9th Barcelona Initiative for Gravitation (BIG) Meeting

Friday 15 May 2026, 09:45 → 18:00 Europe/Madrid

Alberto Lobo Room (Institut de Ciències de l'Espai (ICE, CSIC))

Carlos Sopuerta (Institute of Space Sciences), Sascha Husa (Institute of Space Sciences (ICE, CSIC))

The main goal of this meeting is to bring together researchers working in the different aspects of gravitation in the great area of Barcelona.

09:45 → 10:15 Registration: Welcome to the 9th BIG meeting

Registration

Convener: Carlos Sopuerta (Institute of Space Sciences)

10:15 → 12:00 Morning Session

Conveners: Carlos Sopuerta (Institute of Space Sciences), Sascha Husa (Institute of Space Sciences)

10:15 Probing BH and PBH populations with Laser ranging

Gravitational-wave (GW) signals in the microhertz (μHz) frequency range offer a unique window into both astrophysical and cosmological phenomena, yet their origin remains debated. Lunar Laser Ranging (LLR) detectors, which monitor the Earth-Moon orbital resonance, provide a precise method to distinguish between these sources by probing individual GW events. Leveraging the supermassive black hole binary (SMBHB) population from the European PTA’s second data release, we estimate that LLR could detect between ~10⁻⁴ and ~6×10⁻² chirping binaries over 10–30 years, with no other known astrophysical sources expected to produce such signals. Even two detections would challenge a purely astrophysical interpretation, positioning LLR as a powerful tool for uncovering new compact-object populations. Additionally, scalar-induced GWs from the collapse of primordial curvature perturbations into planetary-mass primordial black holes (PBHs) may generate a stochastic background in the μHz range, detectable by LLR and Satellite Laser Ranging. A null detection could constrain PBH abundances, accounting for electroweak phase transition effects, and complement microlensing observations like those from the HSC collaboration.

Speaker: Dr Miguel Etienne Vanvlasselaer (ICCUB)

11:00 Evaporating black holes: how the burden of their memory stabilizes them

The memory burden effect describes how an object's stored information resists its own decay. I will show how this mechanism can halt Hawking evaporation and dynamically stabilize black holes against complete decay. Importantly, the phenomenon is not exclusive to gravity: it can arise in generic quantum many-body systems and renormalizable field theories, underscoring its broader theoretical relevance. I will then discuss the phenomenological consequences, focusing on potential signatures in the early Universe and today. In particular, memory-stabilized black holes can open a new dark-matter window and produce distinctive high-energy cosmic-ray signals, while leaving correlated imprints on the CMB and other cosmological observables. I will conclude with emerging gravitational-wave implications, including the possibility that mergers of black holes probe the underlying information sector through modified ringdown dynamics.

Speaker: Dr Michael Zantedeschi (IFAE)

12:00 → 13:00 Pizza Seminar: ICE Pizza Seminar

ICE Pizza Seminar

12:00 The meaning of “Big Bang”

What was the Big Bang? Everybody knows (better, thinks to know!). The most widespread (and erroneous) belief is that it was a colossal explosion that took place at the very beginning of the cosmos, and that it launched all the matter and energy concentrated (or “shrunken”) in a “primordial atom” --of unbelievable density and temperature-- into all regions of the universe. It is an enormous task, no matter how hard we scientists try, to erase this image, as spectacular as it is radically false, and replace it with that of the true Big Bang. But, again, what was it? There are nowadays several different cosmological concepts sharing this very name (singularity, inflation stage, whole models, a TV series, ...). And even the best descriptions of this issue, provided by the allmighty AIs still miss important aspects of what Sir Fred Hoyle had in his mind when he pronounced these two words. It was not a mere insult, as widely stated, it had a lot more insight in it!

Speaker: Emili Elizalde (Institute of Space Sciences (ICE, CSIC))

13:00 → 14:30 Lunch Break

14:30 → 17:00 Afternoon Session

14:30 Massive black hole binaries in the era of gravitational wave detectors: A challenge for galaxy formation models?

The inevitable galaxy mergers and the existence of massive black holes (>1e5 Msun) at the centre of most galaxies suggests that galaxies may harbour more than one MBH. After a galaxy merger, two MBHs can eventually get sufficiently close to interact gravitationally and form a massive black hole binary (MBHB). According to general relativity, MBHBs emit gravitational waves (GWs) at different frequencies depending on their masses, and separations. Pulsar Timing Array (PTA) experiments target the nHz regime, and recently reported strong evidence of an unexpected loud stochastic GW background produced most likely by a population of >1e8 Msun MBHBs. The future GW space telescope Laser Interferometer Space Antenna (LISA, to be launched in 2034) targets a complementary window, detecting MBHBs of 1e4-1e7 Msun emitting GWs at mHz. The detection of MBHBs systems will be crucial, as their properties and demographics can provide, for the very first time, tight constraints on the formation and evolution of MBHs. Furthermore, the joint detection of these systems across the GW and electromagnetic spectrum will open a new era of multimessenger astronomy. In this talk, I will present the properties and hosts of the MBHBs detected by the PTA and LISA experiments, as well as the challenges associated with using them in multimessenger studies. To this end, I will introduce the L-Galaxies semi-analytical model, a code that encompasses all the relevant physical processes governing the assembly of galaxies, MBHs, and MBHBs, while coupling them with the dark matter merger trees derived from N-body cosmological simulations.

Speaker: Dr David Izquierdo (Institute of Space Sciences (ICE, CSIC))

15:30 Coffee Break

16:00 End of the BIG Season Discussion

Plans for the Future
New ideas and proposals