Publications (155)533.17 Total impact

Article: Helical Inflation and Cosmic Strings
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ABSTRACT: Recent BICEP2 detection of lowmultipole Bmode polarization anisotropy in the cosmic microwave background radiation supports the inflationary universe scenario and suggests a large inflation field range. The latter can be achieved with axion fields in the framework of string theory. We present such a helical model which reduces to the (quadratic) chaotic inflation model in the limit. Its slightly smaller tensor/scalar ratio $r$ provides a signature of the periodic nature of axion potentials. As axions are intimately related to strings/vortices and strings are ubiquitous in string theory, we explore the possibility that cosmic strings may be contributing to the Bmode polarization anisotropy observed.04/2014; 
Article: Helical Inflation and Cosmic Strings
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ABSTRACT: Recent BICEP2 detection of lowmultipole Bmode polarization anisotropy in the cosmic microwave background radiation supports the inflationary universe scenario and suggests a large inflaton field range. The latter feature can be achieved with axion fields in the framework of string theory. We present such a helical model which naturally becomes a model with a single cosine potential, and which in turn reduces to the (quadratic) chaotic inflation model in the superPlanckian limit. The slightly smaller tensor/scalar ratio $r$ of models of this type provides a signature of the periodic nature of an axion potential. We present a simple way to quantify this distinctive feature. As axions are intimately related to strings/vortices and strings are ubiquitous in string theory, we explore the possibility that cosmic strings may be contributing to the Bmode polarization anisotropy observed.03/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We study a racetrack model in the presence of the leading alpha'correction in flux compactification in Type IIB string theory, for the purpose of getting conceivable deSitter vacua in the large compactified volume approximation. Unlike the K\"ahler Uplift model studied previously, the alpha'correction is more controllable for the metastable deSitter vacua in the racetrack case since the constraint on the compactified volume size is very much relaxed. We find that the vacuum energy density \Lambda for deSitter vacua approaches zero exponentially as the volume grows. We also analyze properties of the probability distribution of \Lambda in this class of models. As in other cases studied earlier, the probability distribution again peaks sharply at \Lambda=0. We also study the Racetrack K\"ahler Uplift model in the SwissCheese type model.Journal of High Energy Physics 05/2013; 2013(7). · 5.62 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the probability distribution P(\Lambda) of the cosmological constant \Lambda in a specific set of KKLT type models of supersymmetric IIB vacua. We show that, as we sweep through the quantized flux values in this flux compactification, P(\Lambda) behaves divergent at \Lambda =0^ and the median magnitude of \Lambda drops exponentially as the number of complex structure moduli h^{2,1} increases. Also, owing to the hierarchical and approximate noscale structure, the probability of having a positive Hessian (mass squared matrix) approaches unity as h^{2,1} increases.Physics Letters B 11/2012; 723(4). · 4.57 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Based on the properties of probability distributions of functions of random variables, we proposed earlier a simple stringy mechanism that prefers the metastable vacua with a small cosmological constant \Lambda. As an illustration of this approach, we study in this paper particularly simple but nontrivial models of the K\"ahler uplift in the large volume flux compactification scenario in Type IIB string theory, where all parameters introduced in the model are treated either as fixed constants motivated by physics, or as random variables with some given uniform probability distributions. We determine the value w_0 of the superpotential W_0 at the supersymmetric minima, and find that the resulting probability distribution P(w_0) peaks at w_0=0; furthermore, this peaking behavior strengthens as the number of complex structure moduli increases. The resulting probability distribution P(\Lambda) for metastable vacua also peaks as \Lambda > 0, for both positive and negative \Lambda. This peaking/divergent behavior of P(\Lambda) strengthens as the number of moduli increases. In some scenarios for \Lambda > 0, the likely value of \Lambda decreases exponentially as the number of moduli increases. The light cosmological moduli issue accompanying a very small \Lambda is also mentioned.Journal of Cosmology and Astroparticle Physics 09/2012; 2013(02). · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Based on the probability distributions of products of random variables, we propose a simple stringy mechanism that prefers the metastable vacua with a small cosmological constant. We state some relevant properties of the probability distributions of functions of random variables. We then illustrate the mechanism within the flux compactification models in Type IIB string theory. As a result of the stringy dynamics, we argue that the generic probability distribution for the metastable vacua typically peaks with a divergent behavior at the zero value of the cosmological constant. However, its suppression in the single modulus model studied here is modest.Journal of Cosmology and Astroparticle Physics 04/2012; 2012(08). · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study Coleman–de Luccia tunneling in some detail. We show that, for a single scalar field potential with a true and a false vacuum, there are four types of tunneling, depending on the properties of the potential. A general tunneling process involves a combination of thermal (Gibbons–Hawking temperature) fluctuation part way up the barrier followed by quantum tunneling. The thinwall approximation is a special limit of the case (of only quantum tunneling) where inside the nucleation bubble is the true vacuum while the outside reaches the false vacuum. Hawking–Moss tunneling is the (only thermal fluctuation) limit of the case where the inside of the bubble does not reach the true vacuum at the moment of its creation, and the outside is cut off by the de Sitter horizon before it reaches the false vacuum. A typical tunneling process is a combination of thermal and quantum tunnelings. We estimate the tunneling rate for this case and find that the corrections to the Hawking–Moss formula can be large. In all cases, we see that the Euclidean action of the bounce decreases rapidly as the vacuum energy density increases, signaling that the tunneling is not exponentially suppressed. This phenomenon may be interpreted as a finite temperature effect due to the Gibbons–Hawking temperature of the de Sitter space. As an application, we discuss the implication of this tunneling property to the cosmic landscape.International Journal of Modern Physics A 01/2012; 25(05). · 1.13 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The spin structure construction of fourdimensional fermionic string models of the heterotic type is extended by considering a generalized form of the worldsheet supercurrent. The rules for model building are given and illustrated with two sets of examples: the original spin structure construction and the Z3 asymmetric orbifold.International Journal of Modern Physics A 01/2012; 03(01). · 1.13 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We review the construction and classification of threefamily grand unified models within the framework of asymmetric orbifolds in perturbative heterotic superstring. We give a detailed survey of all such models which is organized to aid analysis of their phenomenological properties. We compute tree level superpotentials for these models. These superpotentials are used to analyze the issues of proton stability (doublet–triplet splitting and Rparityviolating terms) and Yukawa mass matrices. To have agreement with phenomenological data all these models seem to require a certain degree of finetuning. We also analyze the possible patterns of supersymmetry breaking in these models. We find that the supersymmetry breaking scale comes out either too high to explain the electroweak hierarchy problem, or below the electroweak scale unless some degree of finetuning is involved. Thus, none of the models at hand seem to be phenomenologically flawless.International Journal of Modern Physics A 01/2012; 13(15). · 1.13 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The search for classically stable Type IIA deSitter vacua typically starts with an ansatz that gives AntideSitter supersymmetric vacua and then raises the cosmological constant by modifying the compactification. As one raises the cosmological constant, the couplings typically destabilize the classically stable vacuum, so the probability that this approach will lead to a classically stable deSitter vacuum is Gaussianly suppressed. This suggests that classically stable deSitter vacua in string theory (at least in the Type IIA region), especially those with relatively high cosmological constants, are very rare. The probability that a typical deSitter extremum is classically stable (i.e., tachyonfree) is argued to be Gaussianly suppressed as a function of the number of moduli.Journal of High Energy Physics 12/2011; 2012(4). · 5.62 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We argue that classical transitions can be the key to explaining the long standing puzzle of the fast AB phase transition observed in superfluid Helium 3 while standard theory expects it to be unobservably slow. Collisions between domain walls are shown to be capable of reaching phases inaccessible through homogenous nucleation on the measured timescales. We demonstrate qualitative agreements with prior observations and provide a definite, distinctive prediction that could be verified through future experiments or, perhaps, a specific analysis of existing data.10/2011;  [Show abstract] [Hide abstract]
ABSTRACT: The A phase and the B phase of superfluid He3 are well studied, both theoretically and experimentally. The decay time scale of the A phase to the B phase of a typical supercooled superfluid 3HeA sample is calculated to be 1020,000 years or longer, yet the actual firstorder phase transition of supercooled A phase happens very rapidly (in seconds to minutes) in the laboratory. We propose that this very fast phase transition puzzle can be explained by the resonant tunneling effect in field theory, which generically happens since the degeneracies of both the A and the B phases are lifted by many small interaction effects. This explanation predicts the existence of peaks in the A→B transition rate for certain values of the temperature, pressure, and magnetic field. Away from these peaks, the transition simply will not happen.Physical review. B, Condensed matter 01/2011; 84(18). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We comment on Weinberg’s interesting analysis of asymptotically safe inflation [ S. Weinberg Phys. Rev. D 81 083535 (2010)]. We find that even if the gravity theory exhibits an ultraviolet fixed point, the energy scale during inflation is way too low to drive the theory close to the fixed point value. We choose the specific renormalization group flow away from the fixed point towards the infrared region that reproduces the Newton’s constant and today’s cosmological constant. We follow this renormalization group flow path to scales below the Planck scale to study the stability of the inflationary scenario. Again, we find that some finetuning is necessary to get enough e folds of inflation in the asymptotically safe inflationary scenario.Physical review D: Particles and fields 12/2010; 82(12).  [Show abstract] [Hide abstract]
ABSTRACT: We propose a new way to implement an inflationary prior to a cosmological dataset that incorporates the inflationary observables at arbitrary order. This approach employs an exponential form for the Hubble parameter $H(\phi)$ without taking the slowroll approximation. At lowest nontrivial order, this $H(\phi)$ has the unique property that it is the solution to the brachistochrone problem for inflation.Journal of Cosmology and Astroparticle Physics 12/2010; 4(04). · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Recently, BjerrumBohr, Damgaard, Feng, and Sondergaard derived a set of new interesting quadratic identities of the YangMills (YM) tree scattering amplitudes, besides BernCarrascoJohansson (BCJ) identities. Here we comment that these quadratic identities of YM amplitudes actually follow directly from the KLT (KawaiLewellenTye) relation for gravitondilatonaxion scattering amplitudes (in fourdimensional spacetime). This clarifies their physical origin and also provides a simpler version of the new identities. We also comment that the recently discovered BCJ identities of YM helicity amplitudes, at least for the maximal helicityviolating case, can be verified by using (repeatedly) the Schouten identity. We also point out additional quadratic identities that can be written down from the KLT relations.Physical review D: Particles and fields 10/2010; 82(8).  [Show abstract] [Hide abstract]
ABSTRACT: Recently, Bern, Carrasco and Johansson conjectured dual identities inside the gluon tree scattering amplitudes. In this paper, we use the properties of the heterotic string and open string tree scattering amplitudes to refine and derive these dual identities. These identities can be carried over to loop amplitudes using the unitarity method. Furthermore, given the $M$gluon (as well as gluongluino) tree amplitudes, $M$graviton (as well as gravitongravitino) tree scattering amplitudes can be written down immediately, avoiding the derivation of Feynman rules and the evaluation of Feynman diagrams for graviton scattering amplitudes.Journal of High Energy Physics 03/2010; · 5.62 Impact Factor 
Article: A meandering inflaton
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ABSTRACT: If the cosmological inflationary scenario took place in the cosmic landscape in string theory, the inflaton, the scalar mode responsible for inflation, would have meandered in a complicated multidimensional potential. We show that this meandering property naturally leads to many efolds of inflation, a necessary condition for a successful inflationary scenario. This behavior also leads to fluctuations in the primordial power spectrum of the cosmic microwave background radiation, which may be detected in a near future cosmic variance limited experiment like PLANCK.Physics Letters B 01/2010; · 4.57 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The resonant tunneling phenomenon is well understood in quantum mechanics. We argue why a similar phenomenon must be present in quantum field theory. We then use the functional Schr\"odinger method to show how resonant tunneling through multiple barriers takes place in quantum field theory with a single scalar field. We also show how this phenomenon in scalar quantum field theory can lead to an exponential enhancement of the singlebarrier tunneling rate. Our analysis is carried out in the thinwall approximation. Comment: 25 pages, 11 figures10/2009;  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by the possibility of inflation in the cosmic landscape, which may be approximated by a complicated potential, we study the density perturbations in multifield inflation with a random potential. The random potential causes the inflaton to undergo a Brownian motion with a drift in the Ddimensional field space. To quantify such an effect, we employ a stochastic approach to evaluate the twopoint and threepoint functions of primordial perturbations. We find that in the weakly random scenario the resulting power spectrum resembles that of the single field slowroll case, with up to 2% more red tilt. The strongly random scenario, leads to rich phenomenologies, such as primordial fluctuations in the power spectrum on all angular scales. Such features may already be hiding in the error bars of observed CMB TT (as well as TE and EE) power spectrum and can be detected or falsified with more data coming in the future. The tensor power spectrum itself is free of fluctuations and the tensor to scalar ratio is enhanced. In addition a large negative running of the power spectral index is possible. NonGaussianity is generically suppressed by the growth of adiabatic perturbations on superhorizon scales, but can possibly be enhanced by resonant effects or arise from the entropic perturbations during the onset of (p)reheating. The formalism developed in this paper can be applied to a wide class of multifield inflation models including, e.g. the Nflation scenario.Journal of Cosmology and Astroparticle Physics 01/2009; · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Cosmic string networks generate cosmological perturbations actively throughout the history of the universe. Thus, the string sourced anisotropy of the cosmic microwave background is not affected by Silk damping as much as the anisotropy seeded by inflation. The spectrum of perturbations generated by strings does not match the observed CMB spectrum on large angular scales (l<1000) and is bounded to contribute no more than 10% of the total power on those scales. However, when this bound is marginally saturated, the anisotropy created by cosmic strings on small angular scales l>2000 will dominate over that created by the primary inflationary perturbations. This range of angular scales in the CMB is presently being measured by a number of experiments; their results will test this prediction of cosmic string networks soon. Comment: 5 pages, 2 figures; improved fitting formula at 1000<l<3500, typos fixedJournal of Cosmology and Astroparticle Physics 04/2008; · 6.04 Impact Factor
Publication Stats
8k  Citations  
533.17  Total Impact Points  
Top Journals
Institutions

2011–2013

The Hong Kong University of Science and Technology
 Institute for Advanced Study (IAS)
Chiulung, Kowloon City, Hong Kong


1979–2012

Cornell University
 • Laboratory for Elementary Particle Physics
 • Center for Radiophysics and Space Research (CRSR)
Ithaca, New York, United States


2008

Perimeter Institute for Theoretical Physics
Waterloo, Ontario, Canada


2006

Columbia University
 Department of Physics
New York City, New York, United States


1996–1999

Harvard University
Cambridge, Massachusetts, United States


1992

McGill University
 Department of Physics
Montréal, Quebec, Canada


1978–1985

Fermi National Accelerator Laboratory (Fermilab)
Batavia, Illinois, United States


1976–1977

Stanford University
 Stanford Linear Accelerator Center
Palo Alto, CA, United States


1971–1975

Massachusetts Institute of Technology
 Laboratory for Nuclear Science
Cambridge, MA, United States
