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.
Greg,
The problem isn’t with the math for measuring redshift, but the reality of what is being measured. As you describe it, redshift is due to the classic Doppler Effect of motion in a stable medium/geometry causing light/sound waves to be shifted(red or blue) relative to the rate the source is receding/approaching. The problem with applying this concept to cosmological redshift is that if it is classic Doppler Effect, where the redshift is caused by recessional velocity in a stable medium/geometry of space, then it would appear that we/the Milky Way, are at the center of the universe, because all the other galaxies appear to be directly redshifted away from us. Big Bang Theory was amended by saying that it is space itself that is expanding and everything is the center of its own perspective. This is a reasonable solution, except for the point I’ve been raising, that if space is expanding, then our most fundamental measure of it, the speed of light should expand along with it, otherwise C is measuring a stable, not an expanding dimension of space. The solution really does lay at the heart of Relativity, as Lawrence keeps hinting at, but as it is currently formulated, Relativity Theory hasn’t fully escaped classical concepts of time and space. As I keep pointing out, three dimensions are a coordinate system, not space itself and every clock is its own dimension of time, since time is a consequence of motion, not the cause of it. What this means is that cosmological redshift is due to relativistic effects, ie. a cosmological constant, which is, just as Einstein originally proposed it, a balance to gravity that results in a stable universe, not one which is actually expanding or contracting.
That’s not “as I describe it” at all. Cosmological redshift is best described in terms of expanding space, and I have no idea why you imagine that I’m describing it in terms of relative motion. As others have explained, and as I entirely agree, relative motion is a poorly defined concept for objects separated by cosmological distances.
Whether you think the speed of light “should” or “should not” increase as part of your notion of what it means for space to expand is beside the point. The thing is, there is a perfectly coherent notion of space expanding without the speed of light changing, and that notion is the one asserted by cosmologists. This is clearest in the case of a finite universe, when you can talk about the total volume of the universe at any instant of cosmological time, and that volume is increasing. The speed of light stays the same, and as a consequence the transit time for light between any two galaxies increases. This notion of expansion is perfectly coherent as an idea, and perfectly consistent with observation.
I’m sorry, but this is simply false. General relativity makes perfectly clear predictions for redshift in a stable universe where a cosmological constant balances gravity: it says that there are none. So there are no relativistic effects in the situation your propose that would explain the observed redshifts.
Killing vectors are problematic in cosmology. If a spacetime has a Killing vector K = K^a nabla_a, then the Killing theorem is that K as a differential operator acts on the metric as
K*g_{ab} = 0,
which is defined according to the Lie derivative. For the metric as a two-entry device:
g_{ab} = g(X_a,X_b)
K*g_{ab} = Lie_K g(X_a,X_b) = 0.
Now that we have set that up let us consider a line element for a cosmology. The FRW type of line element
ds^2 = g_{ab}dx^a dx^b
ds^2 = -dt^2 + R^2(dr^2 + r^2 d(angular stuff)^2),
where R is the “radius” with R = R(t). Now let us propose there exists a timelike Killing vector K_t = A(&/&t), & = partial. It is not hard to see that this K_t will not give zero on the metric! So there is no time-Killing vector. Now the Killing vector projects onto the geodesic flow-lines in the space. A timelike Killing vector will project on the energy component of a four momentum so that K_t E = constant. But we can’t impose this on the above cosmology.
As strange as this might appear it means that energy conservation can only be said to be a local effect. Now, when it comes to the “dark energy,” there is the equation of state e = -p, for p a pressure. The pressure term is spacelike and it is possible to say, “Well a spatial Killing vector K_i can be found, and so I will then say that e = -p imposes an energy conservation on the cosmology.” But before we start drinking shots of tequila over our cosmic coup, remember that e = -p is artificial in a way. We are treating the cosmological constant as a field source, which removes the Einsteinian condition of the manifold (spacetime) and thus violates some aspects of general covariance. So our identification e = -p can only at best be a local result and not one which pertains to the whole cosmology.
This is a bit odd, for in physics we are taught all sorts of nice things about energy conservation. But in cosmology it turns out that energy conservation is not something which can be well established. We have a notion of time, and the standard logic is that the generator of time is the Hamiltonian, or energy. Think in quantum mechanics where exp(-iHt) is the time development operator and so forth. Yet here we see that the idea of global energy conservation can’t be substantiated, and the definition of time in cosmology is local as well.
Lawrence B. Crowell
No, but the spacelike Killing vector does a perfectly good job at deriving the cosmological redshift, which was what I was using it for.
The lack of a global energy in GR — except in asymptotically flat spacetimes — is just one of many headaches for quantum gravity, but classical cosmology gets along just fine without it.
I looked at your discussion above, and it does appears as if you are appealing more to momentum Killing vectors, in that the ratio of momenta are going to be given by the reciprocal of their ratio Killing vector components.
As for this being a problem for quantum gravity, that depends. In one way this lack of global energy might be seen as an ally.
Lawrence B. Crowell
Greg,
Is that due to other galaxies redshifted as if they are all moving directly away from the Milky Way?
Increasing volume isn’t proof of expanding space, since space outside this volume logically becomes space inside it. Do cosmologists know so much that they can categorically say that space doesn’t exist, except as the carpet of this proposed expanding universe rolls out? The fact that the speed of light doesn’t change is proof that space as measured by C and that inside the bubble of an expanding universe are not the same. So which defines the real dimensions of space, that which is stable, or that which is expanding?
That’s the hitch. Because there is no relativistic explanation for why light would be redshifted, current cosmology assumes the entire universe is expanding. In order to support this assumption, increasingly exotic formulae are conceived to explain the various loose ends, from Inflation to Dark Energy. Yet all it would take to knock the whole house of cards down would be an explanation for relativistic redshift. Since gravity causes space to contract and mass eventually breaks down and radiates the energy back out, I suspect it would have something to do with the effect of radiation on light waves. Of course, it’s much easier to go along with the crowd, especially if you are trying to make a living at this, but it is a hobby for me, so I have the freedom to consider all the options.
A few points to J.M’s post. There is no space that the universe is expanding into. There is no boundary to the universe which demarks some sort of “front” for the expansion of the universe.
To say there is no relativistic explanation of redshift probably needs to be qualified. The redshift is not strictly a special relativistic motion of bodies, though that can be used as an approximation for galaxies somewhat local to ours. The expansion of space is a matter of general relativity, so in that sense it is a relativistic effect.
Einstein originally imposed the cosmological constant and the term /g_{ab} as a way of keeping the universe static. One must realize that in 1915-20 the universe was thought to be the galaxy, and debate raged about whether galaxies, Messier objects, were just more local nebua. So Einstein’s assumption was that the universe was some sort of static space. Hubble came along as showed not only were galaxies beyond the Milky way, but that they were expanding away. Einstein revoked his /-term and called it his greatest blunder. Now the term is being resurrected in a different guise.
In a static universe with / set to keep things stationary there would be no redshift, and Hubble’s observation of galactic redshift blew the static cosmos idea to bits — literally in a big bang.
Lawrence B. Crowell
John
There’s a moderately useful, but not perfect analogy which goes like this:
Big Bang = South Pole
Time = angle from south pole to some circle of latitude
Universe at a given cosmological time = some circle of latitude
World line of a galaxy = some meridian
World line of a free-falling body moving from galaxy to galaxy = portion of a great circle other than a meridian, but inclined closer to a meridian than to the circles of latitude
Velocity someone in a galaxy measures for free-falling body that leaves or enters that galaxy = tan(angle between the meridian for the galaxy and the great circle for the free-falling body)
This is an aid to visualising some of the concepts in cosmology, but it doesn’t function as a precise computational tool. Also, at the risk of stating the obvious, the interior of the Earth and the space around the Earth in this analogy don’t correspond to anything. The universe isn’t curved by being embedded within some larger, flat, higher-dimensional space; it’s just intrinsically curved.
Galaxies are moving away from each other in the sense that meridians on the Earth are moving away from each other. It takes longer to travel from 10 deg W to 20 deg W as you move away from the south pole and get closer to the equator, because the circles of latitude are getting larger.
As I mentioned in #50, there’s a nice analogue of redshift: your east-west component of velocity drops as you move from the antarctic towards the tropics along an inclined great circle, becoming multiplied by the ratio of the circumference of the circle of latitude you left behind (i.e. the size of the universe when you left) to the circumferences of the circle of latitude you’ve arrived at (i.e. the size of the universe when you reach your destination). Something analogous happens to momentum in the real universe.
If you try to construct paths on a plane that match this behaviour in detail, you’ll find that you can’t (as map-makers have known for centuries). Similarly, you can’t match the detailed behaviour of galaxies by positing that they are just moving away from each other in some pre-existing flat, empty space.
In the analogy, circles of latitude get bigger, but there is no “space outside them” which is being converted to “space inside them”. That is what is meant by expanding space.
It’s actually very simple: all local ways of measuring distance will agree on the result. At 85 degrees South, let a billion people spread out evenly on the circle of latitude measure the distance to their neighbour by any sensible means. Because there’s little curvature on that scale (a distance of about half a millimetre from neighbour to neighbour), the result will be virtually independent of their strategy. For example, they can send a great circle to their neighbour at a 45-degree angle to their meridian, and have the neighbour respond the same way, and then they halve the distance they travelled along their own meridian between sending out this “light signal” to their neighbour and getting a reply. When all the inter-neighbour distances are summed, that is an excellent estimate of the size of the circle of latitude (aka size of universe).
When they repeat the experiment at 80 degrees South, they will get a larger inter-neighbour distance, and hence a larger total size for the circle of latitude.
Note that the role of the fixed speed of light here is played by sending out the great circle at an angle of 45 degrees each time. This won’t give sensible results over vast distances, but for checking on your neighbour half a millimetre away it will work extremely well.
What you mean by this (for people who haven’t followed previous comments closely — if in fact there is anyone but the two of us still bothering to read the soggy dregs of this thread at all) is “an explanation for relativistic redshift in a static universe“.
The hitch, for you, is that we already know how relativity works, and that it doesn’t predict redshift in a static universe. So rather than using the adjective “relativistic” here, you really ought to be saying “completely new theory of space, time and gravity which explains all other observations, and gives a redshift in a static universe”.
You’re free to consider options, but you’ve neither described a coherent alternative theory of space, time and gravity, nor motivated any need for such a theory. If you ever do come up with a testable alternative theory that can be used to make numerical predictions of what people will measure in any situation, then maybe someone will be willing to put it to the test. Right now you’ve neither provided that theory nor identified any need for it.
Lawrence,
Einstein originally proposed the cosmological constant because his calculations showed that gravity would cause the entire universe to collapse to a point, so he added an arbitrary factor to balance it. Since gravity is contraction, the CC is effectively an expansion. That it was resurrected to explain dark energy would seem to prove this is accepted.
If one is only looking at bodies in the local galaxy, it would make sense to estimate they are all falling into a vortex. All those other galaxies are also gravitational wells, pulling mass and its attendant spacetime into the central black hole. So this is a shrinking of spacetime. Redshift shows the spacetime between galaxies is expanding. If we have contraction and expansion and they are roughly in balance, as in the concept of Omega=1, as was explained by Hawking, in ABHoT, (Tests by COBE and WMAPP seemed to show this balance exists.) then where is the additional expansion for the entire universe to expand? It would seem from this perspective, that Hubble found Einstein’s Cosmological Constant. It just happened to be between galaxies, not in them. Given that what Peremutter, et al, found in ’98, as well as subsequent studies, redshift matches the predictions for a cosmological constant far better then it did for any predictions based of Big Bang theory.
Greg,
It’s not that I don’t have a reasonably good understanding of Big Bang theory. I’ve been following its evolution since the ’70’s and took for granted that it was a sensible explanation of the universe. It was reading Hawking’s ‘Brief History of Time,’ in’89, where he gave a short description of Omega, that I began to question it.
A compounding effect would also cause this, so that the further light travels, the faster the source appears to recede.
This still doesn’t explain why the speed of light is stable, irrespective of the size of the universe. If all space is squeezed into that point at the south pole and expands from there, how is it that we have an extremely stable measure of space by which to measure the increasing distance???? If the entire universe was the size of a beachball and all space was squeezed into there, it should take light just as long to cross the same proportion of this space then, as whatever it has expanded to now, or it isn’t expanding space, it’s increasing distance. The speed of light is about 186 thousand miles a second, so whatever the age of the universe, it would seem a mile is a mile is a mile.
That redshift is due to an expansion of space that is opposite the contraction of gravity, so that the two balance out and there is no overall expansion. (Thanks for taking the time to debate this. It’s more mentally engaging then the day job.)
It did predict the need for a cosmological constant to maintain a stable universe. It seems the effect of gravity contracting space gets completely overlooked in the glare of an expanding universe! If they are in rough balance, where does the additional expansion to increase the size of the universe come from???? It’s not a static universe, it seems to be a stable universe, where the space expanding between galaxies falls into galaxies!!!!
It isn’t a matter of testability, but of interpretation. Big Bang Theory predicted a completely different rate of expansion then what has been observed, but there is no questioning of theory, the predicted energy density of the universe is simply expanded several times over to fill the gap in the theory. If Bush had as much leeway as cosmologists, we would still be looking for WMD’s in Iraq!!! I suspect we will still be looking for dark energy several decades from now and spent enormous amounts of money doing so.
I’ve been watching this thread develop- with interest.
I believe a correct overall understanding of the geometry of the universe AND the relationship between the way the universe actually exists compared to the way it is observed and measured is the key to understanding dark energy.
The Planck Realm is everywhere and is entangled with what we, from our frame of reference, call energy densitiy black holes of assorted “sizes”.
If we add an additional three-space to the cosmic geometry (a slightly modified Schwarzschild “mirror geometry” with a Planck Realm, massed event horizon rather than a point mass) and assume space in the universe to be locally flat but marginally closed globally…Omega total 1.02, we obtain a universe which is finite in mass yet everywhere…there is nothing outside it.
Because this two-sphere geometry has two sets of global coordinates which fit the GR equations, postitive and negative, within this model the big bang becomes the equivalent of a super massive white hole, and photons (which do work from an entropy standpoint), become the inverse of the singular Planck Realm- in which entropy is presently observed to increase.
This universe we observe only exists on event horizon surfaces of varying sizes…as “particles” and bodies made of these energy density particles of varying sizes and composition…all based on the great proportion. At its heart, everything in the universe is singular, but depending on the coordinates from which the universe is observed, the universe might not seem to exist at all or be as vast as we observe it to be…13.7BLY in radius.
A singular Earth would (and eternally does) have an event horizon surface the size of a golf ball, but its overall gravitational moment eternally remains 1G when measured 4,000 miles from its center, whether is is particulate as at our coordinates in space and time, or as it existed at the big bang- and will exist again, with the rest of the universe in the future.
We observe motion and change-time- when we observe (cross-read) a general, everywhere proper time pulse of the universe which occurs 2.8 trillion times per Earth second, remotely in extreme gravitational time dilation. From our frame. we know relativistically that time is a quatity which is dependent on the relationships of events and observers, yet we fail to recognize the eternal nature of the whole…that everything…all information exists permanently, but is observed more “slowly” and sequentially from differing frames of reference.
The distant universe which we see as progressively younger with “distance” and emerging from the big bang, is, within the second three space simultaneously collapsing to a singularity. From our frame, the universe is “eating itself alive” consuming itself progressively as black holes add to the cumulative mass of the dark energy Planck Realm at the overal expense of particulate existence.
There is no stange “dark energy” at all. What we are observing is the process of simultaneous emergence from a big bang and collapse to singularity of the same universe. The effect: we, from our frame, observe an eternal existence as both a phenomenon of expansion and a cosmic contraction. The doppler effects are real. So is acceleration outward, just as our 4D particulate reality is real. Relativistically, the way we observe the universe IS our reality. We can build a technology on the way the universe is observed and measured. We occupy an accelerated frame of reference- and observe an Earth and universe which are, themselves the result of relativistic effects,
At some point in “time” in the future, as the mass of our cosmic “hemisphere” increases, the Planck Realm wil rise to meet us. At a critical point, what we now observe as almost perfectly flat space will almost instantly cease to exist- and what is remaining of particulate reality with it. However, also simultaneously, the 3 space which has been filling with the matter and energy of our half of the universe will remain- as the universe always exists, everywhere and forever, preserving all information in CPT reversal. As quickly as our “hemisphere” dissappears, it will reappear in opposite polarity, in a hot big bang, pretty much as described in the standard model.
How can we measure the fact that circles of latitude are getting bigger? Very easily: we use measuring rods, or we look at the return of “signals” that we bounce off our neighbours on different meridians at a fixed angle. Measuring rods measure metres, not degrees of longitude; they don’t expand as we move from the south pole towards the equator, conspiring to conceal the expansion of the circles of latitude from us.
Equally, there is nothing to be puzzled about with the situation in cosmology — unless you’ve taken the universe-as-inflating-balloon metaphor too much to heart, and you imagine that all measures of distance are somehow painted in ink on the balloon.
Distance doesn’t work like that. We do not define the kilometre as a fixed fraction of the total size of the universe. Although nobody these days would talk about a “standard measuring rod”, that’s only because some fields demand ultra-high precision; for everyday purposes there’s no reason we couldn’t define “one metre” as the length of a particular metal rod. Atoms, and rods made of atoms, do not get bigger as part of the cosmological expansion (nor does Brooklyn). Nor is the speed of light magically compelled to change in step with the expansion; why do you imagine that it would be? These measures of distance remain unchanged, while the universe grows larger.
If you don’t like the phrase “space expanding” because it conjures up consequences for you that nobody is actually suggesting, well, go ahead and mentally call it something else. I’m happy to concede that “space expanding” is an imperfect, potentially confusing colloquialism. But the fact that you don’t happen to like a phrase isn’t enough to render the scenario for which cosmologists employ it logically flawed in any way.
The trouble with this claim is that when an expansion and contraction balance out and leave no overall expansion, they also leave no overall redshift.
You seem to have some misconception that galaxies could be “disposing of” the space that would otherwise be created by cosmological expansion. I’m afraid this just doesn’t work.
Yes, the presence of matter in the universe acts to slow the expansion, and in the absence of a CC and with omega > 1 would eventually reverse the expansion and cause a collapse. But galaxies are not collapsing, or at least not on any timescale that would allow them to do what you suggest they’re doing.
There are many individual red and blue gravitational shifts involved with the passage of light from a distant galaxy to us: the light climbs out of the gravitational well of a star, then a galaxy, then a galactic cluster, crosses inter-cluster space, then suffers blue shifts as it descends into our cluster, galaxy, solar system, and planetary gravitational fields. But the fact that the net result is a red shift means that there must have been a net expansion of the universe in the time it took for the light to reach us.
That’s dead wrong. What you propose would have observable consequences, and they are very different from what is observed. Apart from the overall killer that your model would actually lead to no cosmological red shifts, it would also imply that we on Earth would see significant blue shifts from everything else in the galaxy. That’s how a collapsing region of space would look.
As for the nature of dark matter and energy, I have no stake in any particular outcome, and I’ll be interested in whatever does or doesn’t eventually show up. But however cynically you choose to view the positing of previously undetected forms of matter, you need to be aware that what you are suggesting is an even worse fit for the observational data than a Big Bang model without recourse to dark matter or dark energy. You’re not solving any problem, you’re just making the discrepancy far worse.
Sam
As a mere mortal, I can only handle one Einstein at a time, so I’ll have to resist the temptation to give a detailed review of your truck-load of cosmic wisdom.
Greg, given good weed, we all become Einsteins and it often takes a good thrashing from a patient person like you before we learn that the things we view as problems with the orthodoxy are simply the product of our own lack of understanding.
Some of this is coming in pretty fast and furious. I will try to make a few comments here that are as brief as possible.
Things like distance and time are parameterizable. IN other words you can demark time with different units of “seconds,” say if we lived on mars or some other planet with a different frequency of orbit and so forth. We mark out months and weeks based on the lunar phase, but aliens on some other world might arrive at a different set of units. So how one chooses the unit of length or time is completely arbitrary and freely chosen by the observer or analyst. By extension the frame on operates on is a matter of choice as well.
On of the nice things about electromagnetism is that the fine structure constant alpha = e^2/hbar c is a pure number ~ 1/137 with no units. No matter where you are this is the same.
There is the bit about fractal geometry, where any subset is similar to a larger set or the whole set. The most rivial fractal is a straight line. Without units attached any segment of it looks like any larger or smaller segment. Much the same holds with time and distance. How there is the bit about the Planck distance, which can be found by equating the deBroglie wavelength of a black hole equal to its horizon circumference. This unit is
L_p = sqrt{G hbar/c^3}
Now this is an absolute unit of length, which even with arbitrary units reflects a discrete unit. There are similar units for time, Planck areas, even Planck voltages! Now if you rescaled the speed of light you would find that this unit is adjusted, but so would all the length scales you have. In doing so you find that Poincare’s curious observation holds: if the size of things were changed in some way how would you know it? So there is underlying this a conformal structure, which gets into Malcedena’s AdS/CFT and quantum holography and so forth.
So the speed of light is simply the speed of light and it is what it is on any local frame is “fixed,” and any idea of adjusting in for a local frame is just some conformal shift which leads to an invariance. The speed of light is “stable,” though I am not sure what is meant by this, in that it is simply “fixed.” Also in a pure unit sense, say if we equate distance with time the speed of light is then “unitless,” and so similar to the fine structure constant is then simply a constant.
Lawrence B. Crowell
HeeHee – Yes rw I do appreciate Greg’s patience (and erudition) and must admit I too was rather too much a handful of a “weed-smoking” Cocktail Einstein. (Greg, I hope you can forgive me for truly being obnoxiously persistent about something I should have at least let go for a good while – maybe I should get a job at “Gitmo” and I wonder what do you Aussies think of Americans’ love of torture? I guess you’ve had your personal fill, heh.) You bring up what is a good subject, the popularization, teaching, and debate about science, and the interactions of experts, semi-professionals (like science-fiction writers who do know a lot about real physics 😉 ) and amateurs and semi-amateurs who have some education and interaction with the scientific world but aren’t up there yet or may never be (like me.)
We can all do better in making these kinds of interactions. First, it is a bad temptation to tend to perceive that any “amateurish” sounding query is just clumsy ignorance of this or that (which it often is, but the good questions are all you have to bother with.) For example, my first point about gravitomagnetism was not based on the specific applicability to high speeds of the low-speed equations as was imagined, but rather on the symmetry requirement that the effects – whatever they were – of mass flowing one way should be canceled out by the same mass flow in the other direction. Well, maybe that is wrong too, but it isn’t the first kind of mistake that is explicitly warned against in a ready resource. Also, if someone wants to know how something fits into X framing of the issue, like “how does it fall relative to an actual structure” wouldn’t a good teacher first just answer that to get that satisfied, and then explain perhaps why it doesn’t in turn show Y (such as that the equivalence principle is thereby wrong, etc.)? When people present arguments using words like “acceleration” and different values for same under different conditions, that does involve semantics and needs elaboration. My mistake was to not retreat and work over the explanations and caveats right when shown them, not to have insisted to begin with that apparent contradictions and problems of meaning need to be resolved.
I won’t give up on making points about physics, but will try to make the Socratic part more truly Socratic – there is indeed a role for that. (To ask good questions to bring forth explanations, which is not the same as claiming that such and such *is* the case, and yes even to press for what happens in such and such a case that you might think isn’t “relevant” etc.) I think that many of the questions posed by Sam Cox and John Merryman are starts at good expressions of Socratic method. All they need is to realize they aren’t demonstrating conclusions thereby, but stimulating you patient and helpful folks to explain why or why not, etc, for which we are grateful for your time, effort, and tolerance of irritation!
Yet, I don’t agree that “the things we view as problems with the orthodoxy are simply the product of our own lack of understanding.” That presumes that orthodoxy must be right, and that is not warranted. More often, our thinking there’s a problem will be a misunderstanding, but it does not *have to be.* A non-specialist is unlikely to make headway in that regard, but the questions a good “student” can ask at least generate insight into what is confident, what isn’t (e.g. aren’t some of you arguing here over whether energy is really conserved in GR, how to define it, etc?)
One thing though, you folks need to know and accept if you aren’t going to be big hypocrites. A bit of good weed and/or scientific degree/s makes many thinkers become Platos and think they are experts at framing or even making final judgments on all philosophical issues like whether the universe is contingent or ontologically self-contained, how to think about unobserved entities like “other universes”, God (as something responsible for the universe/s apart from any sectarian complications), the framing of the anthropic principle as a conceptual issue not just what physical laws are involved etc. Over and over again, I see the same folks (some of whom so wary and weary of “amateurs” poking into their physics turf) just digging and expounding heartily and very confidently into such frontier philosophical questions despite their makeshift handling of the issues and frequent offering of fallacious boners and ignorant misframings of every type. I can assure you, as someone who has studied some of that – many or most of you (and that includes very much the OPs here) have little idea of what you are talking about at the quality level. You don’t appreciate when a philosophical screwdriver can’t or shouldn’t be used to open a philosophical can, etc. In other words, I know how Greg feels but applied to philosophical reasoning instead of science.
Ironically, I myself was getting irritated at amateurish digs at philosophical cases for God etc in the midst of metaphysical rambling about frontier physical questions, when I spouted off about “emperors not having clothes” – If you are going to use metaphysically loaded phrases like “illusion” to describe physical events, and remember that sort of phrase does *not* have rigorous physical content, then be prepared for a “philosophical” response in like manner. Some issues in science are like that.
Please don’t tell me that studying science somehow automatically makes you an expert at general reasoning about issues etc – these issues require special concentration like other fields. Really, after seeing idiocies like “the cosmic teapot” around (see my take-down elsewhere if you want), it is clear that many of the neo-atheist skeptics (and yes I mean the big boys!) are philosophically illiterate, and I have had similar luck getting those I argue with to appreciate such things as Greg must feel he’s had a handful from me. How ironic.
To elaborate on this issue that just about everybody else, in case you folks around here didn’t know: We don’t like physicists etc. thinking that they automatically make good philosophers, economists, theologians, political scientists, etc. Really, isn’t that “cocktail party” philosophy, economics, theology, politics, etc? But here I see all these posts about what God means, about why the world is friendly to the formation of life, of which candidate is better, etc – The OP and the commenters don’t show a lot of contrition and humbleness as I can see, about not being experts in those fields, although expecting such from others talking about science. (Ironically, our big brawl about gravity was in a thread titled “Over to You, Mitt”! Most of the argument was about the reasonableness of believing in a contingent versus self-sufficient universe, etc.)
They go ahead a ply their weed-soaked or whatever form of amateurish expounding, apparently quite sure that they are doing quite intelligently. Really, I am irked by the gall of presumable non-physician Jason Dick to blithely expound on the “rationality” of something complex and out of his field like acupuncture, etc. (not to mention every philosophical question that ever comes up, in such good company!) OK, it’s a free for all and anyone can offer opinions on such topics, but don’t feel shocked when others want to play that game with scientific subjects. Sauce for the goose …
Happy Holidays, whatever they are …
Well, I must give some credit: many of the posts do pose non-science questions in good Socratic form, but I still see examples of presumption (“the answer happens to be no” etc.) In any case, the commenters are more the offenders of the sort I tweaked at here, than the OPs.
Greg,
I’m not Einstein and this isn’t new physics. Actually it is quite old physics, convection specifically. Heat expands until it cools off and cold contracts until it starts to heat back up. Is there any similarity between this and gravitation/radiation? I’m not a scientist. I’m a freaking farmer, for Pan’s sake! I like it simple. Ockham’s razor appeals to me.
No, but if they are physical objects, they may well contract, subjected to extreme gravity. The topography of a sphere still doesn’t mean increasing distance and expanding space are the same thing, since it is the stable distance of the measuring rod that we use to measure this increase.
I don’t. I’m using this to show why saying space expands because we can measure it, is really only increasing distance in stable space.
No. Currently a kilometer is defined as a fixed fraction of the speed of light. So if two galaxies that were 100 million lightyears apart grow to be 200 million lightyears apart, that an increasing quantity of a stable measure.
Cosmologists employed it to explain how the universe can expand without the Milky Way being at the center of the universe. Frankly I have no real problem with space expanding, or contracting, just as I have no problem with the speed of light slowing under various conditions. My problem is with the idea that the entire universe is expanding due to one side of this convective cycle being ignored.
I’m not saying they are collapsing, because they are constantly consuming more energy/mass and the time and space it defines. They are the bottom of the cycle, where mass contracts to the degree it heats back up and radiates out.
There is a redshift for the specific photons which interact with the collection plates on a telescope. To the extent this light is redshifted, it has lost energy. Did it lose this energy because the source is receding, or could there be other reasons? Every wave must fill an increasing volume of space, the further it travels from its source. Could there be an incremental interaction between crossing light waves? It wouldn’t take much, given the distances involved. If the universe isn’t expanding, then the expansion of space would possibly result in additional pressure on gravitational systems. Possibly the reason the outer bands of galaxies spin faster then Newton’s laws predict isn’t due to dark matter, or modified gravity, but from external pressure. The fact is that we don’t know what we don’t know and current cosmology has an awful lot riding on the only explanation for redshift being recession.
It’s mass that collapses. Radiation expands. We see mass being pulled around gravitational vortexes. And we only see it due to the radiation escaping. If it was the radiation that was falling into gravity wells, then maybe it would be blueshifted. Radiation that can’t escape gravity isn’t blue, it’s black.
Actually stars are the primary turnaround for mass being turned back into energy.
Another problem with gravity being modeled as a curvature of spacetime, rather then as a property of mass is that if you go to the center of a massive object, the pressure is very great, but the gravity would balance out. Consider what this would mean if it were applied to galaxies. Possibly there is no gravitational body at the center, only extreme pressure. It would explain the jets of charged particles being ejected out the poles of galaxies.
Lawrence,
That’s why the speed of light is considered the most standard unit. That’s why the length of metric units are define relative to C, not the other way around.
So something measured as moving away on a scale of C is increasing distance relative to our most basic measure of space.
As I recall, Inflation theory says space expanded at much faster then the speed of light because it was space itself that was expanding, carrying the light along with it. So it would seem that light was traveling faster then the speed of light. Prior to Inflation, would the speed of light be similar to what it is now?
I’m not trying to be as hard-headed as I may seem, frankly I feel these are legitimate questions about a theory that seems far more inclined to make extreme projections when theory doesn’t meet observations, than to examine its own assumptions. That there is a deep psychological basis and history of situations where people, even very intelligent people, double down on bad hands, rather accept losses, makes me very leery. I can understand that those who take these ideas seriously think raising questions is bad form, but that is a danger sign in itself. Belief is a very powerful drug.
Hi Sean. Quite frankly, seems to me that your remarks on negative pressure are a bit confused. The straw-man argument that you object to “negative pressure is kind of like anti-gravity” is certainly a problem, because it is wrong. Pressure is a source for gravity, and the gravitational force opposes the force due to the pressure of the field that is the source. The extra gravitational force due to the pressure of radiation gives rise to a stronger gravitational field in a universe dominated by radiation (i.e. relativistic particles), and it contributes to such things as black hole formation in relativistic fluids. With ordinary relativistic particles, the pressure adds to the attractive force due to gravity, but the pressure opposes the gravitational force. With dark energy, the situation is simply reversed. It is the “mechanical” dark energy force that “sucks” and the gravitational force due to the dark energy that is repulsive.
I agree that you don’t really need the negative pressure discussion to consider the situation of a homogeneous and isotropic medium, but it becomes quite important if you consider models in which the dark energy is not homogeneous. It has been quite a while since I have worked in theoretical cosmology myself, but my sense is that there is a lot of confusion about this in the current dark energy literature. In particular, if you try to set up a field that gives rise to inhomogeneous negative energy, the inhomogeneity in the (negative) pressure will give rise to large non-gravitational forces that will likely convert some of the potential energy in the “dark energy” field to kinetic energy, which will give a contribution to the gravitational field of the opposite sign.
Perhaps the best way to see the importance of “negative pressure” as a source of gravity is to consider the cases of topological defects that have tension (or negative pressure) in one or two dimensions: cosmic strings and domain walls, respectively. For a static cosmic string, the gravitational effects of the string energy density and tension exactly cancel, leaving no attractive or repulsive gravitational force. The spacetime outside the string is locally flat. However, it is not globally flat, and a circle around the string will yield 360 degrees minus a deficit angle proportional to the string tension.
Domain walls provide an even more interesting example, and they are discussed in a very nice paper by Ipser and Sikivie (1984, Phys Rev D30, 712). The 2-d tension of the domain wall now dominates over the energy density and the overall force is repulsive. Observers on opposite sides of the wall feel a constant acceleration with respect to each other in the direction perpendicular to the wall. In the case of a localized wall, such as a sphere, observers near the wall see the same force as in the infinite plane wall case. But the wall tension causes it to collapse, and as it accelerates due to the tension, the potential energy stored in the wall is converted to kinetic energy, so the repulsive force goes away. An outside observer sees a Schwarzschild field (as she must due to the symmetry).
So Sean, I hope that you will mend your ways and realize that the problem is not the use of the term “negative pressure” to describe the tension in the dark energy field. The problem is the failure to distinguish between the attractive “mechanical” force caused by the field tension and the repulsive gravitational force that this tension serves as the source for.
John
Er, no, there isn’t. Just because you know a small amount about convection doesn’t mean everything else in the universe is really convection in disguise.
Ockham’s razor doesn’t mean: “Any theory I don’t understand because the mathematics is too difficult must be false, and should be replaced by something I find simpler and more intuitive, despite the fact that all the evidence contradicts the simpler theory.”
Since there is no “convective cycle”, the problem is yours alone.
The rest of your comments are full of errors too numerous to address, when the answers will wash over you with no effect.
Questions are welcome, but you pay no attention to the answers, and you seem to be positively drunk on belief. Why do you take your “convection” ideas at all seriously? Only because you dreamt them up yourself; it’s certainly got nothing to do with either logic or observation.
For all your attempts to construct some kind of sociological theory for a conspiracy in favour of the status quo, that whole idea is remarkably sociologically naive. There is nothing a cosmologist would love more than to overturn conventional wisdom (and as a non-academic amateur myself, with no stake in the status quo, if I had a chance of fame and glory by collaborating with you and lending mathematical rigour to your stunning revelation, I’d seize the opportunity with both hands.)
Unfortunately, as dozens of people have no doubt told you already, you are not the bearer of the great Reality Check that cosmology needed to kick it up the arse and wake it from its delusions. You just don’t know what you’re talking about, and you’re too proud and lazy to learn what the thousands of honest, hard-working people who came before you have done.
Greg, your patience is impressive. There is a surprisingly clear difference between people who don’t know something and are trying to learn, and people who don’t know something and can’t be bothered learning it and feel strongly that their personal ideas should nevertheless be taken seriously by those people who have bothered to learn. I’m not sure if I’ve ever seen a person in the second camp convert to the first.
David, it’s not precisely clear to me where we are disagreeing. I was making a purely pedagogical point, not a physics one. Nobody is arguing that only the 00 component of the energy-momentum tensor is important. I was just arguing that the easiest way to explain why dark energy makes the universe accelerate is to look directly at the evolution of the energy density, and not get the pressure involved at all.
Of course, if the dark energy (or a topological defect or what have you) has spatial gradients, the spacelike terms in the energy-momentum tensor will be important. But this is a great example of where focusing on “w” leads you astray. People begin to think (incorrectly, of course, and the careful people don’t think this way) that p = w ρ is an equation of state, and should have something to do with sounds speeds etc. To get any of that stuff right, you should look at the equations of motion that are derived from the Lagrangian of your underlying theory, not think in terms of p and ρ.
Greg Egans remarks about Einstein reminded me of the famous debate put- down by Bentson in his debate with Dan Quale a while back. Dan was making himself out to be some great guy and compared himself to John F. Kennedy, to which Bentson replied: I’ve known John Kennedy for many years. I have worked with John Kennedy…in fact, John Kennedy was a friend of mine…and you sir are NO Jack Kennedy! (Gales of laughter and applause from the audience) Bentson lost the election, but no one ever questioned the truth of his remark that evening, either. In fact Dan Quale’s bitter rejoinder that Bentson should have known better than talk like that, at once betrayed his lack of a sense of humor and proper modesty in human relations…and showed immediately that Bentsons observation about his character was all too true.
I thought that Sean had some very interesting thoughts on the issue of dark energy and strongly agree with most of his points. Dark Energy is definately “stuff”. Sean made no real attempt, it seemed to me anyway, to put the subject of Dark Energy to rest once and for all, but he tossed the subject out for ideas and comment…Dark Energy is an important topic in cosmology these days.
Einstein was a very modest person. I recall his noting once that he had no special talents, but that he was very curious. I’m sure everyone on this thread shares both Einsteins modesty and his curiousity. However Einstein also once observed that if his theories were proved wrong that he would have indeed known that God himself was incorrect! It is a very human characteristic to form opinions and beliefs of any and every kind, and then defend them with a passionate faith in their veracity.
Hawking concluded some years ago that: “The universe just IS”. That comment, brief it is, can be taken a number of different ways. However when Einstein’s counterintuitive theories were reduced to mathematics and verified to many places, folks slowly came to understand that Einseins counterintuitive ideas were not those of a stubborn crank, but rather of a man of widely unrecognized genius.
Science still lacks key peices of the cosmic puzzle. However. we have a lot of factual information too…information about the veracity of special and general relativity, of the correctness of the principles of quantum mechanics, the age of the universe, the omega total, the nature of the power spectum etc. Working within the envelope of the information we have at our disposal, we can and should, I believe conceptualize and put our ideas to the test, not only of academic criticism but by equipment designed to confirm and falsify ideas.
I think, it was in that spirit that Sean started this thread…
There’s nothing wrong with regarding p = w ? as a linearization of the equation of state that is valid for sufficiently low densities. The actual equation of state depends of course on the details of the matter Lagrangian. In certain cases, e.g. a scalar field, the equation of state is precisely given by p = w ? (w = 1 for a massless quadratic scalar field, w = -1 for a static inflaton-like scalar field).
The coupled equations of motion for gravity and matter are often intractable. One therefore has little choice but to proceed by extracting an effective equation of state from the matter Lagrangian. Sometimes this leads to a dilute fluid approximation like p = w ?. Discussions of sound waves are perfectly legitimate, as long that their density variations are within the regime of validity of the approximate equation of state.
I have to completely agree with David Bennett. Your effort to reduce confusion (which is normally successful) has in this particular case increased it.
Greg, Sean,
I can understand and respect your professional positions. In my business, horse racing, people who don’t know what they are doing are a major source of income and frequently lose their shirts. Given that, I don’t waste much time raising these issues, except as a hobby.
That said, my doubts about BBT preceded both the discovery that large scale structure exists to the very edge of our viewing ability and the need to add dark energy, both of which suggest the current finite model of the universe doesn’t effectively contain all the facts and has been stretched to accomodate them, so I have felt vindicated in the past and suspect I will in the future, but since I don’t want to wear out my welcome here, I’ll drop the discussion.
Not to cause too much aggravation, but I would like to put in a plug for convection as a sort of everymans TOE. It does provide a useful model for understanding all sorts of political, social, economic type issues, where fresh energy is constantly rising up and displacing the old order, even as it carries it along, while the old, cold way falls back down, providing sustenance for more fresh growth. It also provides a useful analogy for life itself, as dynamic youth rises up, while crusty age crumbles.
For us dummies who don’t have the brains or initative to go get a formal education in modern physics, it also provides an interesting analogy for the relationship of Quantum Mechanics to Relativity. QM being the microcosmic indeterminate radiant energy that is constantly expanding out, presumably into new dimensions and universes, while Relativity describes the structured, deterministic, macrocosmic reality that is suffering entropy and gravitational collapse. Obviously this is not for professional consumption, just for those of us unable to grasp the facts and our own inadequacies.
Anyone wishing to consider the analogy further might consider Complexity Theory, with its modeling of the relationships between bottom up process and top down order.
To throw a bit of an added complexity to this topic it appears that the anisotropy of the CMB has an anomalous hole. This little hole was first announced last August and there has been some buzz over this. A recent article by Cruz, Turok, Vielva, Martinez-Gonzalez, Hobson in Science, vol 318, 7 Dec 2007 statistically argues that this “hole” is a texture which might be a latent signature of a cosmic string. The paper can be accessed at:
http://arxiv.org/PS_cache/arxiv/pdf/0710/0710.5737v1.pdf
It appears that the big bang is the ultimate scattering experiment, and these data are information from which we might understand quantum gravity.
Lawrence B. Crowell