Below is a quote from the Mindscape presented by theoretical physicist Sean Carroll. Here, two physicists, Carroll and Leonard Susskind are discussing the fine-tuning of parameters and the existence of the multiverse.
//1:02:46 SC: Well, let me bring it back to sort of wrap things up, here. Let’s bring it back to where we started in string theory, because there’s another obvious set of questions about gravity and space time, which are the cosmological questions.
1:02:58 LS: Oh yes.
1:03:00 SC: The universe, and so forth. And you alluded cagily to the idea that we should take seriously things that we can’t, even in principle, observe, if they’re predicted by our theories. One example of that is the cosmological multiverse, which seems to be part of the string theory story. Do you still think that that’s true?
1:03:16 LS: I think so. I even wrote a book about it.
1:03:19 SC: Yeah, The Cosmic Landscape.
1:03:21 LS: Right.
1:03:21 SC: We’ll plug that book too.
1:03:23 LS: Yeah. Let me put it this way, I don’t think anybody has a better idea for resolving some of the great puzzles of cosmology, the great theoretical puzzles of cosmology. In particular the very, very strange, what are called the fine-tunings, that we seem to see in nature. That parameters are extremely finely adjusted and we don’t know why. There’s of course, a lot of almost anger at this idea of a multiverse and so forth.
1:03:57 SC: We’ve both felt it, yes. [chuckle]
1:03:58 LS: Yeah, we both felt it. My answer is always, “Yeah, what do you have that’s better?” And the answer is never anybody has anything better. So, it’s the best idea we have right now for understanding why the parameters of nature are what they are. We don’t have a better idea. And that’s about all I would say about it with real conviction. Could it turn out to be wrong? I suppose so, but I don’t see how. To say that I think it could turn out to be wrong, is to say that I see some other possibility, and I don’t.
1:04:30 SC: And so just to be clear. The “it” in this case is the idea that there are regions of space very far away…
1:04:34 LS: Yes, yes.
1:04:34 SC: Where the local laws of physics look different, that’s the multiverse we’re thinking about.//1
Here Leonard Susskind is saying that some parameters are extremely finely adjusted and that they don't know why. And he is saying that there is no better idea for resolving this great puzzle of cosmology than the multiverse. He is also saying that the multiverse is the best idea that they have right now for understanding why the parameters of nature are what they are.
It is also a fact that there is no evidence for any multiverse up till now. Moreover, inflation theory was developed in the 20th-century when the prevailing view among the scientists was that space-time was fundamental. But in the 21st-century the prevailing view among them is that it is not fundamental but emergent.
Hyun Seok Yang in his article 'Emergent Spacetime: Reality or Illusion?' 2 has shown that in the case of emergent space-time there would be inflation, but there would be no eternal inflation. Inflation will stop after generating only one universe. So in the case of emergent space-time, there would be no multiverse.
That means God and the multiverse have the same status regarding evidence. Atheists claim that there is no evidence for the existence of God and scientists have also failed to provide any evidence for the multiverse.
So, when scientists are saying that the multiverse is the best idea that they have right now for understanding why the parameters of nature are what they are, that means here they are actually bringing in something unknown to us for explaining something known to us.
As per the atheists, theists’ argument for God is this: We don't know, therefore God.
Physicists’ argument for the multiverse is this: We don't know, therefore the multiverse.
Reference:
1. https://www.youtube.com/watch?v=XTSdPSOcdjI&t=3620s
2. http://arXiv.org/abs/1504.00464