Occam's Machete: Cosmology

If Roger Zelazny had written the Amber books today I’m sure he would have had a Courts of Entropy instead of Chaos.  But I digress…

The Holographic Principal states that a 5-D universe can be encoded holographically on a 4-D universe.  Just as a 3-D object can be captured in a flat hologram.   The implications are that the supertring universe of 5 dimensions can be seen by thinking of our own universe as a hologram.  Using this principal some really smart guys (including George Smoot of Nobel fame) proposed on arXiv that dark energy in our universe is only a holographic affect of an entropic force in higher dimensions.

They develop their theory and use it to predict the amount of dark energy we should see in our universe.  Unless there are mistakes in their reasoning, which I cannot judge, the most interesting thing is how close the prediction is to the dark energy seen in the supernova data—you know the data that showed our universe is expanding at an accelerating rate.  The other interesting thing is this how people are beginning to find observable predictions in String/M-theory.

I’m going to have to pay more attention to this Entropy stuff.

BTW Motl has some thoughts on this paper too.

Entropy is big lately.  A new paper on arxiv (The Entropic Lanscape, Bousso and Harnik) uses entropic principles to derive predictions about such things as the cosmological constant and the nature of entropic radiation.  These predictions match well with observations in our corner of the universe and give a framework that applies to any part of the universe, even in other vacua.  This paper relates to examinations of the vacuum lanscape that usually end up relying on anthropic arguments.  In other words things are the way they are because we wouldn’t be here to observe them if they were different.  Bousso and Harnik replace arguments like that with ones that favor the maximiztion of entropy.

Also in a comment to a previous post here Nisheeth points to his arxiv article (The relativity of theory, Nisheeth) where he describes a framework for deriving physical laws from information-theoretic first principles.  He too relies on the maximiztion of entropy.

Cosmic Inflation attempts to explain why the universe is so uniform by positing that the universe may have expanded faster than the speed of light, at least for a time.  It is a very successful theory which allows us to think of the forces as having not changed with time.  But what if Gravity itself has changed with time?  What if the early universe had little or no gravity and later, through a sort of phase change (like water turning to ice) gravity was switched on?  Could that be an alternative way to explain the state of the universe today and if so how might it have happened?  Those are the questions that Brian Greene, Kurt Hinterbichler, Simon Judes, and Maulik K. Parikh try to address in a new arXiv paper.

The basic premise is that things are uniform now in the universe and if the universe had expanded at a constant rate with gravity acting all the time the laws would predict a more lumpy universe.  Cosmic inflation questions whether the universe expanded at a constant rate.  If the early universe expanded faster that the speed of light then the clumping effect of gravity would have been frozen out for a time.  Then when expansion slowed down the effect would become more pronounced.  As an alternative the new paper questions whether gravity was uniformly powerful the whole time.  If a gas is unaffected by gravity the gas will distribute uniformly in space but when gravity is added the gas atoms will eventually form into clouds, condense into stars, and collapse into black holes.  The new paper suggests that gravity itself might have been weaker for a time, leading to less clumping.

We tend to imagine our surroundings on geological or cosmological time scales as being governed by gradual change.  If things do change the changes must certainly be brought about by constant laws. We think of the speed of light and the strength of the forces as being constant.  But as we are able to see at greater time scales we have found over and over that things we thought were constant do change—stars die, galaxies form, hot rocks flows like water, and the universe expands at an accelerating pace.  There are a growing number of people asking whether the laws of nature also change.  Until we ask this we aren’t done looking at the possibilities so I was excited to read the paper.

Luboš Motl lambasts the idea here.