Over the weekend, Sean Carroll put up a blog post to address common misconceptions about cosmology. I understood most of his points, but was confused when I saw this one:
Dark energy is not a new force; it’s a new substance. The force causing the universe to accelerate is gravity.
Carroll was referring to the accelerating expansion of the universe. But gravity causing the acceleration, instead of dark energy? I asked in a comment, along with at least one other commenter, how this could be so. Carroll was kind enough to respond to us:
Gravity causes the universe to accelerate because gravity is not always attractive. Roughly speaking, the “source of gravity” is the energy density of a fluid plus three times the pressure of that fluid. Ordinary substances have positive energy and pressure, so gravity attracts. But vacuum energy has negative pressure, equal in size but opposite in sign to its energy. So the net effect is to push things apart.
I had always been under the impression that dark energy was simply the unknown force behind the accelerating expansion, a force I understood to be in opposition to gravity. However, it appears that dark energy actually affects gravity by causing it, on cosmological scales, to be repulsive, to repel distant parts of the universe apart from each other.
The force behind this appears to be negative pressure. Pressure, it turns out, is a source of gravity. Brian Greene in his book, The Fabric of the Cosmos, explains that pressure, in the sense of outward pushing, like what you might find with a coiled spring, is a form of energy, and energy generates gravity. Negative pressure, such as the tension in a rubber band that wants to contract when it’s stretched out, is energy going in a different direction. Negative pressure actually has a negative effect on the attractive force of gravity, causing it to be repulsive.
Albert Einstein understood this when he first formulated his Cosmological Constant to explain why gravity didn’t cause the universe to collapse. After Edwin Hubble’s discovery that the universe was in fact expanding, Einstein would regard the Cosmological Constant as his greatest blunder. It’s therefore ironic that several decades later it became useful again with the discovery that the expansion of the universe was actually accelerating.
So gravity can be repulsive, and dark energy, an energy apparently permeating all of space, due to its negative pressure, brings out this repulsive nature. Many of you who are more knowledgeable about physics no doubt already understood this, but it was a major revelation to me.
I briefly wondered if this might be a way to achieve the anti-gravity capabilities that often show up in science fiction. But after giving it some thought, no, it wouldn’t.
The problem is that most of what generates the gravity that attracts, say, a flying car, to the Earth is Earth’s overall mass. In order to overcome this with repulsive gravity, the car would have to generate so much negative pressure that it would cause the car to generate more repulsive gravity than the Earth’s attractive gravity. Such a force would violently repel everything around it, push the Earth out of its orbit, and probably cause a host of other catastrophes. Not exactly a practical solution.
Still, this is a fascinating effect and I learned something new!
SelfAwarePatterns 
Hmm… I know I’ve heard something about this before, but I got the impression at the time that this was not the consensus view among physicists, so I didn’t look into it further. But I may have gotten the wrong impression, or maybe there’s new evidence to support the idea.
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Just to be clear, this is not MOND (modified Newtonian dynamics), the idea that our understanding of gravity needs to change to account for the appearance of missing energy out there.
But our understanding of the effects of pressure on gravity, at least according to Greene, appears to be a part of general relativity going back to Einstein. Based on the discussion on Carroll’s post, it’s not particularly speculative. Of course, its role in the expansion of the universe could be considered speculative.
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Coiled springs and rubber band allegories……. I think that’s a bit of a stretch. Ideas like this coming from Carroll don’t necessarily surprise me. Excuse me if I find any “thing” that theoretical physicists state speculative. It’s all speculative, speculation is the “art” of theoretical physics. The year is 2019 C.E. and we are still living in the dark ages, physicists still cannot account for causation.
More smoke and mirrors, but hey, that’s why they get paid the big bucks while the rest of us are suppose to march in unison, loyal, faithful church members :).
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I think skepticism is good, but it pays to remember that the device you’re reading this on is a result of physics theories. And general relativity is among the most confirmed theories in science.
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That’s the whole point Mike, physics theory is great for unlocking the mystery of the “classical” structure of matter and thereby being able to manipulate those structures to make gadgets. But physics is absolutely bankrupt when it comes to understanding the “underlying form” of those same structures.
General relativity??… that’s a schoolboy’s vivid imagination at work that has absolutely no basis in Reality. The only thing its good for is making relatively accurate calculations that we utilize for our “gadgets”. In contrast, the quantum world clearly demonstrates that general relativity is BS. But if control is the objective instead of truth, then hey, I’m good with that. It’s just that I’m not a very good church member and I get a little riled up when the priesthood makes visionary claims that have absolutely no basis in Reality.
The most confirmed theory in physics is the fact that every morning the sun will rise in the eastern horizon. Too bad we can’t utilize that paradigm for making more gadgets. We need to face the music and grow up, we still live in the dark ages when it comes to causality and consciousness. To tell you the truth, I am blown away that homo sapiens have an affinity for bizarre and wild theories and yet, the same homo sapiens have a pathological aversion to Occam’s razor and the principle of parsimony. Maybe someone would be kind enough to explain that phenomenal to me. Just thinking out loud here Mike…
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Lee,
I’m afraid that the ability to make accurate predictions is the best you get in science. In fact, I think it’s the best we ever get, period. Scientific theories are about predictive models. But your brain is constantly building models of your environment, predictive models. There is no understanding of reality we can have that is not, fundamentally, anything other than a predictive model.
Now, in the case of scientific theories in particular, there are often predictions that are testable, and ones that aren’t. Often the ones that aren’t are assumed to be inevitable consequences of the ones that are, but reality is absurd and frequently overthrows our little paradigms.
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“There is no understanding of reality we can have that is not, fundamentally, anything other than a predictive model.”
I concur with this statement counselor. My model contains five axioms, three definitive ontological distinctions, two universal theorems and a single reality-appearance event horizon. Now, if one were to ask if my model gives us more control, the answer is no. The inverse is actually the case. It is because of that important distinction that my model will be of no interest to homo sapiens because of our pathological obsession with control. I’m no dreamer Mike, I’m a hard-core pragmatist.
So what does my model provide? First and foremost, it moves us out of the dark ages of mysticism (a model that is currently shared by both science and religion) and into a description of reality that is parsimonious and grounded firmly in what I call the immortal principle, which is nothing more than transcendental idealism revision 1.0. Second, my model is testable, it provides consistency, and is absolutely inclusive, with zero paradoxes or contradictions. It’s a potential paradigm shift that rivals the geo-centric universe and the flat earth syndrome.
If disclosed, my model would not be rejected based solely upon its merits, because its merits are without refute. My model would be rejected because it does not correspond to the discrete, binary system of rationality, even though it corresponds concisely and succinctly with the linear, continuous system of reasoning. The world is not ready, and I am no god damned messiah…
Good luck…
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I think that most theoretical physicists would chuckle at the idea that they are paid big bucks.
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our scientific knowledge of gravity and dark energy is impressive … even miraculous.
Unfortunately, a stupid Pole doesn’t understand anything about it. So I once wrote …
This incredible power of repulsive gravity that can push the supercluster of millions of galaxies together with the awesome power of their gravity. (Only how …).
So maybe to give some thought to terrible doubts of the repulsive gravity: (a little puzzle now). Which way repulsive gravity (+ dark energy), must to push our nearest superclusters of galaxies?
Towards us or in the opposite direction?
Please forgive my simplistic reasoning …
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I think what’s happening is that dark energy is generating repulsive gravity throughout space, that is generally driving everything apart. Galaxies and galactic clusters stay together because their local attractive gravity dominates over the overall repulsive gravity pervading the universe. But the distances between galactic clusters are great enough that dark energy and repulsive gravity win out in between.
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To confuse you further, Stan, dark energy grows as the universe expands, because space gets larger 😉
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I also read this sometimes in several books, but then forget about these things. I read about three physics books a year, but after a month I forget the specifics of the details again. I really like to read them, but it’s hard to remember especially if you don’t understand the underlying math. Sean Carol also has a nice podcast nowadays called Mindscape.
What interest me nowadays is with how much mass the universe actually started. The process of inflation can be started with one gram of matter and once started it will create all mass we see and the corresponding negative energy, thus net energy is still zero. I still have to coincidentally stumble on some more information about that, since I never actively search about physics. Keep up your posting
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Thanks! Greene also discussed that negative pressure is a factor in the theory of cosmic inflation, which was also new to me. I definitely need to do some reading on my own. I’m starting to think my knowledge in this area is a bit dated.
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Hi Mike,
This is really interesting. I found this video below that I thought was insightful–had enough depth to be interesting, but I could still follow it. It reviews the basic cosmological equations that Carroll was referring to in his explanation to you, so you can see what terms are involved. I’m intrigued by the fact that the basic idea is that as the universe expands, the energy density (the infamous cosmological constant) remains the same, which requires the creation of energy essentially. As a given volume of space expands (or stretches), for the energy density to be constant in the original volume, more dark energy must come into being. It’s as if space is thinning out, but the energy isn’t thinning out with it.
The hardest part to visualize is what pressure actually means in a relativistic sense. Interestingly enough, when particles in a given volume are moving very fast, this causes the “pressure” to increase and also the gravitational attraction. I guess this makes sense because as we try to accelerate anything to speeds approximating the speed of light the mass becomes infinite, unless I’m mistaken. Which I could be. But it would make sense that for normal matter, if the radiating particles have a high “pressure” then the apparent mass of a given volume would be higher and the attractive component of gravity would increase. That kind of made sense to me.
But in the video the narrator goes on to say that the negative pressure of dark energy isn’t because the dark energy is really slow or anything. He lost me at that point. I’d be curious what you get out of that portion of the explanation if you watch this. I think he is basically saying, maybe without quite saying it, that something is putting work into the universe to push it out, and that is dark energy.
Michael
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Hi Michael,
Thanks for the video! Just finished watching, but I’m pretty much in the same place as you.
The conservation of energy definitely seems to be out the window when we’re talking about the overall cosmos. Where is that energy coming from? Keeping in mind that question assumes it has to come from somewhere, I don’t think we know, a mystery as deep as anything else in science.
As I understand it, in general relativity, mass is just one of the sources of gravity. Energy in general, whether mass or other forms, is what generates gravity. Brian Greene described pressure as essentially another form of energy. But I’m not clear on what that means for negative pressure. He never uses the phrase “negative energy”, but if pressure being energy is what gets it into club gravity-generation, then it seems like the energy involved in negative pressure must be different in some manner.
That said, it’s interesting that the equation treats pressure separately from the rest of the energy. That implies that there’s something special about it. I’d be interested to know details on what makes it special.
I kind of got the impression from the narrator that negative pressure was about the inherent vacuumness of space. Given that “nature abhors a vacuum”, maybe that’s enough to generate the effect we’re talking about? I don’t know.
I’m struck by his statement that it comes down to the math, even though we can’t intuitively understand it. This isn’t quite as stark in that regard as the quantum stuff, but it seems like it’s in the same ballpark. Physics is about consistent math and predicting observations. The rest, as unsatisfying as it sounds, is basically a narrative we tell ourselves about what happening behind the math.
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My understanding (which might be wrong – dark energy is well beyond what I was taught at university-level GR) is that energy conservation still applies, and that the creation of dark energy as space expands is balanced by the negative work done due to the expansion of space. I suggest you google this to find out more, as I am on shaky ground here.
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Thanks Steve. I’m beginning to think even more that we have mathematics that work but that the explanations for why they work are increasingly ad hoc. I’m hoping to go through Greene’s book (probably skipping the string theory parts) after I finish another book I’m reading. Maybe I’ll come out of it with some kind of coherent understanding.
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I’m a big fan of that PBS SpaceTime series; it’s really excellent!
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Mike
Michael wrote (this makes sense because as we try to accelerate anything to speeds approximating the speed of light the mass becomes infinite)
Of course this is true and it makes sense, only what it has to do with expansion, negative pressure or pushing. What I do not understand?
Can you help me understand this?
I hope Michael will not be angry!?
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Stan,
My interpretation of Michael’s remarks is he was just trying to make sense how pressure relates to gravity. The speed the particle’s are moving contribute to the pressure in whatever volume they’re contained in, and that this seems to fit with their speed, under special relativity, also contributing to their mass. (Michael, if he sees, this, may be able to provide corrections.)
Of course, the negative pressure part is strange. I’ll admit I’m still working on that one myself.
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Yes, that’s it Mike. I’m way over my pay grade here, but if we think of pressure being related to the energetic content of individual particles, such that two volumes of space with the same total inertial mass within them–(the same quantity of the same types of particles, let’s say)–might have differences in pressure because the velocities of the particles were higher in one volume than the other. The relativistic effect is that the masses with the higher velocities have a higher apparent mass, and thus a greater gravitational effect. This was all I could think of as an explanation that related higher pressures to stronger gravity. The key is that what physicists are calling higher pressure is (I’m guessing…) probably related to higher velocities, and with relativistic effects, that would mean higher apparent mass.
The negative pressure is still not something for which I have any real sense. Even in the video I posted here, which was really interesting, I felt as though the narrator flip-flopped about whether space was stretching, and this dark energy was filling in behind it, or this energy was actually the cause of space stretching. I’m at the state of thinking they’re correlated, but without knowing whether one or the other is actually at cause.
It’s like any equation. When you write F=ma, you can’t tell by looking at the math what is the cause and what is the effect. You only know that were you to measure those parameters they should display the relationships defined by the math. But when you apply that equation to a particular situation with which you are familiar, you can have an intuition about it. You know if you hit a ball with a particular force, the acceleration is the effect. Likewise, you know that if you decelerate an elevator, the force that the people inside it experience is the effect. But it’s kind of a bad example because it required a force to decelerate the elevator in the first place. So for most things in a Newtonian world, it seems the applied force is the cause. One obvious exception is gravity. That’s about the only thing I can think of where the acceleration exists as the cause of a force. With dark matter, I don’t know which way physicists think about it… whether the dark energy drives expansion, or the expansion “sucks” dark energy into the “void” it leaves in its wake (which sounds a lot like a negative pressure).
MIchael
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Mike, Michael.
my misunderstanding arises from the fact that
although it is true of particle physics and many other physical processes, this makes pretty sense … only that in cases of expansion mechanisms, ie. negative pressure or dark energy, in any case we have NO speeds exceeding the speed of light.
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I think that the so called “Dark energy” is actually new space being created, when a new finest fundamental charge comes into being (“new” matter) during an absorption of a discrete electric field. A graviton is composed of two discrete electric fields coming from opposite finest fundamental charges. A graviton can pull down on two temporarily appearing finest fundamental opposite charges within light by two discrete electric fields attractively being absorbed by the finest fundamental opposite charges. The photon of light goes on but two finest fundamental opposite charges remain behind (new matter along with new space). A photon of light is a wave (traveling at the speed of light) composed of a large number of finest fundamental charges appearing and disappearing. See my 18 Mar 2019 book, Gravity-Wheel Unveiled, 2 GRAVITY BASED ON DISCRETE ELECTRIC FIELDS, 11.2 Cosmology, and subsection Dark Energy within 15.6 Solutions to Prior Unsolved Problems. At gravityunveiled.home.blog you can download a .pdf file of my book for free. My book solves a great many different puzzles or problems. Energy is never conserved, when a finest fundamental charge absorbs a discrete electric field. The potential energies are a fiction to keep up the pretense of energy always being conserved. In practice, it is useful for many computations but the “potential energies” indicate that there will be later absorption of discrete fields by matter. 20190322 – Alden E. Park
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