Dark Energy Survey

“We have a complete inventory of the universe, and it makes no sense.” -Sean Carroll

Since the late 1920’s, scientists have been aware of the fact that our universe is expanding. This well-known fact, as well as the assumption that the universe is full of matter and that gravity is the dominant force with which matter interacts, provided for three basic models for the fate of the universe: Either (1) the gravitational effects would win out with expansion and result in an ultimate collapse of the universe–like a spring expanding out, but reversing direction and relaxing back in; (2) the expansion will ever so slightly outperform gravity and the universe will expand at a constant rate; or (3) the rate of expansion will exactly balance out with gravity, and the universe will come to a standstill.

However, these models came crashing down when in 1998, two independent teams discovered that the expansion of the universe was neither slowing down, nor coming to a standstill, but was in fact, accelerating. Using Type Ia supernova, otherwise known as “standard candles,” both teams noticed a difference between the expected brightness of the supernovae given a deceleration model, and their observed brightness. The supernovae were dimmer, and therefore farther away, than expected. This discovery by the Riess [1] and Perlmutter [2] teams began a revolutionary era in cosmology. A new component was required to account for this acceleration, which Michael Turner named, “Dark Energy.”

Dark energy is an enigma. Thought to make up around 69% of the energy density of the universe according to most recent measurements [3], we know that it should have negative pressure due to its repulsive nature. Even more mysterious, however, is that dark energy does not dilute as the universe expands. This means that it cannot be thought of as a collection of particles (as baryonic matter and dark matter are), but instead as a property of spacetime itself [4].

When including dark matter, it appears, then, that we do not know the nature of almost 95% of the universe. Specifically, one of the most striking questions is what the evolution of dark energy looks like, which if we can understand, will give us hints about the recent past and possible fate of our universe. Here is where ambitious experiments such as the Dark Energy Survey (DES) undoubtedly prove useful. DES will improve the characterization of dark energy evolution parameters using a number of tools. More specifically, it will allow us to characterize the Large Scale Structure (LSS) of the universe by making use of a very robust analytical tool called Counts-in-Cells (CiC) [5].

An incredible facet of cosmology today is that we can reconcile what we observe in the universe with theoretical predictions if we follow the Standard Cosmological Model, or the ΛCDM model. It predicts that (1) the universe started with the Big Bang which assumes isotropy and homogeneity, (2) dark matter is comprised of cold, slowly moving particles which describes clustering in the Large Scale Structure, and (3) requires a need for Λ, the cosmological constant we associate with dark energy and the accelerated expansion of space. These factors interact with each other in an interesting number of ways, including the interplay between the growing matter density perturbations and the expansion rate of the universe.

[3] Planck Collaboration and Ade, P. A. R. and Aghanim, N. and Armitage-Caplan, C. and Arnaud, M. and Ashdown, M. and Atrio-Barandela, F. and Aumont, J. and Baccigalupi, C. and Banday, A. J. and et al. Planck 2015 results. XIII. Cosmological parameters, Astronomy and Astrophysics, 2015.