Existence of God

The Second Law of Thermodynamics

Another important concept in this subject is the second law of thermodynamics. This law states that in a closed system, entropy is almost always positive. In physics, entropy is another word for disorder. In other words, over time, things always get more disorganized. Examples of this include a hot cup of coffee that will eventually cool off, or a plate that falls on the flow and breaks, organic material decomposing, growing older, etc. A great example is a pool table with fifteen pool balls racked up before a game of eight ball. At the first strike, the cue ball causes the fifteen balls to scatter around the table. Now consider a video clip of this played both forwards when the balls are scattered and backwards when the balls come together. If asked which clip is forwards, most people would choose the clip where the balls scatter. Why? This is because it is the most probable. It is possible that the balls came together using strings or something we don’t know about, but which is more likely? Disorder is always more likely. This is positive entropy according to the second law of thermodynamics.

The Bouncing Universe Scenario

The bouncing universe theory is that the universe has undergone infinite cycles of expanding and contracting and the big bang is simply the beginning of the latest cycle. This means there was no “beginning of the universe.” This theory does not fall under the standard big bang theory, but under “Past-extended Big Bang Models” or PBBMs. There are three main problems with this theory.

Firstly, is the amount light left over from the big bang. This is called Cosmic Background Radiation (CBR). CBR makes up 99% of the light in our universe, while light from stars makes up the other 1%. Every time the universe collapsed, light from all of the stars would get scattered, reabsorbed by matter, and re-emitted. So if this happened a thousand times, there should be a thousand times more CBR than light from our current stars. If this happened a million times, there should be a million times more CBR than light from our current stars. However, there is only about a hundred times CBR than light from our current starts, therefore the universe had to have a beginning.

Second, is the second law of thermodynamics. In each cycle, entropy should increase, meaning the universe loses energy and eventually can no longer produce stars. However, entropy right after “our” big bang was so low that the odds of it being so low were 1 in 10^{10^123}. Therefore, it is not possible for there to have been anything before the big bang.

Thirdly, is the idea of increasing volume with each cycle due to radiation. Radiation from each previous cycle accumulates in each new cycle, increasing the pressure, and thus increasing the volume of the universe. If each cycle has an increase in volume, that means as we go back in time, the universes get smaller and smaller, to the point where there must have been a beginning.

The Eternal Inflation Scenario

The enternal inflation scenario is another PBBM. In this scenario, the universe expands at an exponential rate, doubling in size every 10^-40 seconds. Einstein’s equations of gravity proves that this rate of expansion would mean the universe has no “beginning.” However, we know that our universe expands very slowly and takes billions of years to increase a significant amount. So how does this theory explain this? It says that our universe is simply one island among many islands. Therefore, the big bang was simply the start of our “island.” It then says that most of the islands are expanding at an exponential rate, while a few islands (including ours) are expanding very slowly. This sounds good, but has been recently disproved with the Borde-Vilenkin-Guth Theorem or BVG Theorem.

In 2003, Borde, Vilenkin, and Guth theorized that any universe with an expansion greater than zero must have a beginning. I will not discuss the details of the proof, but here is quote from Vilenkin that gives an illustrative example using velocity:

“Suppose, for example, that [a] space traveler has just zoomed by the earth at the speed of 100,000 kilometers per second and is now headed toward a distant galaxy, about a billion light-years away. That galaxy is moving away from us at a speed of 20,000 kilometers per second, so when the space traveler catches up with it, the observers there will see him moving at 80,000 kilometers per second. If the velocity of the space traveler relative to the spectators gets smaller and smaller into the future, then if follows that his velocity should get larger and larger as we follow his history into the past. In this limit, his velocity should get arbitrarily close to the speed of light. – Vilenkin, Alexander, The Vacuum Energy Crisis, 2006.

This is a great proof that can be applied to many universe theories in physics because of its simplicity. Any universe that expands must have had a starting point.

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