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Measuring Dark Energy with Gamma-Ray Bursts

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

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  1. Measuring Dark Energy with Gamma-Ray Bursts Fayin Wang and Z. G. Dai Astronomy Department of Nanjing University 2008 Nanjing GRB Conference

  2. 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

  3. 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

  4. 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

  5. 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

  6. Outline (for topic 3) • Introduction • Measuring dark energy and cosmic expansion • Conclusion 2008 Nanjing GRB Conference

  7. Accelerating Universe Riess et al. (1998): 50 SNe Ia Perlmutter et al. (1999): 42 high-z SNe Ia 2008 Nanjing GRB Conference

  8. WMAP: the best-fit cosmological model • m=0.270.04 • =0.730.04 • T=1.020.02 2008 Nanjing GRB Conference

  9. 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

  10. 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

  11. 2008 Nanjing GRB Conference

  12. CDM w=w0 Dai, Liang & Xu (2004) 2008 Nanjing GRB Conference

  13. Ghirlanda et al. (2004b) 2008 Nanjing GRB Conference

  14. Liang-Zhang Relation (Liang & Zhang 2005 ApJ, 633, 611) Optical time break 2008 Nanjing GRB Conference

  15. Wang & Dai (2006, MNRAS, 368, 371): w=-1 (left); w=w0 (right) 2008 Nanjing GRB Conference

  16. Schaefer (2007) 2008 Nanjing GRB Conference

  17. 2008 Nanjing GRB Conference

  18. Wang, Dai & Zhu (2007, ApJ, 667, 1) 2008 Nanjing GRB Conference

  19. W(z)=w0 model 2008 Nanjing GRB Conference

  20. w(z)=w0+w1z/(1+z) Conclusion: these models are consistent with theΛCDM model. 2008 Nanjing GRB Conference

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

  26. Thank you! 2008 Nanjing GRB Conference

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