CosmoVerse holds regular seminars on the cosmological tensions, focusing on both new measurements and proposed solutions.
CosmoVerse Seminars
José Luis Bernal
Elsa Teixeira
Caroline Huang
Frank Qu Zhu
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.
Luca Amendola
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?
Past Seminars
Sherry Suyu
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.
Anjan Sen
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.
Jens Chluba
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.
Dan Scolnic
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.
Vivian Miranda
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.
Tessa Baker
Gravitational wave (GW) sirens are a group of methods used to constrain cosmological parameters and test the nature of gravity on large scales. In this talk I’ll introduce the main categories of GW sirens — bright, dark, and spectral —explaining how they differ, and how we work around the lack of further multimessenger events after GW170817.
We’ll take stock of where the current constraints from GW sirens stand. We’ll then explore how we can enhance these constraints through galaxy catalogue completion, i.e. reverse-engineering the distribution of faint galaxies missed by surveys.
Finally, we’ll look at what lies ahead in the short-term and long-term for cosmology using GW sirens. With tens of thousands (or more) of GW sources in hand, can we use GWs as ‘just another tracer’ of large-scale structure?
Khaled Said
The Dark Energy Spectroscopic Instrument (DESI) Peculiar Velocity Survey aims to measure the peculiar velocities of early- and late-type galaxies within the DESI footprint using both the Fundamental Plane and Tully-Fisher relations. These direct measurements promise to tighten constraints on the growth rate by a factor of 2.5 at z=0.1 compared to redshift-space distortions alone. I’ll present our method for assessing stellar velocity dispersions from DESI spectra and establishing the Fundamental Plane. After calibrating our sample using SBF, we constructed the Hubble diagram and estimated a Hubble constant of H0 = 76.05 ± 0.35 (statistical) ± 0.49 (systematic FP) ± 4.86 (statistical due to calibration) km s^-1 Mpc^-1. I’ll discuss key systematics and their impacts and look ahead to upcoming DESI releases and future surveys like WALLABY and 4HS.
Glenn Starkman
We have long celebrated the great success of cosmology in predicting the observed properties of the cosmic microwave background. And yet, for well over two decades there has been consistent and slowly mounting evidence that on large scales the CMB is anomalous. The evidence is consistent from experiment to experiment and it implies the violation of statistical isotropy. I will discuss this evidence, why it is so compelling, and where it may be pointing us.
Leandros Perivolaropoulos
I review the current status of the Hubble tension emphasizing the following:
1. Current measurements of H_0 indicate a tension not between early and late time measurements of H_0 but between distance ladder based measurements which favor a high value of H0 and all other measurements which favor a low value of H0. These measurements include sound horizon based measurements and one step measurements that are independent of the sound horizon involving both late and early time physics. The one step sound horizon free measurements (31), are consistent with each other (\chi^2/dof=1) provided that two outliers (TDCOSMO I and MCP-SH0ES) are removed one of which has been shown to be plagued with systematics (TDCOSMO I).
2. Models for the resolution of the Hubble tension using H(z) deformations, are faced with serious challenges. The most important challenge is the inconsistency between BAO and SnIa distances at low redshifts if SnIa are calibrated with distance ladder and BAO with the sound horizon scale. These data can not be simultaneously fit by this class of models. In addition these models can not be consistent with late time one step H_0 measurements that favor low values of H_0.
3. Models for the resolution of the Hubble tension using pre recombination physics that decreases the sound horizon scale, are also faced with serious challenges. The most important challenge is that even if they manage to consistently increase the predicted value of H_0, they will still be unable to fit one step measurements of H_0 that are independent of the sound horizon scale and favor a lower value of H_0.
4. Based on the above it becomes highly likely that there is a problem with distance ladder measurements which is the only class of measurements based on local astrophysics (redshifts between 0 and 0.01 or distances between 0 and 40Mpc and times between present and 150Myrs ago). I thus argue that the Hubble tension may indicate a new phenomenon in the above scales which may either manifest as a common unknown systematic effect in diverse distance ladder based measurements or a fundamental physics transition which affects the above local scales or times.
Nils Schöneberg
Early-time solutions to the Hubble tensions currently appear to be our best shot at finding a theoretical model that would reconcile the current measurements from the local distance ladder and the CMB while remaining in agreement with other late-time probes. In this talk I focus on those solutions that modify the redshift of recombination, such as through early variations of fundamental constants, quickly review the mechanism of these kinds of solutions, and discuss their viability with regard to current data. While the parameter space is continually closing in on these kinds of models, a shift of the electron mass in the early universe stubbornly remains a surprisingly good candidate model to explain the Hubble tension. We even find some evidence that current BAO data from DESI might favor such a model.