10.1117/2.1200809.1288 Mapping the accelerating expansion of the universe Francisco J. Castander A new wide-field optical camera will obtain images of tens of millions of galaxies in multiple filters to determine their distances and investigate the nature of dark energy. In the past decade our understanding of the universe has im- proved considerably. We now have a detailed picture of its gen- eral composition and are getting to grips with evolutionary sce- narios. Overall, modern cosmological observations lead to the conclusion that we live in a nearly ‘flat’ universe where ordinary matter only represents approximately 5% of the total energy– matter content, ‘cold dark matter’ constitutes ∼20%, and the re- maining ∼75% of unknown makeup is referred to as ‘dark en- ergy.’ The latter causes the accelerated expansion of the universe, and understanding its nature is one of the most fundamental problems in contemporary physics and cosmology. From an observational point of view we can investigate the properties of dark energy by studying how the universe expands and how the observed large-scale structures form and evolve. Supernovae, the cosmic-microwave-background radiation, the distribution of both individual galaxies and clusters of galaxies, and deformations of the light emitted by distant stellar systems due to ‘gravitational lensing’ by massive foreground objects pro- vide the best observational probes. In particular, the observation of a characteristic length scale in the distribution of galaxies in the early universe is one of the most promising diagnostic tools in this field because of its robustness to small changes due to systematic biases. 1, 2 The early universe is thought to have been fairly homoge- neous, with tiny matter fluctuations that subsequently grew to form galaxies. When photons and baryons fell into these den- sity fluctuations, pressure produced acoustic waves. As the uni- verse continued to expand, photons and baryons decoupled but the scale of these baryon-acoustic oscillations (BAOs) remained imprinted on the distribution of the next generations of galax- ies. This scale provides a standard ruler which can be used to measure the expansion of the universe and may thus lead to insights into the basic characteristics of the mysterious dark Figure 1. A possible implementation of the filter system for the ‘Physics of the Accelerating Universe’ camera. Wavelengths are expressed in units of angstroms. The transmission curve is based on an airmass-1.3 atmosphere, two reflections at aluminum surfaces, the CCD quantum efficiency, and the expected transmission of the individual filters. This particular realization includes 41 100 ˚ A-wide narrow-band filters cov- ering the wavelength range from 4200 to 8400 ˚ A, supplemented by two wider filters (a u-band and a 350 ˚ A-wide filter) at the blue end and three filters (a 200 ˚ A-wide, a narrow z, and a narrow Y-band filter) in the red. The final camera design may be slightly different. energy. 3 Since the BAO signal in the distribution of galaxies is rather weak, large numbers of galaxies are needed to obtain a robust measurement. The BAO scale was recently measured in spectroscopic 4, 5 and photometric 6 galaxy surveys. This has triggered a significant effort to develop new astronomical in- strumentation aimed at obtaining more detailed BAO measure- ments. To improve upon current BAO results, significant volumes of space need to be sampled, requiring deep large-area surveys. Ac- curate galaxy positions are needed to recover the full 3D matter Continued on next page