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.
Congratulations to Giovanni Piccoli!
Congratulations to Giovanni Piccoli, who has been offered a PhD position at the University of Zurich, in the group of Jaiyul Yoo! Giovanni will be leaving my group in September to move to Zurich. Congratulations Giovanni, good luck for this new adventure in your life, and looking forward to reading your works!
Negative cosmological constant and JWST paper (part 2) published in JCAP!
My paper with Nicola Menci, Shahnawaz Adil, Upala Mukhopadhyay, and Anjan Sen, where we robustly compare a dark energy model featuring a negative cosmological constant against JWST data (see this earlier news item), has now officially been published in JCAP! The full bibliographic coordinates for the paper are JCAP 2407 (2024) 072. Here is the link to the paper (which is published Open Access).
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.
Scale-invariant inflation paper published in JCAP!
My paper on scale-invariant inflation with Chiara Cecchini, Mariaveronica De Angelis, William Giarè, and Max Rinaldi, which I previously reported on in an earlier news item, has now officially been published in JCAP! The full bibliographic coordinates for the paper are JCAP 2407 (2024) 058. Here is the link to the paper (which is published Open Access).
Negative cosmological constant and JWST paper (part 2) accepted in JCAP!
My paper with with with Nicola Menci, Shahnawaz Adil, Upala Mukhopadhyay, and Anjan Sen, where we robustly compare a dark energy model featuring a negative cosmological constant against JWST data (see this earlier news item), has been accepted for publication in JCAP! The results are basically unchanged compared to our earlier version, with certain aspects of our analysis better clarified. You can read the preprint version of the paper on arXiv: 2401.12659.
Visit by Valerio Faraoni
For the next couple of weeks we have the great pleasure of hosting Valerio Faraoni, currently a Full Professor at Bishop’s University in Canada, and arguably one of the world experts on all things related to gravity. Valerio will also be delivering a seminar by the title of “A bird's eye view of the first-order thermodynamics of scalar-tensor gravity”. Welcome Valerio!
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.
Visit by Benjamin Knorr
This week we have the pleasure to host Benjamin Knorr, currently a postdoc at NORDITA and arguably one of the world experts on asymptotic safety, a highly non-trivial generalization of the idea of perturbative renormalization. Benjamin also delivered a very nice talk by the title of “Asymptotic safety meets field redefinitions”. Welcome Benjamin!
New impact factor for Physics of the Dark Universe
Physics of the Dark Universe, the journal for which I am Editor, performed once again very well at the 2024 release of the annual Journal Citation Reports. Our updated impact factor (IF) for 2023 is 5.0 (the number reflects works published in 2021 and 2022, so still before my time)! This number has gone a bit down from our 2022 one, which was 5.5, yet remains high for the standards of the field. In fact, we are competitive with the other top-tier journals in cosmology and high-energy physics, such as PRD (4.6), PLB (4.3), JCAP (5.3), ApJ (4.8), and MNRAS (4.7), whose IFs all fall in the 4.5-5.5 ballpark. Huge thank you to everyone who helped us grow!
Scale-invariant inflation paper accepted in JCAP!
My paper with with with Chiara Cecchini, Mariaveronica De Angelis, William Giarè, and Max Rinaldi, where we tested a theoretically very well-motivated classically scale-invariant inflationary model against current cosmological data (see this earlier news item), has been accepted for publication in JCAP! There were a few minor changes in order to better clarify a few aspects of our analysis, but the results are completely unchanged with respect to the earlier version. You can read the preprint version of the paper on arXiv: 2403.04316.
Visit by Leonardo Giani
We are delighted to have Leonardo (Leo) Giani visiting us once more! Leo is a postdoc at the University of Queensland, where he is working on a bunch of very interesting things gravity- and cosmology-related, including our recent work on Laniakea. We took the opportunity to catch up on a few ideas which had been hanging around (and are turning into student projects), while Leo also gave a seminar by the title of “Cosmology from the point of view of an almost spherical cow”.
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.
Visit by Luca Visinelli
We are delighted to have Luca Visinelli visiting us once more! Luca is a Professor at the Tsung-Dao Lee Insittute in Shanghai Jiao Tong University: he is a very well-known scientist with broad research interests spanning dark matter, dark energy, and black holes. It was a very enjoyable visit during which we took the opportunity to catch up on our ongoing projects (especially replying to pending referee reports!) and enjoy the surroundings of Povo, while Luca also gave a seminar by the title of “Theoretical motivations for a light boson and its phenomenology”.
Visit to Camerino
Today and for the next two days I am visiting the University of Camerino, where I will be giving a talk on “Searching for dark energy off the beaten track” (see poster below, slides here). I will be hosted by Orlando Luongo, whom I look forward to meeting in person after reading his works for many years. I look forward to a nice visit in a charming city!
State of the dark energy equation of state paper published in JCAP!
My paper on the dark energy equation of state with Luis Escamilla, William Giarè, Eleonora Di Valentino, and Rafael Nunes, which I previously reported on in an earlier news item, has now officially been published in JCAP! The full bibliographic coordinates for the paper are JCAP 2405 (2024) 091. Here is the link to the paper (which is published Open Access).
Teaching ends today (for now)
My teaching duties for the spring semester end today, after having covered in depth the second law of thermodynamics. The next month will be busy serving on various committees, after which I look forward to a couple of months almost entirely dedicated to research!
Visit by Francesco Di Filippo
We’re excited to welcome our latest visitor: Francesco Di Filippo from Charles University in Prague! Francesco, hosted by Max Rinaldi, is currently a postdoc at Charles University in Prague, where he has been doing a lot of interesting work especially on regular black holes and possible instabilities (or not) thereof, together with a bunch of experts in the field including Stefano Liberati and Matt Visser - he will be delivering a seminar at TIFPA by the title of “Non-singular black holes: Open issues and implications”. Welcome Francesco!
