A brief history of the multiverse

inflationary universeAndrei Linde
The theory of the inflationary multiverse changes the way we think about our place in the world.
According to its most popular version, our world may consist of infinitely many exponentially large parts, exhibiting different sets of low-energy laws of physics. Since these parts are extremely large, the interior of each of them behaves as if it were a separate universe, practically unaffected by the rest of the world.
This picture, combined with the theory of eternal inflation and anthropic considerations, may help to solve many difficult problems of modern physics, including the cosmological constant problem.
In this article I will briefly describe this theory and provide links to the some hard to find papers written during the first few years of the development of the inflationary multiverse scenario….
Read more at http://arxiv.org/pdf/1512.01203v1.pdf


In a Multiverse, What Are the Odds?

Testing the multiverse hypothesis requires measuring whether our universe is statistically typical among the infinite variety of universes. But infinity does a number on statistics.

In modern physics is to be believed, we shouldn’t be here. The meager dose of energy infusing empty space, which at higher levels would rip the cosmos apart, is a trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion times tinier than theory predicts. And the minuscule mass of the Higgs boson, whose relative smallness allows big structures such as galaxies and humans to form, falls roughly 100 quadrillion times short of expectations. Dialing up either of these constants even a little would render the universe unlivable…
… read more at http://www.quantamagazine.org/20141103-in-a-multiverse-what-are-the-odds/


Brian Greene: Welcome to the Multiverse

The latest developments in cosmology point toward the possibility that our universe is merely one of billions.

“What really interests me is whether God had any choice in creating the world.”

That’s how Albert Einstein, in his characteristically poetic way, asked whether our universe is the only possible universe.
The reference to God is easily misread, as Einstein’s question wasn’t theological. Instead, Einstein wanted to know whether the laws of physics necessarily yield a unique universe—ours—filled with galaxies, stars, and planets. Or instead, like each year’s assortment of new cars on the dealer’s lot, could the laws allow for universes with a wide range of different features? And if so, is the majestic reality we’ve come to know—through powerful telescopes and mammoth particle colliders—the product of some random process, a cosmic roll of the dice that selected our features from a menu of possibilities? Or is there a deeper explanation for why things are the way they are?

In Einstein’s day, the possibility that our universe could have turned out differently was a mind-bender that physicists might have bandied about long after the day’s more serious research was done. But recently, the question has shifted from the outskirts of physics to the mainstream. And rather than merely imagining that our universe might have had different properties, proponents of three independent developments now suggest that there are other universes, separate from ours, most made from different kinds of particles and governed by different forces, populating an astoundingly vast cosmos.

The multiverse, as this vast cosmos is called, is one of the most polarizing concepts to have emerged from physics in decades, inspiring heated arguments between those who propose that it is the next phase in our understanding of reality, and those who claim that it is utter nonsense, a travesty born of theoreticians letting their imaginations run wild.

So which is it? And why should we care? Grasping the answer requires that we first come to grips with the big bang…..
Read more: www.thedailybeast.com


Neutrinos and multiverses: a new cosmology beckons

You wait decades for discoveries that could revolutionise physics, then three come along at once

“THE universe is not only queerer than we suppose, but queerer than we can suppose,” as geneticist J. B. S. Haldane once remarked. In recent decades, physicists have done their best to prove Haldane wrong, by supposing some very queer universes indeed.

Their speculations may seem fantastical, but they are well motivated. Physics poses some formidable questions that we are so far unable to answer. Why is the universe dominated by matter not antimatter? Why does our universe appear to be “fine-tuned” with just the right properties to give rise to galaxies, stars, planets, life and physicists?

The existing edifice of physics, built upon the twin foundations of general relativity and quantum mechanics, is clearly in need of renovation. We have been waiting for years for cracks to appear that might tell us how to go about it. But up to now, nature has remained stubbornly unmoved.

In the past few weeks, however, promising cracks have opened up. In September came stunning news of neutrinos travelling faster than the speed of light. Sceptics withheld judgement but now a new analysis has affirmed the initial result (see “More data shows neutrinos still faster than light”). We still await independent verification – doubts have already been cast – but if it holds up the implications are enormous, opening the door to a new and very different picture of the cosmos.

No less tantalising is a report that particles called mesons decay differently from their antimatter counterparts, anti-mesons (see “LHC antimatter anomaly hints at new physics”). If this result stands up, it would go a long way towards explaining why we have more matter than antimatter. More importantly, it would prise open the standard model of particle physics – which cannot explain the result – and point the way to yet more new physics.

The widest crack of all concerns a theory once considered outlandish but now reluctantly accepted as the orthodoxy. Almost everything in modern physics, from standard cosmology and quantum mechanics to string theory, points to the existence of multiple universes – maybe 10500 of them, maybe an infinite number (see “The ultimate guide to the multiverse”).

If our universe is just one of many, that solves the “fine-tuning” problem at a stroke: we find ourselves in a universe whose laws are compatible with life because it couldn’t be any other way. And that would just be the start of a multiverse-fuelled knowledge revolution.

Conclusive evidence may be close at hand. Theorists predict that our universe might once have collided with others. These collisions could have left dents in the cosmic microwave background, the universe’s first light, which the European Space Agency’s Planck satellite is mapping with exquisite precision. The results are eagerly awaited, and could trigger a revolution not unlike the ones unleashed by Copernicus’s idea that the Earth is not the centre of the solar system and Edwin Hubble’s discovery that our galaxy is just one among many in an expanding universe.

These are exciting, possibly epoch-making, times. Our understanding of the universe stands on the brink of being remade once again. The universe may indeed be queerer than we can suppose, but that was never going to stop us from trying

Read also: The Ultimate Guide to the Multiverse

How to spot a multiverse

All-sky survey of the cosmic microwave background taken by the ESA's Planck space mission. (Courtesy: ESA)

How can we tell if another universe has collided with our own? Physicists in Canada and the US believe they have the answer – it would leave “a unique and highly characteristic” imprint in the microwave background that pervades the cosmos. The physicists claim that the prediction can be tested using existing and future space telescopes, which contradicts a widespread view that the existence of a multiverse is untestable.

Chuck Bennett, an astrophysicist at Johns Hopkins University in Maryland, US, who was not involved with the study, believes the prediction helps bring multiverse theory into the realms of conventional, falsifiable science. “Science relies on being able to falsify ideas through experiment or observations of nature,” he says. “The fact that these potentialities exist enables us to call this ‘science’. That, to me, is a significant statement.”

The possibility of a multiverse comes from both string theory and inflation theory, the idea that our universe underwent a rapid expansion just after the Big Bang. Inflation theory does a good job of explaining why space is fairly smooth on large scales, but researchers can’t explain what started the expansion and what stopped it. These problems have led physicists to consider the possibility that inflation could occur at other places and times, generating new universes in addition to our own.

Metaphysical problem

The idea of a multiverse is highly controversial. One problem is metaphysical: the universe seems big already, without having to contend with a potentially infinite number of others. Yet perhaps a bigger problem is scientific. If observations are limited to our own observable universe, how can scientists test whether a bigger multiverse exists? The answer to that has been that, from time to time, another universe in the multiverse might collide through ours, leaving a “wake” in its path. But figuring out precisely what such a wake would look like hasn’t been easy.

Now, however, Kris Sigurdson of the University of British Columbia in Vancouver and others say they have calculated the detailed features of a cosmic wake. They have considered the possibility that our universe collided with another before our inflation period, because, they say, the latter would have erased the wake’s evidence. Even though this happened more than 13 billion years ago, the wake would have been preserved in the cosmic microwave background (CMB), which was formed some 380,000 years into the universe’s existence.

Look for a ‘double peak’

The focus of the prediction is in the polarization of photons in the CMB. Photons have two transverse polarization states, and any that come from a certain region in the CMB might be mostly in the same polarization state, or in a mix of both. Sigurdson and colleagues calculate that, providing the wake was big enough, it ought to imprint the CMB with a characteristic “double peak”: two close rings where the photons sway towards a single polarization state.

The prediction is not strictly the first to arise from multiverse theory. In 2007 researchers at the University of California at Santa Cruz, US, also suggested that a cosmic wake could imprint itself on the CMB; then, earlier this year, a group led by Hiranya Peiris of University College London found hints that this prediction was true. But these predicted features were too vague, say Sigurdson and colleagues, and might have existed in the CMB anyway.

Evidence for string theory?

“[Our] features represent the first verifiable prediction of the multiverse paradigm,” write Sigurdson and colleagues in their preprint, which they uploaded to the arXiv server last month. “A detection of a bubble collision would confirm the existence of the multiverse, provide compelling evidence for the string theory landscape, and sharpen out picture of the universe and its origins.” Physics World was unable to speak to the researchers about their preprint because they are submitting it to a journal that employs an embargo policy.

If the prediction is correct, it should be possible to test it in upcoming data from the European Space Agency’s Planck space observatory and future CMB missions, say the researchers. Yet Bennett, the principal investigator on NASA’s Wilkinson Microwave Anisotropy Probe, another CMB space observatory, thinks the detection of a cosmic wake would nonetheless be “extremely unlikely”. He says the amplitude of a wake would have to be just right: too small and we wouldn’t see it; too big and it would probably have had severe consequences for our universe’s structure. The number of collisions would also have to be “fine-tuned”, he says.

Infinite number of wakes

“The claim seems to be that we might see one or two wakes in our sky, but why one or two?” he adds. “Why not none or an infinite number? In fact, if bubble collisions were common we would not be alive to discuss the question.”

Cosmologist Arjun Berera at the University of Edinburgh, UK, also thinks the idea of a multiverse – and by extension Sigurdson and colleagues’ prediction – is speculative. But he notes that a positive detection would be “spectacular”. “Such a case would offer suggestive evidence in support of string theory,” he says. “On the other hand, no evidence in the CMB data for a collision between two universes would not rule out string theory, it would simply extend the widely held belief in the field that string theory is unfalsifiable.”

The research is described in arXiv:1109.3473.