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Open access funding provided by Istituto Nazionale di Astrofisica within the CRUI-CARE Agreement. Publisher Copyright: © 2021, The Author(s). Sky survey telescopes and powerful targeted telescopes play complementary roles in astronomy. In order to investigate the nature and characteristics of the motions of very faint objects, a flexibly-pointed instrument capable of high astrometric accuracy is an ideal complement to current astrometric surveys and a unique tool for precision astrophysics. Such a space-based mission will push the frontier of precision astrometry from evidence of Earth-mass habitable worlds around the nearest stars, to distant Milky Way objects, and out to the Local Group of galaxies. As we enter the era of the James Webb Space Telescope and the new ground-based, adaptive-optics-enabled giant telescopes, by obtaining these high precision measurements on key objects that Gaia could not reach, a mission that focuses on high precision astrometry science can consolidate our theoretical understanding of the local Universe, enable extrapolation of physical processes to remote redshifts, and derive a much more consistent picture of cosmological evolution and the likely fate of our cosmos. Already several missions have been proposed to address the science case of faint objects in motion using high precision astrometry missions: NEAT proposed for the ESA M3 opportunity, micro-NEAT for the S1 opportunity, and Theia for the M4 and M5 opportunities. Additional new mission configurations adapted with technological innovations could be envisioned to pursue accurate measurements of these extremely small motions. The goal of this White Paper is to address the fundamental science questions that are at stake when we focus on the motions of faint sky objects and to briefly review instrumentation and mission profiles. publishersversion published

Galaxy clusters are dominated by dark matter, and may have a larger proportion of surviving substructure than, e.g, field galaxies. Due to the presence of galaxy clusters in relative proximity and their high dark matter content, they are promising targets for the indirect detection of dark matter via Gamma-rays. Indeed, dedicated studies of sets of up to 100 clusters have been made previously, so far with no clear indication of a dark matter signal. Here we report on Gamma-ray observations of some 26,000 galaxy clusters based on Pass-7 Fermi Large Area Telescope (LAT) data, with clusters selected from the Tully 2MASS Groups catalog. None of these clusters is significantly detected in Gamma-rays, and we present Gamma-ray flux upper limits between 20 GeV and 500 GeV. We estimate the dark matter content of each of the clusters in these catalogs, and constrain the dark matter annihilation cross section, by analyzing Fermi-LAT data from the directions of the clusters. We set some of the tightest cluster-based constraints to date on the annihilation of dark matter particles with masses between 20 GeV and 500 GeV for annihilation to a gamma-ray line. Our cluster based constraints are not yet as strong as bounds placed using the Galactic Center, although an uncertainty still exists regarding the "boost factor" from cluster substructure, where we have chosen a rather conservative value. Our analysis, given this choice of possible boost, is not yet sensitive enough to fully rule out typical realistic DM candidates, especially if the gamma-ray line is not a dominant annihilation mode., 21 pages, 10 Figures, and 3 Tables

Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, $\Lambda$CDM. 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 $\Lambda$CDM 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., Comment: 99 pages, 8 figures. Version published in Physics of the Dark Universe

The inner profile of Dark Matter (DM) halos remains one of the central problems in small-scale cosmology. At present, the problem can not be resolved in dwarf spheroidal galaxies due to a degeneracy between the DM profile and the velocity anisotropy beta of the stellar population. We discuss a method which can break the degeneracy by exploiting 3D positions and 1D line-of-sight (LOS) velocities. With the full 3D spatial information, we can determine precisely what fraction of each stars LOS motion is in the radial and tangential direction. This enables us to infer the anisotropy parameter beta directly from the data. The method is particularly effective if the galaxy is highly anisotropic. Finally, we argue that such a test could be applied to Sagittarius and potentially other dwarfs with RR Lyrae providing the necessary depth information., 10 pages, 3 figures Accepted in MNRAS. Extended introduction to match accepted version. Main conclusions unchanged

In SuperCool Inflation (SCI), a technically natural and thermal effect gives a graceful exit to old inflation. The Universe starts off hot and trapped in a false vacuum. The Universe supercools and inflates solving the horizon and flatness problems. The inflaton couples to a set of QCD like fermions. When the fermions' non-Abelian gauge group freezes, the Yukawa terms generate a tadpole for the inflaton, which removes the barrier. Inflation ends, and the Universe rapidly reheats. The thermal effect is technically natural in the same way that the QCD scale is technically natural. In fact, Witten used a similar mechanism to drive the Electro-Weak (EW) phase transition; critically, no scalar field drives inflation, which allows SCI to avoid eternal inflation and the measure problem. SCI also works at scales, which can be probed in the lab, and could be connected to EW symmetry breaking. Finally, we introduce a light spectator field to generate density perturbations, which match the CMB. The light field does not affect the inflationary dynamics and can potentially generate non-Gaussianities and isocurvature perturbations observable with Planck., 16 pages, 6 figures

Recently, observations by PAMELA, the Fermi Gamma Ray Space Telescope, and other cosmic ray experiments have generated a great deal of interest in dark matter (DM) particles which annihilate at a high rate to leptons. In this letter, we explore the possibility of using large volume neutrino telescopes, such as IceCube, to constrain such models; specifically we consider signals due to DM annihilation in the inner Milky Way. We find that, if Dark Matter annihilations are responsible for the signals observed by PAMELA and FGST, then IceCube (in conjunction with the planned low threshold extension, DeepCore) should detect or exclude the corresponding neutrino signal from the inner Milky Way with a few years of observation., 4 pages, 1 Figure, and 2 Tables

The first phase of stellar evolution in the history of the universe may be Dark Stars, powered by dark matter heating rather than by fusion. Weakly interacting massive particles, which are their own antiparticles, can annihilate and provide an important heat source for the first stars in the universe. This and the previous contribution present the story of Dark Stars. In this second part, we describe the structure of Dark Stars and predict that they are very massive ($\sim 800 M_\odot$), cool (6000 K), bright ($\sim 10^6 L_\odot$), long-lived ($\sim 10^6$ years), and probable precursors to (otherwise unexplained) supermassive black holes. Later, once the initial dark matter fuel runs out and fusion sets in, dark matter annihilation can predominate again if the scattering cross section is strong enough, so that a Dark Star is born again., Comment: 3 pages, 1 figure, and conference proceeding for IDM Sweden

Dark Stars are the very first phase of stellar evolution in the history of the universe: the first stars to form (typically at redshifts $z \sim 10-50$) are powered by heating from dark matter (DM) annihilation instead of fusion (if the DM is made of particles which are their own antiparticles). We find equilibrium polytropic configurations for these stars; we start from the time DM heating becomes important ($M \sim 1-10 M_\odot$) and build up the star via accretion up to 1000 M$_\odot$. The dark stars, with an assumed particle mass of 100 GeV, are found to have luminosities of a few times $10^6$ L$_\odot$, surface temperatures of 4000--10,000 K, radii $\sim 10^{14}$ cm, lifetimes of at least $ 0.5$ Myr, and are predicted to show lines of atomic and molecular hydrogen. Dark stars look quite different from standard metal-free stars without DM heating: they are far more massive (e.g. $\sim 800 M_\odot$ for 100 GeV WIMPs), cooler, and larger, and can be distinguished in future observations, possibly even by JWST or TMT., 14 pages, 1 figure, 1 table, shortened manuscript for publication, updated mansucript in accordance with referee's report

Chain inflation takes place in the string theory landscape as the universe tunnels rapidly through a series of ever lower energy vacua such as may be characterized by quantized changes in four form fluxes. The string landscape may be well suited to an early period of rapid tunneling, as required by chain inflation, followed by a later period of slow tunneling, such as may be required to explain today's dark energy and small cosmological constant. Each tunneling event (which can alternatively be thought of as a nucleation of branes) provides a fraction of an e-folding of inflation, so that hundreds of tunneling events provide the requisite amount of inflation. A specific example from M-theory compactification on manifolds with non-trivial three-cycles is presented., 7 pages, revtex