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Cosmological simulations of galaxy formation are limited by finite computational resources. We draw from the ongoing rapid advances in artificial intelligence (AI; specifically deep learning) to address this problem. Neural networks have been developed to learn from high-resolution (HR) image data and then make accurate superresolution (SR) versions of different low-resolution (LR) images. We apply such techniques to LR cosmological N-body simulations, generating SR versions. Specifically, we are able to enhance the simulation resolution by generating 512 times more particles and predicting their displacements from the initial positions. Therefore, our results can be viewed as simulation realizations themselves, rather than projections, e.g., to their density fields. Furthermore, the generation process is stochastic, enabling us to sample the small-scale modes conditioning on the large-scale environment. Our model learns from only 16 pairs of small-volume LR-HR simulations and is then able to generate SR simulations that successfully reproduce the HR matter power spectrum to percent level up to 16 h −1Mpc and the HR halo mass function to within 10% down to 1011 M ☉. We successfully deploy the model in a box 1,000 times larger than the training simulation box, showing that high-resolution mock surveys can be generated rapidly. We conclude that AI assistance has the potential to revolutionize modeling of small-scale galaxy-formation physics in large cosmological volumes.

Galaxy protoclusters, which will eventually grow into the massive clusters we see in the local Universe, are usually traced by locating overdensities of galaxies 1 . Large spectroscopic surveys of distant galaxies now exist, but their sensitivity depends mainly on a galaxy’s star-formation activity and dust content rather than its mass. Tracers of massive protoclusters that do not rely on their galaxy constituents are therefore needed. Here we report observations of Lyman-α absorption in the spectra of a dense grid of background galaxies 2 , 3 , which we use to locate a substantial number of candidate protoclusters at redshifts 2.2 to 2.8 through their intergalactic gas. We find that the structures producing the most absorption, most of which were previously unknown, contain surprisingly few galaxies compared with the dark-matter content of their analogues in cosmological simulations 4 , 5 . Nearly all of the structures are expected to be protoclusters, and we infer that half of their expected galaxy members are missing from our survey because they are unusually dim at rest-frame ultraviolet wavelengths. We attribute this to an unexpectedly strong and early influence of the protocluster environment 6 , 7 on the evolution of these galaxies that reduced their star formation or increased their dust content. Lyman-α absorption observations from the Las Campanas Observatory are used to find a population of ultraviolet-dim protoclusters that contain few galaxies compared with their analogues in cosmological simulations.

Extremely red quasars (ERQs) are an interesting sample of quasars in the Baryon Oscillation Spectroscopic Sample (BOSS) in the redshift range of \\(2.0 - 3.4\\) and have extreme red colours of \\(i-W3\\ge4.6\\). Core ERQs have strong CIV emission lines with rest equivalent width of \\(\\ge100\\)\\AA. Many core ERQs also have CIV line profiles with peculiar boxy shapes which distinguish them from normal blue quasars. We show, using a combination of kernel density estimation and local outlier factor analyses on a space of the \\(i-W3\\) colour, CIV rest equivalent width and line kurtosis, that core ERQs likely represent a separate population rather than a smooth transition between normal blue quasars and the quasars in the tail of the colour-REW distribution. We apply our analyses to find new criteria for selecting ERQs in this 3D parameter space. Our final selection produces \\(133\\) quasars, which are \\emph{three} times more likely to have a visually verified CIV broad absorption line feature than the previous core ERQ sample. We further show that our newly selected sample are extreme objects in the intersection of the WISE AGN catalogue with the MILLIQUAS quasar catalogue in the colour-colour space of (\\(W1-W2\\), \\(W2-W3\\)). This paper validates an improved selection method for red quasars which can be applied to future datasets such as the quasar catalogue from the Dark Energy Spectroscopic Instrument (DESI).

Observations of metal absorption lines in the spectra of QSOs out to z > 6 are providing an important probe into the enrichment and ionization state of the intergalactic medium (IGM) at the tail end of reionization. Using simulations with four different feedback models, including the Illustris and Sherwood simulations, we investigate how the overall incidence rate and equivalent width distribution of metal-line absorbers varies with the galactic wind scheme. The low-ionization absorbers are reasonably insensitive to the feedback implementation, with all models reasonably close to the observed incidence rate of O i absorbers. However, all of our models struggle to reproduce the observations of C iv, which is probing overdensities close to the mean at z ~ 6, suggesting that the metals are not being transported out into the IGM efficiently enough in these simulations.

I use mathematical models and numerical simulations to constrain cosmological inflation, the seeds of structure, and the mass of the neutrino. I revisit arguments that simple models of inflation with a small red tilt in the scalar power spectrum generically yield an observable tensor spectrum. I show that criteria for fine-tuning based upon the algebraic simplicity of the potential depend strongly upon the assumptions they incorporate about the potential. Furthermore, several models with algebraically simple potentials require carefully tuned initial field configurations. I demonstrate the existence of potentials with vanishingly small tensor amplitudes which are natural in terms of both their algebraic form and initial conditions. I thus argue that proposed experiments which make highly sensitive measurements of the tensor amplitude cannot definitively rule out the inflationary paradigm. The overshoot problem is the need to make the initial kinetic energy of the inflaton small enough to ensure slow-roll. I investigate claims that brane inflation solves the overshoot problem through microphysical restrictions on the phase space of initial conditions. By carrying out a comprehensive analysis of the parameter space allowed by the latest advances in brane inflation model-building, I find that the vast majority of the phase space of initial conditions is still dominated by overshoot trajectories. Current results from the Lyman-α forest assume that the primordial power spectrum of density perturbations follows a simple power-law form with running. I perform a large suite of numerical simulations, using them to calibrate a minimally parametric framework for describing the power spectrum. Combined with cross-validation this framework allows me to directly reconstruct the power spectrum shape, a consistency check on the standard model. I find no evidence for deviation from scale-invariance, but current Lyman-? data do not have sufficient statistical power to robustly probe the shape of the power spectrum at these scales. In contrast, the ongoing Baryon Oscillation Sky Survey will be able to do so with high precision. I perform an extensive suite of N-body simulations of the matter power spectrum, probing deep into the non-linear regime while incorporating massive neutrinos. I compare my results to the widely used HALOFIT approximation, and find that in the strongly nonlinear regime it significantly over-predicts the suppression due to the free-streaming of neutrinos. Most published constraints are not affected, as they have used HALOFIT only in the linear or mildly non-linear regime. However, my results are important for future galaxy and weak lensing surveys.

We make forecasts for the impact a future \"midband\" space-based gravitational wave experiment, most sensitive to \\(10^{-2}- 10\\) Hz, could have on potential detections of cosmological stochastic gravitational wave backgrounds (SGWBs). Specific proposed midband experiments considered are TianGo, B-DECIGO and AEDGE. We propose a combined power-law integrated sensitivity (CPLS) curve combining GW experiments over different frequency bands, which shows the midband improves sensitivity to SGWBs by up to two orders of magnitude at \\(10^{-2} - 10\\) Hz. We consider GW emission from cosmic strings and phase transitions as benchmark examples of cosmological SGWBs. We explicitly model various astrophysical SGWB sources, most importantly from unresolved black hole mergers. Using Markov Chain Monte Carlo, we demonstrated that midband experiments can, when combined with LIGO A+ and LISA, significantly improve sensitivities to cosmological SGWBs and better separate them from astrophysical SGWBs. In particular, we forecast that a midband experiment improves sensitivity to cosmic string tension \\(G\\mu\\) by up to a factor of \\(10\\), driven by improved component separation from astrophysical sources. For phase transitions, a midband experiment can detect signals peaking at \\(0.1 - 1\\) Hz, which for our fiducial model corresponds to early Universe temperatures of \\(T_*\\sim 10^4 - 10^6\\) GeV, generally beyond the reach of LIGO and LISA. The midband closes an energy gap and better captures characteristic spectral shape information. It thus substantially improves measurement of the properties of phase transitions at lower energies of \\(T_* \\sim O(10^3)\\) GeV, potentially relevant to new physics at the electroweak scale, whereas in this energy range LISA alone will detect an excess but not effectively measure the phase transition parameters. Our modelling code and chains are publicly available.

We demonstrate that enhanced early galaxy formation can generically arise in axion-like particle (ALP) dark matter (DM) models with a delayed onset of axion field oscillation. In these models, the formation of localized massive objects enhances structure formation, potentially addressing the excess recently observed by the James Webb Space Telescope (JWST), while remaining consistent with existing constraints. We identify viable parameter space with the ALP mass in the range of \\(10^{-22}~{\\rm eV}

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