# Simons Observatory - Goals

### Develop infrastructure and instrumentation in preparation for the next generation of CMB experiments.

The SO will study and dene a path that best supports the ultimate goals of the CMB community for CMB-S4. The existing ACT and SA site in Chile will be significantly expanded with several new telescopes and receivers as well as increased infrastructure forming the basis for the Chilean component of CMB-S4. SO will also provide a platform on which we demonstrate technologies, establishing them for use in CMB-S4.

### Deepen our understanding of the first moments of the Big Bang.

If the tensor to scalar ratio r is > 0.01, then the SO will make the definitive measurement of gravitational waves, considered the surest evidence of inflation. A detection would transform physicists' understanding of the early universe and high energy physics at the GUT scale. With its wide frequency range and sky coverage, the SO can test the isotropy, frequency spectrum and scale-dependence of the gravitational wave signal, and have the ability to internally delens the signal. By measuring the small-scale polarization anisotropy, the SO will also significantly improve our measurement of the scalar spectral index of primordial fluctuations, directly connecting to early universe models.

### Accurately and independently determine the basic parameters of our universe.

The SO will make a cosmic-variance-limited measurement of the polarization out to l = 2000 over 15,000 deg2. By measuring 3 times more polarization modes than Planck, it will provide an independent check of our basic cosmological model and more accurately determine the age, composition and geometry of the universe, and its statistical properties very early in its history.

### Determine fundamental properties of neutrinos.

Via measurements of the growth rate of structure, the SO will strongly constrain the sum of neutrino masses. Known to be at least m = 60 meV, the SO targets a few-sigma detection of the mass sum in combination with low-redshift data. With improved small-scale polarization measurements, the SO will also measure the effective number of neutrino species to the percent level, testing neutrino interactions and the conditions in the early universe. While we quote the constraint on effective number of neutrino species, our measurements are sensitive to all relativistic species so will be sensitive to any light particles even those that decouple before the QCD phase transition.

### Seek cosmological signatures of dark matter.

By measuring the CMB polarization and the growth of structure, the SO will uniquely constrain dark matter annihilation, dark matter interactions, and signficantly improve constraints on light axions as dark matter.

### Find > 1 galaxy cluster per square degree in a mass-limited SZ survey over half the sky.

With overlapping data from HSC, DES, LSST, eROSITA and other surveys, the SO survey will transform the study of clusters. The SO will have a unique combination of resolution, sensitivity, and sky access to produce this half-sky catalog. We will be able to calibrate the mass of these clusters using both our own CMB lensing data and optical lensing data. These calibrated cluster counts will provide an independent measurement of neutrino mass.

### Improve measurements of dark energy properties and test whether cosmic acceleration is due to the breakdown of General Relativity.

The SO will trace the growth rate of structure using three independent measurements: (1) CMB lensing and its cross-correlation with optical lensing surveys; (2) cluster counts, calibrated by HSC, DES, and LSST lensing measurements; and (3) the galaxy momentum field, via the kinematic SZ (kSZ) effect. By correlating the momentum field with the density field traced by BOSS, DESI and PFS, the SO will make measurements of f(z) and the Alcock-Pacynzski effect that are not limited by cosmic variance.

### Trace the relationship between dark matter halos and their ionized gas.

The combination of SO and the upcoming DESI survey will make a high S/N detection of the cross-correlation between galaxy halos and the large-scale distribution of ionized baryons. This measurement will trace the effects of galaxy feedback as a function of redshift.

### Provide critical confirmation of the LSST galaxy shear calibration,

through cross- correlations of the CMB lensing map with galaxy shear and galaxy clustering, amplifying the science impact of the highest priority project in the 2010 Decadal Survey.

### Determine the duration of reionization.

The SO will make a very high S/N detection of the Ostriker-Vishniac effect in the CMB intensity, and will use higher order statistics to constrain the spatial variation of reionization.

### Constrain parity and Lorentz-violating effects.

Chern-Simons cosmic birefringence can be measured with precise limits on cosmic polarization rotation. A detection of the rotation of the plane of linear polarization of the CMB would be a remarkable glimpse into the fundamental laws of nature. Such an effect is predicted in many theories of beyond the standard model physics. The SO will provide ten times better constraints on these effects than current observations provide.

on 20th April 2016