Modified Gravity on Trial

Informazioni relative al progetto

ModGravTrial

ID dell’accordo di sovvenzione: 801781

DOI

10.3030/801781

Progetto chiuso

Data della firma CE 5 Ottobre 2018

Data di avvio 1 Gennaio 2019

Data di completamento 31 Dicembre 2024

Finanziato da

• EXCELLENT SCIENCE - European Research Council (ERC)

Costo totale

€ 1 500 000,00

Contributo UE

€ 1 500 000,00

Coordinato da

RUPRECHT-KARLS-UNIVERSITAET HEIDELBERG
Germany

Periodic Reporting for period 4 - ModGravTrial (Modified Gravity on Trial)

Periodo di rendicontazione: 2023-05-01 al 2024-12-31

Sintesi del contesto e degli obiettivi generali del progetto

Our research explores some of the most profound and open questions in physics:

*What makes gravity fundamentally different from the other forces of nature?
*What are the essential features that define gravity?
*How can we reconcile Einstein’s theory of gravity with quantum mechanics?
*And how can we extract deep insights about the universe from what we observe in cosmology and particle physics?
These questions lie at the heart of our work, and the ERC Starting Grant is instrumental in helping us address them.

Our research is organized into four main areas:

1) Rethinking Gravity through New Lenses:
We study gravity from different mathematical and physical perspectives to find simpler ways to tackle complex problems in Einstein’s theory, such as how to define energy in a gravitational field or how to describe gravity at the quantum level.
2) Exploring Theories Beyond Einstein:
Going beyond general relativity requires care: many alternative gravity theories are mathematically inconsistent or physically problematic. We have constructed broad classes of these extended theories and rigorously test them to ensure they make sense—checking for things like stability, causality, and mathematical soundness.
3) Linking Gravity to Quantum and String Theory:
We treat gravity as the low-energy shadow of a deeper quantum or string theory. Our research looks at how quantum effects might subtly change gravity at large scales, how these effects can be calculated, and what this means for our understanding of the early universe and fundamental physics.
4) Connecting Theory to Observations:
Any viable theory of gravity must ultimately match what we see in the sky and in experiments. We study the real-world implications of our theoretical work—how it affects gravitational waves, the formation of cosmic structures, the behavior of dense astrophysical objects, and possible new physics beyond the known particles.

These research efforts are structured into six interconnected work packages supported by the ERC grant.

We have developed two powerful ways to describe gravity: one based on geometry and another using field theory. On the geometrical side, we’ve shown that gravity can be explained entirely through a property called non-metricity. This fresh perspective has major implications for understanding gravitational energy and entropy. It also opens the door to promising new theories, such as f(Q) gravity, which could offer alternatives to Einstein’s general relativity. On the field theory side, we’ve explored what happens when gravity includes a massive force-carrying field—specifically a vector field, as seen in Generalized Proca theories. Our research spans a wide range: from ensuring these theories are stable at the quantum level to studying how they might affect gravitational waves. Together, these approaches help us deepen our understanding of gravity and explore how it might behave differently from what we currently expect.

Lavoro eseguito dall’inizio del progetto fino alla fine del periodo coperto dalla relazione e principali risultati finora ottenuti

In modern cosmology, one of the key challenges is to develop a theory of gravity that not only fits our observations but also holds up under the rules of quantum physics. These so-called effective field theories of gravity are delicate: their parameters must be carefully fine-tuned to avoid internal inconsistencies.

We studied several of the most promising gravity theories—like Galileons, Horndeski, and Generalized Proca—to see if they remain stable when quantum effects are taken into account. If quantum corrections (known as “loops”) change the interactions too much, they can cause a breakdown in the theory, leading to unwanted and unphysical effects called ghost instabilities.

Our team performed an in-depth analysis of these quantum effects and showed that Horndeski and Generalized Proca theories remain stable—meaning their basic parameters don’t need unnatural fine-tuning. This result is a major step in proving these models are physically viable.

With this solid theoretical foundation, we moved on to explore how these models could shape the universe. We systematically classified which versions of these theories can explain key cosmic events—like the early inflationary period and the current accelerated expansion (dark energy). We also applied them to astrophysics, uncovering new types of black hole solutions and predicting how these theories could leave detectable traces in gravitational waves from merging stars or black holes.

Progressi oltre lo stato dell’arte e potenziale impatto previsto (incluso l’impatto socioeconomico e le implicazioni sociali più ampie del progetto fino ad ora)

Our understanding of gravity—and with it, the origin, structure, and evolution of the universe—has advanced dramatically in recent years. These breakthroughs would not have been possible without continuous efforts to develop cutting-edge ground-based and space-based experiments. Each new generation of these experiments not only demanded major technological innovation but also required sophisticated data analysis techniques to handle the enormous volumes of data with ever-greater precision.

The ESA satellite Planck, for instance, has given us the most accurate measurements to date of tiny temperature variations in the Cosmic Microwave Background (CMB), the afterglow of the Big Bang. Combined with other observations, these measurements have allowed scientists to build a precise picture of how the large-scale structure of the universe formed over time. Our research group used these data to test various theories of gravity, particularly in the early and linear phases of cosmic evolution.

The next step in this quest is Euclid, ESA’s new satellite mission. To fully interpret its data, we will need highly advanced tools—especially powerful simulations that model how cosmic structures, such as galaxies and clusters, form in more complex and realistic scenarios. Our team developed novel simulations that go beyond existing models by incorporating effects from more general gravity theories.

Gravitational wave (GW) astronomy is another exciting new window into the universe. Since LIGO’s historic first detection in 2015, we’ve been able to "listen" to ripples in spacetime caused by cosmic events like black hole collisions. These signals are incredibly subtle—LIGO can detect changes smaller than the width of a hydrogen atom over the distance between the Earth and the Sun. ESA’s upcoming LISA mission, a space-based gravitational wave observatory, will take this sensitivity even further.

To interpret GW data correctly, we must understand in detail how compact objects like black holes merge—a task that requires precise theoretical predictions. Through this ERC project, our group has played a key role in unlocking the scientific potential of both Euclid and LISA. These two flagship missions represent milestones in ESA’s long-term strategy, and we are proud to be actively contributing to their success.

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