Fivebrane Structures

A new jewel recently hit the arxiv: arXiv:0805.0564v3. I'll comment more about this later.

Tachyonic Inflation

It is well known that inflation provides the most compelling solution to many long-standing problems of the big bang model (horizon, flatness, monopoles, etc.) The source of inflation is a scalar field (the inflaton field), which in light of modern theories, can be based on a Dirac-Borne-Infeld action rather than the conventional Klein-Gordon action, allowing the inflaton field to take the form of a tachyon field. A rolling tachyon field is usually associated with unstable D-brane configurations, where decay of such D-branes (via tachyon condensation) produces a pressureless gas with finite energy density resembling classical dust. In their April 6th paper (arXiv:0904.1032v1), del Campo et al. investigate such a tachyonic inflationary universe model in the context of intermediate inflation.

All Loop Finiteness of D=4 N=8 Supergravity?

Yesterday, a striking new paper arXiv:0903.4630v1 emerged on the arxiv by the well-known Renata Kallosh. Kallosh builds on the unitary cut and pure spinor methods which have established 3-loop superfiniteness and suggested the onset of divergences starting from the 9-loop order or even all loop finiteness, respectively. She specifically investigates N=8 supergravity on the light cone and makes her prediction for the actual computations using Feynman light-cone supergraphs. It is noted that light-cone superfield amplitudes have a non-local structure in the transverse directions leading to the increase of the delay of divergences with the increasing number of legs in the loop-amplitudes. The delay of divergences leads to the all loop finiteness prediction for N =8 supergravity.

Dyonic Giant Magnons on CP^3

Kalousios et al. posted a paper (arXiv:0902.3179) today investigating a new dyonic magnon solution on CP^3. Fans of twistor theory will recall that CP^3 is actually projective twistor space. However, here the context is N=6 superconformal Chern-Simons theory with SU(N)xSU(N) symmetry, which in the large N,k limit is dual to type IIA string theory on AdS_4 x CP^3. The present paper generalizes Maldacena's giant magnon construction where it is a particular open string configuration on an R x S^2 subset of AdS_5 x S^5. This is done by noticing the equations of motion for a string on R x CP^3 are classically integrable, and supplementing them with the Virasoro constraints. Various solutions are given, such as a pointlike string moving along the equator of an S^2 in CP^3 and another on a CP^1 which is a rotation of the ordinary Hofman-Maldacena giant magnon.

Looking back at twistor string theory, Witten wanted to consider the open string B-model on CP^3, but was hindered by the fact that CP^3 is not a Calabi-Yau manifold, hence giving an anomalous R-symmetry. For this reason he considered supermanifolds of the form CP^{3|N}, which is Calabi-Yau if and only if N=4.

In the present case, type IIA string theory has no such restrictions, so it might be that Kalousios et al. have laid the foundation for a new twistor string theory on ordinary CP^3, where the perturbative expansion of the superconformal N=6 theory could be related to a D-instanton expansion of the dual string theory on AdS_4 x CP^3.

Ed Witten - Branes, Instantons, And Taub-NUT Spaces

For those wondering what Ed Witten has been up to, you'll likely enjoy the new arxiv paper arXiv:0902.0948.

Blackfolds, Twistor Strings and Cosmic Strings

Today's (Wed. Feb 4th) new arxiv HEP papers included some noteworthy topics, such as black holes with novel horizon geometries, N=8 twistor supergravity and cosmic superstrings. The abstracts are as follows (feel free to comment):


Blackfolds

We argue that the main feature behind novel properties of higher-dimensional black holes, compared to four-dimensional ones, is that their horizons can have two characteristic lengths of very different size. We develop a long-distance worldvolume effective theory that captures the black hole dynamics at scales much larger than the short scale. In this limit the black hole is regarded as a blackfold: a black brane (possibly boosted locally) whose worldvolume spans a curved submanifold of the spacetime. This approach reveals black objects with novel horizon geometries and topologies more complex than the black ring, but more generally it provides a new organizing framework for the dynamics of higher-dimensional black holes.


New Twistor String Theories Revisited

A gauged version of Berkovits twistor string theory featuring the particle content of N=8 supergravity was suggested by Abou-Zeid, Hull and Mason. The equations of motion for a particular multiplet in the modified theory are examined on the level of basic twistor fields and thereby shown to imply the vanishing of the negative helicity graviton on-shell. Additionally, the restrictions emerging from the equation of motion for the new gauge field \bar{B} reveal the chiral nature of interactions in theories constructed in this manner. Moreover, a particular amplitude in Berkovits open string theory is shown to be in agreement with the corresponding result in Einstein gravity.


Cosmic Strings and Cosmic Superstrings

In these lectures, I review the current status of cosmic strings and cosmic superstrings. I first discuss topological defects in the context of Grand Unified Theories, focusing in particular in cosmic strings arising as gauge theory solitons. I discuss the reconciliation between cosmic strings and cosmological inflation, I review cosmic string dynamics, cosmic string thermodynamics and cosmic string gravity, which leads to a number of interesting observational signatures. I then proceed with the notion of cosmic superstrings arising at the end of brane inflation, within the context of brane-world cosmological models inspired from string theory. I discuss the differences between cosmic superstrings and their solitonic analogues, I review our current understanding about the evolution of cosmic superstring networks, and I then briefly describe the variety of observational consequences, which may help us to get an insight into the stringy description of our Universe.

Geometry and Topology for Theoretical Physicists

Happy 2009 to all U-duality readers! Today was the first day of class for the Winter term of Caltech's math sequence for theoretical physicists. This math sequence consists of three classes covering the modern mathematics a young string theorist should know. For the Fall term the main textbook was Nakahara's Geometry Topology and Physics, 2nd Edition.


This term, the main text is Morita's Geometry of Differential forms.


Today's lecture jumped right in to Čech cohomology, which can be thought of as a type of 'unifying' cohomology in the sense that it is equivalent to de Rham, simplicial and singular cohomology for well-behaved choices of topological space X. de Rham cohomology arises most frequently for physicists, but since it involves solving differential equations, it is not always the most pleasant for computations. Simplicial cohomology is probably the most computation-friendly cohomology, so Čech cohomology can be thought of as a kind of 'bridge' for translating physics problems from de Rham to a simpler (pun intended) simplicial cohomology setting. The proof of the equivalence of de Rham and Čech cohomology is most enlightening in this respect and involves forming a square of commutative (or anticommutative if you wish) mappings between vector spaces of bi-degree forms e.g. (p,q where total degree is p+q), with the vertical direction increasing via the de Rham coboundary operator, d, and horizontal direction via the Čech coboundary operator, delta. One can then define a new coboundary operator D=d+delta and show it satisfies D^2=0. I'll sketch the proof in more depth in another post, for those interested. I'll also refer the interested reader to another classic text on cohomology, namely Bott and Tu's, Differential Forms in Algebraic Topology.

Holographic Baryons

Today I came across a nice talk by Shigeki Sugimoto entitled Properties of Baryons in Holographic QCD. Using Gauge/String duality, the talk explores the implications of modeling baryons as wrapped D4-branes or equivalently, instantons on D8-branes. The talk is largely based on the pre-print arXiv:0806.3122.

Status of Superstring and M-Theory

A new paper, entitled "Status of Superstring and M-Theory" has recently appeared on the arxiv, written by none other than one of the fathers of string theory, John H. Schwarz. The paper is very readable and covers the history of string theory, as well as the dualities that imply the existence of an 11-dimensional M-theory underlying the various string theories and supergravity. Other topics include Flux Compactifications, Warped Compactification, Brane Worlds, String Cosmology, M-theory on G2-Manifolds and F-theory Local Models.

F-theory phenomenology (pioneered by Vafa et al) is one of the promising approaches toward connecting stringy physics to the standard model. John explains that "[t]he new proposal, which has given the subject a new lease on life, is to focus on models in which one can define a limit in which gravity is decoupled. The criterion is that it should be possible to make the dimensions transverse to the 4-cycles wrapped by the 7-branes arbitrarily large. Equivalently, it should be possible to contract the 4-cycles to points while holding the six-dimensional volume fixed. Such contractible 4-cycles must be positive curvature Kahler manifolds. These are fully classified and are given by manifolds called del Pezzo manifolds (or del Pezzo surfaces), which are denoted dP_n. The integer n takes the values 0 ≤ n ≤ 8.9 The del Pezzos have a close relationship with the exceptional Lie algebras E_n. The basic idea is that they contain 2-cycles whose intersections are characterized by the E_n Dynkin diagram. By this type of F-theory construction, one can construct an SU(5) or SO(10) SUSY-GUT model. Constructions that involve 7-branes of various types are much more subtle – and also more interesting than ones that only involve D7-branes. D7-branes are mutually local. A stack of N of them gives U(N) gauge symmetry. Matter fields at intersections (due to stretched open strings) are bifundamental. However, different kinds of 7-branes are mutually nonlocal. As a result, there are stacks (corresponding to the ADE classification of singularities) that can give U(N), SO(2N) or even E_N gauge symmetry."

So we see that F-theory, along with the use of del Pezzo surfaces enhances the usual symmetry groups one recoveres from stacks of branes. (Such symmetry groups describe the freedom the string has in choosing which brane in the stack to end on.) Phenomenologically, a configuration with E_6 symmetry would be particularly interesting.

Meteor Lights up Western Canada Skies

The meteor that streaked through the darkening skies over Alberta and Saskatchewan at about 5:30 p.m. Calgary time, likely weighed between one and 10 tons and shone brightly enough to be seen over an area 700 km (435 miles) wide.

Alan Hildebrand (a planetary scientist at the University of Calgary) said the meteor may have broken into hundreds of smaller meteorites that likely landed in central Saskatchewan near that province's border with Alberta.

Black Holes, Qubits and Octonions

In perhaps what is the most up-to-date paper on the quantum computation-extremal black hole-division algebra correspondence, Duff et al review all previous conjectures and suggest an alternative interpretation for which the black hole charges are identified with components of reduced density matrices. The density matrix formulation of quantum mechanics is discussed and compared to the Jordan algebraic quantum mechanics, showing the two approaches are essentially equivalent. The exceptional Jordan algebra forbids a traditional Hilbert space formulation, hence the corresponding D=5 and D=4 supergravities remain somewhat of a mystery.

LHC First beam a success

Today, the first beam in the Large Hadron Collider (LHC) at CERN was successfully steered around the full 27 kilometres of the world’s most powerful particle accelerator this morning.

“It’s a fantastic moment,” said LHC project leader Lyn Evans, “we can now look forward to a new era of understanding about the origins and evolution of the universe.”

The LHC is capable of proping the energy region around 1 TeV by colliding together 7 TeV proton beams, which should reveal new physics that will address the following questions:

1) Is there a Higgs particle?

2) Is there supersymmetry (SUSY)?

3) What is dark matter?

4) Where has all the antimatter gone?

5) Why are there only six quarks?


More at press.web.cern.ch and scitech.

The LHC countdown is on!

LHC First Beam on
10 September 2008

On 10 September scientists at CERN in Geneva, Switzerland will attempt for the first time to circulate a beam in the Large Hadron Collider. The LHC is the world’s most powerful particle accelerator, and will produce beams seven times more energetic, and around 30 times more intense than any previous machine when it reaches design performance.

The first injection of a beam is scheduled for 9:30 CET (+9 hours from Pacific Standard Time), and will be preceded by a planning meeting that will be relayed to the Globe from 9:00.

The Live webcast can be found here.

Edward Witten for VP?

After years of work on a candidate theory of quantum gravity, there's word that the father of M-theory may well be Obama's future VP candidate. After all, it is a well-known fact that Witten was actively involved in former democratic presidential candidate George McGovern's campaign. Moreover, he has published articles in The New Republic and The Nation and has donated more than $50,000 in democratic campaign contributions from 2000-2008. Witten's desire for change clearly rivals that of any other VP hopeful. Not to mention, he's pretty darned smart. So, look out Chet Edwards and Joe Biden, your VP hopes are hanging by a string. ;)

NASA Confirms Water on Mars

NASA scientists said on Thursday they had definitive proof that water exists on Mars after tests on ice found on the planet in June by the Phoenix Mars Lander.

Until now, the evidence for ice has been circumstantial. That was based on photos Phoenix took of a hard splotchy area near its landing site and changes it saw in a trench.

The robot heated up ice in one of its instruments earlier this week. Scientists say the chemical test confirms the presence of ice near the Martian north pole.

Read more at NASA.gov

msnbc

Tree Quantum Field Theory

Gurau et. al. posted a new paper today on arxiv (arxiv.org/0807.4122 [hep-th]), proposing a new approach to quantum field theory: marked trees.

We propose a new formalism for quantum field theory which is neither based on functional integrals, nor on Feynman graphs, but on marked trees. This formalism is constructive, i.e. it computes correlation functions through convergent rather than divergent expansions. It applies both to Fermionic and Bosonic theories. It is compatible with the renormalization group, and it allows to define non-perturbatively {\it differential} renormalization group equations. It accommodates any general stable polynomial Lagrangian. It can equally well treat noncommutative models or matrix models such as the Grosse-Wulkenhaar model. Perhaps most importantly it removes the space-time background from its central place in QFT, paving the way for a nonperturbative definition of field theory in noninteger dimension.

SU(5) Grand Unified Model from Non-perturbative String Theory

Mirjam Cvetic and Timo Weigand posted a new paper today (arXiv:0807.3953v2 [hep-th]) on symmetry breaking and SU(5) GUT model building from supergravity/non-perturbative string theory. The abstract is as follows:

We propose a robust supergravity model of dynamical supersymmetry breaking and gauge mediation, and a natural embedding in non-perturbative string theory with D-branes. A chiral field (and its mirror) charged under "anomalous" U(1)'s acts as a Polonyi field whose hierarchical Polonyi-term can be generated by string instantons. Further quartic superpotential terms arise naturally as a tree-level decoupling effect of massive string states. A robust supersymmetry breaking minimum allows for gauge mediation with soft masses at the TeV scale, which we realise for a globally consistent SU(5) GUT model of Type I string theory, with a D1-instanton inducing the Polonyi term.

Former Google Engineers Launch Cuil Search Engine

A former Google employee and her husband launched a new search engine Monday called Cuil (pronounced "cool", http://www.cuil.com), aiming to topple Google by indexing more Web pages than the search giant.

Cuil, of Menlo Park, California, is led by Anna Patterson , a former leader of Google's search index and her husband, Tom Costello, who researched and developed search engines at Stanford University and IBM. The two, president and CEO, respectively, met at Stanford.

Russell Power, the third cofounder of the group, also worked at Google on search indexing, Web rankings, and spam detection. He works as vice president of engineering at Cuil.

The company, which shaved an 'L' off its name to become Cuil, said it has indexed 120 billion Web pages and can provide results organized by ideas with complete privacy for users.

Google on Friday said it had discovered 1 trillion unique Web pages on the Internet, but did not give an updated number on how many of those pages it has indexed.

Cuil said its search engine goes beyond traditional approaches by analyzing the context of each page and the concepts behind each query so it can provide better rankings by content rather than popularity. Cuil then organizes similar results into groups and sorts them by category. It also offers tabs to clarify subjects, as well as suggestions on how to refine searches.

Cuil isn't the first Google rival to launch this year. Wikia Search, a highly anticipated search engine from Wikipedia founder Jimmy Wales, made its official debut in January . Wikia Search hopes to provide better search results by allowing a community of users to index pages by using their Web page rankings and other suggestions, as well as its own indexing of the Web.

InfoWorld

The Dark Knight's Suit

In "The Dark Knight," Batman complains that he needs a better suit - and that's a concern for the real-life knights in the U.S. military as well. The Pentagon would love to have Wayne Enterprises' secret for lighter, more flexible body armor. Nanocomp Technologies based in New Hampshire, is among several companies working on carbon-nanotube composites for military applications.

"We're really focused on trying to create layers of protection that would improve things for our troops," Peter Antoinette, Nanocomp's president and chief executive officer, told me. "It would take a number of years before you could order up a suit, and then a billionaire would have to pay seven figures for a suit that would work the way they do in the movies."

As an initial step, Nanocomp is working on nanotubes for next-generation wiring in satellites and aircraft. Carbon nanotubes are highly conductive and could replace copper wire in settings where reducing weight is crucial. "We're less than one-tenth the weight of copper, so if you can take 1,000 pounds off these satellites or aircraft, you'd be saving a huge amount of money," Antoinette said.

Commercialization of nanotube wiring could begin as early as next year, Antoinette said. He added that Nanocomp's materials are already undergoing military testing, and body-armor applications could start emerging in 2010 or so.

Kakalios agreed that nanotubes are a technology to watch: "Compared to steel cables, it's about 100 times stronger. Whether you can make this in large enough quantities, in long enough length scales ... that work is still in progress."

CosmicLog

Northern Lights Explained

NASA released findings Thursday that indicate magnetic explosions about one-third of the way to the moon cause the northern lights, or aurora borealis, to dance across the sky in spectacular shapes and colors.

A fleet of five small satellites, called Themis, observed the beginning of a geomagnetic storm in February, while ground observatories recorded the brightening of the northern lights.

A team led by UCLA scientist Vassilis Angelopoulos confirmed that the observed storm about 80,000 miles from Earth was triggered by a phenomenon known as magnetic reconnection. Every so often, the Earth's magnetic field lines are stretched like rubber bands by solar energy, snap, are thrown back to Earth and reconnect, in effect creating a short circuit.

This stored-up energy powers the northern and southern lights, Angelopoulos said.

LA Times