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).

Internal Call for Research 2023 grant application

I just submitted an application for an UniTrento Internal Call for Research 2023 grant, funded by the Autonomous Province of Trento (PAT). I based my scientific case largely on the ERC StG application I submitted last year, obviously after taking into account reviewer feedback. I should hear back in October and, if successful, this would allow me to hire a couple of postdocs - I’m keeping my fingers crossed!

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 PTA paper published in JHEAp!

My single-author paper studying an inflationary interpretation of the signal observed by PTA experiments, which I previously reported on in an earlier news item, has now officially been published in JHEAp! The full bibliographic coordinates for the paper are JHEAp 39 (2023) 81. Here is the link to the paper (which is published Open Access).

Sgr A* horizon-scale tests paper published in CQG!

My paper with Rittick Roy, Yu-Dai Tsai, Luca Visinelli, Misba Afrin, Alireza Allahyari, Parth Bambhaniya, Dipanjan Dey, Sushant Ghosh, Pankaj Joshi, Kimet Jusufi, Mohsen Khodadi, Rahul Kumar Walia, Ali Övgün, and Cosimo Bambi on horizon-scale tests of gravity theories and fundamental physics from the EHT image of Sgr A*, which I previously reported on in an earlier news item, has now officially been published in CQG! The full bibliographic coordinates for the paper are Class. Quant. Grav. 40 (2023) 165007. Here is the link to the paper (which is published Open Access).

Congratulations to Davide Pedrotti!

Congratulations to Davide Pedrotti, who has won one of the highly competitive “free topic” PhD positions at the University of Trento, financed by INFN! Davide will begin his position at the start of November, and will keep working within our group. Congratulations Davide, and looking forward to continuining working together!

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.

Inflationary gravitational waves and PTA paper accepted in JHEAp!

My single-author paper where I examine an inflationary interpretation of the signal observed by PTA experiments (see this earlier news item) has been accepted for publication in JHEAp! For once, after papers which went through extremely long journeys, this was a very quick turnaround, as the referee report clearly highlighted the timeliness and importance of the results. You can read the preprint version of the paper on arXiv: 2306.16912.

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.

New impact factor for Physics of the Dark Universe

The 2023 release of the annual Journal Citation Reports from Clarivate Analytics has now been made public, and with it the updated impact factors (IFs) for several journals. I am thrilled to announce that the journal of which I am Editor, Physics of the Dark Universe, performed extremely well, with a 2022 IF of 5.5! For the full list of IFs, see this document. This confirms once more the high impact of papers published in PDU, which places us at a level comparable to journals such as PRD (5.0), PLB (4.4), JCAP (6.4), ApJ (4.9), and MNRAS (4.8), just to mention a few. Please consider submitting your next paper to PDU!

Three recent talks

In the past three days I delivered three (online) seminars, all by the title of Seven hints that early-time new physics alone is not sufficient to solve the Hubble tension. I gave the talks “at” IPM, Tehran (Iran); the Institute of Advanced Studies in Science and Technology of Babes-Bolyai University (Romania), within Tiberiu Harko’s highly recommended seminar series in Gravitation, Cosmology, and Astrophysics; and at the conference Recent Advances in Mathematical Sciences and Interdisciplinary Areas 2023 at GLA University (India). I basically reported about a work in progress which I expect will be out in the next three months (so stay tuned!), and all the slides are available on my talks page. A pictorial summary of the work, made by my wife, is shown below.

Tonale Winter School on Cosmology

I’m pleased to announce that I will be lecturing at the Tonale Winter School on Cosmology later this year. This is a well-known cosmology winter school addressed to advanced MSc students, PhD students, and junior postdocs, which started in 2007 and takes place in Passo del Tonale, Italy. I myself attended as a student in 2014. This year I will deliver 4 lectures on cosmological tensions. More details, including the official school page, will follow later!

Teaching ends today (for now)

Today marks the end of the teaching period, which will restart in September. Now of course it’s time for exams (already the coming Monday)! While it was truly nice to be able interact with many bright young minds, teaching two and a half courses was also particularly heavy, I’m not gonna lie, so I really look forward to a couple of months where I will be able to mostly focus on research. So, hopefully, several new papers should come out in the next months. Stay tuned!

Giovanni Piccoli joins my group!

My group is expanding, and as of today includes also Giovanni Piccoli. He will be working on his Master’s thesis under my supervision, and we plan to explore novel cosmological and terrestrial tests of the (very) small-scale matter power spectrum - no spoilers, you’ll have to wait and see what we come up with! Welcome Giovanni, and looking forward to our work together!

Sgr A* horizon-scale tests paper accepted in CQG!

I’m really excited that my paper on horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope (EHT) image of Sagittarius A* (Sgr A*) has been accepted for publication in CQG! This is a huge paper I led with 14 other authors spread throughout the world (our countries of affiliation include Italy, UK, China, USA, India, Iran, Canada, South Africa, North Macedonia, and Turkey): Rittick Roy, Yu-Dai Tsai, Luca Visinelli, Misba Afrin, Alireza Allahyari, Parth Bambhaniya, Dipanjan Dey, Sushant Ghosh, Pankaj Joshi, Kimet Jusufi, Mohsen Khodadi, Rahul Kumar Walia, Ali Övgün, and Cosimo Bambi. We used the EHT image of Sgr A* to test over 50 well-motivated theoretical scenarios, ranging from theories of gravity, novel fundamental physics frameworks, and black hole mimickers such as naked singularities and wormholes. This paper was initially (see v1) written only with Rittick, Yu-Dai, and Luca, tirelessly and also shamelessly ambulance-chased over a weekend following the exciting EHT announcement on May 12, 2022, but was then substantially polished and extended throughout summer 2022 with the addition of all the new authors - it is easily one of the most impactful papers I’ve ever written and at the same time likely the paper that drained the most energy out of me, and I’m really relieved to finally see it accepted for publication, after a very long refereeing process (one of the referee reports was a 4-page long pdf in 10pt font!). You can find the preprint version of the paper on arXiv: 2205.07787.

Visit by Michael Zantedeschi

This week we have Michael Zantedeschi, currently a postdoc at the Tsung-Dao Lee Institute, Shanghai Jiao Tong University (in the group of Luca Visinelli), visiting us for a couple of days. Michael is actually not new to Trento, having been an undergrad here (although he arrived just after I finished my undergrad, so we never overlapped). While we aren’t directly working together (yet), we will be brainstorming ideas related to screened dark energy, as well as the mysterious cosmological constant. Welcome Michael!