Theoretical Challenges in Large Scale Structure

In the last ten years, a wealth of observational data has revolutionized cosmology, allowing to pin down the cosmological parameters with an amazing precision. In the currently favored cosmological model, the wide range of cosmological structures we observe today formed from the nonlinear evolution of tiny fluctuations which were imprinted during an inflationary phase in the early Universe. While cosmology has already offered a number of outstanding achievements, such as the detection of primeval fluctuations in the cosmic microwave background (CMB) and the discovery of dark energy, a number of key questions remain unanswered, including:

  • What is the nature of dark energy?
  • Is it Einstein’s cosmological constant, a different substance with unusual equation of state, or perhaps a signature of a breakdown of Einstein’s General Relativity on cosmological scales?
  • Are the initial conditions in the Universe compatible with the theory of inflation, or do they require an alternative description?
  • Are there primordial non-Gaussianities that would invalidate the simplest models of inflation?
  • What is the mass of neutrinos?
  • Are the masses of neutrino families degenerate?
  • Are there more that 3 neutrino families?

Cosmological observations of the CMB have played a leading role in recent years, a process that will culminate with the Planck resuts to be announced in March 2013. However, the future of observational cosmology is likely to be dominated by the large scale structure (LSS) of the Universe, which encodes information on the clustering of dark matter as a function of time and scale and, therefore, is sensitive to physical quantities such as the neutrino mass or the dark energy equation of state. There are currently two major approaches to the LSS: the first consists in measuring galaxy positions and relating these to the underlying dark matter clustering. This method, called galaxy clustering, has good signal, but the theoretical modeling suffers from uncertainties arising from our lack of understanding of galaxy bias. The second consists in measuring the weak gravitational lensing induced by the dark matter through the distortion of distant galaxy images. This technique, called weak lensing, furnishes a more direct probe of the dark matter distribution. However, it suffers from a lower overall signal and from various observational systematics.

The question of whether these methods can achieve their promise depends on whether the aforementioned theoretical and observational uncertainties can be resolved. This is the subject of the proposed conference. One of the main conference topics is on theoretical aspects, because a lack of progress in this area could seriously hinder the promised achievements of the future surveys. There are many theoretical aspects that need to be understood better if the connection between fundamental physics and LSS observables is to be fully exploited. Among the open theoretical questions are understanding the nonlinear clustering of dark matter and the bias of tracers such as galaxies or Ly-alpha absorption lines, quantifying the effects of baryons and feedback from galaxy formation on the weak lensing observables, modeling the signature of redshift space distortions, assessing the information content of high order correlations (mostly bispectrum and trispectrum) of galaxy clustering and weak lensing observables.

Another focus will be on new methods that have been developed in recent years. Among them are primordial non-Gaussianity in the 2-point and higher order correlations of biased tracers as a probe of inflation, methodologies that can reduce the impact of traditional limitations (such as sampling variance or shot noise), the use of reconstruction techniques to increase the information on cosmological parameters or the combination of several LSS techniques (e.g. weak lensing, galaxy clustering, cluster counts) to improve constraints. With a suitable application of these methods, the future surveys may be able to extend their initially planned objectives and have a far greater impact than originally expected.

The third broad topic of the conference will be on the challenges and limitations imposed by the data. Galaxy clustering data is contaminated by the presence of stars, variable selection functions etc., while weak lensing measurements are contaminated by distortions of the point spread function, systematics in the extraction of galaxy ellipticities, uncertainties in the determination of photometric redshifts etc. All these sources of systematic errors may impact the desired sensitivity of the survey and, therefore, must be understood thoroughly.

More info: LSS13