Fine tuning specifics: universe expansion speed (Introduction)

by David Turell , Thursday, March 03, 2016, 22:17 (3187 days ago) @ David Turell

Acts as a protection against gamma ray bursts which could kill life by moving planets out to the safer areas of galaxies:- http://www.sciencemag.org/news/2016/02/conditions-life-may-hinge-how-fast-universe-expa... dense environments, you have many explosions, and you're too close to them,” says cosmologist and theoretical physicist Raul Jimenez of the University of Barcelona in Spain and an author on the new study. “It's best to be in the outskirts, or in regions that have not been highly populated by small galaxies—and that's exactly where the Milky Way is.”-***-"Jimenez and colleagues tackled one, large-scale facet of the anthropic principle. They used a computer model to run simulations of the universe expanding and accelerating at many different speeds. They then measured how changing the cosmological constant affected the universe's density, paying particular attention to what that meant about gamma ray bursts raining down radiation on stars and planets.-"As it turns out, our universe seems to get it just about right. The existing cosmological constant means the rate of expansion is large enough that it minimizes planets' exposure to gamma ray bursts, but small enough to form lots of hydrogen-burning stars around which life can exist. (A faster expansion rate would make it hard for gas clouds to collapse into stars.)-"Jimenez says the expansion of the universe played a bigger role in creating habitable worlds than he expected. “It was surprising to me that you do need the cosmological constant to clear out the region and make it more suburbanlike,” he says.-"Beyond what they reveal about the potential for life in our galaxy and beyond, the findings offer a new nugget of insight into one of the biggest puzzles in cosmology: why the cosmological constant is what it is, says cosmologist Alan Heavens, director of the Imperial Centre for Inference and Cosmology at Imperial College London.-***-"Lee Smolin, a theoretical physicist at Perimeter Institute for Theoretical Physics in Waterloo, Canada, and a skeptic of the anthropic principle, says the paper's argument is a novel one and that on first reading he didn't see any obvious mistakes. “I've not heard it before, so they're to be praised for making a new argument,” he says.-"However, he adds, all truly anthropic arguments to date fall back on fallacies or circular reasoning. For example, many tend to cherry-pick by looking only at one variable in the development of life at a time; looking at several variables at once could lead to a different conclusion.-"Jimenez says the next step is to investigate whether gamma ray bursts are really as devastating to life as scientists believe. His team's work has shown only that exposure to such massive bursts of radiation would almost certainly peel away a planet's protective ozone layer. “Is this going to be catastrophic to life?” he says. “I think so, but it may be that life is more resilient than we think.'”-Comment: Further work may answer Smolin's comment.