Universes and timelines are two different things, coming from two separate multiversal theories, with completely different principles.
The concept of multipe universes comes from the Bubble Universe Theory, which implies that there are multiple (scientifically potentially infinite but there's literally no way to know mathematically speaking) universes existing in their own "bubbles". In this theory, they all interact on a larger, separate plane of existence that we haven't really figured out yet, and can bump into each other... potentially causing events such as the Big Bang. Potentially. Theoretically.
EACH UNIVERSE HAS ITS OWN TIMELINES. A DIFFERENT TIMELINE IS NOT A DIFFERENT UNIVERSE.
Timelines, in concept, would comply to the "Many Worlds" theory, which utilizes principals such as the Heisenberg principle (you cannot know where a particle is and how fast it is moving at the same time, same thing with time and energy) to come to that conclusion that in every moment, simultaneously, multiple possibilities both exist and are happening. Schrodinger's cat type shit, except the cat is always both dead and alive, and the cat is also the butterfly effect. Heisenberg's Zombie Butterfly Cat, if you will.
Conceptually speaking, they're both by definition different universes, but each Bubble Universe has its own set of Many Worlds universes. And what we conceptually call "timelines" fall under the Many Worlds theory.
In my opinion, there are likely an infinite number of timelines and a finite number of universes rather than vice versa, but it's so theoretical at this point that my scientific opinion is as credible as any fantasy novel plot.
There's also a third commonly considered multiverse theory, which we're the closest to actively proving, involving the concept of dark matter. Dark matter makes up about half of our universe's matter, but we don't really know what it is. Some scientists theorize that it's evidence of a sort of "mirror universe". This one is actually pretty close to being proven or disproven. It also would apply to every timeline, if we assume those also exist, creating a nesting doll concept of universes: in each Many Worlds timeline there is a Mirror Universe, and in each Bubble Universe, there are potentially infinite Many Worlds timelines.
Again, all of this is conceptual and HIGHLY theoretical (for now), but I'm tired of people using timelines and universes interchangeably. They're not. It's also grossly oversimplified, so do your research thoroughly if you decide to take my word for this. My description is basic addition compared to the calculus of the real thing.
Thank you for coming to my Ted Talk. Any questions?
