49526339
Brane Formation and Cosmological Constraint on the Number of Extra Dimensions
Special relativity is generalized to extra dimensions and quantized energy levels of particles are obtained. By calculating the probability of particles' motion in extra dimensions at high temperature of the early universe, it is proposed that the branes may have not existed since the very beginning of the universe, but formed later. Meanwhile, before the formation, particles of the universe may have filled in the whole bulk, not just on the branes. This scenario differs from that in the standard big bang cosmology in which all particles are assumed to be in the 4D spacetime. So, in brane models, whether our universe began from a 4D big bang singularity is questionable. A cosmological constraint on the number of extra dimensions is also given which favors N ≥ 7.
11595061
DARK ENERGY IN GLOBAL BRANE UNIVERSE
We discuss the exact solutions of brane universes and the results indicate that the Friedmann equations on the branes are modified with a new density term. Then, we assume the new term as the density of dark energy. Using Wetterich's parametrization equation of state (EOS) of dark energy, we obtain that the new term varies with the redshift z. Finally, the evolutions of the mass density parameter Ω2, dark energy density parameter Ωx and deceleration parameter q2 are studied.
2265424
THE COSMOLOGICAL CONSTANT PROBLEM AND KALUZA–KLEIN THEORY
We present technical results which extend previous work and show that the cosmological constant of general relativity is an artefact of the reduction to 4D of 5D Kaluza–Klein theory (or 10D superstrings and 11D supergravity). We argue that the distinction between matter and vacuum is artificial in the context of ND field theory. The concept of a cosmological "constant" (which measures the energy density of the vacuum in 4D) should be replaced by that of a series of variable fields whose sum is determined by a solution of ND field equations in a well-defined manner.
53459258
Reconstruction of Five-Dimensional Bounce Cosmological Models from Deceleration Factor
In this paper, we consider a class of five-dimensional Ricci-flat vacuum solutions, which contain two arbitrary functions μ(t) and ν(t). It is shown that μ(t) can be rewritten as a new arbitrary function f(z) in terms of redshift z and the f(z) can be determined by choosing particular deceleration parameters q(z) which gives early deceleration and late time acceleration. In this way, the 5D cosmological model can be reconstructed and the evolution of the universe can be determined.
118582285
The dynamic behavior of quantum statistical entropy in 5D Ricci-flat black string with thin-layer approach
Abstract In this Letter, the statistical–mechanical entropies of 5D Ricci-flat black string is calculated through the wave modes of the quantum field with improved thin-layer brick-wall method. The modes along the fifth dimension are semi-classically quantized by Randall–Sundrum mass relationship. We use the two-dimensional area to describe this black string's entropy which, in the small-mass approximation, is a linear sum of the area of the black hole horizon and the cosmological horizon. The proportionality coefficients of entropy are discretized with quantized extra dimensional modes. It should be noted that the small-mass approximation used in our calculation is naturally justified by the assumption that the two branes are located far apart.
14287684
Correspondence Between DGP Brane Cosmology and $5D$ Ricci-flat Cosmology
We discuss the correspondence between the DGP brane cosmology and 5D Ricci-flat cosmology by letting their metrics equal each other. By this correspondence, a specific geometrical property of the arbitrary integral constant I in DGP metric is given and it is related to the curvature of 5D bulk. At the same time, the relation of arbitrary functions μ and ν in a class of Ricci-flat solutions is obtained from DGP brane metric.
119533356
SU(5) monopoles and the dual standard model
We find the spectrum of magnetic monopoles produced in the symmetry breaking SU(5){r_arrow}[SU(3){times}SU(2){times}U(1){prime}]/Z{sub 6} by constructing classical bound states of the fundamental monopoles. The spectrum of monopoles is found to correspond to the spectrum of one family of standard model fermions and, hence, is a starting point for constructing the dual standard model. At this level, however, there is an extra monopole state {emdash} the {open_quotes}diquark{close_quotes} monopole {emdash} with no corresponding standard model fermion. If the SU(3) factor now breaks down to Z{sub 3}, the monopoles with nontrivial SU(3) charge get confined by strings in SU(3) singlets. Another outcome of this symmetry breaking is that the diquark monopole becomes unstable (metastable) to fragmentation into fundamental monopoles and the one-one correspondence with the standard model fermions is restored. We discuss the fate of the monopoles if the [SU(2){times}U(1){prime}]/Z{sub 2} factor breaks down to U(1){sub Q} by a Higgs mechanism as in the electroweak model. Here we find that monopoles that are misaligned with the vacuum get connected by strings even though the electroweak symmetry breaking does not admit topological strings. We discuss the lowest order quantum corrections to the mass spectrum of monopoles. {copyright} {ital 1997} {ital The American Physical Society}
15969913
Perturbed Electroweak Strings and Fermion Zero Modes
Abstract The effect of perturbations of electroweak strings on quark zero modes is studied in 2+1 and 3+1 dimensions. As first discovered by Naculich, it is found that the bosonic perturbations that destabilize the string give a mass to the zero modes and also lift their degeneracy. The effect of the zero modes on the stability of the string is discussed qualitatively and, based on general arguments, we conjecture that the fermionic vacuum instability found by Naculich should lead to a distortion of the bosonic string but not be responsible for decay.
119113707
f(R) gravities, Killing spinor equations, “BPS” domain walls and cosmology
A bstractWe derive the condition on f(R) gravities that admit Killing spinor equations and construct explicit such examples. The Killing spinor equations can be used to reduce the fourth-order differential equations of motion to the first order for both the domain wall and FLRW cosmological solutions. We obtain exact “BPS” domain walls that describe the smooth Randall-Sundrum II, AdS wormholes and the RG flow from IR to UV. We also obtain exact smooth cosmological solutions that describe the evolution from an inflationary starting point with a larger cosmological constant to an ever-expanding universe with a smaller cosmological constant. In addition, We find exact smooth solutions of pre-big bang models, bouncing or crunching universes. An important feature is that the scalar curvature R of all these metrics is varying rather than a constant. Another intriguing feature is that there are two different f(R) gravities that give rise to the same “BPS” solution. We also study linearized f(R) gravities in (A)dS vacua.
1499155
Structure and mutagenic conversion of E1 dehydrase: at the crossroads of dehydration, amino transfer, and epimerization.
Pyridoxal 5'-phosphate (PLP) and pyridoxamine 5'-phosphate (PMP) are highly versatile coenzymes whose importance is well recognized. The capability of PLP/PMP-dependent enzymes to catalyze a diverse array of chemical reactions is attributed to fine-tuning of the cofactor-substrate interactions in the active site. CDP-6-deoxy-L-threo-D-glycero-4-hexulose 3-dehydrase (E1), along with its reductase (E3), catalyzes the C-3 deoxygenation of CDP-4-keto-6-deoxy-D-glucose to form the dehydrated product, CDP-4-keto-3,6-dideoxy- d-glucose, in the ascarylose biosynthetic pathway. This product is the progenitor to most 3,6-dideoxyhexoses, which are the major antigenic determinants of many Gram-negative pathogens. The dimeric [2Fe-2S] protein, E 1, cloned from Yersinia pseudotuberculosis, is the only known enzyme whose catalysis involves the direct participation of PMP in one-electron redox chemistry. E1 also contains an unusual [2Fe-2S] cluster with a previously unknown binding motif (C-X 57-C-X 1-C-X 7-C). Herein we report the first X-ray crystal structure of E1, which exhibits an aspartate aminotransferase (AAT) fold. A comparison of the E1 active site architecture with homologous structures uncovers residues critical for the dehydration versus transamination activity. Site-directed mutagenesis of four E1 residues, D194H, Y217H, H220K, and F345H, converted E 1 from a PMP-dependent dehydrase to a PLP/glutamate-dependent aminotransferase. The E1 quadruple mutant, having been conferred this altered enzyme activity, can transaminate the natural substrate to CDP-4,6-dideoxy-4-amino-D-galactose without E3. Taken together, these results provide the molecular basis of the functional switch of E1 toward dehydration, epimerization, and transamination. The insights gained from these studies can be used for the development of inhibitors of disease-relevant PLP/PMP-dependent enzymes.
14591909
Closed strings from decaying D-branes
We compute the emission of closed string radiation from homogeneous rolling tachyons. For an unstable decaying Dp-brane the radiated energy is infinite to leading order for p ≤ 2 and finite for 2$>p > 2. The closed string state produced by a decaying brane is closely related to the state produced by D-instantons at a critical Euclidean distance from t = 0. In the case of a D0 brane one can cutoff this divergence so that we get a finite energy final state which would be the state that the brane decays into.