CosmoVerse seminars

The universe may feature large-scale inhomogeneities and isotropies that cannot be explained by the standard model of cosmology. After reviewing present observational constraints on inhomogeneity around us, I will present the BEHOMO project (Cosmology BEyond HOMOgeneity and Isotropy), whose aim is to study, via numerical cosmology methods, the evolution of the large-scale structure on an inhomogeneous background. I will describe the BEHOMO suite of 68 simulations and data products, and I will present the preliminary results regarding the growth of structures. Specifically, I will show that the zero-shear approximation is able to accurately describe the angular power spectrum of the simulations that have mildly nonlinear LTB structures.

Type Ia Supernovae (SNe Ia) are considered the most reliable standard candles and they have played an invaluable role in cosmology since the discovery of the Universe’s accelerated expansion. During the last decades, the SNe Ia samples have been improved in number, redshift coverage, calibration methodology, and systematics treatment. These efforts led to the most recent “Pantheon” (2018) and “Pantheon +” (2022) releases, which enable to constrain cosmological parameters more precisely than previous samples. In this era of precision cosmology, the community strives to find new ways to reduce uncertainties on cosmological parameters. To this end, we start our investigation even from the likelihood assumption of Gaussianity, implicitly used in this domain. Indeed, the usual practice involves constraining parameters through a Gaussian distance moduli likelihood. This method relies on the implicit assumption that the difference between the distance moduli measured and the ones expected from the cosmological model is Gaussianly distributed. In this work, we test this hypothesis for both the Pantheon and Pantheon + releases. We find that in both cases this requirement is not fulfilled and the actual underlying distributions are a logistic and a Student’s t distribution for the Pantheon and Pantheon + data, respectively. When we apply these new likelihoods fitting a flat ΛCDM model, we significantly reduce the uncertainties on the matter density ΩM and the Hubble constant H0 of ∼ 40%. As a result, the Hubble tension is increased at > 5 σ level. This boosts the SNe Ia power in constraining cosmological parameters, thus representing a huge step forward to shed light on the current debated tensions in cosmology. This analysis has been extended also to the GRBs and BAO and results show a decrease in terms of cosmological parameters also when these probes are added.

Quantitative solutions to the Hubble tension have been toyed with in exact inhomogeneous cosmologies for decades. Fundamentally we must ask:

• Can the average expansion history of matter and structures in the observed Universe be realistically and rigorously reduced to a Friedmann-Lemaitre-Robertson-Walker (FLRW) model?
• Can observed departures from average FLRW expansion always be exactly reduced to a local Lorentz boosts, i.e., peculiar velocities, of the source, observer and intervening structures?

It is a direct consequence of the general relativity that the answer to the above questions is no. However, since the standard Λ Cold Dark Matter cosmology can explain so much, any realistic alternative must give answers which are (i) theoretically compelling; (ii) agree with observational tests of ΛCDM within instrumental, statistical and systematic uncertainties; (iii) explain observed tensions and anomalies; (iv) make predictions for new experiments.

The Timescape cosmology has provided such a framework since 2007 but its predictions had to wait for the level of precision now available in the 2020s from multi-messenger astronomy, and complementary missions from DES, DESI, … to Euclid and JWST. Notably, independent projections made for the Euclid satellite in 2014 indicate that within 5 years either the FLRW expansion history, or any realistic non-FLRW alternative, will be falsified.

Firstly, by a thorough reanalysis of the Pantheon+ catalogue of 1535 unique supernove Ia (arXiv:2311.01438) – including details of light-curves, bootstrapped resampling, convergence by both Bayesian and complementary frequentist statistical methods – we find a self-consistent resolution of debates about the basic cosmological paradigm in which:

• the “Hubble tension” is resolved;
• the Timescape (with backreaction and no cosmological constant) is a marginally better fit than Lambda Cold Dark Matter (LCDM);
• empirical astrophysical puzzles (“Hubble bubble”, galactic dust properties) are resolved;
• hidden detail in the light-curve data is revealed by a new empirical scheme which admits non-FLRW average expansion.

Secondly, application of similar methodology to the largest strong gravitational lens catalogue (arXiv:2403.11997) favours Timescape. For Singular Isothermal Sphere lens profiles, models with FLRW spatial curvature fit better as the free parameter approaches an unphysical empty universe, ΩM0 → 0. By contrast, the Timescape single free parameter, the void fraction, is driven to 𝑓v0 → 0.73, within the uncertainty found from Planck CMB data, and from Sne Ia tests.

Thirdly, empirical data-driven approaches to the construction of void finders are adapted to develop rigorous tools for novel approaches to void statistics in full numerical general relativity simulations (arXiv:2403.15134). Within the limits of resolution of the simulations, the results are again consistent with Timescape parameters.

We welcome discussion about open questions, current limitations, ongoing investigations and further observational tests.

I will present the results from the supernova cosmology key paper with the completed Dark Energy Survey.  With a brand new sample of ~1500 type Ia supernova, which approximately quintuples the cosmology-quality high-redshift (z > 0.5) supernovae in the literature, we find tight constraints on dark energy and dark matter.  In addition to the standard cosmological analyses I will present our explorations of the deeper implications into the nature of dark energy such as testing its time variability (and maybe even measuring cool effects like cosmological time dilation).  I’ll introduce a new parameter Q_H (in analogy to S_8 for lensing) that takes into account the degeneracy direction in the w-Omega_M plane for SNe.  Finally, I’ll  show to what extent our published Hubble diagram is cosmological model independent, and how to use the data to test your own favourite cosmological model.

The standard LCDM model gives a successful description of many astrophysical observations. However, in the past few years a tension has developed between local determinations of the Hubble constant and the value predicted from early universe probes. If confirmed, this so-called Hubble Tension, would require additional physical ingredients beyond LCDM, e.g. early dark energy, or new particles. After describing the tension, I will provide an update of a 25-year long effort to measure the expansion history of the universe and thus the Hubble constant using gravitational time delays, highlighting recent results based on lensed quasars from the TDCOSMO collaboration (the union of H0licow/STRIDES/SHARP collaborations), and from multiply imaged supernovae, including Refsdal. I will show new JWST HST Keck and VLT data that we are currently analysing with the goal of achieving 1-2% precision and accuracy on H0 and thus contribute to the resolution of the Hubble tension.

The possibility that the vacuum energy density (VED) could be a running quantity in the expanding Universe is intuitively more reasonable than just a rigid cosmological constant for the entire cosmic history. In fact, it may be more than a mere intuition. In the context of the running vacuum model (RVM), one finds that the VED evolves as a power series of the Hubble rate, H(t), and its time derivatives. This is not an ad hoc structure for it emerges from the result of detailed calculations in quantum field theory (QFT) in cosmological (FLRW) spacetime. The powers of H are just quantum effects from the vacuum fluctuations. As a result, the RVM i) predicts the running of both the cosmological `constant’ and the gravitational`constant’; ii) contributes to alleviate major issues such as the fine tuning conundrum within the cosmological constant problem; iii) furnishes a new mechanism for inflation; and iv) it implies that the (quantum) vacuum has a dynamical equation of state parameter (rather than the rigid value -1, as usually assumed), thus mimicking quintessence or phantom DE and potentially rendering the traditional DE fields expendable. Last, but not least, the RVM has a positive bearing on easing the current phenomenological tensions within the concordance model. In this talk, I will show that all of these properties are actually a generic prediction of QFT in FLRW spacetime. While this framework stems from old semi-qualitative ideas, only recently it has been possible to substantiate it after identifying an appropriate renormalization framework for tackling the issue of vacuum energy in the context of QFT and its connection with Cosmology.

Baryon Acoustic Oscillations are considered one of the most powerful cosmological probes. They are assumed to provide distance measures independent of a specific cosmological model. At the same time the obtained distances are considered agnostic with respect to other cosmological observations. However, in current measurements, the inference is done assuming parameter values of a fiducial LCDM model and employing prescriptions tested to be unbiased only within some LCDM fiducial cosmologies. Moreover the procedure is plagued by the ambiguity of choosing a specific correlation function model-template to measure cosmological distances.
Does this comply with the requirement of model and parameter independent distances useful, for instance, to characterize cosmological tensions?
In this talk I will review the subject, answer compelling questions and explore new promising research directions.

It appears by now established that solving the Hubble tension requires new physics operating at early times, i.e. prior to recombination. But is that really the end of the story? Based on Miller’s law (which states that the number of objects the average person can hold in working memory is about seven), I will present seven independent hints pointing towards the fact that the Hubble tension requires more than just early-time new physics, and will discuss my personal thoughts about what the most promising scenarios might be moving forward (also keeping in mind the S8 tension).

Differing measurements of the expansion rate of the Universe have given rise to an observational dilemma in cosmology commonly referred to as the Hubble tension. A possible solution is provided by the model of New Early Dark Energy. Here, a scalar field’s false vacuum energy plays the role of an early dark energy component that leads to a short repulsive boost close to matter-radiation equality before it decays through a fast, triggered first-order phase transition. I will outline a particular microphysical implementation, highlight the importance of a trigger mechanism, and discuss the model’s phenomenology. Finally, I will report on recent results and show how the same physics can address another observational challenge relating to the large-scale structure of the Universe.

A couple of days before the US holiday, Thanksgiving, I will give thanks for the new James Webb Space Telescope and share some early results from it. Specifically I will focus on the increased resolution and sensitivity of JWST in the context of local distance measurements in the hosts of 7 SN Ia and direct comparisons between past results from the Hubble Space Telescope and JWST. We find that systematic errors in HST Cepheid photometry do not play a significant role in the decade-long Hubble Tension, leaving us without a source of error to explain its presence.

Modern surveys provide access to high-quality measurements on large areas of the sky, sampling the galaxy distribution in detail also in the emptiest regions, voids. Void cosmology is becoming an increasingly active sector of galaxy clustering analysis: by measuring void properties, such as density profiles or void number counts, it is possible to constrain cosmological parameters. Cosmic voids are particularly sensitive to the properties of dark energy and neutrinos, and are a powerful tool to test modifications of the laws of general relativity. Studying voids provides a novel perspective to unravel the unsolved mysteries of our Universe.
In this talk I introduce cosmic voids as a tool for cosmology, I present recent results, and discuss the perspective of voids on the rising cosmology tensions. I also discuss current challenges and future developments in the field.

Although most of cosmologists believe that we live in a Λ𝐶𝐷𝑀 Universe, the phenomenon of the Dark Matter continues to baffle the researchers: the underlying dark particle has escaped, so far, the detection and its astrophysical role appears to be very complex and entangled with that of the standard luminous particles. As an example, in disc systems, there is a number of well known scaling laws, connecting among them the structural properties of the dark and the luminous matter, that are too complex to be arisen in a scenario in which these two mass components do not “talk” to each other but just share the same gravitational field. We propose that, in order to proceed efficiently, alongside with abandoning the current Λ𝐶𝐷𝑀 scenario, we need also to shift the Paradigm from which such scenario has emerged.
Then, we advocate for a Paradigm, according to which, we are poised to search for DM scenarios without requiring that: (a) they naturally come from (known) “first principles” (b) they obey to the Occam razor idea (c) they have the bonus to lead us towards the solution of presently open big issues of fundamental Physics. On the other side, the proper search shall: (i) give precedence to observations and to the experimental results, wherever these may lead us (ii) consider the possibility that the Physics behind the Dark Matter phenomenon be disconnected from the Physics we know and and that, furthermore, iii) the actual scenario does not comply with the usual canons of beauty.
An immediate application of this paradigm leads us to a Scenario featuring a direct interaction between Dark and Standard Model particles that has finely shaped the inner regions of galaxies.

I will review recent work aimed at measuring the Hubble constant to 1% using the extragalactic distance ladder, with a focus on stellar standard candles. In particular, I will present progress in the understanding of Gaia parallax systematics unraveled by asteroseismology, the most accurate absolute calibration of Cepheids based on Gaia parallaxes of star clusters, and the improvements to Cepheid photometry enabled by the unprecedented near-infrared spatial resolution of the James Webb Space Telescope. In turn, I will present recent improvements to the Tip of the Red Giant Branch calibration in the Large Magellanic Cloud.

Can modern cosmological observations be reconciled with a general-relativistic Universe without an anti-gravitating energy source? Usually, the answer to this question by cosmologists is in the negative, and it is commonly believed that the observed excess dimming of light from supernovae relative to the predictions of the Milne model is evidence for dark energy. In this talk I will illustrate why this intuition does not generally hold once the symmetries of the Friedman-Lemaitre-Robertson-Walker metric are broken. This opens up an avenue of research into general-relativistic space-time solutions without dark energy that may be competitive cosmological models. I will discuss the geometrical constraints that such space-times must necessarily satisfy in order to conform with cosmological observations.

Weak gravitational lensing is a powerful probe of the matter distribution in the recent Universe and thus a key probe for dark energy and gravity. I will give a status of weak lensing cosmology in ΛCDM and tests of gravity, putting the results we obtained in arxiv:2207.05766 into perspective and highlighting the current slight tensions in S8 and ∑0 with the Cosmic Microwave Background. I will then discuss ongoing progress and outlooks to complete our understanding of such tensions in the experimental landscape of the coming decade. Finally, I will show how the method we proposed in arxiv:2110.13171 can be used for the purpose of exploring solutions to the S8 tension. I will end by showing a preliminary version of an online page to centralize cosmological results and thus facilitate the visualization of cosmological tensions.

I will present recent results from a full re-analysis of TRGB measurements in the first and second rung of the distance ladder.  I will show how the tip brightness varies across fields of the galaxies, and present a 5sigma relation between the tip brightness and the measured contrast ratio.  I will show how this relation can be used to ’standardize’ TRGB measurements, reaching a state-of-the-art dispersion across fields.  I will then talk about how we combine measurements of the TRGB in NGC 4258 with measurements in supernova hosts, and combine this with Pantheon+.  I will discuss how the resulting value is consistent with the SH0ES value of the Hubble constant, and this is partly due to the homogenous treatment of TRGB, as well as the usage of the Pantheon+ supernova catalog.  Finally, I will place this result in the context of other H0 measurements.

I will discuss the model-independent requirements for solving the H_0 and \sigma_8 tensions simultaneously.

Inconsistencies have arisen with theory based values of the Hubble constant, which fundamentally is a relation between galaxy velocities and distances.  Unfortunately, there is no single way of measuring distances over the range accessed by stellar parallaxes and the regime of cosmic expansion unconfused by peculiar motions.  Instead, there is recourse to the distance ladder.  A variety of methodologies can be laced together, each accurate within a limited domain but with substantial overlaps between techniques.   The construction of the ladder has a long and tenuous history but is reaching a fruitful conclusion.

Dipole cosmology is the maximally Copernican generalization of the FLRW paradigm that can incorporate bulk flows in the cosmic fluid. In this paper, we first discuss how multiple fluid components with independent flows can be realized in this set up. This is the necessary step to promote “tilted” Bianchi cosmologies to a viable framework for cosmological model building involving fluid mixtures (as in FLRW). We present a dipole \lcdm\ model which has radiation and matter with independent flows, with (or without) a positive cosmological constant. A remarkable feature of models containing radiation (including dipole $\Lambda$CDM) is that the relative flow between radiation and matter can increase at late times, which can contribute to eg., the CMB dipole. This can happen generically in the space of initial conditions. We discuss the significance of this observation for late time cosmic tensions.

General Theory of Relativity needs at least one modification – the Cosmological Constant. Yet there are possibilities for other modified theories of gravity to explain the accelerated expansion. In this talk I’m going to discuss the impact of Modified Gravity on the two-body problem. In particular, with the latest observational constraints from the galactic center and the S-stars, binary pulsars and the Milky and Andromeda dynamics.

The distribution of galaxies on large scales is a sensitive probe of fundamental physics. In particular, the structure of this distribution depends on properties of dark matter and the dynamics of the early universe. Understanding this dependence, however, is a challenging task because the observed galaxy distribution is modulated by a variety of non-linear effects. I will present new theoretical tools that have allowed for a systematic analytic description of these effects. These tools play a central role in a new program of extracting cosmological information from large scale galaxy surveys. I will share some results of this program from several independent analyses of the public data from the Baryon acoustic Oscillation Spectroscopic Survey. These results include new measurements of the Hubble constant, the growth of structure, and constrains on dark matter models. I will discuss these results with a particular emphasis on the H0 and S8 tensions.

What happens when cosmological models break down? I will argue that the cosmological parameters, which are integration constants from the perspective of mathematics, pick up redshift dependence. This marks a fundamental clash between mathematics and observation, a conflict which forces one to either work with a unpredictive model or simply throw it away. I will provide evidence for this evolution in the Lambda-CDM model in the late Universe, the simplest setting to look for evolution. Evolution must be found if cosmological tensions are physical, otherwise one concludes that systematics are at play.

The Hubble tension is arguably the biggest open question in modern cosmology. While it is the most significant signature of new cosmological physics, it is imperative to test whether unknown systematics are at play. In my talk I will summarise our recent work on developing the distance ladder with novel probes like the tip of the red giant branch to use Type Ia supernovae (SNe Ia) from the Zwicky Transient Facility for measuring the Hubble constant. 

Strong gravitational lensing is an independent channel with which SNe Ia weigh in on the tension. Since lensed SNe measure time delay distances this probe has complete independent systematics to the local distance ladder and hence, is a powerful way to measure H0. I will talk about our recent work with wide-field surveys to discover and characterise lensed SNe. 

An interlinked problem in cosmology today is the test for whether the universe is isotropic. Our recent work shows some indications for potential deviations from isotropy and forecasts suggest the exciting possibility to strongly confirm or refute this claim.

The Cosmological Principle – that the Universe is statistically isotropic and homogeneous – leads to a critical consistency test of the standard model of cosmology: the rest-frames of matter and the CMB must coincide. Estimates of the kinematic dipole in radio continuum surveys lead to an agreement in direction with the CMB dipole, but an anomalously large velocity. Recent estimates from the CATWise2020 quasar sample put this velocity tension at > 4 sigma. I highlight a fundamental theoretical systematic: the standard formula for extracting the velocity from the matter dipole is not correct. This may not resolve the tension, but the correct formula needs to be applied before a robust claim can be made.

The most stringent local measurement of the Hubble constant from Cepheid-calibrated Type Ia supernovae differs from the value inferred via the cosmic microwave background radiation (Planck + LCDM) by more than five sigmas. This so-called “Hubble tension” has been confirmed by other independent methods and thus does not appear to be a possible consequence of systematic errors. In this talk, I will describe an independent approach to test the second and third rungs of the distance-ladder method using Type II supernovae. Finally, I will also present a method that does not require any external calibration and can be used to directly measure H0.

The current expansion rate of the Universe is captured by the so-called Hubble constant, or its dimensionless equivalent, “little h”, which is a key parameter in the, extremely successful, standard model of cosmology. 

The Hubble constant  relates measurements of the expansion history of the Universe to its components, and “little h” appears in all astrophysical quantities which measurement or calibration somewhat  depend on the background cosmology. 

There are many different ways to constrain H_0 or little h and they are fully equivalent only  within a model. I will recap different approaches to measure h and discuss what they mean  in both a model-dependent and model-independent way.

In its most basic form, the highly successful ΛCDM cosmology can be encapsulated in six parameters. Once these parameters are specified, so too is a wide variety of phenomena, from fluctuations in the microwave background to the growth of structure to the evolution of the expansion rate of the Universe. While the model is in good agreement with the vast majority of observations, I will discuss two ΛCDM predictions that may be in tension with data: the properties of the earliest galaxy candidates that have been revealed with JWST and the current expansion rate of the Universe (the Hubble constant). The resolution of each of these discrepancies is currently unclear, and I will discuss the prospects for making progress on each separately. I will also speculate about the intriguing possibility of a common solution for these issues and how this might be tested in the near future, including the use of precision stellar age measurements in the local Universe.

Probes of the large scale structures can give us the much needed insight into the nature of the dark Universe. KiDS is a purpose-built gravitational lensing survey with high quality images and a wide photometric coverage, resulting in very high fidelity data. Combining weak lensing data with other probes of the large scale structures, such as galaxy clustering, enables us to break degeneracies in cosmological parameters and control the systematics in the data. In this talk I will review KiDS results and its combinations with other surveys and discuss the prospects for the KiDS final data release and what we can expect to see.

A violation of parity symmetry in electromagnetism would rotate the polarisation of the cosmic microwave background (CMB) in an effect known as cosmic birefringence. In the past, attempts to measure isotropic cosmic birefringence have been limited by the uncertainty in the calibration of the instrument’s polarisation angle. In this talk I will present the novel methodology that allowed us to bypass that limitation by using Galactic foreground emission as our calibrator. Its application to WMAP and Planck data yields a birefringence angle of β≈0.3º, with a statistical significance of 3σ. This measurement could be explained by a Chern-Simons coupling between photons and a pseudoscalar field like those predicted by ultra-light axionlike particles or Early Dark Energy. High-precision measurements of the CMB polarisation will allow us to distinguish between these two effects, potentially shedding more light on the Hubble tension.

Measurements of weak gravitational lensing at low redshifts (z≲0.5−1), quantified by the parameter S_8, favor weaker matter clustering than that expected from the standard ΛCDM cosmological model with parameters determined by cosmic microwave background (CMB) measurements. However, the amplitude of matter clustering at higher redshifts, as probed by lensing of the CMB, is consistent with ΛCDM. This apparent paradox suggests a connection between the S_8 tension and the transition from matter to dark-energy domination. In this talk, I will show that the tension can be resolved by introducing a friction between dark matter and dark energy without altering the tightly constrained expansion history. The low-S_8 measurements favor (at ≳3σ, in this one parameter model) a non-zero drag leading to a suppression of low-redshift power right around the transition from matter to dark-energy domination.

Type Ia supernovae are a central pillar of cosmology that is vital to the adoption of the LambdaCDM “Standard Model of cosmology”. However, they may also elucidate potential gaps in the theory. I will present recent results of the distance ladder constructed from SH0ES observations of Cepheids in combination with the sample of 1550 Pantheon+ supernovae. Ruthless attention paid to systematic uncertainties and recent leaps in progress of SNIa analyses have culminated in a 1km/s/Mpc constraint of the value of the Hubble constant, which has now crossed the significant 5 sigma threshold of discrepancy when compared to that of Planck+LambdaCDM.

In this talk I will discuss the results of the full shape analysis of two-point clustering measurements from BOSS galaxy sample combined with the quasar sample from eBOSS, analysed using an updated recipe for the non-linear matter power spectrum and the non-local bias parameters. I will focus on the cosmological parameters that are not defined through Hubble parameter, h, and highlight how this parameter space not only allows us to appropriately and accurately compare constraints from different probes when assessing consistency, but also enables tight constraints in the extended cosmologies with free dark energy equation of state parameter w. I will present the resulting constraints in LCDM cosmologies, as well as a number of extended models where w is allowed to vary freely and show how clustering can be complimented by external probes, namely, CMB (Planck) and weak lensing (3x2pt measurements from DES Y1).

Acoustic peaks in the Cosmic Microwave Background (CMB) temperature spectrum as observed by the Planck satellite appear to be smoother than our expectation from the standard model lensing effect. This anomalous effect can be also mimicked by a spatially closed Universe with a very low value of Hubble constant that consequently aggravates the already existing discordance between cosmological observations. I will talk about a signature from the early Universe, a particular form of oscillation in the primordial spectrum of quantum fluctuations with a characteristic frequency, that solves all these anomalies. Interestingly, this form of the primordial spectrum resolves or substantially subsides, various tensions in the standard model of cosmology in fitting different observations, namely Planck CMB, clustering and weak lensing shear measurements from several large scale structure surveys, local measurements of Hubble constant, and recently estimated age of the Universe from globular clusters. I will also talk about how to generate this signature from inflation.

I will discuss the role that dark matter may play in relieving tensions in cosmology and how this effect can (and may already) be measured from the Integrated Sachs-Wolfe effect from cosmic voids. I consider a phenomenological model of dark matter with an equation-of-state that is negative and changing at late times, a model degenerate with interacting dark matter — dark energy models. I show this couples the H0 and σ8 tensions, providing an explanation for both simultaneously, while also providing an explanation for the anomalously large integrated Sachs-Wolfe (ISW) effect from cosmic voids. Observations of high ISW from cosmic voids may therefore be evidence that dark matter plays a significant role in the H0 and σ8 tensions. I predict the ISW from cosmic voids to be a factor of ~ 2 greater in this model than what is expected from the standard model ΛCDM. I will also discuss the general prospects for new-physics in cosmology and emphasise the need for new-physics solutions to tensions to be tested and validated with other anomalies and unique predictions/consequences.