Hubble Constant H0 Research Articles

Inflation model selection meets dark radiation

We investigate how inflation model selection is affected by the presence of additional free-streaming relativistic degrees of freedom, i.e. dark radiation. We perform a full Bayesian analysis of both inflation parameters and cosmological parameters taking reheating into account self-consistently. We compute the Bayesian evidence for a few representative inflation scenarios in both the standard ΛCDM model and an extension including dark radiation parametrised by its effective number of relativistic species Neff. Using a minimal dataset (Planck low-ℓ polarisation, temperature power spectrum and lensing reconstruction), we find that the observational status of most inflationary models is unchanged. The exceptions are potentials such as power-law inflation that predict large values for the scalar spectral index that can only be realised when Neff is allowed to vary. Adding baryon acoustic oscillations data and the B-mode data from BICEP2/Keck makes power-law inflation disfavoured, while adding local measurements of the Hubble constant H0 makes power-law inflation slightly favoured compared to the best single-field plateau potentials. This illustrates how the dark radiation solution to the H0 tension would have deep consequences for inflation model selection.

Hubble’s Constant and Flat Rotation Curves of Stars: Are Dark Matter and Energy Needed?

Although dark energy and dark matter have not yet been detected, they are believed to comprise the majority of the universe. Observations of the flat rotation curve of galaxies may be explained by dark matter and dark energy. This article, using Newton’s laws and Einstein’s theory of gravitation, shows that it is possible to define a new term, called E0, variable in time and space, of which one of its limits is the Hubble constant H0. I show that E0 is strongly linked to an explanation of the flat rotation curve of galaxies. This strong correlation between Hubble’s constant H0 and E0 enables us to solve the mystery of the surplus of gravity that is stabilizing the universe.

Constraining the time evolution of dark energy, curvature and neutrino properties with cosmic chronometers

We use the latest compilation of observational Hubble parameter measurements estimated with the differential evolution of cosmic chronometers, in the redshift range 0<z<2, to place constraints on cosmological parameters. We used a Markov-Chain Monte-Carlo approach to sample the parameter space for the cosmic chronometers dataset alone and in combination with other state-of-the art cosmological measurements: CMB data from the latest Planck 2015 release, the most recent estimate of the Hubble constant H0, a compilation of recent baryon acoustic oscillation data, and the latest type Ia cosmological supernovae sample. From late-Universe probes alone (z<2) we find that w0 = −0.9 ± 0.18 and wa = −0.5 ± 1.7, and when combining also Planck 2015 data we obtain w0=−0.98± 0.11 and wa=−0.30±0.4. These new constraints imply that nearly all quintessence models are disfavoured by the data; only phantom models or a pure cosmological constant are favoured. This is a remarkable finding as it imposes severe constraints on the nature of dark energy. For the curvature our constraints are Ωk = 0.003 ± 0.003, considering also CMB data. We also find that H(z) data from cosmic chronometers are important to constrain parameters that do no affect directly the expansion history, by breaking or reducing degeneracies with other parameters. We find that Neff = 3.17 ± 0.15, thus excluding the possibility of an extra (sterile) neutrino at more than 5 σ, and put competitive limits on the sum of neutrino masses, Σ mν< 0.27 eV at 95% confidence level. Finally, we constrain the redshift evolution of dark energy by exploring separately the early and late-Universe, and find a dark energy equation of state evolution w(z) consistent with that in the ΛCDM model at the ± 0.4 level over the entire redshift range 0 < z < 2.

H0LiCOW – V. New COSMOGRAIL time delays of HE 0435−1223:H0to 3.8 per cent precision from strong lensing in a flat ΛCDM model

We present a new measurement of the Hubble Constant H0 and other cosmological parameters based on the joint analysis of three multiply-imaged quasar systems with measured gravitational time delays. First, we measure the time delay of HE0435-1223 from 13-year light curves obtained as part of the COSMOGRAIL project. Companion papers detail the modeling of the main deflectors and line of sight effects, and how these data are combined to determine the time-delay distance of HE 0435-1223. Crucially, the measurements are carried out blindly with respect to cosmological parameters in order to avoid confirmation bias. We then combine the time-delay distance of HE0435-1223 with previous measurements from systems B1608+656 and RXJ1131-1231 to create a Time Delay Strong Lensing probe (TDSL). In flat $\Lambda$CDM with free matter and energy density, we find $H_0$ = 71.9 +2.4 -3.0 km/s/Mpc and $\Omega_{\Lambda}$ = 0.62 +0.24 -0.35 . This measurement is completely independent of, and in agreement with, the local distance ladder measurements of H0. We explore more general cosmological models combining TDSL with other probes, illustrating its power to break degeneracies inherent to other methods. The TDSL and Planck joint constraints are $H_0$ = 69.2 +1.4 -2.2 km/s/Mpc, $\Omega_{\Lambda}$ = 0.70 +0.01 -0.01 and $\Omega_k$ = 0.003 +0.004 -0.006 in open $\Lambda$CDM and $H_0$ = 79.0 +4.4 -4.2 km/s/Mpc, $\Omega_{de}$ = 0.77 +0.02 -0.03 and $w$ = -1.38 +0.14 -0.16 in flat $w$CDM. Combined with Planck and Baryon Acoustic Oscillation data, when relaxing the constraints on the numbers of relativistic species we find $N_{eff}$ = 3.34 +0.21 -0.21 and when relaxing the total mass of neutrinos we find 0.182 eV. In an open $w$CDM in combination with Planck and CMB lensing we find $H_0$ = 77.9 +5.0 -4.2 km/s/Mpc, $\Omega_{de}$ = 0.77 +0.03 -0.03, $\Omega_k$ = -0.003 +0.004 -0.004 and $w$ = -1.37 +0.18 -0.23.

WHAT ARE THE Omh2 (z1, z2) AND Om (z1, z2) DIAGNOSTICS TELLING US IN LIGHT OF H(z) DATA?

ABSTRACT The two-point diagnostics Om(z i , z j ) and Omh 2(z i , z j ) have been introduced as an interesting tool for testing the validity of the Λ cold dark matter (ΛCDM) model. Recently, Sahni et al. combined two independent measurements of H(z) from baryon acoustic oscillation (BAO) data with the value of the Hubble constant H 0, and used the second of these diagnostics to test the ΛCDM (a constant equation-of-state parameter for dark energy) model. Their result indicated a considerable tension between observations and predictions of the ΛCDM model. Since reliable data concerning the expansion rates of the universe at different redshifts H(z) are crucial for the successful application of this method, we investigate both two-point diagnostics on the most comprehensive set of N = 36 measurements of H(z) from BAOs and the differential ages (DAs) of passively evolving galaxies. We discuss the uncertainties of the two-point diagnostics and find that they are strongly non-Gaussian and follow the patterns deeply rooted in their very construction. Therefore we propose that non-parametric median statistics is the most appropriate way of treating this problem. Our results support the claims that ΛCDM is in tension with H(z) data according to the two-point diagnostics developed by Shafieloo, Sahni, and Starobinsky. However, other alternatives to the ΛCDM model, such as the wCDM or Chevalier–Polarski–Linder models, perform even worse. We also note that there are serious systematic differences between the BAO and DA methods that ought to be better understood before H(z) measurements can compete with other probes methods.

PARALLAX OF GALACTIC CEPHEIDS FROM SPATIALLY SCANNING THE WIDE FIELD CAMERA 3 ON THE HUBBLE SPACE TELESCOPE: THE CASE OF SS CANIS MAJORIS

ABSTRACT We present a high-precision measurement of the parallax for the 12-day Cepheid SS Canis Majoris, obtained via spatial scanning with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). Spatial scanning enables astrometric measurements with a precision of 20–40 μas, an order of magnitude better than pointed observations. SS CMa is the second Cepheid targeted for parallax measurement with HST and is the first of a sample of 18 long-period (≳10 days) Cepheids selected in order to improve the calibration of their period–luminosity relation and eventually permit a determination of the Hubble constant H 0 to better than 2%. The parallax of SS CMa is found to be 348 ± 38 μas, corresponding to a distance of 2.9 ± 0.3 kpc. We also present a refinement of the static geometric distortion of WFC3 obtained using spatial scanning observations of calibration fields, with a typical magnitude ≲0.01 pixels on scales of 100 pixels.

Testing the interaction between dark energy and dark matter with H(z) data

With the Markov Chain Monte Carlo (MCMC) method, we constrain an interactive dark energy model by combing the up-to-date observational data of Hubble parameter H(z) with the 7-year baryon acoustic oscillation (BAO) data, and the cosmic microwave background (CMB) data observed by the Planck satellite. Under the joint constraint of the three kinds of data, the best-fit values of the model parameters and their 1-σ errors are obtained as follows: the energy density Ωm=0.266-0.028+0.028(1σ), the interaction factor γ=0.090-0.098+0.100(1σ), the parameter of state equation of dark matter wX=-1.307-0.269+0.263(1σ), and the Hubble Constant H0=7420-4.56+4.66(1σ), where the coupling parameter γ > 0 means that the energy is transferred from dark matter to dark energy, and the coincidence problem in the Lambda-Cold Dark Matter (ΛCDM) model is slightly alleviated in the 1σ range. For comparisons, we constrain the same model with the BAO+CMB observations and H(z) data separately. The results are as follows: (1) The H(z) data could put stricter constraint on the parameter γ than the BAO+CMB observations. (2) The ΛCDM model is best fitted, and the coupling parameter γ is correlated with parameters Ωm and H0. (3) The inconsistency of the constraint results of H0 between the local distance ladder measurements and the Planck observations can be alleviated after taking account of the interaction between dark energy and dark matter.

Constraining the local variance of H0 from directional analyses

We evaluate the local variance of the Hubble Constant H0 with low-z Type Ia Supernovae (SNe). Our analyses are performed using a hemispherical comparison method in order to test whether taking the bulk flow motion into account can reconcile the measurement of the Hubble Constant H0 from standard candles (H0 = 73.8±2.4 km s-1 Mpc -1) with that of the Planck's Cosmic Microwave Background data (H0 = 67.8 ± 0.9km s-1 Mpc-1). We obtain that H0 ranges from 68.9±0.5 km s-1 Mpc-1 to 71.2±0.7 km s-1 Mpc-1 through the celestial sphere (1σ uncertainty), implying a Hubble Constant maximal variance of δH0 = (2.30±0.86) km s-1 Mpc-1 towards the (l,b) = (315°,27°) direction. Interestingly, this result agrees with the bulk flow direction estimates found in the literature, as well as previous evaluations of the H0 variance due to the presence of nearby inhomogeneities. We assess the statistical significance of this result with different prescriptions of Monte Carlo simulations, obtaining moderate statistical significance, i.e., 68.7% confidence level (CL) for such variance. Furthermore, we test the hypothesis of a higher H0 value in the presence of a bulk flow velocity dipole, finding some evidence for this result which, however, cannot be claimed to be significant due to the current large uncertainty in the SNe distance modulus. Then, we conclude that the tension between different H0 determinations can plausibly be caused to the bulk flow motion of the local Universe, even though the current incompleteness of the SNe data set, both in terms of celestial coverage and distance uncertainties, does not allow a high statistical significance for these results or a definitive conclusion about this issue.

THE MEGAMASER COSMOLOGY PROJECT. VIII. A GEOMETRIC DISTANCE TO NGC 5765b

ABSTRACT As part of the Megamaser Cosmology Project, here we present a new geometric distance measurement to the megamaser galaxy NGC 5765b. Through a series of very long baseline interferometry observations, we have confirmed the water masers trace a thin, sub-parsec Keplerian disk around the nucleus, implying an enclosed mass of 4.55 ± 0.40 × 107 M ⊙. Meanwhile, from single-dish monitoring of the maser spectra over two years, we measured the secular drifts of maser features near the systemic velocity of the galaxy with rates between 0.5 and 1.2 km s−1 yr−1. Fitting a warped, thin-disk model to these measurements, we determine a Hubble Constant H 0 of 66.0 ± 6.0 km s−1 Mpc−1 with an angular-diameter distance to NGC 5765b of 126.3 ± 11.6 Mpc. Apart from the distance measurement, we also investigate some physical properties related to the maser disk in NGC 5765b. The high-velocity features are spatially distributed into several clumps, which may indicate the existence of a spiral density wave associated with the accretion disk. For the redshifted features, the envelope defined by the peak maser intensities increases with radius. The profile of the systemic masers in NGC 5765b is smooth and shows almost no structural changes over the two years of monitoring time, which differs from the more variable case of NGC 4258.

Cosmic flows and the expansion of the local Universe from non-linear phase–space reconstructions

Abstract In this work, we investigate the impact of cosmic flows and density perturbations on Hubble constant H0 measurements using non-linear phase–space reconstructions of the Local Universe (LU). In particular, we rely on a set of 25 precise constrained N-body simulations based on Bayesian initial conditions reconstructions of the LU using the Two-Micron Redshift Survey galaxy sample within distances of about 90 h−1 Mpc. These have been randomly extended up to volumes enclosing distances of 360 h−1 Mpc with augmented Lagrangian perturbation theory (750 simulations in total), accounting in this way for gravitational mode coupling from larger scales, correcting for periodic boundary effects, and estimating systematics of missing attractors (σlarge = 134 s−1 km). We report on Local Group (LG) speed reconstructions, which for the first time are compatible with those derived from cosmic microwave background-dipole measurements: |vLG| = 685 ± 137 s−1 km. The direction (l, b) = (260$_{.}^{\circ}$5 ± 13$_{.}^{\circ}$3, 39$_{.}^{\circ}$1 ± 10$_{.}^{\circ}$4) is found to be compatible with the observations after considering the variance of large scales. Considering this effect of large scales, our local bulk flow estimations assuming a Λ cold dark matter model are compatible with the most recent estimates based on velocity data derived from the Tully–Fisher relation. We focus on low-redshift supernova measurements out to 0.01 &amp;lt; z &amp;lt; 0.025, which have been found to disagree with probes at larger distances. Our analysis indicates that there are two effects related to cosmic variance contributing to this tension. The first one is caused by the anisotropic distribution of supernovae, which aligns with the velocity dipole and hence induces a systematic boost in H0. The second one is due to the inhomogeneous matter fluctuations in the LU. In particular, a divergent region surrounding the Virgo Supercluster is responsible for an additional positive bias in H0. Taking these effects into account yields a correction of ΔH0 = -1.76 ± 0.21 s− 1 km Mpc− 1, thereby reducing the tension between local probes and more distant probes. Effectively H0 is lower by about 2 per cent.

Investigation of Homogeneity and Matter Distribution on Large Scales Using Large Quasar Groups**Supported by the National Natural Science Foundation of China under Grant No. 30000-41030521

We use 20 large quasar group (LQG) samples in Park et al. (2015) to investigate the homogeneity of the 0.3 ≲ z ≲ 1.6 Universe (z denotes the redshift). For comparison, we also employ the 12 LQGs samples at 0.5 ≲ z ≲ 2 in Komberg et al. (1996) to do the analysis. We calculate the bias factor b and the two-point correlation function ξLQG for such groups for three different density profiles of the LQG dark matter halos, i.e. the isothermal profile, the Navarro–Frenk–White (NFW) profile, and the (gravitational) lensing profile. We consider the ΛCDM (cold dark matter plus a cosmological constant Λ) underlying matter power spectrum with Ωm = 0.28, ΩΛ = 0.72, the Hubble constant H0 = 100 h·km·s−1· Mpc−1 with h = 0.72. Dividing the samples into three redshift bins, we find that the LQGs with higher redshift are more biased and correlated than those with lower redshift. The homogeneity scale RH of the LQG distribution is also deduced from theory. It is defined as the comoving radius of the sphere inside which the number of LQGs N(< r) is proportional to r3 within 1%, or equivalently above which the correlation dimension of the sample D2 is within 1% of D2 = 3. For Park et al.'s samples and the NFW dark matter halo profile, the homogeneity scales of the LQG distribution are RH ⋍ 247 h−1· Mpc for 0.2 < z ≤ 0.6, RH ⋍ 360 h−1· Mpc for 0.6 < z ≤ 1.2, and RH ⋍ 480 h−1· Mpc for 1.2 < z ≲ 1.6. The maximum extent of the LQG samples are beyond RH in each bin, showing that the LQG samples are not homogeneously distributed on such a scale, i.e. a length range of ∼ 500 h−1. Mpc and a mass scale of ∼1014M⊙. The possibilities of a top-down structure formation process as was predicted by the hot/warm dark matter (WDM) scenarios and the redshift evolution of bias factor b and correlation amplitude ξLQG of the LQGs as a consequence of the cosmic expansion are both discussed. Different results were obtained based on the LQG sample in Komberg et al. (1996) and the possible reasons for such differences were discussed.

CMB and BAO constraints for an induced gravity dark energy model with a quartic potential

We study the predictions for structure formation in an induced gravity dark energy model with a quartic potential. By developing a dedicated Einstein-Boltzmann code, we study self-consistently the dynamics of homogeneous cosmology and of linear perturbations without using any parametrization. By evolving linear perturbations with initial conditions in the radiation era, we accurately recover the quasi-static analytic approximation in the matter dominated era. We use PLANCK 2013 data and a compilation of baryonic acoustic oscillation (BAO) data to constrain the coupling γ to the Ricci curvature and the other cosmological parameters. By connecting the gravitational constant in the Einstein equation to the one measured in a Cavendish-like experiment, we find γ < 0.0012 at 95% CL with PLANCK 2013 and BAO data. This is the tightest cosmological constraint on γ and on the corresponding derived post-Newtonian parameters. Because of a degeneracy between γ and the Hubble constant H0, we show how larger values for γ are allowed, but not preferred at a significant statistical level, when local measurements of H0 are combined in the analysis with PLANCK 2013 data.

Effects of a Time-Varying Color-Luminosity Parameter β on the Cosmological Constraints of Modified Gravity Models

It has been found that, for the Supernova Legacy Survey three-year (SNLS3) data, there is strong evidence for the redshift-evolution of color-luminosity parameter β. In previous studies, only dark energy (DE) models are used to explore the effects of a time-varying β on parameter estimation. In this paper, we extend the discussions to the case of modified gravity (MG), by considering Dvali—Gabadadze—Porrati (DGP) model, power-law type f(T) model and exponential type f(T) model. In addition to the SNLS3 data, we also use the latest Planck distance priors data, the galaxy clustering (GC) data extracted from Sloan Digital Sky Survey (SDSS) data release 7 (DR7) and Baryon Oscillation Spectroscopic Survey (BOSS), as well as the direct measurement of Hubble constant H0 from the Hubble Space Telescope (HST) observation. We find that, for both cases of using the supernova (SN) data alone and using the combination of all data, adding a parameter of β can reduce χ2 by ∼ 36 for all the MG models, showing that a constant β is ruled out at 6σ confidence level (CL). Moreover, we find that a time-varying β always yields a larger fractional matter density Ωm0 and a smaller reduced Hubble constant h; in addition, it significantly changes the shapes of 1σ and 2σ confidence regions of various MG models, and thus corrects systematic bias for the parameter estimation. These conclusions are consistent with the results of DE models, showing that β's evolution is completely independent of the cosmological models in the background. Therefore, our work highlights the importance of considering the evolution of β in the cosmology-fits.

The Value of H0 from Gaussian Processes

AbstractA new non-parametric method based on Gaussian Processes (GP) was proposed recently to measure the Hubble constant H0. The freedom in this approach comes in the chosen covariance function, which determines how smooth the process is and how nearby points are correlated. We perform coverage tests with a thousand mock samples within the ΛCDM model in order to determine what covariance function provides the least biased results. The function Matérn(5/2) is the best with sligthly higher errors than other covariance functions, although much more stable when compared to standard parametric analyses.

The tension on the cosmological parameters from different observational data

Planck measurements of the cosmic microwave background power spectra find a lower value of the Hubble constant H0 and a higher value of the fractional matter energy density Ωm0 for the concordance ΛCDM model, and these results are in tension with other measurements. The Planck group argued that the tension came either from some sources of unknown systematic errors in some astrophysical measurements or the wrong ΛCDM model applied in fitting the data. We studied the reason for the tension on H0 from different measurements by considering two dynamical dark energy models. We found that there is no tension between different data, the constraint on H0 is almost unchanged for different dark energy models and the tension with the local measurements remains when the error bar on H0 is tightened to be around 1. We argue that the tension on H0 is not caused by the fitting model.

Cosmological evidence for leptonic asymmetry after Planck

Recently, the PLANCK satellite found a larger and most precise value of the matter energy density, that impacts on the present values of other cosmological parameters such as the Hubble constant H0, the present cluster abundances S8, and the age of the Universe tU. The existing tension between PLANCK determination of these parameters in the frame of the base ΛCDM model and their determination from other measurements generated lively discussions, one possible interpretation being that some sources of systematic errors in cosmological measurements are not completely understood. An alternative interpretation is related to the fact that the CMB observations, that probe the high redshift Universe are interpreted in terms of cosmological parameters at present time by extrapolation within the base ΛCDM model that can be inadequate or incomplete. In this paper we quantify this tension by exploring several extensions of the base ΛCDM model that include the leptonic asymmetry. We set bounds on the radiation content of the Universe and neutrino properties by using the latest cosmological measurements, imposing also self-consistent BBN constraints on the primordial helium abundance. For all asymmetric cosmological models we find the preference of cosmological data for smaller values of active and sterile neutrino masses. This increases the tension between cosmological and short baseline neutrino oscillation data that favors a sterile neutrino with the mass of around 1 eV. For the case of degenerate massive neutrinos, we find that the discrepancies with the local determinations of H0, and tU are alleviated at ∼ 1.3σ level while S8 is in agreement with its determination from CFHTLenS survey data at ∼ 1σ and with the prediction of cluster mass-observation relation at ∼ 0.5σ. We also find 2σ statistical preference of the cosmological data for the leptonic asymmetric models involving three massive neutrino species and neutrino direct mass hierarchy. We conclude that the current cosmological data favor the leptonic asymmetric extension of the base ΛCDM model and normal neutrino mass hierarchy over the models with additional sterile neutrino species and/or inverted neutrino mass hierarchy.

Cepheid theoretical models and observations in HST/WFC3 filters: the effect on the Hubble constant H0

We present a complete theoretical scenario for classical Cepheids in the most commonly used HST/WFC3 filters, going from optical (F555W, F606W and F814W) to near-infrared (F160W) bands. The importance of such a study is related to the recent release of new classical Cepheids observed with HST/WFC3 in 8 distant galaxies where SNIa are hosted. These observations have posed sound constraints to the current distance scale with uncertainties on the Hubble constant Ho smaller than 3%. Our models explore a large range of metallicity and Helium content, thus providing a robust and unique theoretical tool for describing these new and future HST/WFC3 observations. As expected, the Period-Luminosity (PL) relation in F160W filter is linear and slightly dependent on the metallicity when compared with optical bands, thus it seems the most accurate tool to constrain extragalactic distances with Cepheids. We compare the pulsation properties of Cepheids observed with HST/WFC3-IR with our theoretical scenario and we discuss the agreement with the predicted Instability Strip for all the investigated galaxy samples including the case of NGC4258. Finally, adopting our theoretical F160W PL relation for Z=0.02 and log P>1.0, we derive new distance moduli. In particular, for NGC 4258, we derive a distance modulus mu0 = 29.345 +- 0.004 mag with a sigma = 0.34 mag, which is in very good agreement with the geometrical maser value. Moreover, using the obtained distance moduli, we estimate the Hubble constant value, Ho=76.0 +- 1.9 km s-1 Mpc-1 in excellent agreement with the most recent literature values.

New constraints on cosmological parameters and neutrino properties using the expansion rate of the Universe to z ∼ 1.75

We have assembled a compilation of observational Hubble parameter measurements estimated with the differential evolution of cosmic chronometers, in the redshift range 0 < z < 1.75. This sample has been used, in combination with CMB data and with the most recent estimate of the Hubble constant H0, to derive new constraints on several cosmological parameters. The new Hubble parameter data are very useful to break some of the parameter degeneracies present in CMB-only analysis, and to constrain possible deviations from the standard (minimal) flat ΛCDM model. The H(z) data are especially valuable in constraining Ωk and ΩDE in models that allow a variation of those parameters, yielding constraints that are competitive with those obtained using Supernovae and/or baryon acoustic oscillations.We also find that our H(z) data are important to constrain parameters that do no affect directly the expansion history, by breaking or reducing degeneracies with other parameters. We find that Nrel = 3.45±0.33 using WMAP 7-years data in combination with South Pole Telescope data and our H(z) determinations (Nrel = 3.71±0.45 using Atacama Cosmology Telescope data instead of South Pole Telescope). We exclude Nrel > 4 at 95% CL (74% CL) using the same datasets combinations. We also put competitive limits on the sum of neutrino masses, Σmν < 0.24 eV at 68% confidence level. These results have been proven to be extremely robust to many possible systematic effects, such as the initial choice of stellar population synthesis model adopted to estimate H(z) and the progenitor-bias.

Cosmology and the Hubble Constant: On the Megamaser Cosmology Project (MCP)

AbstractThe Hubble constant H0 describes not only the expansion of local space at redshift z ~ 0, but is also a fundamental parameter determining the evolution of the universe. Recent measurements of H0 anchored on Cepheid observations have reached a precision of several percent. However, this problem is so important that confirmation from several methods is needed to better constrain H0 and, with it, dark energy and the curvature of space. A particularly direct method involves the determination of distances to local galaxies far enough to be part of the Hubble flow through water vapor (H2O) masers orbiting nuclear supermassive black holes. The goal of this article is to describe the relevance of H0 with respect to fundamental cosmological questions and to summarize recent progress of the ‘Megamaser Cosmology Project’ (MCP) related to the Hubble constant.

SNLS3: CONSTRAINTS ON DARK ENERGY COMBINING THE SUPERNOVA LEGACY SURVEY THREE-YEAR DATA WITH OTHER PROBES

We present observational constraints on the nature of dark energy using the Supernova Legacy Survey three year sample (SNLS3) of Guy et al. (2010) and Conley et al. (2011). We use the 472 SNe Ia in this sample, accounting for recently discovered correlations between SN Ia luminosity and host galaxy properties, and include the effects of all identified systematic uncertainties directly in the cosmological fits. Combining the SNLS3 data with the full WMAP7 power spectrum, the Sloan Digital Sky Survey luminous red galaxy power spectrum, and a prior on the Hubble constant H0 from SHOES, in a flat universe we find omega_m=0.269+/-0.015 and w=-1.061+0.069-0.068 -- a 6.5% measure of the dark energy equation-of-state parameter w. The statistical and systematic uncertainties are approximately equal, with the systematic uncertainties dominated by the photometric calibration of the SN Ia fluxes -- without these calibration effects, systematics contribute only a ~2% error in w. When relaxing the assumption of flatness, we find omega_m=0.271+/-0.015, omega_k=-0.002+/-0.006, and w=-1.069+0.091-0.092. Parameterizing the time evolution of w as w(a)=w_0+w_a(1-a), gives w_0=-0.905+/-0.196, w_a=-0.984+1.094-1.097 in a flat universe. All of our results are consistent with a flat, w=-1 universe. The size of the SNLS3 sample allows various tests to be performed with the SNe segregated according to their light curve and host galaxy properties. We find that the cosmological constraints derived from these different sub-samples are consistent. There is evidence that the coefficient, beta, relating SN Ia luminosity and color, varies with host parameters at >4sigma significance (in addition to the known SN luminosity--host relation); however this has only a small effect on the cosmological results and is currently a sub-dominant systematic.