66 lines
6.8 KiB
Typst
66 lines
6.8 KiB
Typst
#import "helpers.typ": *
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= Introduction
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The earliest cosmological events (such as the formation of the first astrophysical objects - stars, galaxies, black holes...) have a profound influence on the evolution of the universe. Though poorly understood, these events have shaped the characteristics of our current universe, including the structure and distribution of matter itself.
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// Citation about an overview paper on ionization vs structure formation.
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Despite the milestones achieved in observational cosmology, many aspects of the early universe and its dark ages remain difficult to probe. While traditional measurements provide insights into relatively recent epochs, and the cosmic microwave background (CMB) serves as an early snapshot of the universe, the dark ages are incompatible with direct observations. They represent a critical link between the late-time universe and the primordial conditions that has remained largely unexplored.
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The epoch of reionization (EOR) spans the time period from the end of the dark ages until the time when the universe is fully ionized again. It is a period of complex interactions between matter and radiation but it is crucial to understand as it sets the stage for the subsequent evolution of the universe.
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// including the formation of galaxies and large-scale structures.
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// It simultaneously is affected by the fundamental mechanisms and also affects the subsequent evolution of the universe.
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Beyond its impact on the late universe, a detailed understanding of the reionization process has been shown to provide new and competitive constraints on the current cosmological model (e.g
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@Mao_2008
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@McQuinn_2006
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@schneider2023cosmologicalforecast21cmpower
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).
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Understanding and being able to model the EOR is therefore crucial for a comprehensive picture of cosmology.
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The dark ages of the universe refer to the period after recombination where the primordial atoms remained neutral. They are characterized by the total lack of sources of radiation (beyond the radiation background). The dominant interactions during that period were either gravitational or due to the cooling of the primordial gas. The formation of the first stars was obstructed by the lack of efficient cooling mechanisms in the absence of heavier nuclei. With the simplest cooling channel being the deexcitation of atomic hydrogen, the gas inside a virialized structure can only collapse if the enclosed mass is high enough. This so called atomic cooling limit sets a minimum mass for the halos that can host star formation at around $10^8 M_(dot.circle)$. Other cooling channels such as the deexcitation of molecular hydrogen were suppressed by the emission of photons from the first stars.
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// => argument that there is no "galaxy" in that sense below
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The first stars mark the end of the dark ages. These so called population III stars were metal-free and their short lifespan ended in supernovae that enriched and heated the surrounding gas in the intergalactic medium (IGM).
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// Citation about Pop III stars and their role in the cosmic dawn.
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which...
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During the cosmic dawn ...
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The large amounts of neutral hydrogen in the intergalactic medium during the dark ages and cosmic dawn allow for an additional mode of observation: the 21-cm line emission. Due to the hyperfine transition of neutral hydrogen there is a characteristic emission or absorption of photons at a frequency of $1420 "MHz"$. The strength of this signal depends on the local conditions, in particular the redshifting of the photons allows to probe different epochs through the observed frequency.
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The main purpose of simulations is to constrain EOR observables, in particular the 21-cm signal.
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// Keep the below?
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Combined with upcoming observations from ... these simulations will generate a wealth of information about the early universe, at a range of redshifts that has previously been inaccessible. With the highest sensitivity and resolution forecasted for these observations, the simulations must be able to capture the full dynamic range of the interactions, from the small-scale physics of star formation and feedback to the large-scale structure of the universe.
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State of the art simulations need to implement a range of physical processes, including gravitational interactions, hydrodynamics, radiative transfer, and feedback mechanisms. Prominent examples include the THESAN simulations
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#cite(<Kannan_2021>, form: "normal")
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#cite(<Garaldi_2022>, form: "normal")
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#cite(<Smith_2022>, form: "normal")
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and ... .
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Another approach is to use ray-tracing algorithms which give detailed descriptions of the radiative transfer.
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// C2ray?
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These methods are computationally expensive, which limits their applicability for large-scale simulations.
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// Shortcomings of similar codes (as noted in #beorn paper). => justification for the development of #beorn (@Schaeffer_2023).
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This work presents #beorn, the _Bubbles during the Epoch of Reionization Numerical simulator_ by @Schaeffer_2023, and the refinements we make to achieve self-consistency.
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// not clear!
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In its simplest description #beorn is the implementation of the "halo model of reionization" by @schneider2023cosmologicalforecast21cmpower. In this model the radiative interactions are treated as spherically symmetric around a halo-scale source. This effectively reduces the dimensionality of the radiative transfer problem. #beorn uses the one-dimensional (1-d) profiles generated by this model to paint the 3-d space around sources which are obtained from a large-scale #nbody simulation. A distinguishing feature of #beorn is the self-consistent treatment of the growth of individual sources over the course of the simulation. The first iteration of #beorn focused on the effect of emitted photons whereas this work focuses on the effects of gravitational mass accretion. We show that the radiation profiles are sensitive to the growth rate of the sources and that an accurate treatment of the source growth has an impact on the resulting 21-cm signal.
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// Mention that this is treated in more detail in @procedure
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This report is structured as follows: @procedure describes the details of the simulation procedure, including the underlying model. @halo_mass_history explains how mass evolution is modeled and its impact on the profiles.
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// not any profiles.
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In @implementation we give an overview of the implementation of the modeling assumed by #beorn and the steps required to produce a full 3-d lightcone simulation.
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// self-consistent treatment of mass accretion.
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@validation details the validation we perform on the refined procedure and in @results we compare the resulting signals to quantify the impact of different models of mass accretion. @conclusion summarizes our findings and discusses potential future improvements.
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Other points to mention
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- wouthuysen
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- cold reionization
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- comoving distances - check consistency
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how #beorn compares to traditional approaches
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