximum strength of vacuum energy.
In the Nov. 17 issue of the journal Physical Review Letters, Starkman and Trotta argue there are many other equally valid anthropic approaches to calculating the cosmological constant that lead to wrong answers.
For instance, Starkman and Trotta note that in order to live and thus view the universe, humans need to collect and expend energy. Using this rationale, humans should prefer a universe that is flying apart as slowly as possible, making it easier to go out and collect energy to expend. In such a universe, the cosmological constant should be as low as possible, even lower than the value seen now.
"The specifications used to allow life are rather arbitrary and can lead to very different expected values for fundamental constants," Starkman told SPACE.com.
Anthropic advocates are not swayed.
"The observed value of the cosmological constant, which has been predicted from anthropic considerations, has so far no other plausible explanation," said theoretical physicist Alexander Vilenkin, director of the Institute of Cosmology at Tufts University.
"The best way to make progress is to continue to try to understand the nature of dark energy by observations and experiments," said senior astrophysicist Mario Livio at the Space Telescope Science Institute in Baltimore. "Those, in turn, will allow for theoretical progress as well."
This article is part of SPACE.com's weekly Mystery Monday series.
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