LSST as a facility for fundamental physics research

The LSST total effective system throughput, AΩ = 318 m² deg²,is nearly two orders of magnitude larger than that of any existing facility. LSST will enable a wide variety of complementary scientific investigations, all utilizing a common database. Of particular interest to particle physics, LSST will probe the physics of dark energy in multiple ways, as well as a measurement of the neutrino mass down to the 10 milli eV level. This will be done via data on the shapes, positions, and distances of billions of galaxies, plus a million supernovae. The physics of dark matter will be probed via the strong gravitational lenses LSST will discover. When combined with cosmic microwave background data, measurement of the dark matter power spectrum through weak lensing and baryon acoustic oscillations will provide tight constraints on models of dark energy. Other complementary LSST probes of dark energy physics include counts of giant clusters of dark matter vs cosmic epoch, and the distances to supernovae vs cosmic epoch.

LSST's constraints on the nature of dark energy are also uniquely accurate. By measuring the gravitational lens distorted shapes of billions of galaxies as a function of angle on the sky and redshift, as well as their correlations in 3-space, LSST will constrain at least six eigenmodes of the dark energy equation of state. These data are uniquely capable of breaking the degeneracy between models of the origin of the recent acceleration in the expansion of the universe: dark energy as a diffuse component in the stress-energy, or dark energy as a manifestation of modifications to spacetime-gravity on large scales.

Starting in 2012, data from LSST will be analyzed for a wide range of phenomena. By separately tracing the development mass structure and rate of expansion of the universe, these data will address the physics of dark matter and dark energy, the possible existence of modified gravity on large scales, large extra dimensions, the neutrino mass, supersymmetry, and possible self interaction of dark matter particles.

LSST and New Physics

In the last decade, cosmologists have converged on a "standard model" of cosmology. This so-called "concordance model" posits two mysterious new components: non-baryonic dark matter and dark energy. Our universe appears to be composed mainly of unknown forms of non-luminous mass-energy: 96 percent of the mass-energy is "dark". The existence of non-baryonic dark matter implies the existence of a totally new sector of particle physics, which dominates the matter inventory of the Universe.

Even more striking is evidence for accelerating cosmic expansion. This is clear and robust evidence for physics beyond the standard model at a friction point between our current paradigms: at the interface between quantum mechanics and gravitation. The physics that produces the observed accelerating cosmic expansion is a complete mystery. Is it "dark energy" arising from quantum fluctuations in the vacuum, or is it new gravitational physics? In either case, the LSST is uniquely capable of addressing the underlying physics by exploiting a diversity of cosmic probes:

• Weak lensing of galaxies vs redshift, which probes both the evolution of structure over cosmic time and dimensionless ratios of distances vs cosmic time, thereby setting multiple independent strong constraints on the nature of dark energy,
• Strong lensing of galaxies, quasars and supernovae, which probes the nature of dark matter in galaxy and cluster halos, as well as the evolution of the underlying geometry,
• Correlations of galaxies in 3-space vs cosmic epoch ("Baryon Acoustic Oscillations") utilizes the "standard ruler" of the peak in the angular correlation of dark matter revealed in the temperature anisotropies in the cosmic microwave background.
• Counts of clusters of dark matter (via weak gravitational lensing combined with the optical data) are an exponentially sensitive probe of the equation of state of dark energy.
• Supernovae are a useful complementary technique for probing the cosmic era when dark energy becomes dominant.
• The resulting precision measurements of the power spectra vs cosmic time will usefully constrain the sum neutrino mass.
• By simultaneously measuring mass growth and curvature, LSST data can tell us whether the recent acceleration is due to dark energy or modified gravity.

While it is certainly the case that these measurements are observational rather than experimental, we simply must go where the signal for new fundamental physics is non-zero. Astrophysical observations are indeed susceptible to sources of systematic error, but the LSST is being specifically designed and engineered to minimize and control systematics. The diverse techniques listed above have different susceptibilities to systematic uncertainties, and this multi-pronged approach will help us discriminate fundamental physics from artifacts. Much of the power of the LSST comes from the fact that the measurements will be obtained from the same basic set of observations, using a powerful facility that is optimized for this purpose.

The ultimate systematics test will be agreement from multiple methods with completely different systematic error susceptibilities. We can check if the various integrals over the Hubble expansion H(z) are consistent, e.g., the luminosity distance, the comoving volume, and the development of structure, all as a function of redshift. Today we parametrize our ignorance of the physics of dark energy as a scaling of the cosmic pressure to energy density ratio w(z) with scale factor a(z): p/ρ = w₀ + w_a (1-a). Perhaps we will not be able to fit all the data with one free function. We have no trusted dark energy theory guide, so we want every possible handle on w(z): baryon acoustic oscillations, supernovae and weak lensing probe w(z) very differently. Even the statistical errors are different. Moreover, the multiple weak lensing probes of dark energy which use the same shear data each sample the time history of the dark energy density with more normal modes than SN data.

The LSST will be operating in an era when we will asking new questions. While it is obviously impossible to predict with confidence just what these might be, three illustrative examples of the additional scientific "reach" of LSST are:

1. Testing the isotropy of the Hubble diagram using both weak lens tomography, baryon acoustic oscillations (BAO), and supernovae. If the cosmic acceleration is the result of some very recent symmetry breaking at a new energy scale, it might not have happened at exactly the same time in our horizon volume. The LSST's all-sky cosmic shear and BAO data and supernova sample will probe the details of the expansion history since z~0.5, when the dark energy began to dominate the expansion. By measuring w(θ,ψ) we can perform a differential test of this idea, immune from systematics due to supernova evolution, etc.
2. Testing the law of gravitation over cosmic scales. Armed with knowledge of the Hubble parameter H(z), gravitationally lensed systems can be considered as tests of the gravitational force law over cosmic distances. Speculative ideas (Dvali et al, 2003) have linked the cosmic acceleration to string-inspired modifications of gravity that also produce effects on smaller scales.
3. WMAP CMB anisotropy data indicate a deficit in power on large angular scales. LSST is uniquely able to measure the mass power spectrum on such scales.