Periodic Reporting for period 1 - MGCODE (Modified Gravity: from Compact Objects to Dark Energy)
Reporting period: 2023-01-01 to 2025-05-31
Einstein's theory of General Relativity has been very successful in describing gravitational physics at a wide range of scales, from the astrophysical to the cosmological. At the largest of these cosmological scales, however, in order to do so the theory requires the inclusion of unknown matter and energy components, which have remained otherwise undetected by other means so far. Due to their mysterious nature, they are commonly referred to as Dark Matter and Dark Energy, respectively. The latter is especially intriguing, as it must have exotic properties in order to fill in the role of feeding the observed accelerating expansion of the universe. What is the true nature of Dark Energy remains a fundamental open question in physics.
A tantalizing possibility is that Dark Energy is not just yet another contribution to the energy budget of the universe, even if it seems to be the majority one, but rather a manifestation of gravity itself behaving differently than expected. This would mean a departure from the theory of General Relativity at the largest of cosmological distances. Nonetheless, testing this hypothesis is not an easy task as the only place where the effect of Dark Energy has been observed so far is the accelerated expansion of the universe. This fact, coupled with the sheer amount of possible ways to extend the theory of General Relativity has been a limiting factor in understanding the nature of Dark Energy from cosmological observations alone. The MGCODE project aimed to find new signatures of Dark Energy in a very different place, at the much shorter scales of black holes. The very strong gravity in their vicinity gives rise to a regime that still remains largely untested, and which is only recently becoming accessible through the detection of gravitational waves produced by the collision of pairs of black holes, neutron stars, and/or other compact objects. For this reason, it is an interesting place to look for deviations from the theory of General Relativity, and while this is certainly usually considered by the astrophysics community, the possible connection to deviations at cosmological scales is often overlooked. Establishing this connection between modifications of the theory of gravity at the very different astrophysical and cosmological scales can lead to powerful novel constraints on the properties of gravity.
This project was successful in finding concrete examples among a wide range of candidate modified gravity theories where Dark Energy, the cause for the accelerated expansion of the universe, also produces observable deviations from what the theory of General Relativity predicts about black-hole physics.
A tantalizing possibility is that Dark Energy is not just yet another contribution to the energy budget of the universe, even if it seems to be the majority one, but rather a manifestation of gravity itself behaving differently than expected. This would mean a departure from the theory of General Relativity at the largest of cosmological distances. Nonetheless, testing this hypothesis is not an easy task as the only place where the effect of Dark Energy has been observed so far is the accelerated expansion of the universe. This fact, coupled with the sheer amount of possible ways to extend the theory of General Relativity has been a limiting factor in understanding the nature of Dark Energy from cosmological observations alone. The MGCODE project aimed to find new signatures of Dark Energy in a very different place, at the much shorter scales of black holes. The very strong gravity in their vicinity gives rise to a regime that still remains largely untested, and which is only recently becoming accessible through the detection of gravitational waves produced by the collision of pairs of black holes, neutron stars, and/or other compact objects. For this reason, it is an interesting place to look for deviations from the theory of General Relativity, and while this is certainly usually considered by the astrophysics community, the possible connection to deviations at cosmological scales is often overlooked. Establishing this connection between modifications of the theory of gravity at the very different astrophysical and cosmological scales can lead to powerful novel constraints on the properties of gravity.
This project was successful in finding concrete examples among a wide range of candidate modified gravity theories where Dark Energy, the cause for the accelerated expansion of the universe, also produces observable deviations from what the theory of General Relativity predicts about black-hole physics.
To achieve its goals, this project focused on Modified Gravity models that can successfully explain the accelerated expansion of the universe, or in other words that contain a Dark Energy component. By studying the properties of black holes in such conditions, and comparing them with their General Relativistic cousins, it was possible during the project to identify features of the way in which gravity works that could set them apart. One important such effect is when the black hole drags around a “cloud” of Dark Energy with it when it moves, producing ripples in the underlying sea of Dark Energy. This slows the black hole down, for example, as it spirals around another, causing an observable signature in the gravitational waves produced leading up to a collision. This type of effect can be observed with the planned future Laser Interferometer Space Antenna (LISA), opening interesting prospects of searching for Dark Energy in this scenario.
Not all ways of extending the theory of General Relativity to include a Dark Energy component are the same. In fact, there is a huge array of possibilities and not all will exhibit observable effects near black holes. For this reason, this project also considered other possibilities, such as a randomly changing gravitational constant, alternative geometrical features of spacetime, and more. The project achieved results suggesting that these more exotic types of Modified Gravity theories can indeed exhibit interesting phenomena both in cosmology and in astrophysics. As they have so far received much less attention, further studies are required to properly study Dark Energy and black holes in this context.
Yet another path followed in this project involved studying the internal consistency of a given Modified Gravity theory with fundamental principles in physics. This approach originates from Particle Physics where it has had great success, allowing to significantly restrict how models describing new physics can be built. Applying the same logic to cosmological Modified Gravity is promising, but faces great technical challenges. This project managed to contribute some initial steps in this direction.
Not all ways of extending the theory of General Relativity to include a Dark Energy component are the same. In fact, there is a huge array of possibilities and not all will exhibit observable effects near black holes. For this reason, this project also considered other possibilities, such as a randomly changing gravitational constant, alternative geometrical features of spacetime, and more. The project achieved results suggesting that these more exotic types of Modified Gravity theories can indeed exhibit interesting phenomena both in cosmology and in astrophysics. As they have so far received much less attention, further studies are required to properly study Dark Energy and black holes in this context.
Yet another path followed in this project involved studying the internal consistency of a given Modified Gravity theory with fundamental principles in physics. This approach originates from Particle Physics where it has had great success, allowing to significantly restrict how models describing new physics can be built. Applying the same logic to cosmological Modified Gravity is promising, but faces great technical challenges. This project managed to contribute some initial steps in this direction.
The MGCODE project has advanced the current knowledge in this field at the crossroads between cosmology and astrophysics in several ways, making definite progress towards understanding the nature of Dark Energy as a modification of gravity and its prospects of being tested in an astrophysical context.
In this regard, a key aspect has been the discovery of the Dark Energy “cloud” phenomenon around black holes. While similar effects have been investigated in the past with a focus on purely astrophysical modifications of gravity, it is not usually studied in connection to Dark Energy. This follows the common misconception that cosmological physics cannot influence astrophysical systems due to the very different scales characterizing each. This project proved this is not always the case by constructing concrete examples within Modified Gravity theories where such influence not only exists, but it is very important and should not be neglected. In some specific cases this influence introduces such strong differences to what is already known about black holes that the theory has to be discarded outright. In other cases, it predicts concrete observable signatures waiting to be tested by future gravitational-wave observatories.
The project also contributed technical improvements in various related areas, including on the theoretical formulations of alternative theories, as well as the characterization of internal consistency tests, among others.
All these developments constitute significant steps forward beyond the state of the art, opening new avenues to test Dark Energy beyond cosmological physics, which was the main scientific goal of MGCODE.
In this regard, a key aspect has been the discovery of the Dark Energy “cloud” phenomenon around black holes. While similar effects have been investigated in the past with a focus on purely astrophysical modifications of gravity, it is not usually studied in connection to Dark Energy. This follows the common misconception that cosmological physics cannot influence astrophysical systems due to the very different scales characterizing each. This project proved this is not always the case by constructing concrete examples within Modified Gravity theories where such influence not only exists, but it is very important and should not be neglected. In some specific cases this influence introduces such strong differences to what is already known about black holes that the theory has to be discarded outright. In other cases, it predicts concrete observable signatures waiting to be tested by future gravitational-wave observatories.
The project also contributed technical improvements in various related areas, including on the theoretical formulations of alternative theories, as well as the characterization of internal consistency tests, among others.
All these developments constitute significant steps forward beyond the state of the art, opening new avenues to test Dark Energy beyond cosmological physics, which was the main scientific goal of MGCODE.