A Cosmological Ruler

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Science  24 Sep 2004:
Vol. 305, Issue 5692, pp. 1875
DOI: 10.1126/science.305.5692.1875e

Measuring the distance to far- away astronomical objects is difficult. Astronomers use a standard candle, which is an object of a known luminosity, to estimate distance, where the intensity of light from a source of constant luminosity decreases as a function of the square of the distance. Type Ia supernovae are used as standard candles because these thermonuclear explosions of carbon-oxygen white dwarfs are thought to represent a relatively uniform mass and composition. When they explode, they are extremely bright, with a maximum absolute magnitude that decays in a consistent fashion as a function of time, allowing the luminosity to be estimated accurately.

Two papers by Ghirlanda et al. suggest that gamma-ray bursts (GRBs) can be used as standard candles. They found a correlation between the peak energy and the collimation-corrected energy, which removes the scatter in luminosity estimates used to determine distance. Combining 15 GRBs with supernovae data yields a cosmological model consistent with recent cosmic microwave background estimates. GRBs can be measured to a redshift of about 10, whereas the supernovae reach redshifts up to 1 and the CMB is measured at a redshift of about 1000, so GRBs can provide an important link between the deep and shallow universe. — LR

Astrophys.J. astro-ph/0405602; 613, L13 (2004).

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