Seminars

CosmoVerse holds regular seminars on the cosmological tensions, focusing on both new measurements and proposed solutions.

CosmoVerse Seminars

Indranil Banik
ICG Portsmouth
Wednesday - April 30, 2025 @ 10:30am CET
Title: The local void solution to the Hubble tension

Cosmology is currently in a crisis known as the Hubble tension, the observation that redshift increases with distance about 10% faster than expected in the ΛCDM standard cosmological paradigm with parameters calibrated to fit the CMB anisotropies. A promising explanation for this is that we live near the centre of a large local underdensity or void. This is suggested by observations of source number counts across the whole electromagnetic spectrum, with near-infrared results implying that the density is about 20% below average out to 300 Mpc across 90% of the sky and most of the galaxy luminosity function (ApJ, 775, 62). Outflows from this KBC void can induce enough extra redshift to plausibly solve the Hubble tension (MNRAS, 499, 2845). I will discuss various tests of this proposal. At low redshift, the bulk flow of galaxies traces the average velocity of matter within a sphere centred on our location. The observed bulk flow curve is in good agreement with the void model predictions (MNRAS, 527, 4388). Looking further out, it is possible to infer the H0 parameter from data in a narrow redshift range centred on z. Such an empirical H0(z) curve agrees quite well with expectations in the void model, which predicts a return to the CMB-derived H0 beyond the void (MNRAS, 536, 3232). This result is related to recently submitted work on baryon acoustic oscillations (BAOs), which show a deviation from ΛCDM expectations (Arxiv:2501.17934). I will explain how the BAO observables would be affected by a local void. I will then present BAO results compiled over the last twenty years. These results fit better if the local void is included, thanks to good agreement with ΛCDM at high redshift but a persistent anomaly at lower redshift.

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Luca Amendola
University of Heidelberg
Thursday - May 15, 2025 @ 5:00pm CET
Title: Cosmological Geometry and Gravity with non-linear clustering

A new avenue was recently developed for analyzing large-scale structure data which does not depend on assumptions about the power spectrum shape, the specific background expansion, or the growth function. In this talk I discuss how this model-independent methodology can be applied to answer three fundamental questions: a) is space curve?; b) is gravity Einsteinian?; c) is the equivalence principle violated?

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Florian Beutler
University of Edinburgh
Thursday - June 5, 2025 @ 5:00pm CET
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Adrià Gómez-Valent
University of Barcelona
Wednesday - June 18, 2025 @ 10:30am CET
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Taylor Hoyt
Thursday - July 3, 2025 @ 5:00pm CET
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Pedro Ferreira
Oxford University
Thursday - July 17, 2025 @ 5:00pm CET
Title: How do we determine the nature of dark energy?

We seem to be faced with an impossible task: to determine the precise microphysical model of dark energy. I will argue that, nevertheless, it may be possible to determine certain features of the nature of dark energy. In particular, and in light of recent cosmological data, I will show that it is unlikely that the dark energy is a simple, minimally coupled, “thawing”, scalar field. This means that there is scant evidence for, for example, rolling scalar fields like axions or other such simple models. A careful analysis seems to indicate a very strong preference (on any measure) for a non-minimally couple scalar field which brings with it a host of undesirable properties: time varying Newton’s constant and fifth forces. We will show that there are ways of getting around these problems, but we are left with the unwelcome requirement of further new physics on non-cosmological scales. I will conclude by surveying the narrow range of options that are available to explain current cosmological data.

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Marika Asgari
Thursday - July 24, 2025 @ 5:00pm CET
Title: Cosmology with the final KiDS data release

The Kilo Degree Survey (KiDS) has collected all of its images which form the basis of its 5th and final data release (KiDS-Legacy). In this talk I will summarise the resulting cosmic shear analysis from KiDS-Legacy and detail the various systematic and consistency tests that we performed to ensure its robustness. We find that with our new analysis choices and extra data the tension in S8 with respect to CMB data from Planck-Legacy has reduced to less than 1 sigma (assuming a flat-LCDM model), rendering it insignificant. At the end of this talk, I will point to the remaining cosmological analyses that are expected to be released in the near future.

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Past Seminars

Frank Qu Zhu
SLAC and Stanford University, USA
Thursday - April 17, 2025 @ 5:00pm CET
Title: Cosmic Structure Growth Through the Lens of the CMB

The cosmic microwave background (CMB) serves as a unique backlight for tracing the growth of cosmic structures. By measuring arc-minute-scale deflections experienced by CMB photons due to gravitational lensing, we can map matter distributions at high redshifts. This lensing signal provides a powerful probe of fundamental physics—such as the sum of neutrino masses—and enables consistency tests of the standard cosmological model by comparing observed and predicted large-scale structure growth.
In this talk, I will review recent and forthcoming advances in this rapidly evolving field, with a particular focus on the DR6 CMB lensing results from the Atacama Cosmology Telescope (ACT). I will also discuss ongoing efforts to produce state-of-the-art lensing maps using the final data release of ACT by combining night-time, daytime, and large-scale CMB observations from Planck. I will then explore the transformative potential of the Simons Observatory and discuss the implications of these lensing measurements in the context of understanding cosmic structure growth and addressing the S₈ tension.

Abstract
Maria Vincenzi
University of Oxford, UK
Thursday - April 3, 2025 @ 5:00pm CET
Title: Cosmological results from the Dark Energy Survey Supernova program

The Dark Energy Survey Supernova sample is the largest and deepest Type Ia Supernova (SN Ia) sample from a single telescope to date. It includes 1600 photometrically identified SNe Ia with high-quality multi-band light curves and spectroscopic redshifts. With a redshift range spanning between 0.1 to 1.2 and a well-defined selection function, this SN sample constitutes an ideal dataset for cosmology. In my talk, I will present the cosmological results from this unique sample and show that DES SNe, combined with publicly available low-z SN samples, provides excellent constraints on the Dark Energy equation of state from SN Ia. I will briefly discuss how the DES-SN compares to previous SN cosmological results and how these measurements, combined with BAO and CMB, seem to provide tantalizing evidences for evolving dark energy. Looking towards the future, I will discuss how future SN surveys like Rubin and Roman will revolutionise SN Ia cosmology and potentially answer some of the most pressing questions of modern cosmology.

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Caroline Huang
Harvard-Smithsonian CfA, USA
Thursday - March 20, 2025 @ 5:00pm CET
Title: The Hubble Constant from Mira Variables

The present-day expansion rate of the universe, known as the Hubble Constant, is one of the few directly measurable cosmological parameters. In recent years, the persistent disagreement between the value of the Hubble Constant obtained from direct distance-ladder measurements to nearby galaxies and the value inferred from observations of the Cosmic Microwave Background (CMB), assuming a standard Lambda-CDM cosmological model, has become one of the strongest indications of new physics. While the most precise distance ladder currently uses Cepheids, independent measurements of distances made with other standard candles can serve as a cross-check for systematics in distance measurements, helping to either solidify or resolve the tension. 

One such alternative precision distance indicator and calibrator of Type Ia supernovae (SNe) is Mira variables – highly evolved, asymptotic giant branch stars – which are astrophysically distinct from the more commonly used Cepheids or the Tip of the Red Giant Branch (TRGB). In addition to being highly luminous and ubiquitous, Miras can be detected and characterized using only near-infrared and infrared observations, which is particularly advantageous in the era of the JWST and the Roman. In my talk, I will discuss the current progress in the development of the Mira distance ladder and its Hubble Constant measurements, with a focus on recent HST and JWST observations of Mira variables in M101, the nearest recent host galaxy of a Type Ia supernova.

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Elsa Teixeira
LUPM/Université de Montpellier
Wednesday - March 5, 2025 @ 10:30am CET
Title: Illuminating cosmic tensions with a coupled scalar dark sector

The persistent discrepancy between theoretical predictions of the standard cosmological model and precision measurements from various observational probes remains a significant challenge in modern cosmology. Over the past decade, mounting evidence for persistent discrepancies in the inferred values of cosmological parameters derived from both model-dependent and -independent methodologies has motivated the proposal of alternatives to the standard paradigm. In this talk, I will focus on the exploration of potential missing physics within the standard model, particularly the enigmatic dark sector comprising dark matter and dark energy, and any potential interactions between them. Leveraging on phenomenology considerations and fluid approximations for the physical nature of the dark sector and its underlying dynamics, we assess the viability of various models in reconciling the observed cosmological tensions.

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José Luis Bernal
Cantabrian Institute of Physics
Thursday - February 20, 2025 @ 5:00pm CET
Title: Current status of Line-Intensity Mapping and promises for the future

Line-Intensity mapping (LIM) uses the integrated flux along the line of sight with relatively low-aperture telescopes, recovering radial information targeting known spectral lines discarding the continuum emission. Mapping the intensity fluctuations of an array of lines from HI 21cm to optical-UV lines offers a unique opportunity to probe redshifts well beyond the reach of other cosmological observations, access regimes that cannot be explored otherwise, and exploit the enormous potential of cross-correlations with other measurements. This promises to deepen our understanding of various questions related to galaxy formation and evolution, cosmology, and fundamental physics. In this talk I will cover the current status of LIM, current experiments and discuss the main challenges to fulfill its promise, and will mention the potential that it holds for probing physics beyond the standard model in the future.

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Sherry Suyu
Technical University of Munich, Germany
Wednesday - February 5, 2025 @ 10:30am CET
Title: Cosmology with Strongly Lensed Supernovae

Strongly lensed supernovae (SNe) are emerging as a new probe of cosmology and SN.  The time delays between the multiple images of a lensed SN can be used to determine the Hubble constant (H0) that sets the expansion rate of the Universe.  An independent determination of H0 is important to ascertain the possible need of new physics beyond the standard cosmological model, given the tension in current H0 measurements.  While strongly lensed SNe are rare, the first lensed SN systems are being discovered in the past few years.  I will give an overview of these first discoveries and their cosmological results.  Future surveys, particularly the Rubin Observatory Legacy Survey of Space and Time, are expected to yield hundreds of such exciting events.  I will highlight a new program aimed to find and study lensed SNe for cosmology and stellar physics.

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Anjan Sen
Center for Theoretical Physics, Jamia Millia Islamia, New Delhi, India
Tuesday - January 28, 2025 @ 10:30am CET
Title: Decoding Cosmic Mysteries: Unorthodox approaches towards New Physics

Cosmology is currently in a fascinating phase of exploration. For a considerable period, the ΛCDM model has served as our leading framework for understanding the observable Universe. However, tensions and anomalies across various cosmological observations have cast doubt on its status as the definitive model. Moreover, recent findings from DESI-DR1 indicate significant deviations from the ΛCDM model. 

In this presentation, I shall discuss a few alternative approaches aimed to investigate these departures from the ΛCDM paradigm and exploring potential new physics in cosmological observations. These approaches are motivated either by theoretical challenges in modeling dark energy or by the motivation to study the robustness of the claims for new exotic physics at cosmological scales, as suggested in current literature.

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Jens Chluba
University of Manchester
Thursday - January 23, 2025 @ 5:00pm CET
Title: CMB spectral distortions and how they might help with the Hubble tension

Studies of the cosmic microwave background (CMB) have been instrumental in establishing the cosmological concordance model. Exciting times are ahead with upcoming CMB experiments targeting polarisation patterns of the CMB. However, a completely new frontier in CMB science can be explored using CMB spectral distortions. These signals are created by non-equilibrium processes in the primordial plasma that cause tiny departures of the CMB energy spectrum (not to be confused with the CMB power spectra) from that of a perfect blackbody. The last precise measurements of the CMB spectrum date back to COBE/FIRAS, which established the close blackbody nature of the CMB. With modern technology, we can not only improve these measurements by several orders of magnitude, but we can also explore the full spectro-spatial structure of the CMB with CMB distortion anisotropies. These novel observables will open up unexplored regimes in our understanding of the early universe, shedding new light on standard cosmological processes and new physics and even the origin of the Hubble tension. In my talk, I will highlight some of the recent advances in distortion science with an eye on the many synergistic opportunities that have emerged.

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Dan Scolnic
Duke University, USA
Thursday - December 12, 2024 @ 5:00pm CET
Title: The Hubble Tension in our own Backyard: DESI and the Nearness of the Coma Cluster

In this talk, I will discuss a new local distance-ladder measurement.  The Dark Energy Spectroscopic Instrument (DESI) collaboration measured a tight relation between the Hubble constant (H0) and the distance to the Coma cluster using the fundamental plane (FP) relation of the deepest, most homogeneous sample of early-type galaxies. To determine H0, we measure the distance to Coma by several independent routes each with its own geometric reference. We measure the most precise distance to Coma from Type Ia Supernovae (SNe Ia) and find a distance that results in H0=76.5±2.2 km/s/Mpc from the DESI FP relation. From a broad array of distance estimates compiled back to 1990, it is hard to see how Coma could be located as far as the Planck+ΛCDM prediction.  By extending the Hubble diagram to Coma, a well-studied location in our own backyard whose distance was in good accord well before the Hubble Tension, DESI indicates a more pervasive conflict between our knowledge of local distances and cosmological expectations. I will go over these results, as well as some other techniques for the final rung in the distance ladder.

Abstract
Vivian Miranda
Stony Brook University, US
Thursday - November 28, 2024 @ 5:00pm CET
Title: What Lies Beyond LCDM: New Physics in an Era of Cosmological Tensions

The standard model of cosmology, LCDM, is built upon precise phenomenological hypotheses on how the early, intermediate, and late-time Universes behave. The Cosmic Microwave Background, combined with multiple sets of optical data available from astronomical observatories, can constrain the six free parameters of LCDM; comparing these constraints has unveiled multiple tensions. Among these tensions is the S8 discrepancy, which may indicate that the Cosmological Constant does not adequately describe the late-time cosmic acceleration. A clear demonstration that late-time Dark Energy is not the cosmological constant would have profound implications for our understanding of nature’s primary forms of energy. On the other hand, the tension in the values of the Hubble constant points towards LCDM requiring changes in the early cosmos, perhaps an early Dark Energy component. In this talk, we review the status of the Smooth Paradigm of Cosmic Acceleration, given recent observational results. We will revisit whether time changes in the linear growth factor can alleviate the S8 tensions, as the Smooth Paradigm predicts only such changes. Finally, we will glance at our efforts to advance inferences involving optical lensing and clustering in photometric surveys as probes of Dark Energy beyond the Smooth Paradigm, fulfilling their potential as robust laboratories for testing new physics.

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