But the problem is that is what so many atheists call the idea of God, an absurd idea with no evidence to support it. Mostly it is a philosophical error. They are expecting to find physical evidence for God. There is no good reason to expect that. But we do expect to find metaphysical evidence for God. That the world should have the look and feel of being designed by a loving creator. That may or may not have physical corollaries that can be scientifically tested. But leaving aside what evidence you should expect the objection has been adding God is the size of the idea is out of proportion to the evidence. This is often described as a violation of Ockham's razor. But does not the multiple universe theory now violate Ockham's razor. It is an interesting question. One guy who argues it does not write this:
William of Ockham, 1285-1349(?) English philosopher and one of the founders of logic, proposed a maxim for judging theories which says that hypotheses should not be multiplied beyond necessity. This is known as Ockham's razor and is interpreted, today, as meaning that to account for any set of facts the simplest theories are to be preferred over more complex ones. Many-worlds is viewed as unnecessarily complex, by some, by requiring the existence of a multiplicity of worlds to explain what we see, at any time, in just one world.Now there are a few philosophical assumptions behind Ockham's razor that people often don't understand. St Thomas Aquinas believed in the essence of things. That when we see a dog that it was an imperfect representation of the essence of dogness. Ockham didn't believe that such an essence existed. He just thought our ideas about dogs were merely a name for what we experienced of dogs.It is a philosophy known as nominalism because it reduces everything to a simple name. Now given the assumption of nominalism Ockham's razor makes sense. If we don't have to worry about what is true about the essence of dogness and just want to name what we know about dogs then the simplest description for that is to be preferred. There is no truth out there that we have to describe. We are just composing a theory that fits our observations. So we can pick theories and picking simple ones is better.
But when we are trying to determine truth this fails. If I know somebody traveled from New York to Los Angles and that is all I knew it would be best for me to assume he made the trip in the simplest possible way, maybe a direct flight. But in truth he might have made a very complex trip. He might have driven for a while, got on a boat, rode a bus, maybe biked a few hundred miles. That might be the truth. Now absent any other evidence the simple theory is to be preferred. But in the case where there is an essential truth behind the theory Ockham's razor does not guarantee you truth about that essence. So there is an implicit assumption of nominalism. That is that such truth does not matter.
This is to mistake what is meant by "complex". Here's an example. Analysis of starlight reveals that starlight is very similar to faint sunlight, both with spectroscopic absorption and emission lines. Assuming the universality of physical law we are led to conclude that other stars and worlds are scattered, in great numbers, across the cosmos. The theory that "the stars are distant suns" is the simplest theory and so to be preferred by Ockham's Razor to other geocentric theories.First of all, this is precisely the opposite of what you are assuming with multiple universes. That is the observed reality of physical laws being constant. The theory is such laws are not constant in these other universes. So we are not just asserting more of the same like we do with stars. We are asserting something new. That is complex as well as large.
The other think to note is that God is not complex. God is simple. He is vast but He is not complex. Complex things can be divided into parts. God cannot be divided. See this post on divine simplicity. So making this distinction really seems to make Ockham's razor oppose multiple universes more and God less.
Similarly many-worlds is the simplest and most economical quantum theory because it proposes that same laws of physics apply to animate observers as has been observed for inanimate objects. The multiplicity of worlds predicted by the theory is not a weakness of many-worlds, any more than the multiplicity of stars are for astronomers, since the non-interacting worlds emerge from a simpler theory.But the same physical laws would mean the same physical constants. That does not help explain the fine tuning at all.
As an historical aside it is worth noting that Ockham's razor was also falsely used to argue in favour of the older heliocentric theories against Galileo's notion of the vastness of the cosmos. The notion of vast empty interstellar spaces was too uneconomical to be believable to the Medieval mind. Again they were confusing the notion of vastness with complexity .)One rule for atheists in these debates is to work in as many references to Galileo as possible. Notice the swipe at the "Medieval mind".