### De mundi systemate

by @ulaulaman about #IsaacNewton #physics #gravity
I published this post some years ago (archived version), but for unilateral decision of the online publisher, it is deleted, so I decide to recover it.
It's the third pard o Newton's Philosophiae Naturalis Principia Mathematica, one of the most famous physics tractatus (latin world to treatise). In 2010 Dave Richeson proposes An amazing paragraph from Euler's Introductio, a wonderful post in which he extraxts some Euler's quotes from Introductio in analysin infinitorum (Introduction to analysis of the infinite). So, because I'm working on a learning project about gravity, I decide to propose an analouge post about De mundi systemate (On worlds' system). For english version of the following Newton's quotes I'll ask help to Andrew Motte.
We began with Propositio II (Proposition II):

### The infinite inflation and the end of time

by @ulaulaman about #cosmology #mathematics #inflation #Hawking #AlanGuth
I published this post some years ago (archived version), but for unilateral decision of the online publisher, it is deleted, so I decide to recover it.
In the early years of the 3rd millennium there was a discussion about eternal inflation. This theoric ipothesis was introduce by Alan Guth and other physicists. In particular you can read Guth's paper Eternal Inflation(1):
The basic workings of inflationary models are summarized, along with the arguments that strongly suggest that our universe is the product of inflation. It is argued that essentially all inflationary models lead to (future-)eternal inflation, which implies that an infinite number of pocket universes are produced. Although the other pocket universes are unobservable, their existence nonetheless has consequences for the way that we evaluate theories and extract consequences from them. The question of whether the universe had a beginning is discussed but not definitively answered. It appears likely, however, that eternally inflating universes do require a beginning.
We have a lot of observations that confirms not only the big bang theory, but also the inflation period: in some time after the first expansion of the universe, there is a faster expansion of space time. The most important observation that supports inflation is the anisotropy of the cosmic background radiation (we could add also the absence of magnetic monopole...).
The background of eternal inflation ipothesis is the existence of repulsive-gravity material, that is unstable and decay with an exponential law (like any radiactive atom). In every decay process the volume of repulsive-gravity material grow instead decrease and prodece a never ending series of pocket universes(1, 2):
In Cosmology from the Top Down, a talk presented at Davis Inflation Meeting in 2003, Stephen Hawking speak about some criticism on eternal inflation: