Heterotic Cosmic Strings

We show that all three conditions for the cosmological relevance of heterotic cosmic strings, the right tension, stability and a production mechanism at the end of inflation, can be met in the strongly coupled M-theory regime. Whereas cosmic strings generated from weakly coupled heterotic strings have the well known problems posed by Witten in 1985, we show that strings arising from M5-branes wrapped around 4-cycles (divisors) of a Calabi-Yau in heterotic M-theory compactifications, solve these problems in an elegant fashion.Comment: 25 pages, v2: section and references adde

Signal peptide peptidases and gamma-secretase: Cousins of the same protease family?

Signal peptide peptidase (SPIP) is an unusual aspartyl protease, which mediates clearance of signal peptides by proteolysis within the endoplasmic reticulum (ER). Like presenilins, which provide the proteolytically active subunit of the,gamma-secretase complex, SPP contains a conserved GxGD motif in its C-terminal domain which is critical for its activity. While SPIP is known to be an aspartyl protease of the GxGD type, several presenilin homologues/SPP-like proteins (PSHs/ SPPL) of unknown function have been identified by database searches. In contrast to SPP and SPPL3, which are both restricted to the endoplasmic reticulum, SPPL2b is targeted through the secretory pathway to endosomes/lysosomes. As suggested by the differential subcellular localization of SPPL2b and SPPL3 distinct phenotypes were found upon antisense gripNA-mediated knockdown in zebrafish. spp and sppl3 knockdowns in zebrafish result in cell death within the central nervous system, whereas reduction of sppl2b expression causes erythrocyte accumulation in an enlarged caudal vein. Moreover, expression of D/A mutants of the putative C-terminal active sites of spp, sppl2, and spp13 produced phenocopies of the respective knockdown phenotypes. These data suggest that all investigated PSHs/SPPLs are members of the novel family of GxGD aspartyl proteases. More recently, it was shown that SPPL2b utilizes multiple intramembrane cleavages to liberate the TNF(x intracellular domain into the cytosol and to release the C-terminal counterpart into the lumen. These findings suggest common principles of intramembrane proteolysis by GxGD type aspartyl proteases. In this article,we will review the similarities of SPPs and gamma-secretase based on recent findings by us and others

Third order perturbations of a zero-pressure cosmological medium: Pure general relativistic nonlinear effects

We consider a general relativistic zero-pressure irrotational cosmological medium perturbed to the third order. We assume a flat Friedmann background but include the cosmological constant. We ignore the rotational perturbation which decays in expanding phase. In our previous studies we discovered that, to the second-order perturbation, except for the gravitational wave contributions, the relativistic equations coincide exactly with the previously known Newtonian ones. Since the Newtonian second-order equations are fully nonlinear, any nonvanishing third and higher order terms in the relativistic analyses are supposed to be pure relativistic corrections. In this work we derive such correction terms appearing in the third order. Continuing our success in the second-order perturbations we take the comoving gauge. We discover that the third-order correction terms are of $\phi_v$-order higher than the second-order terms where $\phi_v$ is a gauge-invariant combination related to the three-space curvature perturbation in the comoving gauge; compared with the Newtonian potential we have $\delta \Phi \sim {3 \over 5} \phi_v$ to the linear order. Therefore, the pure general relativistic effects are of $varphi_v$-order higher than the Newtonian ones. The corrections terms are independent of the horizon scale and depend only on the linear order gravitational potential perturbation strength. From the temperature anisotropy of cosmic microwave background we have ${\delta T \over T} \sim {1 \over 3} \delta \Phi \sim {1 \over 5} \phi_v \sim 10^{-5}$. Therefore, our present result reinforces our previous important practical implication that near current era one can use the large-scale Newtonian numerical simulation more reliably even as the simulation scale approaches near the horizon.Comment: 9 pages, no figur

Second-order perturbations of cosmological fluids: Relativistic effects of pressure, multi-component, curvature, and rotation

We present general relativistic correction terms appearing in Newton's gravity to the second-order perturbations of cosmological fluids. In our previous work we have shown that to the second-order perturbations, the density and velocity perturbation equations of general relativistic zero-pressure, irrotational, single-component fluid in a flat background coincide exactly with the ones known in Newton's theory. Here, we present the general relativistic second-order correction terms arising due to (i) pressure, (ii) multi-component, (iii) background curvature, and (iv) rotation. In case of multi-component zero-pressure, irrotational fluids under the flat background, we effectively do not have relativistic correction terms, thus the relativistic result again coincides with the Newtonian ones. In the other three cases we generally have pure general relativistic correction terms. In case of pressure, the relativistic corrections appear even in the level of background and linear perturbation equations. In the presence of background curvature, or rotation, pure relativistic correction terms directly appear in the Newtonian equations of motion of density and velocity perturbations to the second order. In the small-scale limit (far inside the horizon), relativistic equations including the rotation coincide with the ones in Newton's gravity.Comment: 41 pages, no figur

Classification and Moduli Kahler Potentials of G_2 Manifolds

Compact manifolds of G_2 holonomy may be constructed by dividing a seven-torus by some discrete symmetry group and then blowing up the singularities of the resulting orbifold. We classify possible group elements that may be used in this construction and use this classification to find a set of possible orbifold groups. We then derive the moduli Kahler potential for M-theory on the resulting class of G_2 manifolds with blown up co-dimension four singularities.Comment: 30 pages, Latex, references adde

Second order perturbations of a zero-pressure cosmological medium: Proofs of the relativistic-Newtonian correspondence

The dynamic world model and its linear perturbations were first studied in Einstein's gravity. In the system without pressure the relativistic equations coincide exactly with the later known ones in Newton's gravity. Here we prove that, except for the gravitational wave contribution, even to the second-order perturbations, equations for the relativistic irrotational zero-pressure fluid in a flat Friedmann background coincide exactly with the previously known Newtonian equations. Thus, to the second order, we correctly identify the relativistic density and velocity perturbation variables, and we expand the range of applicability of the Newtonian medium without pressure to all cosmological scales including the super-horizon scale. In the relativistic analyses, however, we do not have a relativistic variable which corresponds to the Newtonian potential to the second order. Mixed usage of different gauge conditions is useful to make such proofs and to examine the result with perspective. We also present the gravitational wave equation to the second order. Since our correspondence includes the cosmological constant, our results are relevant to currently favoured cosmology. Our result has an important practical implication that one can use the large-scale Newtonian numerical simulation more reliably even as the simulation scale approaches near horizon.Comment: 10 pages, no figur

Friedmann-like equations for High Energy Area of Universe

In this paper, evolution of the high energy area of universe, through the scenario of 5 dimensional (5D) universe, has been studied. For this purpose, we solve Einstein equations for 5D metric and 5D perfect fuid to derive Friedmann-like equations. Then we obtain the evolution of scale factor and energy density with respect to both space-like and time-like extra dimensions. We obtain the novel equations for the space-like extra dimension and show that the matter with zero pressure cannot exist in the bulk. Also, for dark energy fuid and vacuum fluid, we have both accelerated expansion and contraction in the bulk.Comment: 9 pages, Accepted to publication in IJTP 26 June 2012. arXiv admin note: substantial text overlap with arXiv:1202.497

Unification, KK-thresholds and the top Yukawa coupling in F-theory GUTs

In a class of F-theory SU(5) GUTs the low energy chiral mass spectrum is obtained from rank one fermion mass textures with a hierarchical structure organised by U(1) symmetries embedded in the exceptional E_8 group. In these theories chiral fields reside on matter `curves' and the tree level masses are computed from integrals of overlapping wavefuctions of the particles at the triple intersection points. This calculation requires knowledge of the exact form of the wavefuctions. In this work we propose a way to obtain a reliable estimate of the various quantities which determine the strength of the Yukawa couplings. We use previous analysis of KK threshold effects to determine the (ratios of) heavy mass scales of the theory which are involved in the normalization of the wave functions. We consider similar effects from the chiral spectrum of these models and discuss possible constraints on the emerging matter content. In this approach, we find that the Yukawa couplings can be determined solely from the U(1) charges of the states in the `intersection' and the torsion which is a topological invariant quantity. We apply the results to a viable SU(5) model with minimal spectrum which satisfies all the constraints imposed by our analysis. We use renormalization group analysis to estimate the top and bottom masses and find that they are in agreement with the experimental values.Comment: 28 pages, 2 figure

Bulk Scale Factor at Very Early Universe

In this paper we propose a higher dimensional Cosmology based on FRW model and brane-world scenario. We consider the warp factor in the brane-world scenario as a scale factor in 5-dimensional generalized FRW metric, which is called as bulk scale factor, and obtain the evolution of it with space-like and time-like extra dimensions. It is then showed that, additional space-like dimensions can produce exponentially bulk scale factor under repulsive strong gravitational force in the empty universe at a very early stage.Comment: 7 pages, October 201

Conductivity of CuO3_3-Chains: Disorder versus Electron-Phonon Coupling

The optical conductivity of the CuO$_3$-chains, a subsystem of the 1-2-3 materials, is dominated by a broad peak in the mid-infrared ($\omega \approx 0.2$eV), and a slowly falling high-frequency tail. The 1D $t$-$J$-model is proposed as the relevant low-energy Hamiltonian describing the intrinsic electronic structure of the CuO$_3$-chains. However, due to charge-spin decoupling, this model alone cannot reproduce the observed \sw. We consider two additional scattering mechanisms: (i) Disregarding the not so crucial spin degrees of freedom, the inclusion of strong potential disorder yields excellent agreement with experiment, but suffers from the unreasonable value of the disorder strength necessary for the fit. (ii) Moderately strong polaronic electron-phonon coupling to the mode involving Cu(1)-O(4) stretching, can be modeled within a 1D Holstein Hamiltonian of spinless fermions. Using a variational approximation for the phonon Hilbert space, we diagonalize the Hamiltonian exactly on finite lattices. As a result of the experimental hole density $\approx 1/2$, the chains can exhibit strong charge-density-wave (CDW) correlations, driven by phonon-mediated polaron-polaron interactions. In the vicinity of half filling, charge motion is identified as arising from moving domain walls, \ie defects in the CDW. Incorporating the effect of vacancy disorder by choosing open boundary conditions, good agreement with the experimental spectra is found. In particular, a high-frequency tail arises as a consequence of the polaron-polaron interactions.Comment: 42 pages, ETH-TH/93-31 (Postscript