When cosmologists came up with cosmic inflation, the idea that the early universe ballooned exponentially in a moment, they quickly realised they may have got more than they bargained for. Inflation can happen anywhere in space and time. It happened in our patch of the universe a long time ago, and it made our corner of the universe very large, but there could be different parts of the universe where it’s still going on.
This scenario, known as eternal inflation, produces a pantheon of different “bubble” universes, all crowded together, with more budding off all the time. Welcome to the inflationary multiverse. There is no way to observe or measure it because all the bubble universes it contains lie outside the limits of our observable universe. Instead, many cosmologists are convinced it exists because it is a logical consequence of two theories, inflation and quantum mechanics, that have been demonstrated to be valid to varying degrees.
Not being able to see them hasn’t prevented people from speculating about how many universes there might be, and what they might contain.
With the standard-issue inflationary multiverse, the number of universes is endless. What we find in each one could be something wildly different from the universe we know. This idea of a cosmic pick-and-mix grew out of attempts to explain gravity in the same way as the other three forces of nature, as a quantum force. These string theories replace familiar point-like particles with tiny vibrating strings that exist in multiple dimensions – normally 10 or 11 of them, depending on your preferred version– and predict a vast landscape of at least 10500 different possibilities for how physics might look in the myriad bubbles of the inflationary multiverse. Each would have different physical laws and different values for the constants of nature.
Or maybe there is just one other universe, and we have already seen tangible evidence of its existence. In 2016, the Antarctic Impulsive Transient Antenna (ANITA) detected a high-energy particle that instead of heading in from space, appeared to be blasting out of Earth. Two years later, it made a second such discovery. One explanation is that the particle might have come from a parallel universe created concurrently with our own, but travelling backwards in time. (New Scientist, December 2020).
My comment: makes all our alarums and excursions, attempts at political coups, illnesses and political buffoonery seem rather petty. I wish I could have been one of the clever scientists who spend their lives theorizing about universes. It takes imagination ( with a capital “I”).
A good and harmless way of achieving ataraxia.