arXiv subjects

Yun Wang

Publications and source records attributed to Yun Wang.

At least 361 records · Page 20Linked to original sources

arXiv2004

Probing Dark Energy Using Its Density Instead of Its Equation of State

The variation of dark energy density with redshift, $ρ_X(z)$, provides a critical clue to the nature of dark energy. Since $ρ_X(z)$ depends on the dark energy equation of state $w_X(z)$ through an integral, $ρ_X(z)$ can be constrained more tightly than $w_X(z)$ given the same observational data. We demonstrate this explicitly using current type Ia supernova (SN Ia) data [the Tonry/Barris sample], together with the Cosmic Microwave Background (CMB) shift parameter from CMB data (WMAP, CBI, and ACBAR), and the large scale structure (LSS) growth factor from 2dF galaxy survey data. We assume a flat universe, and use Markov Chain Monte Carlo (MCMC) technique in our analysis. We find that, while $w_X(z)$ extracted from current data is consistent with a cosmological constant at 68% C.L., $ρ_X(z)$ (which has far smaller uncertainties) is not. Our results clearly show the advantage of using $ρ_X(z)$, instead of $w_X(z)$, to probe dark energy.

astro-ph↗

arXiv2004

Probing the Evolution of the Dark Energy Density with Future Supernova Surveys

The time dependence of the dark energy density can be an important clue to the nature of dark energy in the universe. We show that future supernova data from dedicated telescopes (such as SNAP), when combined with data of nearby supernovae, can be used to determine how the dark energy density $ρ_X(z)$ depends on redshift, if $ρ_X(z)$ is not too close to a constant. For quantitative comparison, we have done an extensive study of a number of dark energy models. Based on these models we have simulated data sets in order to show that we can indeed reconstruct the correct sign of the time dependence of the dark energy density, outside of a degeneracy region centered on $1+w_0 = -w_1 z_{max}/3$ (where $z_{max}$ is the maximum redshift of the survey, e.g., $z_{max}=1.7$ for SNAP). We emphasize that, given the same data, one can obtain much more information about the dark energy density directly (and its time dependence) than about its equation of state.

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arXiv2004

Model-Independent Constraints on Dark Energy Density from Flux-averaging Analysis of Type Ia Supernova Data

We reconstruct the dark energy density $ρ_X(z)$ as a free function from current type Ia supernova (SN Ia) data (Tonry et al. 2003; Barris et al. 2003; Knop et al. 2003), together with the Cosmic Microwave Background (CMB) shift parameter from CMB data (WMAP, CBI, and ACBAR), and the large scale structure (LSS) growth factor from 2dF galaxy survey data. We parametrize $ρ_X(z)$ as a continuous function, given by interpolating its amplitudes at equally spaced $z$ values in the redshift range covered by SN Ia data, and a constant at larger $z$ (where $ρ_X(z)$ is only weakly constrained by CMB data). We assume a flat universe, and use the Markov Chain Monte Carlo (MCMC) technique in our analysis. We find that the dark energy density $ρ_X(z)$ is constant for $0 \la z \la 0.5$ and increases with redshift $z$ for $0.5 \la z \la 1$ at 68.3% confidence level, but is consistent with a constant at 95% confidence level. For comparison, we also give constraints on a constant equation of state for the dark energy. Flux-averaging of SN Ia data is required to yield cosmological parameter constraints that are free of the bias induced by weak gravitational lensing \citep{Wang00b}. We set up a consistent framework for flux-averaging analysis of SN Ia data, based on \cite{Wang00b}. We find that flux-averaging of SN Ia data leads to slightly lower $Ω_m$ and smaller time-variation in $ρ_X(z)$. This suggests that a significant increase in the number of SNe Ia from deep SN surveys on a dedicated telescope \citep{Wang00a} is needed to place a robust constraint on the time-dependence of the dark energy density.

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arXiv2003

Direct Wavelet Expansion of the Primordial Power Spectrum

In order to constrain and possibly detect unusual physics during inflation, we allow the power spectrum of primordial matter density fluctuations, P_{in}(k), to be an arbitrary function in the estimation of cosmological parameters from data. The multi-resolution and good localization properties of orthogonal wavelets make them suitable for detecting features in P_{in}(k). We expand P_{in}(k) directly in wavelet basis functions. The likelihood of the data is thus a function of the wavelet coefficients of P_{in}(k), as well as the H_0, Ω_b h^2, $Ω_c h^2 and the τ_{ri}, in a flat $Λ$CDM cosmology. We derive constraints on these parameters from CMB anisotropy data (WMAP, CBI, and ACBAR) and large scale structure (LSS) data (2dFGRS and PSCZ) using the Markov Chain Monte Carlo (MCMC) technique. The direct wavelet expansion method is different and complimentary to the wavelet band power method of Mukherjee & Wang (2003a,b), and results from the two methods are consistent. In addition, as we demonstrate, the direct wavelet expansion method has the advantage that once the wavelet coefficients have been constrained, the reconstruction of P_{in}(k) can be effectively denoised, i.e., P_{in}(k) can be reconstructed using only the coefficients that, say, deviate from zero at greater than 1σ. In doing so the essential properties of P_{in}(k) are retained. The reconstruction also suffers much less from the correlated errors of binning methods. The shape of the primordial power spectrum, as reconstructed in detail here, reveals an interesting new feature at 0.001 \la k/{Mpc}^{-1} \la 0.005. It will be interesting to see if this feature is confirmed by future data. The reconstructed and denoised P_{in}(k) is favored over the scale-invariant and power-law forms at \ga 1σ.

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arXiv2003

Model-Independent Reconstruction of the Primordial Power Spectrum from WMAP Data

Reconstructing the shape of the primordial power spectrum in a model independent way from cosmological data is a useful consistency check on what is usually assumed regarding early universe physics. It is also our primary window to unknown physics during the inflationary era. Using a power-law form for the primordial power spectrum $P_{in}(k)$ and constraining the scalar spectral index and its running, \cite{Peiris03} found that the first year WMAP data seem to indicate a preferred scale in $P_{in}(k)$. We use two complementary methods: the wavelet band powers method of \cite{pia1}, and the top-hat binning method of \cite{Wang99} to reconstruct $P_{in}(k)$ as a free function from CMB data alone (WMAP, CBI, and ACBAR), or from CMB data together with large scale structure data (2dFGRS and PCSZ). The shape of the reconstructed $P_{in}(k)$ is consistent with scale-invariance, although it allows some indication of a preferred scale at $k \sim 0.01$Mpc$^{-1}$. While consistent with the possible evidence for a running of the scalar spectral index found by the WMAP team, our results highlight the need of more stringent and independent constraints on cosmological parameters (the Hubble constant in particular) in order to more definitively constrain deviations of $P_{in}(k)$ from scale-invariance without making assumptions about the inflationary model.

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arXiv2003

Future Type Ia Supernova Data as Tests of Dark Energy from Modified Friedmann Equations

In the Cardassian model, dark energy density arises from modifications to the Friedmann equation, which becomes $H^2 = g(\rhom)$, where $g(\rhom)$ is a new function of the energy density. The universe is flat, matter dominated, and accelerating. The distance redshift relation predictions of generalized Cardassian models can be very different from generic quintessence models, and can be differentiated with data from upcoming pencil beam surveys of Type Ia Supernovae such as SNAP. We have found the interesting result that, once $Ω_m$ is known to 10% accuracy, SNAP will be able to determine the sign of the time dependence of the dark energy density. Knowledge of this sign (which is related to the weak energy condition) will provide a first discrimination between various cosmological models that fit the current observational data (cosmological constant, quintessence, Cardassian expansion). Further, we have performed Monte Carlo simulations to illustrate how well one can reproduce the form of the dark energy density with SNAP. To be concrete we study a class of two parameter ($n$,$q$) generalized Cardassian models that includes the original Cardassian model (parametrized by $n$ only) as a special case. Examples are given of MP Cardassian models that fit current supernovae and CMB data, and prospects for differentiating between MP Cardassian and other models in future data are discussed. We also note that some Cardassian models can satisfy the weak energy condition $w>-1$ even with a dark energy component that has an effective equation of state $w_X < -1$.

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arXiv2003

Wavelet Band Powers of the Primordial Power Spectrum from CMB Data

Measuring the primordial matter power spectrum is our primary means of probing unknown physics in the very early universe. We allow the primordial power spectrum to be an arbitrary function, and parametrize it in terms of wavelet band powers. Current cosmological data correspond to 11 such wavelet bands. We derive constraints on these band powers as well as $H_0$, $Ω_b h^2$ and $Ω_m h^2$ from current Cosmic Microwave Background Anisotropy (CMB) data using the Markov Chain Monte Carlo (MCMC) technique. Our results indicate a feature in the primordial power spectrum at $ 0.008 \la k/(h {Mpc}^{-1}) \la 0.1$. MAP and Planck data should allow us to put tighter constraints on the primordial power spectrum.

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arXiv2003

Dark energy effects on the Lyman-alpha forest

In quintessence models, the dark energy content of the universe is described by a slowly rolling scalar field whose pressure and energy density obey an equation of state of the form p=w $ρ$; w is in general a function of time such that w<-1/3, in order to drive the observed acceleration of the Universe today. The cosmological constant model LCDM corresponds to the limiting case w=-1. In this paper, we explore the prospects of using the Lyman-alpha forest to constrain w, using semi-analytical techniques to model the intergalactic medium (IGM). A different value of w changes both the growth factor and the Hubble parameter as a function of time. The resulting change in the optical depth distribution affects the optical depth power spectrum, the number of regions of high transmission per unit redshift and the cross-correlation coefficient of spectra of quasar pairs. These can be detected in current data, provided we have independent estimates of the thermal state of the IGM, its ionization parameter and the baryon density.

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arXiv2002

How Sensitive Are Weak Lensing Statistics to Dark Energy Content?

Future weak lensing surveys will directly probe the clustering of dark matter, in addition to providing a test for various cosmological models. Recent studies have provided us with the tools which can be used to construct the complete probability distribution function for convergence fields. It is also possible to construct the bias associated with the hot-spots in convergence maps. These techniques can be used in both the quasi-linear and the highly nonlinear regimes using various well developed numerical methods. We use these results here to study the weak lensing statistics of cosmological models with dark energy. We study how well various classes of dark energy models can be distinguished from models with a cosmological constant. We find that the ratio of the square root of the variance of convergence is complementary to the convergence skewness $S_3$ in probing dark energy equation of state; it can be used to predict the expected difference in weak lensing statistics between various dark energy models, and for choosing optimized smoothing angles to constrain a given class of dark energy models. Our results should be useful for probing dark energy using future weak lensing data with high statistics from galaxy weak lensing surveys and supernova pencil beam surveys.

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arXiv2002

A universal probability distribution function for weak-lensing amplification

We present an approximate form for the weak lensing magnification distribution of standard candles, valid for all cosmological models, with arbitrary matter distributions, over all redshifts. Our results are based on a universal probability distribution function (UPDF), $P(η)$, for the reduced convergence, $η$. For a given cosmological model, the magnification probability distribution, $P(μ)$, at redshift $z$ is related to the UPDF by $P(μ)=P(η)/2|κ_{min}|$, where $η=1+(μ-1)/(2|κ_{min}|)$, and $κ_{min}$ (the minimum convergence) can be directly computed from the cosmological parameters ($Ω_m$ and $Ω_Λ$). We show that the UPDF can be well approximated by a three-parameter stretched Gaussian distribution, where the values of the three parameters depend only on $ξ_η$, the variance of $η$. In short, all possible weak lensing probability distributions can be well approximated by a one-parameter family. We establish this family, normalizing to the numerical ray-shooting results for a $Λ$CDM model by Wambsganss et al. (1997). Each alternative cosmological model is then described by a single function $ξ_η(z)$. We find that this method gives $P(μ)$ in excellent agreement with numerical ray-tracing and three-dimensional shear matrix calculations, and provide numerical fits for three representative models (SCDM, $Λ$CDM, and OCDM). Our results provide an easy, accurate, and efficient method to calculate the weak lensing magnification distribution of standard candles, and should be useful in the analysis of future high-redshift supernova data.

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arXiv2002

Model Independent Primordial Power Spectrum from Maxima, Boomerang, and DASI Data

A model-independent determination of the primordial power spectrum of matter density fluctuations could uniquely probe physics of the very early universe, and provide powerful constraints on inflationary models. We parametrize the primordial power spectrum $A_s^2(k)$ as an arbitrary function, and deduce its binned amplitude from the cosmic microwave background radiation anisotropy (CMB) measurements of Maxima, Boomerang, and DASI. We find that for a flat universe with $A_s^2(k)=1$ (scale-invariant) for scales $k<0.001 $h/Mpc, the primordial power spectrum is marginally consistent with a scale-invariant Harrison-Zeldovich spectrum. However, we deduce a rise in power compared to a scale-invariant power spectrum for $0.001 h/{Mpc} \la k \la 0.01 h/{Mpc}$. Our results are consistent with large-scale structure data, and seem to suggest that the current observational data allow for the possibility of unusual physics in the very early universe.

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arXiv2001

Unbiased Estimate of Dark Energy Density from Type Ia Supernova Data

Type Ia supernovae (SNe Ia) are currently the best probes of the dark energy in the universe. To constrain the nature of dark energy in a model-independent manner, we allow the density of dark energy, $ρ_X(z)$, to be an arbitrary function of redshift. Using simulated data from a space-based supernova pencil beam survey, we find that by optimizing the number of parameters used to parametrize the dimensionless dark energy density, $f(z)=ρ_X(z)/ρ_X(z=0)$, we can obtain an unbiased estimate of both f(z) and $Ω_m$ (assuming a flat universe and that the weak energy condition is satisfied). A plausible supernova pencil beam survey (with a square degree field of view and for an observational duration of one year) can yield about 2000 SNe Ia with $0\le z \le 2$. Such a survey in space would yield SN peak luminosities with a combined intrinsic and observational dispersion of $σ(m_{int})=0.16$ mag. We find that for such an idealized survey, $Ω_m$ can be measured to 10% accuracy, and f(z) can be estimated to $\sim$ 20% to $z \sim 1.5$, and $\sim$ 20-40% to $z \sim 2$, depending on the time dependence of the true dark energy density. Dark energy densities which vary more slowly can be more accurately measured. For the anticipated SNAP mission, $Ω_m$ can be measured to 14% accuracy, and f(z) can be estimated to $\sim$ 20% to $z \sim 1.2$. Our results suggest that SNAP may gain much sensitivity to the time-dependence of f(z) and $Ω_m$ by devoting more observational time to the central pencil beam fields to obtain more SNe Ia at z>1.2. We also find that Monte Carlo analysis gives a more accurate estimate of the dark energy density than the maximum likelihood analysis. (abridged)

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arXiv2001

Prospects for Constraining Cosmology with the Extragalactic Cosmic Microwave Background Temperature

Observers have demonstrated that it is now feasible to measure the cosmic microwave background (CMB) temperature at high redshifts. We explore the possible constraints on cosmology which might ultimately be derived from such measurements. Besides providing a consistency check on standard and alternative cosmologies, possibilities include: constraints on the inhomogeneity and anisotropy of the universe at intermediate redshift $z ^<_\sim 10$; an independent probe of peculiar motions with respect to the Hubble flow; and constraining the epoch of reionization. We argue that the best possibility is as a probe of peculiar motions. We show, however, that the current measurement uncertainty ($ΔT= \pm 0.002 $K) in the local present absolute CMB temperature imposes intrinsic limits on the use of such CMB temperature measurements as a cosmological probe. At best, anisotropies at intermediate redshift could only be constrained at a level of $^>_\sim 0.1%$ and peculiar motions could only be determined to an uncertainty of $^>_\sim 311$ km s$^{-1}$. If the high $z$ CMB temperature can only be measured with a precision comparable to the uncertainty of the local interstellar CMB temperature, then peculiar motions could be determined to an uncertainty of $1101 (1+z)^{-1} [ΔT_{CMB}(z)/0.01 K] km s^{-1}$.

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arXiv2001

Constraints on extra dimensions from cosmological and terrestrial measurements

If quantum fields exist in extra compact dimensions, they will give rise to a quantum vacuum or Casimir energy. That vacuum energy will manifest itself as a cosmological constant. The fact that supernova and cosmic microwave background data indicate that the cosmological constant is of the same order as the critical mass density to close the universe supplies a lower bound on the size of the extra dimensions. Recent laboratory constraints on deviations from Newton's law place an upper limit. The allowed region is so small as to suggest that either extra compact dimensions do not exist, or their properties are about to be tightly constrained by experimental data.

hep-ph↗

arXiv2001

Measuring Time-Dependence of Dark Energy Density from Type Ia Supernova Data

Observations of high redshift supernovae imply an accelerating Universe which can only be explained by an unusual energy component such as vacuum energy or quintessence. To assess the ability of current and future supernova data to constrain the properties of the dark energy, we allow its density to have arbitrary time-dependence, $ρ_X(z)$. This leads to an equation of state for the dark energy, $w_X(z)=p_X(z)/ρ_X(z)$, which is a free function of redshift $z$. We find that current type Ia supernova (SNe Ia) data are consistent with a cosmological constant, with large uncertainties at $z\ga 0.5$. We show that $ρ_X(z)/ρ_X(z=0)$ can be measured reasonably well to about $z=1.5$ using type Ia supernova data from realistic future SN Ia pencil beam surveys, provided that the weak energy condition (energy density of matter is nonnegative for any observer) is imposed. While it is only possible to differentiate between different models (say, quintessence and k-essence) at $z \la 1.5$ using realistic data, the correct trend in the time-dependence of the dark energy density can be clearly detected out to $z=2$, even in the presence of plausible systematic effects. This would allow us to determine whether the dark energy is a cosmological constant, or some exotic form of energy with a time-dependent density.

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arXiv2000

Self-lensing of a Singular Isothermal Sphere

Many astrophysical systems can be approximated as isothermal spheres. In an isothermal sphere, the ``foreground'' objects can act as lenses on ``background'' objects in the same distribution. We study gravitational lensing by a singular isothermal sphere analytically. Our results may have interesting applications.

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arXiv2000

Flux-averaging Analysis of Type Ia Supernova Data

Because of flux conservation, flux-averaging {\it justifies} the use of the distance-redshift relation for a smooth universe in the analysis of type Ia supernova (SN Ia) data. We have combined the SN Ia data from the High-$z$ SN Search and the Supernova Cosmology Project, and binned the combined data by flux-averaging in redshift intervals of $Δz=0.05$ and $Δz=0.1$. We find that the unbinned data yield a Hubble constant of $H_0=65\pm 1 $km$ $s$^{-1} $Mpc$^{-1}$ (statistical error only), a matter density fraction of $Ω_m=0.7\pm0.4$, and a vacuum energy density fraction of $Ω_Λ=1.2\pm0.5$. The binned data for $Δz=0.1$ yield $H_0=65\pm 1 $km$ $s$^{-1} $Mpc$^{-1}$ (statistical error only), $Ω_m=0.3\pm0.6$, and $Ω_Λ=0.7\pm0.7$. Our results are not sensitive to the redshift bin size. Flux-averaging leads to less biased estimates of the cosmological parameters by reducing the bias due to systematic effects such as weak lensing. Comparison of the data of 18 SNe Ia published by both groups yields a mean SN Ia peak absolute magnitude of $M_B=-19.33\pm 0.25$. The internal dispersion of each data set is about 0.20 magnitude in the {\it calibrated} SN Ia peak absolute magnitudes. The difference in analysis techniques introduces an additional uncertainty of about 0.15 magnitude. If the SNe Ia peak absolute luminosity changes with redshift due to evolution, our ability to measure the cosmological parameters from SN Ia data will be significantly diminished.

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arXiv1999

Supernova pencil beam survey

Type Ia supernovae (SNe Ia) can be calibrated to be good standard candles at cosmological distances. We propose a supernova pencil beam survey that could yield between dozens to hundreds of SNe Ia in redshift bins of 0.1 up to $z=1.5$, which would compliment space based SN searches, and enable the proper consideration of the systematic uncertainties of SNe Ia as standard candles, in particular, luminosity evolution and gravitational lensing. We simulate SNe Ia luminosities by adding weak lensing noise (using empirical fitting formulae) and scatter in SN Ia absolute magnitudes to standard candles placed at random redshifts. We show that flux-averaging is powerful in reducing the combined noise due to gravitational lensing and scatter in SN Ia absolute magnitudes. The SN number count is not sensitive to matter distribution in the universe; it can be used to test models of cosmology or to measure the SN rate. The SN pencil beam survey can yield a wealth of data which should enable accurate determination of the cosmological parameters and the SN rate, and provide valuable information on the formation and evolution of galaxies. The SN pencil beam survey can be accomplished on a dedicated 4 meter telescope with a square degree field of view. This telescope can be used to conduct other important observational projects compatible with the SN pencil beam survey, such as QSOs, Kuiper belt objects, and in particular, weak lensing measurements of field galaxies, and the search for gamma-ray burst afterglows.

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