arXiv subjects

Hao Wei

Publications and source records attributed to Hao Wei.

At least 109 records · Page 6Linked to original sources

arXiv2014

Cosmological Applications of Pade Approximant

As is well known, in mathematics, any function could be approximated by the Padé approximant. The Padé approximant is the best approximation of a function by a rational function of given order. In fact, the Padé approximant often gives better approximation of the function than truncating its Taylor series, and it may still work where the Taylor series does not converge. In the present work, we consider the Padé approximant in two issues. First, we obtain the analytical approximation of the luminosity distance for the flat XCDM model, and find that the relative error is fairly small. Second, we propose several parameterizations for the equation-of-state parameter (EoS) of dark energy based on the Padé approximant. They are well motivated from the mathematical and physical points of view. We confront these EoS parameterizations with the latest observational data, and find that they can work well. In these practices, we show that the Padé approximant could be an useful tool in cosmology, and it deserves further investigation.

astro-ph.CO↗

arXiv2013

Indistinguishability of Warm Dark Matter, Modified Gravity, and Coupled Cold Dark Matter

The current accelerated expansion of our universe could be due to an unknown energy component with negative pressure (dark energy) or a modification to general relativity (modified gravity). On the other hand, recently warm dark matter (WDM) remarkably rose as an alternative of cold dark matter (CDM). Obviously, it is of interest to distinguish these different types of models. In fact, many attempts have been made in the literature. However, in the present work, we show that WDM, modified gravity and coupled CDM form a trinity, namely, they are indistinguishable by using the cosmological observations of both cosmic expansion history and growth history. Therefore, to break this degeneracy, the other complementary probes beyond the ones of cosmic expansion history and growth history are required.

astro-ph.CO↗

arXiv2013

Cosmological Constraints on Variable Warm Dark Matter

Although $Λ$CDM model is very successful in many aspects, it has been seriously challenged. Recently, warm dark matter (WDM) remarkably rose as an alternative of cold dark matter (CDM). In the literature, many attempts have been made to determine the equation-of-state parameter (EoS) of WDM. However, in most of the previous works, it is usually assumed that the EoS of dark matter (DM) is constant (and usually the EoS of dark energy is also constant). Obviously, this assumption is fairly restrictive. It is more natural to assume a variable EoS for WDM (and dark energy). In the present work, we try to constrain the EoS of variable WDM with the current cosmological observations. We find that the best fits indicate WDM, while CDM is still consistent with the current observational data. However, $Λ$CDM is still better than WDM models from the viewpoint of goodness-of-fit. So, in order to distinguish WDM and CDM, the further observations on the small/galactic scale are required. On the other hand, in this work we also consider WDM whose EoS is constant, while the role of dark energy is played by various models. We find that the cosmological constraint on the constant EoS of WDM is fairly robust.

astro-ph.CO↗

arXiv2012

Quasi-Rip: A New Type of Rip Model without Cosmic Doomsday

The fate of our universe is an unceasing topic of cosmology and the human being. The discovery of the current accelerated expansion of the universe significantly changed our view of the fate of the universe. Recently, some interesting scenarios concerning the fate of the universe attracted much attention in the community, namely the so-called "Little Rip" and "Pseudo-Rip". It is worth noting that all the Big Rip, Little Rip and Pseudo-Rip arise from the assumption that the dark energy density $ρ(a)$ is monotonically increasing. In the present work, we are interested to investigate what will happen if this assumption is broken, and then propose a so-called "Quasi-Rip" scenario, which is driven by a type of quintom dark energy. In this work, we consider an explicit model of Quasi-Rip in detail. We show that Quasi-Rip has an unique feature different from Big Rip, Little Rip and Pseudo-Rip. Our universe has a chance to be rebuilt from the ashes after the terrible rip. This might be the last hope in the "hopeless" rip.

gr-qc↗

arXiv2012

Pilgrim Dark Energy

In the present work, we reconsider the idea of holographic dark energy. One of its key points is the formation of the black hole. And then, we propose the so-called "pilgrim dark energy" based on the speculation that the repulsive force contributed by the phantom-like dark energy ($w<-1$) is strong enough to prevent the formation of the black hole. We also consider the cosmological constraints on pilgrim dark energy by using the latest observational data. Of course, one can instead regard pilgrim dark energy as a purely phenomenological model without any physical motivation. We also briefly discuss this issue.

gr-qc↗

arXiv2012

Constraining $f(T)$ Theories with the Varying Gravitational Constant

As is well known, a varying effective gravitational "constant" is one of the common features of most modified gravity theories. Of course, as a modified gravity theory, $f(T)$ theory is not an exception. Noting that the observational constraint on the varying gravitational "constant" is very tight, in the present work we try to constrain $f(T)$ theories with the varying gravitational "constant". We find that the allowed model parameter $n$ or $β$ has been significantly shrunk to a very narrow range around zero. In fact, the results improve the previous constraints by an order of magnitude.

gr-qc↗

arXiv2012

Dynamics of Teleparallel Dark Energy

Recently, Geng et al. proposed to allow a non-minimal coupling between quintessence and gravity in the framework of teleparallel gravity, motivated by the similar one in the framework of General Relativity (GR). They found that this non-minimally coupled quintessence in the framework of teleparallel gravity has a richer structure, and named it "teleparallel dark energy". In the present work, we note that there might be a deep and unknown connection between teleparallel dark energy and Elko spinor dark energy. Motivated by this observation and the previous results of Elko spinor dark energy, we try to study the dynamics of teleparallel dark energy. We find that there exist only some dark-energy-dominated de Sitter attractors. Unfortunately, no scaling attractor has been found, even when we allow the possible interaction between teleparallel dark energy and matter. However, we note that $w$ at the critical points is in agreement with observations (in particular, the fact that $w=-1$ independently of $ξ$ is a great advantage).

gr-qc↗

arXiv2012

Noether Symmetry in $f(T)$ Theory

As is well known, symmetry plays an important role in the theoretical physics. In particular, the well-known Noether symmetry is an useful tool to select models motivated at a fundamental level, and find the exact solution to the given Lagrangian. In the present work, we try to consider Noether symmetry in $f(T)$ theory. At first, we briefly discuss the Lagrangian formalism of $f(T)$ theory. In particular, the point-like Lagrangian is explicitly constructed. Based on this Lagrangian, the explicit form of $f(T)$ theory and the corresponding exact solution are found by requiring Noether symmetry. In the resulting $f(T)=μT^n$ theory, the universe experiences a power-law expansion $a(t)\sim t^{2n/3}$. Furthermore, we consider the physical quantities corresponding to the exact solution, and find that if $n>3/2$ the expansion of our universe can be accelerated without invoking dark energy. Also, we test the exact solution of this $f(T)$ theory with the latest Union2 Type Ia Supernovae (SNIa) dataset which consists of 557 SNIa, and find that it can be well consistent with the observational data in fact.

gr-qc↗

arXiv2011

Cosmological Constraints on the Sign-Changeable Interactions

Recently, Cai and Su [Phys. Rev. D {\bf 81}, 103514 (2010)] found that the sign of interaction $Q$ in the dark sector changed in the approximate redshift range of $0.45\,\lsim\, z\,\lsim\, 0.9$, by using a model-independent method to deal with the observational data. In fact, this result raises a remarkable problem, since most of the familiar interactions cannot change their signs in the whole cosmic history. Motivated by the work of Cai and Su, we have proposed a new type of interaction in a previous work [H. Wei, Nucl. Phys. B {\bf 845}, 381 (2011)]. The key ingredient is the deceleration parameter $q$ in the interaction $Q$, and hence the interaction $Q$ can change its sign when our universe changes from deceleration ($q>0$) to acceleration ($q<0$). In the present work, we consider the cosmological constraints on this new type of sign-changeable interactions, by using the latest observational data. We find that the cosmological constraints on the model parameters are fairly tight. In particular, the key parameter $β$ can be constrained to a narrow range.

gr-qc↗

arXiv2011

$f(T)$ Theories and Varying Fine Structure Constant

In analogy to $f(R)$ theory, recently a new modified gravity theory, namely the so-called $f(T)$ theory, has been proposed to drive the current accelerated expansion without invoking dark energy. In the present work, by extending Bisabr's idea, we try to constrain $f(T)$ theories with the varying fine structure "constant", $α\equiv e^2/\hbar c$. We find that the constraints on $f(T)$ theories from the observational $Δα/α$ data are very severe. In fact, they make $f(T)$ theories almost indistinguishable from $Λ$CDM model.

gr-qc↗

arXiv2011

Dark Energy Cosmology with the Alternative Cosmic Microwave Background Data

Recently, in a series of works by Liu and Li (L&L), they claimed that there exists a timing asynchrony of $-25.6\,$ms between the spacecraft attitude and radiometer output timestamps in the original raw WMAP time-ordered data (TOD). L&L reprocessed the WMAP data while the aforementioned timing asynchrony has been corrected, and they obtained an alternative CMB map in which the quadrupole dropped to nearly zero. In the present work, we try to see the implications to dark energy cosmology if L&L are right. While L&L claimed that there is a bug in the WMAP pipeline which leads to significantly different cosmological parameters, an interesting question naturally arises, namely, how robust is the current dark energy cosmology with respect to systematic errors and bugs? So, in this work, we adopt the alternative CMB data of L&L as a strawman to study the robustness of dark energy predictions.

astro-ph.CO↗

arXiv2011

Cosmological Evolution of Quintessence and Phantom with a New Type of Interaction in Dark Sector

In the present work, motivated by the work of Cai and Su [Phys. Rev. D {\bf 81}, 103514 (2010)], we propose a new type of interaction in dark sector, which can change its sign when our universe changes from deceleration to acceleration. We consider the cosmological evolution of quintessence and phantom with this type of interaction, and find that there are some scaling attractors which can help to alleviate the cosmological coincidence problem. Our results also show that this new type of interaction can bring new features to cosmology.

gr-qc↗

arXiv2010

Spinor Dark Energy and Cosmological Coincidence Problem

Recently, the so-called Elko spinor field has been proposed to be a candidate of dark energy. It is a non-standard spinor and has unusual properties. When the Elko spinor field is used in cosmology, its unusual properties could bring some interesting consequences. In the present work, we discuss the cosmological coincidence problem in the spinor dark energy models by using the dynamical system method. Our results show that the cosmological coincidence problem should be taken to heart in the investigations of spinor dark energy models.

gr-qc↗

arXiv2010

Cosmological Constraints on the Modified Entropic Force Model

Very recently, Verlinde considered a theory in which space is emergent through a holographic scenario, and proposed that gravity can be explained as an entropic force caused by changes in the information associated with the positions of material bodies. Then, motivated by the Debye model in thermodynamics which is very successful in very low temperatures, Gao modified the entropic force scenario. The modified entropic force (MEF) model is in fact a modified gravity model, and the universe can be accelerated without dark energy. In the present work, we consider the cosmological constraints on the MEF model, and successfully constrain the model parameters to a narrow range. We also discuss many other issues of the MEF model. In particular, we clearly reveal the implicit root to accelerate the universe in the MEF model.

gr-qc↗

arXiv2010

Observational Constraints on Cosmological Models with the Updated Long Gamma-Ray Bursts

In the present work, by the help of the newly released Union2 compilation which consists of 557 Type Ia supernovae (SNIa), we calibrate 109 long Gamma-Ray Bursts (GRBs) with the well-known Amati relation, using the cosmology-independent calibration method proposed by Liang {\it et al.}. We have obtained 59 calibrated high-redshift GRBs which can be used to constrain cosmological models without the circularity problem (we call them ``Hymnium'' GRBs sample for convenience). Then, we consider the joint constraints on 7 cosmological models from the latest observational data, namely, the combination of 557 Union2 SNIa dataset, 59 calibrated Hymnium GRBs dataset (obtained in this work), the shift parameter $R$ from the WMAP 7-year data, and the distance parameter $A$ of the measurement of the baryon acoustic oscillation (BAO) peak in the distribution of SDSS luminous red galaxies. We also briefly consider the comparison of these 7 cosmological models.

astro-ph.CO↗

arXiv2010

Revisiting the Cosmological Constraints on the Interacting Dark Energy Models

In this work, we consider the cosmological constraints on the interacting dark energy models. We generalize the models considered previously by Guo {\it et al.}, Costa and Alcaniz, and try to discuss two general types of models: type I models are characterized by $ρ_{_X}/ρ_m=f(a)$ and $f(a)$ can be any function of scale factor $a$, whereas type II models are characterized by $ρ_m=ρ_{m0}\,a^{-3+ε(a)}$ and $ε(a)$ can be any function of $a$. We obtain the cosmological constraints on the type I and II models with power-law, CPL-like, logarithmic $f(a)$ and $ε(a)$ by using the latest observational data.

gr-qc↗

arXiv2010

Tension in the Recent Type Ia Supernovae Datasets

In the present work, we investigate the tension in the recent Type Ia supernovae (SNIa) datasets Constitution and Union. We show that they are in tension not only with the observations of the cosmic microwave background (CMB) anisotropy and the baryon acoustic oscillations (BAO), but also with other SNIa datasets such as Davis and SNLS. Then, we find the main sources responsible for the tension. Further, we make this more robust by employing the method of random truncation. Based on the results of this work, we suggest two truncated versions of the Union and Constitution datasets, namely the UnionT and ConstitutionT SNIa samples, whose behaviors are more regular.

astro-ph.CO↗

arXiv2009

Varying Alpha Driven by the Dirac-Born-Infeld Scalar Field

Since about ten years ago, varying $α$ theories attracted many attentions, mainly due to the first observational evidence from the quasar absorption spectra that the fine structure ``constant'' might change with cosmological time. In this work, we investigate the cosmic evolution of $α$ driven by the Dirac-Born-Infeld (DBI) scalar field. To be general, we consider various couplings between the DBI scalar field and the electromagnetic field. We also confront the resulting $Δα/α$ with the observational constraints, and find that various cosmological evolution histories of $Δα/α$ are allowed. Comparing with the case of varying $α$ driven by quintessence, the corresponding constraints on the parameters of coupling have been relaxed, thanks to the relativistic correction of the DBI scalar field.

gr-qc↗