How Many Universes Are There?

SPEAKER_1 (0:00)

This episode is brought to you by Accenture. When you're advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales. Using automation, analytics, and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

SPEAKER_2 (0:31)

This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome, that's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up required compatibility and availability varies 18 plus.

Matt O'Dowd (0:56)

Space is big, but it's peanuts compared to the eternally inflating multiverse.

But just how many bubbled universes does the eternally inflating multiverse contain? What are they like? And most importantly, what can they tell us about aliens?

Imagine it. The observable part of our universe is 93 billion light-years across, and that's just a small fraction of the stuff created in our Big Bang. But in the eternal inflation picture, ours is just one among uncountable bubbled universes, bubbles that are continuously appearing and growing within a vastly larger space-time that itself expands at an exponentially accelerating rate. A greater inflationary space-time whose expansion never ends.

We looked at the bizarre idea of eternal inflation in recent episodes, but we stopped short of exploring the full implications of this proposition. Those implications are, frankly, completely nuts. Some may also be true. Here's the scenario. The default state of the greater universe or multiverse is to expand exponentially due to the vacuum of space itself having a large and constant energy density. Energy locked into something called the inflaton field. Within that inflating space, tiny patches stop inflating. The inflaton field in that patch loses its energy and so accelerating expansion stops there. Within each bubble we get a new big bang that kicks off a more slowly expanding universe.

But that bubble has an edge, and the edge spreads into the surrounding inflating space time at the speed of light causing inflation to stop within the growing bubble. If lots of bubbles form, they could collide with each other to produce a connected network of non-inflating universes.

But the little patches where inflation persists are expanding so fast that they quickly dominate the non-inflating network of bubbles, and so the cycle of inflation continues eternally. Some questions spring to mind. I mean, besides, what? For example, how many bubbled universes can be made this way? How often do they collide? Are those universes similar to each other, or wildly different? I'll give you an idea of our best thinking for all of these. Let's start with the first two, because these are basically the answers to our recent challenge question. Now, to fully answer these, we would need to know the true physics of the inflaton field. But we can figure some stuff out just based on the exponential nature of expansion. For example, in the challenge question, I asked you to calculate the relative number of new bubbled universes that form.

SPEAKER_1 (3:56)

This episode is brought to you by Accenture. When you're advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales. Using automation, analytics, and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

SPEAKER_2 (4:27)

This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome? That's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up required compatibility and availability varies 18 plus.

Matt O'Dowd (4:57)

We assume that there's a fixed but unknown probability that a bubble universe will form in any given volume of inflating space. That means the number of bubbles forming at each point in time should be proportional to the volume of the inflating universe at that time. So the rate at which that volume is increasing is the same as the rate at which the bubble universes multiply.