I tend to agree with Sabine Hossenfelder: "The simplest explanation for dark energy is that it's a constant of nature, the cosmological constant, end of story. I don't know why people find this so hard to accept, it's the neatest possible law-of-nature that one can think of: A constant of nature!" (source: https://twitter.com/skdh/status/1626125552917569538)

The cosmological constant, which appears in a version of the Einstein's equations of general relativity (on which Einstein himself had mixed feelings), is the energy density of empty space. If we take it as a fundamental constant of nature there's no need to explain it further.

It's only the simplest explanation if you're an inductivist. In fact, that Einstein's equations work either with or without a cosmological constant means that the cosmological constant can't explain anything unless we can explain why that version of the equation is correct. And unless you're an inductivist, "it fits the observations" is not an explanation.

I guess I must be an inductivist then, since to me "it fits the observations" is the primary justification of a physical law. Einstein's equations work either with or without a cosmological constant. They also work with either that specific value of G or any other value. In both cases, it is observation that determines the value of G and whether the cosmological constant must be included.

Imagine an equivalent situation 300 years ago: Why are there 8 planets in the universe? The "simplest explanation" is that it's just a fundamental constant. And our equations of orbital dynamics are compatible with any number of planets, even zero. Since it fits our observations, it's justified.

There's no explanation in this answer. It fits all known observations, but any arbitrary addition to a theory can easily be made to fit observations. The true answer, which is actually an explanation, depends on an improved theory of the universe. The theory tells us that the universe is more than just the solar system, and it explains that our solar system having 8 planets is largely incidental (and it also tells us that, for example, a solar system can't have -1 or 0.75 or 10000 planets).

This is the same situation we're in today with the cosmological constant. We know that there's an improved theory of spacetime and gravity (that's at minimum compatible with quantum theory) we don't have yet. What are the chances that theory will have the exact same possibilities for a cosmological constant? And we know that the improved theory will allow us to make new observations, including many that wouldn't even occur or make sense to us with our present theories.

Maybe the improved theory will explain the cosmological constant and explain why it had a particular value in terms of something deeper. Or maybe it'll explain that what we've observed as the expansion rate of the universe is just an incidental property of our local region of spacetime (like the number of planets). Or maybe it'll explain that the universe isn't even expanding. It's impossible to know without knowing the theory itself.

Sure, but we have been trying to explain it in terms of vacuum energy density. https://ned.ipac.caltech.edu/level5/Carroll2/Carroll1_3.html (which turns out to be wildly wrong.) I don't see any problem with a different idea, especially if there is some type of data to support it. (Black holes getting more massive as the universe expands... I'm still trying to get my head around the idea.)

Reasonable as a phenomenology. Gravity would have two constants of nature, one for attraction and another for repulsive gravity (dark energy). QED has one fine-structure constant for pos+ and neg- charges

We seem to live in a quiet, leafy suburb of the Universe. The really interesting stuff is all happening downtown, but I wouldn’t want to live there. -- Bob (graboyes.substack.com)

Motion of massive objects causes pressure in the sense included in the stress-energy tensor. Just like the particles in a gas give it a positive pressure, so does the the motion of the edge of a flywheel. If you include this effect, the total stress in a flywheel would be about the same as one that simply had internal stress because its edge was longer than 2pi times its radius, so rotation shouldn't give a solution to negative total pressure.

Nitpick: I think it doesn’t make sense to say “in the buildings and machines around you, far more material by mass is under compression than tension”. A free standing wall has vertical compression, but horizontally internally it is mostly tension holding it together. A horizontal beam matches the tension at the bottom to the compression at the top, but also has molecular tension keeping the beam one solid piece. Loaded walls are similar.

I'm not understanding the mechanism here. How does negative energy inside a black hole cause the universe to expand? Is there a simpleish explanation for that, or do I just have to take it as a given?

This seems to be proposing a single point source (central galactic black holes) that both attracts stars in the galaxy and nearby local groups galaxies ... while simultaneously repelling very distant galaxies. How do those net forces work, from the same point source? Gravity falls off with an inverse square law. Is negative pressure supposed to be more far reaching than that, perhaps starting at a low strength but not following an inverse square law? How does the negative pressure force decline with distance?

I haven't read the papers, but since the universal expansion is proposed to be increasing the central objects masses, that should dominate locally as it follows an inverse square law, whereas the repulsion is a universal constant, exerting a certain rate of expansion in each unit of space (increasing linearly with distance), which will dominate at much larger distances.

You're describing the more common theory, that dark energy is merely an intrinsic property of space itself, and therefore the more space, the more dark energy (and thus the more repulsion). I understand that theory.

But Robin is describing a very different theory: "dark energy is actually black holes". This new theory seems to require that repulsion does not automatically come with additional space, but instead is concentrated at the point sources (galactic central black holes). Please also study this quote: "expansion could be accelerating because of a few very small but very dense pockets of matter with negative pressure, even if all the other nearly empty volume has zero or positive pressure". The new theory proposes that additional empty space does not have negative pressure; only the black holes do.

So your explanation doesn't seem to answer my question about the theory Robin is describing.

There seem to be at least 2 claims being made in the articles I can access. First, that a subset of black holes, particularly supermassive black holes, are somehow "filled with" vacuum energy and do not contain a gravitational singularity, and as a result (or cause?) their mass is somehow increasing in proportion to the volume of the *entire* cosmos, not just their local region of space. And second, that this somehow accelerates the on-large-scales-uniform universal expansion.

I don't see how either of those can work without FTL interaction of some sort, but maybe there's a way.

That said, it doesn't look like the original paper postulates any causal mechanism for the coupling? And just shows a correlation that they find suggestive?

Ahhh – so this wouldn't imply any different long-term evolution of galaxies, i.e. them eventually being causally isolated (from other gravitationally bound galactic clusters)?

I tend to agree with Sabine Hossenfelder: "The simplest explanation for dark energy is that it's a constant of nature, the cosmological constant, end of story. I don't know why people find this so hard to accept, it's the neatest possible law-of-nature that one can think of: A constant of nature!" (source: https://twitter.com/skdh/status/1626125552917569538)

The cosmological constant, which appears in a version of the Einstein's equations of general relativity (on which Einstein himself had mixed feelings), is the energy density of empty space. If we take it as a fundamental constant of nature there's no need to explain it further.

It's only the simplest explanation if you're an inductivist. In fact, that Einstein's equations work either with or without a cosmological constant means that the cosmological constant can't explain anything unless we can explain why that version of the equation is correct. And unless you're an inductivist, "it fits the observations" is not an explanation.

I guess I must be an inductivist then, since to me "it fits the observations" is the primary justification of a physical law. Einstein's equations work either with or without a cosmological constant. They also work with either that specific value of G or any other value. In both cases, it is observation that determines the value of G and whether the cosmological constant must be included.

Imagine an equivalent situation 300 years ago: Why are there 8 planets in the universe? The "simplest explanation" is that it's just a fundamental constant. And our equations of orbital dynamics are compatible with any number of planets, even zero. Since it fits our observations, it's justified.

There's no explanation in this answer. It fits all known observations, but any arbitrary addition to a theory can easily be made to fit observations. The true answer, which is actually an explanation, depends on an improved theory of the universe. The theory tells us that the universe is more than just the solar system, and it explains that our solar system having 8 planets is largely incidental (and it also tells us that, for example, a solar system can't have -1 or 0.75 or 10000 planets).

This is the same situation we're in today with the cosmological constant. We know that there's an improved theory of spacetime and gravity (that's at minimum compatible with quantum theory) we don't have yet. What are the chances that theory will have the exact same possibilities for a cosmological constant? And we know that the improved theory will allow us to make new observations, including many that wouldn't even occur or make sense to us with our present theories.

Maybe the improved theory will explain the cosmological constant and explain why it had a particular value in terms of something deeper. Or maybe it'll explain that what we've observed as the expansion rate of the universe is just an incidental property of our local region of spacetime (like the number of planets). Or maybe it'll explain that the universe isn't even expanding. It's impossible to know without knowing the theory itself.

Neptune was only discovered in 1846.

Sure, but we have been trying to explain it in terms of vacuum energy density. https://ned.ipac.caltech.edu/level5/Carroll2/Carroll1_3.html (which turns out to be wildly wrong.) I don't see any problem with a different idea, especially if there is some type of data to support it. (Black holes getting more massive as the universe expands... I'm still trying to get my head around the idea.)

Reasonable as a phenomenology. Gravity would have two constants of nature, one for attraction and another for repulsive gravity (dark energy). QED has one fine-structure constant for pos+ and neg- charges

edited Feb 19, 2023Like (quantum) gravity is (thought to be) associated with spin-2 bosons and QED/electroweak is associated with spin-1 bosons...

If this was true shouldn't we expect black hole gravity to be weaker than their mass implies, as they expand space around them?

Also shouldn't we see more expansion inside galaxies than between galaxies?

We seem to live in a quiet, leafy suburb of the Universe. The really interesting stuff is all happening downtown, but I wouldn’t want to live there. -- Bob (graboyes.substack.com)

Motion of massive objects causes pressure in the sense included in the stress-energy tensor. Just like the particles in a gas give it a positive pressure, so does the the motion of the edge of a flywheel. If you include this effect, the total stress in a flywheel would be about the same as one that simply had internal stress because its edge was longer than 2pi times its radius, so rotation shouldn't give a solution to negative total pressure.

Nitpick: I think it doesn’t make sense to say “in the buildings and machines around you, far more material by mass is under compression than tension”. A free standing wall has vertical compression, but horizontally internally it is mostly tension holding it together. A horizontal beam matches the tension at the bottom to the compression at the top, but also has molecular tension keeping the beam one solid piece. Loaded walls are similar.

I'm not understanding the mechanism here. How does negative energy inside a black hole cause the universe to expand? Is there a simpleish explanation for that, or do I just have to take it as a given?

I love the way you write about astronomy. Not often I get to say that I found a really complex line of reasoning nevertheless easy to follow!

This seems to be proposing a single point source (central galactic black holes) that both attracts stars in the galaxy and nearby local groups galaxies ... while simultaneously repelling very distant galaxies. How do those net forces work, from the same point source? Gravity falls off with an inverse square law. Is negative pressure supposed to be more far reaching than that, perhaps starting at a low strength but not following an inverse square law? How does the negative pressure force decline with distance?

I haven't read the papers, but since the universal expansion is proposed to be increasing the central objects masses, that should dominate locally as it follows an inverse square law, whereas the repulsion is a universal constant, exerting a certain rate of expansion in each unit of space (increasing linearly with distance), which will dominate at much larger distances.

You're describing the more common theory, that dark energy is merely an intrinsic property of space itself, and therefore the more space, the more dark energy (and thus the more repulsion). I understand that theory.

But Robin is describing a very different theory: "dark energy is actually black holes". This new theory seems to require that repulsion does not automatically come with additional space, but instead is concentrated at the point sources (galactic central black holes). Please also study this quote: "expansion could be accelerating because of a few very small but very dense pockets of matter with negative pressure, even if all the other nearly empty volume has zero or positive pressure". The new theory proposes that additional empty space does not have negative pressure; only the black holes do.

So your explanation doesn't seem to answer my question about the theory Robin is describing.

There seem to be at least 2 claims being made in the articles I can access. First, that a subset of black holes, particularly supermassive black holes, are somehow "filled with" vacuum energy and do not contain a gravitational singularity, and as a result (or cause?) their mass is somehow increasing in proportion to the volume of the *entire* cosmos, not just their local region of space. And second, that this somehow accelerates the on-large-scales-uniform universal expansion.

I don't see how either of those can work without FTL interaction of some sort, but maybe there's a way.

That said, it doesn't look like the original paper postulates any causal mechanism for the coupling? And just shows a correlation that they find suggestive?

Wouldn't this imply that expansion is 'lumpy', i.e. varies with the distribution of negative pressure stuff?

Yes, but only on the scale of the typical distance between these objects.

Ahhh – so this wouldn't imply any different long-term evolution of galaxies, i.e. them eventually being causally isolated (from other gravitationally bound galactic clusters)?

A typical 'exciting' move is to relate two enigmas together.

Examples - neutrinos and dark energy, dark matter and dark energy, Higgs boson and dark energy

So now another example is black holes and dark energy. Hide one puzzle inside of another puzzle.

No natural explanation why this mechanism keeps 'negative pressure' of dark energy constant as a function of Universe's size?!

If the negative pressure is at max possible level, general relativity predicts an exponential expansion with a constant energy density.

Where would this constant energy density come from? Unless it is postulated in general relativity as a cosmological constant anyway. A reference?

NB - i am not an expert, but a decent former physicist

too many interpretations without experimental cross-links

For instance, http://www.homepages.ucl.ac.uk/~ucapola/CLrev.pdf