arXiv subjects

Benjamin D. Wandelt

Publications and source records attributed to Benjamin D. Wandelt.

At least 73 records · Page 4Linked to original sources

arXiv2021

The trouble beyond $H_0$ and the new cosmic triangles

The distance ladder using supernovae yields higher values of the Hubble constant $H_0$ than those inferred from measurements of the cosmic microwave background (CMB) and galaxy surveys, a discrepancy that has come to be known as the `Hubble tension'. This has motivated the exploration of extensions to the standard cosmological model in which higher values of $H_0$ can be obtained from CMB measurements and galaxy surveys. The trouble, however, goes beyond $H_0$; such modifications affect other quantities, too. In particular, their effects on cosmic times are usually neglected. We explore here the implications that measurements of the age $t_{\rm U}$ of the Universe, such as a recent inference from the age of the oldest globular clusters, can have for potential solutions to the $H_0$ tension. The value of $H_0$ inferred from the CMB and galaxy surveys is related to the sound horizon at CMB decoupling (or at radiation drag), but it is also related to the matter density and to $t_{\rm U}$. Given this observation, we show how model-independent measurements may support or disfavor proposed new-physics solutions to the Hubble tension. Finally, we argue that cosmological measurements today provide constraints that, within a given cosmological model, represent an over-constrained system, offering a powerful diagnostic tool of consistency. We propose the use of ternary plots to simultaneously visualize independent constraints on key quantities related to $H_0$ like $t_{\rm U}$, the sound horizon at radiation drag, and the matter density parameter. We envision that this representation will help find a solution to the trouble of and beyond $H_0$.

astro-ph.CO↗

arXiv2021

Accurate precision Cosmology with redshift unknown gravitational wave sources

Gravitational waves can provide an accurate measurement of the luminosity distance to the source, but cannot provide the source redshift unless the degeneracy between mass and redshift can be broken. This makes it essential to infer the redshift of the source independently to measure the expansion history of the Universe. We show that by exploiting the clustering scale of the gravitational wave sources with galaxies of known redshift, we can infer the expansion history from redshift unknown gravitational wave sources. By using gravitational wave sources of unknown redshift that are detectable from the network of gravitational wave detectors with Advanced LIGO design sensitivity, we will be able to obtain accurate and precise measurements of the local Hubble constant, the expansion history of the universe, and the gravitational wave bias parameter, which captures the distribution of gravitational wave sources with respect to the redshift tracer distribution. While we showcase its application to low redshift gravitational waves, this technique will be applicable also to the high redshift gravitational wave sources detectable from Laser Interferometer Space Antenna (LISA), Cosmic Explorer (CE), and Einstein Telescope (ET). Moreover, this method will also be applicable to samples of supernovae and fast radio bursts with unknown or photometric redshifts.

astro-ph.CO↗

arXiv2021

SSSpaNG! Stellar Spectra as Sparse, data-driven, Non-Gaussian processes

Upcoming million-star spectroscopic surveys have the potential to revolutionize our view of the formation and chemical evolution of the Milky Way. Realizing this potential requires automated approaches to optimize estimates of stellar properties, such as chemical element abundances, from the spectra. The volume and quality of the observations strongly motivate that these approaches should be data-driven. With this in mind, we introduce SSSpaNG: a data-driven non-Gaussian Process model of stellar spectra. We demonstrate the capabilities of SSSpaNG using a sample of APOGEE red clump stars, whose model parameters we infer via Gibbs sampling. Pooling information between stars to infer their covariance, we permit clear identification of the correlations between spectral pixels. Harnessing these correlations, we infer the true spectrum of each star, inpainting missing regions and denoising by a factor of at least 2 for stars with signal-to-noise of ~20. As we marginalize over the covariance matrix of the spectra, the effective prior on these true spectra is non-Gaussian and sparsifying, favouring typically small but occasionally large excursions from the mean. The high-fidelity inferred spectra produced will enable improved elemental abundance measurements for individual stars. Our model also allows us to quantify the information gained by observing portions of a star's spectrum, and thereby define the most mutually informative spectral regions. Using 25 windows centred on elemental absorption lines, we demonstrate that the iron-peak and alpha-process elements are particularly mutually informative for these spectra, and that the majority of information about a target window is contained in the 10-or-so most informative windows. Such mutual-information estimates have the potential to inform models of nucleosynthetic yields and the design of future observations.

astro-ph.SR↗

arXiv2021

Testing the general theory of relativity using gravitational wave propagation from dark standard sirens

Alternative theories of gravity predict modifications in the propagation of gravitational waves (GW) through space-time. One of the smoking-gun predictions of such theories is the change in the GW luminosity distance to GW sources as a function of redshift relative to the electromagnetic (EM) luminosity distance expected from EM probes. We propose a multi-messenger test of the theory of general relativity from the propagation of gravitational waves by combining EM and GW observations to resolve these issues from GW sources without EM counterparts (which are also referred to as dark standard sirens). By using the relation between the geometric distances accessible from baryon acoustic oscillation measurements, and luminosity distance measurements from the GW sources, we can measure any deviation from the general theory of relativity via the GW sources of unknown redshift that will be detectable by networks of GW detectors such as LIGO, Virgo, and KAGRA. Using this technique, the fiducial value of the frictional term can be measured to a precision $Ξ_0=0.98^{+0.04}_{-0.23}$ after marginalizing over redshift dependence, cosmological parameters, and GW bias parameters with $\sim 3500$ dark standard sirens of masses $30\,\rm M_\odot$ each distributed up to redshift $z=0.5$. For a fixed redshift dependence, a value of $Ξ_0=0.99^{+0.02}_{-0.02}$ can be measured with a similar number of dark sirens. Application of our methodology to the far more numerous dark standard sirens detectable with next generation GW detectors, such as LISA, Einstein Telescope and Cosmic Explorer, will allow achievement of higher accuracy than possible from use of bright standard sirens.

astro-ph.CO↗

arXiv2020

Velocity correction for Hubble constant measurements from standard sirens

Gravitational wave (GW) sources are an excellent probe of the luminosity distance and offer a novel measure of the Hubble constant, $H_0$. This estimation of $H_0$ from standard sirens requires an accurate estimation of the cosmological redshift of the host galaxy of the GW source, after correcting for its peculiar velocity. Absence of an accurate peculiar velocity correction affects both the precision and accuracy of the measurement of $H_0$, particularly for nearby sources. We propose a framework to incorporate such a peculiar velocity correction for GW sources. A first implementation of our method to the event GW170817 combined with the Very Large Baseline Interferometry (VLBI) observation leads to a revised value of $H_0= 68.3^{+ 4.6}_{-4.5}$ km/s/Mpc. While this revision is minor, it demonstrates that our method makes it possible for obtaining an unbiased and accurate measurements of $H_0$ at the precision required for the standard siren cosmology.

astro-ph.CO↗

arXiv2020

Precision cosmology with voids in the final BOSS data

We report novel cosmological constraints obtained from cosmic voids in the final BOSS DR12 dataset. They arise from the joint analysis of geometric and dynamic distortions of average void shapes (i.e., the stacked void-galaxy cross-correlation function) in redshift space. Our model uses tomographic deprojection to infer real-space void profiles and self-consistently accounts for the Alcock-Paczynski (AP) effect and redshift-space distortions (RSD) without any prior assumptions on cosmology or structure formation. It is derived from first physical principles and provides an extremely good description of the data at linear perturbation order. We validate this model with the help of mock catalogs and apply it to the final BOSS data to constrain the RSD and AP parameters $f/b$ and $D_AH/c$, where $f$ is the linear growth rate, $b$ the linear galaxy bias, $D_A$ the comoving angular diameter distance, $H$ the Hubble rate, and $c$ the speed of light. In addition, we include two nuisance parameters in our analysis to marginalize over potential systematics. We obtain $f/b=0.540\pm0.091$ and $D_AH/c=0.588\pm0.004$ from the full void sample at a mean redshift of $z=0.51$. In a flat $Λ$CDM cosmology, this implies $Ω_\mathrm{m}=0.312\pm0.020$ for the present-day matter density parameter. When we use additional information from the survey mocks to calibrate our model, these constraints improve to $f/b=0.347\pm0.023$, $D_AH/c=0.588\pm0.003$, and $Ω_\mathrm{m}=0.310\pm0.017$. However, we emphasize that the calibration depends on the specific model of cosmology and structure formation assumed in the mocks, so the calibrated results should be considered less robust. Nevertheless, our calibration-independent constraints are among the tightest of their kind to date, demonstrating the immense potential of using cosmic voids for cosmology in current and future data.

astro-ph.CO↗

arXiv2020

Solving high-dimensional parameter inference: marginal posterior densities & Moment Networks

High-dimensional probability density estimation for inference suffers from the "curse of dimensionality". For many physical inference problems, the full posterior distribution is unwieldy and seldom used in practice. Instead, we propose direct estimation of lower-dimensional marginal distributions, bypassing high-dimensional density estimation or high-dimensional Markov chain Monte Carlo (MCMC) sampling. By evaluating the two-dimensional marginal posteriors we can unveil the full-dimensional parameter covariance structure. We additionally propose constructing a simple hierarchy of fast neural regression models, called Moment Networks, that compute increasing moments of any desired lower-dimensional marginal posterior density; these reproduce exact results from analytic posteriors and those obtained from Masked Autoregressive Flows. We demonstrate marginal posterior density estimation using high-dimensional LIGO-like gravitational wave time series and describe applications for problems of fundamental cosmology.

stat.ML↗

arXiv2020

Neural networks as optimal estimators to marginalize over baryonic effects

Many different studies have shown that a wealth of cosmological information resides on small, non-linear scales. Unfortunately, there are two challenges to overcome to utilize that information. First, we do not know the optimal estimator that will allow us to retrieve the maximum information. Second, baryonic effects impact that regime significantly and in a poorly understood manner. Ideally, we would like to use an estimator that extracts the maximum cosmological information while marginalizing over baryonic effects. In this work we show that neural networks can achieve that. We made use of data where the maximum amount of cosmological information is known: power spectra and 2D Gaussian density fields. We also contaminate the data with simplified baryonic effects and train neural networks to predict the value of the cosmological parameters. For this data, we show that neural networks can 1) extract the maximum available cosmological information, 2) marginalize over baryonic effects, and 3) extract cosmological information that is buried in the regime dominated by baryonic physics. We also show that neural networks learn the priors of the data they are trained on. We conclude that a promising strategy to maximize the scientific return of cosmological experiments is to train neural networks on state-of-the-art numerical simulations with different strengths and implementations of baryonic effects.

astro-ph.CO↗

arXiv2020

Inferring the Age of the Universe with Globular Clusters

We present an estimate of the absolute age of 68 galactic globular clusters obtained by exploiting the distribution of stars in the full color-magnitude diagram. In particular, we jointly estimate the absolute age, distance, reddening, metallicity ([Fe/H]) and [$α$/Fe] of each cluster, imposing priors motivated by independent observations; we also estimate possible systematics from stellar modeling. Our derived distances for the globular cluster sample are in agreement with those obtained from GAIA using main-sequence dwarf stars (where available), and the inferred ages are in good agreement with those previously published. The novelty of our approach is that, with the adopted priors, we are able to estimate robustly these parameters from the globular cluster color-magnitude diagram. We find that the average age of the oldest globular clusters is $t_{\rm GC}=13.32 \pm 0.1 {\rm (stat.)} \pm 0.5 {\rm (sys.)}$, at 68\% confidence level, including systematic uncertainties from stellar modeling. These measurements can be used to infer the age of the Universe, largely independently of the cosmological parameters: we find an age of the Universe $t_{\rm U}=13.5^{+0.16}_{-0.14} {\rm (stat.)} \pm 0.5 ({\rm sys.})$ at 68\% confidence level, accounting for the formation time of globular clusters and its uncertainty. This value is compatible with $13.8 \pm 0.02$ Gyr, the cosmological model-dependent value inferred by the Planck mission assuming the $Λ$CDM model.

astro-ph.CO↗

arXiv2020

Cosmology Intertwined III: $f σ_8$ and $S_8$

The standard $Λ$ Cold Dark Matter cosmological model provides a wonderful fit to current cosmological data, but a few tensions and anomalies became statistically significant with the latest data analyses. While these anomalies could be due to the presence of systematic errors in the experiments, they could also indicate the need for new physics beyond the standard model. In this Letter of Interest we focus on the tension of the Planck data with weak lensing measurements and redshift surveys, about the value of the matter energy density $Ω_m$, and the amplitude or rate of the growth of structure ($σ_8,fσ_8$). We list a few interesting models for solving this tension, and we discuss the importance of trying to fit with a single model a full array of data and not just one parameter at a time.

astro-ph.CO↗

arXiv2020

Cosmology Intertwined II: The Hubble Constant Tension

The current cosmological probes have provided a fantastic confirmation of the standard $Λ$ Cold Dark Matter cosmological model, that has been constrained with unprecedented accuracy. However, with the increase of the experimental sensitivity a few statistically significant tensions between different independent cosmological datasets emerged. While these tensions can be in portion the result of systematic errors, the persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the need for new physics. In this Letter of Interest we will focus on the $4.4σ$ tension between the Planck estimate of the Hubble constant $H_0$ and the SH0ES collaboration measurements. After showing the $H_0$ evaluations made from different teams using different methods and geometric calibrations, we will list a few interesting new physics models that could solve this tension and discuss how the next decade experiments will be crucial.

astro-ph.CO↗

arXiv2020

Cosmology Intertwined I: Perspectives for the Next Decade

The standard $Λ$ Cold Dark Matter cosmological model provides an amazing description of a wide range of astrophysical and astronomical data. However, there are a few big open questions, that make the standard model look like a first-order approximation to a more realistic scenario that still needs to be fully understood. In this Letter of Interest we will list a few important goals that need to be addressed in the next decade, also taking into account the current discordances present between the different cosmological probes, as the Hubble constant $H_0$ value, the $σ_8 - S_8$ tension, and the anomalies present in the Planck results. Finally, we will give an overview of upgraded experiments and next-generation space-missions and facilities on Earth, that will be of crucial importance to address all these questions.

astro-ph.CO↗

arXiv2020

Cosmology Intertwined IV: The Age of the Universe and its Curvature

A precise measurement of the curvature of the Universe is of primeval importance for cosmology since it could not only confirm the paradigm of primordial inflation but also help in discriminating between different early Universe scenarios. The recent observations, while broadly consistent with a spatially flat standard $Λ$ Cold Dark Matter ($Λ$CDM) model, are showing tensions that still allow (and, in some cases, even suggest) a few percent deviations from a flat universe. In particular, the Planck Cosmic Microwave Background power spectra, assuming the nominal likelihood, prefer a closed universe at more than 99\% confidence level. While new physics could be in action, this anomaly may be the result of an unresolved systematic error or just a statistical fluctuation. However, since a positive curvature allows a larger age of the Universe, an accurate determination of the age of the oldest objects provides a smoking gun in confirming or falsifying the current flat $Λ$CDM model.

astro-ph.CO↗

arXiv2020

First measurement of the Hubble parameter from bright binary black hole GW190521

The Zwicky Transient Facility (ZTF) reported the event "ZTF19abanrhr" as a candidate electromagnetic (EM) counterpart at a redshift $z=0.438$ to the gravitational wave (GW) emission from the binary black hole merger GW190521. Assuming that ZTF19abanrhr is the {\it bona fide} EM counterpart to GW190521, and using the GW luminosity distance estimate from three different waveforms NRSur7dq4, SEOBNRv4PHM, and IMRPhenomPv3HM, we report a measurement of the Hubble constant $H_0= 50.4_{-19.5}^{+28.1}$ km/s/Mpc, $ 62.2_{-19.7}^{+29.5}$ km/s/Mpc, and $ 43.1_{-11.4}^{+24.6}$ km/s/Mpc (median along with $68\%$ credible interval) respectively after marginalizing over matter density $Ω_m$ (or dark energy equation of state $w_0$) assuming the flat LCDM (or wCDM) model. Combining our results with the binary neutron star event GW170817 with its redshift measurement alone, as well as with its inclination angle inferred from Very Large Baseline Interferometry (VLBI), we find $H_0= 67.6_{-4.2}^{+4.3}$ km/s/Mpc, $Ω_m= 0.47_{-0.27}^{+0.34}$, and $w_0= -1.17_{-0.57}^{+0.68}$ (median along with $68\%$ credible interval) providing the most stringent measurement on $H_0$ and the first estimation on $Ω_m$ and $w_0$ from bright standard siren. In the future, $1.3\%$ measurement of $H_0=68$ km/s/Mpc and $28\%$ measurement of $w_0=-1$ is possible from about $200$ GW190521-like sources.

astro-ph.CO↗

arXiv2020

Multi-messenger tests of gravity with weakly lensed gravitational waves

General relativity (GR) predicts concordant trajectories for photons and gravitational waves (GW). We propose a new multi-messenger avenue (GW-CMB-CMB) to prove this aspect of fundamental physics by cross-correlating the GW signal of astrophysical origin with the lensing field derived from the cosmic microwave background (CMB). This new window will allow robust measurement of the prediction from GR with high signal-to-noise and will be able to unveil the true nature of gravity using the GW sources detected by missions such as the Laser Interferometer Space Antenna (LISA), Einstein Telescope and Cosmic Explorer.

astro-ph.CO↗

arXiv2020

Super-resolution emulator of cosmological simulations using deep physical models

We present an extension of our recently developed Wasserstein optimized model to emulate accurate high-resolution features from computationally cheaper low-resolution cosmological simulations. Our deep physical modelling technique relies on restricted neural networks to perform a mapping of the distribution of the low-resolution cosmic density field to the space of the high-resolution small-scale structures. We constrain our network using a single triplet of high-resolution initial conditions and the corresponding low- and high-resolution evolved dark matter simulations from the Quijote suite of simulations. We exploit the information content of the high-resolution initial conditions as a well constructed prior distribution from which the network emulates the small-scale structures. Once fitted, our physical model yields emulated high-resolution simulations at low computational cost, while also providing some insights about how the large-scale modes affect the small-scale structure in real space.

astro-ph.CO↗

arXiv2020

Probing the theory of gravity with gravitational lensing of gravitational waves and galaxy surveys

The cross-correlation of gravitational wave strain with upcoming galaxy surveys probe theories of gravity in a new way. This method enables testing the theory of gravity by combining the effects from both gravitational lensing of gravitational waves and the propagation of gravitational waves in spacetime. We find that within 10 years, the combination of the Advanced-LIGO and VIRGO detector networks with planned galaxy surveys should detect weak gravitational lensing of gravitational waves in the low redshift Universe ($z<0.5$). With the next generation gravitational wave experiments such as Voyager, LISA, Cosmic-Explorer and Einstein Telescope, we can extend this test of the theory of gravity to larger redshifts by exploiting the synergies between electromagnetic wave and gravitational wave probes.

astro-ph.CO↗

arXiv2020

How to measure CMB spectral distortions with an imaging telescope

We propose the implementation of an imaging telescope in combination with an inter-frequency calibrator to measure the spectral shape of the microwave sky by exploiting the differences in the sky intensity between multiple pairs of frequency channels. By jointly sampling the cosmological and foreground parameters in a Bayesian framework for $600$ instrument configurations, we determine the minimum calibration accuracy required in order to obtain measurements of spectral distortions and simultaneously measure spectral and spatial fluctuations of the CMB. We demonstrate the feasibility of this technique for a CMB mission like PICO (Probe of Inflation and Cosmic Origins), and show that a $10$-$σ$ measurement of the $y$-distortion along with a two orders of magnitude improvement on the FIRAS (Far-Infrared Absolute Spectrophotometer) $μ$-distortion limit is feasible from this technique. We argue that longer term applications may be envisaged at even higher sensitivity, capable of attaining the $μ$-distortion that provide a robust prediction of the $Λ$CDM model and even primordial recombination lines of hydrogen and helium, in the context of the ESA (European Space Agency) Voyage 2035-2050 program.

astro-ph.CO↗