Occam's Machete: Cosmology

The year of Entropy continues with a paper referencing Verlinde but describing gravity and dark energy as effects of entanglement entropy. Read it on arXiv.

Something that has alway bothered me is the opinion expressed by moralistic politicians that faith and belief are virtues. Why? Doesn’t the skeptic who searches for evidence and rejects wishful thinking embody real virtue?  Would you be persuaded by someone’s belief that aliens crashed in a volcano and spread to infect all of humanity causing our combined ill-fortunes?  Oh, wait some people do believe that even though most of us would call that belief silly.  It is not backed by evidence and without evidence is not worthy of serious consideration. Which brings me to the quote of the day.

“I don’t ‘believe’ in string theory”, Brian Greene (the popular face of string theory and author of The Elegant Universe) in a Discover interview here.

Of course he goes on to make the distinction between belief and the virtuous pursuit of scientific skepticism in examining string theory. This balance of conviction and skepticism is the hallmark of good science. Greene is clearly a proponent of string theory and has written much about it but he is making a point about thought hygiene that ought to be applied broadly.

So the next time you are asked to vote for a politician who professes “faith” ask yourself if that lack of intellectual rigor is a good thing in a leader.  Faith is generally not a virtue.

Click to download the movie-all 18M of it.

There’s more news on that weird sucking from outside the visible universe.  This was first noted by Alexander Kashlinsky at Goddard Space Flight Center and I wrote about it here.  The team has now tracked the flow to twice the distance reported back in 2008—to 2.5 billion light years away.  This video shows the flow for different distances from the Earth.  Unfortunately they don’t know yet for sure whether the flow is away from us or towards us.  Everyone assumes it is away and there is some data to say that this assumption is true.  At least if it is flowing away from us we can blame it on something outside our light horizon, something really really big.  The galaxy clusters studied are travelling at a million miles per hour.  Read about the new data here.

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.

Some folks at Colgate (Patrick Crotty, Daniel Schult, Ken Segall) have proposed a Josephson Junction array that simulates many of the physical properties of a biological neuron but operates orders of magnitude faster.  Now if that doesn’t get the SciFi writer in you going nothing will.

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.

Lately I’ve seen a several papers describing how one fundamental way of measuring the universe or another is actually emergent from a different way of looking at things.  There are those that claim time does not exist in any fundamental way, we are just looking at things wrong. So what is fundamental?

Entropy.

Understand entropy and gravity emerges.  Understand it in a moving frame and General Relativity emerges.  The idea is published on arxiv (On the Origin of Gravity and the Laws of Newton, Erik Verlinde) and even made it to Slashdot.  Woit surveys other entropy related ideas.

As a computer scientist there is something appealing in the notion that information is fundamental and that all the laws of nature are derived from the way information works.

Update: There is a long post from Verlinde on Motl’s blog here.  It illuminates some of his ideas and explains their history.  If you can stomach Motl’s bullying read the comments too, there are many.

Motl has a nice post about the place of thought experiments in physics. How do you run an experiment when you have to have a train that moves at the speed of light and is smaller than the Planck scale—run a thought experiment.

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.

Sean Carroll of Cosmic Variance has another great post about an arXiv paper that raises questions about GR.  The gist is that the curvature of space and time should be equal according to GR (though I’m not so sure about that one) and looking back in time using gravitational lensing shows them to be different.  Sean explains it best here.