New Light on Dark Matter
Under this proposal, we have been undertaking a calibration of rate of change of the expansion rate of the Universe as a function of cosmic look-back time using the high-precision standard candles, Type Ia supernovae, as observed in their rest-frame near-infrared wavelengths. The apparent acceleration of the Universe, as discovered earlier using these same types of supernovae, was both unanticipated and extremely profound in its implications. Not only does the acceleration mean that the Universe is unbound, but it also implies the existence of a new constituent of the Universe (so-called 'dark energy') that is many orders of magnitude stronger than what physicists can easily accommodate in their standard theories of particle physics. A result with such wide-ranging and important implications must be checked, and all sources of systematic error and uncertainty must be evaluated and accounted for. At increasingly higher redshifts the objects being observed are seen at earlier cosmic times and the radiation that reaches Earth is shifted to longer and longer wavelengths. What leaves a supernova event at one time in the past as an optical photon is downgraded by cosmic expansion into a red or infrared photon by the time it is detected here. Optical images of distant supernovae seen now, began their lives as ultraviolet photons. The ultraviolet properties of nearby supernovae are not well understood, so comparing supernova across time and space becomes complicated and uncertain. Moreover it is well known that the systematic effects of interstellar dust are larger and more variable from place to place in the ultraviolet than they are at longer wavelengths. To mitigate both the uncertainty of the ultraviolet calibration and the certainty of variable dust extinction along the line of sight, the Carnegie Supernova Program (CSP) has been observing the distant supernovae at groundbased infrared wavelengths that more closely match restframe (emitted) optical wavelengths at the supernova event itself. Not only does this allow us to compare local (calibrating) supernovae with distant supernovae without the uncertainty of shifting between uncertain physical regimes, but it also actively reduces both the impact and the uncertainty of interstellar dust on the apparent magnitudes of the tagert supernovae and their calibrators. Infrared radiation penetrates dust and gas much more efficiently than optical and ultraviolet photons. This has been possible because Carnegie operates very large-aperture telescopes at the Las Campanas Observatory in Chile, equipped with state-of-the-art, wide-field near-infrared detectors capable of detecting and measuring distant supernovae (discovered by collaborating surveys) early in their evolution. With support from the DOE through this grant the Carnegie Supernova Project has observed 70 Type Ia supernovae from Chile obtaining near-infrared light curves which, when combined with the discovery images provide high-quality data on the rest-frame, near-infrared magnitudes of these supernovae at the time of maximum light. The peak luminosity of Type Ia supernovae can then be used to estimate their distances (once corrected for decline rate and residual reddening effects). Those distances when compared to their expansion velocities give us the systematic departures from pure Hubble expansion that lie at the heart of the detection of dark energy in the Universe. A paper summarizing the techniques and methods used by the CSP in measuring high-redshift supernovae is in the final stages of circulating amongst the team members. We expect to submit it to the Astrophysical Journal before the end of 2008. Half of the data on the full sample observed supernovae has been fully reduced for this paper. We already have a new measurement of the dark energy contribution to cosmic acceleration. We find a value of w = -1.05 {+-} 0.08 (statistical) {+-} 0.08 (systematic). This value is consistent with, but completely independent of and has a smaller systematic uncertainty than, other studies to date.
- Research Organization:
- Carnegie Inst. of Washington, Washington, DC (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- FG02-07ER41512
- OSTI ID:
- 944923
- Report Number(s):
- DE-FG02-07ER41512- Final Report; TRN: US1102446
- Country of Publication:
- United States
- Language:
- English
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