The next generation "Stage-4" ground-based cosmic microwave background (CMB) experiment, CMB-S4, consisting of dedicated telescopes equipped with highly sensitive superconducting cameras operating at the South Pole, the high Chilean Atacama plateau, and possibly northern hemisphere sites, will provide a dramatic leap forward in our understanding of the fundamental nature of space and time and the evolution of the Universe. CMB-S4 will be designed to cross critical thresholds in testing inflation, determining the number and masses of the neutrinos, constraining possible new light relic particles, providing precise constraints on the nature of dark energy, and testing general relativity on large scales.CMB-S4 is intended to be the definitive ground-based CMB project. It will deliver a highly constraining data set with which any model for the origin of the primordial fluctuations-be it inflation or an alternative theory-and their evolution to the structure seen in the Universe today must be consistent. While we have learned a great deal from CMB measurements, including discoveries that have pointed the way to new physics, we have only begun to tap the information encoded in CMB polarization, CMB lensing and other secondary effects. The discovery space from these and other yet to be imagined effects will be maximized by designing CMB-S4 to produce high-fidelity maps, which will also ensure enormous legacy value for CMB-S4. CMB-S4 is the logical successor to the Stage-3 CMB projects which will operate over the next few years. For maximum impact, CMB-S4 should be implemented on a schedule that allows a transition from Stage 3 to Stage 4 that is as seamless and as timely as possible, preserving the expertise in the community and ensuring a continued stream of CMB science results. This timing is also necessary to ensure the optimum synergistic enhancement of the science return from contemporaneous optical surveys (e.g., LSST, DESI, Euclid and WFIRST). Information learned from the ongoing Stage-3 experiments can be easily incorporated into CMB-S4 with little or no impact on its design. In particular, additional information on the properties of Galactic foregrounds would inform the detailed distribution of detectors among frequency bands in CMB-S4. The sensitivity and fidelity of the multiple band foreground measurements needed to realize the goals of CMB-S4 will be provided by CMB-S4 itself, at frequencies just below and above those of the main CMB channels. This timeline is possible because CMB-S4 will use proven existing technology that has been developed and demonstrated by the CMB experimental groups over the last decade. There are, to be sure, considerable technical challenges presented by the required scaling-up of the instrumentation and by the scope and complexity of the data analysis and interpretation. CMB-S4 will require: scaled-up superconducting detector arrays with well-understood and robust material properties and processing techniques; high-throughput mmwave telescopes and optics with unprecedented precisi...

We consider the possibility that the black-hole (BH) binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses 20 M M bh 100 M where primordial black holes (PBHs) may constitute the dark matter. If two BHs in a galactic halo pass sufficiently close, they radiate enough energy in gravitational waves to become gravitationally bound. The bound BHs will rapidly spiral inward due to emission of gravitational radiation and ultimately merge. Uncertainties in the rate for such events arise from our imprecise knowledge of the phase-space structure of galactic halos on the smallest scales. Still, reasonable estimates span a range that overlaps the 2 − 53 Gpc −3 yr −1 rate estimated from GW150914, thus raising the possibility that LIGO has detected PBH dark matter. PBH mergers are likely to be distributed spatially more like dark matter than luminous matter and have no optical nor neutrino counterparts. They may be distinguished from mergers of BHs from more traditional astrophysical sources through the observed mass spectrum, their high ellipticities, or their stochastic gravitational wave background. Next generation experiments will be invaluable in performing these tests.The nature of the dark matter (DM) is one of the most longstanding and puzzling questions in physics. Cosmological measurements have now determined with exquisite precision the abundance of DM [1, 2], and from both observations and numerical simulations we know quite a bit about its distribution in Galactic halos. Still, the nature of the DM remains a mystery. Given the efficacy with which weakly-interacting massive particlesfor many years the favored particle-theory explanationhave eluded detection, it may be warranted to consider other possibilities for DM. Primordial black holes (PBHs) are one such possibility [3-6].Here we consider whether the two ∼ 30 M black holes detected by LIGO [7] could plausibly be PBHs. There is a window for PBHs to be DM if the BH mass is in the range 20 M M 100 M [8,9]. Lower masses are excluded by microlensing surveys [10][11][12]. Higher masses would disrupt wide binaries [9,13,14]. It has been argued that PBHs in this mass range are excluded by CMB constraints [15,16]. However, these constraints require modeling of several complex physical processes, including the accretion of gas onto a moving BH, the conversion of the accreted mass to a luminosity, the self-consistent feedback of the BH radiation on the accretion process, and the deposition of the radiated energy as heat in the photon-baryon plasma. A significant (and difficult to quantify) uncertainty should therefore be associated with this upper limit [17], and it seems worthwhile to examine whether PBHs in this mass range could have other observational consequences.In this Letter, we show that if DM consists of ∼ 30 M BHs, then the rate for mergers of such PBHs falls within the merger rate inferred from GW150914. In any galactic halo, there is a chance two BHs will undergo a hard scatter, lose energy to a s...

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We explore the effects of elastic scattering between dark matter and baryons on the 21-cm signal during the dark ages. In particular, we consider a dark-matter-baryon interaction with a cross section of the form σ = σ0v −4 , in which case the effect of the drag force between the dark mater and baryon fluids grows with time. We show that, as opposed to what was previously thought, this effect heats up the baryons due to the relative velocity between dark matter and baryons. This creates an additional source of fluctuations, which can potentially make interactions easier to detect by 21-cm measurements than by using the cosmic microwave background and the Lyman-α forest. Our forecasts show that the magnitude of the cross section can be probed to σ0 ∼ 3 × 10

The dynamics of our Universe is strongly influenced by pervasive-albeit elusive-dark matter, with a total mass about five times the mass of all the baryons. Despite this, its origin and composition remain a mystery. All evidence for dark matter relies on its gravitational pull on baryons, and thus such evidence does not require any non-gravitational coupling between baryons and dark matter. Nonetheless, some small coupling would explain the comparable cosmic abundances of dark matter and baryons , as well as solving structure-formation puzzles in the pure cold-dark-matter models . A vast array of observations has been unable to find conclusive evidence for any non-gravitational interactions of baryons with dark matter. Recent observations by the EDGES collaboration, however, suggest that during the cosmic dawn, roughly 200 million years after the Big Bang, the baryonic temperature was half of its expected value . This observation is difficult to reconcile with the standard cosmological model but could be explained if baryons are cooled down by interactions with dark matter, as expected if their interaction rate grows steeply at low velocities . Here we report that if a small fraction-less than one per cent-of the dark matter has a mini-charge, a million times smaller than the charge on the electron, and a mass in the range of 1-100 times the electron mass, then the data from the EDGES experiment can be explained while remaining consistent with all other observations. We also show that the entirety of the dark matter cannot have a mini-charge.

The possibility that part of the dark matter is made of massive compact halo objects (MACHOs) remains poorly constrained over a wide range of masses, and especially in the 20 − 100 M window. We show that strong gravitational lensing of extragalactic fast radio bursts (FRBs) by MACHOs of masses larger than ∼ 20 M would result in repeated FRBs with an observable time delay. Strong lensing of a FRB by a lens of mass ML induces two images, separated by a typical time delay ∼ few ×(ML/30 M ) milliseconds. Considering the expected FRB detection rate by upcoming experiments, such as CHIME, of 104 FRBs per year, we should observe from tens to hundreds of repeated bursts yearly, if MACHOs in this window make up all the dark matter. A null search for echoes with just 10 4 FRBs would constrain the fraction fDM of dark matter in MACHOs to fDM 0.08 for ML 20 M .Although observations indicate that dark matter accounts for a significant share of the energy density of our Universe [1], we do not know its composition. Longtime candidates to make up the dark matter are massive compact halo objects (MACHOs) [2]. They were originally proposed to be as light as 10 −7 M and as heavy as the first stars (∼ 10 3 M ) [3]. Over the years, different experiments have progressively constrained the fraction f DM of dark matter that can reside in MACHOs with a given mass, placing tight upper bounds over most of the vast range above. High-mass ( 100 M ) MACHOs, for example, are constrained by the fact that they would perturb wide stellar binaries in our Galaxy [4]. Meanwhile, lower-mass ( 20M ) MACHOs are effectively ruled out as the sole component of Galactic dark matter, as they would create artificial variability in stars, due to gravitational microlensing [5][6][7].However, there remains a window of masses between 20 and 100 M , where the constraints are weaker, and in which arguably all the cosmological dark matter could be in the form of MACHOs [6][7][8][9][10]. This is a particularly interesting window, as it has been recently argued in Ref.[11] that if primordial black holes (PBHs) [12,13] in the ∼ 30 M mass range are the constituents of dark matter, they form binaries in halos, coalesce, and emit observable gravitational waves, with an event rate consistent with the published LIGO detection [14].In this Letter we propose to use the strong lensing of fast radio bursts (FRBs) to probe MACHOs of masses 20 M , including PBHs, and either confirm that they make up the dark matter or close this window. FRBs are strong radio bursts with a very short duration, which makes them ideal as microlensing targets. Their temporal width is increased by the dispersion measure (DM), which measures the time delay of photons with different radio frequencies due to scattering by free electrons on their way to Earth. All detected FRBs to date possess high DMs, which yield burst widths of ∼ 1 − 10 ms [15][16][17][18][19][20][21][22][23][24]. These values of the DM are several times larger than the expected contribution from free electrons within the Milky Way ...

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A measurement of primordial non-gaussianity will be of paramount importance to distinguish between different models of inflation. Cosmic microwave background (CMB) anisotropy observations have set unprecedented bounds on the non-gaussianity parameter fNL but the interesting regime fNL 1 is beyond their reach. Brightness-temperature fluctuations in the 21-cm line during the dark ages (z ∼ 30 − 100) are a promising successor to CMB studies, giving access to a much larger number of modes. They are, however, intrinsically non-linear, which results in secondary non-gaussianities orders of magnitude larger than the sought-after primordial signal. In this paper we carefully compute the primary and secondary bispectra of 21-cm fluctuations on small scales. We use the flat-sky formalism, which greatly simplifies the analysis, while still being very accurate on small angular scales. We show that the secondary bispectrum is highly degenerate with the primordial one, and argue that even percent-level uncertainties in the amplitude of the former lead to a bias of order ∆fNL ∼ 10. To tackle this problem we carry out a detailed Fisher analysis, marginalizing over the amplitudes of a few smooth redshift-dependent coefficients characterizing the secondary bispectrum. We find that the signal-to-noise ratio for a single redshift slice is reduced by a factor of ∼ 5 in comparison to a case without secondary non-gaussianities. Setting aside foreground contamination, we forecast that a cosmic-variance-limited experiment observing 21-cm fluctuations over 30 ≤ z ≤ 100 with a 0.1-MHz bandwidth and 0.1-arcminute angular resolution could achieve a sensitivity of order f local NL ∼ 0.03, f equil NL ∼ 0.04 and f ortho NL ∼ 0.03.