Dark Energy

Several observations indicate an apparent acceleration of the expansion rate of the universe (see insert on the right), namely the universe seems to be expanding faster now than in the past. Understanding the expansion history has become one of the outstanding problems in cosmology.

If the universe consisted only of matter and radiation one would expect the expansion to be slowing down due to the influence of gravity. If this is not the case there should be something else counteracting and overtaking the gravitational influence.

The leading explanations for this effect are either that the Universe is dominated by a negative pressure energy component (called dark energy), or that gravity deviates from General Relativity at large scales. In either case this demonstrates that some of our well established theories of particles and/or gravity are incomplete, if not incorrect.

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Detecting dark energy

We do not know what dark energy is, so it is not surprise that we do not know how to detect it directly. The detection is indirect, through its effect on observable quantities that we can indeed measure. The expansion history is the most clear example.

Observationally there are several probes of the universe's expansion history, some quite direct, others less so. It is often necessary to combine several of them to extract conclusions on the existence of dark energy. The different probes are also affected by different systematic uncertainties, and this adds to the need of carrying out independent experiments related to dark energy.

In PAU we have focused on the measurement of Baryon Acoustic Oscillations. These are fluctuations on the large scale mass density in the universe, which can be inferred from the measurement of the galaxy density over huge regions of space.