Very happy to see my latest paper with Guan-Wen Yuan, Anne Davis, Maurizio Giannotti, Luca Visinelli, and Julia Vogel out on arXiv - kudos to Guan-Wen who did all the heavy lifting! This paper is essentially a merger of two previous papers of mine: in 2021 Luca, Anne, and I argued that dark matter direct detection experiment such as XENON1T could detect chameleons, light particles potentially related to dark energy, produced in the Sun (see paper in PRD), whereas later in 2024 some of us drastically improved the model for solar chameleon production, which was previously grossly incomplete to say the least (see paper in PRD and this earlier news item). What we did here was to combine the tools built in these two papers, applying the results to newer and much more sensitive data from XENONnT, and confirming the extremely important message that terrestrial dark matter experiments are well suited to probe screened dark energy at no extra experimental cost. We clarified various previously unclear points, and derived a new upper limit on a combination of chameleon couplings (specifically the conformal coupling to photons and the disformal coupling to electrons), valid for essentially all values of the chameleon power-law index n, making it one of the most widely applicable chameleon bounds to date. You can read the results in the preprint we just posted on arXiv: 2511.01655.
Hawking radiation from regular black holes
New paper with Marco Calzà and Max Rinaldi out today, making it yet one more paper 100% produced within the Theoretical Gravitation and Cosmology Group led by myself and Max! We study the issue of Hawking radiation from regular black holes (BHs): most, if not all, of the regular BH solutions usually studied are non-vacuum solutions, i.e. their Ricci scalar R is non-zero. In this case a possible non-minimal coupling of scalar fields to curvature, strongly motivated by QFT in curved space-time considerations, can become relevant, yet has almost always been ignored. What we showed is that including this non-minimal coupling can drastically alter the resulting Hawking radiation spectrum, even by orders of magnitude, leading to potentially observable effects. You can read our results in the preprint we just posted on arXiv: 2510.12257.
BAO and the Hubble tension no-go theorem
I’m extremely excited to finally see my latest paper with Davide Pedrotti, Luis Escamilla, Valerio Marra, and Leandros Perivolaropoulos, finally out on the arXiv - kudos to Davide who did all the heavy-lifting in what I do not hesitate to place among the three most important papers I ever wrote! This paper is about what we could call the “Hubble tension no-go theorem”, which forbids purely post-recombination solutions due to the constraints BAO impose on the product of the sound horizon and the Hubble constant. However, the pipeline from which BAO measurements are obtained requires assuming a fiducial cosmological model, with the choice falling on ΛCDM: many have therefore wondered whether we can trust these measurements when testing late-time modifications to ΛCDM, and this is the most frequently invoked loophole to the no-go theorem. In this work we have played devil’s advocate and showed that, even if BAO measurements were to be grossly (and unrealistically) biased by fiducial cosmology assumptions, this would still not be sufficient to rescue post-recombination solutions to the Hubble tension, primarily because of the extremely tight constraints on the (unnormalized) shape of the expansion history from unanchored SNeIa, whose role in the Hubble tension so far has been underappreciated. You can read our results in the preprint we just posted on arXiv: 2510.01974.
Screened dark energy and non-local quantum correlations
Very happy to see my latest preprint with Fabiano Feleppa and Gaetano Lambiase from the University of Salerno finally out on the arXiv - kudos to Fabiano who did all the heavy-lifting and an excellent job! We focused on so-called screened dark energy/modified gravity models, where the would-be fifth force associated to a new light scalar degree of freedom responsible for cosmic acceleration is dynamically suppressed through, you guessed it, screening mechanisms, such as the chameleon, symmetron, and dilaton ones. We showed that these mechanisms can leave their imprint in non-local Bell-type correlations between pairs of entangled particles, leading to potential new observational signatures in currently unconstrained regions of parameter space. These will not be easy to detect, but if anything that’s a challenge for future generations of experimentalists! You can read our results in the preprint we just posted on arXiv: 2508.18448.
Primordial regular black holes (part 3)
Very happy to have posted my first new paper in about 10 months (excluding the CosmoVerse white paper) today (after several months spent applying for grants and teaching, with very little research done), together with Marco Calzà, Davide Pedrotti, and Guan-Wen Yuan, making it yet another paper entirely produced within my group! This is the third chapter in the series on primordial regular black holes as potential dark matter candidates (see the earlier part 1 and part 2, both published in PRD - paper 1 and paper 2), where we consider a completely different metric, which we call the Zhang-Lewandowski-Ma-Yang (ZLMY) BH, after the names of the authors which first proposed it here. This BH has completely different properties with respect to the ones we had considered earlier: it has a very strong theoretical foundation within an effective canonical (loop) quantum gravity framework which addresses the important issue of general covariance, whereas from the phenomenological perspective it is one of the few BHs which is hotter with respect to the Schwarzschild BH. We find that this leads to a smaller asteroid mass window where ZLMY PBHs can account for all the DM, smaller relative to Schwarzschild PBHs of course, a result which completely reverses the trend we found earlier, when the asteroid mass window was significantly enlarged and evaporation constraints could in principle be bypassed. You can read our results in the preprint we just posted on arXiv: 2507.02396.
CosmoVerse white paper
Very happy to see the CosmoVerse white paper, produced within the CosmoVerse COST Action, out today! The white paper discusses in detail the state-of-the-art regarding a number of observational tensions in cosmology (among all the Hubble tension) and how these may be solved either through new fundamental physics or systematics. Huge congrats are due to my friends and colleagues Eleonora Di Valentino and Jackson Levi Said for leading this massive effort featuring over 400 pages and over 500 authors, which required significant coordination and management skills. I myself was one of the Section Coordinators, for section 4.1.3 on extra relativistic degrees of freedom. You can read our results in the preprint we just posted on arXiv: 2504.01669.
Primordial regular black holes (part 2)
A truly busy day today, as with Marco Calzà and Davide Pedrotti we posted not one but two papers! In our first paper, covered in this other news item and motivated by the fact that all studies on primordial black holes (PBHs) consider Schwarzschild and Kerr BHs which feature curvature singularities, we took a first step towards studying primordial regular BHs as dark matter (DM) candidates, focusing on phenomenological tr-symmetric metrics. In this paper, we extend our pilot study to non-tr-symmetric metrics, which complicate our work by a fair margin. Aside from the well-known Simpson-Visser metric, the space-times we studied include two metrics inspired by Loop Quantum Gravity, more specifically the Peltola-Kunstatter and D’Ambrosio-Rovelli ones, and in all three cases we find that the “asteroid mass window” where all the DM can be made of PBHs is enlarged. You can read our results in the preprint we just posted on arXiv: 2409.02807.
Primordial regular black holes (part 1)
I would be lying if I didn’t say I am particularly proud of this new paper which appeared today with Marco Calzà and Davide Pedrotti, making it another made in Trentino paper and, especially, another paper entirely produced within my group (note that this paper is for 2/3 made within the Valle dei Laghi, which is where both Marco and Davide originally come from)! There are a huge number of studies on primordial black holes (PBHs) as potential dark matter (DM) candidates, yet virtually (almost) all of these works consider Schwarzschild or Kerr BHs, which suffer from a few well-known problems, including the presence of curvature singularities. In today’s paper we therefore took a first step towards characterizing primordial regular BHs (which, on the contrary, do not feature curvature singularities) as DM candidates, finding that they can potentially lead to a larger “asteroid mass window” where PBHs can make up all the DM. Today’s pilot study is focused on so-called tr-symmetric metrics, which include the well-known Bardeen and Hayward regular BHs, whereas we have also put out a companion paper (covered in this other news item), where we study non-tr-symmetric metrics, which also include metrics inspired from Loop Quantum Gravity. You can read our results in the preprint we just posted on arXiv: 2409.02804.
Stochastic gravitational wave background from cosmologically coupled black holes
Very happy to see my latest paper with Marco Calzà, Francesco Gianesello, and Max Rinaldi out! This is a 100% “made in Trentino” paper, and more precisely made within the Theoretical Gravitation and Cosmology Group led by myself and Max. At some point in 2023, the possibility that dark energy could be sourced by cosmologically coupled black holes (BHs), whose mass increases in time through purely cosmological growth even in the absence of accretion and merger events, received a lot of interest, especially given the possibility that signatures of such a coupling could have been observed in the growth of supermassive BHs in red-sequence elliptical galaxies. In today’s paper we show that mergers of such cosmologically coupled BHs would lead to a stochastic gravitational wave background whose strength is significantly larger (up to an order of magnitude stronger!) than the standard one from mergers of uncoupled BHs, with very interesting implications for the signal observed last year by pulsar timing arrays (among which NANOGrav, EPTA, PPTA, and CPTA), which is a bit too strong to be easily explainable by mergers of standard BHs. You can read our results in the preprint we just posted on arXiv: 2409.01801.
Hubble tension, Ωm, and ωc
A very busy week, with another paper produced almost entirely within my group, just in time for wrapping up for holidays! With Davide Pedrotti, Jun-Qian Jiang, Luis Escamilla, and Simony Santos da Costa, we argue that the Hubble tension is inherently multidimensional, and that the matter density parameter Ωm and cold dark matter physical density ωc play key roles. In particular, we analytically explained why any model aiming to solve the Hubble tension will inevitably lead to an increase in ωc (because both Ωm and ωb are precisely calibrated by BAO and/or SNeIa, and BBN respectively) and, by extension, S8 (with potential problems for the S8 discrepancy), and explicitly verified that this holds on real data. We then argued that if cosmologists interested in solving the Hubble tension could ask for just one present from Father Christmas…well, then they really should wish to know the value of Ωm chosen by Nature - or, in practical terms, they should wish for a calibration of Ωm which is as reliable and model-independent as possible, and we put forward some ideas on how to achieve this. You can read our results in the preprint we just posted on arXiv: 2408.04530.
EHT and mimetic gravity
Another exciting paper out today! With Mohsen Khodadi and Javad Firouzjaee, we show that the Event Horizon Telescope (EHT) observations basically rule out compact objects in mimetic gravity, a framework of modified gravity which has received a lot of interest as a candidate for explaining both dark matter and dark energy, and first proposed (explicitly) by Mukhanov and Chamseddine in 2013. Mohsen and collaborators studied compact objects in mimetic gravity in 2020, finding them to be highly non-trivial: in practice, the theory supports only a naked singularity, and a black hole obtained through a particular gluing procedure. What we showed in today’s paper is that the shadow properties of both these space-times are pathological, since the naked singularity does not cast a shadow, whereas the black hole casts a shadow which is way too small: for these reasons, both compact objects (and by extension mimetic gravity or, more precisely, the baseline version proposed in 2013) appear to be excluded by the EHT images of M87* and Sgr A*. You can read our results in the preprint we just posted on arXiv: 2408.03241.
Non-parametric expansion history reconstruction and Hubble tension after DESI
I’m particularly excited and proud about today’s paper, which is the first one written entirely and exclusively with members of my group: visiting PhD student Jun-Qian Jiang (who did all the heavy lifting), PhD student Davide Pedrotti, and postdoc Simony Santos da Costa. What we did has been to perform a non-parametric reconstruction of the late-time expansion history in light of DESI BAO data, combined with various SNeIa datasets, using both interpolation and Gaussian Process reconstruction techniques, and studying implications for the Hubble tension. We find that the shape of the expansion history cannot deviate more than 10% from that of ΛCDM, but within that 10% we find interesting deviations which could hint to a non-monotonic/oscillatory behaviour of the expansion rate (and therefore of the dark energy component). I really believe that this can be a pretty important paper, as it represents a much needed revisitation in light of DESI of two seminal papers in the expansion history reconstrcution and Hubble tension literature: the famous “Trouble with H0” paper by Bernal, Verde, & Riess, and the equally famous “Sounds discordant” paper by Aylor and collaborators. You can read our results in the preprint we just posted on arXiv: 2408.02365.
Post-DESI neutrino cosmology
Another paper out today! Led by Jun-Qian Jiang and William Giarè (both of whom did a huge amount of work), and together with Stefano Gariazzo, Maria Giovanna Dainotti, Eleonora Di Valentino, Olga Mena, Davide Pedrotti, and Simony Santos da Costa, we investigate the status of (positive) neutrino mass cosmology after the latest DESI measurements. We find very tight upper limits on the sum of the neutrino masses, a strong preference for the normal ordering, and a significant tension with terrestrial observations, all of which we carefully quantify. We also studied the impact of allowing the dark energy component to be non-phantom, which makes all the previous conclusions somewhat stronger, and highlights an interesting synergy between laboratory experiments aimed at determining the neutrino mass ordering, and the nature of dark energy. You can read our results in the preprint we just posted on arXiv: 2407.18047.
Attempting to solve the Hubble tension combining varying electron mass and ΛsCDM
I’m very, very glad to see my latest work with Yo Toda, William Giarè, Emre Özülker, and Eleonora Di Valentino finally out on arXiv after several months of hard work (especially by the first author Yo who did all the heavy lifting)! The idea is motivated by my seven hints paper (see this earlier news item) arguing that solving the Hubble tension may require a combination of pre- and post-recombination new physics, and here we attempted to construct precisely such a combination, focusing on models which individually worked well in the pre- and post-recombination era so far: a spatially uniform time-varying electron mass in a non-spatially flat Universe, and the ΛsCDM model, featuring a late-time sign-switching cosmological constant (from negative to positive). In the end the idea didn’t quite work, but we still decided to document our attempt because we learned a great deal about potential difficulties and drew what we hope are important general lessons for future endeavours: for the record, these are reported between Pages 10 and 13 and, spoiler, Ωm plays a crucial role in explaining why this didn’t work. A fun fact Özgür Akarsu reminded me of is that around minute 44 of my Universe Today Podcast interview with Fraser Cain back in November (see this earlier news item) I actually gave a very detailed spoiler of this project, as I had spoken to Yo a few days back so we had just gotten it started (and I had quite high hopes on this working out, which it certainly did in the sense of learning a whole lot of new things, although not in the way I initially expected). You can read our results in the preprint we just posted on arXiv: 2407.01173.
Solar chameleons revisited: 2024 edition
I’m extremely happy to see our latest work led by the brilliant Tomás (Tom) O'Shea, together with Anne Davis, my fellow countryman (we are both from Terracina) Maurizio Giannotti, Luca Visinelli, and Julia Vogel, out on arXiv! This was a technical tour-de-force led by Tom where we revisited the issue of production of chameleons (which could be relevant to the dark energy problem) in the Sun, whose state-of-the-art dated back to 2012, and partially motivated by the earlier work of myself, Luca, and Anne. The resulting spectrum includes a number of previously overlooked contributions which turn out to make an important difference, and our results can be extremely relevant for future experiments such as IAXO. While in our work we only included the contribution from transverse photons, the study of longitudinal plasmons is certainly relevant and in progress. You can read our results in the preprint we just posted on arXiv: 2406.01691.
Quasinormal modes-shadow correspondence for rotating regular black holes
I’m truly thrilled to see my latest preprint with my PhD student Davide Pedrotti, which also happens to be Davide’s first paper, out on arXiv! This work is the one I was anticipating in an earlier news item, and is based on part of Davide’s MSc thesis - so, needless to say, kudos to Davide who did all the hard work! There is a well-known correspondence between black hole quasinormal modes (QNMs) in the eikonal limit (ℓ>>1), and the size of BH shadows: this correspondence has been extensively studied for spherically symmetric space-times, but the extension to rotating space-times is non-trivial, and has only been worked out either only for equatorial QNMs (m=±ℓ), or for general QNMs but limited to the Kerr metric. What we did with Davide was to extend this correspondence to more general rotating space-times, then testing it explicitly on the rotating regular Bardeen and Hayward BHs, while also discussing the conditions under which the correspondence holds within general rotating space-times (basically the Hamilton-Jacobi and Klein-Gordon equations have to be separable). You can read our results in the preprint we just posted on arXiv (with what I think is a pretty cool title): 2404.07589.
Scale-invariant inflation meets cosmological data
Very happy to see my latest preprint with Chiara Cecchini, Mariaveronica De Angelis, William Giarè, and Max Rinaldi finally out on the arXiv - kudos especially to the three younger collaborators (Chiara, Mariaveronica, and William) who did all the heavy-lifting! We studied a theoretically very well-motivated classically scale-invariant inflationary model, quadratic in curvature and featuring a scalar field non-minimally coupled to gravity, where inflation occurs in the transition between two de Sitter regimes, during which dynamical breaking of scale-invariance occurs and the Planck mass emerges. We show that the model is in excellent agreement with current CMB data, and that it makes a highly testable prediction for the amplitude of primordial tensor modes: r≳0.003. Given its very specific predictions, near-future CMB experiments can therefore make or break scale-invariant inflation - we argued that this, in combination with its strong theoretical motivation, makes the model an interesting benchmark to add when studying future tests of inflation from CMB data. You can read our results in the preprint we just posted on arXiv: 2403.04316.
Negative cosmological constant and JWST (part 2)
Together with Nicola Menci, Shahnawaz Adil, Upala Mukhopadhyay, and Anjan Sen, today we posted a new preprint which is basically the sequel to our earlier negative cosmological constant and JWST paper published in JCAP. What we did here, in no small part thanks to Nicola’s contribution, was to perform a more thorough analysis of JWST data, which significantly strengthens our earlier conclusions and shows that a dark energy model featuring a negative cosmological constant is a very interesting candidate model in light of the JWST observations. One notable addition was our study not only of photometric observations, but also spectroscopic observations from the FRESCO survey, which again confirm the earlier results and at the same time make them much more robust. It was great fun working on this paper, and I learned a lot about high-redshift galaxies! You can read our results in the preprint we just posted on arXiv: 2401.12659.
Laniakea and the Hubble tension
Extremely excited about my latest work with Leo Giani, Cullan Howlett, Khaled Said, and Tam Davis (all four from the University of Queensland), where we study the impact of Laniakea, the supercluster hosting the Milky Way (also known as our home in the Cosmos) on local cosmological measurements and in particular measurements of the Hubble constant. Our initial hope was that taking into account the local inhomogeneities and anisotropies induced by Laniakea could help alleviate the Hubble tension - surprisingly, we found the opposite! The reason in short is that Laniakea is on average overdense compared to the cosmological background in which it resides, so its effect is the opposite of the prototype one would need to alleviate the Hubble tension locally (e.g. a void) - in other words, if one accounts for Laniakea’s impact on distances when inferring the Hubble constant locally, one should find an even higher Hubble constant, by an amount which we quantify exactly. Congratulations to Leo, who did basically all the heavy-lifting on this paper (incidentally this is what we were working on when he visited), which I expect can become a very important one! You can read our results in the preprint we just posted on arXiv: 2311.00215.
Constraints on fifth forces from OSIRIS-REx tracking data for Bennu
Really excited about the new preprint we just posted with Yu-Dai Tsai, Davide Farnocchia, Marco Micheli, and Luca Visinelli, where we use OSIRIS-REx tracking data for the asteroid Bennu to set new constraints on fifth forces and ultralight dark matter - in certain ranges of mediator mass these are among the tightest constraints ever obtained! This is a follow-up to our earlier paper in JCAP where we only considered the effect of such particles on the asteroid orbital precession. In our new preprint we are instead using real tracking data from the OSIRIS-REx mission and ground-based optical and radar telescopes, gathering as much information as possible on Bennu’s full trajectory. The timing is, to say the least, perfect, as in the past days OSIRIS-REx has been making headlines, being on its way back to Earth with a sample of Bennu. You can read our results in the preprint we just posted on arXiv: 2309.13106.
