Supernova Cosmology Project Research Articles

Probing the Ricci dark energy model with perturbations by using WMAP seven-year cosmic microwave background measurements, baryon acoustic oscillations, and type Ia supernovae

In this paper, we investigate the Ricci dark energy model with perturbations through the joint constraints of current cosmological data sets from dynamical and geometrical perspectives. We use the full cosmic microwave background information from WMAP seven-year data, the baryon acoustic oscillations from the Sloan Digital Sky Survey and the Two Degree Galaxy Redshift Survey, and type Ia supernovae from the Union2 compilation of the Supernova Cosmology Project Collaboration. A global constraint is performed by employing the Markov chain Monte Carlo method. With the best-fitting results, we show the differences of cosmic microwave background power spectra and background evolutions for the cosmological constant model and Ricci dark energy model with perturbations.

Unification of Dark Matter and Dark Energy in a Modified Entropic Force Model

In Verlinde's entropic force scenario of gravity, Newton's laws and Einstein equations can be obtained from the first principles and general assumptions. However, the equipartition law of energy is invalid at very low temperatures. We show clearly that the threshold of the equipartition law of energy is related with horizon of the universe. Thus, a one-dimensional Debye (ODD) model in the direction of radius of the modified entropic force (MEF) may be suitable in description of the accelerated expanding universe. We present a Friedmann cosmic dynamical model in the ODD-MEF framework. We examine carefully constraints on the ODD-MEF model from the Union2 compilation of the Supernova Cosmology Project (SCP) collaboration, the data from the observation of the large-scale structure (LSS) and the cosmic microwave background (CMB), i.e. SNe Ia+LSS+CMB. The combined numerical analysis gives the best-fit value of the model parameters ζ ≃ 10−9 and Ωm0 = 0.224, with χ2min = 591.156. The corresponding age of the universe agrees with the result of D. Spergel et al. [J.M. Bardeen, B. Carter, and S.W. Hawking, Commun. Math. Phys. 31 (1973) 161] at 95% confidence level. The numerical result also yields an accelerated expanding universe without invoking any kind of dark energy. Taking ζ(≡ 2πωD/H0) as a running parameter associated with the structure scale r, we obtain a possible unified scenario of the asymptotic flatness of the radial velocity dispersion of spiral galaxies, the accelerated expanding universe and the Pioneer 10/11 anomaly in the entropic force framework of Verlinde.

Observational constraints on exponential gravity

We study the observational constraints on the exponential gravity model of $f(R)=\ensuremath{-}\ensuremath{\beta}{R}_{s}(1\ensuremath{-}{e}^{\ensuremath{-}R/{R}_{s}})$. We use the latest observational data including Supernova Cosmology Project Union2 compilation, Two-Degree Field Galaxy Redshift Survey, Sloan Digital Sky Survey Data Release 7, and Seven-Year Wilkinson Microwave Anisotropy Probe in our analysis. From these observations, we obtain a lower bound on the model parameter $\ensuremath{\beta}$ at 1.27 (95% C.L.) but no appreciable upper bound. The constraint on the present matter density parameter is $0.245<{\ensuremath{\Omega}}_{m}^{0}<0.311$ (95% C.L.). We also find out the best-fit value of model parameters on several cases.

CONSTRAINING DUST AND COLOR VARIATIONS OF HIGH-z SNe USING NICMOS ON THE HUBBLE SPACE TELESCOPE *

We present data from the Supernova Cosmology Project for five high redshift Type Ia supernovae (SNe Ia) that were obtained using the NICMOS infrared camera on the Hubble Space Telescope. We add two SNe from this sample to a rest-frame I-band Hubble diagram, doubling the number of high redshift supernovae on this diagram. This I-band Hubble diagram is consistent with a flat universe (ΩM, ΩΛ) = (0.29, 0.71). A homogeneous distribution of large grain dust in the intergalactic medium (replenishing dust) is incompatible with the data and is excluded at the 5σ confidence level, if the SN host galaxy reddening is corrected assuming RV = 1.75. We use both optical and infrared observations to compare photometric properties of distant SNe Ia with those of nearby objects. We find generally good agreement with the expected color evolution for all SNe except the highest redshift SN in our sample (SN 1997ek at z = 0.863) which shows a peculiar color behavior. We also present spectra obtained from ground-based telescopes for type identification and determination of redshift. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555. The observations are associated with program GO-07850.

Quantitative comparison between type Ia supernova spectra at low and high redshifts: a case study

We develop a method to measure the strength of the absorption features in Type Ia supernova (SN Ia) spectra and use it to make a quantitative comparison between the spectra of Type Ia supernovae at low and high redshifts. In this case study, we apply the method to 12 high-redshift (0.212 ≤ z ≤ 0.912) SNe Ia observed by the Supernova Cosmology Project . Through measurements of the strengths of these features and of the blueshift of the absorption minimum in Ca ii H&K, we show that the spectra of the high-redshift SNe Ia are quantitatively similar to spectra of nearby SNe Ia (z < 0.15). One supernova in our high redshift sample, SN 2002fd at z=0.279, is found to have spectral characteristics that are associated with peculiar SN 1991T/SN 1999aa-like supernovae.

Improvements to the Image Processing of Hubble Space Telescope NICMOS Observations with Multiple Readouts11Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5‐26555. These observations are associated with programs GO‐8088 and GO‐9075.

ABSTRACTWe report on improvements made to the standard NICMOS processing pipeline. The calculation of the uncertainties on the signal accumulation rate has been modified to include the statistical correlations between the consecutive readouts. This leads to a ∼30% correction in the photometric weight of individual pixels containing faint objects. In order to correct a problem with the existing cosmic‐ray rejection algorithm, we have developed and implemented a joint fit procedure, where the accumulating signal is fit as linear functions of time with the same rate both before and after the cosmic‐ray (CR) impact. This procedure leads to significantly smaller uncertainty in the count rate for pixels affected by CRs. We also show that regions neighboring CRs found by the standard NICMOS pipeline are systematically brighter. This interpixel correlation substantially increases the footprint of CR impacts and is treated for the first time by our new pipeline. The new processing is most relevant for photometry from deep observations of faint targets, for which accurate, optimal, and unbiased uncertainty estimates are important. We present examples of these improvements for deep NIC2 images of a high‐redshift supernova from the Supernova Cosmology Project. The net improvement is a factor of 2 reduction in the number of 3 σ photometric outliers.

Spectra of High-Redshift Type Ia Supernovae and a Comparison with Their Low-Redshift Counterparts

We present spectra for 14 high-redshift (0.17 < z < 0.83) supernovae, which were discovered by the Supernova Cosmology Project as part of a campaign to measure cosmological parameters. The spectra are used to determine the redshift and classify the supernova type, essential information if the supernovae are to be used for cosmological studies. Redshifts were derived either from the spectrum of the host galaxy or from the spectrum of the supernova itself. We present evidence that these supernovae are of Type Ia by matching to spectra of nearby supernovae. We find that the dates of the spectra relative to maximum light determined from this fitting process are consistent with the dates determined from the photometric light curves, and moreover the spectral time-sequence for SNe Type Ia at low and high redshift is indistinguishable. We also show that the expansion velocities measured from blueshifted CaHK are consistent with those measured for low-redshift Type Ia supernovae. From these first-level quantitative comparisons we find no evidence for evolution in SNIa properties between these low- and high-redshift samples. Thus even though our samples may not be complete, we conclude that there is a population of SNe Ia at high redshift whose spectral properties match those at low redshift.

Einstein’s Mistakes

Science sets itself apart from other paths to truth by recognizing that even its greate practitioners sometimes err.

CONSTRAINTS ON A NEW ALTERNATIVE MODEL TO DARK ENERGY

The recent type Ia supernova data suggest that the Universe is accelerating now and had decelerated in recent past. This may provide the evidence that the standard Friedmann equation needs to be modified. We analyze in detail a new model in the context of modified Friedmann equation using the supernova data published by the High-z Supernova Search Team and the Supernova Cosmology Project. The new model explains recent acceleration and past deceleration. Furthermore, the acceleration of the expansion of the Universe is almost zero in the future.

Verifying the Use of Supernovae as Probes of the Cosmic Expansion

We present preliminary results of a follow-up survey which aims to characterise in detail those galaxies which hosted Type Ia supernovae found by the Supernova Cosmology Project. Our survey has two components: Hubble Space Telescope imaging with STIS and Keck spectroscopy with ESI, the goal being to classify each host galaxy into one of three broad morphological/spectral classes and hence to investigate the dependence of supernovae properties on host galaxy type over a large range in redshift. Of particular interest is the supernova Hubble diagram characterised by host galaxy class which suggests that most of the scatter arises from those occurring in late-type irregulars. Supernovae hosted by (presumed dust-free) E/SO galaxies closely follow the adopted SCP cosmological model. Although larger datasets are required, we cannot yet find any significant difference in the light curves of distant supernovae hosted in different galaxy types.

Weak lensing of the high-redshift SNIa sample

The current sample of high-redshift Type Ia supernovae (SNeIa), which combines results from two teams, the High-z Supernova Search Team and the Supernova Cosmology Project, is analysed for the effects of weak lensing. After correcting supernovae (SN) magnitudes for cosmological distances, assuming recently published, homogeneous distance and error estimates, we find that brighter SNe are preferentially found behind regions (5–15 arcmin radius) that are overdense in foreground, z∼ 0.1 galaxies. This is consistent with the interpretation that SN fluxes are magnified by foreground galaxy excess and demagnified by foreground galaxy deficit, compared with a smooth universe case. The difference between most magnified and most demagnified SNe is approximately 0.3–0.4 mag. The effect is significant at the >99 per cent level. Simple modelling reveals that the slope of the relation between supernova magnitude and foreground galaxy density depends on the amount and distribution of matter along the line of sight to the sources, but does not depend on the specifics of the galaxy biasing scheme.

Cosmological constraints on Chaplygin gas dark energy from galaxy cluster x-ray and supernova data

The recent observational evidence for the present accelerated stage of the Universe has stimulated renewed interest in alternative cosmologies. In general, such models contain an unknown negative-pressure dark component that explains the supernova results and reconciles the inflationary flatness prediction $({\ensuremath{\Omega}}_{T}=1)$ and the cosmic microwave background measurements with the dynamical estimates of the quantity of matter in the Universe $({\ensuremath{\Omega}}_{m}\ensuremath{\simeq}0.3\ifmmode\pm\else\textpm\fi{}0.1).$ In this paper we study some observational consequences of a dark energy candidate, the so-called generalized Chaplygin gas, which is characterized by an equation of state ${p}_{C}=\ensuremath{-}A/{\ensuremath{\rho}}_{C}^{\ensuremath{\alpha}},$ where A and $\ensuremath{\alpha}$ are positive constants. We investigate the prospects for constraining the equation of state of this dark energy component by combining Chandra observations of the x-ray luminosity of galaxy clusters, independent measurements of the baryonic matter density, the latest measurements of the Hubble parameter as given by the HST Key Project, and data of the Supernova Cosmology Project. We show that very stringent constraints on the model parameters can be obtained from this combination of observational data.

The redshift distribution of type Ia supernovae: constraints on progenitors and cosmic star formation history

We use the redshift distribution of type-Ia supernovae (SNe) discovered by the Supernova Cosmology Project to constrain the star formation history (SFH) of the Universe and SN Ia progenitor models. Given some of the recent determinations of the SFH, the observed SN Ia redshift distribution indicates a long (>~1 h^-1 Gyr) mean delay time between the formation of a stellar population and the explosion of some of its members as SNe Ia. For example, if the Madau et al. (1998) SFH is assumed, the delay time tau is constrained to be tau > 1.7 (tau > 0.7) h^-1 Gyr at the 95%(99%) confidence level (CL). SFHs that rise at high redshift, similar to those advocated by Lanzetta et al. (2002), are inconsistent with the data at the 95% CL unless tau > 2.5 h^-1 Gyr. Long time delays disfavor progenitor models such as edge-lit detonation of a white dwarf accreting from a giant donor, and the carbon core ignition of a white dwarf passing the Chandrasekhar mass due to accretion from a subgiant. The SN Ia delay may be shorter, thereby relaxing some of these constraints, if the field star formation rate falls, between z=1 and the present, less sharply than implied, e.g., by the original Madau plot. We show that the discovery of larger samples of high-z SNe Ia by forthcoming observational projects should yield strong constraints on the progenitor models and the SFH. In a companion paper (astro-ph/0309797), we demonstrate that if SNe Ia produce most of the iron in galaxy clusters, and the stars in clusters formed at z~2, the SN Ia delay time must be lower than 2 Gyr. If so, then the Lanzetta et al. (2002) SFH will be ruled out by the data presented here.

New Constraints on ΩM, ΩΛ, andwfrom an Independent Set of 11 High‐Redshift Supernovae Observed with theHubble Space Telescope

We report measurements of $\Omega_M$, $\Omega_\Lambda$, and w from eleven supernovae at z=0.36-0.86 with high-quality lightcurves measured using WFPC-2 on the HST. This is an independent set of high-redshift supernovae that confirms previous supernova evidence for an accelerating Universe. Combined with earlier Supernova Cosmology Project data, the new supernovae yield a flat-universe measurement of the mass density $\Omega_M=0.25^{+0.07}_{-0.06}$ (statistical) $\pm0.04$ (identified systematics), or equivalently, a cosmological constant of $\Omega_\Lambda=0.75^{+0.06}_{-0.07}$ (statistical) $\pm0.04$ (identified systematics). When the supernova results are combined with independent flat-universe measurements of $\Omega_M$ from CMB and galaxy redshift distortion data, they provide a measurement of $w=-1.05^{+0.15}_{-0.20}$ (statistical) $\pm0.09$ (identified systematic), if w is assumed to be constant in time. The new data offer greatly improved color measurements of the high-redshift supernovae, and hence improved host-galaxy extinction estimates. These extinction measurements show no anomalous negative E(B-V) at high redshift. The precision of the measurements is such that it is possible to perform a host-galaxy extinction correction directly for individual supernovae without any assumptions or priors on the parent E(B-V) distribution. Our cosmological fits using full extinction corrections confirm that dark energy is required with $P(\Omega_\Lambda>0)>0.99$, a result consistent with previous and current supernova analyses which rely upon the identification of a low-extinction subset or prior assumptions concerning the intrinsic extinction distribution.

The Hubble diagram of type Ia supernovae as a function of host galaxy morphology

We present new results on the Hubble diagram of distant type Ia supernovae (SNe Ia) segregated according to the type of host galaxy. This makes it possible to check earlier evidence for a cosmological constant by explicitly comparing SNe residing in galaxies likely to contain negligible dust with the larger sample. The cosmological parameters derived from these SNe Ia hosted by presumed dust-free early-type galaxies support earlier claims for a cosmological constant, which we demonstrate at ≃5σ significance, and the internal extinction implied is small even for late-type systems (AB < 0.2). Thus, our data demonstrate that host galaxy extinction is unlikely to systematically dim distant SNe Ia in a manner that would produce a spurious cosmological constant. Our analysis is based on new Hubble Space Telescope STIS ‘snapshot’ images and Keck-II echellette spectroscopy at the locations of the SNe, spanning the redshift range 0 < z < 0.8. Selecting from the sample discovered by the Supernova Cosmology Project (SCP), we classify the host galaxies of 39 distant SNe using the combination of STIS imaging, Keck spectroscopy and ground-based broad-band photometry. The distant data are analysed in comparison with a low-redshift sample of 25 SNe Ia re-calibrated according to the precepts of the SCP. The scatter observed in the SNe Ia Hubble diagrams correlates closely with host galaxy morphology. We find this scatter is smallest for SNe Ia occurring in early-type hosts and largest for those occurring in late-type galaxies. Moreover, SNe residing in late-type hosts appear ≃ 0.14 ± 0.09 mag fainter in their light-curve-width-corrected luminosity than those in early-type hosts, as expected if a modest amount of dust extinction is a contributing factor. As in previous studies, these results are broadly independent of whether corrections based upon SN light-curve shapes are performed. We also use our high-redshift data set to search for morphological dependences in the SNe light curves, as are sometimes seen in low-redshift samples. No significant trends are found, possibly because the range of light-curve widths is too limited.

On the Reality of the Accelerating Universe

Two groups recently deduced the positive value for the cosmological constant, concluding at a high (≥99%) confidence level that the universe should be accelerating. This conclusion followed from the statistical analysis of dozens of high-redshift supernovae. In this paper this conclusion is discussed. From the conservative frequentist's point of view, the validity of the null hypothesis of the zero cosmological constant is tested by the classical statistical χ2 test for the 60 supernovae listed in Perlmutter et al. This sample contains 42 objects discovered in the frame of the Supernova Cosmology Project and 18 low-redshift objects detected earlier. Excluding the event SN 1997O, which is doubtlessly an outlier, one obtains the following result: the probability for seeing a worse χ2—if the null hypothesis is true—is in the 5%-8% range, a value that does not indicate significant evidence against the null. Furthermore, if one excludes five possible outliers, as proposed by Perlmutter et al., then the sample of 54 supernovae is in excellent accordance with the null hypothesis. It also seems that supernovae from the High-z Supernova Search Team do not change the acceptance of the null hypothesis. This means that the rejection of the Einstein equations with zero cosmological constant—based on the supernova data alone—is still premature.

The Distant Type Ia Supernova Rate

We present a measurement of the rate of distant Type Ia supernovae derived using 4 large subsets of data from the Supernova Cosmology Project. Within this fiducial sample,which surveyed about 12 square degrees, thirty-eight supernovae were detected at redshifts 0.25--0.85. In a spatially flat cosmological model consistent with the results obtained by the Supernova Cosmology Project, we derive a rest-frame Type Ia supernova rate at a mean red shift z {approx_equal} 0.55 of 1.53 {sub -0.25}{sub -0.31}{sup 0.28}{sup 0.32} x 10{sup -4} h{sup 3} Mpc{sup -3} yr{sup -1} or 0.58{sub -0.09}{sub -0.09}{sup +0.10}{sup +0.10} h{sup 2} SNu(1 SNu = 1 supernova per century per 10{sup 10} L{sub B}sun), where the first uncertainty is statistical and the second includes systematic effects. The dependence of the rate on the assumed cosmological parameters is studied and the redshift dependence of the rate per unit comoving volume is contrasted with local estimates in the context of possible cosmic star formation histories and progenitor models.

Value of the Cosmological Constant in the Cosmological Relativity Theory

It is shown that the cosmological relativity theory predicts the value Λ = 1.934 × 10−35s−2 for the cosmological constant. This value of Λ is in excellent agreement with the measurements recently obtained by the High-Z Supernova Team and the Supernova Cosmology Project.

Analyses of Type Ia Supernova Data in Cosmological Models with a Local Void

The data for type Ia supernovae obtained by the High-z SN Search Team and Supernova Cosmology Project are analyzed using inhomogeneous cosmological models with a local void on scales of about 200 Mpc, to derive the best-fit values of model parameters and the confidence contours. The χ2 fitting is found to be slightly better than that in homogeneous models. It is shown that (1) the best-fit values are most sensitive to the ratio R of the outer Hubble constant, H0II, to the inner Hubble constant, H0I, (2) the best-fit outer density parameter, Ω0II, and cosmological constant parameter, λ0II, are, respectively, increasing and decreasing functions of R, and (3) (Ω0II, λ0II) can be (1, 0) for R ≈0.8. Moreover, it is shown that these models are naturally consistent with the new supernova data (SN1997ff) with z = 1.7.

The Stanford Cluster Search: Scope, Method, and Preliminary Results11The Hobby‐Eberly Telescope (HET) is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig‐Maximillians‐Universität München, and Georg‐August‐Universität Göttingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly.

ABSTRACTWe describe the scientific motivation behind, and the methodology of, the Stanford Cluster Search (StaCS), a program to compile a catalog of optically selected galaxy clusters at intermediate and high (0.3≲z≲1) redshifts. The clusters are identified using a matched filter algorithm applied to deep CCD images covering ∼60 deg2 of sky. These images are obtained from several data archives, principally that of the Berkeley Supernova Cosmology Project of Perlmutter et al. Potential clusters are confirmed with spectroscopic observations at the 9.2 m Hobby‐Eberly Telescope. Follow‐up observations at optical, submillimeter, and X‐ray wavelengths are planned in order to estimate cluster masses. Our long‐term scientific goal is to measure the cluster number density as a function of mass and redshift, n(M, z), which is sensitive to the cosmological density parameter Ωm and the amplitude of density fluctuations σ8. The combined data set will contain clusters ranging over an order of magnitude in mass and allow constraints on these parameters accurate to ∼10%. We present our first spectroscopically confirmed cluster candidates and describe how to access them electronically.