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!

ERC Starting Grant evaluation report

Today I received the evaluation report for Step 1 of my ERC Starting Grant application, which was unsurprisingly unsuccessful - as I promised in an earlier news item, I have decided to openly report the outcome of the evaluation to help normalize otherwise taboo discussions about lack of success in academia. I received a “B” (proposal of high quality but not sufficient to pass to Step 2 of the evaluation), which means I am blocked from applying for a further year. The comments from the 4 reviewers were actually extremely positive (their ratings included 7 “exceptional”, 3 “excellent”, and 2 “very good”), and the high-risk high-gain nature of the project was clearly appreciated. The main issue identified by more than one reviewers was that I probably put too much at once - a subset of the proposal focusing on 2/3 of the ideas I put out there would probably have stood a much better chance. This was a very useful experience, the reviewer comments were very helpful and I have to say somewhat reassuring, so at this point there’s not much more I can do than roll up my sleeves, while looking forward to trying again in 2024 taking this feedback into account!

PhD positions at the University of Trento

The selection for admission to the 39th cycle of the PhD in Physics at the University of Trento is now open, see this link. For interested students not familiar with the procedure, this is done at a centralized (departmental) level, rather than by individual supervisors, through a committee of (typically) four people, which varies each year. There are a total of 22+1 positions, 19+1 of which with scholarship, of which 15+1 on a specific topic and the remaining 4 “free”: since I am not funding any “specific topic” position, students interested in working with me will have to compete for the 4 free fellowships, which means that there is no guarantee I will actually get to have an incoming PhD student this year - note that competition for these free fellowships is typically extremely tough, and it is recommended that if you are interested in working with someone specific, you should explicitly mention this in the cover letter. For a rough guideline on possible topics one could work on with me, please see this link (Section “First call 39° cycle - Other research topics”), and refer to the topics “Dark matter and dark energy in the era of precision multi-messenger cosmology and cosmic tensions (Vagnozzi)” and “Black holes as windows onto fundamental physics (Vagnozzi)” (note, however, that these are not binding). The deadline for applying is May 31 at 16:00 Italy time - please spread the word, and apply if you are interested!

Visit by Leonardo Giani

More visitors! This week we have Leonardo (Leo) Giani, currently a postdoc at the University of Queensland, visiting us for a couple of days. Leo and I are working on a very exciting project, which (spoiler) has to do with Laniakea, our home in the Cosmos, while also discussing other ideas more or less related to dark energy and cosmological tensions. Leo will also be delivering a seminar by the title of “Doctor suggests a new diet for Non-local gravity”. Welcome Leo!