Generating scale-invariant tensor perturbations in the non-inflationary universe

It is believed that the recent detection of large tensor perturbations strongly favors the inflation scenario in the early universe. This common sense depends on the assumption that Einstein's general relativity is valid at the early universe. In this paper we show that nearly scale-invariant primordial tensor perturbations can be generated during a contracting phase before the radiation dominated epoch if the theory of gravity is modified by the scalar-tensor theory at that time. The scale-invariance protects the tensor perturbations from suppressing at large scales and they may have significant amplitudes to fit BICEP2's result. We construct a model to achieve this purpose and show that the universe can bounce to the hot big bang after long time contraction, and at almost the same time the theory of gravity approaches to general relativity through stabilizing the scalar field. Theoretically, such models are dual to inflation models if we change to the frame in which the theory of gravity is general relativity. Dual models are related by the conformal transformations. With this study we reinforce the point that only the conformal invariant quantities such as the scalar and tensor perturbations are physical. How did the background evolve before the radiation time depends on the frame and has no physical meaning. It is impossible to distinguish different pictures by later time cosmological probes.Comment: 7 pages, 4 figures. An error in the numerical results and some typos in the text were corrected, but general conclusions are unchange

Detecting relic gravitational waves in the CMB: The contamination caused by the cosmological birefringence

The B-mode polarization of the cosmic microwave background (CMB) radiation is an excellent information channel for the detection of relic gravitational waves. However, the detection is contaminated by the B-mode polarization generated by some other effects. In this paper, we discuss the contaminations caused by the cosmological birefringence, which converts the CMB E-mode to the B-mode, and forms the effective noise for the detection of gravitational waves. We find that this contamination is significant, if the rotation angle is large. However, this kind of B-mode can be properly de-rotated, and the effective noises can be greatly reduced. We find that, comparing with the contaminations caused by cosmic weak lensing, the residual polarization generated by the cosmological birefringence is negligible for the detection of relic gravitational waves in the CMB.Comment: 9 pages, 4 figures, references added, PLB accepte

Cosmological CPT violating effect on CMB polarization

A dark energy scalar (or a function of the Ricci scalar) coupled with the derivative to the matter fields will violate the $CPT$ symmetry during the expansion of the Universe. This type of cosmological $CPT$ violation helps to generate the baryon number asymmetry and gives rise to the rotation of the photon polarization which can be measured in the astrophysical and cosmological observations, especially the experiments of the cosmic microwave background radiation. In this paper, we derive the rotation angle in a fully general relativistic way and present the rotation formulas used for the cosmic microwave background data analysis. Our formulas include the corrections from the spatial fluctuations of the scalar field. We also estimate the magnitude of these corrections in a class of dynamical dark energy models for quintessential baryo/leptogenesis.Comment: 12 page

Entropic mechanisms with generalized scalar fields in the Ekpyrotic universe

For the Ekpyrotic universe, the entropic mechanisms with minimal couplings, which have been used to generate nearly scale invariant primordial perturbations, was proved to be unstable. To overcome this difficulty, some non-minimal coupling entropic models were proposed. In this paper we extend these studies to the cases where all the scalar fields have non-standard kinetic terms. We first prove that in these general cases, without non-minimal couplings the entropic models are still unstable. The condition for the stability conflicts with the requirement for achieving scale invariant perturbations. Then we study the non-minimal coupling models and show that at least for some simple cases these models are stable and able to generate the primordial perturbations consistent with current observations.Comment: 11 pages, PLB version, minor typos were correcte

Improving Performance of BitTorrent Network through Incentive Mechanism

Content Distribution via Internet has become increasingly popular right now. Compared with centralized file distribution system using FTP and HTTP protocols, P2P (Peer to Peer) is more cost-effective. Furthermore, it could help save on bandwidth costs and to handle pick demands. Among many P2P protocols, BitTorrent is one of the most popular protocols right now. The BitTorrent network uses tit-for-tat as a method of seeking Pareto efficiency. However, seeders who contribute more to BitTorrent network than leechers are not incentivized to stay online and upload to others. This paper discusses an incentive mechanism which rewards seeder, who stays in a BitTorrent network and uploads to other peers, with better download speed in its further download process. Experimental results with the incentive mechanism and an analysis of the result are also discussed in this paper