* Astronomy

Blobrana · L

RE: Standard Candles

Blobrana · L

Cosmology Standard Candle Not So Standard After All

Astronomers have turned up the first direct proof that "standard candles" used to illuminate the size of the universe, termed Cepheids, shrink in mass, making them not quite as standard as once thought. The findings, made with NASA's Spitzer Space Telescope, will help astronomers make even more precise measurements of the size, age and expansion rate of our universe.
Standard candles are astronomical objects that make up the rungs of the so-called cosmic distance ladder, a tool for measuring the distances to farther and farther galaxies. The ladder's first rung consists of pulsating stars called Cepheid variables, or Cepheids for short. Measurements of the distances to these stars from Earth are critical in making precise measurements of even more distant objects. Each rung on the ladder depends on the previous one, so without accurate Cepheid measurements, the whole cosmic distance ladder would come unhinged.
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Blobrana · L

Title: Reducing distance errors for standard candles and standard sirens with weak-lensing shear and flexion maps
Authors: Stefan Hilbert, Jonathan R. Gair, Lindsay J. King

Gravitational lensing induces significant errors in the measured distances to high-redshift standard candles and standard sirens such as type-Ia supernovae, gamma-ray bursts, and merging supermassive black hole binaries. There will therefore be a significant benefit from correcting for the lensing error by using independent and accurate estimates of the lensing magnification. We investigate how accurately the magnification can be inferred from convergence maps reconstructed from galaxy shear and flexion data. We employ ray-tracing through the Millennium Simulation to simulate lensing observations in large fields, and perform a weak-lensing reconstruction on these fields. We identify optimal ways to filter the reconstructed convergence maps and to convert them to magnification maps. We find that a shear survey with 100 galaxies/arcmin² can help to reduce the lensing-induced distance errors for standard candles/sirens at redshifts z=1.5 (z=5) on average by 20% (10%), whereas a futuristic survey with shear and flexion estimates from 500 galaxies/arcmin² yields much larger reductions of 50% (35%). For redshifts z>=3, a further improvement by 5% can be achieved, if the individual redshifts of the galaxies are used in the reconstruction. Moreover, the reconstruction allows one to identify regions for which the convergence is low, and in which an error reduction by up to 75% can be achieved.

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Blobrana · L

Tip of the red giant branch (TRGB) is a primary distance indicator used in astronomy. It uses the luminosity of the brightest red giant branch stars in a galaxy to gauge the distance to that galaxy.

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Blobrana · L

"These are galaxies that might contain as few as a thousand stars, and those stars are being pulled out into the halo of our Milky Way" - UA astronomer Ed Olszewski.

UA astronomy professor Jill Bechtold and MMTO astronomer Tim Pickering are also on the project.

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Blobrana · L

Cosmology's Best Standard Candles Get Even Better
Members of the international Nearby Supernova Factory (SNfactory), a collaboration among the U.S. Department of Energy's Lawrence Berkeley National Laboratory, a consortium of French laboratories, and Yale University, have found a new technique that establishes the intrinsic brightness of Type Ia supernovae more accurately than ever before. These exploding stars are the best standard candles for measuring cosmic distances, the tools that made the discovery of dark energy possible.
SNfactory member Stephen Bailey, formerly at Berkeley Lab and now at the Laboratory of Nuclear and High-Energy Physics (LPNHE) in Paris, France, searched the spectra of 58 Type Ia supernovae in the SNfactory's dataset and found a key spectroscopic ratio. Simply by measuring the ratio of the flux (visible power, or brightness) between two specific regions in the spectrum of a Type Ia supernova taken on a single night, that supernova's distance can be determined to better than 6 percent uncertainty.
The new brightness-ratio correction appears to hold no matter what the supernova's age or metallicity (mix of elements), its type of host galaxy, or how much it has been dimmed by intervening dust.

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