Accurate distance gauge out to z ~ 4 alleged (AGN reverberation) Because AGN (active galactic nuclei) are so bright, it would be nice if we could tell their intrinsic luminosity or "wattage". Then it would be like having a "standard candle" to tell distance with, by seeing how less bright it looks at our remove from it. These people claim to have found a way to tell the inherent wattage of an AGN by measuring a time-lag which tells them the radius of a certain surrounding photo-ionized region. This surrounding region is called the BLR (broad line region). If their method is confirmed it would be very important for cosmology. One would then have a direct way to gauge the distance out beyond where we can judge it using Type IA supernovae as standard candles. Let's hope it is confirmed! Then we will come to understand the cosmos much better. http://arxiv.org/abs/1109.4632 A new cosmological distance measure using AGN D. Watson (1), K. D. Denney (1), M. Vestergaard (1), T. M. Davis (2) ((1) Dark Cosmology Centre, U. Copenhagen, (2) U. Queensland) (Submitted on 21 Sep 2011) Accurate distances to celestial objects are key to establishing the age and energy density of the Universe and the nature of dark energy. A distance measure using active galactic nuclei (AGN) has been sought for more than forty years, as they are extremely luminous and can be observed at very large distances. We report here the discovery of an accurate luminosity distance measure using AGN. We use the tight relationship between the luminosity of an AGN and the radius of its broad line region established via reverberation mapping to determine the luminosity distances to a sample of 38 AGN. All reliable distance measures up to now have been limited to moderate redshift -- AGN will, for the first time, allow distances to be estimated to z~4, where variations of dark energy and alternate gravity theories can be probed. Apj in press; 5 pages, 3 figures One of these authors is Tamara Davis. She was Charley Lineweaver's PhD student and wrote an excellent thesis in 2003 IIRC. She co-authored the SciAm article I have in my signature, which explains misconceptions people have about expansion cosmology (aka "big bang" although this is a misleading nickname). I'm definitely a Lineweaver-Davis fan and urge you to read the SciAm article---the "charley" link. Also since she is onboard with this AGN thing I tend to give it good odds of turning out right. Others: Marianne Vestergaard, Kelly Denney, Darach Watson at the Copenhagen center for dark cosmology http://www.dark-cosmology.dk/~darach/About%20Me.html [Broken] Darach must be an Irish name, new to me. This team has quite a bit of credibility as I see it. Also see their figure. The correlation of this measure with known distances is good. What I want to understand better is this delay time tau. AGN are black holes with stuff spiraling in. And their output fluctuates depending on how much hot gas is on the way in. The light emitted is thermal, as the gas heats up as it spirals in, and glows---with a continuum spectrum. This is the main source of luminosity and it is what we want to gauge. Now this hot central glow falls off as the square of the radius outwards into the surrounding clouds and how luminous it is determines how far out it can excite secondary radiation. If we knew the radius we could tell the luminosity. But we can tell the radius by the delay time of a flare-up. The central glow flares up first, and then the secondary radiation happens a little while later because the light takes time to get out to the surrounding BLR gas clouds. I think that is roughly the idea. You can read the explanation on page 1. It is called the "reverberation" method. The BLR radiation is a kind of "echo" of the initial flareup. So you have to observe the AGN and record when things happen and this way you learn the absolute luminosity. Then you see how much dimmer it looks to us, and that tells the distance.