My paper with Jun-Qian Jiang, Davide Pedrotti, and Simony Santos da Costa, where we perform a non-parametric reconstruction of the late-time expansion history in light of DESI BAO data (see this earlier news item), has now officially been published in PRD! The full bibliographic coordinates for the paper are Phys. Rev. D 110 (2024) 123519. Here is the link to the paper.
Non-parametric expansion history reconstruction paper accepted in PRD!
My paper with Jun-Qian Jiang, Davide Pedrotti, and Simony Santos da Costa (first one entirely produced within my group), where we perform a non-parametric reconstruction of the late-time expansion history in light of DESI BAO data (see this earlier news item), has been accepted for publication in PRD! Compared to the earlier version, there have been minor changes to the plots and title, but our main results are unaltered. You can read the preprint version of the paper on arXiv: 2408.02365.
Varying electron mass and ΛsCDM paper published in PDU!
My paper with Yo Toda, William Giarè, Emre Özülker, and Eleonora Di Valentino, where we attempt to solve the Hubble tension combining 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 (see this earlier news item), has now officially been published in PDU! The full bibliographic coordinates for the paper are Phys. Dark Univ. 46 (2024) 101676. Here is the link to the paper (which is published Open Access).
Varying electron mass and ΛsCDM paper accepted in PDU!
My paper with Yo Toda, William Giarè, Emre Özülker, and Eleonora Di Valentino, where we attempt to solve the Hubble tension combining 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 (see this earlier news item), has been accepted for publication in PDU! Minor revisions compared to the previous version include a small change to the title, small updates to the figures and tables for consistency, and especially a proper Bayesian evidence-based model comparison analysis. You can read the preprint version of the paper on arXiv: 2407.01173.
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.
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.
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 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”.
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).
State of the dark energy equation of state paper accepted in JCAP!
My paper with with Luis Escamilla, William Giarè, Eleonora Di Valentino, and Rafael Nunes, where we present state-of-the-art constraints on the dark energy equation of state from a number of cosmological probes (see this earlier news item), has been accepted for publication in JCAP! There were a few changes, mostly making a few parts of the discussion clearer, but the results are otherwise completely unchanged compared to the previous version we posted this summer. You can read the preprint version of the paper on arXiv: 2307.14802.
Jun-Qian Jiang joins my group!
I’m very happy to welcome my latest group member, Jun-Qian Jiang (江俊钱)! Jun-Qian is currently a PhD student at the University of Chinese Academy of Sciences under the supervision of Prof. Yun-Song Piao, and has been doing a lot of interesting and very diversified work in the fields of cosmological tensions, inflation, gravitational waves, and so on. He joins my group for the next 6 months as a long-term visiting PhD student. We still have to figure out what we will be working on, but it likely will have to do with cosmological tensions and possibly implications for inflation. Welcome Jun-Qian, and I hope you will enjoy your stay in Italy!
Tonale Winter School
This week I’m travelling to Passo del Tonale to lecture at the 2023 Tonale Winter School on Cosmology. The weather is fortunately fantastic, with clear skies and a lot of snow (below is the view from my hotel window)! For those of you who might be interested, below are the slides I will use during the lectures, and the questions which will be discussed in the working group sessions:
Lecture 1: Basics of theoretical and observational cosmology [Slides]
Lecture 2: Measuring the Hubble constant – the Hubble tension [Slides]
Lecture 3: How (not) to solve the Hubble tension? [Slides]
Lecture 4: Other tensions and challenges for ΛCDM [Slides]
I have really fond memories from the time I attended as a student in 2014 (fun fact: my roommate was Vivian Poulin, now one of the big names in the Hubble tension game), and I’m looking forward to an exciting week!
Davide Pedrotti (re)joins my group!
Today Davide Pedrotti, after an excellent MSc defense, (re)joins my group as a PhD student. We will be starting some very cool work on cosmological tensions, while also carrying on Davide’s earlier research interests in black hole physics. Stay tuned in particular for our upcoming paper on the connection between quasinormal modes and shadows for rotating regular BHs, which constituted a major part of Davide’s MSc thesis! Welcome (back) Davide!
Seven hints paper makes cover page of September issue!
My seven hints paper - Universe 9 (2023) 393 - has been selected as cover story for the September 2023 issue (Volume 9, Issue 9) of Universe! The production team did a very nice job in designing the cover story image, which you can find here. My paper was also selected as a Feature Paper and an Editor’s Choice. I also recommend to take a look at the Special Issue where my paper was published, which features a bunch of other really interesting papers!
PhD defense of Tiziano Schiavone
Today I had the pleasure of serving as external committee member for the PhD thesis discussion of Tiziano Schiavone, a PhD student at the University of Pisa supervised by Giovanni Marozzi, Giovanni Montani, and Giuseppe Fanizza. Tiziano wrote an excellent thesis by the title of “Large-scale structure of the Universe in General Relativity and beyond”, which on the one hand studied the impact of local inhomogeneities on cosmological observables, and on the other hand examined ways to distinguish between ΛCDM and competing cosmological models, particularly those based on modifications to gravity, especially in light of cosmological tensions. The other two committee members were my collaborator Eleonora Di Valentino and Scilla Degl’Innocenti. Unfortunately I could not be present in person for the first ever PhD defense of which I am committe member (I would have loved to visit the beautiful city of Pisa after many years). Tiziano’s defense was excellent (the final mark we gave was “ottimo”, which roughly translates to something between “very good” and “excellent”), and he will now be moving to the University of Lisbon as a Della Riccia fellow for his first postdoc - congratulations Tiziano!
Corfu2023 Workshop on Tensions in Cosmology
Straight after the XXV SIGRAV conference, I am travelling to Corfu, Greece, to attend the Corfu2023 Workshop on Tensions in Cosmology. The workshop, as the name suggests, is focused on cosmological tensions, with special attention to the Hubble tension. I have been particularly looking forward to this workshop, firstly because it will be an occasion to catch up with some friends colleagues I haven’t seen in a long time (e.g. Vivian Poulin), and finally meet several people I’ve written many papers with (e.g. Eleonora Di Valentino) or frequently interacted with online (Leandros Perivolaropoulos, Jackson Levi Said, and Özgür Akarsu, to name but a few), but never actually met in person, but also because I’ve never been to Greece before. I will also be delivering a plenary talk on “Seven hints that early-time new physics alone is not sufficient to solve the Hubble tension” (whose slides you can find here), obviously focused on my “seven hints” paper. I look forward to lots of fruitful discussions, and interesting workshop, and a series of exciting social events (including a boat trip and a traditional Greek dinner)!
Media coverage for seven hints paper
My seven hints paper has been picked up by Universe Today, who write a nice piece which you can find here:
www.universetoday.com/163026/its-going-to-take-more-than-early-dark-energy-to-resolve-the-hubble-tension/
The same piece was later picked up by Phys.org. Enjoy the read! 😃
Seven hints that early-time new physics alone is not sufficient to solve the Hubble tension
I’m very excited to share that my latest single-author paper (on which I already gave three talks) has now been published in Universe (in a Special Issue guest edited by Eleonora Di Valentino, Leandros Perivolaropoulos, and Jackson Levi Said)! This is an opinion paper where I argue that the Hubble tension is even nastier than it looks and that, if we insist on it requiring new physics, it will not be enough to add early-time (i.e. pre-recombination) new physics - instead, I present seven reasons in favor of my argument that one should combine early- and late-time new physics, and potentially local new physics as well. The choice of number seven is motivated by Miller’s law, which states that the number of objects the average person can hold in working memory is 7±2. The full bibliographic coordinates for the paper are Universe 9 (2023) 393, and you can also find it in preprint form on arXiv: 2308.16628. Here is the link to the paper (which is published Open Access).
The state of the dark energy equation of state
The dark energy equation of state w is one of the cosmological parameters a number of next-generation surveys aim to measure particularly well, and it is therefore quite surprising that there wasn’t a single paper after the 2003 Melchiorri-Mersini-Ödman-Trodden paper (“The state of the dark energy equation of state”) comprehensively discussing state-of-the-art constraints on w from a number of probes (rather, various papers usually focus on one probe at a time), especially in light of the possible impact of w on cosmological tensions. In today’s new preprint with Luis Escamilla, William Giarè (yes, this was one of the main things William and I worked on during his visit to Trento), Eleonora Di Valentino, and Rafael Nunes, we therefore found it very timely to provide a snapshot of the state of the dark energy equation of state, circa 2023 of course. What we found confirmed a suspicion I have had for a long time, i.e. that current constraints on w (when including data from the CMB) cluster around w~-1.03, and in any case just into the phantom regime. Why is this? We haven’t been able to provide a clear answer, but hopefully you will find some interesting discussions on this and other points in the preprint we just posted on arXiv: 2307.14802.
Tonale Winter School registration open
Registration for the 2023 Tonale Winter School on Cosmology, where I will be lecturing, is now open. Besides cosmological tensions, the other topics covered this year are stochastic gravitational waves backgrounds, full-sky surveys, and the effective field theory of structure formation. Please see the official school page for further details. Only a maximum of 40 participants will be accepted and, given the large number of applications usually received, it is strongly recommended to register as soon as possible. Note that I am not involved in the selection process, so please reach out to the organizers if you have any questions.
