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Astrophysicists Identify the Habitable Regions of the Entire Universe
KentuckyFC writes It's not just star systems and galaxies that have habitable zones--regions where conditions are suitable for life to evolve. Astrophysicists have now identified the entire universe's habitable zones. Their approach starts by considering the radiation produced by gamma ray bursts in events such as the death of stars and the collisions between black holes and so on. Astrobiologists have long known that these events are capable of causing mass extinctions by stripping a planet of its ozone layer and exposing the surface to lethal levels of radiation. The likelihood of being hit depends on the density of stars, which is why the center of galaxies are thought to be inhospitable to life. The new work focuses on the threat galaxies pose to each other, which turns out to be considerable when they are densely packed together. Astronomers know that the distribution of galaxies is a kind of web-like structure with dense knots of them connected by filaments interspersed with voids where galaxies are rare. The team says that life-friendly galaxies are most likely to exist in the low density regions of the universe in the voids and filaments of the cosmic web. The Milky Way is in one of these low density regions with Andromeda too far away to pose any threat. But conditions might not be so life friendly in our nearest knot of galaxies called the Virgo supercluster."
No one "assumed" that. There's a fuck-ton of evidence for the big bang and universal expansion. And the current model is a reaction to that information.
Off the top of my head:
1. Cosmic microwave background radiation matching what the models suggested there would be had the universe expanded from a singularity
2. Redshift of distant objects being proportional to their distance
3. Overall concentration of elements heavier than those produced in a super-nova matching expected levels for the super-high energy environment immediately following the big bang
4. The apparent distances of the furthest objects in the "visible universe" corresponding to about 14 billion light years in distance
Now before all these observations, the de facto assumption was a Newtonian model, where gravity and momentum kept things in a constant near perfect eternal balance, with the universe having an unknown age. When hubble came up with the Big Bang, he was attempting to address the discrepancies that model had with reality.