For life to flourish, the universe had to meet a nesting doll of conditions. The quantum fluctuations that led to the Big Bang had to be just right to kickstart the cosmological chemistry necessary for star, galaxy and planetary formation. Eventually, conditions on one of these planets — Earth — were just right to support the rise of organic matter, leading to the evolution of life. Leading to us.
Was the universe tuned to support life? Or was it cosmological happenstance?
Scientists and philosophers have pondered this question, but a definitive conclusion is hard to come by. After all, how do you experimentally test a philosophical concept like the anthropic principle? The anthropic principle states that since we can observe the universe, it must have evolved with the conditions necessary to support intelligent life.
“Our existence tells us something about the past history of the universe,” said Nemanja Kaloper, a professor in the Department of Physics and Astronomy. “Certain things had to happen in a certain way to produce the conditions we now encounter."
In a paper published in the Journal of Cosmology and Astroparticle Physics, Kaloper and Alexander Westphal, a professor at the German Electron Synchrotron, propose a way to potentially test the anthropic principle.
Cosmic inflation, dark matter and fuzzy axions
The researchers’ proposal requires three conditions that must be met to falsify the anthropic principle.
First, cosmic inflation must be proved. This theory describes the universe’s superluminal expansion that occurred less than a second after the Big Bang.
Second, the existence of fuzzy axions — a theoretical particle with a mass infinitesimal compared to an electron — must be proved.
And third, those fuzzy axions can’t be the building blocks of dark matter, an unknown, unobservable substance that physicists estimate to be five times more prevalent in the universe than ordinary matter.
The ability to test for these three conditions hinges on comparing theoretical physical models of the universe, including cosmological parameters like the expansion rate of the universe, to astronomical observations that will be possible within the next decade thanks to advanced scientific satellites.
To detect signatures of cosmic inflation, Kaloper referenced the LiteBIRD satellite, a proposed space observatory led by the Japan Aerospace Exploration Agency with a launch date of 2032.
“The LiteBIRD will look for something called primordial gravitational waves, which is a very specific type of signature left from this early stage of cosmic inflation,” Kaloper said.
If such signatures are detected, it bolsters, if not proves, that the cosmic inflation theory is sound.
Proof of the existence of fuzzy axions could come from future observations of black holes, Kaloper said. According to current theory, clouds of fuzzy axions surround black holes, affecting their rotational energy. Future observatory missions may be able to detect axion-induced effects based on measures of a black hole’s properties.
Stacking the nesting dolls for life in the universe
These imminent technologies may help scientists move the anthropic principle from philosophical conjecture to testable hypothesis.
“With early inflation and the black holes that have spins affected by axions, we can actually come up with a situation where we in fact have a fairly firm prediction for when the anthropic principle actually fails,” Kaloper said.
For the anthropic principle to be falsified, fuzzy axions would have to be ruled out as dark matter candidates, according to Kaloper’s and Westphal’s proposal. Future direct dark matter searches may discover that most dark matter is in fact not the fuzzy axion.
“If we lived in such a multiverse that underwent this stage of inflation, that has many ultralight (fuzzy) axions, many places would in fact not be conducive to life,” Kaloper said. “Inflation would generate too much dark matter, and so you would have too fast galaxy formation, too many black holes. Nasty stuff would happen.”
Finding such evidence would suggest that the universe’s initial conditions, the ones that led to the development of intelligent life, are exceptionally rare in the multiverse. They’re not the factory settings.
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