Bouncing cosmologies

A proposed alternative to the inflationary paradigm is the idea that the universe underwent a bounce: a transition from  a period of contraction to expansion. If the bounce is nonsingular then quantum effects are subdominant and the dynamics classical. An important open question is what happens to nonsingular bouncing cosmologies in the non-perturbative regime, in particular what happens to primordial black holes during the bounce. This is the question we set out to answer in this project. What we found is that for sufficiently large black holes the black hole apparent horizon can disappear during the contraction phase. Despite this, we show that most of the local cosmological evolution remains largely unaffected by the presence of the black hole and that, independently of the black hole’s initial mass, the black hole’s event horizon persists throughout the bounce.  

The figure and movie show a black hole and cosmological horizon merging  in a nonsingular bouncing universe.

Robustness inflation to initial conditions

Inflation, a period of time where the universe expanded very rapidly, is the currently widely accepted paradigm to explain the high degree of flatness and homogeneity observed in our universe. An often repeated criticism of inflation is that while it is true that if inflation starts it will smooth out any inhomogeneities, in order for inflation to begin in the first place one requires homogeneity over several Hubble scales. In this work, I want to understand the conditions under which inflation may start. Introducing and comparing several different ways of constructing cosmological initial conditions with inhomogeneous scalar field and time derivative profiles I evolved such initial conditions with  large inhomogeneities in both single- and two-field inflationary models. I show that a universe that is initially dominated by large gradient and kinetic energies can still transition to an inflationary period, even when the scale of the inhomogeneities is comparable to the Hubble radius and some regions collapse to form black holes. This suggest inflation can arise from highly inhomgeneous conditions and solve the cosmic initial condition problem. 

The figures below show the kinetic, gradient and potential  energy density of two different examples where the universe is initially dominated by gradient and kinetic energy of scalar field but transitions to accelerated expansion.  

Nonlinear dynamics of flux compactifications

Determining the physical mechanism(s) responsible for the accelerated expansion of the Universe both at early times, in the inflationary era described above and at late times in our current epoch of dark energy domination are among the most important challenges in modern cosmology. One proposed framework for  tackling this  is string theory. A major technical problem are the extra dimensions introduced to make string theory a consistent theory of quantum gravity.  One of the paradigm explaining why we cannot observe the extra dimensions is compactification, where the spatial dimensions are too  small to be observed.  In this work we study the evolution of the size and shape of a compactification which in this picture can provide  the mechanism for inflation and dark energy.  We show that initially homogeneous flux compactifications are unstable to dynamically forming warped compactifications. In some cases, we find that the warping process can serve as a toy-model  of slow-roll inflation, while in other instances, we find solutions that eventually evolve to a singular state.

Below is a cartoon of the solutions we study and movie of an unstable homogeneous solution that becomes  singular.   

Black hole mergers in modified theories of gravity

The recent breakthrough discovery of gravitational waves opened a very unique way of probing the dynamics of spacetime and allowed for unprecedented tests of general relativity in the strong field regime. However, testing our theory of gravity relies on accurate waveforms that cover the inspiral, merger and ringdown of compact binaries, which has been a major theoretical and technical challenge for many theories of interest. In this work I use recent advances in solving the full Einstein field equations in Horndeski theories to evolve black hole binaries in Einstein scalar Gauss Bonnet gravity, which introduces modifications to gravity at small curvature scales. We answer questions such as what does the gravitational wave signal look like in a specific theory of modified gravity and compare our numerical results to Post-Newtonian calculations.

The figure shower the gravitational wave signal during the inspiral of different binaries, while the movie shows the scalar field surrounding two equal mass black holes in quasi-circular inspiral. 

Constraining extra dimensions with future Gravitational Wave space-based detectors.

The first observation of Gravitational Waves from a neutron star binary coalescence and the identification of an electromagnetic counterpart opened a new era of GW and multi-messenger astronomy. In the 2030s, when the space-based interferometer LISA will be online  we will be capable of detecting large numbers of  coalescing compact binaries at large cosmological redshifts.  The propagation of gravitational waves at cosmological distances offers a new way to constrain cosmological parameters and test cosmological models beyond the framework  of General Relativity. In this work, I consider a phenomenological model for higher dimensional theories with large extra dimensions  and forecast the capability of LISA to constrain these theories  from future multi-messenger observations of massive binary black hole  mergers. We found that the extent to which LISA will be able to place  limits on the number of spacetime dimensions and other cosmological  parameters characterising the modified theory of gravity will  strongly depend on the actual number and redshift distribution of sources, together with uncertainty on GW measurements.

CV.

My CV can be found here.

Contact.

email: maxence.corman@aei.mpg.de

address: 

Max Planck Institute for Gravitational Physics

Am Muhlenberg 1

14476, Potsdam

Germany