Why parallel universes

But there are very different ways that multiple universes might come about, and one of the most mind-blowing — the Many-Worlds formulation of quantum physics — is also one of the most plausible. Rather, it refers to a collection of regions of space, so far away that they are unobservable to us, where conditions of very different. There may be different particles, different forces, even a different number of dimensions of space from what we see around us. It arises naturally as a consequence of other speculative ideas, including string theory and cosmological inflation.

But exactly because those ideas are themselves speculative, the cosmological multiverse should be thought of as speculative-squared.

And they arise naturally from the simplest version of our most solidly tested physical theory, quantum mechanics. The many worlds of quantum mechanics, I would argue, are probably there. Not everyone agrees with me about this. To see why, we have to think about how quantum mechanics works.

Consider an electron, which is an elementary particle that has a certain fixed amount of a quantity called spin. That would be weird enough as it is — why only two possible answers?

We can use the wave function to calculate the probability of each measurement outcome. Here, multiverse proponents Alexander Vilenkin and Max Tegmark offer counterpoints, explaining why the multiverse would account for so many features of our universe—and how it might be tested.

The universe as we know it originated in a great explosion that we call the big bang. For nearly a century cosmologists have been studying the aftermath of this explosion: how the universe expanded and cooled down, and how galaxies were gradually pulled together by gravity. The nature of the bang itself has come into focus only relatively recently. It is the subject of the theory of inflation, which was developed in the early s by Alan Guth, Andrei Linde and others, and has led to a radically new global view of the universe.

Inflation is a period of super-fast, accelerated expansion in early cosmic history. It is so fast that in a fraction of a second a tiny subatomic speck of space is blown to dimensions much greater than the entire currently observable region.

At the end of inflation, the energy that drove the expansion ignites a hot fireball of particles and radiation. This is what we call the big bang. The end of inflation is triggered by quantum, probabilistic processes and does not occur everywhere at once. In our cosmic neighborhood, inflation ended The new regions appear as tiny, microscopic bubbles and immediately start to grow.

The bubbles keep growing without bound; in the meantime they are driven apart by the inflationary expansion, making room for more bubbles to form. This never-ending process is called eternal inflation. We live in one of the bubbles and can observe only a small part of it. No matter how fast we travel, we cannot catch up with the expanding boundaries of our bubble, so for all practical purposes we live in a self-contained bubble universe.

The theory of inflation explained some otherwise mysterious features of the big bang, which simply had to be postulated before. It also made a number of testable predictions, which were then spectacularly confirmed by observations. By now inflation has become the leading cosmological paradigm. Another key aspect of the new worldview derives from string theory, which is at present our best candidate for the fundamental theory of nature.

String theory admits an immense number of solutions describing bubble universes with diverse physical properties. Now combine this with the theory of inflation.

Each bubble type has a certain probability to form in the inflating space. So inevitably, an unlimited number of bubbles of all possible types will be formed in the course of eternal inflation. This picture of the universe, or multiverse , as it is called, explains the long-standing mystery of why the constants of nature appear to be fine-tuned for the emergence of life.

The reason is that intelligent observers exist only in those rare bubbles in which, by pure chance, the constants happen to be just right for life to evolve. The rest of the multiverse remains barren, but no one is there to complain about that. Some of my physicist colleagues find the multiverse theory alarming. Any theory in physics stands or falls depending on whether its predictions agree with the data.

But how can we verify the existence of other bubble universes? Paul Steinhardt and George Ellis have argued, for example, that the multiverse theory is unscientific, because it cannot be tested, even in principle.

Surprisingly, observational tests of the multiverse picture may in fact be possible. Anthony Aguirre, Matt Johnson, Matt Kleban and others have pointed out that a collision of our expanding bubble with another bubble in the multiverse would produce an imprint in the cosmic background radiation—a round spot of higher or lower radiation intensity. A detection of such a spot with the predicted intensity profile would provide direct evidence for the existence of other bubble universes. The search is now on, but unfortunately there is no guarantee that a bubble collision has occurred within our cosmic horizon.

There is also another approach that one can follow. The idea is to use our theoretical model of the multiverse to predict the constants of nature that we can expect to measure in our local region. If the constants vary from one bubble universe to another, their local values cannot be predicted with certainty, but we can still make statistical predictions. We can derive from the theory what values of the constants are most likely to be measured by a typical observer in the multiverse.

Assuming that we are typical—the assumption that I called the principle of mediocrity —we can then predict the likely values of the constants in our bubble. Steven Weinberg has noted that in regions where dark energy is large, it causes the universe to expand very fast, preventing mater from clumping into galaxies and stars. Observers are not likely to evolve in such regions.

Calculations showed that most galaxies and therefore most observers are in regions where the dark energy is about the same as the density of matter at the epoch of galaxy formation. The prediction is therefore that a similar value should be observed in our part of the universe. For the most part, physicists did not take these ideas seriously, but much to their surprise, dark energy of roughly the expected magnitude was detected in astronomical observations in the late s.

That is, if the universe that we live in goes on forever, there are only so many ways that the building blocks of matter can arrange themselves as they assemble across infinite space.

Eventually, any finite number of particle types must repeat a particular arrangement. Hypothetically, in a big enough space, those particles must repeat arrangements as large as entire solar systems and galaxies. So, your entire life might be repeated elsewhere in the universe, down to what you ate for breakfast yesterday. At least, that's the theory. But if the universe began at a finite point, as nearly every physicist agrees that it did, an alternate version of you likely doesn't exist, according to astrophysicist Ethan Siegel's Medium article.

According to Siegel, "the number of possible outcomes from particles in any Universe interacting with one another tends towards infinity faster than the number of possible Universes increases due to inflation.

In a relatively recent addition to the pantheon of multiverse theories, researchers from the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, have proposed that the universe began at the Big Bang — and on the opposite side of the Big Bang timeline, stretching backwards in time, a universe once existed that was the exact mirror image of our own.

That means everything — protons, electrons, even actions like cracking an egg — would be reversed. Antiprotons and positively charged electrons would make up atoms, while eggs would un-crack and make their way back inside chickens.

Eventually, that universe would shrink down, presumably to a singularity, before expanding out into our own universe. Seen another way, both universes were created at the Big Bang and exploded simultaneously backward and forward in time.

Our universe grew exponentially in the first moments of its existence, but was this expansion uniform? If not, it suggests different regions of space grew at different rates — and may be isolated from one another. How are the laws of the universe so exact? Some propose that this happened only by chance — we are the one universe out of many that happened to get the numbers right. What is beyond the edge of the observable space around us?

No one knows for sure, and until we do which could be never , the thought that ou universe extends indefinitely is an interesting one. There is no way for us to ever test theories of the multiverse.

We will never see beyond the observable universe, so if there is no way to disprove the theories, should they even be given credence?

Sometimes, the simplest ideas are the best. Some physicists argue that we don't need the multiverse theory at all.

It doesn't solve any paradoxes, and only creates complications. Not only can we not disprove any multiverse theory, we can't prove them either.

We currently have no evidence that multiverses exists, and everything we can see suggests there is just one universe — our own.