# Chapter 28: The Big Bang

These last two chapters are tightly connected and fall under the overall title of cosmology. Cosmology is the study of the universe as a whole, how it began, how it evolved, and how or if it will end.

## 28.1 The Age of the Universe

From our understanding of gravity, the universe must be either expanding against gravity, or contracting due to gravity. The observation by Hubble and company that all the galaxies are moving away from us is consistent with the idea that the universe is expanding.

### The Hubble Time

If we now imagine going backward in time, if all the galaxies move further apart as time advances, then they will be closer together as we go back in time. The further back we go, they closer together they will get, until finally all the galaxies in the universe meet at one place. Simply stated, this is the idea behind the big bang theory. If the universe is expanding, and apparently has been doing so for as long as we can determing, then it must have started out from a very compact region of space some time in the past. This time corresponds to the beginning of the universe.

We can estimate when this occured based on the speed with which galaxies are moving apart, and their separation. This is simply the rule that the time it takes to get somewhere is the distance divided by the speed:

time = distance / speed
If you want to travel from Detroit to Chicago, about 300 miles away, at 60 miles per hour, then the trip will take (300 miles)/(60 mph) = 5 hours.

According to Hubble, galaxies move apart at speed v that is proportional to their separation d:

v = H×d
Therefore, the time it took any pair of galaxies to reach their present separation (the time since the beginning of the universe) is:
t = d / v = d / H×d = 1/H
The age of the universe is the inverse of the Hubble constant!

### Comparing Ages

With the recent results of the Wilkinson Microwave Anisotropy Probe (WMAP), the age of the universe is now known to be 13.7 billion years with about 1% accuracy.

## 28.5 What is the Universe Really Made Of?

See Figure 28.17 and Table 28.4.

## 28.6 The Inflationary Universe

Skip this section.

## 28.7 The Anthropic Principle

The anthropic principle states that the reason the universe has turned out the way it has (the reason the physical laws that govern the universe are what they are) is because this is the (only) set that allows complex life to evolve with the ability to ask why the universe is the way it is.

While the anthropic principle may contain a grain of truth, it is the easy way out of answering the question.