1 / 17

Bright High z SnIa: A Challenge for LCDM?

Bright High z SnIa: A Challenge for LCDM?. Based on arXiv:0811.2802 L. Perivolaropoulos and A. Shafieloo. Arman Shafieloo Particle Physics Seminar, 17 th February 09 Oxford Theoretical Physics. Introduction:.

adah
Download Presentation

Bright High z SnIa: A Challenge for LCDM?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Bright High z SnIa: A Challenge for LCDM? Based on arXiv:0811.2802 L. Perivolaropoulos and A. Shafieloo Arman Shafieloo Particle Physics Seminar, 17th February 09 Oxford Theoretical Physics

  2. Introduction: • Current cosmological observations indicate that the universe expands today with acceleration. • The driving agent of this acceleration is what is called “dark energy”, a uniformly distributed component constituting about 70% of total energy density and having negative pressure. • The nature of the “dark-energy” phenomenon remains to be unknown and is one of the biggest puzzles in modern cosmology.

  3. Dark Energy Models • Cosmological constant • Quintessence and k-essence (scalar fields) • Exotic matter (Chaplygin gas, phantom, etc.) • Braneworlds (higher-dimensional theories) • ….. But which one is really responsible for the acceleration of the expanding universe?!

  4. The most direct indication for the current accelerating expansion of the universe comes from the accumulating type Ia supernovae data: Supernovae Ia as Standard Candles Dark Energy Most general form

  5. BUT! The observed luminosity distances of supernovae are not so accurate. To calculate the Hubble parameter and the equation of state of dark energy, we should use the first and second derivatives of this data, which enlarges the errors by huge factors. Even future supernovae data (like SNAP) will not be that accurate to be used directly to calculate these cosmological parameters.

  6. Y. Gong, JCAP 2005

  7. Dealing with observational uncertainties in matter density • Small uncertainties in the value of matter density may affect the reconstruction exercise quiet dramatically. • Hubble parameter is not affected to a very high degree by the value of matter density. • Any uncertainties in matter density is bound to affect the reconstructed w(z).

  8. Sahni, Shafieloo, Starobinsky, PRD 2008

  9. Binned Normalized Difference Statistics (BND) • It is directly applicable on the distance moduli data. • It is a “yes-no” statistic for each model. No comparison with alternative models or parameterizations. • It focuses on specific features of the data with respect with best fit model. • It is insensitive to the uncertainties of the matter or curvature densities.

  10. Method 1. Assume a model. Obtain the best fit parameters of the mode to the data and the corresponding distance moduli. 2. Construct the “error normalized difference” 3. Construct “binned normalized difference” 4. Increase the bin size “N” until “Q(N)” changes sign for the first time. 5. Generate many realization of the data, assuming the model with its best fit parameters as the fiducial model. 6. Repeat the analysis for each realization of the data. 7. Find out the fraction of realizations leading to redshift of crossing less than or equal to the redshift of crossing in the actual case.

  11. Results • Models: Phantom Divide Line (PDL) Lambda Cold Dark Matter (LCDM) • Data: Gold 2006 (182 SnIa) Union 2008 (307 SnIa)

  12. Union08 Data LCDM PDL

  13. Gold data (2006) LCDM PDL 2.2% Consistency 32.1% Consistency

  14. Union data (2008) LCDM PDL 5.3% Consistency Consistent

  15. Consistency at low redshifts Consistent Consistent LCDM PDL Union08 Data Start point: z=0.8

  16. Effect of unknown systematics 12.1% Consistency LCDM Union08 Data Assuming extra systematic errors:

  17. Summary: • According to the BND statistic, Gold06 and Union08 datasets have probability 2.2% and 5.3% to have emerged in the context of LCDM cosmology. • Inconsistency between the data and LCDM model exist at the high redshifts. At low redshifts BND statistic does not show any inconsistency. The tension must be due to data points at high redshifts that seems to be systematically brighter than LCDM predictions. • The inconsistency can be interpreted either as: • More deceleration at high z than expected in the context of LCDM. • Statistical fluctuation. • Systematic effect perhaps due to a mild SnIa evolution at high z. • Our results indicates a potential challenge for LCDM cosmology and provides a motivation for obtaining additional SnIa data at high redshifts z > 1.

More Related