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TOPIC: Superstrings


Posts: 131433
RE: Superstrings

Title: Does string theory predict an open universe?
Authors: R. Buniy, S. Hsu, A. Zee

It has been claimed that the string landscape predicts an open universe, with negative curvature. The prediction is a consequence of a large number of metastable string vacua, and the properties of the Coleman--De Luccia instanton which describes vacuum tunnelling. We examine the robustness of this claim, which is of particular importance since it seems to be string theory's sole claim to falsifiability. We find that, due to subleading tunnelling processes, the prediction is sensitive to unknown properties of the landscape. Under plausible assumptions, universes like ours are as likely to be closed as open.

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Title: Spacetime foam at a TeV
Authors: Luis A. Anchordoqui

Motivated by recent interest in TeV-scale gravity and especially by the possibility of fast baryon decay mediated by virtual black holes, we study another dangerous aspect of spacetime foam interactions: lepton flavour violation. We correlate existing limits on gravity-induced decoherence in the neutrino sector with a lower bound on the scale of quantum gravity, and find that if spacetime foam interactions do not allow an S-matrix description the UV cutoff is well beyond the electroweak scale. This suggests that string theory provides the appropriate framework for description of quantum gravity at the TeV-scale.

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Brane Inflation

Title: Is Brane Inflation Eternal?
Authors: Xingang Chen, Sash Sarangi, S.-H. Henry Tye, Jiajun Xu

In this paper, Researchers show that eternal inflation of the random walk type is generically absent in the brane inflationary scenario. Depending on how the brane inflationary universe originated, eternal inflation of the false vacuum type is still quite possible. Since the inflation is the position of the D3-brane relative to the anti-D3-brane inside the compactified bulk with finite size, its value is bounded.
In DBI inflation, the warped space also restricts the amplitude of the scalar fluctuation. These upper bounds impose strong constraints on the possibility of eternal inflation. The researchers find that eternal inflation due to the random walk of the inflaton field is absent in both the KKLMMT slow roll scenario and the DBI scenario. A more careful analysis for the slow-roll case is also presented using the Langevin equation, which gives very similar results.
They discuss possible ways to obtain eternal inflation of the random walk type in brane inflation. In the multi-throat brane inflationary scenario, the branes may be generated by quantum tunnelling and roll out the throat. Eternal inflation of the false vacuum type inevitably happens in this scenario due to the tunnelling process. Since these scenarios have different cosmological predictions, more data from the cosmic microwave background radiation will hopefully select the specific scenario our universe has gone through.

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Hidden Spatial Dimensions

A tiny, artificial solar system could reveal hidden spatial dimensions and test alternative theories of gravity, a new study suggests. If the system's "planets" moved slightly differently than expected from standard gravity, it would signal the presence of new physical phenomena – which have proven very difficult to test.
Numerous theories that attempt to unify all the forces of physics into one cohesive model call for hidden spatial dimensions in addition to the three we can sense. In some of these theories, gravity would leak into the extra dimensions – explaining why it is a relatively weak force in the universe we know.

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Posts: 131433
Big Bang

How did the Universe begin? Many scientists would regard this as one of the most profound questions of all. But to Stephen Hawking, who has perhaps come closer than anyone to answering it, the question doesn't in fact even exist.

Hawking, based at the University of Cambridge, UK, and his colleague Thomas Hertog of the European Laboratory for Particle Physics at CERN in Geneva, Switzerland, are about to publish a paper claiming that the Universe had no unique beginning1. Instead, they argue, it began in just about every way imaginable (and maybe some that aren't).
Out of this profusion of beginnings, the vast majority withered away without leaving any real imprint on the Universe we know today. Only a tiny fraction of them blended to make the current cosmos, Hawking and Hertog claim.
That, they insist, is the only possible conclusion if we are to take quantum physics seriously.

"Quantum mechanics forbids a single history" - Thomas Hertog.

The researchers' theory comes in response to a problem raised by 'string theory', one of the best hopes for a theory of everything. String theory permits innumerable different kinds of universe, most of them very different from the one we inhabit. Some physicists suspect that an unknown factor will turn up that rules out most of these universes.
But Hawking and Hertog say that the countless 'alternative worlds' of string theory may actually have existed. We should picture the Universe in the first instants of the Big Bang as a superposition of all these possibilities, they say; like a projection of billions of movies played on top of one another.

This might sound odd, but it is precisely the view adopted by quantum theory. Think of a particle of light reaching our eye from a lamp. Common sense suggests that it simply travels in a straight line from the bulb to the eye. But to make correct predictions about the particle's behaviour, quantum mechanics must consider all other possible paths too, including ones in which, say, the photon bounces around the walls thousands of times before reaching us.

This summation of all paths, proposed in the 1960s by physicist Richard Feynman and others, is the only way to explain some of the bizarre properties of quantum particles, such as their apparent ability to be in two places at once. The key point is that not all paths contribute equally to the photon's behaviour: the straight-line trajectory dominates over the indirect ones.
Hertog argues that the same must be true of the path through time that took the Universe into its current state. We must regard it as a sum over all possible histories.
He and Hawking call their theory 'top-down' cosmology, because instead of looking for some fundamental set of initial physical laws under which our Universe unfolded, it starts 'at the top', with what we see today, and works backwards to see what the initial set of possibilities might have been. In effect the present 'selects' the past.

Within just a few seconds after the Big Bang, a single history had already come to dominate the Universe, he explains. So from the 'classical' viewpoint of big objects such as stars and galaxies, things happened only one way after that point. Other 'histories', say, one in which the Earth formed only 4,000 years ago, have made no significant contribution to this cosmic evolution.
But in the first instants of the Big Bang, there existed a superposition of ever more different versions of the Universe, instead of a unique history. And most crucially, Hertog says that "our current Universe has features frozen in from this early quantum mixture".

In other words, some of these alternative histories have left their imprint behind. This is why Hertog and Hawking insist that their 'top-down' cosmology is testable. The theory predicts the pattern of the variations in intensity of microwave background radiation, the afterglow of the Big Bang now imprinted on the sky, which reveal fluctuations in the fireball of the nascent Universe. These variations are minute, but space-based detectors have measured them ever more accurately over the past several years.
As the two researchers work out top-down cosmology in more detail, they hope to be able to calculate the spectrum of these microwave fluctuations and compare it with observations.
The theory also suggests an answer to the puzzle of why some of the 'constants of nature' seem finely tuned to a value that allows life to evolve. If we start from where we are now, it is obvious that the current Universe must 'select' those histories that lead to these conditions. Otherwise we simply wouldn't be here.




Posts: 131433
Extra Dimensions

Researchers at Northeastern University and the University of California, Irvine say that scientists might soon have evidence for extra dimensions and other exotic predictions of string theory. Early results from a neutrino detector at the South Pole, called AMANDA, show that ghostlike particles from space could serve as probes to a world beyond our familiar three dimensions, the research team says.

No more than a dozen high-energy neutrinos have been detected so far. However, the current detection rate and energy range indicate that AMANDA's larger successor, called IceCube, now under construction, could provide the first evidence for string theory and other theories that attempt to build upon our current understanding of the universe.

An article describing this work appears in the current issue of Physical Review Letters. The authors are: Luis Anchordoqui, associate research scientist in the Physics Department at Northeastern University; Haim Goldberg, professor in the Physics Department at Northeastern University; and Jonathan Feng, associate professor in the Department of Physics and Astronomy at University of California, Irvine. The evidence, they say, would come from how neutrinos interact with other forms of matter on Earth.

"To find clues to support string theory and other bold, new theories, we need to study how matter interacts at extreme energies. Human-made particle accelerators on Earth cannot yet generate these energies, but nature can in the form of the highest-energy neutrinos" - Luis Anchordoqui.

In recent decades, new theories have developed - such as string theory, extra dimensions and supersymmetry - to bridge the gap between the two most successful theories of the 20th century, general relativity and quantum mechanics. Quantum mechanics describes three of the fundamental forces of nature: electromagnetism, strong forces (binding atomic nuclei) and weak forces (seen in radioactivity). It is, however, incompatible with Einstein's general relativity, the leading description of the fourth force, gravity. Scientists hope to find one unified theory to provide a quantum description of all four forces.

Clues to unification lie at extreme energies. On Earth, human-made particle accelerators have already produced energies at which electromagnetic forces and weak forces are indistinguishable. Scientists have ideas about how the next generation of accelerators will reveal that strong forces are indistinguishable from the weak and electromagnetic at yet higher energies. Yet to probe deeper to see gravity's connection to the other three forces, still higher energies are needed.
Extragalactic sources can serve as the ultimate cosmic accelerator, and that neutrinos from these sources smacking into protons can release energies in the realm where the first clues to string theory could be revealed.
Neutrinos are elementary particles similar to electrons, but they are far less massive, have neutral charge, and hardly interact with matter. They are among the most abundant particles in the universe; untold billions pass through our bodies every second. Most of the neutrinos reaching Earth are lower-energy particles from the sun.

AMANDA, funded by the National Science Foundation, attempts to detect neutrinos raining down from above but also coming "up" through the Earth. Neutrinos are so weakly interacting that some can pass through the entire Earth unscathed. The total number of "down" and "up" neutrinos is uncertain; however, barring exotic effects, the relative detection rates are well known.
AMANDA detectors are positioned deep in the Antarctic ice. The NSF-funded IceCube has a similar design, only it has about six times more detectors covering a volume of one cubic kilometre. A neutrino smashing into atoms in the ice will emit a brief, telltale blue light; and using the detectors, scientists can determine the direction where the neutrino came from and its energy.
The key to the work presented here is that the scientists are comparing "down" to "up" detections and looking for discrepancies in the detection rate, evidence of an exotic effect predicted by new theories.

"String theory and other possibilities can distort the relative numbers of 'down' and 'up' neutrinos. For example, extra dimensions may cause neutrinos to create microscopic black holes, which instantly evaporate and create spectacular showers of particles in the Earth's atmosphere and in the Antarctic ice cap. This increases the number of 'down' neutrinos detected. At the same time, the creation of black holes causes 'up' neutrinos to be caught in the Earth's crust, reducing the number of 'up' neutrinos. The relative 'up' and 'down' rates provide evidence for distortions in neutrino properties that are predicted by new theories" - Jonathan Feng.

"The neutrinos accelerated in the cosmos to energies unattainable on Earth can detect the 'footprint' of new physics. The 'body' responsible for the footprint can then emerge through complementary experiments at the new generation of human-made colliders. On all fronts, it is an exciting era in high-energy physics" - Haim Goldberg

Source Northeastern University



Posts: 131433
RE: Superstrings

Ancient neutrinos could put string theory and quantum loop gravity to the test
Author: Joy Christian

Tiny but ageing neutrinos can be used to test the very foundations of quantum theory at unprecedented cosmological time scales
Must Schrödinger’s Cat be fat enough for us to detect the possible phenomenon of gravity-driven collapse of the wave-function? In the 1950s, Feynman suggested that the infamous Schrödinger’s Cat paradox would be resolved if gravity can be implicated for the collapse of the wave-function.

But, from Feynman in the 1950s to Penrose today, physicists have always taken for granted that one has to look for quantum superpositions of sufficiently macroscopic objects to detect such an effect due to gravity, since only the large mass of such macroscopic objects could distort the very fabric of space-time in accordance with Einstein's theory of gravity, thereby inducing the quantum mechanical wave-function to collapse.

In this paper, however, researchers prove that the idea of Feynman and Penrose can be tested more decisively by observing neutrinos---provided they have been born just after the Big Bang.
In other words, they show that such tiny but ageing neutrinos can be used to test the very foundations of quantum theory at unprecedented cosmological time scales.
Researchers have pointed out that the Diosi-Penrose ansatz for gravity-induced quantum state reduction can be tested by observing oscillations in the flavour ratios of neutrinos originated at cosmological distances.
Since such a test would be almost free of environmental decoherence, testing the ansatz by means of a next generation neutrino detector such as IceCube would be much cleaner than by experiments proposed so far involving superpositions of macroscopic systems.
The proposed microscopic test would also examine the universality of superposition principle at unprecedented cosmological scales.

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Posts: 131433

Physicists who work with a concept called string theory envision our universe as an eerie place with at least nine spatial dimensions, six of them hidden from us, perhaps curled up in some way so they are undetectable.
The big question is why we experience the universe in only three spatial dimensions instead of four, or six, or nine.
Two theoretical researchers from the University of Washington and Harvard University think they might have found the answer. They believe the way our universe started and then diluted as it expanded - what they call the relaxation principle - favoured formation of three- and seven-dimensional realities.
The one we happen to experience has three dimensions.

"That's what comes out when you do the math" - Andreas Karch, a University of Washington assistant professor of physics and lead author of a new paper that details the theory.

Karch and his collaborator, Lisa Randall, a physics professor at Harvard, set out to model how the universe was arranged right after it began in the big bang, and then watch how the cosmos evolved as it expanded and diluted. The only assumptions were that it started with a generally smooth configuration, with numerous structures - called membranes, or "branes" - that existed in various spatial dimensions from one to nine, all of them large and none curled up.

The researchers allowed the cosmos to evolve naturally, without making any additional assumptions. They found that as the branes diluted, the ones that survived displayed three dimensions or seven dimensions.
In our universe, everything we see and experience is stuck to one of those branes, and for it to result in a three-dimensional universe the brane must be three-dimensional.
Other realities, either three- or seven-dimensional, could be hidden from our perception in the universe.

"There are regions that feel 3D. There are regions that feel 5D. There are regions that feel 9D. These extra dimensions are infinitely large. We just happen to be in a place that feels 3D to us" - Andreas Karch.

In our world, forces such as electromagnetism only recognize three dimensions and behave according to our laws of physics, their strength diminishing with distance. Gravity, however, cuts across all dimensions, even those not recognized in our world. But they theorize that the force of gravity is localized and, with seven branes, gravity would diminish far more quickly with distance than it does in our three-dimensional world.

"We know there are people in our three-brane existence. In this case we will assume there are people somewhere nearby in a seven-brane existence. The people in the three-brane would have a far more interesting world, with more complex structures" - Andreas Karch.

With gravity diminishing rapidly with distance, a seven-dimensional existence would not have planets with stable orbits around their sun.

"I am not precisely sure what a universe with such a short-range gravity would look like, mostly because it is always difficult to imagine how life would develop under completely different circumstances. But in any case, planetary systems as we know them wouldn't form. The possibility of stable orbits is what makes the three-dimensional world more interesting" - Andreas Karch.

Karch and Randall detail their work in the October edition of Physical Review Letters, published by the American Physical Society. The research was supported by grants from the U.S. Department of Energy and the National Science Foundation.
Karch said they hope the work will spark extensive scientific exploration of many other questions involving string theory, extra dimensions and the evolution of the cosmos.




Posts: 131433

The universe has more dimensions than the four we perceive (three spatial dimensions plus time).
If there were 10 dimensions the equations linking gravity and atomic forces into a "theory of everything" would work out just fine. For that reason, extradimensional physics is very attractive.

We live in a 10-dimensional realm, with the other six dimensions compactified.
In a paper due to be published in Physical Review Letters, two physicists propose that the kind of universe we live in represents one of the most likely results from a "battle of the branes."

In cosmological parlance, a two-dimensional space, or membrane, is a "2-brane." A line is a 1-brane, a particle is a 0-brane, and we perceive space as a 3-brane.
Harvard's Lisa Randall and the University of Washington's Andreas Karch did a mathematical analysis of a scenario in which an expanding 10-dimensional space holds a variety of branes.

If two branes intersect, they are annihilated and their energy is dissipated.
"The net result of all this is that you reduce the number of branes" - Andreas Karch.

And because of the 10-dimensional geometry, some types of branes are more likely to survive than others.

This is more understandable if you bring it down to a 3-D level, kind of like a brane version of Edwin Abbott's "Flatland": Imagine you have an expanding container containing a whole bunch of spaced-out particles as well as widening membranes and lengthening lines. If two objects in the container intersect, they both disappear. The membranes will cancel each other out, unless they're precisely parallel. However, more of the lines will survive, and almost all of the particles will be unaffected.
When you bring it back up to 10-D (actually "nine-plus-one" D, since it is nine spatial dimensions plus time), the math indicates that the 3-branes like ours hold a special status.

"A 3-brane is the largest brane that doesn't get destroyed by its cousins" - Andreas Karch.

The findings are consistent with the idea that our whole universe is a 3-D region within a wider, flatter 10-D realm. Such regions "could be like sinkholes in which gravity is localized". And ultimately, that could help explain why gravity works the way it does.

The other class of dimensional space that shows a good survival rate is the 7-brane — which doesn't apply to our particular sinkhole in the cosmic roadway, but turns out to match the expectations of string theorists.

"Several versions of string theories require the existence of 3-D and 7-D branes; indeed, the particles that constitute matter — such as quarks and electrons — can be considered open strings with one end planted on a 3-D brane and the other end planted on a 7-D brane" - Phillip Schewe and Ben Stein of the American Institute of Physics.

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RE: String Theory

A team of astrophysicists claims to have identified evidence that space is six-dimensional.

Joseph Silk of the University of Oxford, UK, and his co-workers say that these extra spatial dimensions can be inferred from the perplexing behaviour of dark matter. This mysterious stuff cannot be seen, but its presence in galaxies is betrayed by the gravitational tug that it exerts on visible stars.

Silk and his colleagues looked at how dark matter behaves differently in small galaxies and large clusters of galaxies. In the smaller ones, dark matter seems to be attracted to itself quite strongly. But in the large galactic clusters this doesn't seem to be the case: strongly interacting dark matter should produce cores of dark material bigger than those that are actually there, as deduced from the way the cluster spins.

One explanation, they say, is that three extra dimensions, in addition to the three spatial ones to which we are accustomed, are altering the effects of gravity over very short distances of about a nanometre.

The team argues that such astronomical observations of dark matter provide the first potential evidence for extra dimensions. Others are supportive, but unconvinced. Lisa Randall, a Harvard physicist who has explored the possibility of extra spatial dimensions, says "Even if their idea works, which it probably does, it may be an overstatement to use these observations as evidence of extra dimensions."

Silk himself acknowledges that the proposal is "extremely speculative".

Physicists have suspected for years that 'hidden' dimensions exist, largely because they seem to be predicted by string theory, the current favourite for a theory of fundamental subatomic particles.

These extra dimensions are generally thought to be tiny: many billions of times smaller than atoms. This would make these dimensions very hard to detect, explaining why the Universe looks as if it has just three. Physicists such as Randall, however, have proposed that some extra dimensions might be relatively big, but inaccessible to us.

The extra dimensions that Silk and colleagues say they have identified are likewise 'big', at about a nanometre across. In other words, they say, the Universe is only about a nanometre wide in these three 'directions'.

They argue that the force of gravity does not obey Isaac Newton's famous laws over small distances, where these dimensions come into play. This has never been tested experimentally: no one has measured how gravity behaves over distances below about a hundredth of a millimetre.

This variation in gravity, says Silk, could be why dark matter behaves differently in different galactic environments.

According to one interpretation of the astronomical observations, dark matter, which is thought to account for 85% of all the mass in the Universe but not to be made from the known fundamental particles, seems to attract itself through some unknown force. And this attraction seems to be stronger in dwarf galaxies than in galactic clusters. This is very odd: it is rather as if apples were to fall faster from single trees than from trees in an orchard.

But the attraction isn't due to an unknown force, argue Silk and his colleagues, but to the effect of extra dimensions on gravity. And because dark matter particles are accelerated to higher speeds in massive galactic clusters than in dwarf galaxies, they spend less time close to each other, so the effects of these extra dimensions are felt less.

There are other ways of explaining the puzzling dark-matter distributions, admits Silk's colleague Ue-Li Pen of the University of Toronto in Canada. For example, one could assume that the rate at which stars explode, as supernovae, was quite different in the past.

"Personally, I think changing the supernovae rate is more conservative than changing the number of spatial dimensions" - Ue-Li Pen.
He thinks that invoking extra dimensions is such an exciting idea that it is worth investigating, "even if it is a long shot" - Ue-Li Pen.

The most popular versions of string theory suggest that there are as many as eight extra dimensions, not just three. But thankfully this needn't be a problem. There's no reason why, in addition to the three large extra dimensions predicted by Silk and colleagues, there might not be several other small ones too.

Bosonic string theories are 26-dimensional, while classic superstring and M-theories turn out to involve 10 or 11 dimensions. However new sting theories now have reduced that number to 6 dimensions using Penroses twister space.

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superstring theory is a theory of all fundamental physical interactions. While in principle able to explain all phenomena, it has in practice been very challenging to find any observable predictions. One generic prediction is the existence of extra dimensions in addition to our familiar 3-dimensional space. These extra dimensions had been thought to be extremely tiny (of order the Planck length about 10^-33cm), until in recent years the idea of large extra dimensions was proposed to address the “hierarchy problem”—a problem associated with the factor of the 10^17 huge difference between the Planck scale and the weak interaction scale.
In the Arkani-Hamed, Dimopoulos and Dvali (ADD) scenario, the electro-weak and Planck energies are the same, and the large extra dimensions explain the apparent discrepancy in strength when measured on macroscopic scales.
In the context of string cosmology, the fundamental scale would be about 1TeV, corresponding to 10^-17cm. There are 3+1 cosmological size dimensions, n large extra dimensions, and 6 - n fundamental scale dimensions.
The size R of these large extra dimensions depends on the number of large extra dimensions n. Gravity would start to deviate from Newton’s inverse square law at small distance scales r less than R. For n = 2, R is about 1mm.
This opens a new window for experimental tests of string theory and searches for extra dimensions, by precise measurements of the gravitational force at sub-mm scales.
Tremendous efforts have been made in the past few years in testing Newton’s inverse square law at small scales and searching for evidence of the large extra dimensions. Currently, the measurements are reaching micron scales and no deviation from Newton’s law has been found from about 1cm down to about 10^-3cm.
While string scenarios and extra dimensions have not yet been tested experimentally so far in the laboratory, recent astronomical observations of dark matter, on the other hand, may shed light on the issue. It should be noted that any connection of string scenarios to observable phenomena would be an exciting possibility deserving further investigation.
The existence of dark matter was first realized by Zwicky nearly 70 years ago, and it is now well established that dark matter constitutes the major matter component of the universe. Although its nature remains a mystery, most cosmologists and particle physicists believe that dark matter is likely to be a new species of elementary particle that is neutral, long-lived, cold (or nonrelativistic), and collisionless (i.e. dark matter particles have very little interactions with themselves as well as with ordinary matter). This “standard” picture of collisionless cold dark matter (CCDM) has gained great success in explaining the origin and evolution of cosmic structures on large scales.
However, the CCDM model is facing a potential challenge in recent years from observations on galactic and sub-galactic scales. Numerical simulations of the CCDM model predict that the density profiles of dark matter halos should exhibit a cuspy core in which the density rises sharply as the distance from the centre decreases. In contrast, observations of systems of dark matter, ranging from dwarf galaxies, low surface brightness galaxies [17, 18] to galaxies comparable in mass with the Milky Way, indicate that the central density profiles are probably much less cuspy than predicted. Clusters of galaxies also seem to reveal a near isothermal core, although there is considerable scatter.
The plausible discrepancies between theory and observations, although still vigorously debated, have stimulated many attempts to understand the nature of dark matter and to modify the CCDM model, among which one of the more popular schemes is the self-interacting cold dark matter model. As proposed by Spergel and Steinhardt, the above conflicts can be readily resolved if the cold dark matter particles are self-interacting with a large scattering cross section Oxx/md = 8 × 10^-(25-22)cm^2/GeV, where md is the mass of dark matter particles.

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