Focus on Dark Energy

Abstract

The 1998 discovery that the universe is accelerating set off an enormous amount of activity, both theoretical and observational. The original result, from two groups observing the Hubble diagram of Type Ia supernovae, has since been verified by a variety of independent types of observations. It seems clear that our universe really is accelerating; what remains a mystery is why.The most straightforward explanation for the universe's acceleration is the presence of a dark energy component comprising 70% of the universe. In order to fit the data, dark energy must have two features: it should be smoothly distributed, so as not to show up in dynamical studies of galaxies and clusters, and its density should be nearly constant as the universe expands, to provide the persistent impulse necessary to make the universe accelerate.The simplest candidate for dark energy is vacuum energy, equivalent to Einstein's cosmological constant. Simple estimates from quantum field theory indicate that vacuum energy should exist – indeed, in an amount larger than what we observe by a factor of 10120. This discrepancy, the 'cosmological constant problem', led to a widespread assumption that some mysterious mechanism worked to set the vacuum energy precisely to zero. If the dark energy really is a cosmological constant, we must find a mechanism to suppress its natural value without driving it all the way to vanishing.Alternatively, the dark energy could be a dynamical field, albeit one that changes very gradually with time. Such a case is observationally distinguishable, at least in principle, from that of a truly constant vacuum energy. Constraints on the evolution of the dark-energy density come from a variety of measurements, and improving the precision of these techniques is a major goal of the next decade in cosmology.Most dramatically, there might not be any dark energy at all, even if the universe is accelerating – a possibility that is well-explored in this focus issue of New Journal of Physics. Two possibilities present themselves. On the one hand, general relativity could break down on cosmological scales, forcing us to a new theory of gravity. In that case, we may use other observed phenomena to put limits on the way in which gravity could deviate from Einstein's theory. On the other hand, general relativity could be correct, but differ in its true predictions from the simple approximations we are used to applying. Such a possibility is both dramatically different from more conventional approaches, and yet radically conservative, attempting to explain all of the accumulated observations with nothing more than matter particles and ordinary general relativity. Only a great deal of additional theoretical investigation and observational progress will be able to distinguish which of these possibilities explains the behaviour of our universe on large scales.The articles below represent the first contributions and further additions will appear.Focus on Dark Energy ContentsCosmic clocks, cosmic variance and cosmic averages David L WiltshireDark energy, a cosmological constant, and type Ia supernovae Lawrence M Krauss, Katherine Jones-Smith and Dragan HutererCosmological dark energy: prospects for a dynamical theory Ignatios Antoniadis, Pawel O Mazur and Emil MottolaPredictive power of strong coupling in theories with large distance modified gravity G Dvali Constraints on dynamical dark energy: an update Alessandro Melchiorri, Barbara Paciello, Paolo Serra and Anze Slosar Scaling solutions to 6D gauged chiral supergravity Andrew Tolley, C P Burgess, Claudia de Rham and D Hoover Modified-source gravity and cosmological structure formation Sean Carroll, I Sawicki, A Silvestri and M Trodden On cosmic acceleration without dark energy E W Kolb, Sabino Matarrese and A Riotto Sean Carroll, California Institute of Technology, Pasadena, USA