Dark Energy and the Overall Structure of the Universe
While we now have a general picture of the basic constituents that make up the Universe, there is no understanding of the precise nature of the major components: dark energy, 72% and dark matter, 23%.
It has only become apparent in the last decade that dark energy exists and that the Universe is dominated by it, an unknown effect that pushes the Universe to ever‐faster expansion. The outstanding question facing cosmology today is, without doubt: What is dark energy? The answer affects not only cosmology and astrophysics in the broadest sense, but is crucial to the nature of the quantum vacuum, gravity, or their mutual interaction.
Dark energy is typically modeled as a negative‐pressure fluid, a new, previously unknown energy density. It may also be gravitational physics, an extension of general relativity. The different classes of energy density models are classified according to their equations of state, namely the relation between energy density ε and pressure P: P = wε. In fact, while we have some understanding of dark matter, its interactions, and the structures it forms, we know so little about dark energy that even if this equation is an accurate representation of it, we are still very uncertain about the parameters and whether it evolves with time.
DARK continues to investigate the pressing question of the nature of dark energy. And while very large projects are potentially very significant, we will also use our expertise in selected smaller projects .