COSMOLOGY WITH THE NEW GENERATION OF GALAXY SURVEYS

My research proposal focuses on three main projects, which in my opinion represent the most exciting fields of contemporary cosmology. The first project discusses the measurement of non-Gaussianity in the distribution of density fluctuations in the Universe. Density fluctuations in the early Universe have been shown to follow a Gaussian form meaning that the distributions of densities around the mean density can be described by only one number, the standard deviation. The current standard model of cosmology includes an inflationary period in the very early Universe, which seeded the large-scale density distributions we observe today. The Gaussian nature of the early density fluctuations is a direct consequence of the inflationary physics. While the simplest model of inflation (single field slow roll inflation) is expected to produce almost perfect Gaussian density fluctuations, other inflationary models can produce significant levels of non-Gaussianity. Such non-Gaussianity would show up as excess large-scale clustering in the galaxy distribution, and hence we can use the distribution of galaxies to test physics that happened within the first second of our Universe's existence.
The second project I discuss in my proposal concerns the sum of the neutrino masses. Study of neutrino oscillations has shown that neutrinos cannot be massless, as proposed in the standard model of particle physics. Therefore neutrinos carry a certain fraction of the matter content of the Universe, which impacts the matter clustering of the Universe. Most cosmic neutrinos are produced during the Big Bang and therefore have a close to relativistic velocity, which allows them to escape the gravitational potential of low mass dark matter haloes. This reduces the clustering of matter on small scales and provides a signal for the measurement of the sum of the neutrino masses using galaxy survey datasets. The cosmological constraints on the sum of the neutrino masses are now more than one order of magnitude lower compared to laboratory experiments and it is expected that the next generation of galaxy surveys will provide a detection of this parameter.
Finally, the third project discussed in my proposal addresses the relative velocity effect. The relative velocity effect describes a relative velocity between baryonic matter and cold dark matter due to physics that happened in the first 300 000 years of the Universe's existence. The relative velocity of these components changes the clustering on very large scales and potentially modifies the Baryon Acoustic Oscillation (BAO) scale. The BAO scale is a special scale in the distribution of galaxies. In our Universe, there are more galaxy pairs separated by 150 Mpc compared to the number of galaxy pairs at 140 Mpc or 160 Mpc. This special scale can be used to map out the expansion history of the Universe using galaxy redshift surveys. The measurement of the BAO scale has emerged as one of the most powerful tools of current cosmology and represents the main science driver for the next generation of galaxy surveys. Given the potential impact of the relative velocity on the BAO scale, studying and measuring the relative velocity effect is essential to ensure the correct interpretation of the BAO scale. Furthermore, a first detection of the relative velocity effect has the potential to alter our understanding of galaxy formation.