<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">["Nemesis", previous page] This leaves an object that is massive but emits very little electromagnetic radiation.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Dr. Van Flandern response: That is rather contradictory. Massive implies hot, and hot implies radiating abundantly.
<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">["Nemesis"] Possible candidates could be a brown dwarf, or a collapsed object like a neutron star, a supernova remnant. I favor the latter possibility myself.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Dr. TVF: Doesn't that require that the Sun had a companion that went supernova? That would have wiped out the planets and changed the Sun drastically from a normal G-type star.
<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">["Nemesis"] The best way to detect it may be through occultation of background stars. This latter method may be the only way to pick up a collapsed object, or maybe gravitational lensing of background objects.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Dr. TVF: The gravitational lensing should indeed be strong and evident. But the whole sky has been surveyed many times, which is how we get proper motions of stars. And region where gravitational lensing was going on would distort all the proper motions in that vicinity. No such region has been seen. So no dark companion with significant gravity exists. (par.) There is now yet another direct test for this, one that I also mentioned to Cruttenden. Pulsar timings allow us to detect any unknown accelerations of the Sun's motion through space, because they would displace the Earth a bit closer to pulsars in some direction, but farther away from pulsars in the opposite direction. So the arrival times of pulsar signals would be changed by a certain predictable pattern across the sky. This has long been known as a test for the possible existence of undiscovered planets of significant mass. No such displacement signal is seen, meaning the Sun is not undergoing any significant acceleration from an unknown cause. -|Tom|-
Joe Keller's comment:
A more detailed version of the following information, was posted piecemeal, subsequent to the above 2006 post by Dr. Van Flandern, by me on the "Requiem for Relativity" thread.
The last statement, about pulsar timing, especially refers to the Oct. 2005 article of Zakamska & Tremaine of Princeton, which to my knowledge has not been superseded in accuracy. They claimed only the ability to detect a Jupiter at 200 AU (or a solar mass at 200*sqrt(1000)=6000AU=0.1 light year). These authors mixed nonparametric and parametric statistics, which might have caused them to overestimate the sensitivity of their test. Also, my own investigation has revealed that millisecond pulsars (commonly a kiloparsec distant, and who knows what is really happening in the vast intervening space?) show a median Pdot/P which happens to equal the Hubble constant. This peculiar and unexplained coincidence of Pdot/P with the so-called "Hubble expansion", casts doubt on the simple model of acceleration which underlies Zakamska and Tremaine's conclusion.
Regarding gravitational lensing, Gaudi & Bloom, Astrophysical Journal 635:711+, Dec. 2005, state in their abstract:
"...Gaia (launch date 2011). A Jupiter-mass object at 2000 AU is detectable by Gaia over the whole sky above 5 sigma, with even stronger constraints if it lies near the ecliptic plane. ..."
Until now I've omitted microlensing from my Barbarossa discussion, because I saw this article. Here is a claim that sometime after 2011, a planned satellite better than Hipparcos *will be* able to detect a Jupiter at 2000AU. Surely Gaia will exceed Hipparcos not only in accuracy but in the number of stars observed; and Hipparcos exceeded ground-based astrometry in accuracy. I haven't yet found any article claiming that Hipparcos has ruled out a companion of any mass at any distance in any part of the sky (if anyone knows of one, please tell me!).
Good point about the supernova remnant, Dr. Van Flandern!
Regarding mass and temperature, the state of the art (1990s) calculation in the mainstream literature, is that a 4.6 billion yr old brown dwarf is roughly the same temperature whether it is slightly above or slightly below the critical mass for (brief) nuclear reaction ignition. The present temperature of the brown dwarf is very sensitive to the (very small) theoretical thermal conductivity of the degenerate matter. However, in 4.6 billion years, there's a lot that can go wrong with a theoretical model of bulk physical properties, untestable in the lab. Unexpected mechanisms of convection might occur; or, gravitational energy might be deposited mainly on the surface, not the interior, to begin with, depending on the mechanism of accretion.
Dr. Van Flandern, I'd like to work for Mr. Cruttenden!
