High-redshift Supernova Data Research Articles

The Speed of Light Is Not Constant in Basic Big Bang Theory

Starting from the basic assumptions and equations of Big Bang theory, we present a simple mathematical proof that this theory implies a varying (decreasing) speed of light, contrary to what is generally accepted. We consider General Relativity, the first Friedmann equation and the Friedmann-Lema?tre- Robertson-Walker (FLRW) metric for a Comoving Observer. It is shown explicitly that the Horizon and Flatness Problems are solved, taking away an important argument for the need of Cosmic Inflation. A decrease of 2.1 cm/s per year of the present-day speed of light is predicted. This is consistent with the observed acceleration of the expansion of the Universe, as determined from high-redshift supernova data. The calculation does not use any quantum processes, and no adjustable parameters or fine tuning are introduced. It is argued that more precise laboratory measurements of the present-day speed of light (and its evolution) should be carried out. Also it is argued that the combination of the FLRW metric and Einstein’s field equations of General Relativity is inconsistent, because the FLRW metric implies a variable speed of light, and Einstein’s field equations use a constant speed of light. If we accept standard Big Bang theory (and thus the combination of General Relativity and the FLRW metric), a variable speed of light must be allowed in the Friedmann equation, and therefore also, more generally, in Einstein’s field equations of General Relativity. The explicit form of this time dependence will then be determined by the specific problem.

Early dark energy is not excluded by current large-scale structure data

We revisit the impact of early dark energy (EDE) on galaxy clustering using BOSS galaxy power spectra, analyzed using the effective field theory (EFT) of large-scale structure (LSS), and anisotropies of the cosmic microwave background (CMB) from Planck. Recent studies found that these data place stringent constraints on the maximum abundance of EDE allowed in the Universe. We argue here that their conclusions are a consequence of their choice of priors on the EDE parameter space, rather than any disagreement between the data and the model. For example, when considering EFT-LSS, CMB, and high-redshift supernovae data we find the EDE and $\Lambda$CDM models can provide statistically indistinguishable fits ($\Delta \chi^2 = 0.12$) with a relatively large value for the maximum fraction of energy density in the EDE ($f_{\rm ede} = 0.09$) and Hubble constant ($H_0 = 71$ km/s/Mpc) in the EDE model. Moreover, we demonstrate that the constraining power added from the inclusion of EFT-LSS traces to the potential tension between the power-spectrum amplitudes $A_s$ derived from BOSS and from Planck that arises even within the context of $\Lambda$CDM. Until this is better understood, caution should be used when interpreting EFT-BOSS+Planck constraints to models beyond $\Lambda$CDM. These findings suggest that EDE still provides a potential resolution to the Hubble tension and that it is worthwhile to test the predictions of EDE with future data-sets and further study its theoretical possibilities.

Testing unimodular gravity

We consider models of gravitation that are based on unimodular general coordinate transformations (GCT). These transformations include only those which do not change the determinant of the metric. We treat the determinant as a separate field which transforms as a scalar under unimodular GCT. We consider a class of such theories. In general, these theories do not transform covariantly under the full GCT. We characterize the violation of general coordinate invariance by introducing a new parameter. We show that the theory is consistent with observations for a wide range of this parameter. This parameter may serve as a test for possible violations of general coordinate invariance. We also consider the cosmic evolution within the framework of these models. We show that in general we do not obtain consistent cosmological solutions if we assume the standard cosmological constant or the standard form of non-relativistic matter. We propose a suitable generalization which is consistent with cosmology. We fit the resulting model to the high redshift supernova data. We find that we can obtain a good fit to this data even if include only a single component, either cosmological constant or non-relativistic matter.

Direction dependence in supernova data: constraining isotropy

We revise and extend the extreme value statistic, introduced in \cite{gup08}, to study directional dependence in the high redshift supernova data; arising either from departures from the cosmological principle or due to direction dependent statistical systematics in e data. We introduce a likelihood function that analytically marginalises over the Hubble constant, and use it to extend our previous statistic. We also introduce a new statistic that is sensitive to direction dependence arising from living off-centre inside a large void, as well as previously mentioned reasons for anisotropy. We show that for large data sets this statistic has a limiting form that can be computed analytically. We apply our statistics to the gold data sets from \cite{rie04} and \cite{rie07}, as in our previous work. Our revision and extension of previous statistic shows that 1) the effect of marginalsing over Hubble constant instead of using its best fit value has only a marginal effect on our results. However, correction of errors in our previous work reduce the level of non-Gaussianity in the 2004 gold data that was found in our earlier work. The revised results for the 2007 gold data show that the data is consistent with isotropy and Gaussianity. Our second statistic confirms these results.

Tracing the equation of state and the density of the cosmological constant alongz

We investigate the equation of statew(z) ina non-parametric form using the latest compilations of the luminosity distance from SNe Ia at highz. We combine the inverse problem approach with a Monte Carlo method to scan the spaceof priors. In the light of the latest high redshift supernova data sets, we reconstructw(z). A comparison between a sample including the latest results atz>1 and a sample without those results shows the improvement achieved through observations of very highz supernovae. We present the prospects for measuring the variation of dark energy density alongz by this method.

Direction dependence and non-Gaussianity in the high-redshift supernova data

The most detailed constraints on the accelerating expansion of the universe and details of nature of dark energy are derived from the high redshift supernova data, assuming that the errors in the measurements are Gaussian in nature. There is a possibility that there are direction dependent systematics in the data, either due to uncorrected, known physical processes or because there are tiny departures from the cosmological principle, making the universe slightly anisotropic. To investigate this possibility we introduce a statistic based on the extreme value theory and apply it to the gold data set from Riess et al. (2004). Our analysis indicates a systematic, direction dependent non-gaussianity at about one sigma level.

Bayesian Evidence for a cosmological constant using new high-redshift supernova data

We carry out a Bayesian model selection analysis of different dark energy parametrizations using the recent luminosity distance data of high redshift supernovae from Riess et al. 2007 and from the new ESSENCE Supernova Survey. Including complementary cosmological datasets, we found substantial evidence (�ln(E) � 1) against a time-varying dark energy equation of state parameter, and against phantom dark energy models. We find a small preference for a standard cosmological constant over accelerating non-phantom models where w is constant, but allowed to vary in the range 1 to 0.33.

Confronting dark energy models with astrophysical data: Non-equilibrium vs. conventional cosmologies

We discuss fits of cosmological dark energy models to the available data on high-redshift supernovae. We consider a conventional model with cold dark matter and a cosmological constant (ΛCDM), a model invoking super-horizon perturbations (SHCDM) and models based on Liouville strings in which dark energy is provided by a rolling dilaton field (Q-cosmology). We show that a complete treatment of Q-cosmology requires a careful discussion of non-equilibrium situations (off-shell effects). The two main high-redshift supernova data sets give compatible constraints on ΛCDM and the other models. We recover the well-known result that ΛCDM fits very well the combined supernova data sets, as does the super-horizon model. We discuss the model-dependent off-shell corrections to the Q-cosmology model that are relevant to the supernova data, and show that this model fits the data equally well. This analysis could be extended to other aspects of cosmological phenomenology, in particular to the CMB and baryon acoustic oscillations, which have so far been treated using on-shell models.

Confronting quintessence models with recent high-redshift supernovae data

We confront the predictions of different quintessence models with recent measurements of the luminosity distance from two sets of supernovae type Ia. In particular, we consider the 157 SNe Ia in the Gold dataset with $zl1.7$, and the more recent data containing 71 supernovae obtained by the Supernova Legacy Survey (SNLS) with $zl1$. We numerically solve the evolution equations for the Ratra-Peebles inverse power-law model, the double exponential and the hyperbolic cosine quintessence models. We obtain confidence regions from the two datasets in the ${\ensuremath{\Omega}}_{M}\ensuremath{-}\ensuremath{\alpha}$ and ${\ensuremath{\Omega}}_{M}\ensuremath{-}{w}_{\ensuremath{\phi}}$ planes for the different models and compare their predictions with dark energy models with constant equation of state.

Optimal extraction of cosmological information from supernova data in the presence of calibration uncertainties

We present a new technique to extract the cosmological information from high-redshift supernova data in the presence of calibration errors and extinction due to dust. While in the traditional technique the distance modulus of each supernova is determined separately, in our approach we determine all distance moduli at once, in a process that achieves a significant degree of self-calibration. The result is a much reduced sensitivity of the cosmological parameters to the calibration uncertainties. As an example, for a strawman mission similar to that outlined in the SNAP satellite proposal, the increased precision obtained with the new approach is roughly equivalent to a factor of five decrease in the calibration uncertainty.

DYNAMICS OF FERROMAGNETIC WALLS: GRAVITATIONAL PROPERTIES

We discuss a new mechanism which allows domain walls produced during the primordial electroweak phase transition. We show that the effective surface tension of these domain walls can be made vanishingly small due to a peculiar magnetic condensation induced by fermion zero modes localized on the wall. We find that in the perfect gas approximation the domain wall network behaves like a radiation gas. We consider the recent high-red shift supernova data and we find that the corresponding Hubble diagram is compatible with the presence in the Universe of an ideal gas of ferromagnetic domain walls. We show that our domain wall gas induces a completely negligible contribution to the large-scale anisotropy of the microwave background radiation.

Late universe expansion dominated by domain walls and dissipative dark matter

In this paper we show that a universe dominated by two components, namely, domain walls and dissipative dark matter, such that the former slowly decays into the latter, may drive power-law cosmological acceleration consistent with the high redshift supernovae data, while the ratio between the energy densities of both components remains fixed at late times thus solving the coincidence problem of present acceleration. Likewise, we estimate the aforesaid ratio at early times.

Exact model of conformal quintessence

A non--minimally coupled quintessence model is investigated and the conditions for a stationary solution to the coincidence problem are obtained. For a conformally coupled scalar field and dissipative matter, a general solution possessing late acceleration is found. It fits rather well the high redshift supernovae data and gives a good prediction of the age of the Universe. Likewise, the cold dark matter component dominates the cosmological perturbations at late times albeit they decrease with expansion.

Constraints on Cardassian expansion

High-redshift supernovae and cosmic microwave background (CMB) data are used to constrain the Cardassian expansion model of Freese & Lewis, a cosmology in which a modification to the Friedmann equation gives rise to a flat, matter-dominated universe which is undergoing a phase of accelerated expansion. In particular, the precision of the positions of the Doppler peaks in the CMB angular power spectrum provided by the Wilkinson Microwave Anisotropy Probe tightly constrains the cosmology. The available parameter space is further constrained by various high-redshift supernova data sets taken from Tonry et al., a sample of 230 supernovae collated from the literature, in which fits to the distance and extinction have been recomputed where possible and a consistent zero-point has been applied. In addition, the Cardassian model can also be loosely constrained by inferred upper limits on the epoch at which the Cardassian term in the modified Friedmann equation begins to dominate the expansion (z eq ). Using these methods, a Cardassian cosmology is constrained at the 2σ level to 0.19 1, as opposed to the supernova data which support a high-Ω m , low-n cosmology.

Is the present expansion of the Universe really accelerating?

The current observations are usually explained by an accelerating expansion of the present Universe. However, with the present quality of the supernovae Ia data, the allowed parameter space is wide enough to accommodate decelerating models as well. This is shown by considering a particular example of the dark energy equation of state w Φ ≡ p Φ /ρ Φ = -1/3, which is equivalent to modifying the geometrical curvature index k of the standard cosmology by shifting it to (k - a), where a is a constant. The resulting decelerating model is consistent with the recent cosmic microwave background observations made by WMAP, as well as with the high-redshift supernovae Ia data including SN 1997ff at z = 1.755. It is also consistent with the newly discovered supernovae SN 2002dc at z = 0.475 and SN 2002dd at z = 0.95, which have a general tendency to improve the fit.

Cosmological parameter estimation in the quintessence paradigm

We present cosmological parameter constraints on flat cosmologies dominated by dark energy using various cosmo- logical data including the recent Archeops angular power spectrum measurements. A likelihood analysis of the existing Cosmic Microwave Background data shows that in the absence of further prior, dark energy is not required. This comes from the fact that degeneracies exist among the various cosmological parameters constrained by the Cosmic Microwave Background. We found that there is a degeneracy in a combination of the Hubble parameter H0 and of the dark energy equation of state parame- terwQ ,b ut thatwQ is not correlated with the primordial index n of scalar fluctuations and the baryon content bh 2 . The preferred primordial index is n= 0:95 0:05 (68%) and baryon contentbh 2 = 0:021 0:003. Adding constraints on the amplitude of matter fluctuations on small scales, 8, obtained from clusters abundance or weak lensing data may allow degenaracies to be broken, although present-day systematic uncertainties do not allow for firm conclusions as yet. Further addition of the Hubble Space Telescope measurements of the local distance scale and of the high redshift supernovae data allow one to obtain tight constraints. When these constraints are combined we find that the amount of dark energy is 0:7 +0:10 0:07 (95% C.L.) and that its equation of state is very close to those of the vacuum:wQ 95% C.L.). In no case do we find that quintessence is prefered over the classical cosmological constant, although robust data on8 might rapidly shed light on this important issue.

Discriminating between models for the dark energy

Recent measurements suggest our universe has a substantial dark energy component, which is usually interpreted in terms of a cosmological constant. Here we examine how much the form of this dark energy can be modified while still retaining an acceptable fit to the high redshift supernova data. We first consider changes in the dark energy equation of state and then explore a model in which the dark energy is interpreted as a fluid with a bulk viscosity.

SNOC: A Monte-Carlo simulation package for high-zsupernova observations

We present a Monte-Carlo package for simulation of high-redshift supernova data, SNOC. Optical and near-infrared photons from supernovae are ray-traced over cosmological distances from the simulated host galaxy to the observer at Earth. The distances to the sources are calculated from user provided cosmological parameters in a Friedmann-Lemaˆ itre universe, allowing for arbitrary forms of dark energy. The code takes into account gravitational interactions (lensing) and extinction by dust, both in the host galaxy and in the line-of-sight. The user can also choose to include exotic eects like a hypothetical attenuation due to photon-axion oscillations. SNOC is primarily useful for estimations of cosmological parameter uncertainties from studies of apparent brightness of type Ia supernovae vs redshift, with special emphasis on potential systematic eects. It can also be used to compute standard cosmological quantities like luminosity distance, lookback time and age of the universe in any Friedmann-Lemaˆ itre model with or without quintessence.

Constraining the dark universe

We combine complementary datasets to constrain dark energy. Using standard Big Bang Nucleosynthesis and the observed abundances of primordial nuclides to put constraints on Ω Q at temperatures near T ∼ 1 MeV, we find the strong constraint Ω Q (MeV) < 0.045 at 2σ c.l.. Under the assumption of flatness, using results from Cosmic Microwave Background (CMB) anisotropy measurements, high redshift supernovae (SN-Ia) observations and data from local cluster abundances we put a new constraint on the equation of state parameter w Q < −0.85 at 68% c.l..

A Universal Probability Distribution Function for Weak-lensing Amplification

We present an approximate form for the weak lensing magnification distribution of standard candles, valid for all cosmological models, with arbitrary matter distributions, over all redshifts. Our results are based on a universal probability distribution function (UPDF), $P(\eta)$, for the reduced convergence, $\eta$. For a given cosmological model, the magnification probability distribution, $P(\mu)$, at redshift $z$ is related to the UPDF by $P(\mu)=P(\eta)/2|\kappa_{min}|$, where $\eta=1+(\mu-1)/(2|\kappa_{min}|)$, and $\kappa_{min}$ (the minimum convergence) can be directly computed from the cosmological parameters ($\Omega_m$ and $\Omega_\Lambda$). We show that the UPDF can be well approximated by a three-parameter stretched Gaussian distribution, where the values of the three parameters depend only on $\xi_\eta$, the variance of $\eta$. In short, all possible weak lensing probability distributions can be well approximated by a one-parameter family. We establish this family, normalizing to the numerical ray-shooting results for a ${\Lambda}$CDM model by Wambsganss et al. (1997). Each alternative cosmological model is then described by a single function $\xi_\eta(z)$. We find that this method gives $P(\mu)$ in excellent agreement with numerical ray-tracing and three-dimensional shear matrix calculations, and provide numerical fits for three representative models (SCDM, $\Lambda$CDM, and OCDM). Our results provide an easy, accurate, and efficient method to calculate the weak lensing magnification distribution of standard candles, and should be useful in the analysis of future high-redshift supernova data.