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The observed value of the Higgs mass indicates an instability of the Higgs scalar at large energy scales, and hence also at large field values. In the context of early universe cosmology, this is often considered to lead to problems. Here, we point out that we can use the instability of the Higgs field to generate an ekpyrotic phase of contraction. In the context of string theory, it is possible that at very high energy densities, extra states become massless leading to an -brane which causes the transition between a contracting phase in the past and the current expanding phase. Thus, the Higgs field may be useable to generate a nonsingular bouncing cosmology in which the anisotropy problem of usual bouncing scenarios is mitigated. [ABSTRACT FROM AUTHOR]

A self-consistent global fitting method based on the Markov Chain Monte Carlo technique to study the dark matter (DM) property associated with the cosmic ray electron/positron excesses was developed in our previous work. In this work we further improve the previous study to include the hadronic branching ratio of DM annihilation/decay. The PAMELA $\bar{p}/p$ data are employed to constrain the hadronic branching ratio. We find that the 95% (2σ) upper limits of the quark branching ratio allowed by the PAMELA $\bar{p}/p$ data is ~0.032 for DM annihilation and ~0.044 for DM decay, respectively. This result shows that the DM coupling to pure leptons is indeed favored by the current data. Based on the global fitting results, we further study the neutrino emission from DM in the galactic center. Our predicted neutrino flux is some smaller than previous works since the constraint from γ-rays is involved. However, it is still capable to be detected by the forthcoming neutrino detector such as IceCube. The improved points of the present study compared with previous works include: (1) the DM parameters, both the particle physical ones and astrophysical ones, are derived in a global fitting way, (2) constraints from various species of data sets, including γ-rays and antiprotons are included, and (3) the expectation of neutrino emission is fully self-consistent. [ABSTRACT FROM AUTHOR]

In this paper, we study the possibility of model building of cyclic universe with Quintom matter in the framework of Loop Quantum Cosmology. After a general demonstration, we provide two examples, one with double-fluid and another double-scalar field, to show how such a scenario is obtained. Analytical and numerical calculations are both presented in the paper. [ABSTRACT FROM AUTHOR]

Null Energy Condition (NEC) requires the equation of state (EoS) of the universe wu satisfy wu ≥ -1, which implies, for instance in a universe with matter and dark energy dominating wu = wmΩm + wdeΩde = wdeΩde ≥ -1. In this paper we study constraints on the dark energy models from the requirement of the NEC. We will show that with Ωde ~ 0.7, wde < -1 at present epoch is possible. However, NEC excludes the possibility of wde < -1 forever as happened in the Phantom model, but if wde < -1 stays for a short period of time as predicted in the Quintom theory, NEC can be satisfied. We take three examples of Quintom models of dark energy, namely the phenomenological EoS, the two-scalar-field model and the single scalar model with a modified Dirac–Born–Infeld (DBI) Lagrangian to show how this happens. [ABSTRACT FROM AUTHOR]

In this paper we study in detail the sound speed — $c_s^2$, which is directly related to the classical perturbations — of dynamical dark energy (DE). We consider especially the case with an equation of state crossing the cosmological constant boundary, and show its implications for cosmic microwave background (CMB) anisotropy. With the present observational data on the CMB, the Type Ia supernova (SNIa) and galaxy clustering, we perform a global analysis to constrain the sound speed of DE, using the Markov chain Monte Carlo method. We find that the sound speed of DE is weakly constrained by current observations, and thus futuristic precision measurements of the CMB on a very large angular scale (low multipoles) are necessary. [ABSTRACT FROM AUTHOR]

We study the stability of the effective Higgs potential in the split supersymmetry and Little Higgs models. In particular, we study the effects of higher dimensional operators in the effective potential on the Higgs mass predictions. We find that the size and sign of the higher dimensional operators can significantly change the Higgs mass required to maintain vacuum stability in Split SUSY models. In the Little Higgs models the effects of higher dimensional operators can be large because of a relatively lower cutoff scale. Working with a specific model we find that a contribution from the higher dimensional operator with coefficient of O(1) can destabilize the vacuum. [ABSTRACT FROM AUTHOR]

In this paper, we study the possibility of building two-field models of dark energy with equation of state across -1. Specifically we will consider two classes of models: one consists of two scalar fields (quintessence + phantom) and another includes one scalar (phantom) and one spinor field (neutrino). Our studies indicate to some extent that two-field models give rise to a simple realization of the dynamical dark energy model with the equation of state across w=-1. [ABSTRACT FROM AUTHOR]

We consider in this paper a simple oscillating Quintom model of dark energy which has two free parameters and an equation of state oscillating and crossing -1. For low redshifts the equation of state of this model resembles itself similar to the linearly parametrized dark energy, however differs substantially at large redshifts. We fit our model to the observational data separately from the new high redshifts supernova observations from the HST/GOODS program and previous supernova, CMB and galaxy clustering. Our results show that because of the oscillating feature of our model the constraints from observations at large redshifts such as CMB become less stringent. [ABSTRACT FROM AUTHOR]

We study the Randall-Sundrum model with a small 4D effective cosmological constant on the brahe, and drive a corrected radion potential following the Goldberger-Wise mechanism. We then discuss the effect of the 4D effective cosmological constant on the stability of the brane-system, and find that to quintessence determined by updated observation, the proper distance between the two branes required to solve the hierarchy problem can exist. However, during inflation, whether we can get a reasonable hierarchy scale is still uncertain. [ABSTRACT FROM AUTHOR]