Jordi Miralda-Escudé

Cosmology, Galaxies and Large-Scale Structure

Over the last decade, 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 been participating in several large collaborations that aim at discovering additional information on the cause of the acceleration of the expansion. One of them is called Physics of the Accelerated Universe, which is a Consolider project that was awarded to a Spanish collaboration to investigate this phenomenon. The principal project that was proposed in this collaboration is a galaxy imaging survey using narrow-band filters, with the aim of measuring baryon acoustic oscillations in the redshift direction from redshifts obtained from narrow-band photometry. A group of us, however, concluded after some investigations that narrow-band photometry is not the best way to measure the Baryon Acoustic Oscillations.

At present I am interested in the SDSS-III Collaboration, and specifically in the BOSS survey, which will measure Baryon Acoustic Oscillations using spectroscopic redshifts from a survey of Luminous Red Galaxies and from the Lyman Alpha Forest spectra in high-redshift quasars. A description of this project can be found in the BOSS website .

The last mission is called EUCLID, a proposed space mission to ESA that would put a telescope in space to do gravitational lensing measurements of a large number of galaxies over the entire sky to a much better accuracy than can be done from the ground, and to obtain spectra of a large number of objects at high redshift to study galaxy evolution. This mission is in its conception phase and, if it is finally approved, it will be launched many years from now, but it is also exciting to think about long-term science plans.

Publications related to this project

  1. 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'', submitted to JCAP.
    You can find the astro-ph preprint online.
  2. J. Yoo, J. Miralda-Escudé 2009, ''Gravitational Lensing Effects on the Baryonic Acoustic Oscillation Signature in the Redshift-Space Correlation Function'', submitted to ApJL.
    You can find the astro-ph preprint online.

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