In this centennial anniversary of General Relativity, we will hold a conference on Hot Topics in General Relativity and Gravitation with the motivation to emphasize the tremendous progresses that have been made in Astrophysics and in Cosmology since Einstein’s discovery of General Relativity (GR) in 1915.
Nowadays, the space-time dynamics has been enriched with new concepts that appeared in Cosmology, Black Holes Physics and Gravitational Waves. They provide us with enlightening on issues in which GR plays a fundamental role. Faced to these scientific goals to reach, observation technologies have been improved with impressive developments in a large window wavelength and for several types of sources, which initiates the era of multi-messenger astronomy.
The Standard Cosmological Model describes the expansion of the observable universe by pointing out the presence of Dark Matter (DM) as the main component of massive gravitational sources. Also, it is a necessary ingredient for modeling the dynamics of gravitational sources at smaller scales, as for individual galaxies (the rotation curves of spiral galaxies) and galaxies in clusters (e.g., their kinematics and the surrounding gravitational lensing effects). However, High Energy Physics has not yet provides us with the corresponding weakly interacting massive particles, although new particles beyond the standard model might be promising DM candidates. Furthermore, the attempts at explaining the cosmological constant as related to the energy density of vacuum (quantum fluctuations) have failed. Hence, although a non-zero cosmological (universal) constant would be the simplest interpretation of the observed acceleration of the universe expansion, alternative approaches named Dark Energy have been also envisaged. They include modified theories of gravity or unknown non-standard matter fields such as quintessence. The inflationary paradigm successfully accounts the primordial era of the Universe, in agreement with recent observational data. However, while being confirmed at the phenomenological level, it suffers from conceptual difficulties (we still do not know the origin of the inflation). The understanding of these issues will lead to new advances in fundamental physics.
Direct evidence of the existence of Black Holes (BH) has not yet been made but the phenomena they cause in their environment demonstrate their presence and help us to better understand how they work. Their detection can be done either by the interpretation of the trajectories of nearby stars, or by its activity as inferred from the interaction with its immediate environment. Before falling into the black hole, matter revolves around him and starts to heat up considerably by emitting intense radiation; the quasar could be interpreted as such a process. In the absence of interacting matter, the BH can also be stealthy. For example, the BH found at the center of our galaxy is part of the category of supermassive BH while is not active. According to astronomers, most galaxies could harboring one, the largest of which might reach several billion solar masses. Some high-energy astrophysical phenomena can be explained by assuming the existence of a BH but the models have still to be improved, with (possibly) a new physics. The most promising way to detect BHs would be the gravitational waves (GW) by using ground-based and space-based GW detectors, which stands for a new observation window in astronomy. The detailed study of the coalescence of compact binary systems will give not only a wealth of information about astrophysical parameters such as their masses but will also reveal the equation of state at quite high density. The coalescence of BH binaries may give an accurate test of the BH no-hair conjecture in GR. It can be also used to test gravitational theories. Numerical Relativity is a key method in these studies.
More fundamental questions also arise about the gravitational interaction, and in particular, the quantization of Gravity is one of the most important. Several approaches are competing on this issue and they have not yet reached a general approbation. For example, superstring theory reconciles quantum mechanics and Einstein's theory of gravitation but his mathematical construction remains incomplete. On the other hand, AdS/CFT correspondence is another interesting way to study gravitational phenomena in the relation to strongly coupled quantum field theories. It can be extended to gauge/gravity or fluid/gravity correspondence. It has been used to study nuclear and condensed matter physics.
The fundamental subjects in astrophysics, cosmology and high energy physics are closely related to GR. Hence, we will survey the progress and recent developments in some hot topics on GR and related subjects. We will also discuss the improvements on unsolved problems on general relativity and gravitation.