Jun Jie Wei

THE GAMMA-RAY BURST HUBBLE DIAGRAM AND ITS IMPLICATIONS FOR COSMOLOGY

In this paper, we continue to build support for the proposal to use gamma-ray bursts (GRBs) as standard candles in constructing the Hubble Diagram at redshifts beyond the current reach of Type Ia supernova observations. We confirm that correlations among certain spectral and lightcurve features can indeed be used as luminosity indicators, and demonstrate from the most up-to-date GRB sample appropriate for this work that the ΛCDM model optimized with these data is characterized by parameter values consistent with those in the concordance model. Specifically, we find that (Ωm,ΩΛ) ≈ (0.30, 0.70), versus (0.27, 0.73) obtained from the 5-yr WMAP data. We also carry out a comparative analysis between ΛCDM and the Rh = ct Universe and show that the latter is a better fit to the GRB data. We find that the optimal ΛCDM model fits the GRB Hubble Diagram with a reduced χdof ≈ 1.79, whereas the fit using Rh = ct results in a χdof ≈ 1.66. In both cases, about 20% of the events lie at least 2σ away from the best-fit curves, suggesting that either some contamination by non-standard GRB luminosities is unavoidable, or that the errors and intrinsic scatter associated with the data are being underestimated. Subject headings: cosmology: observations, redshift, theory; early universe; gamma-ray bursts: generalIn this paper, we continue to build support for the proposal to use gamma-ray bursts (GRBs) as standard candles in constructing the Hubble diagram at redshifts beyond the current reach of Type Ia supernova observations. We confirm that correlations among certain spectral and light-curve features can indeed be used as luminosity indicators, and demonstrate from the most up-to-date GRB sample appropriate for this work that the Lambda CDM model optimized with these data is characterized by parameter values consistent with those in the concordance model. Specifically, we find that (Omega(m), Omega(Lambda)) approximate to (0.25(-0.06)(+0.05), 0.75(-0.05)(+0.06)), which are consistent, to within 1 sigma, with (0.29, 0.71) obtained from the 9 yr Wilkinson Microwave Anisotropy Probe data. We also carry out a comparative analysis between Lambda CDM and the R-h = ct universe and find that the optimal Lambda CDM model fits the GRB Hubble diagram with a reduced chi(2)(dof) approximate to 2.26, whereas the fit using R-h = ct results in a chi(2)(dof) approximate to 2.14. In both cases, about 20% of the events lie at least 2 sigma away from the best-fit curves, suggesting that either some contamination by non-standard GRB luminosities is unavoidable or that the errors and intrinsic scatter associated with the data are being underestimated. With these optimized fits, we use three statistical tools-the Akaike information criterion, the Kullback information criterion, and the Bayes information criterion-to show that, based on the GRB Hubble diagram, the likelihood of R-h = ct being closer to the correct model is similar to 85%-96%, compared to similar to 4%-15% for Lambda CDM.

Cosmological tests using gamma-ray bursts, the star formation rate and possible abundance evolution

The principal goal of this paper is to use attempts at reconciling the Swift long gamma-ray bursts (LGRBs) with the star formation history (SFH) to compare the predictions of Lambda cold dark matter (Lambda CDM) with those in the R-h = ct Universe. In the context of the former, we confirm that the latest Swift sample of GRBs reveals an increasing evolution in the GRB rate relative to the star formation rate (SFR) at high redshifts. The observed discrepancy between the GRB rate and the SFR may be eliminated by assuming a modest evolution parametrized as (1 + z)(0.8) - perhaps indicating a cosmic evolution in metallicity. However, we find a higher metallicity cut of Z = 0.52 Z(circle dot) than was seen in previous studies, which suggested that LGRBs occur preferentially in metal-poor environments, i.e. Z similar to 0.1-0.3 Z(circle dot). We use a simple power-law approximation to the high-z ( greater than or similar to 3.8) SFH, i.e. R-SF proportional to [(1 + z)/4.8](alpha), to examine how the high-z SFR may be impacted by a possible abundance evolution in the Swift GRB sample. For an expansion history consistent with Lambda CDM, we find that the Swift redshift and luminosity distributions can be reproduced with reasonable accuracy if alpha = -2.41(-2.09)(+1.87). For the R-h = ct Universe, the GRB rate is slightly different from that in Lambda CDM, but also requires an extra evolutionary effect, with a metallicity cut of Z = 0.44 Z(circle dot). Assuming that the SFR and GRB rate are related via an evolving metallicity, we find that the GRB data constrain the slope of the high-z SFR in R-h = ct to be alpha = -3.60(-2.45)(+2.45). Both cosmologies fit the GRB/SFR data rather well. However, in a one-on-one comparison using the Akaike information criterion, the best-fitting R-h = ct model is statistically preferred over the best-fitting Lambda CDM model with a relative probability of similar to 70 per cent versus similar to 30 per cent.

A COMPARATIVE ANALYSIS OF THE SUPERNOVA LEGACY SURVEY SAMPLE WITH ΛCDM AND THE R h = ct UNIVERSE

The use of Type Ia supernovae (SNe Ia) has thus far produced the most reliable measurement of the expansion history of the universe, suggesting that Lambda CDM offers the best explanation for the redshift-luminosity distribution observed in these events. However, analysis of other kinds of sources, such as cosmic chronometers, gamma-ray bursts, and high-z quasars, conflicts with this conclusion, indicating instead that the constant expansion rate implied by the R-h = ct universe is a better fit to the data. The central difficulty with the use of SNe Ia as standard candles is that one must optimize three or four nuisance parameters characterizing supernova (SN) luminosities simultaneously with the parameters of an expansion model. Hence, in comparing competing models, one must reduce the data independently for each. We carry out such a comparison of Lambda CDM and the R-h = ct universe using the SN Legacy Survey sample of 252 SN events, and show that each model fits its individually reduced data very well. However, since R-h = ct has only one free parameter (the Hubble constant), it follows from a standard model selection technique that it is to be preferred over Lambda CDM, the minimalist version of which has three (the Hubble constant, the scaled matter density, and either the spatial curvature constant or the dark energy equation-of-state parameter). We estimate using the Bayes Information Criterion that in a pairwise comparison, the likelihood of R-h = ct is similar to 90%, compared with only similar to 10% for a minimalist form of Lambda CDM, in which dark energy is simply a cosmological constant. Compared to R-h = ct, versions of the standard model with more elaborate parametrizations of dark energy are judged to be even less likely.

A Comparison of Cosmological Models Using Time Delay Lenses

The use of time-delay gravitational lenses to examine the cosmological expansion introduces a new standard ruler with which to test theoretical models. The sample suitable for this kind of work now includes 12 lens systems, which have thus far been used solely for optimizing the parameters of ACDM. In this paper, we broaden the base of support for this new, important cosmic probe by using these observations to carry out a one-on-one comparison between competing models. The currently available sample indicates a likelihood of similar to 70%-80% that the R-h = ct universe is the correct cosmology versus similar to 20%-30% for the standard model. This possibly interesting result reinforces the need to greatly expand the sample of time-delay lenses, e.g., with the successful implementation of the Dark Energy Survey, the VST ATLAS survey, and the Large Synoptic Survey Telescope. In anticipation of a greatly expanded catalog of time-delay lenses identified with these surveys, we have produced synthetic samples to estimate how large they would have to be in order to rule out either model at a similar to 99.7% confidence level. We find that if the real cosmology is ACDM, a sample of similar to 150 time-delay lenses would be sufficient to rule out R-h = ct at this level of accuracy, while similar to 1000 time-delay lenses would be required to rule out ACDM if the real universe is instead R-h = ct. This difference in required sample size reflects the greater number of free parameters available to fit the data with ACDM.

Cosmological tests using the angular size of galaxy clusters

We use measurements of the galaxy-cluster angular size versus redshift to test and compare the standard model (Lambda CDM) and the R-h = ct Universe. We show that the latter fits the data with a reduced chi(2)(dof) = 0.786 for a Hubble constant H-0 = 72.6(-3.4)(+3.8) km s(-1) Mpc(-1), and H-0 is the sole parameter in this model. By comparison, the optimal flat Lambda cold dark matter (Lambda CDM) model, with two free parameters (including Omega(m) = 0.50 and H-0 = 73.9(-9.5)(+10.6) km s(-1) Mpc(-1)), fits the angular-size data with a reduced chi(2)(dof) = 0.806. On the basis of their chi(2)(dof) values alone, both models appear to account for the data very well in spite of the fact that the R-h = ct Universe expands at a constant rate, while Lambda CDM does not. However, because of the different number of free parameters in these models, selection tools, such as the Bayes Information Criterion, favour R-h = ct over Lambda CDM with a likelihood of similar to 86 per cent versus similar to 14 per cent. These results impact the question of galaxy growth at large redshifts. Previous work suggested an inconsistency with the underlying cosmological model unless elliptical and disc galaxies grew in size by a surprisingly large factor similar to 6 from z similar to 3 to 0. The fact that both Lambda CDM and R-h = ct fit the cluster-size measurements quite well casts some doubt on the suggestion that the unexpected result with individual galaxies may be due to the use of an incorrect expansion scenario, rather than astrophysical causes, such as mergers and/or selection effects.

A Comparison of Cosmological Models Using Strong Gravitational Lensing Galaxies

Strongly gravitationally lensed quasar-galaxy systems allow us to compare competing cosmologies as long as one can be reasonably sure of the mass distribution within the intervening lens. In this paper, we assemble a catalog of 69 such systems, and carry out a one-on-one comparison between the standard model, LCDM, and the R_h=ct Universe. We find that both models account for the lens observations quite well, though the precision of these measurements does not appear to be good enough to favor one model over the other. Part of the reason is the so-called bulge-halo conspiracy that, on average, results in a baryonic velocity dispersion within a fraction of the optical effective radius virtually identical to that expected for the whole luminous-dark matter distribution. Given the limitations of doing precision cosmological testing using the current sample, we also carry out Monte Carlo simulations based on the current lens measurements to estimate how large the source catalog would have to be in order to rule out either model at a ~99.7% confidence level. We find that if the real cosmology is LCDM, a sample of ~200 strong gravitational lenses would be sufficient to rule out R_h=ct at this level of accuracy, while ~300 strong gravitational lenses would be required to rule out LCDM if the real Universe were instead R_h=ct. The difference in required sample size reflects the greater number of free parameters available to fit the data with LCDM. We point out that, should the R_h=ct Universe eventually emerge as the correct cosmology, its lack of any free parameters for this kind of work will provide a remarkably powerful probe of the mass structure in lensing galaxies, and a means of better understanding the origin of the bulge-halo conspiracy.

TESTING COSMOLOGICAL MODELS WITH TYPE Ic SUPER LUMINOUS SUPERNOVAE

The use of type Ic Super Luminous Supernovae (SLSN Ic) to examine the cosmological expansion introduces a new standard ruler with which to test theoretical models. The sample suitable for this kind of work now includes 11 SLSNe Ic, which have thus far been used solely in tests involving

The Age-Redshift Relationship of Old Passive Galaxies

\Lambda

The H ii galaxy Hubble diagram strongly favours Rh = ct over ΛCDM

CDM. In this paper, we broaden the base of support for this new, important cosmic probe by using these observations to carry out a one-on-one comparison between the

GRBs and Fundamental Physics

R_{\rm h}=ct