Cosmology, Galaxies and Large-Scale Structure

Over the last two decades, astrophysicists have accumulated new evidence that the expansion of the Universe is accelerating at the present epoch.

We have known since 1965 that the Universe started with the Big Bang, as shown by the Cosmic Microwave Background radiation which has a blackbody spectrum, and it has been expanding throughout its history. Most cosmologists had always assumed that the expansion would be slowing down over time, or decelerating. Naturally, the effect of gravity of all the matter present in the universe, which is attractive, should be to slow down the expansion, and this is indeed predicted by the General Relativity theory for the most simple case of the universe in which it contains only matter and radiation. However, Einstein himself had shown back in 1917 that it is possible to add an additional term to his equations called the cosmological constant, which causes the universe to accelerate its expansion instead. Once it was understood that our universe started with the Big Bang, it was considered unlikely that this term would be present.

It therefore came as a big surprise when evidence for precisely the type of acceleration of the universe predicted by a cosmological constant was found at the turn of the millennium. The evidence was discovered by a combination of the observations of brightness of supernovae at cosmological distances, and from detailed measurements of the intensity fluctuations in the Cosmic Microwave Background. This new evidence has totally undermined the previous idea of many scientists working in the field whereby, by determining the matter density of the universe, we could predict the fate of the universe and tell whether the expansion would continue forever or would turn around and lead to a Big Crunch. Instead, the fate of the universe now depends on the reason for this acceleration, which is completely unknown. Curiously, ever since this discovery was made, theoretical cosmologists have started talking about ``dark energy'', meaning all types of models where a new energy component is present in the universe (in addition to matter and radiation) which has a large negative pressure, and this negative pressure causes an effect analogous to the cosmological constant term in Einstein's equations. Dark energy might have fluctuations affecting the evolution of the other components of the universe. Cosmologists have also started discussing a variety of models of modified gravity, where General Relativity is modified to give rise to the appearance of an accelerated expansion. However, the observations remain stubbornly consistent with a simple cosmological constant.

I have participated in one of the large collaborations that aim at discovering additional information on the cause of the acceleration of the expansion, the SDSS-III Collaboration , specifically in the Baryon OScillation Survey, or the BOSS survey . This is one of the large surveys that measured Baryon Acoustic Oscillations using spectroscopic redshifts from a survey of Luminous Red Galaxies and from the Lyman Alpha Forest spectra in high-redshift quasars.

One of the great things the BOSS survey did was to measure correlations using the Lyman Alpha Forest, which is the absorption caused by intergalactic hydrogen in the spectra of distant quasars. This intergalactic hydrogen traces the large-scale density fluctuations in the Universe that arise from the primordial initial conditions. By studying these correlations of intergalactic hydrogen absorption, we are therefore studying the spatial variations in the matter density that the Universe was born with, the origin of which relates to fundamental principles of physics that governed the early Universe and which we do not yet understand. Moreover, the distribution of intergalactic hydrogen also informs us about the process of galaxy formation that took place starting from these early fluctuations. In fact, the gas we observe in absorption in the spectra of quasars, which is observed in the past because of the great distance at which we measure it, is in the process of collapsing into gravitational potential wells of galaxies that are forming.

The papers mentioned below studied, among other things, the Baryon Acoustic Oscillations that allow us to study these primordial initial fluctuations and the history of expansion of the Universe. In addition, the amplitude of the correlations between the Lyman Alpha forest and the quasars themselves, or the strong absorbers called Damped Lyman Alpha systems (which are identified with forming galaxies), tell us about the spatial distribution of these galaxies in formation and the types of dark matter halos they are associated with.

Publications related to this project

  1. A. Arinyo-i-Prats, J. Miralda-Escudé, et al. 2015, ''The Non-Linear Power Spectrum of the Lyman Alpha Forest'', JCAP, 12, 17 (arXiv:1506.04519).
  2. J. E. Bautista, et al. 2017, ''Measurements of BAO Correlations at $z=2.3$ with SDSS-DR12 Lyman Alpha forests'', A& A, 603, 12 (arXiv:1702.00176).
  3. L. Mas-Ribas, J. Miralda-Escudé, I. Pérez-Ràfols, et al. 2017, ''The Mean Metal-line Absorption Spectrum of Damped Lyman Alpha Systems in BOSS'', ApJ, 846, 4 (arXiv:1610.02711).
  4. T. Venumadhav, L. Dai, J. Miralda-Escudé 2017, ''Gravitational Microlensing during Caustic Crossings'', ApJ, 850, 49 (arXiv:1707.00003).
  5. H. du Mas de Borboux, et al. 2017, ''Baryon Acoustic Oscillations from the complete SDSS-III Lyman Alpha - Quasar cross-correlation function at z=2.4'', A& A, 608, 130 (arXiv:1708.02225).
  6. I. Pérez-Ràfols, A. Font-Ribera, J. Miralda-Escudé, et al. 2018, ''The SDSS-DR12 large-scale cross-correlation of Damped Lyman Alpha Systems with the Lyman Alpha forest'', MNRAS, 473, 3019 (arXiv:1709.00889).
  7. D. Roig, L. Verde, J. Miralda-Escudé , R. Jiménez, C. Pena-Garay 2009, '' Photometric Redshift Optimization for Measurements of the Baryon Acoustic Oscillation Radial Scale'', JCAP, 4, 8.
    You can find the astro-ph preprint online.
  8. J. Yoo, J. Miralda-Escudé 2009, ''Gravitational Lensing Effects on the Baryonic Acoustic Oscillation Signature in the Redshift-Space Correlation Function'', ApJL, 614, L25.
    You can find the astro-ph preprint online.