Cosmology deals with the question of what our Universe as a whole is made of, as well as its history and evolution.  The answers and models that this field provides set the stage for the formation of galaxies, stars, and planets.  Two of the most pressing mysteries in cosmology are the nature of dark matter and dark energy, which appear to make up 96% of the Universe's current energy budget.

Dark Energy vs Modified Gravity

Observations through Supernovae and other probes have shown that the expansion of the Universe is accelerating.  This is a big puzzle for cosmologists, since according to our current understanding of gravity (through Einstein's theory of General Relativity), a Universe filled with just matter and radiation (light) should be decelerating.

Different explanations of this mystery have been postulated: it could be due to a cosmological constant, the simplest explanation which is still consistent with current observations.  It could be due to an exotic form of (loosely speaking) anti-gravitating energy, dubbed Dark Energy, which effectively drives the Universe apart.  Or,  finally, General Relativity could not be the final word on gravity. 

One of the goals of my research is to explore how a Universe governed by a modified form of gravity would look like, and how we can use observations to test the behavior of gravity over huge distances.  One important tool are simulations of structure formation, where a large, representative chunk of the Universe is followed through cosmic time, starting from the tiny initial density fluctuations that we see imprinted in the cosmic microwace background.  The picture above shows a slice through such a simulation which was run to the present time.  The black dots show collapsed structures of dark matter, which host groups and clusters of galaxies (though our simulations do not have sufficient resolution to actually see galaxies form).

One of the fascinating aspects of cosmology is the rich variety of observations that can be used to probe dark matter, dark energy, and gravity:

Galaxy Clusters

Galaxy clusters are the most massive, gravitationally bound structures in the Universe.  Massive clusters, which can contain thousands of galaxies, are exceedingly rare, so clever methods are needed to find them in large sky surveys.  Apart from identifying them
as ensembles of galaxies with optical telescopes, they can also be searched for in X-rays:  the tenuous diffuse gas in massive clusters is heated to millions of degrees by the enormous gravitational pull, making it shine in high energy X-rays.  Clusters can then be found and studied in detail by X-ray satellites such as Chandra and XMM-Newton.  The picture here shows the cluster Abell 3376 as seen in X-rays (yellow), radio (blue), and the optical band (white).  Please click on the picture to read more.

The scarceness of massive clusters is also a virtue:  their abundance responds sensitively to changes in the growth of structure.  Modifications to the behavior of gravity typically have a large impact on cluster abundance.  In 2009,  we used a sample of X-ray emitting clusters together with our simulation results to constrain a modified gravity scenario called f(R) gravity (which attempts to explain the acceleration of the Universe without dark energy).  This work, which was featured in a Chandra press release, is the result of  a collaboration with Wayne Hu from the University of Chicago and Alexey Vikhlinin from the Harvard/Smithsonian Center for Astrophysics.  Please read on at the Chandra site !

Unraveling the initial conditions of the Universe

Many observations, in particular the cosmic microwave background, have now demonstrated beyond a doubt that the Universe started out from a very hot dense and homogeneous state (the “Big Bang”). But why was the Universe so homogeneous (it still is on very large scales today), and how were the initial fluctuations generated that grew into the clusters, galaxies, stars and planets today ?  The leading theoretical contender is the scenario of inflation: the Universe underwent a period of very rapid expansion early on in its history (corresponding to extremely high energy scales). 
Quantum fluctuations were stretched out and “froze in”, later to become the seed fluctuations out of which all structure formed.  Of course, we would like to test this scenario and obtain clues on if, how, and when inflation happened.  One possibility is to look for hints in the statistical distribution of the initial fluctuations, in particular for correlations between fluctuations on different length scales.  Technically, this is called primordial non-Gaussianity. One of the most fascinating aspects of cosmology today is that, through primordial non-Gaussianity, we can study inflation using the distribution of galaxies on large scales, such as observed by the Sloan Digital Sky Survey (above).  In another words, we can use the largest observable scales in the Universe, to probe physics at energy scales orders of magnitude beyond particle accelerators on Earth !

In order to do this however, we need theoretical models that connect the statistics of the initial conditions predicted by inflationary models with the galaxy distribution today.  This is an area of ongoing, intense research.  The calculations done by Vincent Desjacques (Geneva), Donghui Jeong (Johns Hopkins) and me are the most accurate to date, and match the result of cosmological simulations very well. 

Gravitational waves from inflation

Large-scale structure also offers opportunities to search for gravitational waves from inflation, the echo of the Big Bang. This fascinating topic caught my interest over the past year and a half. More to come soon... 

Gravitational Lensing

Gravity affects not only ordinary matter, but light as well.  The intense gravitational pull of a massive foreground galaxy or cluster can distort the image of galaxies that lie far behind the "lens".  The neat property of this gravitational lensing is that it responds equally to all matter.  Thus, even the otherwise elusive dark matter shows up in this effect, making it especially interesting for cosmologists.  The picture on the right shows a very distant galaxy image (blue) strongly distorted by a galaxy in the foreground (yellowish).  Click on the image to obtain more information.

One particular technique, called weak lensing aims to measure the large-scale structure of the Universe through the distortions of background galaxies.  Weak lensing is another promising avenue for probing gravity in cosmology, and has been hailed as potentially the most powerful observational tool in cosmology.  The main downside is the smallness of the effect, distorting any individual background galaxy image by a few percent or less, requiring great care when attempting to measure the effect.  One of the subtleties is that even the way these background galaxies are selected has an impact on the measurement.  This effect which I discovered in 2009 together with colleagues from Chicago, Columbia University, and Brookhaven National Lab, has to be taken into account for upcoming ambitious surveys, such as the Dark Energy Survey.  Please see this "News and Views" article in Nature for a discussion and background.

We have also recently detected weak lensing magnification in the COSMOS survey, using images taken by the Hubble Space Telescope. (...)

Cosmic rays

I have previously worked on ultra high energy cosmic rays, and was a member of the Pierre Auger collaboration.  In particular, I have worked on figuring out the composition of the cosmic rays (are they protons, or heavier nuclei ?) and on comparing hadronic interaction models with the data.  Here are some pictures of simulations of cosmic ray-induced air showers, for various primary particles and energies.