Anthropic Principle: The Universe appears as it does because we exist.

Brandon Carter (1974) coined the phrase Anthropic Principle with the definition

What we can expect to observe must be restricted by the conditions necessary for our presence as observers.

---

We may thus not only occupy a preferred place and/or preferred time, we may also occupy a preferred universe. This uncomfortable idea may follow from several vexing facts about our Universe:

• Horizon problem
• Flatness problem
• Matter/Anti-matter asymmetry
• Cosmological constant
• ....

  The problems in magenta have already been introduced. The Flatness problem and Matter/Anti-Matter asymmetry problem have not yet made an appearance.

It appears we live in a Universe with special properties. Some quantities assume highly improbable values, for example, the flatness of the Universe is disturbing. Several questions are:

• Do these special values simply reflect the random nature of the origins of our Universe (that is, have there been many other universes without these special properties in which life did not develop)?
• Is there a deeper level to the physical nature of the Universe which we do not yet understand (which makes our type of Universe the most probable type of Universe)?
• .....

These questions are interesting in the sense that if we can understand the origins of the above problems, it would imply we understand how the Universe was produced (in the beginning) which would be intellectually satisfying and would also offer the admittedly rather far-fetched notion, that we could engineer the space-time structure of the Universe.

The Anthropic Principle simply notes that if some of the finely-balanced quantities of our Universe were not so finely-tuned, then our Universe would have grossly different properties, properties which would in fact be so different that it is highly likely that life (as we know it) would not develop and we would not be around to ask the question of why the Universe appears special. Selection effects would say that it is only in universes where the conditions are right for life (thus pre-selecting certain universes) that it is possible for the questions of specialness to be posed.

This statement and variants of this statement are the gist of the Anthropic Principle.

computer simulation showing parallel universe.

Multiverses? Note that the Anthropic Principle is probably true and says that there is nothing mysterious about why our Universe is special. However, it does not rule out the possibility that there is a deeper level to our understanding of the Universe which makes our Universe the most probable universe from the plethora of all possible universes. This still may be true but is not required philosophically or scientifically.

computer simulation showing signatures of colliding universes