[Previous entry: ""The Culture""] [Main Index] [Next entry: "more on data privacy and ownership"]
08/22/2004 Archived Entry: "The expanding universe"
An update on the imprisonment of libertarian Anthony Hargis, and an update on revelations about our expanding universe from co-blogger Gordon P....
On Hargis, I refer you to Claire Wolfe's blog.
Gordon P. on our expanding universe....Lately, the physics and astronomy community have been all abuzz over unexpected observations whose analysis apparently suggest that the expansion of the Universe is "accelerating." At best, an "accelerating expansion" implies that the Universe's future will be dark and dismally lonely as the accelerating expansion drives every other galaxy beyond the "Hubble Horizon" (apparent radius of the
"visible" portion of the Universe, i.e., the portion from which light has had time to reach a given observer), causing it to become invisible --- and at worst, the Universe will end in a "Big Rip" singularity that causes every particle to recede to an _INFINITE_ distance from every other particle in a _FINITE_ time, tearing apart all material objects into individual elementary particles.
To try and "explain" the observed Universe, physicists have been forced to introduce _two_ separate unobservable" substances, "Dark Matter" and "Dark Energy," that affect "normal" matter only through gravitational forces, and which together constitute the overwhelming majority of the Universe (30% and over 60%, respectively), with the "normal" matter of one's everyday experience representing only a few percent of the total. We haven't more than the _vaguest_ of clues as to what either of these mysterious "unobservable" substances might be, and _FAR_ too many proposed theories about them --- most of which are currently untestable. These bizarre new "essences" rather disturbingly remind one of other "essences" from the Dark Ages of Science, such as Phlogiston or the Luminiferous Aether. Moreover, "Dark Energy" appears to have deeply implausible properties, such as having a density that is nearly independent of its state of expansion, of exerting a "negative pressure," exerting a "repulsive" effective gravity, and possibly allowing violations of the laws of thermodynamics and causality under certain circumstances.
Even more disturbing, the data appear to demand that our civilization coincides with an extremely improbable era of time: A "Cosmic Coincidence" period that happens to be both only a short (cosmological) time after the "Dark Energy" had begun to dominate over Dark Matter so that the Universe transitioned from "deceleration" to "acceleration," and also the time when the apparent radius of the "Hubble Horizon" of the Universe happens to be comparable to the length-scale associated with the "Dark Energy." Furthermore, extrapolating backward, we find that matter, radiation, and "Dark Energy" all had densities within an order of magnitude or so of each other at the epoch when the Universe transitioned from "deceleration" to "acceleration;" to achieve such a "Triple Cosmic Coincidence" requires a highly implausible degree of "fine tuning" of the Universe's parameters.
(It also implies that if the Universe ends in a "Big Rip," it may do so in a cosmologically "short" amount of time --- perhaps only a few billion years, i.e., even before the Sun runs out of fuel !!!)
Notwithstanding all the above deeply bizarre and counterintuitive properties, the scientific community has accepted the "near-certainty" of the "existence" of "Dark Matter" and "Dark Energy" in a _remarkably_ short time: Only a few decades in the case of "Dark Matter" (which, despite being "invisible," at least had _somewhat_ plausible physical properties), and (paradoxically) less than a decade for the far more implausible case of "Dark Energy" --- when the latter idea _should_ have IMO had every cosmologist and astronomer furiously scratching their heads, and searching for probable sources of systematic error in the analysis of the data, rather than so quickly accepting such a bizarre claim! (Indeed, the whole affair seems to be Yet Another Example of that rather disturbing _lack_ of skepticism exhibited by modern scientists, and their extreme willingness to almost unanimously jump onto the bandwagon of the latest "Standard Model" to come along... :-(
Recently, I learned that it's possible that the source of the systematic error has been found --- and that it is something so _BLINDINGLY_ obvious that if it turns out to be right, thousands of scientists will be slapping themselves in the head and saying "D'oh!"...
One of the more evident features of the Universe is that it is "lumpy." It consists mostly of vast empty spaces separating dense bodies of matter: Galaxies are small compared to intergalactic distances, stars are infinitesimal compared to interstellar distances, planets are microscopic compared to interplanetary distances, etc. Because of current mathematical and computational limitations, we cannot adequately treat the "lumpiness" of the universe --- so we instead approximate it as completely smooth and uniform, and at most treat departures from uniformity as "perturbations" that evolve against this smooth and uniform "background." Up until recently, few people bothered to calculate the "back-reaction" perturbations exert on the smooth "background," but instead treated it as "pre-specified." The logic appears to have been that the first-order effects are negligible, so back-reaction was "unimportant." Apparently, they forgot about two little details: First, that the first-order effects were only "negligible" because they were _constructed_ to be "negligible" --- by _definition_, the spatial average of a perturbation is "zero," causing its first-order effect to drop out. That does _NOT_ mean its effect is "negligible," but rather that one has to carry the approximation to higher order to study the perturbation's effects. Second, they forgot that General relativity, Einstein's theory of Gravitation, is _NONLINEAR_; hence, the effects of material sources of gravitation cannot be neatly superposed.
Recently, Robert Brandenberger et al have examined the higher-order effects of perturbations. They finds that, not surprisingly, since an "underdense" region has _less_ than the "average" cosmic density, "underdense" regions act somewhat like "negative masses" (to the extent that the amplitude of the perturbation is "small" compared to the density of the uniform "background"); hence, "underdense" regions produce an effective "gravitational repulsion," much like "Dark Energy" is supposes to do. Furthermore, since the Universe is "lumpy," there are a =LOT= more "underdense" regions in the Universe than "overdense" regions --- and since GR is nonlinear, the dominance of "underdense" regions has a proportionately larger effect than the "lumps." The net result appears to be that if one stupidly approximates a "lumpy" Universe by a "smooth" Universe, one comes to the false conclusion that it is dominated by some mysterious substance that exerts a "gravitational
repulsion"... Like I said: "D'oh!" :-T
If Brandenberger is right, it will be interesting to see how much longer the current belief in "Dark Energy" lingers on... [For technical details, click or here]