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JournalMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY (4)ASTRONOMY & ASTROPHYSICS (3)AMERICAN JOURNAL OF PHYSICS (2)The Astrophysical Journal (2)ASTROPHYSICAL JOURNAL (1)EPL (1)EPL (Europhysics Letters) (1)EUROPEAN PHYSICAL JOURNAL C (1)INTERNATIONAL JOURNAL OF MODERN PHYSICS A (1)PHYSICAL REVIEW D (1)View MoreAuthors

Univ Arizona, Dept Astron (18)

Univ Arizona, Dept Phys, Appl Math Program (18)

Melia, Fulvio (17)Melia, Fulvio (4) Univ Arizona, Dept Phys (4)Leaf, Kyle (2)Ruan, Cheng-Zong (2)Yennapureddy, Manoj K. (2)Zhang, Tong-Jie (2)Chen, Yu (1)View MoreTypesArticle (18)
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Model selection with strong-lensing systems

Leaf, Kyle; Melia, Fulvio (OXFORD UNIV PRESS, 2018-05-24)

In this paper, we use an unprecedentedly large sample (158) of confirmed strong lens systems for model selection, comparing five well-studied Friedmann–Robertson–Walker cosmologies: ΛCDM, wCDM (the standard model with a variable dark-energy equation of state), the Rh = ct universe, the (empty) Milne cosmology, and the classical Einstein-de Sitter (matter-dominated) universe. We first use these sources to optimize the parameters in the standard model and show that they are consistent with Planck, though the quality of the best fit is not satisfactory. We demonstrate that this is likely due to underreported errors, or to errors yet to be included in this kind of analysis. We suggest that the missing dispersion may be due to scatter about a pure single isothermal sphere (SIS) model that is often assumed for the mass distribution in these lenses. We then use the Bayes information criterion, with the inclusion of a suggested SIS dispersion, to calculate the relative likelihoods and ranking of these models, showing that Milne and Einstein-de Sitter are completely ruled out, while Rh = ct is preferred over ΛCDM/wCDM with a relative probability of ∼73percent versus ∼24percent. The recently reported sample of new strong lens candidates by the Dark Energy Survey, if confirmed, may be able to demonstrate which of these two models is favoured over the other at a level exceeding 3σ.

The maximum angular-diameter distance in cosmology

Melia, Fulvio; Yennapureddy, Manoj K. (OXFORD UNIV PRESS, 2018-07-23)

Unlike other observational signatures in cosmology, the angular-diameter distance dA(z) uniquely reaches a maximum (at zmax) and then shrinks to zero towards the big bang. The location of this turning point depends sensitively on the model, but has been difficult to measure. In this paper, we estimate and use zmax inferred from quasar cores: (1) by employing a sample of 140 objects yielding a much reduced dispersion due to pre-constrained limits on their spectral index and luminosity, (2) by reconstructing dA(z) using Gaussian processes, and (3) comparing the predictions of seven different cosmologies and showing that the measured value of zmax can effectively discriminate between them. We find that zmax = 1.70 ± 0.20 – an important new probe of the Universe’s geometry. The most strongly favoured model is Rh = ct, followed by PlanckΛCDM. Several others, including Milne, Einstein-de Sitter, and Static tired light are strongly rejected. According to these results, the Rh = ct universe, which predicts zmax = 1.718, has a ∼92.8 per cent probability of being the correct cosmology. For consistency, we also carry out model selection based on dA(z) itself. This test confirms that Rh = ct and PlanckΛCDM are among the few models that account for angular-size data better than those that are disfavoured by zmax. The dA(z) comparison, however, is less discerning than that with zmax, due to the additional free parameter, H0. We find that H0 = 63.4 ± 1.2 km s−1 Mpc−1 for Rh = ct, and 69.9 ± 1.5 km s−1 Mpc−1 for ΛCDM. Both are consistent with previously measured values in each model, though they differ from each other by over 4σ. In contrast, model selection based on zmax is independent of H0.

J1342+0928 supports the timeline in the = cosmology

Melia, Fulvio (EDP SCIENCES S A, 2018-07-24)

Aims. The discovery of quasar J1342+0928 (z = 7.54) reinforces the time compression problem associated with the premature formation of structure in A cold dark matter (ACDM). Adopting the Planck parameters, we see this quasar barely 690 Myr after the big bang, no more than several hundred Myr after the transition from Pop III to Pop II star formation. Yet conventional astrophysics would tell us that a 10 M-circle dot seed, created by a Pop II/III supernova, should have taken at least 820 Myr to grow via Eddington-limited accretion. This failure by ACDM constitutes one of its most serious challenges, requiring exotic "fixes", such as anomalously high accretion rates, or the creation of enormously massive (similar to 10(5) M-circle dot) seeds, neither of which is ever seen in the local Universe, or anywhere else for that matter. Indeed, to emphasize this point, J1342+0928 is seen to be accreting at about the Eddington rate, negating any attempt at explaining its unusually high mass due to such exotic means. In this paper, we aim to demonstrate that the discovery of this quasar instead strongly confirms the cosmological timeline predicted by the R-h = Ct Universe. Methods. We assume conventional Eddington-limited accretion and the time versus redshift relation in this model to calculate when a seed needed to start growing as a function of its mass in order to reach the observed mass of J1342+0928 at z = 7.54. Results. Contrary to the tension created in the standard model by the appearance of this massive quasar so early in its history, we find that in the R-h = Ct cosmology, a 10 M-circle dot seed at z similar to 15 (the start of the Epoch of Reionization at t similar to 878 Myr) would have easily grown into an 8 x 10(8) M-circle dot black hole at z = 7.54 (t similar to 1.65 Gyr) via conventional Eddington-limited accretion.

Evidence of a truncated spectrum in the angular correlation function of the cosmic microwave background

Melia, Fulvio; López-Corredoira, M. (EDP SCIENCES S A, 2018-03-09)

Aim. The lack of large-angle correlations in the fluctuations of the cosmic microwave background (CMB) conflicts with predictions of slow-roll inflation. But while probabilities (≲0.24%) for the missing correlations disfavour the conventional picture at ≳3σ, factors not associated with the model itself may be contributing to the tension. Here we aim to show that the absence of large-angle correlations is best explained with the introduction of a non-zero minimum wave number kmin for the fluctuation power spectrum P(k).
Methods. We assumed that quantum fluctuations were generated in the early Universe with a well-defined power spectrum P(k), although with a cut-off kmin ≠ 0. We then re-calculated the angular correlation function of the CMB and compared it with Planck observations.
Results. The Planck 2013 data rule out a zero kmin at a confidence level exceeding 8σ. Whereas purely slow-roll inflation would have stretched all fluctuations beyond the horizon, producing a P(k) with kmin = 0 – and therefore strong correlations at all angles – a kmin ≠ 0 would signal the presence of a maximum wavelength at the time (tdec) of decoupling. This argues against the basic inflationary paradigm, and perhaps even suggests non-inflationary alternatives, for the origin and growth of perturbations in the early Universe. In at least one competing cosmology, the Rh = ct universe, the inferred kmin corresponds to the gravitational radius at tdec.

Testing viable f(R) models with the angular-diameter distance to compact quasar cores

Sultana, Joseph; Melia, Fulvio; Kazanas, Demosthenes (AMER PHYSICAL SOC, 2019-05-06)

We consider here some popular f(R) models generally viewed as possible alternatives to the existence of dark energy in General Relativity. For each of these, we compute the redshift zmax at which the angular diameter distance dA(z) is expected to reach its maximum value. This turning point in dA(z) was recently measured in a model-independent way using compact quasar cores and was found to occur at zmax=1.70±0.20. We compare the predictions of zmax for the f(R) models with this observed value to test their viability at a deeper level than has been attempted thus far, thereby quantifying an important observational difference between such modified gravity scenarios and standard Lambda Cold Dark Matter (ΛCDM) cosmology. Our results show that, while the most popular f(R) models today are consistent with this measurement to within 1σ, the turning point zmax will allow us to prioritize these alternative gravity theories as the measurement precision continues to improve, particularly with regard to how well they mitigate the tension between the predictions of ΛCDM and the observations. For example, while the Hu-Sawicki version of f(R) increases this tension, the Starobinky model reduces it.

Using Spatial Curvature with H II Galaxies and Cosmic Chronometers to Explore the Tension in H 0

Ruan, Cheng-Zong; Melia, Fulvio; Chen, Yu; Zhang, Tong-Jie (IOP PUBLISHING LTD, 2019-08-21)

We present a model-independent measurement of spatial curvature Omega(k) in the Friedmann-Lemaitre-Robertson-Walker universe, based on observations of the Hubble parameter H(z) using cosmic chronometers, and a Gaussian process (GP) reconstruction of the H II galaxy Hubble diagram. We show that the imposition of spatial flatness (i.e., Omega(k) - 0) easily distinguishes between the Hubble constant measured with Planck and that based on the local distance ladder. We find an optimized curvature parameter Omega(k) = -0.120(-0.147)(+0.168) when using the former (i.e., H-0 = 67.66 +/- 0.42 km s(-1) Mpc(-1)), and Omega(k) = -0.298(-0.088)(+0.122) for the latter (H-0 = 73.24 +/- 1.74 km s(-1) Mpc(-1)). The quoted uncertainties are extracted by Monte Carlo sampling, taking into consideration the covariances between the function and its derivative reconstructed by GP. These data therefore reveal that the condition of spatial flatness favors the Planck measurement, while ruling out the locally inferred Hubble constant as a true measure of the large-scale cosmic expansion rate at a confidence level of similar to 3 sigma.

A solution to the electroweak horizon problem in the R-h = ct universe

Melia, Fulvio (SPRINGER, 2018-09)

Particle physics suggests that the Universe may have undergone several phase transitions, including the well- known inflationary event associated with the separation of the strong and electroweak forces in grand unified theories. The accelerated cosmic expansion during this transition, at cosmic time t ∼ 10−36 − 10−33 s, is often viewed as an explanation for the uniformity of the CMB temperature, T , which would otherwise have required inexplicable initial conditions. With the discovery of the Higgs particle, it is now quite likely that the Universe underwent another (elec- troweak) phase transition, at T = 159.5 ± 1.5 GeV – roughly ∼ 10−11 s after the big bang. During this event, the fermions gained mass and the electric force separated from the weak force. There is currently no established explanation, however, for the apparent uniformity of the vacuum expectation value of the Higgs field which, like the uniformity in T , gives rise to its own horizon problem in standard ΛCDM cosmology. We show in this paper that a solution to the electroweak horizon problem may be found in the choice of cosmological model, and demonstrate that this issue does not exist in the alterna- tive Friedmann–Robertson–Walker cosmology known as the Rh = ct universe.

Cosmological test using the Hubble diagram of high-z quasars

Melia, Fulvio (OXFORD UNIV PRESS, 2019-08-01)

It has been known for over three decades that the monochromatic X-ray and UV luminosities in quasars are correlated, though non-linearly. This offers the possibility of using high-z quasars as standard candles for cosmological testing. In this paper, we use a recently assembled, high-quality catalogue of 1598 quasars extending all the way to redshift similar to 6, to compare the predictions of the R-h = ct and Lambda cold dark matter (Lambda CDM) cosmologies. In so doing, we affirm that the parameters characterizing the correlation depend only weakly on the chosen cosmology, and that both models account very well for the data. Unlike Lambda CDM, however, the R-h = ct model has no free parameters for this work, so the Bayesian Information Criterion favours it over Lambda CDM with a relative likelihood of similar to 88 per cent versus similar to 10 per cent. This result is consistent with the outcome of other comparative tests, many of which have shown that R-h = ct is favoured over the standard model based on a diverse range of observations.

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