You are wrong, dhyana.
Multiverse cosmologies are provable; they are just NOT testable.
Multiverse cosmology is theoretical physics, not experimental physics. And theoretical physics rely on mathematical models and mathematical equations, not on tests or evidences.
In the language of science, evidence and proof are two different things.
Most science rely on verifiable evidences or testable predictions, and less on proof. Theoretical physics rely on proof, and "proof" being mathematical models and equations, but lest on empirical evidences and experiments.
The Big Bang cosmology used to be theoretical physics, but have been verified through observations and tests. Observations as in the detections of the cosmic background
If you are saying that ALL scientific theories are "idle speculation", then you really don't understand science at all.
Experimental physics or empirical physics, as stated earlier, have theories that can be tested. Most other scientific branches (physics, chemistry, biology) fall under "empirical" or "experimental" umbrella, because they are testable.
If all theories are just speculation, then there wouldn't be modern technology, like computers, mobile phones, cars, airplanes.
A theory...in the realms of science...is a scientific explanation of the natural and artificial (like computer, electronics, man-made stuffs) worlds, that can be tested, through the scientific method.
In the areas of physics, the theory of Relativity (both Special Relativity and General Relativity), Quantum Physics, and the Big Bang cosmology, all used to be theoretical physics, but since evidences (and therefore verifiable and testable) have been found in all three of them, their theories are no longer "theoretical".
The cosmic microwave background radiation (CMBR) was discovered in the 1960s (and confirmed again recently by the WAMP mission), verified that the universe have been expanding since 13.6 billion years ago. This mean the Big Bang cosmology is testable.
The same can't be said about the multiverse cosmology, because it is untestable, but is provable by mathematical models and equations, hence multiverse cosmology falls under theoretical astrophysics. Other untestable cosmologies are the Big Rip, Big Bounce, Big Crunch, all of which fall under theoretical astrophysics.
Other theoretical physics in modern science, are string theory, superstring theory, M-theory.
Scientific "proof" is mathematical model or mathematical equation; proof is not the same things as evidence, experimentation or test.
Theories that are testable, hence empirical science, can be found in gravity, relativity, evolution, electronics, computers, engineering.
Evolution, like electronics and engineering, have real world application. The study of medicine, diseases (like bacterial and viral diseases), genetics, required understanding of evolution (biology).
So if most theories have real-life or real world application, then those theories are not just "idle speculation". For you to say it is, showed that don't known anything about scientific theory.
I hoped that you grasp what I am saying, because I am tired of theists who deliberately (or deceptively) misuse or misrepresent what science have to say.
Thank you for a well expressed thoughtful, informative post. It is much appreciated. Seriously. I am tired of condescending know-it-alls who feel the need to preach to the unwashed.