What Is Eternal Inflation? Universes Within Universes Featuring Ethan Siegel

In order to get the large scale structure of the Universe we see today, cosmologists have proposed the idea of inflation, that the Universe expanded an enormous amount in the earliest moments. But if inflation really happened, then it has even stranger implications for the nature of the Universe and the search for multiverses.

Ethan Siegel


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We’ve covered the topic of inflation a couple of times in the past, but I’ll give you the short version one more time.

The Big Bang exquisitely explains the expansion of the Universe we see today. When we look out as far as we can, to the edge of the observable Universe we see the afterglow of the Big Bang: the cosmic microwave background radiation.

This light was released the moment the Universe had cooled down a little, and has been traveling for almost 13.8 billion years to reach us. Thanks to the expansion of the Universe, it’s been redshifted to just a few degrees above absolute zero.

When astronomers measure the temperature of this background, it’s incredibly consistent, with only tiny fluctuations measurable with the most sensitive instruments. This means that the entire Universe that we can see had time to transfer temperature to each other before it expanded.

But the original Big Bang Theory suggests that the expansion of the Universe didn’t give the material time to even out its temperature.

In order to explain this, cosmologists developed the concept of inflation. There was a period in the earliest Universe when the energy in matter was bound up in the fabric of space itself. The Universe expanded so quickly, that a region the size of a subatomic particle would have been stretched to the size of the visible Universe in a fraction of a second.

Inflation also answered other challenges that the original Big Bang couldn’t explain, such as the flatness of the Universe, and total lack of monopoles. Like I said, we’ve done a whole video about inflation.

But inflation has introduced its own set of strange ideas, including the concept of “eternal inflation”; that inflation didn’t end for the entire Universe like it did in our local area. There are regions undergoing inflation all over the place, creating multiple universes within our Universe. You know, a multiverse.

I’ll be honest, though, the concept of eternal inflation is beyond my comprehension. And so, in times like this, I like to bring in a ringer.

Today, I’m glad to bring you Dr. Ethan Siegel, an astrophysicist and science writer. His most recent book is Treknology, all about the science of Star Trek. Ethan tackles some of the most complex topics out there in an understandable way, and I could really use his help.



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