Munch: Monday, February 26, 2007

                               


 

 WHERE: 6TH FLOOR CONFERENCE ROOM
WHEN  : 12:30, MONDAY

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


FAQ

What is Munch?
       Munch Archive

The ESSENCE Supernova Survey: Survey Optimization, Observations, and Supernova Photometry  astro-ph/0701043  (suggested by Gajus, after considerable hints and nudges from Josh)

Authors: G. Miknaitis, G. Pignata, A. Rest, W. M. Wood-Vasey, S. Blondin, P. Challis, R. C. Smith, C. W. Stubbs, N. B. Suntzeff, R. J. Foley, T. Matheson, J. L. Tonry, C. Aguilera, J. W. Blackman, A. C. Becker, A. Clocchiatti, R. Covarrubias, T. M. Davis, A. V. Filippenko, A. Garg, P. M. Garnavich, M. Hicken, S. Jha, K. Krisciunas, R. P. Kirshner, B. Leibundgut, W. Li, A. Miceli, G. Narayan, J. L. Prieto, A. G. Riess, M. E. Salvo, B. P. Schmidt, J. Sollerman, J. Spyromilio, A. Zenteno
Comments: Submitted to ApJ. Companion paper to Wood-Vasey et al (2007). Electronic tables available at this http URL
We describe the implementation and optimization of the ESSENCE supernova survey, which we have undertaken to measure the equation of state parameter of the dark energy. We present a method for optimizing the survey exposure times and cadence to maximize our sensitivity to the dark energy equation of state parameter w=P/rho c^2 for a given fixed amount of telescope time. For our survey on the CTIO 4m telescope, measuring the luminosity distances and redshifts for supernovae at modest redshifts (z~0.5 +- 0.2) is optimal for determining w. We describe the data analysis pipeline based on using reliable and robust image subtraction to find supernovae automatically and in near real-time. Since making cosmological inferences with supernovae relies crucially on accurate measurement of their brightnesses, we describe our efforts to establish a thorough calibration of the CTIO 4m natural photometric system. In its first four years, ESSENCE has discovered and spectroscopically confirmed 102 type Ia SNe, at redshifts from 0.10 to 0.78, identified through an impartial, effective methodology for spectroscopic classification and redshift determination. We present the resulting light curves for the all type Ia supernovae found by ESSENCE and used in our measurement of w, presented in Wood-Vasey et al, 2007.

Full-text: PostScript, PDF, or Other formats

Observational Constraints on the Nature of the Dark Energy: First Cosmological Results from the ESSENCE Supernova Survey  astro-ph/0701041  (refer to previous)

Authors: W. M. Wood-Vasey, G. Miknaitis, C. W. Stubbs, S. Jha, A. G. Riess, P. M. Garnavich, R. P. Kirshner, C. Aguilera, A. C. Becker, J. W. Blackman, S. Blondin, P. Challis, A. Clocchiatti, A. Conley, R. Covarrubias, T. M. Davis, A. V. Filippenko, R. J. Foley, A. Garg, M. Hicken, K. Krisciunas, B. Leibundgut, W. Li, T. Matheson, A. Miceli, G. Narayan, G. Pignata, J. L. Prieto, A. Rest, M. E. Salvo, B. P. Schmidt, R. C. Smith, J. Sollerman, J. Spyromilio, J. L. Tonry, N. B. Suntzeff, A. Zenteno
Comments: Submitted to ApJ. 82 pages. 9 figures. 10 tables. Companion paper to Miknaitis et al (2007). Electronic tables and probability surfaces available at this http URL
We present constraints on the dark energy equation-of-state parameter, w=P/(rho c^2), using 60 Type Ia supernovae (SNe Ia) from the ESSENCE supernova survey. We derive a set of constraints on the nature of the dark energy assuming a flat Universe. By including constraints on (Omega_M, w) from baryon acoustic oscillations, we obtain a value for a static equation-of-state parameter w=-1.05^{+0.13}_{-0.12} (stat; 1 sigma) +- 0.11 (sys) and Omega_M=0.274^{+0.033}_{-0.020} (stat; 1 sigma) with a best-fit chi^2/DoF of 0.96. These results are consistent with those reported by the SuperNova Legacy Survey in a similar program measuring supernova distances and redshifts. We evaluate sources of systematic error that afflict supernova observations and present Monte Carlo simulations that explore these effects. Currently, the largest systematic currently with the potential to affect our measurements is the treatment of extinction due to dust in the supernova host galaxies. Combining our set of ESSENCE SNe Ia with the SuperNova Legacy Survey SNe Ia, we obtain a joint constraint of w=-1.07^{+0.09}_{-0.09} (stat; 1 sigma) +- 0.12 (sys), Omega_M=0.267^{+0.028}_{-0.018} (stat; 1 sigma) with a best-fit chi^2/DoF of 0.91. The current SN Ia data are fully consistent with a cosmological constant.

Full-text: PostScript, PDF, or Other formats

Scrutinizing Exotic Cosmological Models Using ESSENCE Supernova Data Combined with Other Cosmological Probes  astro-ph/0701510  (ditto)

Authors: T. M. Davis, E. Mortsell, J. Sollerman, A. C. Becker, S. Blondin, P. Challis, A. Clocchiatti, A. V. Filippenko, R. J. Foley, P. M. Garnavich, S. Jha, K. Krisciunas, R. P. Kirshner, B. Leibundgut, W. Li, T. Matheson, G. Miknaitis, G. Pignata, A. Rest, A. G. Riess, B. P. Schmidt, R. C. Smith, J. Spyromilio, C. W. Stubbs, N. B. Suntzeff, J. L. Tonry, W. M. Wood-Vasey
Comments: Submitted to ApJ. 9 pages, 7 figures
The first cosmological results from the ESSENCE supernova survey (Wood-Vasey et al. 2007) are extended to a wider range of cosmological models including dynamical dark energy and non-standard cosmological models. We fold in a greater number of external data sets such as the recent Higher-z release of high-redshift supernovae (Riess et al. 2007) as well as several complementary cosmological probes. Model comparison statistics such as the Bayesian and Akaike information criteria are applied to gauge the worth of models. These statistics favor models that give a good fit with fewer parameters.
Based on this analysis, the preferred cosmological model is the flat cosmological constant model, where the expansion history of the universe can be adequately described with only one free parameter describing the energy content of the universe. Amongst the more exotic models that provide good fits to the data, we note a preference for models whose best-fit parameters reduce them to the cosmological constant model.

Full-text: PostScript, PDF, or Other formats

Weak Lensing of Baryon Acoustic Oscillations  astro-ph/0702606

Authors: Alberto Vallinotto, Scott Dodelson, Carlo Schimd, Jean-Philippe Uzan
Comments: 10 pages, 4 figures
Report-no: FERMILAB-PUB-07-045-A
Baryon Acoustic Oscillations (BAO) have recently been observed in the distribution of distant galaxies. The height and location of the BAO peak are strong discriminators of cosmological parameters. Here we consider the ways in which weak gravitational lensing distorts the BAO signal. We find two effects that can affect the height of the BAO peak in the correlation function at the percent level but that do not significantly impact the position of the peak and the measurement of the sound horizon. BAO turn out to be robust cosmological standard rulers.

Full-text: PostScript, PDF, or Other formats

The large scale CMB cut-off and the tensor-to-scalar ratio  astro-ph/0701783

Authors: Gavin Nicholson, Carlo R. Contaldi
Comments: 4 pages, 3 figures
A kinetic dominated stage preceding the last N~60 e-foldings of inflation leads to a cut-off in both scalar and tensor primordial spectra on the largest observable scales. We discuss the overall probability of inflationary solutions with a limited number of e-foldings and point out an interesting feature. The tensor-to-scalar ratio in these models grows at large scales. This potentially observable signature could shed some light on the true origin of the low Cosmic Microwave Background (CMB) quadrupole power.

Full-text: PostScript, PDF, or Other formats

The probability distribution function of the SZ power spectrum: an analytical approach  astro-ph/0701879

Authors: Pengjie Zhang (SHAO), Ravi K. Sheth (UPenn)
Comments: 11 pages, 7 figures. Submitted to MNRAS
The Sunyaev Zel'dovich (SZ) signal is highly non-Gaussian, so the SZ power spectrum (along with the mean $y$ parameter) does not provide a complete description of the SZ effect. Therefore, SZ-based constraints on cosmological parameters and on cluster gastrophysics which assume Gaussianity will be biased.
We derive an analytic expression for the $n$-point joint PDF of the SZ power spectrum. Our derivation, which is based on the halo model, has several advantages: it is expressed in an integral form which allows quick computation; it is applicable to any given survey and any given angular scale; it is straightforward to incorporate many of the complexities which arise when modeling the SZ signal. To illustrate, we use our expression to estimate $p(C_\ell)$, the one-point PDF of the SZ power spectrum. For small sky coverage (applicable to BIMA/CBI and the Sunyaev Zel'dovich Array experiments), our analysis shows that $p(C_\ell)$ on the several arc-minute scale is expected to be strongly skewed, peaking at a value well below the mean and with a long tail which extends to tail high $C_\ell$ values. In the limit of large sky coverage (applicable to the South Pole Telescope and Planck), $p(C_\ell)$ approaches a Gaussian form. However, even in this limit, the variance of the power spectrum is very different from the naive Gaussian-based estimate. This is because different $\ell$ models are strongly correlated, making the cosmic variance of the SZ band-power much larger than the naive estimate. Our analysis should also be useful for modeling the PDF of the power spectrum induced by gravitational lensing at large $\ell$.

Full-text: PostScript, PDF, or Other formats

Void Ellipticity Distribution as a Probe of Cosmology  astro-ph/0610520  (suggested by Pasquale)

Authors: Daeseong Park, Jounghun Lee (Seoul Nat'l Univ.)
Comments: final version, published in PRL, all typos corrected in Fig.1 and the reference
Journal-ref: Phys.Rev.Lett. 98 (2007) 081301
DOI: 10.1103/PhysRevLett.98.081301
Cosmic voids refer to the large empty regions in the universe with a very low number density of galaxies. Voids are likely to be severely disturbed by the tidal effect from the surrounding dark matter. We derive a completely analytic model for the void ellipticity distribution from physical principles. We use the spatial distribution of galaxies in a void as a measure of its shape, tracking the trajectory of the void galaxies under the influence of the tidal field using the Lagrangian perturbation theory. Our model implies that the void ellipticity distribution depends sensitively on the cosmological parameters. Testing our model against the high-resolution Millennium Run simulation, we find excellent quantitative agreement of the analytic predictions with the numerical results.

Full-text: PostScript, PDF, or Other formats

Exciting Dark Matter and the INTEGRAL/SPI 511 keV signal                  astro-ph/0702587  (suggested by Josh)

Authors: Douglas P. Finkbeiner (CfA), Neal Weiner (CCPP, NYU)
Comments: 18 pages
We propose a WIMP candidate with an ``excited state'' 1-2 MeV above the ground state, which may be collisionally excited and de-excites by e+e- pair emission. By converting its kinetic energy into pairs, such a particle could produce a substantial fraction of the 511 keV line observed by INTEGRAL/SPI in the inner Milky Way. Only a small fraction of the WIMPs are above threshold, and that fraction drops sharply with galactocentric radius, naturally yielding a radial cutoff, as observed. Even if the scattering probability in the inner kpc is << 1% per Hubble time, enough power is available to produce the ~3*10^42 pairs per second observed in the Galactic bulge. We specify the parameters of a complex scalar field designed to explain the INTEGRAL result, and find that it annihilates chiefly to e+e- and neutrinos, and freezes out with the correct relic density. We discuss possible observational consequences of this model.

Full-text: PostScript, PDF, or Other formats

Probing Dark Matter Substructure with Pulsar Timing  astro-ph/0702546  (suggested by Josh)

Authors: E. R. Siegel, M. P. Hertzberg, J. N. Fry
Comments: 8 pages, 4 figures, submitted to MNRAS
Report-no: MIT-CTP/3814
We demonstrate that pulsar timing measurements may be able to detect the presence of dark matter substructure within our own galaxy. As dark matter substructure transits near the line-of-sight between a pulsar and an observer, the change in the gravitational field will result in a delay of the light-travel-time of photons. We calculate the effect of this delay due to transiting dark matter substructure and find that the effect on pulsar timing ought to be observable for a wide range of substructure masses and density profiles. We find that transiting dark matter substructure with masses above 0.01 solar masses ought to be detectable at present by these means. With small improvements, this method may be able to distinguish between baryonic, thermal non-baryonic, and non-thermal non-baryonic types of dark matter. Additionally, information about structure formation on small scales and the density profiles of galactic dark matter substructure can be extracted via this method.

Full-text: PostScript, PDF, or Other formats

The detectability of baryonic acoustic oscillations in future galaxy surveys          astro-ph/0702543  (suggested by Josh)

Authors: R. Angulo (1), C. M. Baugh (1), C. S. Frenk (1), C. G. Lacey (1) ((1) ICC, Durham)
Comments: 23 pages, 18 figures, submitted to MNRAS
We use N-body simulations of the hierarchical clustering of dark matter and semi-analytical modelling of galaxy formation to assess the detectability of baryonic acoustic oscillations in the power spectrum of galaxies. Our primary simulation has a volume of $2.4 h^{-3} {\rm Gpc}^{3}$, comparable to forthcoming redshift surveys at $z \sim 1$, with sufficient mass resolution to see the galaxies expected in these surveys. We present a step-by-step illustration of the effects which change the form of the galaxy power spectrum on large scales from the simple predictions of linear theory. Nonlinear effects are evident on scales in excess of $100 h^{-1}$Mpc. Nonlinearities, galaxy bias and redshift-space distortions erase some of the acoustic oscillations. We present an improved, robust method to find the equation of state of the dark energy parameter $w$. Our galaxy formation model allows us to construct synthetic galaxy samples with the selection criteria proposed for future surveys. We find a weak systematic difference between the equation of state parameter recovered using galaxies and dark matter. Sampling variance is the dominant source of error despite the huge volume simulated. We use our simulation results to estimate the accuracy with which $w$ will be measured in the future. Pan-STARRS could potentially yield a measurement with an accuracy of $\Delta w = 4-7%$, which is competitive with the proposed WFMOS survey ($\Delta w = 5%$). This represents a factor of two improvement over current constraints on $w$. To achieve $\Delta w \sim 1%$ using acoustic oscillations would require a survey with at least 16 times the effective volume of Pan-STARRS. Thus, it is unlikely that this level of accuracy will be reached by the next generation of galaxy surveys.

Full-text: PostScript, PDF, or Other formats