arXiv subjects

Arman Shafieloo

Publications and source records attributed to Arman Shafieloo.

At least 73 records · Page 4Linked to original sources

arXiv2022

How to use GP: Effects of the mean function and hyperparameter selection on Gaussian Process regression

Gaussian processes have been widely used in cosmology to reconstruct cosmological quantities in a model-independent way. However, the validity of the adopted mean function and hyperparameters, and the dependence of the results on the choice have not been well explored. In this paper, we study the effects of the underlying mean function and the hyperparameter selection on the reconstruction of the distance moduli from type Ia supernovae. We show that the choice of an arbitrary mean function affects the reconstruction: a zero mean function leads to unphysical distance moduli and the best-fit LCDM to biased reconstructions. We propose to marginalize over a family of mean functions and over the hyperparameters to effectively remove their impact on the reconstructions. We further explore the validity and consistency of the results considering different kernel functions and show that our method is unbiased.

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arXiv2022

A novel approach for calculating galaxy rotation curves using spaxel cross-correlation and iterative smoothing

Precise measurements of the internal dynamics of galaxies have proven of great importance for understanding the internal dark matter distribution of galaxies. We present a novel method for measuring the line-of-sight (LOS) velocities across the face of galaxies by cross-correlation of spectral pixels (spaxels) and an iterative method of smoothing. On simulated data the method can accurately recover the input LOS velocities for different types of spectra (absorption line dominated, emission line dominated, and differing shapes of the continuum), and can handle stellar population radial gradients. Most important of all, it continues to provide reliable measurements of LOS velocities with reasonable uncertainties even when the spectra are very low signal-to-noise (approaching $\sim 1$), which is a challenge for traditional template-fitting approaches. We apply our method to data from a real MaNGA galaxy as a demonstration and find promising results with good precision. This novel approach can be complementary to existing methods primarily based on template fitting.

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arXiv2022

Identifying lensed quasars and measuring their time-delays from unresolved light curves

Identifying multiply imaged quasars is challenging due to their low density in the sky and the limited angular resolution of wide field surveys. We show that multiply imaged quasars can be identified using unresolved light curves, without assuming a light curve template or any prior information. After describing our method, we show using simulations that it can attain high precision and recall when we consider high-quality data with negligible noise well below the variability of the light curves. As the noise level increases to that of the Zwicky Transient Facility (ZTF) telescope, we find that precision can remain close to $100\%$ while recall drops to $\sim 60\%$. We also consider some examples from the Time Delay Challenge 1 (TDC1) and demonstrate that the time delays can be accurately recovered from the joint light curve data in realistic observational scenarios. We further demonstrate our method by applying it to publicly available COSMOGRAIL data of the observed lensed quasar SDSS J1226-0006. We identify the system as a lensed quasar based on the unresolved light curve and estimate a time delay in good agreement with the one measured by COSMOGRAIL using the individual image light curves. The technique shows great potential to identify lensed quasars in wide field imaging surveys, especially the soon to be commissioned Vera Rubin Observatory.

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arXiv2022

Out of One, Many: Distinguishing Time Delays from Lensed Supernovae

Gravitationally lensed Type Ia supernovae are an emerging probe with great potential for constraining dark energy, spatial curvature, and the Hubble constant. The multiple images and their time delayed and magnified fluxes may be unresolved, however, blended into a single lightcurve. We demonstrate methods without a fixed source template matching for extracting the individual images, determining whether there are one (no lensing) or two or four (lensed) images, and measuring the time delays between them that are valuable cosmological probes. We find 100% success for determining the number of images for time delays greater than $\sim10$ days.

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arXiv2022

One spectrum to cure them all: Signature from early Universe solves major anomalies and tensions in cosmology

Acoustic peaks in the Cosmic Microwave Background (CMB) temperature spectrum as observed by the Planck satellite appear to be smoother than our expectation from the standard model lensing effect. This anomalous effect can be also mimicked by a spatially closed Universe with a very low value of Hubble constant that consequently aggravates the already existing discordance between cosmological observations. We reconstruct a signature from the early Universe, a particular form of oscillation in the primordial spectrum of quantum fluctuations with a characteristic frequency, that solves all these anomalies. Interestingly, we find this form of the primordial spectrum resolves or substantially subsides, various tensions in the standard model of cosmology in fitting different observations, namely Planck CMB, clustering and weak lensing shear measurements from several large scale structure surveys, local measurements of Hubble constant, and recently estimated age of the Universe from globular clusters. We support our findings phenomenologically, by proposing an analytical form of the primordial spectrum with similar features and demonstrate that it agrees remarkably well with various combinations of cosmological observations. We support further our findings theoretically, by introducing a single scalar field potential for inflation that can generate such a form of the primordial spectrum.

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arXiv2021

Inflation Story: slow-roll and beyond

We present constraints on inflationary dynamics and features in the primordial power spectrum of scalar perturbations using the Cosmic Microwave Background temperature, polarization data from Planck 2018 data release and updated likelihoods. We constrain the slow-roll dynamics using Hilltop Quartic Potential and Starobinsky $R+R^2$ model in the Einstein frame using the Planck 2018 binned Plik likelihood. Using the Hilltop as base potential we construct Whipped Inflation potential to introduce suppression in the scalar power spectrum at large angular scales. We notice marginal (68% C.L.) preference of suppression from the large scale temperature angular power spectrum. However, large-scale E-mode likelihood, based on high frequency instrument cross spectrum, does not support this suppression and in the combined data the preference towards the suppression becomes negligible. Based on the Hilltop and Starobinsky model we construct the Wiggly Whipped Inflation potentials to introduce oscillatory features along with the suppression. We use unbinned data from the recently released CamSpec v12.5 likelihood which updates Planck 2018 results. We compare the Bayesian evidences of the feature models with their baseline slow-roll potentials. We find that the complete slow-roll baseline potential is moderately preferred against potentials which generate features. Compared to Planck 2015 PlikHM bin1 likelihood, we find that the significance of sharp features has decreased owing to the updates in the data analysis pipeline. We also compute the bispectra for the best fit candidates obtained from our analysis.

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arXiv2021

Phantom braneworld and the Hubble tension

Braneworld models with induced gravity exhibit phantom-like behaviour of the effective equation of state of dark energy. They can, therefore, naturally accommodate higher values of $H_0$, preferred by recent local measurements, while satisfying the CMB constraints. We test the background evolution in such phantom braneworld scenarios with the current observational datasets. We find that the phantom braneworld prefers a higher value of $H_0$ even without the R19 prior, thereby providing a much better fit to the local measurements. Although this braneworld model cannot fully satisfy all combinations of cosmological observables, among existing dark energy candidates the phantom brane provides one of the most compelling explanations of cosmic evolution.

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arXiv2021

Testing the Mutual Consistency of the Pantheon and SDSS/eBOSS BAO Data Sets with Gaussian Processes

We test the mutual consistency between the baryon acoustic oscillation measurements from the eBOSS SDSS final release, as well as the Pantheon supernova compilation in a model independent fashion using Gaussian process regression. We also test their joint consistency with the $Λ$CDM model, also in a model independent fashion. We also use Gaussian process regression to reconstruct the expansion history that is preferred by these two datasets. While this methodology finds no significant preference for model flexibility beyond $Λ$CDM, we are able to generate a number of reconstructed expansion histories that fit the data better than the best-fit $Λ$CDM model. These example expansion histories may point the way towards modifications to $Λ$CDM. We also constrain the parameters $Ω_k$ and $H_0r_d$ both with $Λ$CDM and with Gaussian process regression. We find that $H_0r_d =10030 \pm 130$ km/s and $Ω_k = 0.05 \pm 0.10$ for $Λ$CDM and that $H_0r_d = 10040 \pm 140$ km/s and $Ω_k = 0.02 \pm 0.20$ for the Gaussian process case.

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arXiv2021

Bayesian vs Frequentist: Comparing Bayesian model selection with a frequentist approach using the iterative smoothing method

We have developed a frequentist approach for model selection which determines the consistency between any cosmological model and the data using the distribution of likelihoods from the iterative smoothing method. Using this approach, we have shown how confidently we can conclude whether the data support any given model without comparison to a different one. In this current work, we compare our approach with the conventional Bayesian approach based on the estimation of the Bayesian evidence using nested sampling. We use simulated future Roman (formerly WFIRST)-like type Ia supernovae data in our analysis. We discuss the limits of the Bayesian approach for model selection and show how our proposed frequentist approach can perform better in the falsification of individual models. Namely, if the true model is among the candidates being tested in the Bayesian approach, that approach can select the correct model. If all of the options are false, then the Bayesian approach will select merely the least incorrect one. Our approach is designed for such a case and we can conclude that all of the models are false.

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arXiv2021

Generalized Emergent Dark Energy Model and the Hubble Constant Tension

We investigate a generalized form of the phenomenologically emergent dark energy model, known as generalized emergent dark energy (GEDE), introduced by Li and Shafieloo [Astrophys. J. {\bf 902}, 58 (2020)] in light of a series of cosmological probes and considering the evolution of the model at the level of linear perturbations. This model introduces a free parameter $Δ$ that can discriminate between the $Λ$CDM (corresponds to $Δ=0$) or the phenomenologically emergent dark energy (PEDE) (corresponds to $Δ=1$) models, allowing us to determine which model is preferred most by the fit of the observational datasets. We find evidence in favor of the GEDE model for Planck alone and in combination with R19, while the Bayesian model comparison is inconclusive when Supernovae Type Ia or BAO data are included. In particular, we find that $Λ$CDM model is disfavored at more than $2σ$ CL for most of the observational datasets considered in this work and PEDE is in agreement with Planck 2018+BAO+R19 combination within $1σ$ CL.

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arXiv2021

Hubble diagram at higher redshifts: Model independent calibration of quasars

In this paper, we present a model-independent approach to calibrate the largest quasar sample. Calibrating quasar samples is essentially constraining the parameters of the linear relation between the $\log$ of the ultraviolet (UV) and X-ray luminosities. This calibration allows quasars to be used as standardized candles. There is a strong correlation between the parameters characterizing the quasar luminosity relation and the cosmological distances inferred from using quasars as standardized candles. We break this degeneracy by using Gaussian process regression to model-independently reconstruct the expansion history of the Universe from the latest type Ia supernova observations. Using the calibrated quasar dataset, we further reconstruct the expansion history up to redshift of $z\sim 7.5$. Finally, we test the consistency between the calibrated quasar sample and the standard $\rmΛCDM$ model based on the posterior probability distribution of the GP hyperparameters. Our results show that the quasar sample is in good agreement with the standard $\rmΛCDM$ model in the redshift range of the supernova, despite of mildly significant deviations taking place at higher redshifts. Fitting the standard $\rmΛCDM$ model to the calibrated quasar sample, we obtain a high value of the matter density parameter $Ω_m = 0.382^{+0.045}_{-0.042}$, which is marginally consistent with the constraints from other cosmological observations.

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arXiv2021

The Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: a multi-tracer analysis in Fourier space for measuring the cosmic structure growth and expansion rate

We perform a joint BAO and RSD analysis using the eBOSS DR16 LRG and ELG samples in the redshift range of $z\in[0.6,1.1]$, and detect a RSD signal from the cross power spectrum at a $\sim4σ$ confidence level, i.e., $fσ_8=0.317\pm0.080$ at $z_{\rm eff}=0.77$. Based on the chained power spectrum, which is a new development in this work to mitigate the angular systematics, we measurement the BAO distances and growth rate simultaneously at two effective redshifts, namely, $D_{\rm M}/r_{\rm d} \ (z=0.70)=17.96\pm0.51, \ D_{\rm H}/r_{\rm d} \ (z=0.70)=21.22\pm1.20, \ fσ_8 \ (z=0.70) =0.43\pm0.05$, and $D_{\rm M}/r_{\rm d} \ (z=0.845)=18.90\pm0.78, \ D_{\rm H}/r_{\rm d} \ (z=0.845)=20.91\pm2.86, \ fσ_8 \ (z=0.845) =0.30\pm0.08$. Combined with BAO measurements including those from the eBOSS DR16 QSO and Lyman-$α$ sample, our measurement has raised the significance level of a nonzero $Ω_{\rm Λ}$ to $\sim11σ$.

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arXiv2020

Dark Energy as a Critical Phenomenon: a Resolution for Hubble Tension

We propose a dark energy model based on the physics of critical phenomena which is consistent with both the Planck's CMB and the Riess et al.'s local Hubble measurements. In this model the dark energy density behaves like the magnetization of the Ising model. This means the dark energy is an emergent phenomenon and we named it critically emergent dark energy model, CEDE. In CEDE, dark energy emerges at a transition redshift, $z_c$, corresponding to the critical temperature in critical phenomena. Combining the Planck CMB data and local measurement of the Hubble constant from Riess et al. (2019) we find statistically significant support for this transition with respect to the case of very early transition that represents effectively the cosmological constant. This is understandable since CEDE model naturally prefers larger values of Hubble constant consistent with local measurements. Since CEDE prefers a non-trivial transition when we consider both high redshift Planck CMB data and local Hubble constant measurements, we conclude that $H_0$ tension can be a hint for the substructure of the dark energy as a well-studied properties of critical phenomena.

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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.

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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.

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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.

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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.

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arXiv2020

Be It Unresolved: Measuring Time Delays from Lensed Supernovae

Gravitationally lensed Type Ia supernovae may be the next frontier in cosmic probes, able to deliver independent constraints on dark energy, spatial curvature, and the Hubble constant. Measurements of time delays between the multiple images become more incisive due to the standardized candle nature of the source, monitoring for months rather than years, and partial immunity to microlensing. While currently extremely rare, hundreds of such systems should be detected by upcoming time domain surveys. Even more will have the images spatially unresolved, with the observed lightcurve a superposition of time delayed image fluxes. We investigate whether unresolved images can be recognized as lensed sources given only lightcurve information, and whether time delays can be extracted robustly. We develop a method that successfully identifies such systems, with a false positive rate of $\lesssim 5\%$, and measures the time delays with a completeness of $\gtrsim 93\%$ and with a bias of $\lesssim 0.5\%$ for $Δt_{\rm fit} \gtrsim 10$ days.

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