Diagram Of Type Ia Supernovae Research Articles

Redshift evolution and non-universal dispersion of quasar luminosity correlation

ABSTRACT The standard Λ cold dark matter (ΛCDM) model is recently reported to deviate from the high-redshift Hubble diagram of Type Ia supernovae (SNe) and quasars (QSOs) at ∼4σ confidence level. In this work, we combine the PAge approximation (a nearly model-independent parametrization) and a high-quality QSO sample to search for the origins of the deviation. By visualizing the ΛCDM model and the marginalized 3σ constraints of SNe+QSOs into the PAge space, we confirm that the SNe+QSO constraints in both flat and non-flat PAge cases are in remarkable tension with the standard ΛCDM cosmology. Next, we investigate the tension from the perspective of redshift-evolution effects. We find that the QSO correlation coefficient γ calibrated by SNe+low-z QSOs and SNe+high-z QSOs shows ∼2.7σ and ∼4σ tensions in flat and non-flat universes, respectively. The tensions for intrinsic dispersion δ between different data sets are found to be >4σ in both flat and non-flat cases. These results indicate that the QSO luminosity correlation suffers from significant redshift evolution and non-universal intrinsic dispersion. Using a redshift-dependence correlation to build QSO Hubble diagram could lead to biases. Thus, the ∼4σ deviation from the standard ΛCDM probably originates from the redshift-evolution effects and non-universal dispersion of the QSO luminosity correlation rather than new physics.

Peculiar motion of Solar system from the Hubble diagram of supernovae Ia and its implications for cosmology

ABSTRACTPeculiar motion of the Solar system, determined from the dipole anisotropy in the Cosmic Microwave Background Radiation (CMBR), has given a velocity 370 km s−1 along RA = 168°, Dec. = −7°. Subsequent peculiar motion determinations from the number counts, sky brightness, or redshift dipoles observed in large samples of distant radio galaxies and quasars yielded peculiar velocities 2 to 10 times larger than CMBR, though in all cases the directions matched with the CMBR dipole. Here, we introduce a novel technique for determining the peculiar motion from the magnitude-redshift (mB–z) Hubble diagram of Type Ia Supernovae (SN Ia), one of the best standard candles available. We find a peculiar velocity 1.6 ± 0.5 × 103 km s−1, larger than the CMBR value roughly by a factor of four, along RA = 173° ± 12°, Dec. = 10° ± 9°, the direction being within $\stackrel{\lt }{_{\sim }}2\sigma$ of the CMBR dipole. Since a genuine solar motion would not depend upon the method or the data set employed, large discrepancies seen among various dipole amplitudes could imply that these dipoles, including the CMBR one, might not pertain to observer’s peculiar motion. However, a common direction for various dipoles might indicate a preferred direction in the Universe, implying an intrinsic anisotropy, in violation of the cosmological principle, a cornerstone of the modern cosmology.

Tension with the flat ΛCDM model from a high-redshift Hubble diagram of supernovae, quasars, and gamma-ray bursts

In the current framework, the standard parametrization of our Universe is the so-called Lambda cold dark matter (ΛCDM) model. Recently, a ∼4σ tension with the ΛCDM model was shown to exist via a model-independent parametrization of a Hubble diagram of type Ia supernovae (SNe Ia) from the JLA survey and quasars. Model-independent approaches and independent samples over a wide redshift range are key to testing this tension and any possible systematic errors. Here we present an analysis of a combined Hubble diagram of SNe Ia, quasars, and gamma-ray bursts (GRBs) to check the agreement of the quasar and GRB cosmological parameters at high redshifts (z > 2) and to test the concordance flat ΛCDM model with improved statistical accuracy. We build a Hubble diagram with SNe Ia, quasars, and GRBs, where quasars are standardised through the observed non-linear relation between their ultraviolet and X-ray emission and GRBs through the correlation between the spectral peak energy and the isotropic-equivalent radiated energy (the so-called Amati relation). We fit the data with cosmographic models consisting of a fourth-order logarithmic polynomial and a fifth-order linear polynomial, and compare the results with the expectations from a flat ΛCDM model. We confirm the tension between the best-fit cosmographic parameters and the ΛCDM model at ∼4σ with SNe Ia and quasars, at ∼2σ with SNe Ia and GRBs, and at > 4σ with the whole SNe Ia+quasars+GRB data set. The completely independent high-redshift Hubble diagrams of quasars and GRBs are fully consistent with each other, strongly suggesting that the deviation from the standard model is not due to unknown systematic effects but to new physics.

Revisiting Ryskin’s model of cosmic acceleration

Cosmic backreaction as an additional source of the expansion of the universe has been a debate topic since the discovery of cosmic acceleration. The major concern is whether the self interaction of small-scale nonlinear structures would source gravity on very large scales. Gregory Ryskin argued against the additional inclusion of gravitational interaction energy of astronomical objects, whose masses are mostly inferred from gravitational effects and hence should already contain all sources with long-range gravity forces. Ryskin proposed that the backreaction contribution to the energy momentum tensor comes instead from the rest of the universe beyond the observable patch. Ryskin’s model solves the fine-tuning problem and is in good agreement with the Hubble diagram of Type Ia supernovae. In this article we revisit Ryskin’s model and show that it is inconsistent with at least one of the following statements: (i) the universe is matter-dominated at low redshift (z ≲ 2); (ii) the universe is radiation-dominated at sufficiently high redshift; (iii) matter density fluctuations are tiny (≲10−4) at the recombination epoch.

Primordial black holes survive SN lensing constraints

It has been claimed in [arxiv:1712.02240] that massive primordial black holes (PBH) cannot constitute all of the dark matter (DM), because their gravitational-lensing imprint on the Hubble diagram of type Ia supernovae (SN) would be incompatible with present observations. In this note, we critically review those constraints and find several caveats on the analysis. First of all, the constraints on the fraction α of PBH in matter seem to be driven by a very restrictive choice of priors on the cosmological parameters. In particular, the degeneracy between Ωm and α was ignored and thus, by fixing Ωm, transferred the constraining power of SN magnitudes to α. Furthermore, by considering more realistic physical sizes for the type-Ia supernovae, we find an effect on the SN lensing magnification distribution that leads to significantly looser constraints. Moreover, considering a wide mass spectrum of PBH, such as a lognormal distribution, further softens the constraints from SN lensing. Finally, we find that the fraction of PBH that could constitute DM today is bounded by fPBH<1.09(1.38), for JLA (Union 2.1) catalogs, and thus it is perfectly compatible with an all-PBH dark matter scenario in the LIGO band.

A Thermodynamic Point of View on Dark Energy Models

We present a conjugate analysis of two different dark energy models, namely the Barboza–Alcaniz parameterization and the phenomenologically-motivated Hobbit model, investigating both their agreement with observational data and their thermodynamical properties. We successfully fit a wide dataset including the Hubble diagram of Type Ia Supernovae, the Hubble rate expansion parameter as measured from cosmic chronometers, the baryon acoustic oscillations (BAO) standard ruler data and the Planck distance priors. This analysis allows us to constrain the model parameters, thus pointing at the region of the wide parameters space, which is worth focusing on. As a novel step, we exploit the strong connection between gravity and thermodynamics to further check models’ viability by investigating their thermodynamical quantities. In particular, we study whether the cosmological scenario fulfills the generalized second law of thermodynamics, and moreover, we contrast the two models, asking whether the evolution of the total entropy is in agreement with the expectation for a closed system. As a general result, we discuss whether thermodynamic constraints can be a valid complementary way to both constrain dark energy models and differentiate among rival scenarios.

Marginal evidence for cosmic acceleration from Type Ia supernovae.

The ‘standard’ model of cosmology is founded on the basis that the expansion rate of the universe is accelerating at present — as was inferred originally from the Hubble diagram of Type Ia supernovae. There exists now a much bigger database of supernovae so we can perform rigorous statistical tests to check whether these ‘standardisable candles’ indeed indicate cosmic acceleration. Taking account of the empirical procedure by which corrections are made to their absolute magnitudes to allow for the varying shape of the light curve and extinction by dust, we find, rather surprisingly, that the data are still quite consistent with a constant rate of expansion.

Gravity in general relativity, attractive and repulsive contributions

The gravitational equations of Einstein are solved for a sphere filled with a dust gas and floating in infinite empty space. It turns out that the radial acceleration of the gas has negative and positive contributions which may be interpreted as attractive and repulsive gravitational forces respectively. Two cases are considered: the collapse of a gas initially at rest and with uniform density, and the expansion of the gas with initial conditions appropriate to the observed Hubble diagram of Type Ia supernovae. The latter case may be seen as a proposal for a new type of cosmology.

Constructing a cosmological model-independent Hubble diagram of type Ia supernovae with cosmic chronometers

We apply two methods, namely the Gaussian processes and the non-parametric smoothing procedure, to reconstruct the Hubble parameter $H(z)$ as a function of redshift from 15 measurements of the expansion rate obtained from age estimates of passively evolving galaxies. These reconstructions enable us to derive the luminosity distance to a certain redshift $z$, calibrate the light-curve fitting parameters accounting for the (unknown) intrinsic magnitude of type Ia supernova (SNe Ia) and construct cosmological model-independent Hubble diagrams of SNe Ia. In order to test the compatibility between the reconstructed functions of $H(z)$, we perform a statistical analysis considering the latest SNe Ia sample, the so-called JLA compilation. We find that, for the Gaussian processes, the reconstructed functions of Hubble parameter versus redshift, and thus the following analysis on SNe Ia calibrations and cosmological implications, are sensitive to prior mean functions. However, for the non-parametric smoothing method, the reconstructed functions are not dependent on initial guess models, and consistently require high values of $H_0$, which are in excellent agreement with recent measurements of this quantity from Cepheids and other local distance indicators.

The emergence of cosmic repulsion

In cosmology based on general relativity, the universe is modeled as a fluid. The transition from the Einstein field equation to its large-scale (cosmological) version is thus analogous to the transition, for a system consisting of a large number of molecules, from the molecular/kinetic description to thermodynamics and hydrodynamics. The cosmic fluid is an effective continuum defined on the cosmological scales (only); for such a continuum, the appearance of new emergent properties should be expected. (Emergence of space–time and gravity is not discussed here.) When these new properties are calculated, the following predictions result: (a) the universe is spatially flat; (b) its expansion is accelerating; (c) dark energy makes up 75% of the total energy density of the universe; (d) the pressure of dark energy is equal and opposite to its density. All of these are in good agreement with the observational data. Also in favor of the present model are the absence of adjustable parameters, and consistency with the second law of thermodynamics. The distance–redshift relation predicted by the model is in good agreement with the Hubble diagram of Type Ia supernovae.

Observational tests of non-adiabatic Chaplygin gas

In a previous paper [1] it was shown that any dark sector model can be mapped into a non-adiabatic fluid formed by two interacting components, one with zero pressure and the other with equation-of-state parameter ω = -1. It was also shown that the latter does not cluster and, hence, the former is identified as the observed clustering matter. This guarantees that the dark matter power spectrum does not suffer from oscillations or instabilities. It applies in particular to the generalised Chaplygin gas, which was shown to be equivalent to interacting models at both background and perturbation levels. In the present paper we test the non-adiabatic Chaplygin gas against the Hubble diagram of type Ia supernovae, the position of the first acoustic peak in the anisotropy spectrum of the cosmic microwave background and the linear power spectrum of large scale structures. We consider two different compilations of SNe Ia, namely the Constitution and SDSS samples, both calibrated with the MLCS2k2 fitter, and for the power spectrum we use the 2dFGRS catalogue. The model parameters to be adjusted are the present Hubble parameter, the present matter density and the Chaplygin gas parameter α. The joint analysis best fit gives α ≈ - 0.5, which corresponds to a constant-rate energy flux from dark energy to dark matter, with the dark energy density decaying linearly with the Hubble parameter. The ΛCDM model, equivalent to α = 0, stands outside the 3σ confidence interval.

An updated analysis of two classes of f(R) theories of gravity

The observed accelerated cosmic expansion can be a signature of fourth-order gravity theories, where the acceleration of the Universe is a consequence of departures from Einstein General Relativity, rather than the sign of the existence of a fluid with negative pressure. In the fourth-order gravity theories, the gravity Lagrangian is described by an analytic function f(R) of the scalar curvature R subject to the demanding conditions that no detectable deviations from standard GR is observed on the Solar System scale. Here we consider two classes of f(R) theories able to pass Solar System tests and investigate their viability on cosmological scales. To this end, we fit the theories to a large dataset including the combined Hubble diagram of Type Ia Supernovae and Gamma Ray Bursts, the Hubble parameter H(z) data from passively evolving red galaxies, Baryon Acoustic Oscillations extracted from the seventh data release of the Sloan Digital Sky Survey (SDSS) and the distance priors from the Wilkinson Microwave Anisotropy Probe seven years (WMAP7) data. We find that both classes of f(R) fit very well this large dataset with the present-day values of the matter density, Hubble constant and deceleration parameter in agreement with previous estimates; however, the strong degeneracy among the f(R) parameters prevents us from strongly constraining their values. We also derive the growth factor g = dln δ/dln a, with δ = δρM/ρM the matter density perturbation, and show that it can still be well approximated by g(z)∝ΩM(z)γ. We finally constrain γ (on some representative scales) and investigate its redshift dependence to see whether future data can discriminate between these classes of f(R) theories and standard dark energy models.

PROBING COSMIC ACCELERATION WITH GALAXY REDSHIFT SURVEYS

Redshift surveys of galaxies beyond the local Universe (z ≫ 0.1) are opening up new possibilities to understanding the observed acceleration of cosmic expansion, one of the greatest mysteries of modern science. Baryonic Acoustic Oscillations in the galaxy power spectrum (or correlation function), provide us with a standard rod to measure the expansion history H(z). At the same time, redshift-space distortions in the clustering pattern due to galaxy peculiar motions are a measure of the growth rate of structure f(z). The combination of these two quantities, allows us to distinguish whether cosmic acceleration is due to the existence of a "dark energy" in the cosmic budget, or rather requires a modification of General Relativity. These two radically alternative scenarios are degenerate when considering H(z) alone, as yielded, e.g. by the Hubble diagram of Type Ia supernovae. In this short review paper I will mostly concentrate on the latter measurement, whose potential importance in this context has been recently highlighted. Current results are consistent with the simplest GR-based cosmological constant scenario, but error bars are still large. Detailed forecasts show that next-generation deep surveys optimizing the combination of large volumes and good galaxy sampling will be able to use redshift distortions as a key tool to understand the physical origin of cosmic acceleration. Among these, I introduce the newly started VIMOS Public Extragalactic Redshift Survey (VIPERS) at the ESO VLT, which is building at [Formula: see text] a sample comparable to the local 2dFGRS. Expectations from even larger surveys planned from space-borne observatories such as EUCLID will also be mentioned.

New constraints on parametrised modified gravity from correlations of the CMB with large scale structure

We study the effects of modified theories of gravity on the cosmic microwave background (CMB) anisotropies power spectrum, and in particular on its large scales, where the integrated Sachs-Wolfe (ISW) effect is important. Starting with a general parametrisation, we then specialise to f(R) theories and theories with Yukawa-type interactions between dark matter particles. In these models, the evolution of the metric potentials is altered, and the contribution to the ISW effect can differ significantly from that in the standard model of cosmology. We proceed to compare these predictions with observational data for the CMB and the ISW, performing a full Monte Carlo Markov chain (MCMC) analysis. In the case of f(R) theories, the result is an upper limit on the lengthscale associated to the extra scalar degree of freedom characterising these theories. With the addition of data from the Hubble diagram of Type Ia supernovae, we obtain an upper limit on the lengthscale of the theory of B0 < 0.4, or correspondingly λ1 < 1900 Mpc/h at 95% c.l. improving previous CMB constraints. For Yukawa-type models we get a bound on the coupling 0.75 < β1 < 1.25 at the 95% c.l. We also discuss the implications of the assumed priors on the estimation of modified gravity parameters, showing that a marginally less conservative choice improves the f(R) constraints to λ1 < 1400 Mpc/h, corresponding to B0 < 0.2 at 95% c.l.

Cosmography off(R)gravity

It is nowadays accepted that the universe is undergoing a phase of accelerated expansion as tested by the Hubble diagram of type Ia supernovae (SNeIa) and several large scale structure observations. Future SNeIa surveys and other probes will make it possible to better characterize the dynamical state of the universe, renewing the interest in cosmography which allows a model independent analysis of the distance-redshift relation. On the other hand, fourth order theories of gravity, also referred to as $f(R)$ gravity, have attracted a lot of interest since they could be able to explain the accelerated expansion without any dark energy. We show here how it is possible to relate the cosmographic parameters (namely, the deceleration ${q}_{0}$, the jerk ${j}_{0}$, the snap ${s}_{0}$, and the lerk ${l}_{0}$ parameters) to the present-day values of $f(R)$ and its derivatives ${f}^{(n)}(R)={d}^{n}f/d{R}^{n}$ (with $n=1$, 2, 3), thus offering a new tool to constrain such higher order models. Our analysis thus offers the possibility to relate the model independent results coming from cosmography to the theoretically motivated assumptions of $f(R)$ cosmology.

Halting eternal acceleration with an effective negative cosmological constant

In order to solve the problem of eternal acceleration, a model has been recently proposed including both a negative cosmological constant Λ and a scalar field evolving under the action of an exponential potential. We further explore this model by contrasting it against the Hubble diagram of type Ia supernovae, the gas mass fraction in galaxy clusters and the acoustic peak and shift parameters. It turns out that the model is able to fit this large dataset quite well so that we conclude that a negative Λ is indeed allowed and could represent a viable mechanism to halt eternal acceleration. In order to avoid problems with theoretical motivations for both a negative Λ term and the scalar field, we reconstruct the gravity Lagrangian f(R) of a fourth-order theory of gravity predicting the same dynamics (scale factor and Hubble parameter) as the starting model. We thus end up with a f(R) theory able to both fit the data and solve the problem of eternal acceleration without the need of unusual negative Λ and ad hoc scalar fields.

THE VARIATION OF THE GRAVITATIONAL CONSTANT INFERRED FROM THE HUBBLE DIAGRAM OF TYPE Ia SUPERNOVAE

We consider a cosmological model with a variable gravitational constant, G, based on a scalar–tensor theory. Using the recent observational data for the Hubble diagram of type Ia supernovae (SNeIa), we find a phenomenological expression describing the variation of G. The corresponding variation of the fine structure constant α within multidimensional theories is also computed and is shown not to support known constraints on Δα/α.

F(R) theories of gravity in the Palatini approach matched with observations

We investigate the viability of f(R) theories in the framework of the Palatini approach as solutions to the problem of the observed accelerated expansion of the universe. Two physically motivated popular choices for f(R) are considered,: power law, f(R) = β Rn, and logarithmic, f(R) = α ln R. Under the Palatini approach, both Lagrangians give rise to cosmological models comprising only standard matter and undergoing a present phase of accelerated expansion. We use the Hubble diagram of type Ia Supernovae and the data on the gas mass fraction in relaxed galaxy clusters to see whether these models are able to reproduce what is observed and to constrain their parameters. It turns out that they are indeed able to fit the data with values of the Hubble constant and of the matter density parameter in agreement with some model independent estimates, but the today deceleration parameter is higher than what is measured in the concordance ΛCDM model.

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.

The mystery of the cosmic vacuum energy density and the accelerated expansion of the Universe

After a short history of the -term it is explained why the (effective) cosmological constant is expected to obtain contributions from short-distance physics, corresponding to an energy scale of at least 100 GeV. The actual tiny value of the cosmological constant in any natural scale of units represents, therefore, one of the deepest mysteries of present-day fundamental physics. We also briefly discuss recent astronomical evidence for a cosmologically significant vacuum energy density causing an accelerating expansion of the universe. This arises mainly from the Hubble diagram of type Ia supernovae and from the observed temperature fluctuations of the cosmic microwave background radiation. If this should become an established fact, we are also confronted with a disturbing cosmic coincidence problem.