High-redshift Type Ia Supernovae Research Articles

Chemical evolution in a model for the joint formation of quasars and spheroids

Direct and indirect pieces of observational evidence point to a strong connection between high-redshift quasars and their host galaxies. In the framework of a model where the shining of the quasar is the episode that stops the formation of the galactic spheroid inside a virialized halo, it has been proven possible to explain the submillimetre source counts together with their related statistics and the local luminosity function of spheroidal galaxies. The time delay between the virialization and the quasar manifestation required to fit the counts is short and incresing with decresing the host galaxy mass. In this paper we compute the detailed chemical evolution of gas and stars inside virialized haloes in the framework of the same model, taking into account the combined effects of cooling and stellar feedback. Under the assumption of negligible angular momentum, we are able to reproduce the main observed chemical properties of local ellipticals. In particular, by using the same duration of the bursts which are required in order to fit the submillimetre source counts, we recover the observed increase of the Mg/Fe ratio with galactic mass. Since for the most massive objects the assumed duration of the burst is Tburst < 0.6 Gyr, we end up with a picture for elliptical galaxy formation in which massive spheroids complete their assembly at early times, thus resembling a monolithic collapse, whereas smaller galaxies are allowed for a more prolonged star formation, thus allowing for a more complicated evolutionary history. In the framework of the adopted scenario, only quasar activity can provide energies large enough to stop the star formation very soon after virialization in the most massive galactic haloes. The chemical abundance of the gas that we estimate at the end of the burst matches well the metallicity inferred from the quasar spectra. Therefore, the assumption that quasar activity interrupts the main episode of star formation in elliptical galaxies turns out to be quite reasonable. In this scenario, we also point out that non-dusty extremely red objects are the best targets for searching for high-redshift Type Ia supernovae.

Cosmology with self-adjusting vacuum energy density from a renormalization group fixed point

Cosmologies with a time dependent Newton constant and cosmological constant are investigated. The scale dependence of G and Λ is governed by a set of renormalization group equations which is coupled to Einstein's equation in a consistent way. The existence of an infrared attractive renormalization group fixed point is postulated, and the cosmological implications of this assumption are explored. It turns out that in the late Universe the vacuum energy density is automatically adjusted so as to equal precisely the matter energy density, and that the deceleration parameter approaches q =−1/4. This scenario might explain the data from recent observations of high redshift type Ia supernovae and the cosmic microwave background radiation without introducing a quintessence field.

The Feasibility of Detecting X-Ray Halos Due to Intergalactic “Cosmological” Gray Dust

Extinction due to large-diameter intergalactic gray (i.e., nonreddening) dust grains has been identified as a possible mechanism to explain the apparent systematic dimming of high-redshift Type Ia supernovae, an alternative hypothesis to the interpretation of this dimming as evidence of acceleration in the cosmological expansion of the universe. Detection of such dust grains could be achieved, in principle, through observations of the X-ray scattering halos surrounding high-redshift X-ray sources, produced by the intervening dust. I have calculated the expected intensity of such X-ray halos, as well as the halo intensity expected to result from Galactic dust; I find that the cosmological dust halo would be too faint to detect with current X-ray telescopes.

Future supernova probes of quintessence

We investigate the potential of a future supernovae data set, as might be obtained by the proposed SNAP satellite, to discriminate between two possible explanations for the observed dimming of the high redshift type IA supernovae: namely, either (i) a cosmological evolution for which the expansion of the universe has been accelerating for a substantial range of redshifts $z\ensuremath{\sim}1;$ or (ii) an unexpected supernova luminosity evolution over such a redshift range. By evaluating Bayes factors we show that within the context of spatially flat model universes with a dark energy the future SNAP data set should be able to discriminate these two possibilities. Our calculations assume particular cosmological models with a quintessence field in the form of a dynamical pseudo Nambu-Goldstone boson (PNGB), and a simple empirical model of the evolution of peak luminosities of the supernovae sources which has been recently discussed in the literature. We also show that the fiducial SNAP data set, simulated with the assumption of no source evolution, is able to discriminate the PNGB model from a number of other spatially flat quintessence models which have been widely studied in the literature, namely those with inverse power-law, simple exponential and double-exponential potentials.

Probing the structure of dark matter in Galaxy haloes and clusters using supernovae

A new method for measuring gravitational lensing with high redshift type Ia supernovae is investigated. The method utilizes correlations between foreground galaxies and supernova brightnesses to substantially reduce possible systematic errors and increase the signal to noise ratio. It is shown that this lensing signal can be related to the mass, size and substructure of galaxy halos and galaxy clusters if dark matter consists of microscopic particles. This technique may be particularly useful for measuring the size of dark matter halos, a measurement to which the lensing of galaxies is not well suited, and for measuring the level of substructure in galaxy halos, a problematic prediction of the cold dark matter model. The contributions to the signal from galaxy halos and galaxy clusters are modeled and contributions to the noise from fluctuations in the galaxy number counts, galaxy redshift error, dispersion in SN luminosities and sample variance are estimated. The intrinsic distribution of supernova luminosities and its redshift evolution are removed as major sources of uncertainty. The method is found to be complimentary to galaxy--galaxy lensing. The required observations of $\simgt 100$ supernovae have already been proposed for the purposes of cosmological parameter estimation.

Implications for Quintessence Models from MAXIMA-1 and BOOMERANG-98

Prompted by the recent MAXIMA-1 and BOOMERANG-98 measurements of the cosmic microwave background (CMB) anisotropy power spectrum, and motivated by the results from the observation of high-redshift Type Ia supernovae, we investigate CMB anisotropies in quintessence models in order to characterize the nature of the dark energy today. We perform a Bayesian likelihood analysis, using the MAXIMA-1 and BOOMERANG-98 published bandpowers, in combination with COBE/DMR, to explore the space of quintessence parameters: the quintessence energy density \Omega_\phi and equation of state w_\phi. We restrict our analysis to flat, scale-invariant, inflationary adiabatic models. We find that this simple class of inflationary models, with a quintessence component \Omega_\phi < ~0.7, -1 < = w_\phi < ~-0.5, is in good agreement with the data. Within the assumptions of our analysis, pure quintessence models seem to be slightly favored, although the simple cosmological constant scenario is consistent with the data.

Optical Spectra of Type I[CLC]a[/CLC] Supernovae at [CLC][ITAL]z[/ITAL][/CLC] = 0.46 and [CLC][ITAL]z[/ITAL][/CLC] = 1.2

We present optical spectra, obtained with the Keck 10-m telescope, of two high-redshift type Ia supernovae (SNe Ia) discovered by the High-z Supernova Search Team: SN 1999ff at z=0.455 and SN 1999fv at z~1.2, the highest-redshift published SN Ia spectrum. Both SNe were at maximum light when the spectra were taken. We compare our high-z spectra with low-z normal and peculiar SNe Ia as well as with SNe Ic, Ib, and II. There are no significant differences between SN 1999ff and normal SNe Ia at low redshift. SN 1999fv appears to be a SN Ia and does not resemble the most peculiar nearby SNe Ia.

Living with lambda

This talk presents a brief overview of recent results pertaining to the cosmological constant ‘A’. I summarize the observational situation focussing on observations of high redshift Type Ia supernovae which suggest A > 0. Observations of small angular anisotropies in the cosmic microwave background complement Type Ia supernovae observations and both CMB and Sn can be combined to place strong constraints on the value of A. The presence of a small A-term increases the age of the universe and slows down the formation of large scale structure. I also review recent theoretical attempts to generate a small A-term at the current epoch and a model independent approach for determining the cosmic equation of state.

New quantum aspects of a vacuum-dominated universe

In a new model that we proposed, nonperturbative vacuum contributions to the effective action of a free quantized massive scalar field lead to a cosmological solution in which the scalar curvature becomes constant after a time ${t}_{j}$ (when the redshift $z\ensuremath{\sim}1)$ that depends on the mass of the scalar field and its curvature coupling. This spatially flat solution implies an accelerating universe at the present time and gives a good one-parameter fit to high-redshift type Ia supernovae (SNe-Ia) data, and the present age and energy density of the universe. Here we show that the imaginary part of the nonperturbative curvature term that causes the cosmological acceleration implies a particle production rate that agrees with predictions of other methods and extends them to non-zero mass fields. The particle production rate is very small after the transition and is not expected to alter the nature of the cosmological solution. We also show that the equation of state of our model undergoes a transition at ${t}_{j}$ from an equation of state dominated by non-relativistic pressureless matter (without a cosmological constant) to an effective equation of state of mixed radiation and cosmological constant, and we derive the equation of state of the vacuum. Finally, we explain why nonperturbative vacuum effects of this ultralow-mass particle do not significantly change standard early universe cosmology.

The case for the cosmological constant

I present a short overview of current observational results and theoretical models for a cosmological constant. The main motivation for invoking a small cosmological constant (or A-term) at the present epoch has to do with observations of high redshift Type Ia supernovae which suggest an accelerating universe. A flat accelerating universe is strongly favoured by combining supernovae observations with observations of CMB anisotropies on degree scales which give the ‘best-fit’ values ΘA ⋍ 0.7 and Θm ⋍ 0.3. A time dependent cosmological A-term can be generated by scalar field models with exponential and power law potentials. Some of these models can alleviate the ‘fine tuning’ problem which faces the cosmological constant.

Spinodal instabilities and the dark energy problem.

The accelerated expansion of the Universe measured by high redshift Type Ia supernova observations is explained using the nonequilibrium dynamics of naturally soft boson fields. Spinodal instabilities inevitably present in such systems yield a very attractive mechanism for arriving at the required equation of state at late times, while satisfying all the known constraints on models of quintessence.

Tests of the Accelerating Universe with Near‐Infrared Observations of a High‐Redshift Type Ia Supernova

We have measured the rest-frame B,V, and I-band light curves of a high-redshift type Ia supernova (SN Ia), SN 1999Q (z=0.46), using HST and ground-based near-infrared detectors. A goal of this study is the measurement of the color excess, E_{B-I}, which is a sensitive indicator of interstellar or intergalactic dust which could affect recent cosmological measurements from high-redshift SNe Ia. Our observations disfavor a 30% opacity of SN Ia visual light by dust as an alternative to an accelerating Universe. This statement applies to both Galactic-type dust (rejected at the 3.4 sigma confidence level) and greyer dust (grain size > 0.1 microns; rejected at the 2.3 to 2.6 sigma confidence level) as proposed by Aguirre (1999). The rest-frame $I$-band light cur ve shows the secondary maximum a month after B maximum typical of nearby SNe Ia of normal luminosi ty, providing no indication of evolution as a function of redshift out to z~0.5. A n expanded set of similar observations could improve the constraints on any contribution of extragalactic dust to the dimming of high-redshift SNe Ia.

The Rise Times of High- and Low-Redshift Type I[CLC]a[/CLC] Supernovae Are Consistent

We present a self-consistent comparison of the rise times for low- and high-redshift Type Ia supernovae. Following previous studies, the early light curve is modeled using a t-squared law, which is then mated with a modified Leibundgut template light curve. The best-fit t-squared law is determined for ensemble samples of low- and high-redshift supernovae by fitting simultaneously for all light curve parameters for all supernovae in each sample. Our method fully accounts for the non-negligible covariance amongst the light curve fitting parameters, which previous analyses have neglected. Contrary to Riess et al. (1999), we find fair to good agreement between the rise times of the low- and high-redshift Type Ia supernovae. The uncertainty in the rise time of the high-redshift Type Ia supernovae is presently quite large (roughly +/- 1.2 days statistical), making any search for evidence of evolution based on a comparison of rise times premature. Furthermore, systematic effects on rise time determinations from the high-redshift observations, due to the form of the late-time light curve and the manner in which the light curves of these supernovae were sampled, can bias the high-redshift rise time determinations by up to +3.6/-1.9 days under extreme situations. The peak brightnesses - used for cosmology - do not suffer any significant bias, nor any significant increase in uncertainty.

A New Measure of σ8Using the Lensing Dispersion in High‐zType Ia Supernovae

The gravitational lensing magni—cation or demagni—cation due to large-scale structures induces a scatter in peak magnitudes of high-redshift Type Ia supernovae (SNe Ia). The amplitude of the lensing dispersion strongly depends on that of density —uctuations characterized by the parameter. Therefore p 8 the value of is constrained by measuring the dispersion in the peak magnitudes. We examine how p 8 well SN Ia data will provide a constraint on the value of using a likelihood analysis method. It is p 8 found that the number and quality of SN Ia data needed for placing a useful constraint on is attain- p 8 able with the Next Generation Space Telescope. Subject headings: cosmology: observationscosmology: theorygravitational lensing ¨ large-scale structure of universesupernovae: general

The Acceleration of the Universe: Measurements of Cosmological Parameters from Type Ia Supernovae

The fate of the Universe, infinite expansion or a ldquo;big crunchrdquo;, can be determined by measuring the redshifts and brightness of very distant supernovae. These provide a record of changes in the expansion rate of the Universe over the past several billion years. The mass density, ΩM, and cosmological-constant energy density ΩΛ, are measured from a data-set consisting of 42 high-redshift Type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these supernovae, at redshifts between 0.18 and 0.83, are fit jointly with a set of supernovae from the Calán/Tololo Supernova Survey, at redshifts below 0.1, to yield values for the cosmological parameters. We find ΩMflat = 0.28+0.09-0.08 (1σ statistical)+0.05-0.04 (identified systematics). The data are strongly inconsistent with a Λ = 0 flat cosmology, the simplest inflationary universe model. An open, Λ = 0 cosmology also does not fit the data well: the data indicate that the cosmological constant is non-zero and positive, with a confidence of P(Λ>0) = 99%, including the identified systematic uncertainties. Thus, the Universe is found to be accelerating, i.e., q0 = ΩM/2 - ΩΛ < 0. The best-fit age of the universe relative to the Hubble time is t0flat = 14.9+1.4-1.1 (0.63/h) Gyr for a flat cosmology.

Is there an Indication of Evolution of Type [CLC]Ia[/CLC] Supernovae from their Rise Times?

We have compared the risetime for samples of nearby and high-redshift type Ia supernovae (SNe Ia). The fiducial risetime of the nearby SNe Ia is 2.5+/-0.4 days longer than the proemial risetime determined by Goldhaber (1998a,b) for high-redshift SNe Ia from the Supernova Cosmology Project. The statistical likelihood that the two samples have different fiducial risetimes is high (5.8 sigma) and indicates possible evolution between the samples of SNe Ia. We consider the likely effects of several sources of systematic error, but none of these resolves the difference in the risetimes. Currently, we cannot directly determine the impact of the apparent evolution on previous determinations of cosmological parameters.

Evolution of Dust Extinction and Supernova Cosmology.

We have made a quantitative calculation for the systematic evolution of the average extinction by interstellar dust in host galaxies of high-redshift Type Ia supernovae by using a realistic model of photometric and chemical evolution of galaxies and supernova rate histories in various galaxy types. We find that the average B-band extinction <AB> at z approximately 0.5 is typically 0.1-0.2 mag larger than the present value, under a natural assumption that dust optical depth is proportional to gas column density and gas metallicity. This systematic evolution causes average reddening with E(B-V&parr0; approximately 0.025-0.05 mag with the standard extinction curve, and this is comparable with the observational uncertainty of the reddening of high-redshift supernovae. Therefore, our result does not contradict the observations that show no significant reddening in high-z supernovae. However, the difference in apparent magnitude between an open universe and a Lambda-dominated flat universe is only approximately 0.2 mag at z approximately 0.5, and hence this systematic evolution of extinction should be taken into account in a reliable measurement of cosmological parameters. Considering this uncertainty, we show that it is difficult to discriminate between open and Lambda-dominated flat cosmologies from the current data.

Nonperturbative effects of vacuum energy on the recent expansion of the universe

We show that the vacuum energy of a free quantized field of very low mass can significantly alter the recent expansion of the universe. The effective action of the theory is obtained from a non-perturbative sum of scalar curvature terms in the propagator. We numerically investigate the semiclassical Einstein equations derived from it. As a result of non-perturbative quantum effects, the scalar curvature of the matter-dominated universe stops decreasing and approaches a constant value. The universe in our model evolves from an open matter-dominated epoch to a mildly inflating de Sitter expansion. The Hubble constant during the present de Sitter epoch, as well as the time at which the transition occurs from matter-dominated to de Sitter expansion, are determined by the mass of the field and by the present matter density. The model provides a theoretical explanation of the observed recent acceleration of the universe, and gives a good fit to data from high-redshift Type Ia supernovae, with a mass of about 10^{-33} eV, and a current ratio of matter density to critical density, Omega_0 <0.4 . The age of the universe then follows with no further free parameters in the theory, and turns out to be greater than 13 Gyr. The model is spatially open and consistent with the possibility of inflation in the very early universe. Furthermore, our model arises from the standard renormalizable theory of a free quantum field in curved spacetime, and does not require a cosmological constant or the associated fine-tuning.

The Distribution of High‐ and Low‐Redshift Type Ia Supernovae

The distribution of high-redshift Type Ia supernovae (SNe Ia) with respect to projected distance from the center of the host galaxy is studied and compared with the distribution of local SNe. The distribution of high-z SNe Ia is found to be similar to that of the local sample of SNe Ia discovered with CCDs, but different than that of the sample discovered photographically. This is shown to be due to the Shaw effect. These results have implications for the use of SNe Ia to determine cosmological parameters if the local sample of supernovae used to calibrate the light-curve decline relationships is drawn from a sample discovered photographically. A K-S test shows that the probability that the high-redshift SNe of the Supernova Cosmology Project are drawn from the same distribution as the low-redshift calibrators of Riess et al. is 0.1%. This is a potential problem because photographically discovered SNe are preferentially discovered farther away from the galaxy nucleus, where SNe show a lower scatter in absolute magnitude and are on average 0.3 mag fainter than SNe located closer to the center of their host galaxy. This raises questions about whether or not the calibration SNe sample the full range of parameters potentially present in high-redshift SNe Ia. The limited data available suggest that the calibration process is adequate; however, it would be preferable if high-redshift SNe and the low-redshift SNe used to calibrate them were drawn from the same sample, as subtle differences may be important. Data are also presented that suggest that the seeming anti-Malmquist trend noticed by Tammann et al. for SNe Ia in galaxies with Cepheid distances may be due to the location of the SNe in their host galaxies.

Gravitational lensing of high-redshift Type Ia supernovae: a probe of medium-scale structure

The dispersion in the peak luminosities of high redshift type Ia supernovae will change with redshift due to gravitational lensing. This lensing is investigated with an emphasis on the prospects of measuring it and separating it from other possible sources of redshift dependent dispersion. Measuring the lensing induced dispersion would directly constrain the power spectrum of density fluctuations on smaller length scales than are easily probed in any other way. The skew of the magnification distribution is related to the bispectrum of density fluctuations. Using cold dark matter models it is found that the amount and quality of data needed is attainable in a few years. A parameterization of the signal as a power law of the angular size distance to the supernovae is motivated by these models. This information can be used in detecting lensing, detecting other systematic changes in supernovae and calculating the uncertainties in cosmological parameter estimates.