But to achieve such immensity and diversity, wouldn't there still have to be, at a deeper level, some rock-bottom, fundamental "universe-generating laws" to create all the multiple universes in the first place, each of which has its own different laws? Where is bedrock reality? Not every cosmologist is a full convert to the multiverse.
As cosmologist George Ellis told me, "I don't like the word 'multiverse. Moreover, he stresses the basic problem of other domains of space-time. Maybe we are seeing the same patch of space-time over and over again. Einstein's theory [of general relativity] allows this to happen because space-time not only is curved, but also it can have a different connectivity structure.
So maybe we can go for several hundred million light-years [in one direction] and then suddenly we return from that side [to where we started from], just like Pac-Man did in those early computer games. We would be seeing many images, maybe hundreds of images, of the same galaxy. Physicist Paul Davies, director of the Beyond Center for Fundamental Concepts in Science at Arizona State University, said he gives "two cheers [not the full three] for the multiverse," because "although there are good reasons for supposing that what we see may not be all that exists, the hypothesis falls far short of being a complete theory of existence.
You're going to need some laws of physics. All theories of the multiverse assume quantum physics to provide the element of spontaneity, to make the bangs happen.
They assume pre-existing space and time. They assume the normal notion of causality, a whole host of pre-existing conditions. Davies then made his deep point. What about those meta-laws that generate all the universes in the first place?
Where did they come from? Then what about the laws or meta-laws that impose diverse local laws upon each individual universe? How do they work? What is the distribution mechanism? Davies dismissed the idea that "any universe you like is out there somewhere. I think such an idea is just ridiculous and it explains nothing. Having all possible universes is not an explanation, because by invoking everything, you explain nothing.
Davies' critique of the multiverse goes deeper. To explain the universe, he rejects "outside explanations," he said.
Then Davies makes his damning comparison. In fact, I think both explanations — multiverse and God — are pretty much equivalent. Davies said he appreciates all the motivations and mathematics that drive inflation theory, along with the multiple universes that seem the compulsory consequences. But still, he said, he feels that an infinite number of universes does not make sense.
Something's amiss. What's my take? Long out of childhood, but still feeling childlike in the presence of a multiverse, I try to assess the possibilities.
I like to categorize things, to discern scope and breadth. Here are seven possible mechanisms that could generate multiple universes. What's more, these seven mechanisms for generating multiple universes are not mutually exclusive. Several, or even all of them, could be true — and they could nest in various ways, one within others, others within one.
In a multiverse, one cannot avoid infinity, and infinity does strange things. There are two types of possible infinities in a multiverse: Type I: A single universe may be infinite in size e. The consequences of either infinity become bizarre. First of all, even Tegmark's Level I multiverse, assuming it's infinite, must contain everything that's physically possible. This means, for example, that every "Star Wars" scenario really exists out there, including those that didn't make it into the films and even all those the writers didn't think of!
Similarly, as long as there is sufficient space for unending random shufflings of particles and a universe of infinite size certainly has sufficient space , there would have to be a sector of space out there identical to our sector of space, with persons identical to you and to me. I'm not so impressed even by this bizarre proposition.
There would also have to be a sector of space identical to our sector of space except for, say, one hair on the head of one person, which is skewed 1 nanometer to the right. And another sector of space in which all else is the same except for that same hair, which is now skewed 2 nanometers to the left. Then all the hairs on all the people, skewed this way and that way. And then all the things in whole sectors of space, arranged in every possible combination and permutation.
There would be innumerable minute differences and innumerable large differences, with every one a separate sector of space — all enabled because the one infinite universe with infinite sectors of space goes on forever. Obviously, on this vision, randomized particles in the overwhelming majority of vast sectors of space yield nothing much at all.
To be clear, a truly infinite universe means that anything that is not impossible no matter how obscure will happen, must happen and must happen, weirdly, an infinite number of times. An infinite universe goes on forever, not only generating uncountable variations, but also requiring each of the uncountable variations to occur an infinite number of times. That's the strange nature of a true infinity. If multiple universes are real, and especially if a true infinite number of universes really exist, then our worldview changes.
Everything changes. In fact, it may be theoretically impossible to find a valid solution to string theory that includes stable dark energy, says Cumrun Vafa, a Harvard University physicist who led the work on the two papers. String theory is an attempt to describe the whole universe under a single "theory of everything" by adding extra dimensions of spacetime and thinking of particles as miniscule vibrating loops.
Many string theorists contend it is still the most promising direction for pursuing Albert Einstein's dream of uniting his general theory of relativity with the conflicting microscopic world of quantum mechanics. Yet the notion of a string theory landscape that predicts not just one universe but many has put some physicists off.
Some string theorists such as Savdeep Sethi of the University of Chicago welcome the reevaluation that is happening now. I'm really happy to see the paradigm shift away from this belief that we have this proven set of solutions. And Eva Silverstein, a Stanford physicist who also helped build the early landscape models, likewise doubts Vafa and his colleagues' argument.
Juan Maldacena, a theorist at the Institute for Advanced Study, says he also still supports the idea of string theory universes with stable dark energy. And many theorists are perfectly happy with the string theory multiverse. And that is a good thing, he adds. Johannes Kepler originally sought a fundamental reason for why Earth lies the distance it does from the sun. At present, physicists have to rely on two such frameworks.
Quantum theory , which accurately describes the physics of the very small, and general relativity , developed by Albert Einstein , which describes the physics of the enormously large. The trouble boils down to gravity. Coming up with a model that ties up all four forces in one neat package is a long-standing dream for theoretical physicists. This leads us back to the Big Bang. An illustration of our cosmic history, from the Big Bang until the present, within the context of The first Friedmann equation describes all of these epochs, from inflation to the Big Bang to the present and far into the future, perfectly accurately, even today.
But the Big Bang wasn't the very beginning of the Universe! We can only extrapolate back to a certain epoch in time before the Big Bang's predictions break down. There are a number of things we observe in the Universe that the Big Bang can't explain, but a new theory that sets up the Big Bang — cosmic inflation — can.
The quantum fluctuations that occur during inflation get stretched across the Universe, and when This leads, over time, to the large-scale structure in the Universe today, as well as the fluctuations in temperature observed in the CMB. In the s, a large number of theoretical consequences of inflation were worked out, including:. In the s, s and s, these four predictions were observationally confirmed to great precision.
Cosmic inflation is a winner. Inflation causes space to expand exponentially, which can very quickly result in any pre-existing If the Universe is curved, it has a radius of curvature that is at minimum hundreds of times larger than what we can observe. Inflation tells us that, prior to the Big Bang, the Universe wasn't filled with particles, antiparticles and radiation.
Instead, it was filled with energy inherent to space itself, and that energy caused space to expand at a rapid, relentless, and exponential rate. At some point, inflation ends, and all or almost all of that energy gets converted into matter and energy, giving rise to the hot Big Bang. The end of inflation, and what's known as the reheating of our Universe, marks the start of the hot Big Bang.
The Big Bang still happens, but it isn't the very beginning. Inflation predicts the existence of a huge volume of unobservable Universe beyond the part we can But it gives us even more than that. If this were the full story, all we'd have was one extremely large Universe.
It would have the same properties everywhere, the same laws everywhere, and the parts that were beyond our visible horizon would be similar to where we are, but it wouldn't be justifiably called the multiverse.
Until, that is, you remember that everything that physically exists must be inherently quantum in nature. Even inflation, with all the unknowns surrounding it, must be a quantum field. It needs to roll down the metaphorical hill and into the valley, but if it's a quantum field, the spreading-out means it will end in some regions while continuing in others.
Wherever inflation occurs blue cubes , it gives rise to exponentially more regions of space with Even if there are many cubes where inflation ends red Xs , there are far more regions where inflation will continue on into the future.
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