-to indicate uncertainty on the spectral type of individual objects in multiple systems where resolved spectroscopy has not been obtained. c Distance are spectrophotometric (d source = ST) except where a trigonometric parallax is available (d source = Parallax). d Henry et al. (1999) reported the possible existence of a fourth component, LHS 1070D, at a separation of 50 mas; further observations have shown the original apparent detection to be spurious (T. Henry 2006, priv. comm.). e Bouy et al. (2005) report a T dwarf as a possible third component to the system. The T dwarf is not included in the luminosity function estimate.f Variable and/or flaring object that will be discussed in detail in Schmidt et al. (2006, in prep.).g The absolute magnitude for this object, based on a trigonometric parallax, is brighter than our brightest M J bin and is not included in our space density per magnitude analysis. However, it is included in our estimates of the total space densities of ultracool dwarfs. It is also a suspected unresolved binary as discussed in § 5.6.h This object has been resolved into an L/T binary by Burgasser et al. (2005b). The T dwarf is not included in the luminosity function estimate.i This object displays features of low gravity and a spectral type of M9: was used to estimate M J . j Change in spectral type from Paper V.k Suspected low-metallicity dwarf as discussed in § 5.4. l Suspected unresolved binary as discussed in § 5.6.

Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, ΛCDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of ΛCDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled.Keywords: cosmology -dark energy -cosmological constant problem -modified gravitydark matter -early universe Cosmology has been both blessed and cursed by the establishment of a standard model: ΛCDM. On the one hand, the model has turned out to be extremely predictive, explanatory, and observationally robust, providing us with a substantial understanding of the formation of large-scale structure, the state of the early Universe, and the cosmic abundance of different types of matter and energy. It has also survived an impressive battery of precision observational tests -anomalies are few and far between, and their significance is contentious where they do arise -and its predictions are continually being vindicated through the discovery of new effects (B-mode polarization [1] and lensing [2,3] of the cosmic microwave background (CMB), and the kinetic Sunyaev-Zel'dovich effect [4] being some recent examples). These are the hallmarks of a good and valuable physical theory.On the other hand, the model suffers from profound theoretical difficulties. The two largest contributions to the energy content at late times -cold dark matter (CDM) and the cosmological constant (Λ) -have entirely mysterious physical origins. CDM has so far evaded direct detection by laboratory experiments, and so the particle field responsible for it -presumably a manifestation of "beyond the standard model" particle physics -is unknown. Curious discrepancies also appear to exist between the predicted clustering properties of CDM on small scales and observations. The cosmological constant is even more puzzling, giving rise to quite simply the biggest problem in all of fundamental physics: the question of why Λ appears to take such an unnatural value [5,6,7]. Inflation, the theory of the very early Universe, has also been criticized for being fine-tuned and under-predictive [8], and appears to leave many problems either unsolved or fundamentally unresolvable. These problems are indicative of a crisis.From January 14th-17th 2015, we held a conference in Oslo, Norway to surve...

Bimetric theory describes gravitational interactions in the presence of an extra spin-2 field. Previous work has suggested that its cosmological solutions are generically plagued by instabilities. We show that by taking the Planck mass for the second metric, $M_f$, to be small, these instabilities can be pushed back to unobservably early times. In this limit, the theory approaches general relativity with an effective cosmological constant which is, remarkably, determined by the spin-2 interaction scale. This provides a late-time expansion history which is extremely close to $\Lambda$CDM, but with a technically-natural value for the cosmological constant. We find $M_f$ should be no larger than the electroweak scale in order for cosmological perturbations to be stable by big-bang nucleosynthesis. We further show that in this limit the helicity-0 mode is no longer strongly-coupled at low energy scales.Comment: 8+2 pages, 2 tables. Version published in PLB. Minor typo corrections from v