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Necessity of Dark Energy Questioned
ttnuagmada points us to an article about scientist David Wiltshire's suggestion that theorized dark energy is not needed to describe the expansion of the universe. His work challenges assumptions made about the distribution of matter in the universe. Early solutions to general relativity were based on a "smooth distribution" of matter. Wiltshire's approach focuses on a "lumpy" dispersal, which more accurately fits data from modern studies. We have discussed other theories about dark energy in the past. Quoting:
"Through observational projects like the Sloan Digital Sky Survey and the 2 Degree Field survey, we now have a much better picture of the large-scale structure of the universe and we know that galaxies are not uniformly distributed. 'Rather, they are in clusters sprinkled thinly in filaments and "bubble walls" surrounding huge voids hundreds of millions of light-years across,' Wiltshire says.
Sheesh, evil *and* a jerk. -- Jade
As Zapp Brannigan is in lukewarm discussions with the Neutral Planet president, the planet's scientists are holding a lukewarm debate over the possible existence of Grey Matter.
Knowledge is how to play a game, intelligence is how to win, wisdom is knowing what game to play.
Anybody want my mod points?
... can be accessed here: http://arxiv.org/abs/astro-ph/0510059 . A bit less recent (but even more readable) account is http://xxx.lanl.gov/abs/astro-ph/0310342 . The first linked article also mentions the approaches featured in the slashdot post (this is an ongoing business for a while). For starters the flow diagrams in the front pages describing the options might be particularly useful.
Attitudes make the difference between Space and Time: we want to MAX our temporal, and MIN our spatial extension.
Real funny. But what is the question?
"Why is the universe expanding?"
IANAAP (I Am Not An Astro Physicist), but the theory seems like it would make sense. When you see explosions of one kind or another, they aren't uniform. This is probably a bad example, but take the explosion of the Death Star, or even fireworks for that matter, they don't make a perfect sphere. More matter is thrown out in some areas than in others. His explanation for the expansion of the universe was a little over my head though. I knew that the observation of time alters relative to your speed, as well as relative to your proximity to large pulls of gravity, but I didn't quite get everything he was saying. Like I said... IANAAP.
I did some googling and found David Wiltshire's home page which had links to his recent publications. That brought me to this full article which I am guessing is the one that corresponds to what was discussed in the original /. article here.
I had a couple courses in astronomy and cosmology way back in my college days. That said, I can't begin to understand the details. I'm hoping someone with more knowledge and experience could elaborate. Is he really onto something that can dispense with the need for dark energy? And, if he is, am I correct in thinking this would be Nobel-Prize-Candidate-Worthy?
The fact that matter forms bubbles around the voids intuitively make me think that some force is pushing matter away from the center of each void. Perhaps the center of each void is location where mini-inflation events have happened and what we see today is the reslut of these events pushing shells of matter up against each other so that they form filaments and bubbles. Just a though, IANAP though.
It's important that this Dark Matter is licensed by the GPL and also that everyone with a Mac can freely share it ("Information wants to be freeeeeeeeeeeeee").
If you cannot detect something at all with light or gravity effects, then it very likely isn't there. So, the whole dark matter thing is equivalent to calling in the gods to explain the unexplained with something even more inexplicable.
Excuse me, but please get off my Pennisetum Clandestinum, eh!
mother. fucking. electric. universe.
Does this have any implications for long-term scenarios, such as the "Great Rip" ..?
Is the Universe expanding?
There I fixed it!
All theory is gray
It seems like physicists LOVES complex answers, but life prefers the simplest possible solution.
I prefer the "u" in honour as it seems to be missing these days.
And, since mass slows down time, the clocks of observers in voids, where most of the empty space in the universe is, will appear to be ticking faster than the clocks of observers in galaxies.
If Hollywood stars learn of this, they'll start clustering around obese people to slow down time. No wonder Orson Wells was so popular. (Oops, I mentioned "cluster", a no-no on slashdot.)
Table-ized A.I.
Anybody want my mod points?
For starters the flow diagrams in the front pages describing the options...
"God does not use flowcharts!" -an important science dude
Table-ized A.I.
Dark Matter/Energy has always been a ridiculous explanation for gravitational effects that we can't explain. Heres how it works:
In certain large galaxies, there are certain stars and star systems that orbit the center of the galaxy FASTER than what relativity predicts. So, someone who has only studied Newtonian gravitational effects said "DURR, THERE MUST BE SOME MORE MASS IN DER, DURRR" because back then, it was easier to just say "Eh its probably something we cant see" than to say "maybe we need to rethink our theories."
So this idiotic observation has led to decades of research into Dark Matter/Energy, which thanks to String Theory (another bogus facet of modern science), has a possible explanation, and it goes like this:
IF the basis of string theory is true AND
IF a certain subset of string theory is true AND
IF a certain sub-subset of the previous subset is also true
THEN
Dark Matter can be explained.
Now take into account the fact that in SIXTY YEARS of string theory research, not a single experiment has been successfully performed to prove ANY aspect of string theory... and then consider the "successful" observation of Dark Matter last summer, and it becomes fairly obvious that it was a false observation, and was predicted by incredibly incorrect assumptions about the way the universe functions (ex. "If this dog sneezes twice today, then that means that is going to rain tomorrow." No basis in science, but it is difficult to refute because of that fact)
So we should applaud Wiltshire for neglecting the "evidence" that Dark Matter exists, and instead treading the path of the true scientist by deciding to observe and understand the universe, and be humbled by the fact that maybe we just don't understand everything about gravity yet.
I mean, is it really so hard to believe that there is some weird aspect of Gravity that we just don't know about yet? It is the least known of all forms of energy, and we have yet to find any way of "controlling" it, so it's not much of a stretch to say we dont know SQUAT about it.
It seems as though each group at different scales don't even know the others exist. If you want an explanation of why the universe has expanded in a non-uniform way, you need only look to the chaos at the quantum level. Imagine the interactions of a pre- big bang sized universe and what a wrench a little quantum foam throws into the uniformity. When we start up computers powerful enough to measure and predict quantum events, funny that it would likely take quantum mechanics to develop, we could very well retro-simulate the expansion of the universe to, or at, a quantum level and make the link.
Slashdot: Where opinions are just opinions until you have mod points.
The cause is not known i think, because we dont understand jet what nature is made of.
:)
So far it's only an observation that it is expanding.
From what i've read / heard about it, that in all directions it seams roughly expanding at the same speed.
I wonder are we then in the middle ?
On the positive side there is more room for us each minute
I know you're out there. I can feel you now. I know that you're afraid. You're afraid of us. You're afraid of change.
What has always struck me as odd when reading articles about certain theories and explenations is that there seem to be very few people left who actually dare state that "this is something we don't know (yet)" but always seem to have the need to come up with (sometimes) extraorinary theories instead. It seems to me as if some human nature is taking over there; the need to explain everything.
To me, one of the things which indicate someone's awareness of certain topic is also knowing ones own limitations.
from the article
This defines exactly the questions Wiltshire seems to be addressing. His most recent paper on arXiv posted on 24 Dec (from a yet unpublished confrerence contribution) is here:
http://arxiv.org/abs/0712.3984
It seems like a good review. He may be right, but then again he might not. Only careful testing against the observational data will tell. He proposes to outline the differences in observational predictions between his "Fractal Bubble" model and the current Lamba CDM model in a forthcoming paper.
Of course not. I mean, she draws them because her boss says she has to. But use them? That'd be like ... like ... That'd be like reading the spec before doing the creative work.
You can't see ANYTHING from a car, You've got to get out of the goddamned contraption and walk...Edward Abbey
Like gravy and oatmeal. God isn't perfect either. :)
Very helpful.
Warning: this article may contain humor, sarcasm, parody, and perhaps even irony. Read at your own risk.
Obviously these people have never been a necromancer in Heroes of Might and Magic. Dark Energy is essential and you can never have enough.
Why have they been wasting our time with this dark energy stuff for the last decade then? Why posit the dark energy if its only needed to fix a model that was derived with what has for a while now known to be a false assumption? It seems stupid. Instead of endless science articles on dark energy, instead there should have been articles on scientists working to solve pde's with really hard constraints that match modern astronomical observations. I don't get it. Is there more to the story?
The need for dark energy seems to be based on the concept that the Universe is expanding. The Universe is thought to be expanding mainly due to the red shift of light from distant galaxys, this may be wrong based on ...
1) Is it not possible that after travelling huge distances light "slows down" and exhibits a red shift. This would also tie in with the fact that the more distant an object is the faster it appears to be receeding.
2) History proves that what ever a scientist tells you is wrong. Beware of the statement "trust me, I am a scientist" !
JimboL
You really have it backwards. Dark Matter was postulated precisely to explain gravity effects that have been observed. Someone that found out that radium created heat and killed things around it might postulate that there was some form of energy causing this and call it "Invisible Energy" before knowing everything about it. Now we call it radioactive decay and this "Invisible Energy" we call radiation. Sometimes these theories might turn out to be wrong.
I'm a chemist, but I think I can explain this. So, please confirm, is this explanation correct:
After the Michelson-Morley experiment definitively showed that the speed of light is not variable, this observation had to be shoehorned into the framework of physics, and Einstein did this by developing the mathematics to distort the spacetime coordinates to make speed of light appear constant. This meant that space and time had to distort. So, in the voids, "time slows down". If we just naively de-Einsteinify this by making a "coordinate transform" into a pseudo-Galilean view, then it means that light goes faster in the voids. (I'm not saying Einstein is incorrect, mind you; you can also construct force fields for virtual forces like the centrifugal force, even though they don't really exist.) I'll call this the virtual velocity of light.
Now, what we've been assuming is that light has one virtual velocity. Therefore, when we look at a star, the age of light linearly depends on its distance. This is an incorrect assumption. When we look through a void, we see older light than elsewhere, because in the void, the virtual velocity of light is higher. We can, essentially, see into an earlier age by looking through a "lens", a void that is, where the virtual velocity of light is higher. This has an immediate implication with respect to redshift: in an older universe, expansion was faster, giving a higher redshift. Therefore, the relationship between distance and redshift (corrected for expansion of space) should not be linear like we have previously assumed.
So, please explain how this implies that we should see an illusion of an accelerated expansion. I can almost grasp it.
> we now have a much better picture of the large-scale structure of the universe and we know that
> galaxies are not uniformly distributed. 'Rather, they are in clusters sprinkled thinly in
> filaments and "bubble walls" surrounding huge voids hundreds of millions of light-years across,'
I suppose the fat black woman on The View doesn't know this, too.
(-1: Post disagrees with my already-settled worldview) is not a valid mod option.
If the universe is not "flat" then taht could explain apparent acceleration. Need a test to distinguish this from a repulsive force.
There is a little more...when I read the article, the biggest thing that jumped out at me is "perspective."
From our perspective the universe is 15 or so billion Y.O. from a bubble it is 18+ BYO. It's all perspective.
Now if you look out far enough, the universe DOES look homogeneous. So the assumption used in the standard model still has merit but this article suggests that the "local" differences in the universe (local meaning the empty bubles hundreds on Millions of Light years across) are more importatnt than the overall universe.
FYI
Just a small point...
Dark matter JUST means unobserved matter.
There are a LOT of theories about it from the mundane to the esoteric. I have heard dark matter theories suggesting that it is the influence of alternate universes and I have also hear that it is just a lot of very dim brown dwarf stars along with many others possible theories. Both theories explain why there appears to be extra mass around even though we can't see it. Both describe "dark matter" but one needs a huge leap and the other doesn't. The multitude of theories does not destroy the apparent necessity of dark matter.
Also, we CAN directly observe black holes through the Hawkins radition they emmit. Plus their observable interaction on the rest of the universe also proves their existance. The observed gravities indicates that there is an apparent hole in space that is essentially black (a black hole). Therefor black holes exist...this doesn't say anything about the structure. Up until recently it was possible that a cluster of dark neutron stars were causing the gravity effects. About 10 years ago that theory was disproven and now a singularity is the most probably cause to explain the observation of "Black Holes."
Remember, UFO's exist! Unidentified Flying Objects appear all the time. Are they spacecraft flown by martians? I doubt that theory is correct but that doesn't mean that the UFO has suddenly been identified just because you eliminate one theory...
Just a random thought.
Physicists sometimes exaggerate. It helps them to get attention for their research, and credibility for their ideas, and makes it easier for those ideas to get a decent shot at being evaluated properly instead of just getting lost in the noise. It also makes for better news stories and for more excited, enthusiastic students.
Eric Baird
Scientists sometimes speak less rigorously for convenience and breathless reporters get a little carried away.
Conversations get a little taxing if you have to be careful to say "hypothetical non-radiating weakly interacting massive particles" every time. Dark matter rolls of the tongue much better.
Besides, actual dark matter is by far the best explanation we've got at present. If someone comes up with something better (and you can bet lots of people are looking) then we'll refer to that in our verbal shorthand as if it were true.
The wave front of 'now', laying down the universe in the direction of positive time, is like a rubber sheet that has been held back in areas of higher density matter, less so in areas of less denstiy. And thus the sheet would become more distorted as the distribution of matter in the universe changed.
Then much like adaptive optics removes the distortion caused to light by a wiggly atmosphere, there must be something like 'adaptive chronics' to remove the distortion caused to light by a wiggly time, and under that correction the universe does not expand at the rate we think, dark matter is unnecessary and the universe is actually 18 billion years old!
I knew dark matter was bogus, I bet this sends it to the dust bin of bad theories.
It's not always the reporters' fault. Some of the worst statistical misrepresentations I've seen in science have come directly from physics people.
For previous examples, look at the "cheerleading" that happened over research on GR's black holes in the 1960's, and then some of the hype that the string theory guys used to put out. If anything, I thought that the journos sometimes tended to tone down some of the worst claims.
Or look at the unrealistic estimates that researchers have been putting out for decades about how close we are to having commercial fusion reactors, if we just put in another few billion dollars right now ... a lot of those guys must know (and must always have known) that the estimates weren't realistic, but it's been getting them the funding so far.
"Cheerleading" seems to be seen by some in the physics community as a legitimate way of "gaming" the system, provided that they're only misleading the politicians and the wider public who vote those politicians in, and aren't misleading their fellow professionals. But "helpful" misinformation originally intended as harmless PR has a habit of contaminating and corrupting genuine information, and if it's not checked, after a generation or two you can end up with a research field where many of the of the newer intake don't really know what information is real and what isn't.
Yep! Trouble is, as a name, "dark matter" is just too catchy. It's a brilliant name, and it's seductive, and when we get to the point where most of the population have heard of "it", and are aware that scientists study "it", they tend to think, quite naturally, that "it" is something that is known to be real. And in the case of DM, so far, it isn't.
Dark matter isn't like a missing element in the periodic table, or an unseen particle that can account for momentum and energy that disappears in a collision. We don't have a "family" structure that suggests the existence of DM, and we don't have a method of carrying out a reaction and comparing a mismatch between measured quantities in two situations, before and after, to indicate an additional piece of the puzzle.
What we seem to have is a single, consistent mismatch between a theory and experiment, and because we can't see how we could have gotten the calculations wrong, or the theory wrong, we've invented new "stuff" to make up the difference, whose only other derived properties are that it doesn't seem to manifest itself in any other way that we can detect. It's getting perilously close to the old medieval description of a basilisk.
While I really love most of the design ideas behind Einstein's GR, I think that other aspects of it suck. It wasn't designed around modern ideas about cosmology, it wasn't imagined as a truly stand-alone system, and it's damned difficult to find any properly testable predictions for it at medium scales that are distinguishable from, say, updated Newtonian theory. Its predicted properties for horizons don't work properly for horizons caused by cosmological curvature, and its predictions for strong-gravity sources (GR black hole event horizons) don't agree with those of quantum theory or with our basic rules of thermodynamics. In an expanding universe it doesn't support energy conservation (unless you make up more arbitrary additional terms). It crashes at cosmological scales, it's not required for medium-scale work, and we aren't supposed to use it to model small-scale curvature down at the particle scale, because ... it doesn't work there either.
So if people using it keep getting the wrong large-scale gravitational predictions too, why the heck
Eric Baird
I think you're greatly overstating the problems with GR. In fact it works EXTREMELY well, which is why it's quite likely that dark matter is, if not actually some sort of matter, some sort of extra effect, not an error in GR. GR doesn't have to be formulated with modern ideas of cosmology, because modern cosmology is built on GR. Of course you can't find obvious, easy tests of GR in low velocity or low gravity situations! If you could then it would disagree with Newton, and Newton works VERY well too, in it's domain of applicability. That's the pattern. Our really successful theories (Newton, Maxwell, etc) aren't wrong, they're just special cases. General relativity and our current quantum theories must be special cases as well. They MUST be, because they don't get along with each other. Quantum field theory does have some nasty hacks too... like papering over infinities by replacing them with experimentally derived values for particle masses.
Still, neither GR or quantum theory is likely to be wrong, just incomplete. Still, they work VERY well. The missing bits are very unlikely to have any observable effects at anything less than the conditions a tiny fraction of a second after the big bang, which we'll be able to reach in the next generation of particle accelerators.
As for public misunderstanding and cheerleading... well, if you need to convince a politician to build you a machine or fund your lab, how are you going to do it? After WWII when the value of basic research was well recognized you could be realistic. Now you have to paint a very rosy picture or your grant money will get spent on some planes or tanks. You've got to keep the time frame short too. Really, the fusion people have a lot of guts saying 20 years. That's four whole terms for most politicians!
There are two solutions. Scientists can stop talking to the public. No science journalism, journals are ONLY available to faculty and registered students of universities, the whole ivory tower route. OR, education, so that, for example, a public with low science literacy doesn't either automatically believe in hypothetical dark matter, or think it's a stupid, crazy, far out idea.
Says who? The experts and textbook writers who are trying to protect their field, and attract the best and the brightest students to it? How do we judge success? GR has been described as a theorist's paradise but an experimenter's nightmare, and the difficulty of actually demonstrating that the theory is any good has led to some truly rotten misrepresentations.
I'm a fan of most of the basic arguments used to construct current GR, but I think it was a premature implementation of a basically good idea, and the levels of BS I've encountered over the years from from some GR experts have been mind-boggling. I'll accept that general relativity is cool, if we take "GR" to mean a general field of study, or a broad inclusive subject whose predictions are liable to change and evolve as we learn more about it, but "Einstein's general theory" as described in textbooks, treated as an individual scientific theory really does suck in some respects. There are too many areas where it simply doesn't work properly. Where GR1915 is successful, the theory's predictions don't seem to be distinguishable from the results of applying more general arguments, and in the cases where current GR does make new predictions, those usually seem to turn out to be wrong.
As far as experimental verification is concerned, we have three main proofs of GR, none of which actually require GR1915:
(1) Gravitational shifts and time dilation, which Einstein showed in 1911 could be calculated from Newtonian principles, and which can be folded back into the Newtonian calculations to bring other NM predictions into line with GR. Since GTD gave us time-warpage as well as space-warpage, and since both seemed to give basically same predictions for, say the gravitational deflection of light, the only thing we needed to pin down to find the approximate shape of spacetime around the Sun was whether these two calculations should be considered as "equivalent" or "cumlative". If we decided to use the known anomaly in Mercury's orbit (2) as a guide, then the two light-bending effects had to be cumulative, and this then gave us the stronger "GR" predictions for the gravitational deflection of light skimming the Sun (3). The decision to take a geometrical approach, plus the 1911 "gravitational time dilation" idea, plus a guess that Mercury's orbital oddity was relevant, will give us the three "GR" results even if we happen to think that textbook GR is the wrong theory. These three results suggest that some of the initial ingredients used to construct the theory seem to be right, they don't necessarily mean that the theory's other components or construction are correct, or that its extrapolations made from this baseline are going to be valid.
The rest of textbook GR is more flakey.
GR1915's assumed reduction to SR means that it can't deal with acoustic metrics, so technically it isn't a truly general theory. It successfully predicts gravitomagnetic effects, but these aren't compatible with the theory's underlying SR equations of motion, which assume that velocity-dependent distortions don't exist. GR1915 also can't be used as a method to apply the "curvature" paradigm to moving particles, for the same reason, the incompatibility of these distortion effects with an assumed underlying Minkowski metric. This makes our standard theory of curvature incompatible with particle physics, and with quantum mechanics.
As a theory of the very large, GR1915 was originally used to explain why the universe wouldn't show a distance-dependent redshift effect (as we'd expect from the cumulative effects of gravitation), just a few years before we discovered that the Hubble shift did actually exist. We reacted by dropping Einstein's "cosmological constant" and saying that the theory had been right all along, just misapplied. We retrospectively redefined the theory and its pre
Eric Baird
Sorry, I think we've passed into the-moon-landing-was-faked territory. I don't see anything further coming out of this conversation.
No, it's just that when someone says that a scientific theory works "extremely well", I expect them to be able to back up their case with some sort of argument or example. There should be some sort of evidence of a objective assessment process having taken place. Pure assertion doesn't cut it.
As you'll know if you've studied the history of GR, some of the early claims made for the theory's wonderful accuracy were seriously overblown. Some of these might have been honest mistakes, some might have been down to "cheerleading", and some, doubtless, were made honestly by people who'd been misinformed by authority-figures that they trusted, and who were just repeating what they'd been told in good faith.
When Einstein presented gravitational shifting as the third key test of general relativity, he may have been honestly unaware that the effect had already been predicted by Michell back in the Eighteenth Century. When the effect was "definitively established" in ~1924 it was with a dubious experiment by Adams that later led to suspicions of fraud. All the claimed verifications of GR's gravitaitonal redshifts performed before the 1960's are now considered to be basically junk science. People's enthusiasm for "proving" the theory tended to overrride more boring scientific considerations.
When proper verifications were finally carried out (Pound-Rebka-Snider, 1960's), the relief that we finally had a proper verification of gravitational shifting then led to another round of over-enthusiastic claims - an outcome that was indistinguishable from a reworking of Michell's 1783 calculations somehow got presented as "proving" Einstein's GR, to umpteen decimal places. Modern texts now acknowledge that this class of test is more correctly considered a verification of the equivalence principle, rather than of something GR-specific. :(
However, as late as the 1990's I was still coming across GR guys who'd try to convince me that Pound-Snider was a definitive proof of GR. But I'd worked through the math with a pocket calculator, and they hadn't.
Things aren't all bad. There are a few GR people out there who, as well as knowing current GR, have also checked out the background history and the experimental evidence, and who do try to represent current GR honestly, warts-and-all. Cliff Will springs to mind as the prime example.
But there are a whole raft of other people out there who are still working from wildly optimistic statements about GR that were supposed to have been squashed half a century ago, and who think that they still hold. These things may have been told to them by their lecturers, who in turn may have been told them by their lecturers, but they ain't true.
Most of the ambiguity over these tests isn't GR's fault, because in the sort of range that we typically use for "the three tests", GR1915 or any successor theory would be likely to produce indistinguishable or nearly-indistinguishable results.
What it does mean though, is that anyone championing Einstein's general theory today is expected to know this history and be more careful than past writers about not making inflated claims.
I should probably repeat that I do like the idea of a general theory of relativity, and I really do like the idea of modelling relativistic physics as spacetime curvature. I think that general relativity, as a subject is very cool, and that the fundamental idea is spot-on.
But I think that any comments still being made in 2007 that the current version of the theory doesn't need revising when applied to more extreme situations, because it already works "extremely well" are misguided, and I'd invite anyone who believes that current GR is in a good state to go back and check the theory's history and its historical predictio
Eric Baird
Yes, that is exactly what Wiltshire just did. The answer is still "42" for the Hitchhiker's Guide cult, and the question was wrong until Wiltshire came along and fixed it!
Amen, brutha!
All 19 hijackers were known terrorists 09-10-2001. Lack of FBI intelligence does not justify warrantless wiretaps..