Abstract
We use a recently proposed luminosity distance measure for relatively nearby active galactic nuclei (AGNs) to test the predicted expansion of the Universe in the \(R_{\mathrm{h}}=ct\) and \(\varLambda\mathrm{CDM}\) cosmologies. This comparative study is particularly relevant to the question of whether or not the Universe underwent a transition from decelerated to accelerated expansion, which is believed to have occurred—on the basis of Type Ia SN studies—within the redshift range (\(0\lesssim z\lesssim 1.3\)) that will eventually be sampled by these objects. We find that the AGN Hubble Diagram constructed from currently available sources does not support the existence of such a transition. While the scatter in the AGN data is still too large for any firm conclusions to be drawn, the results reported here nonetheless somewhat strengthen similar results of comparative analyses using other types of source. We show that the Akaike, Kullback, and Bayes Information Criteria all consistently yield a likelihood of \(\sim 84\mbox{--}96~\%\) that \(R_{\mathrm{h}}=ct\) is closer to the “true” cosmology than \(\varLambda\mathrm{CDM}\) is, though neither model adequately accounts for the data, suggesting an unnaccounted-for source of scatter.
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Notes
As we shall see shortly, this approach is quite different from that suggested for nearby AGNs, even though both make use of our knowledge concerning the BLR. The high-\(z\) quasar technique is, by necessity, statistical in nature, whereas the nearby AGN method relies on the measurement of fluxes and time lags in individual sources.
The Milne Universe (Milne 1933) is sometimes confused with \(R_{\mathrm{h}}=ct\), but in fact its observables are quite different—and have already been refuted by the observations. Unlike the \(R_{\mathrm{h}}=ct\) Universe, in which the spatial curvature constant is \(k=0\), the Milne universe is empty and has \(k=-1\). As a result, the luminosity distance in Milne is \(d_{L}^{\mathrm{Milne}}=R_{\mathrm{h }}(t_{0})(1+z) \sinh[\ln(1+z)]\), which is not at all consistent with the data (Melia and Shevchuk 2012).
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Acknowledgements
I am grateful to the anonymous referee for very helpful comments that have led to an improvement in the manuscript. I am also grateful to Amherst College for its support through a John Woodruff Simpson Lectureship, and to Purple Mountain Observatory in Nanjing, China, for its hospitality while part of this work was being carried out. This work was partially supported by grant 2012T1J0011 from The Chinese Academy of Sciences Visiting Professorships for Senior International Scientists, and grant GDJ20120491013 from the Chinese State Administration of Foreign Experts Affairs.
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Melia, F. The AGN Hubble Diagram and its implications for cosmology. Astrophys Space Sci 359, 34 (2015). https://doi.org/10.1007/s10509-015-2483-4
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DOI: https://doi.org/10.1007/s10509-015-2483-4