Your Universe is Different than Mine

We used to be taught that the universe was everything there is.  But, over the past few years, it is beginning to have a new meaning.  The universe is now meant to be everything that we can possibly see or experience.  Let me illustrate with a story.  Imagine our protagonist Jack happily living in his little universe.  His astronomer buddies have used their most advanced equipment to peer into the deepest depths of space and have detected things a few billion light years away in all directions.  There could be things beyond that “practical observational horizon”, but we are limited by the state of the art of equipment in the year 2010.

However, there is another horizon beyond “a” which denotes the point at which it would be impossible to see beyond, due to the speed of light.  The light from objects at that distance has been traveling toward us since the beginning of the big bang.  This is our theoretical horizon, beyond which we can never see or detect anything no matter how advanced our equipment becomes.  It should be noted, that this statement presumes that nothing travels faster than the speed of light and even if it did, we would not be able to detect it.  Despite a century of hard evidence supporting Einstein’s famous assumption regarding the limitations of the speed of light, there are a number of physicists who don’t rule out the possibility that this barrier could someday be broken.  But that’s a topic for another post.  Setting such arguments aside, there is then a “theoretical observational horizon,” also known as the Hubble Volume, which is generally accepted to be about 42 billion light years in diameter.  But that doesn’t mean there’s nothing beyond the Hubble Volume.  In fact, the inflationary theory of the big bang allows for quite a bit of the material from the big bang to exist beyond that horizon because the inflationary period was superluminal. (We were just saying…?)  But, for all practical purposes, the Hubble Volume contains all that you can ever know about.  By convention, astronomers call that “The Universe.”

By definition, a Universe depends on what is identified to be its center.  So, for example, Jill, standing on a planet 1 light year away from Jack, actually lives in a slightly different Universe.  One which has one horizon one light year further away from Jack’s in the opposite direction and another horizon one light year closer than Jack’s in the direction toward Jack.  So Jack has some stuff in his Universe that Jill doesn’t have and vice versa.

The choice of a light year between Jack and Jill’s positions was arbitrary.  They could be standing next to each other and still have slightly different Hubble Volumes.  In fact, when you get down to it, we all live in different Universes.