Qing-Guo Huang

The holographic dark energy in a non-flat universe

We study the model for holographic dark energy in a spatially closed universe, generalizing the proposal in hep-th/0403127 for a flat universe. We provide independent arguments for the choice of the parameter c = 1 in the holographic dark energy model. On the one hand, c cannot be less than 1, to avoid violating the second law of thermodynamics. On the other hand, observation suggests c to be very close to 1; it is hard to justify a small deviation of c from 1 if c>1.

Supernova constraints on a holographic dark energy model

In this paper, we use the type Ia supernova data to constrain the model of holographic dark energy. For d = 1, the best fit result is Ωm0 = 0.25, the equation of the state of the holographic dark energy wΛ0 = −0.91 and the transition between the decelerating expansion and the accelerating expansion happened when the cosmological red-shift was zT = 0.72. If we set d as a free parameter, the best fit results are d = 0.21, Ωm0 = 0.46, wΛ0 = −2.67, which sounds like a phantom today, and the transition red-shift is zT = 0.28.

Growth factor in f(T) gravity

We derive the evolution equation of growth factor for the matter over-dense perturbation in f(T) gravity. For instance, we investigate its behavior in power law model at small redshift and compare it to the prediction of Lambda CDM and dark energy with the same equation of state in the framework of Einstein general relativity. We find that the perturbation in f (T) gravity grows slower than that in Einstein general relativity if partial derivative f/partial derivative T > 0 due to the effectively weakened gravity.

Field theory at a Lifshitz point

Abstract We construct the general renormalizable actions for the scalar field and the gauge field at a Lifshitz point characterized by the dynamical critical exponent z . The Lorentz invariance is broken down in the UV region, but is recovered in the IR limit. Even though the theories are UV complete, the speed of light is related to the momentum by z ( k / M ) z − 1 which can go to infinity in the UV limit for z ⩾ 2 . Since the Lorentz invariance is broken down, the dispersion relation is modified and the time delays in gamma-ray bursts can be easily explained. In addition, we also discuss the thermal dynamics and the size of causal patch in a FRW universe for the field theory at a Lifshitz point.

Anthropic principle favours the holographic dark energy

We discuss the anthropic principle when applied to holographic dark energy. We find that if the amplitude of the density fluctuation is variable, the holographic dark energy fares better than the cosmological constant. More generally, the anthropic predictions agree better with observation for dark energy with wΛ = pΛ/ρΛ decreasing over time.

A curvaton with a polynomial potential

In general a weakly self-interacting curvaton field is expected and the curvaton potential takes the polynomial form. The curvaton potential can be dominated by the self-interaction term during the period of inflation if the curvaton field stays at a large vacuum expectation value. We use the

Mass-varying massive gravity

\delta {\cal N}

Large non-Gaussianity implication for curvaton scenario

formalism to calculate the primordial curvature perturbation in the various possible scenarios which make the curvaton model much richer.

CMB power spectrum from noncommutative spacetime

It has recently been shown that the graviton can consistently gain a constant mass without introducing the Boulware-Deser ghost. We propose a gravity model where the graviton mass is set by a scalar field and prove that this model is free of the Boulware-Deser ghost by analyzing its constraint system and showing that two constraints arise. We also initiate the study of the models cosmic background evolution and tentatively discuss possible cosmological implications of this model. In particular, we consider a simple scenario where the scalar field setting the graviton mass is identified with the inflaton and the graviton mass evolves from a high to a low energy scale, giving rise to the current cosmic acceleration.

A geometric description of the non-Gaussianity generated at the end of multi-field inflation

Abstract We argue that the typical energy density of a light scalar field should not be less than H 4 in the inflationary Universe. This requirement implies that the non-Gaussianity parameter f NL is typically bounded by the tensor–scalar ratio r from above, namely f NL ≲ 518 ⋅ r 1 4 . If f NL = 10 2 , inflation occurred around the GUT scale.