NASA and DoE Team On Dark Energy Research 106
Roland Piquepaille writes "NASA and the U.S. Department of Energy have teamed up to operate the future Joint Dark Energy Mission. As you probably know, recent astronomical measurements have showed that about 72% of the total energy in the universe is dark energy, even if scientists don't know much about it, but speculate that it is present almost since the beginning of our Universe more than 13 billion years ago. The JDEM 'mission will make precise measurements of the expansion rate of the universe to understand how this rate has changed with time. These measurements will yield vital clues about the nature of dark energy.' The launch of a spacecraft for the JDEM mission is not planned before 2015."
what i don't get is... (Score:4, Interesting)
Dark Matter/Emergy Does Not Exist (Score:5, Interesting)
I was always a skeptic when it came to Dark Matter(I am not an astronomer, so this all technically an uniformed opinion). But now I know that it really is all a load of idle speculation coupled with incomplete investigation, and an excessive dose of hype. It only took a few minutes of googling to come up with this paper [arxiv.org].
One of the biggest pieces of evidence for Dark Matter is the Galaxy Rotation Problem [wikipedia.org]. Basically the rotations of Galaxies do not behave as astronomers expect them to do, leading to the hypothesis that there is more matter in them that we cannot see, "Dark Matter". The velocity profiles that Astronomers expect to see are Keplerian. That is, they expect star systems in galaxies to behave like planets in solar systems when it comes to orbit speed and distance from the focus of rotation.
The bottom line is, as shown in the paper, this assumption is totally unjustified. The integrals in the 2D galactic disc case do not work out using Shell Theorem [wikipedia.org], which cannot be applied. They are instead quite nasty singular integrals, but twenty minutes with MATLAB and the "QUAD" function will be all it takes to see that basic gravitational theory most certainly does not predict that Galaxies should have Keplerian(Solar System-like) rotation curves, and there is no reason whatsoever for astronomers to assume this. It's all basic mathematical physics well withing the reach of many reading this post.
The galactic rotation problem is not evidence for Dark Matter. It is only evidence of the need for more applied mathematics courses in astronomy undergraduate degrees. Of course the Galactic rotation problem is not the only evidence for Dark Matter, but it is a big part. The other big piece of evidence was the Galactic Cluster mass problem. It's been a while since I read the relevant papers, but as I recall, Zwicky played hard and fast with the virial theorem, in particular making assumptions about the stability of Galactic clusters.
Again of course, I am not an astronomer. I am essentially a lay person in these matters, so my posts and opinions (not only in this thread) should be taken with a pinch of salt. Still, I stand by my overall skepticism of Dark Matter theories, and I stand quite firmly on my objections to the interpretation of the Galactic rotation problem. I expect that in the near future, as our ability to analyse and simulate galatic dynamics improves, Dark Matter will finally be debunked.
Re:It's a silly thing to measure. (Score:3, Interesting)
Space can have a gravitational effect: in general relativity, gravity gravitates. A black hole is a vacuum solution of the Einstein field equations. And if you object to it being vacuum because you can't say what's at the singularity, there are non-singular vacuum solutions too, like gravitational geons [wikipedia.org]. However, they're not stable, so attempts to describe matter as pure space have failed. (Another attempt which also largely failed is to describe particles as wormhole mouths.)
Some people think that dark energy is the gravitational effect of space. See the vacuum energy interpretation of the cosmological constant [wikipedia.org]. That is also different, however, from dark energy being matter, or matter being space.
Matter = space is an intriguing idea, but people have worked on it for over 50 years and haven't made it work. Maybe with a full theory of quantum gravity they could, but I really doubt that would lend any new insight into dark matter or energy, any more than it would suddenly revolutionize our understanding of electrons — the relevant physics would have to be Planck scale and mostly irrelevant except maybe at the Big Bang.
Re:Dark Matter/Emergy Does Not Exist (Score:1, Interesting)
What is the purpose of a theory?
If it is to explain observations already made and to make testable predictions about phenomena that have yet to be observed, then the aether theory (which you admit made pretty accurate predictions) served its purpose. Newton's "Laws" still reign in many domains, as their predictions are accurate to useful limits of observation. General Relativity and Quantum Mechanics are clearly incomplete, yet within certain domains, they produce predictions that match observation to the limits of measurement.
The statement that physics was in a rut for 200 years due to the aether theory ignores the work of Maxwell, Faraday, Watt, and countless others who made advances in electromagnetism, electricity, thermodynamics, etc.
The aether theory was in trouble long before Einstein -- Maxwell had negated its primary reason for existence when he demonstrated that an electromagnetic wave could be self-propagating, requiring no "medium" for propagation. That, after all, was the purpose of aether -- to provide a "medium" for the propagation of light.
Far from being an embarrassment, the aether theory is a pretty decent example of the scientific method in action. We start with the idea that light is componsed of particles. We note that light behaves as if it were a wave. We postulate a medium of propagation for the wave and logically derive its properties (massless, fluid, rigid, invisible, etc.).
Maxwell describes EM waves and determines that they CAN be self-propagating (so we don't NEED a luminiferous aether). Michelson and Morley conduct their experiment to demonstrate that the luminiferous aether does not behave as expected, if it indeed exists at all.
Several decades pass before the idea of a luminiferous aether is abandoned, and it is noteworthy that most theories of the time were consistent with the presence or absence of the aether. The reason it took decades to fully abandon the idea is that it DID explain some things. Of course, eventually, these same things were explained in other ways, and the aether was abandoned as a needless complication (Occam's Razor).
Some of the ideas associated with the aether persist. One cannot hear of General Relativity's "frame dragging" (a rotating, massive body dragging space-time around itself) without thinking of the idea that Earth would drag the aether around itself (an early attempt to explain the results of the Michelson-Morley experiment).
So, the aether theory was developed because it explained what was observed. At some point, there were gaps in what it could explain, and alternate explanations surfaced. After some time, the alternate explanations provided better explanations in all domains, and the aether theory was abandoned. Had the aether theory been simpler than Maxwell's equations or had it produced some results that could not be as easily obtained by other means, we might still be saying "it is helpful in analyzing this situation to postulate an invisible, massless, incompressible, rigid fluid through which light propagates..."