A well-written article that covers some of the ideas and challenges of the multiverse theory. - DG
One universe among many?
An astonishing concept has entered mainstream cosmological thought: physical reality could be hugely more extensive than the patch of space and time traditionally called “the universe.” We’ve learnt that we live in a solar system that is just one planetary system among billions, in one galaxy among billions. But there are signs that a further Copernican demotion confronts us. The entire panorama that astronomers can observe could be a tiny part of the aftermath of our Big Bang, which is itself just one bang among a potentially infinite ensemble. In this grander perspective, what we’ve traditionally called the laws of nature may be no more than parochial bylaws—local manifestations of “bedrock” laws that must be sought at a still deeper level.
Astronomers might seem the most helpless of all scientists. They can’t do experiments on stars and galaxies, and human lives are far too short for us to watch most cosmic objects evolve. But there are some compensating advantages. There are huge numbers of objects in the sky, and one can infer the life-cycle of stars, just as one can infer how trees have grown and will die from one day’s wandering in a forest. Because the light from remote galaxies set out towards us billions of years ago, we can actually (unlike geologists) observe the past. Moreover, we understand enough physics to be able to simulate stars and galaxies—how they form, collide and explode—in the “virtual universe” of a computer.
Powerful telescopes can capture newly formed galaxies whose light set out when the universe was less than 10 per cent of its present age. And high-precision measurements of the “afterglow of creation”—the weak microwaves that warm intergalactic space to a temperature of three degrees above absolute zero—allow us to trace cosmic history back to a time when everything was squeezed hotter and denser than the centre of a star. Such inferences are as evidence-based as anything a geologist might tell you about the history of our Earth. We can confidently trace things back to an era just a nanosecond after the Big Bang, when every particle carried as much energy as can be generated by the huge Large Hadron Collider in Geneva, and the entire observable universe was squeezed into dimensions no larger than the solar system.
But, as always in science, each advance poses new questions. For instance, “Why is the universe expanding the way it is?” and “How did it acquire its observed mix of particles and radiation?” The answers to both lie in the brief instants when our universe was far less than a nanosecond old, and everything in it was far hotter and denser than we can simulate in the lab. We consequently lose any firm foothold in experiment and enter more speculative realms.
An issue of Discover magazine had a cover showing a sphere, with the caption: “The universe when it was a trillionth of a trillionth of a trillionth of a second old—actual size.” According to a popular conjecture, the entire volume we can see with our telescopes “inflated” from a hyper-dense blob no bigger than a tennis ball. How can we firm up such an idea?
The twin pillars of 20th century physics are Einstein’s theory of gravity (general relativity) and quantum theory. But these haven’t yet been unified. In most contexts, their domains of relevance do not overlap. Astronomers can ignore quantum fuzziness when calculating the motions of planets and stars. Conversely, chemists can safely ignore gravitational attraction between atoms because this force is about 40 powers of ten feebler than the electrical forces between them. But during the ultra-compressed earliest instants after the Big Bang, quantum fluctuations could, as it were, shake the entire universe. To tackle the fundamental mystery of what banged and how it banged therefore requires a synthesis of these two great 20th century theories. [continue]