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Measuring Dark Energy with Gamma-Ray Bursts. Fayin Wang and Z. G. Dai Astronomy Department of Nanjing University. Gamma-Ray Burst Cosmology. High-redshift star-formation rate High-redshift intergalactic medium (reionization) Cosmic expansion and dark energy.
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Measuring Dark Energy with Gamma-Ray Bursts Fayin Wang and Z. G. Dai Astronomy Department of Nanjing University 2008 Nanjing GRB Conference
Gamma-Ray Burst Cosmology • High-redshift star-formation rate • High-redshift intergalactic medium(reionization) • Cosmic expansion and dark energy Studies on cosmic structure, evolution, and dark energy with gamma-ray bursts 2008 Nanjing GRB Conference
High-z star-formation rate • SFR results derived from GRBs, particularly the z = 5−7 point, are higher than SFR results obtained with more conventional methods. (Yuksel et al. arXiv:0804.4008) • The examination of the Swift data reveals that GRB rate is nottracing the star formation history directly, instead implying some kind of additional evolution. 2008 Nanjing GRB Conference
Bromm & Loeb (2006): a fraction, ~10% (or ~1%), of all bursts detected by Swift will originate at z>5 (or z>10). 2008 Nanjing GRB Conference
Two advantages of GRBs over the traditional cosmic sources such as quasars (for topics 1 & 2) • The GRB afterglow flux at a given observed time is not expected to fade significantly with increasing redshift. • GRB afterglows have smooth (broken power-law) continuum spectra. Bromm & Loeb (2007) arXiv:0706.2445v2 2008 Nanjing GRB Conference
Outline (for topic 3) • Introduction • Measuring dark energy and cosmic expansion • Conclusion 2008 Nanjing GRB Conference
Accelerating Universe Riess et al. (1998): 50 SNe Ia Perlmutter et al. (1999): 42 high-z SNe Ia 2008 Nanjing GRB Conference
WMAP: the best-fit cosmological model • m=0.270.04 • =0.730.04 • T=1.020.02 2008 Nanjing GRB Conference
Measuring CosmologyAdvantages of GRBs relative to SNeIa 1. GRBs can occur at very high redshifts (up to z~25) and thus could be more helpful in measuring changes in the slope of the Hubble diagram than SNe Ia. 2. Gamma rays are free from dust extinction, so the observed gamma-ray flux should be a direct measurement of the prompt emission energy. So, GRBs are an attractive probe of the universe. 2008 Nanjing GRB Conference
ΩM=0.27 ΩΛ=0.73 Ghirlanda et al. (2004a); Dai, Liang & Xu (2004): a tight correlation with a slope of ~1.5 and a reduced \chi2~0.53. 2008 Nanjing GRB Conference
CDM w=w0 Dai, Liang & Xu (2004) 2008 Nanjing GRB Conference
Ghirlanda et al. (2004b) 2008 Nanjing GRB Conference
Liang-Zhang Relation (Liang & Zhang 2005 ApJ, 633, 611) Optical time break 2008 Nanjing GRB Conference
Wang & Dai (2006, MNRAS, 368, 371): w=-1 (left); w=w0 (right) 2008 Nanjing GRB Conference
Schaefer (2007) 2008 Nanjing GRB Conference
Wang, Dai & Zhu (2007, ApJ, 667, 1) 2008 Nanjing GRB Conference
W(z)=w0 model 2008 Nanjing GRB Conference
w(z)=w0+w1z/(1+z) Conclusion: these models are consistent with theΛCDM model. 2008 Nanjing GRB Conference
Constraining the evolution of dark energy • The behavior of the dark energy equation of state (EOS) is crucial in distinguishing different cosmological models. • We separate the redshifts into 4 bins and assume a constant EOS parameter for dark energy in each bin. 0-0.2, 0.2-0.5, 0.5-1.8, 1.8-6.6 2008 Nanjing GRB Conference
Constraining the evolution of dark energy The evolution of equation of state of dark energy from GRBs and other cosmological probes. Result is consistent with ΛCDM. (Qi, Wang & Lu 2008, A&A, 483, 49) 2008 Nanjing GRB Conference
Cosmography by GRBs • Expanding the Hubble law up to the fourth order in redshift and considering the related luminosity distance • GRBs can constrain cosmographic parameters of the Hubble law at medium-high redshifts. (Capozziello & Izzo arXiv:0806.1120) 2008 Nanjing GRB Conference
The cosmographic parameters are: deceleration, jerk, snap parameters • Using the calibration results of Liang et al. 2008 (see N.Liang’s talk). • We measure q0=-0.84, j0=1.45, s0=3.22 2008 Nanjing GRB Conference
3.Conclusion 1. GRBs may provide a promising probe of the early universe and dark energy. 2. The preferred cosmological model is the flat ΛCDM model from our joint analysis. 3. GRBs may measure the cosmographic parameters. 2008 Nanjing GRB Conference
Thank you! 2008 Nanjing GRB Conference