A blog about physics from a well caffeinated grad student.
I’ll write a more interesting post soon, but for now, I present Lawrence Krauss. Here he gives, what I’ve decided to be, the best one hour overview of cosmology I’ve ever seen. (via richarddawkins.net)
What do you all think about his suggestion that getting a universe from nothing is natural?
No, not the economic kind of inflation… I’m talking about cosmic inflation. You might have heard of it before. I’ll give you the gist of it. There are many problems with the big bang theory. When physicists talk about the big bang, they aren’t really referring to the “bang” part (strangely enough). What they actually mean is the part 10-44 seconds after the “bang”. The big bang theory starts with some specially fine tuned initial conditions for the content and expansion of the universe and just lets it evolve from there to get our universe today. It doesn’t, however, explain what gave rise to those initial conditions. Inflation tries to do just that.
So what is inflation? Inflation is a rapid (exponential) expansion of our universe that we suspect happened just after the creation of our universe. This is, of course, unimaginably faster than the expansion rate of the universe right now. It might seem like a very strange thing for a universe to do, but if we accept the idea, we find that it solves several problems with the classical big bang model. Inflation describes how the universe could become initially so flat and homogeneous but still have small fluctuations that evolved into the galaxies we see today.
One of the strangest qualities about our universe is that it is almost perfectly flat. When I say flat, I don’t mean: like a piece of paper. I’m talking about a generalized idea of flatness. If one measures the curvature of the universe to be positive, then a triangle in this universe will have interior angles adding up to more than 180 degrees. If the curvature of the universe is negative, triangles will have angles adding up to less than 180 degrees. Our universe has a curvature of almost zero, so triangles have angles summing to almost exactly 180 degrees.
Why is a flat universe so strange? It’s because a flat universe is unstable. By that I mean that a positively curved universe will tend to collapse in on itself and a negatively curved universe will tend to expand so quickly that galaxies won’t have time to form. The best I can do at a simple explanation as to why inflation explains this apparent “coincidence” is that it has to do with how the contents of the universe (radiation, matter, dark energy), the curvature, and the expansion rate all relate to each other (the Friedman equations). An exponentially expanding universe will force itself to become more and more flat.
What could cause this inflation, you ask? Well, there are many theories of inflation that attempt to naturally account for an inflationary period, but what it mainly boils down to is the, so called, Dark Energy (aka: Cosmological Constant). That’s right, the same thing that’s confusing astrophysicists and cosmologists about the universe’s current accelerated expansion. Dark Energy, put simply, creates a repulsive gravitational force and results in an accelerated expansion of spacetime. The explanation as to why we are not “inflating” today, is that the Dark Energy today has less of a repulsive gravitational effect than it did in the early universe. Physicists are still trying to (cleanly) explain this transition.
I could keep rambling about how neat inflation is, but I think it’s best for everyone that I stop here.
The Hubble site has created a very snazzy new (flash) page that tries to explain dark energy and the expansion of the universe to the layman. It is definitely worth checking out, and has lots of pretty animations.
The beginning is definitely my favorite part. It has the atmosphere of an epic theatrical trailer, hinting on doomsday scenarios. Though, viewer be warned, even the Hubble site is not without fault. Sean Carroll has voiced his beefs with the accuracy of their explanations in the wonderful blog Cosmic Variance. That’s not to say they are all wrong, they have just chosen some poor wording on occasion.
Neat things to read…
