Astrophysics

Standard models of cosmology use inflation as a mechanism to resolve the isotropy and homogeneity problem of the universe as well as the flatness problem. However, due to various well known problems with the inflationary paradigm, there has been an ongoing search for alternatives. Perhaps the most famous among these is the cyclic universe scenario or scenarios which incorporate bounces. As these scenarios have a contracting phase in the evolution of the universe, it is reasonable to ask whether the problems of homogeneity and isotropy can still be resolved in these scenarios. In my talk, I will focus on the problem of the resolution of isotropy. In the contracting phase of the evolution, the mechanism of ekpyrosis is used in most cosmological scenarios which incorporate a contracting phase to mitigate the problem of anisotropies blowing up on approaching the bounce. I will start by studying anisotropic universes and I shall examine the effect of the addition of ultra-stiff anisotropic pressures on the ekpyrotic phase. I will then consider evolving such anisotropic universes through several cycles with increasing expansion maxima at each successive bounce. This eventually leads to flatness in the isotropic case. My aim will be to see if the resolution of the flatness problem also leads to a simultaneous resolution of the isotropy problem. In the last section of my talk, I will briefly consider the effect of non comoving velocities on the shape of this anisotropic bouncing universe.

Are we standing on the brink of a new scientific revolution that will radically change our views on space, time, and gravity?

In most circumstances, the theories of Einstein and Newton adequately describe gravity, but on cosmological scales, big questions arise, particularly surrounding the nature of dark matter and dark energy.

These questions are ushering in a revolution in theoretical physics – a completely new view on spacetime and gravity. Research in string theory and black hole physics, involving key concepts of quantum information theory, reveals a deep connection between the structure of spacetime and the origin of gravity.

This research suggests that gravity is not a fundamental force of nature, but rather an emergent phenomenon, similar to how temperature is an emergent phenomenon that arises from the movement of particles. That is, gravity is a side-effect, not a cause, of what happens in the universe.

In his public lecture, Dr. Erik Verlinde (University of Amsterdam) will explore the core ideas behind this research, and examine the implications of this fast-emerging revolution in our understanding of the universe.

Thermodynamics is a closed field of research. The laws of thermodynamics, established in the nineteenth century, are still standing unchallenged.  However, they do not include gravity. Inclusion of gravity into the thermodynamical system can significantly modify the expected behavior of the system. We will demonstrate that gravity dynamically induces a maximal temperature that can be reached in a gas of particles.  We will also show how gravity can significantly change the Poincare recurrence theorem, and sometimes even prevent the recurrence from happening.

Many of the multi-planet systems discovered around other stars are maximally packed. This implies that simulations with masses or orbital parameters too far from the actual values will destabilize on short timescales; thus, long-term dynamics allows one to constrain the orbital architectures of many closely packed multi-planet systems. I will present a recent such application in the TRAPPIST-1 system, with 7 Earth-sized planets in the longest resonant chain discovered to date. In this case the complicated resonant phase space structure allows for strong constraints. A central challenge in such studies is the large computational cost of N-body simulations, which preclude a full survey of the high-dimensional parameter space of orbital architectures allowed by observations. I will discuss our recent successes in training machine learning models capable of predicting orbital stability a million times faster than N-body simulations, and the discovery space that this opens up for exoplanet characterization and planet formation studies.