Weak Lensing in the Dark Energy Survey
Tue 14 Nov, 2017 @3PM, level 7
Dr. Daniel Gruen, Einstein Postdoctoral Fellow
SLAC / KIPAC / Stanford University
The Dark Energy Survey has combined analyses of galaxy clustering and weak gravitational lensing two-point correlation functions in its first year (Y1) of observations. The goal of this is to constrain cosmological parameters from lensing measurements of structure in the evolved Universe. The combination of two-point correlation functions provides information on the amplitude of density fluctuations (S8=0.794+0.029-0.027) and the dark energy equation of state (w=-0.80+0.20-0.22) that is competitive with Planck CMB data. When joint with probes of cosmic geometry, it yields the best measurement of these parameters to date. Besides reviewing these results and the technical advances that facilitated them, I will also present work that provides a DES lensing view of higher than second moments of the matter density field.
Harnessing the Power of Gravitational Lensing
Wed 08 Nov, 2017 @12PM, Geoff Opat room, level 6
Dr. Rachael Livermore, Postdoctoral fellow
University of Melbourne
The magnifying power of gravitational lensing allows us to study distant galaxies in unprecedented detail. At moderate redshifts (1 < z < 5) the spatial magnification allows us to examine the kinematics and morphologies of ‘normal’ star-forming galaxies, revealing the processes that lead to the clumpy star formation observed in this epoch. At the highest redshifts (6 < z < 10), the flux magnification from lensing allows us to directly observe dwarf galaxies in the first billion years of the Universe, probing the faint end of the luminosity function where the majority of the ionizing photons that contribute to reionization originate. I will also discuss prospects for JWST in studying the first galaxies and the epoch of reionization.
Insights into binary black hole formation from gravitational waves
Wed 01 Nov, 2017 @12PM, Level 7
Dr. Simon Stevenson, OzGrav postdoctoral fellow
In its first observing run (O1), Advanced LIGO detected gravitational waves from two binary black hole mergers, GW150914 and GW151226, along with a statistically less significant candidate LVT151012. The Advanced LIGO detectors were joined by the Advanced Virgo detector in the recently concluded second observing run (O2). Another two binary black hole mergers, GW170104 and GW170814, have been announced so far from this analysis, with the analysis of the data ongoing. These observations confirmed the existence of merging stellar mass black hole binaries originating in low metallicity environments. How to form merging binary black holes remains an open question theoretically. There are many proposed mechanisms, falling broadly into two families: 1) Isolated binary evolution and 2) dynamical formation in dense stellar environments. We show that comparing the distribution of observed black hole masses and spins to those predicted from population models will allow us to gain insight into how binary black holes form. We examine the measurements of the effective spin parameters for the 5 binary black hole observations published thus far and show that the data already exhibit a mild (2.7 sigma) preference for an isotropic distribution of spins over spins aligned with the binary orbital angular momentum. Alternatively, dimensionless black hole spin magnitudes in these systems could be intrinsically small (average spin magnitude < 0.2), at odds with measurements of black hole spin magnitudes in high mass X-ray binaries. Both of these conclusions have implications for the formation of binary black holes, and the formation of heavy black holes in supernovae.
Probing Fundamental Physics with Strong Gravitational Lensing
Wed 25 Oct, 2017 @12PM, Level 7
Dr. Thomas Collett, Dennis Sciama Fellow
Institute of Cosmology & Gravitation (ICG), Univ. of Portsmouth
Local measurements of the expansion rate are in tension with those inferred from observations of the distant Universe. Is this the first sign of new physics or merely a sign of systematic errors within individual probes? This key question remains unsolved, because there are only a handful of established probes. Here I will talk about how strong gravitational lensing offers a new window on precision cosmology, shining a new light on the dark Universe.
I will present strong lensing constraints on the expansion rate of the Universe and the equation of state of dark energy. I will also show how lensing combined with stellar dynamics yields the most precise test to date of the validity of General Relativity on extragalactic scales.
How standard is the standard candle?
Wed 11 Oct, 2017 @12PM, Level 7
Prof. Jeremy Mould, Professor
Over 25 years ago New York Times science writer Dennis Overbye wrote a book oddly titled Lonely Hearts of the Cosmos, narrating the saga of the measurement of distances in the expanding universe, beginning with Edwin Hubble’s first measurements and culminating in a controversy in the scientific literature between Allan Sandage, who had taken up Hubble’s mantle, and Texas astronomer, Gerard de Vaucouleurs. Stranger still, another NYT writer later reported that the film rights had been taken up, and that Tom Hanks had been invited to play Sandage. No movie was made, however, and the controversy was cleared up a few years later by NASA’s Hubble Space Telescope. The age of the expanding universe is now known to an accuracy of a few percent.
There is now a new urgency for increased accuracy in measurements of the Hubble Constant, the misnomer for the current epoch value of the accelerating expansion rate. Stellar Populations and the Distance Scale, sometimes called the distance ladder, as it ascends from the geometric diameter of the Earth’s orbit, has been joined by a rival ladder, descending from the surface of last scattering of the cosmic background radiation. They were expected to meet in the middle, but there seems to be a gap. Or is there? If there is, the standard model of cosmology, the Nobel prize winning ΛCDM model, may be lacking something important. Is it another neutrino, curvature, or a dark matter-neutrino interaction? Right now, the jury is out, and the race for better data is on. Volume simulations indicate that sample variance is not an issue. Are supernovae really standard candles? The OzDES survey is addressing this issue.
The Hubble Constant from Baryon Acoustic Oscillations (BAO) is a standard ruler, rather than a standard candle like supernovae. The ruler is simply the sound horizon at recombination. Simulations tell us that the linear regime for the evolution of structure pertains, and the ruler is unchanged in length in comoving coordinates to a fraction of a percent over the life of the Universe. The Taipan project is expected to achieve 1% accuracy in H0 (1σ) from redshifts of over a million galaxies in the next 5 years. Between the high redshift of BAO and the low redshift of the Local Group, the Hubble Constant can be measured using gravitational lensing with no reference to stars. The H0liCow collaboration has results in agreement with the stellar pops distance scale. A convincing next step would be two independent stellar populations distance scales (Pop I & Pop II) agreeing to 1%.
HI and metal absorption lines during the Epoch of Reionization
Wed 04 Oct, 2017 @12PM, Level 7
Luz Ángela Garcia, PhD student
In this work, we study the epoch of Reionization (EoR) with metal absorption lines in quasar spectra at high redshift, using high resolution hydrodynamical simulations (an improved version of GADGET-3). For this purpose, we set up the physical conditions of the intergalactic medium (IGM) at the redshift of the EoR, and we post-process the simulations to implement a uniform UV ionizing background for quasars and galaxies (Haardt-Madau 2012), the metal ions with CLOUDY 8.1 and HI self-shielding prescription (Rahmati et al. 2013). We use Voigt profile fitting to compute the column densities of the ions from the synthetic spectra and obtain a statistical distribution of the absorbers. This procedure allows us to study the evolution of the state of the IGM at high redshift, compute the cosmological mass density of CIV and HI and other ions. Our simulations produce absorbers properties that are in good agreement with observations in the literature, especially for the high ionization species.
Furthermore, we are able to reproduce an observed example of an LAE galaxy-CIV absorber pair at z=5.7, proving a physical insight into such systems beyond the limit of current observations. Finally, we vary of the uniform UVB at z~6, and compare directly with observations of different metal ions, in order to constrain the ionizing background at the tail of Reionization.
Stochastic gravitational waves in the reach of aLIGO?
Wed 27 Sep, 2017 @12PM, Level 7
Prof. Csaba Balazs, Professor
Based on a scalar singlet extension of the Standard Model of elementary particles, I present a scenario that features stochastic gravitational waves potentially observable by aLIGO.
A list of recent colloquia, with slides in some cases, can be found here.