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One of the most exciting and enticing topics to speculate about is the idea that our reality — our Universe the way it is and the way we experience it — might not be the only version of events out there. Perhaps there are other Universes, perhaps even with different versions of ourselves, different histories and alternate outcomes, than our own. When it comes to physics, this is one of the most exciting possibilities of all, but it’s far from a certainty. Here’s what the science actually says about whether this could be true or not.

Image credit: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team.

Image credit: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team.

The Universe, as far as the most powerful telescopes can see (even in theory), is vast, huge and massive. Including photons and neutrinos, it contains some 10^90 particles, clumped and clustered together into hundreds-of-billions-to-trillions of galaxies. Each one of those galaxies comes with around a trillion stars inside (on average), and they’re strewn across the cosmos in a sphere some 92 billion light years in diameter, from our perspective. But, despite what our intuition might tell us, that doesn’t mean we’re at the center of a finite Universe. In fact, the evidence indicates something quite to the contrary.

Image credit: ESA and the Planck Collaboration, modified by me for correctness.

Image credit: ESA and the Planck Collaboration, modified by E. Siegel for correctness.

The reason the Universe appears finite in size to us — the reason we can’t see anything that’s more than a specific distance away — isn’t because the Universe is actually finite in size, but is rather because the Universe has only existed in its present state for a finite amount of time. If you learn nothing else about the Big Bang, it should be this: the Universe was not constant in space or in time, but rather has evolved from a more uniform, hotter, denser state to a clumpier, cooler and more diffuse state today.

The observable Universe might be 46 billion light years in all directions from our point of view, but there's certainly more, unobservable Universe just like ours beyond that. Image credit: Wikimedia Commons users Frédéric MICHEL and Azcolvin429, annotated by E. Siegel.

The observable Universe might be 46 billion light years in all directions from our point of view, but there’s certainly more, unobservable Universe just like ours beyond that. Image credit: Wikimedia Commons users Frédéric MICHEL and Azcolvin429, annotated by E. Siegel.

This has given us a rich Universe, replete with many generations of stars, an ultra-cold background of leftover radiation, galaxies expanding away from us ever-more-rapidly the more distant they are, with a limit to how far back we can see. That limit is set by the distance that light has had the ability to travel since the instant of the Big Bang.

But this in no way means that there isn’t more Universe out there beyond the portion that’s accessible to us. In fact, from both observational and theoretical points-of-view, we have every reason to believe there’s plenty more, and perhaps even infinitely more. Observationally, we can measure a few different interesting quantities, including the spatial curvature of the Universe, how smooth and uniform it is in both temperature and density, and how it’s evolved over time.

Inflation set up the hot Big Bang and gave rise to the observable Universe we have access to, but we can only measure the last tiny fraction of a second of inflation's impact on our Universe. Image credit: Bock et al. (2006, astro-ph/0604101); modifications by E. Siegel.

Inflation set up the hot Big Bang and gave rise to the observable Universe we have access to, but we can only measure the last tiny fraction of a second of inflation’s impact on our Universe. Image credit: Bock et al. (2006, astro-ph/0604101); modifications by E. Siegel.

What we find is that the Universe is most consistent with being spatially flat, with being uniform over a volume that’s much greater than the volume of the piece of the Universe observable to us, and therefore probably containing more Universe that’s very similar to our own for hundreds of billions of light years in all directions, beyond what we can see. But theoretically, what we learn is even more tantalizing. You see, we can extrapolate the Big Bang backwards to an arbitrarily hot, dense, expanding state, and what we find is that it didn’t get infinitely hot and dense early on, but rather that — above some energy and before some very early time — there was a phase that preceded the Big Bang, and set it up.

That phase, a period of cosmological inflation, describes a phase of the Universe where rather than being full of matter and radiation, the Universe was filled with energy inherent to space itself: a state that causes the Universe to expand at an exponential rate. This means that rather than having the expansion rate slow as time goes on, at having distant points recede from one another at ever slower speeds, the expansion rate doesn’t drop at all, and distant locations — as time goes on incrementally — get twice as far away, then four times, eight, sixteen, thirty-two, etc.

Image credit: E. Siegel, of how spacetime expands when it’s dominated by Matter, Radiation or energy inherent to space itself.

Image credit: E. Siegel, of how spacetime expands when it’s dominated by Matter, Radiation or energy inherent to space itself.

Because the expansion is not just exponential but also incredibly rapid, “doubling” happens on timescale of around 10^-35 seconds. Meaning, by time 10^-34 seconds have passed, the Universe is around 1000 times its initial size; by time 10^-33 seconds have passed, the Universe is around 10^30 (or 1000^10) times its initial size; by time 10^-32 seconds have passed, the Universe is around 10^300 times its initial size, and so on. Exponential isn’t so powerful because it’s fast; it’s powerful because it’s relentless…Read More at

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