We study constraints imposed by two proposed string Swampland criteria on cosmology. These criteria involve an upper bound on the range traversed by scalar fields as well as a lower bound on |∇ φ V |/V when V > 0. We find that inflationary models are generically in tension with these two criteria. Applying these same criteria to dark energy in the present epoch, we find that specific quintessence models can satisfy these bounds and, at the same time, satisfy current observational constraints. Assuming the two Swampland criteria are valid, we argue that the universe will undergo a phase transition within a few Hubble times. These criteria sharpen the motivation for future measurements of the tensor-to-scalar ratio r and the dark energy equation of state w, and for tests of the equivalence principle for dark matter.1 Note that the proper field range is measured along the minimum loci of the potential for a given effective cutoff scale.

This document on the CMB-S4 Science Case, Reference Design, and Project Plan is the product of a global community of scientists who are united in support of advancing CMB-S4 to cross key thresholds in our understanding of the fundamental nature of space and time and the evolution of the Universe. CMB-S4 is planned to be a joint National Science Foundation (NSF) and Department of Energy (DOE) project, with the construction phase to be funded as an NSF Major Research Equipment and Facilities Construction (MREFC) project and a DOE High Energy Physics (HEP) Major Item of Equipment (MIE) project. At the time of this writing, an interim project office has been constituted and tasked with advancing the CMB-S4 project in the NSF MREFC Preliminary Design Phase and toward DOE Critical Decision CD-1. DOE CD-0 is expected imminently.CMB-S4 has been in development for six years. Through the Snowmass Cosmic Frontier planning process, experimental groups in the cosmic microwave background (CMB) and broader cosmology communities came together to produce two influential CMB planning papers, endorsed by over 90 scientists, that outlined the science case as well as the CMB-S4 instrumental concept [1, 2]. It immediately became clear that an enormous increase in the scale of ground-based CMB experiments would be needed to achieve the exciting thresholdcrossing scientific goals, necessitating a phase change in the ground-based CMB experimental program. To realize CMB-S4, a partnership of the university-based CMB groups, the broader cosmology community, and the national laboratories would be needed.The community proposed CMB-S4 to the 2014 Particle Physics Project Prioritization Process (P5) as a single, community-wide experiment, jointly supported by DOE and NSF. Following P5's recommendation of CMB-S4 under all budget scenarios, the CMB community started in early 2015 to hold biannual workshops -open to CMB scientists from around the world -to develop and refine the concept. Nine workshops have been held to date, typically with 150 to 200 participants. The workshops have focused on developing the unique and vital role of the future ground-based CMB program. This growing CMB-S4 community produced a detailed and influential CMB-S4 Science Book [3] and a CMB-S4 Technology Book [4]. Over 200 scientists contributed to these documents. These and numerous other reports, workshop and working group wiki pages, email lists, and much more may be found at the website http://CMB-S4.org.Soon after the CMB-S4 Science Book was completed in August 2016, DOE and NSF requested the Astronomy and Astrophysics Advisory Committee (AAAC) to convene a Concept Definition Taskforce (CDT) to conduct a CMB-S4 concept study. The resulting report was unanimously accepted in late 2017. 1 One recommendation of the CDT report was that the community should organize itself into a formal collaboration. An Interim Collaboration Coordination Committee was elected by the community to coordinate this process. The resulting draft bylaws were refined at the Spring 2018 CMB-S4...

We analyze Hubble Space Telescope observations of scattering regions in 20 luminous obscured quasars at 0.24 < z < 0.65 (11 new observations and 9 archival ones) observed at rest-frame ∼ 3000Å. We find spectacular 5 − 10 kpc-scale scattering regions in almost all cases. The median scattering efficiency at this wavelength (the ratio of observed to estimated intrinsic flux) is 2.3%, and 73% of the observed flux at this wavelength is due to scattered light, which if unaccounted for may strongly bias estimates of quasar hosts' star formation rates. Modeling these regions as illuminated dusty cones, we estimate the radial density distributions of the interstellar medium as well as the geometric properties of circumnuclear quasar obscuration -inclinations and covering factors. Small derived opening angles (median half-angle and standard deviation 27 o ±9 o ) are inconsistent with a 1:1 type 1 / type 2 ratio. We suggest that quasar obscuration is patchy and that the observer has a ∼ 40% chance of seeing a type 1 source even through the obscuration. We estimate median density profile of the scattering medium to be n H = 0.04 − 0.5 (1kpc/r) 2 cm −3 , depending on the method. Quasars in our sample likely exhibit galaxy-wide winds, but if these consist of optically thick clouds then only a small fraction of the wind mass ( 10%) contributes to scattering.