Bulletin of the American Physical Society
2005 APS April Meeting
Saturday–Tuesday, April 16–19, 2005; Tampa, FL
Session U3: Dark Energy I |
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Sponsoring Units: DAP DPF Chair: Wendy Freedman, Carnegie Observatories Room: Marriott Tampa Waterside Grand Salon A/B |
Monday, April 18, 2005 3:30PM - 4:06PM |
U3.00001: Dark Energy: Measurement by the DEEP2 Redshift Survey Invited Speaker: The DEEP2 redshift survey has now covered $\sim$2.5 degrees$^{2}$ of sky and obtained nearly 40,000 spectra; the survey is nearly finished, and I shall describe what has been accomplished with all that Keck time! One of our fields is the Extended Groth Strip (EGS), a region where deep imaging is being obtained with Chandra, Spitzer, GALEX, VLA, and HST/ACS and will be the subject of Sunyaev-Zel'dovich observations. We will eventually provide 17,000 redshifts. In three other regions, we have used three-color imaging to efficiently select galaxies with magnitude R$_{AB}$$<$24.1 and redshifts in the range 0.7 $<$z$<$1.4. The EGS pointing does not have the preselection. We describe here one method by which DEEP2 can set constraints on the equation of state parameter of the Dark Energy, w. By counting the number of virialized groups and clusters we find in redshift space as a function of their redshift and internal velocity dispersion, we probe both the volume element and the growth of structure at z$\sim$1, each of which depends on w. We find 320 groups in the volume, and show how it measures w, but also depends on the bias in the velocity field of galaxies in clusters, b$_{v}$. Studies of this effect are underway. [Preview Abstract] |
Monday, April 18, 2005 4:06PM - 4:42PM |
U3.00002: Dark Energy and Type Ia Supernovae: Present and Future Invited Speaker: Since the pioneering work of Baade and Zwicky in the 1930s, astronomers have been aware of the possibility of using Type~Ia supernovae to probe the expansion of the universe. However, only in the last 15 years has this potential been fully realized. After briefly recounting the discovery and calibration of the peak luminosity vs. decline rate relation for Type~Ia supernovae which has allowed distance measurements to host galaxies to be made with a precision of 10\% or better, I review recent results from high-redshift observations which confirm that the universe is currently being accelerated by a mysterious dark energy which comprises approximately 70\% of the present energy density of the universe. Current research is focussed on measuring the equation of state parameter `w' of the dark energy to determine if it is consistent with a cosmological constant (w = -1). This effort is reviewed, along with the observational problems which must be overcome to achieve this objective. Finally, the potential of future ground- and space-based programs for probing the nature of the dark energy is discussed. [Preview Abstract] |
Monday, April 18, 2005 4:42PM - 5:18PM |
U3.00003: Type Ia Supernovae: Explosion Models versus Observational Constraints Invited Speaker: To have confidence in using Type~Ia supernovae (SNe~Ia) to determine the expansion history of the universe, and thereby probe the nature of the dark energy, we must advance our understanding of SN~Ia physics. In the standard model a carbon--oxygen white dwarf accretes matter from a companion star, approaches the Chandrasekhar mass, ignites carbon fusion, encounters a thermonuclear instability, and explodes completely. The final kinetic energy of the ejected matter is the energy released by fusion minus the white--dwarf binding energy. The kinetic energy inferred from observations indicates that practically the whole white dwarf undergoes fusion. The peak luminosity depends on the mass of freshly synthesized $^{56}$Ni, which provides a delayed release of energy while decaying through $^{56}$Co to stable $^{56}$Fe. The observed SN~Ia luminosity requires that nearly half of the mass is synthesized to $^{56}$Ni. Spectroscopic observations indicate that the composition structure of the ejected matter is radially stratified, with a core of iron--group elements surrounded by lighter elements such as calcium, silicon, and oxygen. Spherically symmetric (1D) nuclear-hydrodynamical explosion models that meet these requirements have been calculated, by parameterizing the velocity of the burning front. In recent years more self--consistent 3D models have been calculated. Deflagration models, in which the burning front remains subsonic, undergo insufficient fusion and lack the stratified composition structure. Delayed--detonation models, which invoke a transition to supersonic front propagation, fare better, although it is not known whether the transition really can occur. I will discuss the status of explosion models versus observational constraints (mostly spectroscopic), and the challenging task of relating the various observational manifestations of SN~Ia diversity to their physical causes. [Preview Abstract] |
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