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.
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 a 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.
Negative cosmological constant and JWST observations of high-redshift galaxies
Last summer, early observations from the James Webb Space Telescope (JWST) caused quite a stir due to their discovery of a puzzlingly abundant population of extremely massive galaxies at too high redshift, too many to have been in place if the ΛCDM model as we understand it is correct. In today’s new preprint with Shahnawaz Adil, Upala Mukhopadhyay, and Anjan Sen (all three from JMI, and kudos to Shahnawaz and Upala who did all the heavy-lifting!) we study whether these results could be explained by a dark energy model beyond the cosmological constant. In particular we consider a model featuring a negative cosmological constant (anti de Sitter vacuum) with an evolving component (whose energy density is of course positive) on top - this model is phenomenologically motivated from string theory considerations, particularly the swampland program, and the difficulty in constructing consistent de Sitter string vacua. We show that such a model can dramatically alter structure formation and potentially explain how the galaxies seen by JWST could have been in place much earlier than is allowed within ΛCDM. You can read our results in the preprint we just posted on arXiv: 2307.12763.
Inflationary gravitational waves and the pulsar timing array signal
Yesterday was a really exciting and breakthrough day for physics, as four major Pulsar Timing Array (PTA) experiments (NANOGrav, EPTA, PPTA, and CPTA) reported evidence for a stochastic gravitational wave background (SGWB) signal in the nHz range, for which one of the most likely explanations is that of merging supermassive black hole binaries. Today I posted a new single-author paper, where I examine whether the signal could instead have been produced during inflation. The answer is “potentially yes”, although the underlying inflationary model would have to be rather strange, requiring a very blue tilt (~1.8, not something you can get in single-field slow-roll inflation) and a very low reheating scale (at most ~10 GeV). As an aside, I’ve also explicitly written down a bivariate Gaussian approximation to the joint amplitude-tilt posterior for the NANOGrav results, which can come in handy if you want to perform a similar analysis for other models. You can read my results in the preprint I just posted on arXiv (the first since September 2022 - it’s obvious that teaching has come in between 😄): 2306.16912.
