A Cosmic Backdrop
The story of modern cosmology began with the 1963‑1965 discovery of the Cosmic Microwave Background (CMB) by Penzias and Wilson – “the electromagnetic radiation produced when the Universe became transparent to light, about 380 000 years after birth.” To probe still earlier moments, José’s research turns to a different messenger: the Stochastic Gravitational‑Wave Background, a faint “white noise” of space‑time ripples generated as early as 10-32 seconds after the Big Bang.
During his PhD in Porto, he investigated the relics of ancient phase transitions – domain walls (sheet‑like) and cosmic strings (wire‑like). “A good analogy is to think of water freezing, and how ice has these sort of cracks or discontinuities. Strings and walls usually form these complicated tangles, and understanding how they evolve over cosmic history is crucial for observational constraints,” he explains. By running large‑scale simulations on thousands of GPUs, he calibrated evolution models that link those tangled defects to observable imprints in gravitational‑wave backgrounds.
His first postdoc in Helsinki shifted the focus from the remnants to the transitions themselves. “It is more akin to water boiling, where bubbles of vapour expand amidst the water,” he describes. In the early Universe, bubbles of a low‑temperature phase would have grown in a hot plasma, generating sound waves that source gravitational waves. “Approximating these particles as a fluid, any sound waves of this fluid will produce gravitational waves, which will leave a footprint on the stochastic background.” The work required mastering hydrodynamics simulations – “numerically challenging and, at the start of the postdoc, unfamiliar ground.”
José's work at ITA
At ITA José is again using giant simulations, this time to answer two intertwined questions that will be crucial for the upcoming Laser Interferometer Space Antenna (LISA) mission (launch planned for 2034). “I will use large-scale simulations to study gravitational waves emitted at the phase transition itself (if first-order) or the gravitational waves emitted by fossils of the transition (cosmic strings).”
First‑order phase transitions – He will model the turbulent, shock‑dominated fluid dynamics that accompany bubble collisions, and compare the resulting gravitational‑wave spectra with current analytical predictions. “Improving such models ahead of LISA is of paramount importance to make sure we obtain as accurate a prediction as possible.”
Cosmic‑string networks – Two long‑standing simulation approaches make different approximations for how tangled strings evolve, and they have disagreed on key features for decades, leading to order‑of‑magnitude uncertainties in stochastic‑background forecasts. José plans to “take strings from one simulation type and use them as initial conditions in the second one directly,” allowing a head‑to‑head comparison of the two methods.
Both projects involve extending existing codes into uncharted numerical regimes, and confronting analytical models with high‑resolution data. “In numerical work there are plenty of reasons not to sleep at night,” he admits, highlighting the twin challenges of computational difficulty and the need to understand how assumptions shape results.
Why Oslo and ITA?
“I was interested in working in the Nordic countries due to personal and professional reasons. Having adapted to life in Finland, I was also sure that it would be easier to adapt to Norway and Oslo than when I moved from Porto to Helsinki (trust me, that was quite the culture shock!).” José recalls. A conversation with fellow Portuguese Professor David Mota at a conference sparked the idea of applying for the DSTrain fellowship, and “the rest is history”.
Since arriving, José has been “enjoying the welcoming atmosphere of the Institute and all the friendly folk I have met so far.” He also appreciates Oslo’s urban charm combined with easy access to nature: “As a city person I also enjoy Oslo very much, but I would be remiss if I didn’t mention how easy it is to go on hikes.”
Looking Ahead
José expects his time in Oslo to sharpen his numerical expertise and open the door to new topics within early‑Universe Cosmology. “Being immersed in a stimulating environment where discussions with my peers and seniors can occur regularly is incredibly important.” He hopes the collaboration will be “scientifically fruitful but ultimately, also a fun time.”