Hao Wei

One of the puzzles of the dark energy problem is the (first) cosmological coincidence problem, namely, {\em why does our universe begin the accelerated expansion recently? why are we living in an epoch in which the dark energy density and the dust matter energy density are comparable?} On the other hand, cosmological observations hint that the equation-of-state parameter (EoS) of dark energy crossed the phantom divide $w_{de}=-1$ in the near past. Many dark energy models whose EoS can cross the phantom divide have been proposed. However, to our knowledge, these models with crossing the phantom divide only provide the possibility that $w_{de}$ can cross -1. They do not answer another question, namely, {\em why crossing the phantom divide occurs recently?} Since in many existing models whose EoS can cross the phantom divide, $w_{de}$ undulates around -1 randomly, {\em why are we living in an epoch $w_{de}<-1$?} This can be regarded as the second cosmological coincidence problem. In this work, the cosmological evolution of the vector-like dark energy interacting with background perfect fluid is investigated. We find that the first and second cosmological coincidence problems can be alleviated at the same time in this scenario.

One of the puzzles of the dark energy problem is the (first) cosmological coincidence problem, namely, why does our Universe begin the accelerated expansion recently? Why are we living in an epoch in which the dark energy density and the dust matter energy density are comparable? On the other hand, cosmological observations hint that the equation-of-state parameter (EoS) of dark energy crossed the phantom divide $w_{\mathrm{de}}=-1$ in the near past. Many dark energy models whose EoS can cross the phantom divide have been proposed. However, to our knowledge, these models with crossing the phantom divide only provide the possibility that $w_{\mathrm{de}}$ can cross $-1$. They do not answer another question, namely, why crossing the phantom divide occurs recently? Since in many existing models whose EoS can cross the phantom divide, $w_{\mathrm{de}}$ undulates around $-1$ randomly, why are we living in an epoch $w_{\mathrm{de}}<-1$? This can be regarded as the second cosmological coincidence problem. In this work, the cosmological evolution of the vectorlike dark energy interacting with background perfect fluid is investigated. We find that the first and second cosmological coincidence problems can be alleviated at the same time in this scenario.

Recently a lot of attention has been given to building dark energy models in which the equation-of-state parameter $w$ can cross the phantom divide $w=-1$. However, to our knowledge, these models with crossing the phantom divide only provide the possibility that $w$ can cross -1. They do not answer another question: {\em why crossing phantom divide occurs recently?} Since in many existing models whose equation-of-state parameter can cross the phantom divide, $w$ undulates around -1 randomly, {\em why are we living in an epoch $w<-1$?} This can be regarded as the second cosmological coincidence problem. In this note, we propose a possible approach to alleviate this problem within a hybrid dark energy model.

Recently, many dark energy models whose equation-of-state parameter can cross the phantom divide $w_{de}=-1$ have been proposed. In a previous paper [Class. Quant. Grav. {\bf 22}, 3189 (2005); hep-th/0501160], we suggest such a model named hessence, in which a non-canonical complex scalar field plays the role of dark energy. In this work, the cosmological evolution of the hessence dark energy is investigated. We consider two cases: one is the hessnece field with an exponential potential, and the other is with a (inverse) power law potential. We separately investigate the dynamical system with four different interaction forms between hessence and background perfect fluid. It is found that the big rip never appears in the hessence model, even in the most general case, beyond particular potentials and interaction forms.

Recently, many dark energy models whose equation-of-state parameter can cross the phantom divide $w_{\mathrm{de}}=-1$ have been proposed. In a previous paper [H. Wei, R. G. Cai, and D. F. Zeng, Classical Quantum Gravity 22, 3189 (2005)], we suggest such a model named hessence, in which a noncanonical complex scalar field plays the role of dark energy. In this work, the cosmological evolution of the hessence dark energy is investigated. We consider two cases: one is the hessence field with an exponential potential, and the other is with a (inverse) power-law potential. We separately investigate the dynamical system with four different interaction forms between hessence and background perfect fluid. It is found that the big rip never appears in the hessence model, even in the most general case, beyond particular potentials and interaction forms.

Recently a lot of attention has been drawn to build dark energy model in which the equation-of-state parameter $w$ can cross the phantom divide $w=-1$. One of models to realize crossing the phantom divide is called quintom model, in which two real scalar fields appears, one is a normal scalar field and the other is a phantom-type scalar field. In this paper we propose a non-canonical complex scalar field as the dark energy, which we dub ``hessence'', to implement crossing the phantom divide, in a similar sense as the quintom dark energy model. In the hessence model, the dark energy is described by a single field with an internal degree of freedom rather than two independent real scalar fields. However, the hessence is different from an ordinary complex scalar field, we show that the hessence can avoid the difficulty of the Q-balls formation which gives trouble to the spintessence model (An ordinary complex scalar field acts as the dark energy). Furthermore, we find that, by choosing a proper potential, the hessence could correspond to a Chaplygin gas at late times.

In this paper we present a hybrid model of k-essence and chameleon, named as k-chameleon. In this model, due to the chameleon mechanism, the directly strong coupling between the k-chameleon field and matters (cold dark matters and baryons) is allowed. In the radiation dominated epoch, the interaction between the k-chameleon field and background matters can be neglected, the behavior of the k-chameleon therefore is the same as that of the ordinary k-essence. After the onset of matter domination, the strong coupling between the k-chameleon and matters dramatically changes the result of the ordinary k-essence. We find that during the matter-dominated epoch, only two kinds of attractors may exist: one is the familiar {\bf K} attractor and the other is a completely {\em new}, dubbed {\bf C} attractor. Once the universe is attracted into the {\bf C} attractor, the fraction energy densities of the k-chameleon $\Omega_{\phi}$ and dust matter $\Omega_m$ are fixed and comparable, and the universe will undergo a power-law accelerated expansion. One can adjust the model so that the {\bf K} attractor do not appear. Thus, the k-chameleon model provides a natural solution to the cosmological coincidence problem.

In this paper we present a hybrid model of $k$-essence and chameleon, named as $k$-chameleon. In this model, due to the chameleon mechanism, the directly strong coupling between the $k$-chameleon field and matters (cold dark matters and baryons) is allowed. In the radiation-dominated epoch, the interaction between the $k$-chameleon field and background matters can be neglected; the behavior of the $k$-chameleon therefore is the same as that of the ordinary $k$-essence. After the onset of matter domination, the strong coupling between the $k$-chameleon and matters dramatically changes the result of the ordinary $k$-essence. We find that during the matter-dominated epoch, only two kinds of attractors may exist: one is the familiar $\mathbf{K}$ attractor and the other is a completely new, dubbed $\mathbf{C}$ attractor. Once the Universe is attracted into the $\mathbf{C}$ attractor, the fraction energy densities of the $k$-chameleon ${\Omega }_{\varphi }$ and dust matter ${\Omega }_m$ are fixed and comparable, and the Universe will undergo a power-law accelerated expansion. One can adjust the model so that the $\mathbf{K}$ attractor does not appear. Thus, the $k$-chameleon model provides a natural solution to the cosmological coincidence problem.