The Simons Observatory (SO) is a new cosmic microwave background experiment being built on Cerro Toco in Chile, due to begin observations in the early 2020s. We describe the scientific goals of the experiment, motivate the design, and forecast its performance. SO will measure the temperature and polarization anisotropy of the cosmic microwave background in six frequency bands centered at: 27, 39, 93, 145, 225 and 280 GHz. The initial configuration of SO will have three small-aperture 0.5-m telescopes and one large-aperture 6-m telescope, with a total of 60,000 cryogenic bolometers. Our key science goals are to characterize the primordial perturbations, measure the number of relativistic species and the mass of neutrinos, test for deviations from a cosmological constant, improve our understanding of galaxy evolution, and constrain the duration of reionization. The small aperture telescopes will target the largest angular scales observable from Chile, mapping ≈ 10% of the sky to a white noise level of 2 μK-arcmin in combined 93 and 145 GHz bands, to measure the primordial tensor-to-scalar ratio,r, at a target level of σ(r)=0.003. The large aperture telescope will map ≈ 40% of the sky at arcminute angular resolution to an expected white noise level of 6 μK-arcmin in combined 93 and 145 GHz bands, overlapping with the majority of the Large Synoptic Survey Telescope sky region and partially with the Dark Energy Spectroscopic Instrument. With up to an order of magnitude lower polarization noise than maps from thePlancksatellite, the high-resolution sky maps will constrain cosmological parameters derived from the damping tail, gravitational lensing of the microwave background, the primordial bispectrum, and the thermal and kinematic Sunyaev-Zel'dovich effects, and will aid in delensing the large-angle polarization signal to measure the tensor-to-scalar ratio. The survey will also provide a legacy catalog of 16,000 galaxy clusters and more than 20,000 extragalactic sources.

Horizon-scale images of black holes (BHs) and their shadows have opened an unprecedented window onto tests of gravity and fundamental physics in the strong-field regime. We consider a wide range of well-motivated deviations from classical general relativity (GR) BH solutions, and constrain them using the Event Horizon Telescope (EHT) observations of Sagittarius A ∗ (Sgr A ∗ ), connecting the size of the bright ring of emission to that of the underlying BH shadow and exploiting high-precision measurements of Sgr A ∗ ’s mass-to-distance ratio. The scenarios we consider, and whose fundamental parameters we constrain, include various regular BHs, string-inspired space-times, violations of the no-hair theorem driven by additional fields, alternative theories of gravity, novel fundamental physics frameworks, and BH mimickers including well-motivated wormhole and naked singularity space-times. We demonstrate that the EHT image of Sgr A ∗ places particularly stringent constraints on models predicting a shadow size larger than that of a Schwarzschild BH of a given mass, with the resulting limits in some cases surpassing cosmological ones. Our results are among the first tests of fundamental physics from the shadow of Sgr A ∗ and, while the latter appears to be in excellent agreement with the predictions of GR, we have shown that a number of well-motivated alternative scenarios, including BH mimickers, are far from being ruled out at present.

The strong discrepancy between local and early-time (inverse distance ladder) estimates of the Hubble constant $H_0$ could be pointing towards new physics beyond the concordance $\mathrm{\Lambda }\mathrm{CDM}$ model. Several attempts to address this tension through new physics rely on extended cosmological models, featuring extra free parameters beyond the six $\mathrm{\Lambda }\mathrm{CDM}$ parameters. However, marginalizing over additional parameters has the effect of broadening the uncertainties on the inferred parameters (including $H_0$), and it is often the case that within these models the $H_0$ tension is addressed due to larger uncertainties rather than a genuine shift in the central value of $H_0$. In this paper I consider an alternative viewpoint: what happens if a physical theory is able to fix the extra parameters to a specific set of nonstandard values? In this case, the degrees of freedom of the model are reduced with respect to the standard case where the extra parameters are free to vary. Focusing on the dark energy equation of state $w$ and the effective number of relativistic species $N_{\mathrm{eff}}$, I find that physical theories able to fix $w\approx -1.3$ or $N_{\mathrm{eff}}\approx 3.95$ would lead to an estimate of $H_0$ from cosmic microwave background, baryon acoustic oscillation, and type Ia supernovae data in perfect agreement with the local distance ladder estimate, without broadening the uncertainty on the former. These two nonstandard models are, from a model-selection perspective, strongly disfavored with respect to the baseline $\mathrm{\Lambda }\mathrm{CDM}$ model. However, models that predict $N_{\mathrm{eff}}\approx 3.45$ would be able to bring the tension down to $1.5\sigma$ while only being weakly disfavored with respect to $\mathrm{\Lambda }\mathrm{CDM}$, whereas models that predict $w\approx -1.1$ would be able to bring the tension down to $2\sigma$ (at the cost of the preference for $\mathrm{\Lambda }\mathrm{CDM}$ being definite). Finally, I estimate dimensionless multipliers relating variations in $H_0$ to variations in $w$ and $N_{\mathrm{eff}}$, which can be used to swiftly repeat the analysis of this paper in light of future more precise local distance ladder estimates of $H_0$, should the tension persist. As a caveat, these results were obtained from the 2015 Planck data release, but these findings would be qualitatively largely unaffected were I to use more recent data.

The Event Horizon Telescope (EHT) collaboration has recently released the first image of a black hole (BH), opening a new window onto tests of general relativity in the strong field regime. In this paper, we derive constraints on the nature of M87* (the supermassive object at the center of the galaxy M87), exploiting the fact that its shadow appears to be highly circular, and using measurements of its angular size. We first consider the simple case where M87* is assumed to be a Kerr BH. We find that the inferred circularity of M87* excludes Kerr BHs with observation angle ${\theta }_{\mathrm{obs}}\gtrsim 45°$ for dimensionless rotational parameter $0.95\lesssim a_{*}\le 1$ whereas the observation angle is unbounded for $a_{*}\lesssim 0.9$. We then consider the possibility that M87* might be a superspinar, i.e., an object described by the Kerr solution and spinning so fast that it violates the Kerr bound by having $|a_{*}|>1$. We find that, within certain regions of parameter space, the inferred circularity and size of the shadow of M87* do not exclude the possibility that this object might be a superspinar.

Using some of the latest cosmological data sets publicly available, we derive the strongest bounds in the literature on the sum of the three active neutrino masses, $M_{\nu }$, within the assumption of a background flat $\mathrm{\Lambda }\mathrm{CDM}$ cosmology. In the most conservative scheme, combining Planck cosmic microwave background temperature anisotropies and baryon acoustic oscillations (BAO) data, as well as the up-to-date constraint on the optical depth to reionization ($\tau$), the tightest 95% confidence level upper bound we find is $M_{\nu }<0.151\mathrm{eV}$. The addition of Planck high-$\ell$ polarization data, which, however, might still be contaminated by systematics, further tightens the bound to $M_{\nu }<0.118\mathrm{eV}$. A proper model comparison treatment shows that the two aforementioned combinations disfavor the inverted hierarchy at $\sim 64\%\mathrm{C}.\mathrm{L}.$ and $\sim 71\%\mathrm{C}.\mathrm{L}.$, respectively. In addition, we compare the constraining power of measurements of the full-shape galaxy power spectrum versus the BAO signature, from the BOSS survey. Even though the latest BOSS full-shape measurements cover a larger volume and benefit from smaller error bars compared to previous similar measurements, the analysis method commonly adopted results in their constraining power still being less powerful than that of the extracted BAO signal. Our work uses only cosmological data; imposing the constraint $M_{\nu }>0.06\mathrm{eV}$ from oscillations data would raise the quoted upper bounds by $\mathcal{O}(0.1\sigma )$ and would not affect our conclusions.

We explore whether nonstandard dark sector physics might be required to solve the existing cosmological tensions. The properties we consider in combination are (a) an interaction between the dark matter and dark energy components and (b) a dark energy equation of state $w$ different from that of the canonical cosmological constant $w=-1$. In principle, these two parameters are independent. In practice, to avoid early-time, superhorizon instabilities, their allowed parameter spaces are correlated. Moreover, a clear degeneracy exists between these two parameters in the case of cosmic microwave background (CMB) data. We analyze three classes of extended interacting dark energy models in light of the 2019 Planck CMB results and Cepheid-calibrated local distance ladder $H_0$ measurements of Riess et al. (R19), as well as recent baryon acoustic oscillation (BAO) and type Ia supernovae (SNeIa) distance data. We find that in quintessence coupled dark energy models, where $w>-1$, the evidence for a nonzero coupling between the two dark sectors can surpass the $5\sigma$ significance. Moreover, for both $\mathrm{Planck}+\mathrm{BAO}$ or $\mathrm{Planck}+\mathrm{SNeIa}$, we find a preference for $w>-1$ at about three standard deviations. Quintessence models are, therefore, in excellent agreement with current data when an interaction is considered. On the other hand, in phantom coupled dark energy models, there is no such preference for a nonzero dark sector coupling. All the models we consider significantly raise the value of the Hubble constant, easing the $H_0$ tension. In the interacting scenario, the disagreement between $\mathrm{Planck}+\mathrm{BAO}$ and R19 is considerably reduced from $4.3\sigma$ in the case of the $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$) model to about $2.5\sigma$. The addition of low-redshift BAO and SNeIa measurements leaves, therefore, some residual tension with R19 but at a level that could be justified by a statistical fluctuation. Bayesian evidence considerations mildly disfavor both the coupled quintessence and phantom models, while mildly favoring a coupled vacuum scenario, even when late-time datasets are considered. We conclude that nonminimal dark energy cosmologies, such as coupled quintessence, phantom, or vacuum models, are still an interesting route toward softening existing cosmological tensions, even when low-redshift datasets and Bayesian evidence considerations are taken into account.