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Dark energy's weakness may be why supernovae didn't kill us all

The supernova that made the Crab Nebula didn’t kill off humanity, which means we are here to observe the universe as it is.

J. Hester and A. Loll/Arizona State University/NASA/ESA

That you exist in the universe is pretty obvious, at least to yourself. But now researchers have used the fact that human observers are alive—and haven’t been zapped into oblivion by supernova explosions—to account for the puzzling weakness of dark energy, the mysterious force accelerating the expansion of the universe.

“This creates a new link between [dark energy] and astrobiology, which were previously considered to be vastly different fields,” says Tomonori Totani, an astronomer at the University of Tokyo and lead author of the new study.

Most people don’t think of dark energy—the all-permeating force driving apart galaxies—as particularly weak. But based on arguments from quantum mechanics and Albert Einstein’s equations for gravity, scientists estimate that dark energy ought to be at least 120 orders of magnitude stronger than it actually is.

If dark energy were that powerful, it would have quickly driven apart matter in the early universe, preventing the formation of galaxies, stars, and living beings. This has led some scientists to invoke what’s called the anthropic principle, which proposes that the laws of physics in our universe were fine-tuned to produce life.

Along with his colleagues, Totani had previously simulated the evolution of the universe for different dark energy strengths, limiting the models to those that could form galaxies capable of hosting living creatures. They found the expected value of dark energy from such simulations to be 20 to 50 times larger than what is seen in reality. The result was a vast improvement over arguments based on pure physics, though they still couldn’t fully explain dark energy’s observed weakness.

In their new calculations, the researchers took a closer look at models where dark energy was approximately 50 times stronger than it is in our cosmos. Galaxies could arise in such a universe, but only during the very earliest epochs, before the mysterious substance’s full power kicked in and drove everything apart. Because the early universe was quite dense, the galaxies that managed to form would be packed with stars, 10 times denser than galaxies such as our Milky Way.

In these dense galaxies, the average star would be much closer to its neighbors. Massive stars, which live short lives and then explode as incendiary supernovae, would deliver lethal doses of radiation to nearby planets, sterilizing any life that happened to exist—and leaving behind no observers.

The researchers calculated that this effect, which had not been previously considered, would make the universe unfavorable to life. Therefore, the observed weakness of dark energy is why we are here, they report in a 27 April paper published on the preprint repository arXiv. Totani says his proposal could be strengthened if future astrobiologists find that life is far more rare in the densest regions of the galaxy.

Tsvi Piran, an astrophysicist at The Hebrew University of Jerusalem who has also speculated about the anthropic principle’s effect on the bounds of dark energy, says some of study’s assumptions are a bit shaky. For instance, the lethal power of supernovae comes mainly from their gamma ray radiation. But only some of their energy is channeled into such radiation, making supernovae somewhat inefficient killers. An especially powerful subset of supernovae, known as gamma ray bursts, are thought to be more important for destroying life in the universe, though these events are much rarer. That the study did not account for this rarity somewhat undermines Totani’s argument, Piran says.

Invoking the anthropic principle itself is controversial, he adds. “I know people who will walk out of the room once you begin to suggest it,” he says. “Others say this is a meaningful argument that should be taken seriously.”

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