Seminars

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.