Tag Archives: string theory

Whole Lotta Untangling to Do

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The Universe May Be Flooded with a Cobweb Network of Invisible Strings

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We may soon find out whether we live in an axiverse

an abstract image of axion strings

(Image: © Shutterstock)

What if I told you that our universe was flooded with hundreds of kinds of nearly invisible particles and that, long ago, these particles formed a network of universe-spanning strings?

It sounds both trippy and awesome, but it’s actually a prediction of string theory, our best (but frustratingly incomplete) attempt at a theory of everything. These bizarre, albeit hypothetical, little particles are known as axions, and if they can be found, that would mean we all live in a vast “axiverse.”

The best part of this theory is that it’s not just some physicist’s armchair hypothesis, with no possibility of testing. This incomprehensibly huge network of strings may be detectable in the near future with microwave telescopes that are actually being built.

If found, the axiverse would give us a major step up in figuring out the puzzle of … well, all of physics.

A symphony of strings

OK, let’s get down to business. First, we need to get to know the axion a little better. The axion, named by physicist (and, later, Nobel laureate) Frank Wilczek in 1978, gets its name because it’s hypothesized to exist from a certain kind of symmetry-breaking. I know, I know — more jargon. Hold on. Physicists love symmetries — when certain patterns appear in mathematics.

There’s one kind of symmetry, called the CP symmetry, that says that matter and antimatter should behave the same when their coordinates are reversed. But this symmetry doesn’t seem to fit naturally into the theory of the strong nuclear force. One solution to this puzzle is to introduce another symmetry in the universe that “corrects” for this misbehavior. However, this new symmetry only appears at extremely high energies. At everyday low energies, this symmetry disappears, and to account for that, and out pops a new particle — the axion.

Now, we need to turn to string theory, which is our attempt (and has been our main attempt for 50-odd years now) to unify all of the forces of nature, especially gravity, in a single theoretical framework. It’s proven to be an especially thorny problem to solve, due to a variety of factors, not the least of which is that, for string theory to work (in other words, for the mathematics to even have a hope of working out), our universe must have more than the usual three dimensions of space and one of time; there have to be extra spatial dimensions.

These spatial dimensions aren’t visible to the naked eye, of course; otherwise, we would’ve noticed that sort of thing. So the extra dimensions have to be teensy-tiny and curled up on themselves at scales so small that they evade normal efforts to spot them.

What makes this hard is that we’re not exactly sure how these extra dimensions curl up on themselves, and there’s somewhere around 10^200 possible ways to do it.

But what these dimensional arrangements appear to have in common is the existence of axions, which, in string theory, are particles that wind themselves around some of the curled-up dimensions and get stuck.

What’s more, string theory doesn’t predict just one axion but potentially hundreds of different kinds, at a variety of masses, including the axion that might appear in the theoretical predictions of the strong nuclear force.

Silly strings

So, we have lots of new kinds of particles with all sorts of masses. Great! Could axions make up dark matter, which seems to be responsible for giving galaxies most of their mass but can’t be detected by ordinary telescopes? Perhaps; it’s an open question. But axions-as-dark-matter have to face some challenging observational tests, so some researchers instead focus on the lighter end of the axion families, exploring ways to find them.

And when those researchers start digging into the predicted behavior of these featherweight axions in the early universe, they find something truly remarkable. In the earliest moments of the history of our cosmos, the universe went through phase transitions, changing its entire character from exotic, high-energy states to regular low-energy states.

During one of these phase transitions (which happened when the universe was less than a second old), the axions of string theory didn’t appear as particles. Instead, they looked like loops and lines — a network of lightweight, nearly invisible strings crisscrossing the cosmos.

This hypothetical axiverse, filled with a variety of lightweight axion strings, is predicted by no other theory of physics but string theory. So, if we determine that we live in an axiverse, it would be a major boon for string theory.

A shift in the light

How can we search for these axion strings? Models predict that axion strings have very low mass, so light won’t bump into an axion and bend, or axions likely wouldn’t mingle with other particles. There could be millions of axion strings floating through the Milky Way right now, and we wouldn’t see them.

But the universe is old and big, and we can use that to our advantage, especially once we recognize that the universe is also backlit.

The cosmic microwave background (CMB) is the oldest light in the universe, emitted when it was just a baby — about 380,000 years old. This light has soaked the universe for all these billions of years, filtering through the cosmos until it finally hits something, like our microwave telescopes.

So, when we look at the CMB, we see it through billions of light-years’ worth of universe. It’s like looking at a flashlight”s glow through a series of cobwebs: If there is a network of axion strings threaded through the cosmos, we could potentially spot them.

In a recent study, published in the arXiv database on Dec. 5, a trio of researchers calculated the effect an axiverse would have on CMB light. They found that, depending on how a bit of light passes near a particular axion string, the polarization of that light could shift. That’s because the CMB light (and all light) is made of waves of electric and magnetic fields, and the polarization of light tells us how the electric fields are oriented — something that changes when the CMB light encounters an axion. We can measure the polarization of the CMB light by passing the signal through specialized filters, allowing us to pick out this effect.

The researchers found that the total effect on the CMB from a universe full of strings introduced a shift in polarization amounting to around 1%, which is right on the verge of what we can detect today. But future CMB mappers, such as the Cosmic Origins Explorer, Lite (Light) satellite for the studies of B-mode polarization and Inflation from cosmic background Radiation Detection (LiteBIRD), and the Primordial Inflation Explorer (PIXIE) , are currently being designed. These futuristic telescopes would be capable of sniffing out an axiverse. And once those mappers come online, we’ll either find that we live in an axiverse or rule out this particular prediction of string theory.

Either way, there’s a lot to untangle.

Paul M. Sutter is an astrophysicist at The Ohio State University, host of Ask a Spaceman and Space Radio, and author of Your Place in the Universe.

from:    https://www.livescience.com/universe-filled-with-axion-strings.html

Large Hadron Collector -Finding New Dimensions?

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Large Hadron Collider Could Detect Extra Dimensions

March 19, 2015 | by Stephen Luntz

Photo credit: Mopic via Shutterstock. If gravity is draining out of tiny black holes into other dimensions, the LHC may find it

A paper in Physics Letters B has raised the possibility that the Large Hadron Collider (LHC) could make a discovery that would put its previous triumph with the Higgs Boson in the shade. The authors suggest it could detect mini black holes. Such a finding would be a matter of huge significance on its own, but might be an indication of even more important things.

Few ideas from theoretical physics capture the public imagination as much as the “many-worlds hypothesis,” which proposes an infinite number of universes that differ from our own in ways large and small. The idea has provided great fodder for science fiction writers and comedians.

However, according to Professor Mir Faizal from the University of Waterloo, “Normally, when people think of the multiverse, they think of the many-worlds interpretation of quantum mechanics, where every possibility is actualized,” he said to Phys.org. “This cannot be tested and so it is philosophy and not science.” Nonetheless, Faizal considers the test for a different sort of parallel universes almost within our grasp.

“What we mean is real universes in extra dimensions,” says Faizal. “As gravity can flow out of our universe into the extra dimensions, such a model can be tested by the detection of mini black holes at the LHC.”

The idea that the universe may be filled with minute black holes has been proposed to explain puzzles such as the nature of dark matter. However, the energy required to create such objects depends on the number of dimensions the universe has. In a conventional four-dimensional universe, these holes would require 1016 TeV, 15 orders of magnitude beyond the capacity of the LHC to produce.

String theory, on the other hand, proposes 10 dimensions, six of which have been wrapped up so we can’t experience them. Attempts to model such a universe suggest that the energy required to make these tiny black holes would be a great deal smaller, so much so that some scientists believe they should have been detected in experiments the LHC has already run.

So if no detection, no string theory? Not according to Faizal and his co-authors. They argue that the models used to predict the energy of the black holes in a 10-dimensional universe have left out quantum deformation of spacetime that changes gravity slightly.

Whether this deformation is real is a rapidly developing question, but if it is, the paper argues that the black holes will have energy levels much smaller than in a four-dimensional universe, but about twice as large as that detectable for any test run so far. The LHC is designed to reach 14 TeV, but so far has only gone to 5.3 TeV, while the paper thinks the holes might be lurking at 11.9 TeV. In this case, once the LHC reaches its full capacity, we should find them.

Such a discovery would demonstrate the microscale deformation of spacetime, the existence of extra dimensions, parallel universes within them and string theory. If found at the right energy levels, the holes would confirm the team’s interpretation of a new theory on black hole behavior named gravity’s rainbow, after the influential novel. Such an astonishing quadruple revelation would transform physics, although the researchers are already considering the most likely flaws in their work if the holes prove elusive.

 

from:    http://www.iflscience.com/physics/large-hadron-collider-might-reveal-extra-dimensions

Holographic Universe? Of Course!

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The Universe Really Is a Hologram, According to New Simulations

A 10-dimensional theory of gravity makes the same predictions as standard quantum physics in fewer dimensions

By Ron Cowen and Nature magazine

Image: Astronomy Picture of the Day

A team of physicists has provided some of the clearest evidence yet that our universe could be just one big projection.

In 1997, theoretical physicist Juan Maldacena proposed that an audacious model of the Universe in which gravity arises from infinitesimally thin, vibrating strings could be reinterpreted in terms of well-established physics. The mathematically intricate world of strings, which exist in nine dimensions of space plus one of time, would be merely a hologram: the real action would play out in a simpler, flatter cosmos where there is no gravity.

Maldacena’s idea thrilled physicists because it offered a way to put the popular but still unproven theory of strings on solid footing—and because it solved apparent inconsistencies between quantum physics and Einstein’s theory of gravity. It provided physicists with a mathematical Rosetta stone, a “duality,” that allowed them to translate back and forth between the two languages, and solve problems in one model that seemed intractable in the other and vice versa. But although the validity of Maldacena’s ideas has pretty much been taken for granted ever since, a rigorous proof has been elusive.

In two papers posted on the arXiv repository, Yoshifumi Hyakutake of Ibaraki University in Japan and his colleagues now provide, if not an actual proof, at least compelling evidence that Maldacena’s conjecture is true.

In one paper, Hyakutake computes the internal energy of a black hole, the position of its event horizon (the boundary between the black hole and the rest of the Universe), its entropy and other properties based on the predictions of string theory as well as the effects of so-called virtual particles that continuously pop into and out of existence. In the other3, he and his collaborators calculate the internal energy of the corresponding lower-dimensional cosmos with no gravity. The two computer calculations match.

“It seems to be a correct computation,” says Maldacena, who is now at the Institute for Advanced Study in Princeton, New Jersey and who did not contribute to the team’s work.

Regime change

The findings “are an interesting way to test many ideas in quantum gravity and string theory,” Maldacena adds. The two papers, he notes, are the culmination of a series of articles contributed by the Japanese team over the past few years. “The whole sequence of papers is very nice because it tests the dual [nature of the universes] in regimes where there are no analytic tests.”

“They have numerically confirmed, perhaps for the first time, something we were fairly sure had to be true, but was still a conjecture — namely that the thermodynamics of certain black holes can be reproduced from a lower-dimensional universe,” says Leonard Susskind, a theoretical physicist at Stanford University in California who was among the first theoreticians to explore the idea of holographic universes.

Neither of the model universes explored by the Japanese team resembles our own, Maldacena notes. The cosmos with a black hole has ten dimensions, with eight of them forming an eight-dimensional sphere. The lower-dimensional, gravity-free one has but a single dimension, and its menagerie of quantum particles resembles a group of idealized springs, or harmonic oscillators, attached to one another.

Nevertheless, says Maldacena, the numerical proof that these two seemingly disparate worlds are actually identical gives hope that the gravitational properties of our universe can one day be explained by a simpler cosmos purely in terms of quantum theory.

This article is reproduced with permission from the magazine Nature. The article was first published on December 10, 2013.

from:     http://www.scientificamerican.com/article.cfm?id=universe-really-is-a-holo