Certain subsectors of the scientifically-oriented blogosphere are abuzz — abuzz, I say! — about this new presentation on Dark Energy at the Hubblesite. It’s slickly done, and worth checking out, although be warned that a deep voice redolent with mystery will commence speaking as soon as you open the page.
But Ryan Michney at Topography of Ignorance puts his finger on the important thing here, the opening teaser text:
Scientists have found an unexplained force that is changing our universe,
forcing galazies farther and farther apart,
stretching the very fabric of space faster and faster.
If unchecked, this mystery force could be the death of the universe,
tearing even its atoms apart.We call this force dark energy.
Scary! Also, wrong. Not the part about “tearing even its atoms apart,” an allusion to the Big Rip. That’s annoying, because a Big Rip is an extremely unlikely future for a universe even if it is dominated by dark energy, yet people can’t stop putting the idea front and center because it’s provocative. Annoying, but not wrong.
The wrong part is referring to dark energy as a “force,” which it’s not. At least since Isaac Newton, we’ve had a pretty clear idea about the distinction between “stuff” and the forces that act on that stuff. The usual story in physics is that our ideas become increasingly general and sophisticated, and distinctions that were once clear-cut might end up being altered or completely irrelevant. However, the stuff/force distinction has continued to be useful, even as relativity has broadened our definition of “stuff” to include all forms of matter and energy. Indeed, quantum field theory implies that the ingredients of a four-dimensional universe are divided neatly into two types: fermions, which cannot pile on top of each other due to the exclusion principle, and bosons, which can. That’s extremely close to the stuff/force distinction, and indeed we tend to associate the known bosonic fields — gravity, electromagnetism, gluons, and weak vector bosons — with the “forces of nature.” Personally I like to count the Higgs boson as a fifth force rather than a new matter particle, but that’s just because I’m especially fastidious. The well-defined fermion/boson distinction is not precisely equivalent to the more casual stuff/force distinction, because relativity teaches us that the bosonic “force fields” are also sources for the forces themselves. But we think we know the difference between a force and the stuff that is acting as its source.
Anyway, that last paragraph got a bit out of control, but the point remains: you have stuff, and you have forces. And dark energy is definitely “stuff.” It’s not a new force. (There might be a force associated with it, if the dark energy is a light scalar field, but that force is so weak that it’s not been detected, and certainly isn’t responsible for the acceleration of the universe.) In fact, the relevant force is a pretty old one — gravity! Cosmologists consider all kinds of crazy ideas in their efforts to account for dark energy, but in all the sensible theories I’ve heard of, it’s gravity that is the operative force. The dark energy is causing a gravitational field, and an interesting kind of field that causes distant objects to appear to accelerate away from us rather than toward us, but it’s definitely gravity that is doing the forcing here.
Is this a distinction worth making, or just something to kvetch about while we pat ourselves on the back for being smart scientists, misunderstood once again by those hacks in the PR department? I think it is worth making. One of the big obstacles to successfully explaining modern physics to a broad audience is that the English language wasn’t made with physics in mind. How could it have been, when many of the physical concepts weren’t yet invented? Sometimes we invent brand new words to describe new ideas in science, but often we re-purpose existing words to describe concepts for which they originally weren’t intended. It’s understandably confusing, and it’s the least we can do to be careful about how we use the words. One person says “there are four forces of nature…” and another says “we’ve discovered a new force, dark energy…”, and you could hardly blame someone who is paying attention for turning around and asking “Does that mean we have five forces now?” And you’d have to explain “No, we didn’t mean that…” Why not just get it right the first time?
Sometimes the re-purposed meanings are so deeply embedded that we forget they could mean anything different. Anyone who has spoken about “energy” or “dimensions” to a non-specialist audience has come across this language barrier. Just recently it was finally beaten into me how bad “dark” is for describing “dark matter” and “dark energy.” What we mean by “dark” in these cases is “completely transparent to light.” To your average non-physicist, it turns out, “dark” might mean “completely absorbs light.” Which is the opposite! Who knew? That’s why I prefer calling it “smooth tension,” which sounds more Barry White than Public Enemy.
What I would really like to get rid of is any discussion of “negative pressure.” The important thing about dark energy is that it’s persistent — the density (energy per cubic centimeter) remains roughly constant, even as the universe expands. Therefore, according to general relativity, it imparts a perpetual impulse to the expansion of the universe, not one that gradually dilutes away. A constant density leads to a constant expansion rate, which means that the time it takes the universe to double in size is a constant. But if the universe doubles in size every ten billion years or so, what we see is distant galaxies acceleratating away — first they are X parsecs away, then they are 2X parsecs away, then 4X parsecs away, then 8X, etc. The distance grows faster and faster, which we observe as acceleration.
That all makes a sort of sense, and never once did we mention “negative pressure.” But it’s nevertheless true that, in general relativity, there is a relationship between the pressure of a substance and the rate at which its density dilutes away as the universe expands: the more (positive) pressure, the faster it dilutes away. To indulge in a bit of equationry, imagine that the energy density dilutes away as a function of the scale factor as R-n. So for matter, whose density just goes down as the volume goes up, n=3. For a cosmological constant, which doesn’t dilute away at all, n=0. Now let’s call the ratio of the pressure to the density w, so that matter (which has no pressure) has w=0 and the cosmological constant (with pressure equal and opposite to its density) has w=-1. In fact, there is a perfectly lockstep relation between the two quantities:
n = 3(w + 1).
Measuring, or putting limits on, one quantity is precisely equivalent to the other; it’s just a matter of your own preferences how you might want to cast your results.
To me, the parameter n describing how the density evolves is easy to understand and has a straightforward relationship to how the universe expands, which is what we are actually measuring. The parameter w describing the relationship of pressure to energy density is a bit abstract. Certainly, if you haven’t studied general relativity, it’s not at all clear why the pressure should have anything to do with how the universe expands. (Although it does, of course; we’re not debating right and wrong, just how to most clearly translate the physics into English.) But talking about negative pressure is a quick and dirty way to convey the illusion of understanding. The usual legerdemain goes like this: “Gravity feels both energy density and pressure. So negative pressure is kind of like anti-gravity, pushing things apart rather than pulling them together.” Which is completely true, as far as it goes. But if you think about it just a little bit, you start asking what the effect of a “negative pressure” should really be. Doesn’t ordinary positive pressure, after all, tend to push things apart? So shouldn’t negative pressure pull them together? Then you have to apologize and explain that the actual force of this negative pressure can’t be felt at all, since it’s equal in magnitude in every direction, and it’s only the indirect gravitational effect of the negative pressure that is being measured. All true, but not nearly as enlightening as leaving the concept behind altogether.
But I fear we are stuck with it. Cosmologists talk about negative pressure and w all the time, even though it’s confusing and ultimately not what we are measuring anyway. Once I put into motion my nefarious scheme to overthrow the scientific establishment and have myself crowned Emperor of Cosmology, rest assured that instituting a sensible system of nomenclature will be one of my very first acts as sovereign.
In GR, it’s not possible to assign energy and momentum to spacetime curvature with a stress-energy tensor that gives meaning to ideas like “the local energy density due to the gravitational field”.
However, you can still define a “gravitational stress-energy pseudotensor”, which although meaningless at a single point nonetheless gives sensible results when integrated, yielding the total energy and momentum in a region surrounded by flat spacetime, and the fluxes of energy and momentum in or out of such a region.
So energy doesn’t “disappear” when it turns into gravitational radiation, it’s just that the bookkeeping scheme becomes a lot more subtle. In Newtonian physics and SR, you can pick a reference frame and then talk unambiguously about the energy density at any single spacetime event. In GR, you can’t do that, but you can, for example, integrate this pseudotensor over several wavelengths of a gravitational wave to get a meaningful result for the energy and momentum that it’s carrying — a result that will tell you how much energy the wave will deposit in various kinds of gravity wave detectors.
See Misner, Thorne and Wheeler Chapter 20 for details. Here’s a quote from pp 467-468:
Greg – thanks for the explanation. It seems that at least my question was worth asking. I was exasperating to you sometimes, so I appreciate your efforts even more. I have seen some of the best writing in this thread explaining gravitational issues from you, Lawrence Crowell, et al, and you guys aren’t even being paid for it (I suppose!)
Happy Holidays. I suppose the summertime Christmas season comes across a little odd to traditionalists in Australia. It hardly ever snows here in SE Virginia anymore, so machts nichts for me.
PS – In the spirit of good will toward all men and women, the following is cute. Time grows short!
Synchronized Global Orgasm for Peace
Jason,
Einstein proposed the cosmological constant because he calculated gravity would cause space and thus the universe to eventually collapse to a point. Doesn’t this presume the universe is finite in the first place? It would seem that if the universe is infinite, all this gravitational tugging would create a gravitational equilibrium, as everything would be pulled in all directions equally.
Obviously I’m drawing a very simplistic picture, but this seems in general agreement, that radiation is the heating/expansion and gravity is the cooling/contraction. Yes the various interactions are complex, yielding much detail to be sorted out. Obviously radiation also carries away some of these complex gases and the collapsing gases also trap energy that is contracted. It still seems to me that the larger picture is still a cycle. Whatever complex structure does eventually collapse creates its own energy and the pressure and eventual radiation process breaks the complex structure down and radiates it away. What is radiated would seem to lose energy the further it radiates, for various reasons; 1) It runs into obstacles and passes energy to them. 2) The further it travels, the greater the area it must expand to cover. 3) To the extent light is affected by gravity, it must also have some counter-effect, thus has some extremely minute mass. Therefore light as particles might have gravitational effects on each other. Since evry point in open space has radiation crossing it from all directions, it would not be a pure vacuum, but it also may not interfere with the precise path of a photon, while serving to slow it. The combination of all these factors causes radiation to cool and eventually start to clump. Possibly there is a phase transition level that this clumping starts to become obvious, such as 2.7K. A dew point, so to speak.
Thus my earlier point that the reason space within the galaxy isn’t blueshifted. It’s the mass that’s collapsing, not the radiation.
As a satellite, it is in a fairly stable orbit, but presumably the sun will eventually expand and incinerate it anyway and its energy will radiate away, whether it falls in the fire, or the fire comes to it.
Lawrence,
That last paragraph was a response, not a quote.
Einstein proposed the cosmological constant because he calculated gravity would cause space and thus the universe to eventually collapse to a point. Doesn’t this presume the universe is finite in the first place? It would seem that if the universe is infinite, all this gravitational tugging would create a gravitational equilibrium, as everything would be pulled in all directions equally.
John, you can’t think of things as if Newton’s concepts applied, like asking what the net force is on an object. An infinite universe can exist in GR, and still has a tendency to “contract” in the sense of separations everywhere getting smaller with time. Maybe you could imagine it as being like tension in an elastic skin, but I don’t want to start misapplying analogies. Indeed, a finite but closed space would have no edge (and thus no preferred center), meaning that any given bit of mass would naively be expected to have no “net force” on it (from symmetry.) Then, you would expect everything to just sit as it was, or continue at the same relative velocity.
However, in old-fashioned GR such a space will actually suffer net contraction of separations imposed relative to the progression that would apply at constant relative recessions (IOW, the “velocity” of a given galaxy relative to us would decrease over time.) Of course, dark energy has complicated all of that, so now we find instead that the distant galaxy will be going even faster from us in the future. In any case, just intuitively working out apparent framings and implications doesn’t do justice to the way the universe actually works. That way is often very counterintuitive, and often (despite platitudes) only really describable in terms of advanced mathematical concepts.
“An infinite universe can exist in GR, and still has a tendency to “contract” in the sense of separations everywhere getting smaller with time.” – actually, getting bigger and bigger, but not as high a rate as if things continued with constant rate of recession. That’s similar in the finite but bounded universes (cycloid), but they could contract again later.
Neil,
I’m just not convinced the math transcends the logic. This isn’t a matter of wanting to hold on to my own beliefs, because I’ve certainly dropped more then enough beliefs over the years that a few more would mean very little, but the fact is that I’ve grown up around a bunch of first rate salesmen(horsetrading doesn’t get its rep for nothing) and what’s is going on has all the hallmarks of a conjuror’s trick, ie. get everyone so focused on one bright spot that they ignore everything else. The simple fact is that the equations are WAY off course. You don’t suddenly say, “Oh, wait, 70% of the universe much be dark energy to keep the theory on track.”, when your whole shtick is that the equations are the only way to go.
They also try to say that since time is a dimension, change is an illusion! Reality might be an illusion, but change is not, as we are all about to shortly find out, when the credit cards are all finally maxed out and lots of our little illusionary bubbles get popped. Including quite a few of those who thought they could make a living scribbling equations.
What you imagine is some radical new hypothesis is actually “Oh wait, the vacuum energy density that we thought was very low, and might even be exactly zero, is … very low, but isn’t exactly zero: it’s about 6 * 10^-30 the energy density of liquid water.” The only reason this tiny non-zero value comprises 70% of the energy density of the universe at large is because there is a lot of vacuum. And the reason it has taken 90 years to shift the estimate (tentatively) away from zero is because the effect it has on the things we can easily observe are very small. Stop blaming people in 1970 (or 1917) for not knowing what modern instruments would measure.
From the very start of GR, there were three different terms that could appear in the equation linking matter to spacetime geometry:
(1) The stress-energy tensor of matter and radiation
(2) The Einstein tensor, derived from the curvature of spacetime
(3) The metric tensor itself
These terms all share the property that they are “divergence-free tensors”; in the case of (1), the stress-energy tensor, the fact that it is divergence-free is what guarantees local conservation of energy and momentum. So Einstein knew that the most general form his equation could take would be to equate (1) with a sum of constant multiples of (2) and (3).
Assuming the universe was static, he first believed that he would have to fine-tune the “cosmological constant” multiplying (3) to achieve a static outcome. When Hubble’s observations made it clear that the universe was expanding, Einstein realised that he could get a perfectly sensible result by assuming that the cosmological constant was very small, and the simplest small value — zero — could not be ruled out by the observational data at the time.
So the fact is, we’re using the same equations for GR that we’ve been using for 90 years, and in the light of observational evidence, we’ve just (tentatively) refined the value of the cosmological constant from “very small, maybe zero” to “very small, but on the evidence to date about 6 * 10^-30 g/cm^3”. This is no different from using improved observations to refine estimates of the density of baryons — or any other empirical input into the theory that can’t be known from first principles.
No doubt some cosmologists in the past assumed prematurely that the cosmological constant was exactly zero. If the evidence continues to favour a non-zero value, they will — very sensibly — change their minds.
Good rundown of the issues, Greg. Now, the really interesting question – is it possible to get started on “why” the Cosmo constant is not zero, and is about what it is? (Also, how much cred to the idea that lambda is getting stronger with time?) And about that notion that some theory of vacuum energy gave a massive energy density to empty space (was it maybe about one planck mass per cuble one planck-length on a side?) Remember, “10^120 order wrong” etc. So, why “should” it have been that much, why isn’t it, etc., and can we use ZPE somehow etc in principle, even though very small? (Note to any reader, that Casimir force is required by basic QM and is calculated independently of the DE or lambda value.)
Since someone mentioned Arp above in the alternate community, well – red shifts must be real in general I suppose, but isn’t there some support for the peculiar “quantization” of the red shift measurements? Could it be, when the universe was small, standing matter waves set up various harmonics which collected around certain recession rates? (Yeah, the universe supposedly wasn’t a hypersphere anyway, but just curious.)
A lot on anyone’s plate to deal with, whatever you have time for, tx yet again.
Greg,
Thank you for making the effort to explain that to me. I’m not going to pretend I get all the terms, so let me put it in concepts gleaned from popular explanations; That the rate the universe is expanding doesn’t match what has been calculated from gravitational slowing of the expansion, therefore there must be some basic energy content to space itself, which is maintaining this expansion.
Since the further away a source is, the faster it appears to recede, the expansion must have been greater in the past and has been slowing since. Dark energy is that some of this expansion is due to the nature of space itself, not just the initial singularity. More recent measurements show there wasn’t a specific time, originally proposed about 7 billion years ago, when this basic energy of space took over, but that it is a even curve back as far as can be measured, which is currently about 9.5 billion years ago. This later, more accurate measure showed it in line with a potential cosmological constant.
Now my point has been that it all is a cosmological constant and that the further light travels through space, the more this effect affects the rate of wave propagation, thus compounding the effect, so that the further the light travels, the greater the rate of redshift, such that the source appears to be receding at an increasing rate. If the source is far enough away, it will appear to recede at the speed of light and any source beyond that distance will effectively be over the horizon line.
Since gravity curves space and we detect this by the fact that the path of light is bent, this must mean gravity affects light and presumably then, light would have some mass content for gravity to affect. Otherwise gravity would not bend light.
So what would the effect of light on light be, given that light waves continually cross each other in open space? The reason tired light was dismissed was because the path of light was not disturbed by whatever caused it to redshift. but gravity only causes the path of light to bend, it doesn’t otherwise distort or scatter it. Therefore if there is some very minor effect of light on other light, it wouldn’t distort its path, other then to slow it somewhat. That would explain why quasars have a much higher redshift then associated galaxies, since the local radiation is so intense.
As Omega is very close to one, the contraction of space by gravity balances its cosmic expansion, so it seems less complex to consider that expanding radiation and collapsing mass are two sides of a cycle, of which the redshift is as much a component of the radiation, as gravity is of mass.
As for my perspective on the philosophy of modern physics, consider this analogy; Say you have a factory and accounting says there are 8,000 widgets in stock, but the stockroom says it only has 3,000. Most likely the paperwork is ahead of or behind the distribution and you just have to give it time to straighten out. If that doesn’t solve the problem then either the accountants screwed up or the stockroom people misplaced 5,000 widgets. So to solve the issue, you examine both sides of the question. For modern physics, this doesn’t seem to be the case anymore, because the math no longer is simply a model of reality, it is the reality. It has gone from being a tool to being a God and God is never wrong. You can’t say it’s just due to discrepencies between the model and reality, because the model is reality and you can’t say the math is screwed up, cause it’s the word of God. Therefore there must be 5,000 missing widgets and if you want a Nobel on your resume, you look for the widgets, you don’t question the math.
Admittedly I’m more of a stockroom type than an accounts department type, but my model only supposes that light affects other light to an extremely minor degree, but measurable over intergalactic distances, not that the universe began in a faster then light flash and is 96% invisible to everything but the theory.
John, one thing to avoid confusion: You said, “Since the further away a source is, the faster it appears to recede, the expansion must have been greater in the past and has been slowing since.” – The Hubble rule means that right now, or any observer in the past or future, is going to find a galaxy twice as far away moving away at twice the “velocity” (I put in quotation marks since it isn’t a simple matter of things just moving in space, but some analogies apply.) (Or, is even that not so simple anymore?)
That is not to be confused with the change in rates of acceleration (“deceleration” if want to refer to inward accel. vector that way) with time. That in turn is not to be confused with the change in rates of acceleration that would occur even if there wasn’t any dark energy (just from a galaxy being subject to a lower density of matter over time – you can “pretend” that the galaxy is being pulled toward us by the mass in the enclosing figural sphere made by a radius between us and it, to get the right rate in the absence of DE, IIRC. So, both the velocity away from us and the acceleration toward us of a given galaxy were higher in the past, but the devil is in the details.
The important thing to remember is, mass density of the universe changes with time. But if the DE is an intrinsic property of space, it likely stays the same (same value of acceleration/r) which means it will predominate as time goes on. But I hear that Lambda (the factor showing influence of DE) may change, such as increasing with time, which has even more effect in the future. It could literally tear atoms and even particles apart someday by making every point in space like a sort of reverse black hole.
Producing high energy/low entropy?
Milky Way’s black hole seen as particle smasher
Feb. 27, 2007
Courtesy University of Arizona
and World Science staff
Scientists were startled to find in 2004 that the center of our galaxy is emitting gamma rays, the highest-energy form of light. Now astrophysicists say they’ve discovered what might produce these.
A black hole believed to lurk in that place, they propose, could be a cosmic form of particle accelerator—a machine built to smash subatomic particles together in order to understand their components.
The black hole, according to this view, would rev up particles known as protons, parts of the cores of ordinary atoms, and smash them at near-light speeds in to lower energy protons. The collisions would produce gamma rays.
The Large Hadron Collider is expected to be able to accelerate protons to seven trillion electron volts, a measure of energy. Our galaxy’s blackhole whips protons to up to 100 trillion electron volts, according to the new study. That’s all the more impressive because “Our blackhole is pretty inactive compared to massive black holes sitting in other galaxies,” Ballantyne said.
A blackhole is an object so tightly compressed that its own weight creates gravity that sucks in anything within a certain range, including light. Most galaxies are thought to harbor central, huge blackholes dubbed supermassive blackholes.
Powerful, chaotic magnetic fields accelerate protons and other particles near our blackhole to extremely high energies, Ballantyne’s team argued.
The Milky Way blackhole “is one of the most energetic particle accelerators in the galaxy, but it does this by proxy,” Melia said. It cajoles a magnetized plasma, or electrically charged gas that’s “haplessly trapped within its clutches, into slinging protons to unearthly speeds.”
http://www.world-science.net/othernews/070227_blackhole.htm
Neil,
I have absolutely no idea why the cosmological constant has the value it has. There are lots of proposals, most of which involve new and untested particle physics, and which I am not qualified to assess.
John,
Light has two effects on other light. The first is that light, like everything else, is both a source for spacetime curvature, and is affected by spacetime curvature. The second is that there is a tiny probability for two photons to interact and produce particle-antiparticle pairs. Both of these effects are quantified by well-tested, and well-understood theories, and do not explain the cosmological red shift. There is no room for you to insert some new process into the mix, because (a) that would contradict existing observational evidence about the behaviour of light, and (b) contrary to what you assert, it would suffer from all the side-effects of other “tired light” theories, i.e. any effect strong enough to account for the red shift would degrade distant images, which is not what is observed.
BTW, light does not have “mass content” (though it has energy). The fact that the path of light is “bent” does not imply that it has mass, it is a consequence of the fact that there are no straight lines in curved spacetime that anything could follow. If you live on the surface of a sphere, are any of the great circles going to obey Euclidean geometry?
Contrary to your beliefs, people have spent a considerable amount of effort questioning GR, proposing new interactions, etc. The reason these efforts don’t get much press is because they have yielded no results. If you want a Nobel on your resume, you overturn all of existing cosmology. This is, however, easier said than done.
You seem to think it’s some kind of massive con trick to propose that there are forms of matter in the universe that we’re having trouble detecting by anything other than their gravitational effects, but since there is no fundamental reason why everything in the universe should interact with us by any other means, it’s actually not surprising that this is the stuff we’re getting around to finding at this point in history — after all the other easier invisible things, like the non-visible 99% of the electromagnetic spectrum, have been identified. Whether dark matter and energy were 0.1% or 96% of the energy density of the universe is completely beside the point; as I’ve already explained, dark energy is 30 orders of magnitude less dense than the matter around us, so complaining that we would have noticed it earlier if it was real is as silly as complaining that if neutrinos were real, and trillions of them were streaming through our bodies every second, then the Ancient Greeks would have known all about them and calculated their mass to six decimal places.
#137. So what? If you imagine this supports any of your gibberish about cosmic convection, I’m afraid you’re mistaken. Lawrence has already summarised the whole jets thing once, but you paid no attention to him, so there’s no reason for anyone else to waste their time re-explaining it to you.
Neil,
If space expands(due to increasing energy from light radiation), but then falls into gravity wells, it would appear that every galaxy is flying away from each other and the further away, the more space light has to cross, so the more the effect is multiplied and the faster the source appears to recede. This is what we see and we don’t need a singularity, Inflation, or dark energy to explain it.
Well, if we are going to talking about weird notions, try this one:
Time is running out – literally, says scientist
By Tom Chivers and Roger Highfield, Science Editor
Last Updated: 6:01am GMT 18/12/2007
[pieces]
The idea that time itself could cease to be in billions of years – and everything will grind to a halt – has been set out by Professor José Senovilla, Marc Mars and Raül Vera of the University of the Basque Country, Bilbao, and University of Salamanca, Spain.
The motivation for this radical end to time itself is to provide an alternative explanation for “dark energy” – the mysterious antigravitational force that has been suggested to explain a cosmic phenomenon that has baffled scientists.
The problem is that no-one has any idea what dark energy is or where it comes from, and theoreticians around the world have been scrambling to find out what it is, or get rid of it.
The team’s proposal, which will be published in the journal Physical Review D, does away altogether with dark energy. Instead, Prof Senovilla says, the appearance of acceleration is caused by time itself gradually slowing down, like a clock that needs winding.
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“We do not say that the expansion of the universe itself is an illusion,” he explains. “What we say it may be an illusion is the acceleration of this expansion – that is, the possibility that the expansion is, and has been, increasing its rate.”
Instead, if time gradually slows “but we naively kept using our equations to derive the changes of the expansion with respect of ‘a standard flow of time’, then the simple models that we have constructed in our paper show that an “effective accelerated rate of the expansion” takes place.”
http://www.telegraph.co.uk/earth/main.jhtml?xml=/earth/2007/12/18/scitime118.xml
PS – I don’t think that is any worse than the idea that collapses are an illusion, that there are multiple worlds, etc.
Electromagnetic radiation does not cause anything to expand. If the universe is initially expanding, radiation acts to slow down the rate of expansion.
As far as I can tell, you claim to believe in GR. So start paying attention to what it actually implies, not what you wish it did. The scenario you describe is not a solution of the equations of general relativity, nor is it compatible with what we observe, and there is no change you can make to the properties of light that would make these problems go away.
Every now and then you admit that you don’t have any real knowledge of this subject, but you never follow through with the consequences of that admission: you are no more competent to devise and test a cosmological model, or to assess the existing models, than I am to do brain surgery. I happen to believe that anyone on the planet who isn’t intellectually disabled could learn general relativity if they really wanted to, but you’re suffering from a fantasy where your intuition provides a reliable short cut that lets you make judgements about cosmology without ever learning any of the details of the subject.
PS – I don’t think that is any worse than the idea that collapses are an illusion, that there are multiple worlds, etc.
Until you know how to calculate the effective density matrix of a quantum subsystem correlated to inaccessible degrees of freedom, what you think about collapse is about as relevant as Paris Hilton’s thoughts on the Middle East peace process.
Greg,
I realize I’m proposing ideas that have been considered from every possible angle, but;
If what I’m trying to describe amounts to an opposite effect of gravitational curvature, ie. expanding hills between those collapsing gravity wells, why would it degrade the image, since curvature doesn’t?
Gravity lensing also magnifies the light that it bends and thus allows us to see further; http://www.space.com/scienceastronomy/050503_grav_lensing.html
“When a galaxy sits near the line of sight to distant quasars, two things happen (see accompanying figure). The quasars are magnified to look brighter, and their apparent positions in the sky are shifted.”
No one argues that gravitational lensing distorts the image, just shifts its position and magnifies it, so the opposite effect, spead across the volume of space between galaxies, would mostly just make the source appear moving further away, since there is no gravitational vortex to curve it around.
I may seem thick-headed, but then I am thick-headed.
Greg,
So it can slow the expansion, but it can’t slow light?
John, think of the universe as like a balloon with spots on it, being inflated. The separations between spots increases all the time, but there is also the issue of change in the rate of the inflation of the balloon (BTW, I mean “inflation” of the balloon, not the theory of Cosmic Inflation, where the universe expands very much at a very early period of its existence.) Light tries to catch up with receding spots, and covers each little piece of rubber at the same local rate we are familiar with (crosses a set of adjacent spots at “c”.) IIUC, the light radiation in the universe does increase the overall average mass-energy density, which increases the mutual pull of things on each other – but I don’t think there’s any special odd trick about it, is there?
And, well, even Paris Hilton can appreciate that it’s better if they get along with each other over there than not, eh? 😉
I doubt that Paris knows at least that a density matrix is about the statistical properties of the system, and there wouldn’t even be a “statistics” (meaning, patterns of counts or “hits” of some type, right?) if the waves didn’t already have something to collapse them (into places, like scintillations on a screen) in the first place. (IOW, they would just stay waves forever.) Well, that’s just a rough general argument, whither I know not, but I won’t say the alternative represents evil, lunacy, etc, as I would in my wilder days.
John
Firstly, gravitational lensing itself does not produce any frequency shift, either red or blue. If I am observing two stars in the same galaxy a few million light years away, and the view of one of them is subject to gravitational lensing by some compact object in the line of sight, those two stars will still have identical red shifts.
Secondly, the question of image degradation depends on the detailed geometry and magnitude of the effect: a tiny amount of gravitational lensing by a single compact object causes very little distortion of the image, but your proposed “anti-lensing” due to light spread out across intergalactic space (a) is simply not what light does to other light via gravity, and (b) if there was such an effect, it would degrade the image, because it would involve billions of small interactions. That light and gravity are involved doesn’t magically make the image as clean as one lensing event by one galaxy; billions of small interactions will always give a result that is essentially the same as positing that the red shift is due to something like intergalactic dust.
No, it can’t slow light. Nothing can slow light.
Stop positing ad hoc nonsense, and make up your mind what your theory is supposed to be. Half the time you claim to believe in GR, and insist that cosmologists have just neglected what GR itself predicts, but the other half of the time you feel free to assert any magical effect of light and matter on each other that takes your fancy.
Either come up with a coherent theory to replace GR (and which reproduces all its successes), or accept what GR predicts, which is that the radiation in intergalactic space has no chance whatsoever of explaining the cosmological redshift.
I suppose you think it’s meaningless to talk about the statistical properties of the frequency components of a complex electromagnetic waveform, unless the waveform can be literally, physically rendered monochromatic. Just because individual radio receivers couple to narrow frequency bands doesn’t require the whole electromagnetic field across a city to “collapse” into just one of those bands.
Quantum mechanics is a linear theory, and in every situation where we can test that hypothesis, it is validated. If a photon striking point A is a valid solution in a particular setup, and a photon striking point B is also a valid solution, then so is any linear combination of both A and B. Complaining that you’ve never personally witnessed A and B together has no force as an objection once you analyse what’s involved when you observe something; you can’t just magically sense what a quantum mechanical state vector is, there has to be a process by which you become correlated with the system in question. When you end up with distinct perceptions and memories correlated with the outcomes A and B — but no access to the billions of other microscopic degrees of freedom that are also correlated with those outcomes — then your perceptions will contain nothing that identifies the continued coexistence of the two outcomes. You will perceive a collapse, when there has been no collapse.
Now this might or might not be what actually happens, but it is certainly what linear quantum mechanics predicts.