arXiv subjects

Yong-Seon Song

Publications and source records attributed to Yong-Seon Song.

At least 55 records · Page 3Linked to original sources

arXiv2009

Reconstructing the history of structure formation using redshift distortions

Measuring the statistics of galaxy peculiar velocities using redshift-space distortions is an excellent way of probing the history of structure formation. Because galaxies are expected to act as test particles within the flow of matter, this method avoids uncertainties due to an unknown galaxy density bias. We show that the parameter combination measured by redshift-space distortions, $fσ_8^{\rm mass}$ provides a good test of dark energy models, even without the knowledge of bias or $σ_8^{\rm mass}$ required to extract $f$ from this measurement (here $f$ is the logarithmic derivative of the linear growth rate, and $σ_8^{\rm mass}$ is the root-mean-square mass fluctuation in spheres with radius $8h^{-1}$Mpc). We argue that redshift-space distortion measurements will help to determine the physics behind the cosmic acceleration, testing whether it is related to dark energy or modified gravity, and will provide an opportunity to test possible dark energy clumping or coupling between dark energy and dark matter. If we can measure galaxy bias in addition, simultaneous measurement of both the overdensity and velocity fields can be used to test the validity of equivalence principle, through the continuity equation.

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arXiv2009

A step towards testing general relativity using weak gravitational lensing and redshift surveys

Using the linear theory of perturbations in General Relativity, we express a set of consistency relations that can be observationally tested with current and future large scale structure surveys. We then outline a stringent model-independent program to test gravity on cosmological scales. We illustrate the feasibility of such a program by jointly using several observables like peculiar velocities, galaxy clustering and weak gravitational lensing. After addressing possible observational or astrophysical caveats like galaxy bias and redshift uncertainties, we forecast in particular how well one can predict the lensing signal from a cosmic shear survey using an over-lapping galaxy survey. We finally discuss the specific physics probed this way and illustrate how $f(R)$ gravity models would fail such a test.

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arXiv2009

The Case for Deep, Wide-Field Cosmology

Much of the science case for the next generation of deep, wide-field optical/infrared surveys has been driven by the further study of dark energy. This is a laudable goal (and the subject of a companion white paper by Zhan et al.). However, one of the most important lessons of the current generation of surveys is that the interesting science questions at the end of the survey are quite different than they were when the surveys were being planned. The current surveys succeeded in this evolving terrain by being very general tools that could be applied to a number of very fundamental measurements. Likewise, the accessibility of the data enabled the broader cosmological and astronomical community to generate more science than the survey collaborations could alone. With that in mind, we should consider some of the basic physical and cosmological questions that surveys like LSST and JDEM-Wide will be able to address.

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arXiv2009

Consistency test of general relativity from large scale structure of the Universe

We construct a consistency test of General Relativity (GR) on cosmological scales. This test enables us to distinguish between the two alternatives to explain the late-time accelerated expansion of the universe, that is, dark energy models based on GR and modified gravity models without dark energy. We derive the consistency relation in GR which is written only in terms of observables - the Hubble parameter, the density perturbations, the peculiar velocities and the lensing potential. The breakdown of this consistency relation implies that the Newton constant which governs large-scale structure is different from that in the background cosmology, which is a typical feature in modified gravity models. We propose a method to perform this test by reconstructing the weak lensing spectrum from measured density perturbations and peculiar velocities. This reconstruction relies on Poisson's equation in GR to convert the density perturbations to the lensing potential. Hence any inconsistency between the reconstructed lensing spectrum and the measured lensing spectrum indicates the failure of GR on cosmological scales. The difficulties in performing this test using actual observations are discussed.

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arXiv2008

Forecasting Cosmological Constraints from Redshift Surveys

Observations of redshift-space distortions in spectroscopic galaxy surveys offer an attractive method for observing the build-up of cosmological structure, which depends both on the expansion rate of the Universe and our theory of gravity. In this paper we present a formalism for forecasting the constraints on the growth of structure which would arise in an idealized survey. This Fisher matrix based formalism can be used to study the power and aid in the design of future surveys.

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arXiv2008

Detectability of a phantom-like braneworld model with the integrated Sachs-Wolfe effect

We study a braneworld model in which a phantom-like behaviour occurs with only cold dark matter and a cosmological constant, due to a large distance modification of gravity. With the addition of curvature, the geometrical tests are not strict enough to rule out models in which gravity is modified significantly on large scales. We show that this degeneracy in the parameter space is broken by the structure formation tests, such as the integrated Sachs-Wolfe effect, which can probe general relativity on large scales.

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arXiv2007

Large Scale Structure Formation of Normal Branch in DGP Brane World Model

In this paper, we study the large scale structure formation of the normal branch in DGP model (Dvail, Gabadadze and Porrati brane world model) by applying the scaling method developed by Sawicki, Song and Hu for solving the coupled perturbed equations of motion of on-brane and off-brane. There is detectable departure of perturbed gravitational potential from LCDM even at the minimal deviation of the effective equation of state w_eff below -1. The modified perturbed gravitational potential weakens the integrated Sachs-Wolfe effect which is strengthened in the self-accelerating branch DGP model. Additionally, we discuss the validity of the scaling solution in the de Sitter limit at late times.

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arXiv2007

Cosmological Constraints on f(R) Acceleration Models

Models which accelerate the expansion of the universe through the addition of a function of the Ricci scalar f(R) leave a characteristic signature in the large-scale structure of the universe at the Compton wavelength scale of the extra scalar degree of freedom. We search for such a signature in current cosmological data sets: the WMAP cosmic microwave background (CMB) power spectrum, SNLS supernovae distance measures, the SDSS luminous red galaxy power spectrum, and galaxy-CMB angular correlations. Due to theoretical uncertainties in the nonlinear evolution of f(R) models, the galaxy power spectrum conservatively yields only weak constraints on the models despite the strong predicted signature in the linear matter power spectrum. Currently the tightest constraints involve the modification to the integrated Sachs-Wolfe effect from growth of gravitational potentials during the acceleration epoch. This effect is manifest for large Compton wavelengths in enhanced low multipole power in the CMB and anti-correlation between the CMB and tracers of the potential. They place a bound on the Compton wavelength of the field be less than of order the Hubble scale.

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arXiv2007

Large-Scale Tests of the DGP Model

The self-accelerating braneworld model (DGP) can be tested from measurements of the expansion history of the universe and the formation of structure. Current constraints on the expansion history from supernova luminosity distances, the CMB, and the Hubble constant exclude the simplest flat DGP model at about 3sigma. The best-fit open DGP model is, however, only a marginally poorer fit to the data than flat LCDM. Its substantially different expansion history raises structure formation challenges for the model. A dark-energy model with the same expansion history would predict a highly significant discrepancy with the baryon oscillation measurement due the high Hubble constant required and a large enhancement of CMB anisotropies at the lowest multipoles due to the ISW effect. For the DGP model to satisfy these constraints new gravitational phenomena would have to appear at the non-linear and cross-over scales respectively. A prediction of the DGP expansion history in a region where the phenomenology is well understood is that high-redshift galaxies should be substantially correlated with the CMB through the ISW effect. This correlation should provide a sharp test of the DGP model in the future.

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arXiv2007

Near-Horizon Solution for DGP Perturbations

We develop a scaling ansatz for the master equation in Dvali, Gabadadze, Porrati cosmologies, which allows us to solve the equations of motion for perturbations off the brane during periods when the on-brane evolution is scale-free. This allows us to understand the behavior of the gravitational potentials outside the horizon at high redshifts and close to the horizon today. We confirm that the results of Koyama and Maartens are valid at scales relevant for observations such as galaxy-ISW correlation. At larger scales, there is an additional suppression of the potential which reduces the growth rate even further and would strengthen the ISW effect.

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arXiv2007

The Large Scale Structure of f(R) Gravity

We study the evolution of linear cosmological perturbations in f(R) models of accelerated expansion in the physical frame where the gravitational dynamics are fourth order and the matter is minimally coupled. These models predict a rich and testable set of linear phenomena. For each expansion history, fixed empirically by cosmological distance measures, there exists two branches of f(R) solutions that are parameterized by B propto d^2 f/dR^2. For B<0, which include most of the models previously considered, there is a short-timescale instability at high curvature that spoils agreement with high redshift cosmological observables. For the stable B>0 branch, f(R) models can reduce the large-angle CMB anisotropy, alter the shape of the linear matter power spectrum, and qualitatively change the correlations between the CMB and galaxy surveys. All of these phenomena are accessible with current and future data and provide stringent tests of general relativity on cosmological scales.

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arXiv2006

Weighing the Universe with Photometric Redshift Surveys and the Impact on Dark Energy Forecasts

With a wariness of Occam's razor awakened by the discovery of cosmic acceleration, we abandon the usual assumption of zero mean curvature and ask how well it can be determined by planned surveys. We also explore the impact of uncertain mean curvature on forecasts for the performance of planned dark energy probes. We find that weak lensing and photometric baryon acoustic oscillation data, in combination with CMB data, can determine the mean curvature well enough that the residual uncertainty does not degrade constraints on dark energy. We also find that determinations of curvature are highly tolerant of photometric redshift errors.

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arXiv2006

A Cosmological Test of Standard Gravity by Weak Lensing

The large scale structure formation in the standard Einstein gravity at later time is uniquely determined by the expansion history H(z) measured by the geometrical factor. A possible departure from the expected standard structure formation is detectable by growth structure test. We design new deviation parameters to test standard gravity at large scales and estimate the expected errors by weak lensing experiment. The effective Newtonian constant G_N at the present time can be constrained around 10% accuracy by measuring the difference curvature perturbations (Φ-Ψ)/2 within 2% accuracy.

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arXiv2005

Looking for an extra dimension with tomographic cosmic shear

The cosmic acceleration was discovered in one of the brane-based models. We are interested in discriminating this model from the dark energy by tomographic cosmic shear. Growth factors are different in the two models when one adjusts parameters to get nearly identical H(z). The two models could be distinguished with independent determinations of both geometrical factors and the growth factor. We introduce new parameterizations to separate the influence of geometry and the influence of growth on cosmic shear maps. We find that future observations will be able to distinguish between both models.

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arXiv2005

Constraints on supernovae dimming from photon-pseudo scalar coupling

An alternative mechanism that dims high redshift supernovae without cosmic acceleration utilizes an oscillation of photons into a pseudo-scalar particle during transit. Since angular diameter distance measures are immune to the loss of photons, this ambiguity in interpretation can be resolved by combining CMB acoustic peak measurements with the recent baryon oscillation detection in galaxy power spectra. This combination excludes a non-accelerating dark energy species at the 4sigma level regardless of the level of the pseudo-scalar coupling. While solutions still exist with substantial non-cosmological dimming of supernovae, they may be tested with future improvement in baryon oscillation experiments.

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arXiv2003

Dark Energy Tomography

We study how parameter error forecasts for tomographic cosmic shear observations are affected by sky coverage, density of source galaxies, inclusion of CMB experiments, simultaneou fitting of non--dark energy parameters, and the parametrization of the history of the dark energy equation-of-state parameter w(z). We find tomographic shear-shear power spectra on large angular scales (l<1000) inferred from all-sky observations, in combination with Planck, can achieve sigma(w0)=0.06 and sigma(wa)=0.09 assuming the equation-of-state parameter is given by w(z)=w0+wa(1-a(z)) and that nine other matter content and primordial power spectrum parameters are simultaneously fit. Taking parameters other than w0, wa and Omegam to be completely fixed by the CMB we find errors on w0 and wa that are only 10% and 30% better respectively, justifying this common simplifying assumption. We also study `dark energy tomography': reconstuction of w(z) assumed to be constant within each of five independent redshift bins. With smaller-scale information included by use of the Jain & Taylor ratio statistic we find sigma(wi)<0.1 for all five redshift bins and sigma(wi)<0.02 for both bins at z<0.8. Finally, addition of cosmic shear can also reduce errors on quantities already determined well by the CMB. We find the sum of neutrino masses can be determined to +-0.013eV and that the primordial power specrum power-law index, ns, as well as dns/dlnk, can be determined more than a factor of two better than by Planck alone. These improvements may be highly valuable since the lower bound on the sum of neutrino masses is 0.06eV as inferred from atmospheric neutrino oscillations, and slow-roll models of inflation predict non-zero dns/dlnk at the forecasted error levels when |n_S-1|>0.04.

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arXiv2003

Determining Neutrino Mass from the CMB Alone

Distortions of Cosmic Microwave Background (CMB) temperature and polarization maps caused by gravitational lensing, observable with high angular resolution and high sensitivity, can be used to measure the neutrino mass. Assuming two massless species and one with mass m_nu we forecast sigma(m_nu) = 0.15 eV from the Planck satellite and sigma(m_nu)=0.04 eV from observations with twice the angular resolution and about 20 times the sensitivity. A detection is likely at this higher sensitivity since the observation of atmospheric neutrino oscillations require mass-squared differences greater than about (0.04 eV)^2.

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arXiv2003

The Detectability of Departures from the Inflationary Consistency Equation

We study the detectability, given CMB polarization maps, of departures from the inflationary consistency equation, r \equiv T/S \simeq -5 n_T, where T and S are the tensor and scalar contributions to the quadrupole variance, respectively. The consistency equation holds if inflation is driven by a slowly-rolling scalar field. Departures can be caused by: 1) higher-order terms in the expansion in slow-roll parameters, 2) quantum loop corrections or 3) multiple fields. Higher-order corrections in the first two slow-roll parameters are undetectably small. Loop corrections are detectable if they are nearly maximal and r \ga 0.1. Large departures (|Δn_T| \ga 0.1) can be seen if r \ga 0.001. High angular resolution can be important for detecting non-zero r+5n_T, even when not important for detecting non-zero r.

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