Theory Colloquia at the Physics Department of Torino

In summer 2023, pulsar timing array (PTA) collaborations around the globe announced compelling evidence for a nanohertz gravitational-wave background (GWB)—a breakthrough achievement with important implications for astrophysics and particle physics. The most common interpretation of the new signal is that it originates from a cosmic population of supermassive black-hole binaries. At present, it is, however, also conceivable that it represents a gravitational-wave echo of the Big Bang. In my talk, I will focus on the latter possibility and illustrate how PTA data can be used to probe new particle physics in the early Universe. I will conclude with a brief outlook and explain how future PTA data will help in discriminating between a GWB signal of astrophysical origin and a GWB signal from the Big Bang.

Friday, 21st March 2025, 14:30, Sala Wataghin

In this colloquium, I will present recent developments in the study of Conformal Field Theories (CFTs) at finite temperature. The motivation of this work is different applications to holography, black holes in AdS as well as interesting statistical physics systems. After a short overview to the usual (zero Temperature) conformal bootstrap approach to CFTs, I will discuss how the dynamics at finite temperature are constrained by the Kubo-Martin-Schwinger (KMS) condition. I will present explicit sum rules for one-point functions, providing a basis for setting up a numerical bootstrap problem, the power of which I will demonstrate with specific examples. Furthermore, the KMS condition can be used to extract the leading behavior of one-point functions for heavy operators analytically. Lastly, I will show how to generalize this framework to temporal line-defects akin to Polyakov loops in gauge theories.

Thursday, 27th February 2025, 14:30, Aula C

Many interesting theories, including Yang-Mills and string theory, have a Hagedorn regime, where the number of states increases exponentially with the energy. This implies the existence of a "limiting temperature", known as the Hagedorn temperature T_H. The regime where the temperature is slightly smaller than T_H is still somehow mysterious. I will discuss a few properties of this regime, beginning from the actual value of T_H, and wonder about its possible phenomenological relevance.

Friday, 06th December 2024, 14:30, Aula C

The Dark Energy Spectroscopic Instrument (DESI) is the first of a new generation of “Stage-IV” cosmology survey experiments to be collecting data. I will describe the experiment and the cosmological results obtained from the first data release (DR1) – this includes redshifts for more than 6 million galaxies and quasars, a factor of 3x larger than for the previous biggest such survey! These data allow us to precisely measure the expansion rate of the Universe over the last 11 billion years. DESI results released in April 2024 were based on observations of the baryon acoustic oscillation feature, and showed tantalising evidence in favour of a time-varying dark energy equation of state, rejecting the cosmological constant at between 2.5 sigma and 3.9 sigma significance, depending on combinations with external datasets. I will describe these results as well as new ones based on the “full-shape” of the clustering power spectrum including redshift-space distortions, to be publicly released on November 19. Apart from dark energy, I will discuss the implications of DESI results for the Hubble constant, the neutrino mass scale and tests of general relativity.

Friday, 29th November 2024, 14:30, Sala Wataghin

Axions have been introduced in relation to the strong CP problem ofthe QCD and are predicted in many extensions of the Standard Model of particle physics. Depending on the actual value of their mass, axions can play an important role in cosmology, acting as dark matter. The coupling with photons allows for axion-photon mixing in external electromagnetic fields. This effect is exploited for direct searches of axions in laboratory experiments. Axions can be searched also through astrophysical observations. Notably they can be produced in stellar cores leading to an excessive energy loss, that would alter the standard stellar evolution. Furthermore, the two-photon vertex would also induce the mixing with axions for photons emitted by distant astrophysical sources, and propagating in the large-scale cosmic magnetic fields, leading to peculiar observational signature. In this talk I will present an overview of current bounds on axions and discovery opportunities in the planned laboratory and astrophysical experiments.

Friday, 25th October 2024, 14:30, Aula A

Over the past couple of years a clear gravitational anomaly has been reported and confirmed by two independent research groups carefully considering relative velocities, v and internal separations, s, on the plane of the sky, for wide binary star samples from the most recent GAIA catalogue. Over various studies covering a range of sample selection strategies and statistical analysis techniques, a surprising phenomenology has emerged. While the small separation samples for s<2000 au accurately conform to Newtonian expectations, for separations above 3000 au, a clear and systematic departure from Newtonian predictions appears. This high separation regime shows a v proportional to s^(-1/2) scaling, but corresponding to Keplerian orbits under an effective gravitational constant of 1.5G. Given the narrow range of total masses of around 1.6 M_sun in the samples considered, the critical separation at which a change in regime appears corresponds to approaching the a0 threshold, where a0 is the characteristic acceleration scale of MOND, as inferred from galactic rotation curve observations. Further, the precise distribution of wide binary relative velocities measured, closely corresponds to MOND expectations for such solar neighbourhood systems under the external field effect predicted by MOND. Now that a low acceleration validity limit for Newtonian gravity has been found, precisely at the acceleration scales over which the presence of dark matter has been proposed, astrophysical inferences for such hypothetical component become suspect.

Wednesday, 9th October 2024, 11:00, Aula Wataghin