2005.03664
Cosmic variance of the 21-cm global signal
Muñoz, Cyr-Racine
Cosmological measurements of the 21-cm line of neutral hydrogen are poised to dramatically enhance our understanding of the early universe. In particular, both the epochs of reionization and cosmic dawn remain largely uncharted, and the 21-cm signal is one of the few probes to reach them. The simplest 21-cm measurement is the global signal (GS), which corresponds to the averaged absorption or emission of 21-cm photons across the entire sky. While bright radio foregrounds swamp the cosmic signal over the entire frequency range observable, presenting a formidable hurdle, they can in principle be subtracted, given enough sensitivity. Here, however, we point out an additional---and irreducible---source of uncertainty for the 21-cm GS: cosmic variance. The cosmic-variance noise arises from the finite volume of the universe accessible to 21-cm experiments. Due to the cosmological redshifting of 21-cm photons, each observed frequency probes our universe during a particular cosmic age, corresponding to a narrow redshift slice. The presence of large 21-cm fluctuations makes the GS within each slice different than the GS averaged over the entire universe. We estimate the size of this cosmic-variance noise, and find that for a standard scenario it has a size of $\sim 0.1$ mK, which is $\sim 10\%$ of the size of the expected instrumental noise of a year-long experiment. Interestingly, cosmic variance can overtake instrumental noise for scenarios with extreme 21-cm fluctuations, such as those suggested to explain the sharpness of the claimed EDGES detection. Moreover, as large-scale 21-cm fluctuations are coherent over long distances, cosmic variance correlates the measurements of the GS at nearby redshifts, leading to off-diagonal uncertainties that have so far been neglected.
Cosmic variance of the 21-cm global signal
Muñoz, Cyr-Racine
Cosmological measurements of the 21-cm line of neutral hydrogen are poised to dramatically enhance our understanding of the early universe. In particular, both the epochs of reionization and cosmic dawn remain largely uncharted, and the 21-cm signal is one of the few probes to reach them. The simplest 21-cm measurement is the global signal (GS), which corresponds to the averaged absorption or emission of 21-cm photons across the entire sky. While bright radio foregrounds swamp the cosmic signal over the entire frequency range observable, presenting a formidable hurdle, they can in principle be subtracted, given enough sensitivity. Here, however, we point out an additional---and irreducible---source of uncertainty for the 21-cm GS: cosmic variance. The cosmic-variance noise arises from the finite volume of the universe accessible to 21-cm experiments. Due to the cosmological redshifting of 21-cm photons, each observed frequency probes our universe during a particular cosmic age, corresponding to a narrow redshift slice. The presence of large 21-cm fluctuations makes the GS within each slice different than the GS averaged over the entire universe. We estimate the size of this cosmic-variance noise, and find that for a standard scenario it has a size of $\sim 0.1$ mK, which is $\sim 10\%$ of the size of the expected instrumental noise of a year-long experiment. Interestingly, cosmic variance can overtake instrumental noise for scenarios with extreme 21-cm fluctuations, such as those suggested to explain the sharpness of the claimed EDGES detection. Moreover, as large-scale 21-cm fluctuations are coherent over long distances, cosmic variance correlates the measurements of the GS at nearby redshifts, leading to off-diagonal uncertainties that have so far been neglected.
2005.03733
Cataclysmic variables as possible counterparts of ancient Far Eastern guest stars
Hoffmann, Vogt
Continuing our efforts to select possible classical nova candidates among Far Eastern guest stars and to identify them with modern cataclysmic variables (CVs), we present a search for counterparts in 24 promising areas of the sky corresponding to ancient observations between 204 BCE and 1690 CE. These areas have been derived by us in a previous paper. Based on physical entities of the CVs in our areas and reasonable magnitude limits compatible with the distribution of known eruption amplitudes of telescopic classical novae, we present a catalogue of a total of 80 CVs and related targets which could possibly have caused the historical sightings. This list could potentially be reduced by additionally discussing further information more vaguely given in the text. In some cases, we present a detailed discussion of the interpretation of ancient sources confronting them with properties of the brightest CVs in the field. In order to estimate whether this list is representative, we discuss the distribution of CV types in our catalogue of counterparts for the historical events. Compared to the entire sky, the surface density of most CV subtypes in our search fields reveals similar values, except for polars and intermediate polars, i. e. strongly magnetic CVs, for which a significant excess in our search fields was detected. Finally, we give an outlook towards future research in this topic, and add in an Appendix a complete atlas of the celestial maps of all 24 guest star events, displaying the search areas and locations of CVs within them.
2005.04109
Reconstructing the gravitational lensing potential from the Lyman-$\alpha$ Forest
Metcalf, et al
We demonstrate a method for reconstructing the weak lensing potential from the Lyman-$\alpha$ forest data. We derive an optimal estimator for the lensing potential on the sky based on the correlation between pixels in real space. This method effectively deals with irregularly spaced data, holes in the survey, missing data and inhomogeneous noise. We demonstrate an implementation of the method with simulated spectra and weak lensing. It is shown that with a source density of $>\sim 0.5$ per square arcminutes and $\sim 200$ pixels in each spectrum ($\lambda / \Delta\lambda = 1300$) the lensing potential can be reconstructed with high fidelity if the relative absorption in the spectral pixels is signal dominated. When noise dominates the measurement of the absorption in each pixel the noise in the lensing potential is higher, but for reasonable numbers of sources and noise levels and a high fidelity map the lensing potential is obtainable. The lensing estimator could also be applied to lensing of the Cosmic Microwave Background (CMB), 21 cm intensity mapping (IM) or any case in which the correlation function of the source can be accurately estimated.
2005.04207
KiDS+VIKING-450: improved cosmological parameter constraints from redshift calibration with self-orgainising maps
Wright, et al
We present updated cosmological constraints for the KiDS+VIKING-450 cosmic shear dataset (KV450), estimated using redshift distributions and photometric samples defined using self organising maps (SOMs). Our fiducial analysis finds marginal posterior constraints of $S_8\equiv\sigma_8\sqrt{\Omega_{\rm m}/0.3}=0.716^{+0.043}_{-0.038}$; smaller than, but fully consistent with, previous work using this dataset ($|\Delta S_8| = 0.023$). We analyse additional samples and redshift distributions constructed in three ways: excluding certain spectroscopic surveys during redshift calibration, excluding lower-confidence spectroscopic redshifts in redshift calibration, and considering only photometric sources which are jointly calibrated by at least three spectroscopic surveys. In all cases, the method utilised here proves robust: we find a maximal deviation from our fiducial analysis of $|\Delta S_8| \leq 0.009$ for all samples defined and analysed using our SOM. Our largest shift in $S_8$ is found when calibrating redshift distributions without the DEEP2 spectroscopic subset, where we find $S_8=0.707_{-0.042}^{+0.046}$. This difference with respect to the fiducial is both significantly smaller than, and in the opposite direction to, the equivalent shift from previous work. No sample analysed in this work results in a meaningful positive shift in $S_8$ with respect to our fiducial constraints. These results suggest that our improved cosmological parameter estimates are insensitive to pathological misrepresentation of photometric sources by the spectroscopy used for direct redshift calibration, and therefore that this systematic effect cannot be responsible for the observed difference between $S_8$ estimates made with KV450 and Planck CMB probes.
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