07/14 - Young-Wook Lee (Yonsei) - "Evidence for significant luminosity evolution in supernova cosmology"

Abstract -"Supernova (SN) cosmology is based on the assumption that the corrected luminosity of SN Ia would not evolve with redshift. Recently, our age dating of early-type host galaxies (ETGs) from high-quality spectra has shown that this key assumption is most likely in error. It has been argued though that the age-Hubble residual (HR) correlation from ETGs is not confirmed from the age datasets measured from multi-band optical photometry of host galaxies of all morphological types. We find, however, that the statistical analysis involved is affected by "regression dilution bias", severely underestimating both the slope and significance of the age-HR correlation. Remarkably, when we apply regression analysis with a standard posterior sampling method to this dataset comprising a large sample of host galaxies, very significant (4.3 sigma) correlation is obtained between the population age and HR with the slope highly consistent with our previous spectroscopic result from ETGs. Therefore, the luminosity evolution stands up to scrutiny as a serious systematic bias that can fully mimic dark energy in SN cosmology. Since the SN cosmology has long been considered as the first and most direct evidence for the accelerating universe with dark energy, this finding may call for some equally careful investigations for the possibility of an alternative interpretation in other cosmological probes as well".

07/21 - Dan Scolnic (Duke) - "New Advances with Type Ia Supernovae To Measure The Expansion of the Universe"

Abstract - "Type Ia Supernovae (SNe Ia) are critical tools for measuring the current expansion rate of the universe, described by the Hubble Constant, and the accelerating expansion, due to a mysterious `dark energy’. As measurements from SNe Ia continue to be important and exciting, there has been widespread interest on strengths and limitations of using SNe Ia in analyses. Here, I review the latest cosmological results using SNe Ia as well as systematic uncertainties and needed improvements for future analyses. I present a new key insight on the physics of SNe that addresses some of the most confounding issues of the last decade. I discuss the state of the `Hubble Constant Tension’ and upcoming measurements of the local cosmic distance ladder. I then will transition to future experiments like LSST and WFIRST, and show forecasts of the amazing constraints on cosmological parameters with 100x the statistics of current samples.".

07/28 - Eleonora Di Valentino (Manchester) - "Cosmic discordances"

Abstract - "The Cosmic Microwave Background (CMB) temperature and polarization anisotropy measurements from the Planck mission have provided strong confirmation of the LCDM model of structure formation. However, there are a few interesting tensions with other cosmological probes and anomalies in the data that leave the door open to possible extensions to LCDM. The most famous ones are the Hubble constant and the S8 parameter tensions, the Alens anomaly and a curvature of the Universe.
I will review all of them, showing some interesting extended cosmological scenarios, in order to find a new concordance model that could explain the current cosmological data".

08/04 - Myung Gyoon Lee (Seoul National University) - "TRGB Cosmology"

Abstract - "The need for precise and accurate measurements of cosmological parameters is increasing more than ever recently. In particular the Hubble tension has become one of the hottest issues in both observational and theoretical cosmology today. Measuring a distance to galaxies is a fundamental step to cosmography. It is simple in principle, but not in practice. The tip of the red giant branch (TRGB) is a very efficient and precise standard candle for measuring the distances to resolved galaxies. The TRGB is a powerful tool, playing as a rising star in the era of precision cosmology.
In this lecture I introduce what the TRGB is, how to use the TRGB for galaxy distance estimation, and their applications to cosmology".

08/11 - Clare Burrage (Nottingham) - "Testing dark energy models with atom interferometry"

Abstract - "The accelerated expansion of the universe motivates a wide class of scalar field theories that modify gravity on large scales. In regions where the General Relativity has been confirmed by experiment, such theories need a screening mechanism to suppress the new force. I will describe how theories with screening mechanisms can be tested in the laboratory, in particular with atom-interferometry experiments.
I will describe the results of a recent experiment in which we measured the acceleration of an atom toward a macroscopic test mass inside a high vacuum chamber, where the new force is unscreened in some theories. Our measurement shows that the attraction between atoms and the test mass does not differ appreciably from Newtonian gravity. This result places stringent limits on the free parameters in chameleon and symmetron theories of modified gravity."

08/18 - Hitoshi Murayama (Berkeley/IPMU) - "Dark Energy and Dark Matter with Subaru Telescope"

Abstract - "The nature of dark matter and dark energy are the central issues in cosmology today, comprising 95% of the energy density of the current Universe. Recent discussions by string theorists called the swampland conjecture suggest that dark energy is evolving with time. At the same time, the standard cold dark matter paradigm may have some issues at small scales. I will discuss some of my recent ideas in this area, as well as the observational campaign with Subaru Telescope to address these issues."

08/25 - Paul Steinhardt (Princeton) - "Dark Energy Changes Everything"

Abstract - "The talk will describe how the discovery of dark energy is forcing a reconsideration of everything from string theory to inflation to the future evolution of the universe."

09/02 - Gong-Bo Zhao (Chinese Academy of Science) - "Cosmological implications of the eBOSS DR16 observations"

Abstract - "I will review the progress of the eBOSS galaxy survey, and present the data analysis and cosmological implications of the DR16 data sample, which includes ~1M spectra taken for the luminous red galaxies (LRGs), emission line galaxies (ELGs) and clustering quasars (QSOs). eBOSS allows for a multi-tracer analysis, which improves constraints on the RSD and primordial non-Gaussianty, due to a suppression of the cosmic variance on large scales. I’ll focus on the multi-tracer analysis between the LRGs and ELGs and discuss the cosmological result."

09/08 - Adam Riess (Johns Hopkins) - "The Present Expansion rate of the Universe, Evidence of New Physics?"

Abstract - "The Hubble constant remains one of the most important parameters in the cosmological model, setting the size and age scales of the Universe. Present uncertainties in the cosmological model including the nature of dark energy, the properties of neutrinos and the scale of departures from flat geometry can be constrained by measurements of the Hubble constant made to higher precision than was possible with the first generations of Hubble Telescope instruments. A streamlined distance ladder constructed from infrared observations of Cepheids and type Ia supernovae with ruthless attention paid to systematics now provide <2% precision and offer the means to do much better. By steadily improving the precision and accuracy of the Hubble constant, we now see evidence for significant deviations from the standard model, referred to as LambdaCDM, and thus the exciting chance, if true, of discovering new fundamental physics such as exotic dark energy, a new relativistic particle, or a small curvature to name a few possibilities. I will review recent and expected progress."

09/15 - George Efstathiou (Cambridge) - "A Lockdown Perspective on the Hubble Tension"

Abstract - "I review the SH0ES analyses by Riess and collaborators and point out some internal inconsistencies, including a discrepancy between the relative distances inferred from Cepheids of two of the primary geometric distance anchors, the Large Magellanic Cloud (LMC) and NGC 4258. I then ask `what would it take to make SH0ES compatible with early time measurements?'. The answer is a systematic bias of 0.1 - 0.15 mag in the intercept of the Cepheid period-luminosity relations of SH0ES galaxies. Such a bias resolves the Hubble tension, the tension between the distance anchors, and the difference between SH0ES and the tip of the red giant branch (TRGB) distance ladder, as measured and calibrated by Freedman and collaborators. I show that the difference between the TRGB and SH0ES values of H0 is caused mainly by a systematic calibration offset. In the short term, observational efforts should be focussed on improving the calibrations of the distance anchors and nearby galaxies, rather than trying to measure distance moduli to more supernovae host galaxies. I argue that an independent distance estimate to NGC 4258 is particularly critical. With such observations, it should be possible, on a relatively short timescale, to establish definitively whether the Hubble tension really exists."

09/22 - Antonella Palmese (Fermi Lab) - "Gravitational wave cosmology with galaxy surveys"

Abstract - "The synergy between gravitational wave experiments and large galaxy surveys such as the Dark Energy Survey (DES) and the Dark Energy Spectroscopic Instrument (DESI) is most prominent in the standard siren method, which has already enabled several measurements of the Hubble Constant. A standard siren analysis was performed using GW170817 and its host galaxy for the first time, and we have later extended the analysis to compact object binary merger events (GW170814 and the recent GW190814) without an electromagnetic counterpart using a DES galaxy catalog. Interesting constraints on the Hubble Constant are expected to be placed with this method using data from the next LIGO/Virgo/KAGRA observing runs. While galaxies' peculiar velocities represent one of the main sources of systematic uncertainty for standard siren measurements with nearby gravitational wave events, they also contain information about the growth of structure and gravity. In this talk, I will present prospects for measuring peculiar velocities using distances from gravitational wave events to infer cosmological parameters, and discuss how this probe will place competitive constraints on the growth index as a test for General Relativity."

10/06 - Lavinia Heisenberg (ETH Zurich) - "Gravitational drama: the actors on the backstage"

Abstract - " I will introduce gravity from complementary perspectives and ask the main protagonists to join the stage. Based on the assumption that General Relativity is the underlying theory of gravity I will introduce the standard model
of cosmology and discuss the tenacious challenges we are facing within this framework. Starting from the defining key
properties of General Relativity I will explain in which consistent ways these properties can be altered. We will then see
how we can use different observational channels and theoretical consistency checks in order to critically assess the
underlying gravity theory."

10/13 - Luca Amendola (Heidelberg) - "New directions in search of the dark universe"

Abstract - "I will present two new ways to explore dark matter and dark energy. The first one introduces
the possibility of very early structure formation, in which primordial dark matter halos form before recombination. These objects are a sort of intermediate between primordial black holes and ordinary
dark matter subhalos and can induce several observable effects.
In the second part, I will discuss how the 21cm line can offer unique constraints on modified gravity
and dark energy at very high redshifts."

10/20 - Cumrun Vafa (Harvard) - "Topological Gravity as the Early Phase of Our Universe"

Abstract - " Motivated by string dualities we propose topological gravity as the early phase of our universe. The topological nature of this phase naturally leads to the explanation of many of the puzzles of early universe cosmology. A concrete realization of this scenario using Witten's four dimensional topological gravity is considered. This model leads to the power spectrum of CMB fluctuations which is controlled by the conformal anomaly coefficients a, c. In particular the strength of the fluctuation is controlled by 1/a and its tilt by cg^2 where g is the coupling constant of topological gravity. The positivity of c, a consequence of unitarity, leads automatically to an IR tilt for the power spectrum. In contrast with standard inflationary models, this scenario predicts O(1) non-Gaussianities for four-and higher-point correlators and the absence of tensor modes in the CMB fluctuations."

10/27 - Harry Desmond (Oxford) - "Fifth force searches in galaxies"

Abstract - "Fifth forces generically follow from new dynamical fields, and hence are ubiquitous in extensions to the standard model. Broad classes of Lagrangian exhibit "screening mechanisms" which hide the fifth force in high-density environments such as the Milky Way, while keeping it operative on larger scales. I will describe the search for fifth forces which act differently on different components of galaxies, e.g. through screening. First, I model the gravitational environments of the local Universe to determine the screening properties of real galaxies and the strength of the fifth-force field over space. I then use this information to forward-model two signals in galaxy morphology -- displacements of stars and gas and warping of stellar disks -- and hence infer fifth-force parameters with a Bayesian likelihood framework. Taking ~16,000 HI-cross-optical detections from the ALFALFA and SDSS surveys and ~4,000 galactic disk images from the Nasa Sloan Atlas, I set the strongest constraints to date on astrophysical fifth forces. Two particularly interesting applications are to f(R) and models such as coupled quintessence in which the fifth force acts only in the dark sector: for the former I require f_R0 < 1.4x10^-8 in the Hu-Sawicki model, and for the latter a fifth-force strength <10^-4 times that of gravity."

11/06 - Ulf Danielsson (Uppsala) - "A journey out of the swamp riding a dark bubble"

Abstract - "I will discuss how dark energy can be realized using unstable AdS, with our universe riding an expanding bubble of true vacuum. I will discuss how matter on the bubble is induced from hanging strings ending on the bubble. The construction provides a new way to understand the cosmological horizon, and there are also implications for particle physics and black holes."