Reconstructing the Equation of State for Dark Energy In the Double Complex Symmetric Gravitational Theory

We propose to study the accelerating expansion of the universe in the double complex symmetric gravitational theory (DCSGT). The universe we live in is taken as the real part of the whole spacetime ${\cal M}^4_C(J)$ which is double complex. By introducing the spatially flat FRW metric, not only the double Friedmann Equations but also the two constraint conditions $p_J=0$ and $J^2=1$ are obtained. Furthermore, using parametric $D_L(z)$ ansatz, we reconstruct the $\omega^{'}(z)$ and $V(\phi)$ for dark energy from real observational data. We find that in the two cases of $J=i,p_J=0$ and $J=\epsilon,p_J\neq 0$, the corresponding equations of state $\omega^{'}(z)$ remain close to -1 at present ($z=0$) and change from below -1 to above -1. The results illustrate that the whole spacetime, i.e. the double complex spacetime ${\cal M}^4_C(J)$, may be either ordinary complex ($J=i,p_J=0$) or hyperbolic complex ($J=\epsilon,p_J\neq 0$). And the fate of the universe would be Big Rip in the future.Comment: 5 pages, 5 figures, accepted by Commun. Theor. Phy

Results from the High-Z Supernova Search Team

We review the use of Type Ia supernovae for cosmological distance determinations. Low-redshift SNe Ia ($z \lesssim 0.1$) demonstrate that (a) the Hubble expansion is linear, (b) $H_0 = 65 \pm 2$ (statistical) km s$^{-1}$ Mpc$^{-1}$, (c) the bulk motion of the Local Group is consistent with the COBE result, and (d) the properties of dust in other galaxies are similar to those of dust in the Milky Way. We find that the light curves of high-redshift SNe Ia are stretched in a manner consistent with the expansion of space; similarly, their spectra exhibit slower temporal evolution (by a factor of $1 + z$) than those of nearby SNe Ia. The luminosity distances of our 16 high-redshift SNe Ia are, on average, 10--15% farther than expected in a low mass-density ($\Omega_M=0.2$) universe without a cosmological constant. Our analysis strongly supports eternally expanding models with positive cosmological constant and a current acceleration of the expansion. We address many potential sources of systematic error; at present, none of them reconciles the data with $\Omega_\Lambda=0$ and $q_0 \geq 0$. The dynamical age of the Universe is estimated to be $14.2 \pm 1.7$ Gyr, consistent with the ages of globular star clusters.Comment: 17 pages, latex, plus 2 figures, to appear in the Proceedings of the 3rd International Symposium on Sources and Detection of Dark Matter in the Universe (DM98), Feb. 1998, ed. D. Clin

A Redetermination of the Hubble Constant with the Hubble Space Telescope from a Differential Distance Ladder

We report observations of 240 Cepheid variables obtained with the Near Infrared Camera (NICMOS) through the F160W filter on the Hubble Space Telescope (HST). The Cepheids are distributed across six recent hosts of Type Ia supernovae (SNe Ia) and the "maser galaxy" NGC 4258, allowing us to directly calibrate the peak luminosities of the SNe Ia from the precise, geometric distance measurements provided by the masers. New features of our measurement include the use of the same instrument for all Cepheid measurements across the distance ladder and homogeneity of the Cepheid periods and metallicities thus necessitating only a differential measurement of Cepheid fluxes and reducing the largest systematic uncertainties in the determination of the fiducial SN Ia luminosity. The NICMOS measurements reduce differential extinction in the host galaxies by a factor of 5 over past optical data. Combined with an expanded of 240 SNe Ia at z<0.1 which define their magnitude-redshift relation, we find H_0=74.2 +/-3.6, a 4.8% uncertainty including both statistical and systematic errors. We show that the factor of 2.2 improvement in the precision of H_0 is a significant aid to the determination of the equation-of-state of dark energy, w = P/(rho c^2). Combined with the WMAP 5-year measurement of Omega_M h^2, we find w= -1.12 +/- 0.12 independent of high-redshift SNe Ia or baryon acoustic oscillations (BAO). This result is also consistent with analyses based on the combination of high-z SNe Ia and BAO. The constraints on w(z) now with high-z SNe Ia and BAO are consistent with a cosmological constant and improved by a factor of 3 from the refinement in H_0 alone. We show future improvements in H_0 are likely and will further contribute to multi-technique studies of dark energy.Comment: 60 pages, 15 figures Accepted for Publication, ApJ. This is the second of two papers reporting results from a program to determine the Hubble constant to 5% precision from a refurbished distance ladder based on extensive use of differential measurement

Scaling attractors for quintessence in flat universe with cosmological term

For evolution of flat universe, we classify late time and future attractors with scaling behavior of scalar field quintessence in the case of potential, which, at definite values of its parameters and initial data, corresponds to exact scaling in the presence of cosmological constant.Comment: 11 pages, 16 eps-figures, revtex4, reference with comment adde

Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae

The property of dark energy and the physical reason for acceleration of the present universe are two of the most difficult problems in modern cosmology. The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-Ia supernova (SNe Ia) and anisotropy of cosmic microwave background (CMB).The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift, implying the existence of a nearly uniform component of dark energy with negative pressure. We use the ``gold'' sample containing 157 SNe Ia and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift. For a flat universe with the cosmological constant, we measure $\Omega_{M}=0.28_{-0.05}^{+0.04}$, which is consistent with Riess et al. The transition redshift is $z_{T}=0.60_{-0.08}^{+0.06}$. We also discuss several dark energy models that define the $w(z)$ of the parameterized equation of state of dark energy including one parameter and two parameters ($w(z)$ being the ratio of the pressure to energy density). Our calculations show that the accurately calculated transition redshift varies from $z_{T}=0.29_{-0.06}^{+0.07}$ to $z_{T}=0.60_{-0.08}^{+0.06}$ across these models. We also calculate the minimum redshift $z_{c}$ at which the current observations need the universe to accelerate.Comment: 16 pages, 5 figures, 1 tabl

Comment on "Constraining the smoothness parameter and dark energy using observational H(z) data"

In this Comment we discuss a recent analysis by Yu et al. [RAA 11, 125 (2011)] about constraints on the smoothness $\alpha$ parameter and dark energy models using observational $H(z)$ data. It is argued here that their procedure is conceptually inconsistent with the basic assumptions underlying the adopted Dyer-Roeder approach. In order to properly quantify the influence of the $H(z)$ data on the smoothness $\alpha$ parameter, a $\chi^2$-test involving a sample of SNe Ia and $H(z)$ data in the context of a flat $\Lambda$CDM model is reanalyzed. This result is confronted with an earlier approach discussed by Santos et al. (2008) without $H(z)$ data. In the ($\Omega_m, \alpha$) plane, it is found that such parameters are now restricted on the intervals $0.66 \leq \alpha \leq 1.0$ and $0.27 \leq \Omega_m \leq 0.37$ within 95.4% confidence level (2$\sigma$), and, therefore, fully compatible with the homogeneous case. The basic conclusion is that a joint analysis involving $H(z)$ data can indirectly improve our knowledge about the influence of the inhomogeneities. However, this happens only because the $H(z)$ data provide tighter constraints on the matter density parameter $\Omega_m$.Comment: 3 pages, 1 figure, submitted to Research in Astronomy and Astrophysic

Simple Model of Propagating Flame Pulsations

A simple model which exhibits dynamical flame properties in 1D is presented. It is investigated analytically and numerically. The results are applicable to problems of flame propagation in supernovae Ia.Comment: 10 pages, 8 figures, revised version accepted by MNRA

A Preliminary Indication of Evolution of Type Ia Supernovae from their Risetimes

We have compared the risetime for samples of nearby and high-redshift type Ia supernovae (SNe Ia). The fiducial risetime of the nearby SNe Ia is 2.5+/-0.4 days longer than the proemial risetime determined by Goldhaber (1998a,b) for high-redshift SNe Ia from the Supernova Cosmology Project. The statistical likelihood that the two samples have different fiducial risetimes is high (5.8 sigma) and indicates possible evolution between the samples of SNe Ia. We consider the likely effects of several sources of systematic error, but none of these resolves the difference in the risetimes. Currently, we cannot directly determine the impact of the apparent evolution on previous determinations of cosmological parameters.Comment: Accepted by the Astronomical Journal, 11 pages, 5 figure