Location: Guildhall Auditorium

E5 - Wednesday 14:00-15:40 (Emanuela Dimastrogiovanni)

Is it a small world after all? Searching for the shape of the universe


GLENN STARKMAN (Physics, Case Western Reserve University)

We know the average spatial curvature of the universe is very small, but that
is just simplest aspect of its large-scale shape. What is the topology of the universe?
So far our searches for topology have yielded only limits. But when the search space is multi-dimensional,
and the searches are specialized, how constraining are they? Meanwhile there are
tantalizing hints in the large-angle anomalies of the cosmic microwave background
that they may be signs of non-trivial cosmic topology. We will discuss.

Understanding the physical conditions of high redshift metal absorption lines


Teresita Suarez (Astronomy, University of Edinburgh)

The spectra from background quasars give us information regarding the distribution of gas on large scales as well as the distribution of heavy elements in the low and high redshift intergalactic medium (IGM). Metal absorption lines not only work as unique probes of the high-redshift universe but also may work as probes of hydrogen reionisation at z~6.
Dense regions of the IGM might be metal enriched, and low-ionisation absorption lines can be observable in these regions, during the last phase of reionisation.
In this work we study these regions by modelling the absorption systems as individual gas clouds heated and ionised by a radiation field to predict its physical conditions (temperature, density, ionisation state). We study the column density ratios of these metal ions to separate systems into those where collisional ionisation dominates (high temperatures) and those where photoionisation dominates (low temperatures). In this talk, I will present the predicted systems by photoionisation models and I will show how they compare to low- and high- ionisation systems from observations.



Caterina Chiocchetta (Department of Physics and Earth Science, University of Ferrara)

Planck measurements of Cosmic Microwave Background (CMB) temperature anisotropies show some anomalous features with respect to the prediction of the Standard Cosmological Model – Lambda Cold Dark Matter (LCDM). These observations might be an evidence for the presence of an unknown foreground, indicate that our Universe is a rare statistical fluctuation within the LCDM model, or, alternatively, point to the need for modifications of the model itself.
I will focus on one of these anomalies, namely the value of the two-point angular correlation function of temperature fluctuations at large scale, that is anomalously low w.r.t. the prediction of LCDM. I will describe how to exploit the additional information encoded in the polarization of CMB photons to test the ‘rare statistical fluctuation’ hypothesis.
After presenting the procedure followed to generate an ensemble of CMB polarization maps constrained to the observed temperature field, I will show the comparison between the polarization correlation function derived from these maps and from Planck data. I will also discuss the possibility of applying this procedure to future CMB experiments.

Large scale polarization likelihood from low-frequency CMB data


Umberto Natale (Fisica e Scienze della Terra, Università di Ferrara)

We present here a combined real space polarization dataset which combines the Planck 70 GHz channel with the WMAP Ka, Q and V bands. We completely rewrote the analysis pipeline. We started from the raw maps as delivered by the two collaborations, we then built masks, we performed the foreground cleaning through a template fitting and, lastly, we combined the four maps in pixel domain with an inverse noise weighting. Such dataset represents an independent estimation of the reionization optical depth τ and it is suitable for a number of studies not accessible for a spectrum based likelihood.
In particular, using only large scale polarization data, we measure τ = 0.069$^{+0.012}_{-0.011}$
(68% C.L.). When we extend the analysis to also include data from the small scales we estimate an optical depth of τ = 0.074$^{+0.012}_{-0.011}$ (68% C.L.). This corresponds to a mid-point reionization redshift of $z_{re}$ = 7.68 ± 0.79 (68% C.L.)

E6 - Wednesday 16:10-17:50 (Emanuela Dimastrogiovanni)

Searching for new physics with CMB-S4 and other CMB experiments


Levon Pogosian (Physics, Simon Fraser University)

Large scale B-mode patterns in CMB polarization, if detected, would constitute a “smoking gun” signature of primordial gravitational waves generated during an inflationary phase in the early universe. In this talk, I will discuss other sources of B-modes, such as primordial magnetic fields, axion-like fields and cosmic strings, and prospects of isolating their distinguishing features with future CMB measurements such as CMB-S4.

Gravitational lensing analysis from high-precision B-modes


Toshiya Namikawa (DAMTP, The University of Cambridge)

One of the promising cosmological probes in the next decades is the CMB polarization. While CMB temperature anisotropies have been already measured very precisely, CMB polarization, in particular a twisting pattern in the polarization map (B-modes) is still dominated by the statistical noise. The precise measurements of B modes will be a unique way to explore the early universe through primordial gravitational waves. The B-modes also allow us to probe the physics in the late time universe by measuring the gravitational lensing effect. I will talk about my recent efforts on gravitational lensing analysis using high precision B-mode data. I will particularly focus on the first evidence of cross-correlations between CMB polarization lensing and cosmic shear from Subaru HSC and POLARBEAR, and future prospects of a gravitational-wave B-mode detection using a removal of the lensing B-mode noise.

Constraints on thermal state of IGM at the tail end of reionization


Prakash Gaikwad (Kavli Institute for Cosmology, University of Cambridge)

The occurrence of few transmission spikes in QSO absorption spectra at $z>5$
indicates that the reionization process is inhomogeneous. We constrain the
thermal state of IGM at $5.3 < z < 5.9$ by comparing shapes of transmission spikes from 5
unprecedented quality, high redshift, high SNR, high resolution QSO absorption
spectra with that from state-of-the art homogeneous UV background (RELIC) and
radiative transfer (GADGET-3 and ATON) simulations. Our simulations show that
transmission spikes probe large scale, underdense, recently ionized and hotter
cosmic voids. We characterize the shape of transmission spikes in observations
and simulations by fitting inverted flux with multi-component Voigt profile
using our automated code VIPER. We find that the width of the simulated
transmission spikes is very sensitive to the instantaneous temperature of the
reionized IGM. Complimentary to the flux based method, our method is less
sensitive to continuum fitting uncertainty. In homogeneous UV background
simulations, the observed spike width distribution is consistent with hotter
$T_0 sim 12000$ K and isothermal ($gamma sim 1$) temperature density
relation of IGM.
Whereas, in the inhomogeneous late reionization full radiative transfer simulations
where islands of neutral persist to $z sim5.3$ the width distribution of the
observed transmission spikes is consistent with the range of temperature caused by
the spatial fluctuations of the temperature-density relation.

E7 - Thursday 14:00-15:40 (Emanuela Dimastrogiovanni)

Gaseous cosmological structures and metagalactic ultra-violet background fluctuations


Avery Meiksin (Institute for Astronomy, University of Edinburgh)

Measurements and predictions for the structure of the Intergalactic Medium (IGM) have achieved unprecedented levels of precision. A principle remaining uncertainty in models of the IGM is the origin of the metagalactic ultra-violet (UV) background that photoionizes the gas, including its relation to cosmic reionization. This talk discusses how fluctuations in the UV background depend on several key properties of galaxies and Quasi-Stellar Objects, the most plausible candidate sources. It will be shown how these factors may be measured using current and future surveys that probe the large-scale structure of the IGM, such as SDSS, DESI, e-BOSS, CHIME and HIRAX. It will also be shown how the UV background fluctuations themselves may in turn influence the spatial power-spectrum of the sources.

The Lyman-α forest and the nature of the dark matter


Antonella Garzilli (Discovery Center, University of Copenhagen)

The observed Lyman-α flux power spectrum (FPS) is suppressed on scales below ∼ 30 km s−1. This cutoff could be due to the high temperature, T0, and pressure, p0, of the absorbing gas or, alternatively, it could reflect the free streaming of dark matter particles in the early universe. We perform a set of very high resolution cosmological hydrodynamic simulations in which we vary T0, p0 and the amplitude of the dark matter free streaming, and compare the FPS of mock spectra to the data. We show that the location of the dark matter free-streaming cutoff scales differently with redshift than the cutoff produced by thermal ef- fects and is more pronounced at higher redshift. We, therefore, focus on a comparison to the observed FPS at z > 5. We demonstrate that the FPS cutoff can be fit assuming cold dark matter, but it can be equally well fit assuming that the dark matter consists of ∼ 7 keV sterile neutrinos in which case the cutoff is due primarily to the dark matter free streaming.

Parameterizing the Recombination Visibility Function


Nathaniel Starkman (Astronomy and Astrophysics, University of Toronto)

The last scattering surface of the CMB is the peak of the recombination visiblity function. While the peak has been studied fruitfully and in depth by every CMB experiment since COBE, the full shape of the visibility function is very poorly constrained. The full visibility function encodes a great deal of information which cannot be probed by the peak alone. With upcoming experiments, we have the unprecedented opportunity to use the visibility function to provide constraints, the first for some, on cosmological theories. To this end, we explore parameterizations of the visibility function, extending work done on Gaussian parameterizations. We conduct MCMC analysis of these new parameters in conjunction with the standard suite of cosmological parameters to explore their correlations and better understand potential of the visibility function to serve as a model-independent test of ΛCDM and other cosmological theories.

Generation of CMB circular polarisation from mixing among polarisation components


Margherita Lembo (Physics and Earth Science, University of Ferrara)

In the standard scenario, CMB polarisation anisotropies are generated through Compton scattering, that does not produce circular polarization. Therefore, the polarisation field is completely described by the Q and U Stokes parameters, while the V parameter is taken to be zero. If, however, we consider non-standard effects, such as parity-violation, some degree of circular polarisation might be produced starting from pure linear polarisation.
I will describe a formalism that accounts for the mixing of the different polarisation components (Q, U and V) in the evolution of CMB anisotropies. The formalism correctly takes into account the different behaviour under rotation of the Stokes parameters and can be applied regardless of the physical mechanism underlying the mixing of the polarisation components.
I will present a computational tool based upon this formalism that allows to compute the CMB power spectra in the presence of such a mixing acting from the time of recombination until the present. I will discuss some results obtained with this tool, also concerning available and future observations. In particular, this includes non-zero power spectra for circular polarisation, as well as (smaller) modifications in the usual E and B power spectra.

E8 - Thursday 16:10-17:50 (Emanuela Dimastrogiovanni)

The Advanced Atacama Cosmology Telescope: A Sunyaev-Zel’dovich Effect Survey over 15,000 square degrees


Matt Hilton (University of KwaZulu-Natal)

Since 2016, the Advanced Atacama Cosmology Telescope (AdvACT) has been conducting a high-resolution cosmic microwave background survey over more than 15,000 square degrees of the southern sky. In this talk, I will present an update on the AdvACT search for galaxy clusters, using the redshift independent Sunyaev-Zel’dovich effect. This allows us to construct an effectively mass-limited cluster sample, and chart the growth of massive structures over the past 10 billion years. To date, we have assembled a preliminary sample of more than 2600 SZ-selected, optically-confirmed clusters with redshift measurements, reaching to redshift 2. To put this into context, the AdvACT sample is larger than the total number of SZ-selected clusters published to date. I will also give an update on our efforts to improve our knowledge of cluster mass calibration – which is needed to unlock the full statistical power of this cluster sample to constrain cosmological parameters.

Radiation-Lambda Equality and Reionization: Coincidence or Anthropic Prediction?


Daniele Sorini (Institute for Astronomy, University of Edinburgh)

The 10^120 discrepancy between the predicted and observed values of the cosmological constant (Lambda) is one of the major outstanding questions in cosmology. The small non-zero value of Lambda poses coincidence problems within the LCDM framework. For instance, the matter-Lambda equality occurs roughly at present time. Lombriser & Smer-Barreto (2017) pointed out an even more statistically significant coincidence between the reionization epoch and radiation-Lambda equality. We explore this issue further from a multiverse perspective, by developing an analytic model that consistently describes reionization and star formation history in LCDM universes within a wide range of values for the cosmological constant. Our preliminary results suggest that the coincidence between radiation-Lambda equality and the epoch of reionization can occur only within a narrow range of Lambda around the value observed in our Universe. We thus investigate whether our model enables interpreting this result with anthropic arguments, hence improving our understanding of the observed value of the cosmological constant.

CMB B-mode delensing: prospects and challenges for the next generation of experiments


Anton Baleato Lizancos (Institute of Astronomy and KICC, University of Cambridge)

Lensing-induced B-mode polarisation of the cosmic microwave background is a source of noise in searches for a primary signal associated with primordial gravitational waves. To the extent that the distribution of matter responsible for lensing deflections can be mapped, the lensing B-mode can be estimated and subtracted off in a procedure called “delensing”. For the Simons Observatory, this will involve internal reconstruction using quadratic estimators combined with external tracers of the large-scale structure, most notably the cosmic infrared background (CIB). Here, we explore possible biases affecting this procedure. The first bias we consider appears whenever there is an overlap in modes between the field to be delensed and that from which the reconstruction is derived — as in the case of an EB quadratic estimator reconstruction — the delensed power spectrum is artificially suppressed. From an analytic model and simulations, we show that this bias inevitably results in a loss of constraining power and must be avoided by masking overlapping modes. In addition to this, we characterise and suggest methods to mitigate the bias due to residual galactic dust present in CIB maps used for the purpose of B-mode delensing.