搜索结果：Quintessence

We study quintessence cosmologies in the context of scalar-tensor theories of gravity, where a scalar field $\ensuremath{\varphi},$ assumed to provide most of the cosmic energy density today, is nonminimally coupled to the Ricci curvature scalar R. Such ``extended quintessence'' cosmologies have the appealing feature that the same field causing the time (and space) variation of the cosmological constant is the source of a varying Newton constant in the manner of Jordan-Brans-Dicke. We investigate here two classes of models, where the gravitational sector of the Lagrangian is $F(\ensuremath{\varphi})R$ with $F(\ensuremath{\varphi})=\ensuremath{\xi}{\ensuremath{\varphi}}^{2}$ [induced gravity (IG)] and $F(\ensuremath{\varphi})=1+\ensuremath{\xi}{\ensuremath{\varphi}}^{2}$ [nonminimal coupling (NMC)]. As a first application of this idea we consider a specific model, where the quintessence field $\ensuremath{\varphi},$ obeying the simplest inverse power potential, has ${\ensuremath{\Omega}}_{\ensuremath{\varphi}}=0.6$ today, in the context of the cold dark matter scenario for structure formation in the Universe, with scale-invariant adiabatic initial perturbations. We find that, if $\ensuremath{\xi}\ensuremath{\lesssim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ for IG and $\ensuremath{\xi}\ensuremath{\lesssim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}(\sqrt{G}{\ensuremath{\varphi}}_{0}{)}^{\ensuremath{-}1}$ for NMC $({\ensuremath{\varphi}}_{0}$ is the present quintessence value), our quintessence field satisfies the existing solar system experimental constraints. Using linear perturbation theory we then obtain the polarization and temperature anisotropy spectra of the cosmic microwave background (CMB) as well as the matter power spectrum. The perturbation behavior possesses distinctive features, that we name ``QR effects:'' the effective potential arising from the coupling with R adds to the true scalar field potential, altering the cosmic equation of state and enhancing the integrated Sachs-Wolfe effect. As a consequence, part of the CMB anisotropy level on COBE scales is due to the latter effect, and the cosmological perturbation amplitude on smaller scales, including the oscillating region of the CMB spectrum, has reduced power; this effect is evident on CMB polarization and temperature fluctuations, as well as on the matter power-spectrum today. Moreover, the acoustic peaks and the spectrum turnover are displaced to smaller scales, compared to ordinary quintessence models, because of the faster growth of the Hubble length, which, for a fixed value today, delays the horizon crossing of scales larger than the horizon wavelength at matter-radiation equality and slightly decreases the amplitude of the acoustic oscillations. These features could be detected in the upcoming observations on CMB and large-scale structure.

A slow-rolling scalar field (Q\ensuremath{\equiv}quintessence) with potential energy ${V}_{Q}\ensuremath{\sim}(3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\mathrm{eV}{)}^{4}$ has been proposed as the origin of an accelerating universe at present. We investigate the effective potential of Q in the framework of a supergravity model including the quantum corrections induced by generic (nonrenormalizable) couplings of Q to the gauge and charged matter multiplets. It is argued that the K\"ahler potential, superpotential, and gauge kinetic functions of the underlying supergravity model are required to be invariant under the variation of Q with an extremely fine accuracy in order to provide a working quintessence potential. Applying these results for string or M theory, we point out that the heterotic M theory or type-I string axion can be a plausible candidate for quintessence if (i) it does not couple to the instanton number of gauge interactions not weaker than those of the standard model and (ii) the modulus partner $\mathrm{Re}(Z)$ of the periodic quintessence axion $\mathrm{Im}(Z)\ensuremath{\equiv}\mathrm{Im}(Z)+1$ has a large vacuum expectation value: $\mathrm{Re}(Z)\ensuremath{\sim}(1/2\ensuremath{\pi})\mathrm{ln}{(m}_{3/2}^{2}{M}_{\mathrm{Planck}}^{2}{/V}_{Q}).$ It is stressed that such a large $\mathrm{Re}(Z)$ gives the gauge unification scale at around the phenomenologically favored value $3\ifmmode\times\else\texttimes\fi{}{10}^{16}$ GeV. To provide an accelerating universe, the quintessence axion should be near the top of its effective potential at present, which requires severe fine tuning of the initial condition of Q and $\mathrm{Q\ifmmode \dot{}\else \.{}\fi{}}$ in the early universe. We discuss a late time inflation scenario based on the modular and $\mathrm{CP}$ invariance of the moduli effective potential, yielding the required initial condition in a natural manner if the K\"ahler metric of the quintessence axion superfield receives a sizable nonperturbative contribution.

Quintessence is a canonical scalar field introduced to explain the late-time cosmic acceleration. The cosmological dynamics of quintessence is reviewed, paying particular attention to the evolution of the dark energy equation of state w. For the field potentials having tracking and thawing properties, the evolution of w can be known analytically in terms of a few model parameters. Using the analytic expression of w, we constrain quintessence models from the observations of supernovae type Ia, cosmic microwave background, and baryon acoustic oscillations. The tracking freezing models are hardly distinguishable from the LCDM model, whereas in thawing models the today's field equation of state is constrained to be w_0<-0.7 (95 % CL). We also derive an analytic formula for the growth rate of matter density perturbations in dynamical dark energy models, which allows a possibility to put further bounds on w from the measurement of redshift-space distortions in the galaxy power spectrum. Finally we review particle physics models of quintessence- such as those motivated by supersymmetric theories. The field potentials of thawing models based on a pseudo-Nambu-Goldstone boson or on extended supergravity theories have a nice property that a tiny mass of quintessence can be protected against radiative corrections.

Quintessence is a canonical scalar field introduced to explain the late-time cosmic acceleration. The cosmological dynamics of quintessence is reviewed, paying particular attention to the evolution of the dark energy equation of state w. For the field potentials having tracking and thawing properties, the evolution of w can be known analytically in terms of a few model parameters. Using the analytic expression of w, we constrain quintessence models from the observations of supernovae type Ia, cosmic microwave background and baryon acoustic oscillations. The tracking freezing models are hardly distinguishable from the Λ-cold-dark-matter model, whereas in thawing models the today's field equation of state is constrained to be w0 < −0.7 (95% CL). We also derive an analytic formula for the growth rate of matter density perturbations in dynamical dark energy models, which allows a possibility of putting further bounds on w from the measurement of redshift-space distortions in the galaxy power spectrum. Finally, we review particle physics models of quintessence—such as those motivated by supersymmetric theories. The field potentials of thawing models based on a pseudo-Nambu–Goldstone boson or extended supergravity theories have a nice property that a tiny mass of quintessence can be protected against radiative corrections.

We present a comprehensive study of the observational constraints on spatially flat cosmological models containing a mixture of matter and quintessence --- a time varying, spatially inhomogeneous component of the energy density of the universe with negative pressure. Our study also includes the limiting case of a cosmological constant. Low red shift constraints include the Hubble parameter, baryon fraction, cluster abundance, age of the universe, bulk velocity and shape of the mass power spectrum; intermediate red shift constraints are due to type 1a supernovae, gravitational lensing, the Ly-a forest, and the evolution of large scale structure; high red shift constraints are based on cosmic microwave background temperature anisotropy. Mindful of systematic errors, we adopt a conservative approach in applying these constraints. We determine that quintessence models in which the matter density parameter is $0.2 \ls \Omega_m \ls 0.5$ and the effective, density-averaged equation of state is $-1 \le w \ls -0.2$, are consistent with the most reliable, current low red shift and CMB observations at the $2\sigma$ level. Factoring in the constraint due to type 1a SNe, the range for the equation of state is reduced to $-1 \le w \ls -0.4$, where this range represents models consistent with each observational constraint at the 2$\sigma$ level or better (concordance analysis). A combined maximum likelihood analysis suggests a smaller range, $-1 \le w \ls -0.6$. We find that the best-fit and best-motivated quintessence models lie near $\Omega_m \approx 0.33$, $h \approx 0.65$, and spectral index $n_s=1$, with an effective equation of state $w \approx -0.65$ for ``tracker'' quintessence and $w=-1$ for ``creeper'' quintessence. (abstract shortened)

We investigate the cosmological role of a tracking field $\ensuremath{\varphi}$ in extended quintessence scenarios, where the dynamical vacuum energy driving the acceleration of the universe today possesses an explicit coupling with the Ricci scalar R of the form $F(\ensuremath{\varphi})R/2,$ where $F(\ensuremath{\varphi})$ mimics general relativity today, $F({\ensuremath{\varphi}}_{0})=1/8\ensuremath{\pi}G.$ We analyze explicit nonminimally coupled (NMC) models where $F(\ensuremath{\varphi})=1/8\ensuremath{\pi}G+\ensuremath{\xi}({\ensuremath{\varphi}}^{2}\ensuremath{-}{\ensuremath{\varphi}}_{0}^{2}),$ with $\ensuremath{\xi}$ is the coupling constant and ${\ensuremath{\varphi}}_{0}$ is the Q value today. Tracker solutions for these NMC models, with inverse power-law potentials, possess an initial enhancement of the scalar field dynamics, named the R-boost, caused by the effective potential generated by the Ricci scalar in the Klein-Gordon equation. During this phase the field performs a ``gravitational'' slow rolling until the true potential becomes important. We give accurate analytic formulas describing the R-boost, showing that the quintessence energy in this phase scales with the redshift z as ${(1+z)}^{2}.$ When the R-boost ends, the field trajectory matches the tracker solution in minimally coupled theories. We compute perturbations in these tracking extended quintessence models, by integrating the full set of equations for the evolution of linear fluctuations in scalar-tensor theories of gravity, and assuming Gaussian scale-invariant initial perturbations. The integrated Sachs-Wolfe (ISW) effect on the cosmic microwave background (CMB) angular spectrum causes a change $\ensuremath{\delta}{C}_{l}{/C}_{l}\ensuremath{\simeq}6[1\ensuremath{-}8\ensuremath{\pi}GF({\ensuremath{\varphi}}_{\mathrm{dec}})]$ at $l\ensuremath{\lesssim}10,$ where ``dec'' stands for decoupling. Similarly, the CMB acoustic peak multipoles shift compared to ordinary tracking quintessence models by roughly an amount $\ensuremath{\delta}l/l\ensuremath{\simeq}[8\ensuremath{\pi}GF({\ensuremath{\varphi}}_{\mathrm{dec}})\ensuremath{-}1]/8.$ The turnover wave number ${k}_{\mathrm{turn}}$ in the matter power spectrum shifts by an amount $\ensuremath{\delta}{k}_{\mathrm{turn}}{/k}_{\mathrm{turn}}\ensuremath{\simeq}[1\ensuremath{-}8\ensuremath{\pi}GF({\ensuremath{\varphi}}_{\mathrm{eq}})]/2,$ where ``eq'' stands for matter-radiation equivalence. All these corrections may assume positive as well as negative values, depending on the sign of the NMC parameter $\ensuremath{\xi}.$ We show that the above effects can be as large as 10--30 % with respect to equivalent cosmological constant and ordinary tracking quintessence models, respecting all the existing experimental constraints on scalar-tensor theories of gravity. These results demonstrate that the playground where the data of the next decade will have their impact includes the nature of the dark energy in the Universe, as well as the structure of the theory of gravity.

Recent observations suggest that a large fraction of the energy density of the Universe has negative pressure. One explanation is vacuum energy density; another is quintessence in the form of a scalar field slowly evolving down a potential. In either case, a key problem is to explain why the energy density nearly coincides with the matter density today. The densities decrease at different rates as the Universe expands, so coincidence today appears to require that their ratio be set to a specific, infinitesimal value in the early Universe. In this paper, we introduce the notion of a ``tracker field,'' a form of quintessence, and show how it may explain the coincidence, adding new motivation for the quintessence scenario.

A new component of the cosmic medium, a light scalar field or ``quintessence,'' has been proposed recently to explain cosmic acceleration with a dynamical cosmological constant. Such a field is expected to be coupled explicitly to ordinary matter, unless some unknown symmetry prevents it. I investigate the cosmological consequences of a coupled quintessence (CQ) model, assuming an exponential potential and a linear coupling. This model is conformally equivalent to Brans-Dicke Lagrangians with any power-law potential. I evaluate the density perturbations on the cosmic microwave background and on the galaxy distribution at the present and derive bounds on the coupling constant from the comparison with observational data. A novel feature of CQ is that during the matter dominated era the scalar field has a finite and almost constant energy density. This epoch, denoted as \ensuremath{\varphi}MDE, is a saddle point in the dynamical phase space. The \ensuremath{\varphi}MDE is responsible of several differences with respect to uncoupled quintessence: the multipole spectrum of the microwave background is tilted at large angles, the acoustic peaks are shifted, their amplitude is changed, and the present 8 Mpc/h density variance is diminished. The present data constrain the dimensionless coupling constant to $|\ensuremath{\beta}|&lt;~0.1$ assuming ${\ensuremath{\Omega}}_{m}=0.3$ and a primordial fluctuation slope ${n}_{s}=1.$

The issues of quintessence and cosmic acceleration can be discussed in the framework of higher order theories of gravity. We can define effective pressure and energy density directly connected to the Ricci scalar of curvature of a generic fourth order theory and then ask for the conditions to get an accelerated expansion. Exact accelerated expanding solutions can be achieved for several fourth order theories so that we get an alternative scheme to the standard quintessence scalar field, minimally coupled to gravity, usually adopted. We discuss also conformal transformations in order to see the links of quintessence between the Jordan and Einstein frames.

We determine the range of parameter space of an Interacting Quintessence Model that best fits the recent WMAP measurements of Cosmic Microwave Background temperature anisotropies. We only consider cosmological models with zero spatial curvature. We show that if the quintessence scalar field decays into cold dark matter at a rate that brings the ratio of matter to dark energy constant at late times, the cosmological parameters required to fit the CMB data are: dark energy density ${\ensuremath{\Omega}}_{x}=0.43\ifmmode\pm\else\textpm\fi{}0.12$, baryon fraction ${\ensuremath{\Omega}}_{b}=0.08\ifmmode\pm\else\textpm\fi{}0.01$, slope of the matter power spectrum at large scales ${n}_{s}=0.98\ifmmode\pm\else\textpm\fi{}0.02$ and Hubble constant ${H}_{0}=56\ifmmode\pm\else\textpm\fi{}4\text{ }\text{ }\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$. The data prefers a dark energy component with a dimensionless decay rate parameter ${c}^{2}=0.005$ and noninteracting models are consistent with the data only at the $99.9%$ confidence level. Using the Bayesian Information Criteria we show that this extra parameter fits the data better than models with no interaction. The quintessence equation of state parameter is less constrained; i.e., the data sets an upper limit ${w}_{x}\ensuremath{\le}\ensuremath{-}0.86$ at the same level of significance. When the WMAP anisotropy data are combined with supernovae data, the density parameter of dark energy increases to ${\ensuremath{\Omega}}_{x}\ensuremath{\simeq}0.68$ while ${c}^{2}$ augments to $6.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. Models with quintessence decaying into dark matter provide a clean explanation for the coincidence problem and are a viable cosmological model, compatible with observations of the CMB, with testable predictions. Accurate measurements of baryon fraction and/or of matter density independent of the CMB data, would support/disprove these models.

Dynamical vacuum energy or quintessence, a slowly varying and spatially inhomogeneous component of the energy density with negative pressure, is currently consistent with observational data. One potential difficulty with the idea of quintessence is that couplings to ordinary matter should be strongly suppressed so as not to lead to observable time variations of the constants of nature. We further explore the possibility of an explicit coupling between the quintessence field and the curvature. Since such a scalar field gives rise to another gravity force of long range $(\ensuremath{\gtrsim}{H}_{0}^{\ensuremath{-}1}),$ the solar system experiments put a constraint on the nonminimal coupling: $|\ensuremath{\xi}|\ensuremath{\lesssim}{10}^{\ensuremath{-}2}.$

We perform a detailed comparison of the Wilkinson Microwave Anisotropy Probe measurements of the cosmic microwave background (CMB) temperature and polarization anisotropy with the predictions of quintessence cosmological models of dark energy. We consider a wide range of quintessence models, including a constant equation of state, a simply parametrized, time-evolving equation of state, a class of models of early quintessence, and scalar fields with an inverse-power law potential. We also provide a joint fit to the Cosmic Background Imager (CBI) and Arcminute Cosmology Bolometer Array Receiver (ACBAR) CMB data, and the type 1a supernovae. Using these select constraints we identify viable, target models which should prove useful for numerical studies of large scale structure formation, and to rapidly estimate the impact to the concordance region when new or improved observations become available.

Recently, a novel class of models for inflation has been found in which the inflationary dynamics is driven solely by (noncanonical) kinetic terms rather than by potential terms. As an obvious extension, we show that a scalar field with noncanonical kinetic terms alone behaves like an energy component which is time varying and has negative pressure presently, i.e., quintessence. We present a model which has a constant equation of state, that is, a ``kinetic'' counterpart of the Ratra-Peebles model of a quintessence field with a potential term. We make clear the structure of the phase plane and show that the quintessential solution is a late-time attractor. We also give a model for the ``phantom'' component which has an equation of state with $w=p/\ensuremath{\rho}&lt;\ensuremath{-}1.$

Recent observations seem to suggest that our Universe is accelerating, implying that it is dominated by a fluid whose equation of state is negative. Quintessence is a possible explanation. In particular, the concept of tracking solutions permits us to address the fine-tuning and coincidence problems. We study this proposal in the simplest case of an inverse power potential and investigate its robustness to corrections. We show that quintessence is not affected by the one-loop quantum corrections. In the supersymmetric case where the quintessential potential is motivated by nonperturbative effects in gauge theories, we consider the curvature effects and the K\"ahler corrections. We find that the curvature effects are negligible while the K\"ahler corrections modify the early evolution of the quintessence field. Finally we study the supergravity corrections and show that they must be taken into account as $Q\ensuremath{\approx}{m}_{\mathrm{Pl}}$ at small redshifts. We discuss simple supergravity models exhibiting the quintessential behavior. In particular, we propose a model where the scalar potential is given by $V(Q)=({\ensuremath{\Lambda}}^{4+\ensuremath{\alpha}}{/Q}^{\ensuremath{\alpha}}{)e}^{(\ensuremath{\kappa}{/2)Q}^{2}}.$ We argue that the fine-tuning problem can be overcome if $\ensuremath{\alpha}&gt;~11.$ This model leads to ${\ensuremath{\omega}}_{Q}\ensuremath{\approx}\ensuremath{-}0.82$ for ${\ensuremath{\Omega}}_{\mathrm{m}}\ensuremath{\approx}0.3$ which is in good agreement with the presently available data.

The abundance of rich clusters is a strong constraint on the mass power spectrum. The current constraint can be expressed in the form $σ_8 Ω_m^γ = 0.5 \pm 0.1$ where $σ_8$ is the $rms$ mass fluctuation on 8 $h^{-1}$ Mpc scales, $Ω_m$ is the ratio of matter density to the critical density, and $γ$ is model-dependent. In this paper, we determine a general expression for $γ$ that applies to any models with a mixture of cold dark matter plus cosmological constant or quintessence (a time-evolving, spatially-inhomogeneous component with negative pressure) including dependence on the spectral index $n$, the Hubble constant $h$, and the equation-of-state of the quintessence component $w$. The cluster constraint is combined with COBE measurements to identify a spectrum of best-fitting models. The constraint from the evolution of rich clusters is also discussed.

The problem of the cosmic coincidence is a longstanding puzzle. This conundrum may be solved by introducing a coupling between the two dark sectors. In this Letter, we study two cases of the coupled quintessence scenario. (a) Assume that the mass of dark matter particles depends exponentially on the scalar field associated to dark energy and meanwhile the scalar field evolves in an exponential potential; (b) Assume that the mass of dark matter particles depends on a power law function of the scalar field and meanwhile the scalar field evolves in a power law potential. Since the dynamics of this system is dominated by an attractor solution, the mass of dark matter particles is forced to change with time as to ensure that the ratio between the energy densities of dark matter and dark energy becomes a constant at late times, and one thus solve the cosmic coincidence problem naturally. We perform a statefinder diagnostic to both cases of this coupled quintessence scenario. It is shown that the evolving trajectory of this scenario in the s–r diagram is quite different from those of other dark energy models.

The framework for considering the astronomical and cosmological observations in the context of scalar-tensor quintessence in which the quintessence field also accounts for a time dependence of the gravitational constant is developed. The constraints arising from nucleosynthesis, the variation of the constant, and the post-Newtonian measurements are taken into account. A simple model of supernovae is presented in order to extract the dependence of their light curves with the gravitational constant; this implies a correction when fitting the luminosity distance. The properties of perturbations as well as CMB anisotropies are also investigated.

A large number of cosmological studies now suggest that roughly two-thirds of the critical energy density of the Universe exists in a component with negative pressure. If the equation of state of such an energy component varies with time, it should in principle be possible to identify such a variation using cosmological probes over a wide range in redshift. Proper detection of any time variation, however, requires cosmological probes beyond the currently studied range in redshift of $\sim$ 0.1 to 1. We extend our analysis to gravitational lensing statistics at high redshift and suggest that a reliable sample of lensed sources, out to a redshift of $\sim$ 5, can be used to constrain the variation of the equation of state, provided that both the redshift distribution of lensed sources and the selection function involved with the lensed source discovery process are known. An exciting opportunity to catalog an adequate sample of lensed sources (quasars) to probe quintessence is now available with the ongoing Sloan Digital Sky Survey. Writing $w(z)\approx w_0 + z (dw/dz)_0$, we study the expected accuracy to which the equation of state today $w_0$ and its rate of change $(dw/dz)_0$ can simultaneously be constrained. Such a determination can rule out some missing-energy candidates, such as classes of quintessence models or a cosmological constant.

Quintessence, a time-varying energy component that may account for the accelerated expansion of the universe, can be characterized by its equation of state and sound speed. In this paper, we show that if the quintessence density is at least 1% of the critical density at the surface of last scattering the cosmic microwave background anisotropy can distinguish between models whose sound speed is near the speed of light versus near zero, which could be useful in distinguishing competing candidates for dark energy.