One way to try to answer these questions is the anthropic principle. There are two forms of it:

• Weak anthropic principle states that the universe must be compatible with our existence. For example, the laws of nature should be such that the universe could become large and cool, and atomic nuclei and moleculas could form, and nuclear reactions could proceed in the centers of stars etc.
• Strong anthropic principle states that the universe is such as it is because its purpose is to create life. In other words, our presence here and now somehow affected the initial conditions such that we could eventually arise.

If we discard the strong anthropic principle, we must acknowledge that we do not occupy a special place in the universe. This is called the Copernican principle. It does not mean that the universe has no center or special places.

Historically, beginning with Newton, scientists assumed that the universe was homogeneous and isotropic on large scales.

isotropic

means that the universe looks the same in all directions on the sky.

homogeneous

means that the universe looks the same everywhere on large scales.

These two assumptions go well beyond the Copernican principle.

Isotropic but not homogeneous means that there is a special place (center).

Homogeneous but not isotropic means that there is a special direction (axis).

Newton's cosmology

Newton based his cosmology on the cosmological principle, that asserts that all points and directions in the universe are more or less equivalent on sufficiently large scales. That was a big change from the Ptolemaic/Aristotelian cosmology.

Cosmological principle does not only tell that the universe looks the same everywhere and in all directions, but it also states that the laws of nature are the same everywhere in the universe.

Newton's universe was infinite and consisted of stars filling the space more or less uniformly.

But he left Aristotelian time intact. In Newton's cosmology the time is infinite either, and the universe always was and always will be as it is now. This latter proposition is called the perfect cosmological principle.

The perfect cosmological principle states that the universe looks the same at every place in space and at every moment in time (on large scales).

Newtonian cosmology envisions the universe as a huge clock mechanism, which operates indefinitely. The role of Creator was in creating the laws and setting the machine in motion, after that it operated by itself. This type of theology is called deism. To a large extent this is still the theology of science.

In Newton's cosmology space and time are absolute. They exist independently of the material world and are both infinite in every possible direction.

Problems with Newtonian cosmology:

• Gravitational interaction extends instantaneously to infinite distances
• Such a universe cannot exist - it is unstable!
• Problem with entropy

Testing cosmological principles

Cosmological principles were formulated as pure guesses, or, at best, plausible assumptions. However, science is developing, and today we can gather observational data about what yesterday was only a guess. In particular, we can test cosmological principles observationally. ²

Observational data available now strongly contradict the perfect cosmological principle: for example, galaxies are observed to evolve strongly; some 10 billion years ago there existed no galaxies similar to those around us.

Observational data support the cosmological principle per se. This support is based on two pieces of evidence:

• Universe is highly isotropic around us. This is demonstrated by distribution of galaxies, Cosmic Microwave background (abbreviated to CMB), gamma-ray bursters etc.
• Universe is isotropic to about 10% level around another distant point in space. This is enough to prove that the universe is homogeneous on large scales (to about 10%).

Scientists can use atoms in distant galaxies as thermometers to measure the CMB temperature at a remote point in space.

Observational data will be collected within the next few years that will be able to establish homogeneity to about 1% level (or disprove it).

Homogeneity of the universe can be proved!

$${\mbox{observed}\above0pt\mbox{isotropy}} + {\mbox{Copernican}\above0pt\mbox{principle}} = \mbox{homogeneity}$$

$${\mbox{observed}\above0pt\mbox{isotropy}} + {\mbox{observed}\above0pt\mbox{isotropy}} = \mbox{homogeneity}$$

$$\Uparrow\phantom{AA}$$

$$\mbox{(around~another~point)}\phantom{AA}$$