An article in the Wall Street Journal by Hawking and Mlodinow on Why God Did Not Create the Universe begins with:
"According to Viking mythology, eclipses occur when two wolves, Skoll and Hati, catch the sun or moon. At the onset of an eclipse people would make lots of noise, hoping to scare the wolves away. After some time, people must have noticed that the eclipses ended regardless of whether they ran around banging on pots.
"Ignorance of nature's ways led people in ancient times to postulate many myths in an effort to make sense of their world. But eventually, people turned to philosophy, that is, to the use of reason—with a good dose of intuition—to decipher their universe. Today we use reason, mathematics and experimental test—in other words, modern science. "
and ends with
"Our universe seems to be one of many, each with different laws. That multiverse idea is not a notion invented to account for the miracle of fine tuning. It is a consequence predicted by many theories in modern cosmology. If it is true it reduces the strong anthropic principle to the weak one, putting the fine tunings of physical law on the same footing as the environmental factors, for it means that our cosmic habitat—now the entire observable universe—is just one of many.
"Each universe has many possible histories and many possible states. Only a very few would allow creatures like us to exist. Although we are puny and insignificant on the scale of the cosmos, this makes us in a sense the lords of creation."
This article and the hundreds of the comments it elicited brings up a point that I have wanted to discus for some time, and that is the idea of multiverses. The multiverse is a collection of universes that are predicted by certain cosmological theories. Many physicists object to the notion that the existence of the other universes are undetectable, therefore, they would argue that "believing" in a multiverse is akin to accepting God.
This objection is similar to objections that have been waged against other new ideas in science that are now commonplace. For example, Ernst Mach objected to the idea that atoms were "real" even though chemists had indirect evidence a century earlier. His gripe was based on his conviction that if something were real, it could be seen directly. Ludwig Boltzmann, Mach's contemporary, struggled for years to get physicists to accept atoms, a critical ingredient of Boltzmann's thermodynamic theories. As the stalwarts died out, and theories based on atoms were experimentally verified from every possible angle, the new generation of physicists of the early 1900s accepted atoms as real.
These arguments hinge on the question of what is meant by "real." Mach's criteria was that one had to be able to see reality "directly" using, for example, a microscope. Ironically, Galileo's contemporaries did not believe that his views of the heavens were real because the telescope purportedly distorted reality. He proved that the telescope depicted far-away objects in true fidelity by comparing what was observed in his telescope to direct visual observation of terrestrial objects. Today, we have been to the moon and confirmed Galileo's observations of craters and other features.
A more modern view of what is real might be defined by its consequences. Though one cannot see an electron, its existence has astronomically many testable consequences that have been confirmed. Every time we use a computer, watch TV, or drive a car, the existence of the electron is confirmed and reconfirmed.
Einstein never believed in quantum mechanics because of its inherent probabilistic interpretation. He felt that someday, an underlying theory with hidden variables would be found that would make quantum mechanics fully deterministic. In the 1960s, Bell showed very elegantly that hidden variable theories were inconsistent with observations. It would have been interesting to have gotten Einstein's impressions of Bell's work, had Einstein lived a few more years.
A deeper issue with quantum mechanics is the central role of the wavefunction, which cannot be directly measured. Only the consequences of the wavefunction are observable. Once again, because quantum mechanics makes so many accurate predictions in the realm of atoms, molecules, nuclei, and force fields, we accept that wavefunctions exist, not as an object that we can directly sense, but as proxy for some unmeasurable structure of reality.
Getting back to the multiverse, if a cosmological model of the universe predicts the existence of multiverses AND simultaneously predicts everything that we observe in our own universe, then accepting the existence of a multiverse is qualitatively no different than accepting much of what is standard physics of today. The key is that the theory must predict what is observed as well as making new testable predictions. The postulate of the existence of God, on the other hand, does not make any quantitative predictions that can be tested.
Are multiverses equivalent to theology? I would say not. Science seeks answers through hypothesis and experiment. Theology, according to dictionary.com is "the field of study and analysis that treats of god and of God's attributes and relations to the universe; study of divine things or religious truth; divinity." Theology presupposes the existence of God while science accepts a theory only when it is consistent with measurements. If an experiment falsifies a hypothesis, it is discarded. Theologians, on the other hand, take without proof the existence of God.