¨ Research in Cosmology

Determining the nature of DARK ENERGY

The accelerated expansion of our universe, and consequently that it apparently contains a substantial amount of dark energy (DE) are still unexplained. Interpreting and understanding the accelerating universe is arguably one of the major scientific challenges of our times. The challenge can be approached on several levels, from the most practical level: how to interpret the data, through an intermediate level: penomenological models that can be embedded into theories of fundamental physics, to the most profound level: the cosmological constant problem.

One approach is to assume the "standard model" (SM) of cosmology: Lambda-Cold-Dark-Matter (with additional assumptions), and, until forced to change this assumption by some compelling evidence, use it to interpret the data. The SM is consistent with all the currently available data, and the errors in determining its parameters are about 15%. However, when considering other possibilities for interpreting the data, the errors increase substantially, and several possibilities seem to be as good an explanation as the SM. This means that currently, theoretical assumptions are a large source of error, and as the data becomes better and more abundant, such theoretical assumptions would become the largest source of error. There are good reasons to consider possibilities other than the SM. There is the well known approximate equality of the cold matter and the DE densities, which perhaps could be explained by dynamics, and in general, the possibility of time dependent DE in theories of fundamental physics. Another reason for considering alternatives to the SM is to quantify the amount by which the data supports the SM and determine the "theoretical errors" on the results.