Cosmic voids and dark energy

Zeitgeist 2 Minutes

Cosmic voids may cause dark energy, or at least provide an explanation for the Hubble tension.

A team of Iranian scientists have proposed that dark energy may not be something inherent to space itself, but the result of cosmic voids, vast regions of the universe between galactic superclusters and filaments with relatively little matter. 

Paul Sutter provides a quick overview of the idea. In summary, the dynamics of the voids and their surrounding regions may work similar to soap bubbles, a dynamic pushing the void edges, which are the galactic superclusters and filaments, further apart at an increasing rate, leading to an overall accelerated expansion of the universe. Sutter’s article provides a link to the preprint.

What clued me in to this idea was Matt O’Dowd’s discussion in this Space Time video. He goes over it starting at around the ten minute mark. Although the discussion before that about how the lumpiness of galactic distributions of matter may be leading to the Hubble tension is itself pretty interesting. (The Hubble tension is the significant discrepancy between measurements of the expansion rate from the cosmic microwave background versus the movement of galaxies in the more recent universe.)

PBS Space Time: Can The Crisis in Cosmology Be SOLVED With Cosmic Voids?

O’Dowd mentions that the original theories about the universe either needing to be expanding or contracting were based on the assumption that, on the largest scales, the density of matter in the universe is uniform. This assumption appears to be getting challenged by new data, most notably by the discovery of gigantic structures that shouldn’t exist. 

There was also recent confirmation that the universe won’t tear itself apart in a big rip scenario. Interesting times in cosmology. 

O’Dowd’s discussion of the cosmic void origin theory for dark energy includes the interesting point that, if true, it could mean dark energy will change over time, eventually fading. That seems like it would have profound consequences for our current understanding of the far future.

The typical narrative in the last couple of decades is that the expansion would continue indefinitely, utterly isolating regions of the universe from each other forever. Eventually these isolated regions would run out of matter to create stars and would grow cold. At the longest time frames, the universe would eventually undergo heat death.

However, if dark energy fades, then it seems like this picture is undermined. Instead, the expansion might stall at some point. Instead of a long infinitely fading future, we might be back to looking at something like a big crunch scenario, where gravity eventually gets the upper hand again and draws everything back in, with the universe dying as it began, in heat and density. Or a big bounce, putting it into a never ending cyclic model of expansion and contraction.

At the very minimum, it might mean that regions in the far future aren’t nearly as isolated as they would be under the current understanding.

Of course, the role of cosmic voids may just be limited to the Hubble tension issue. Or play no significant role at all. Only time will tell. But it’s worth remembering how much of our current understanding of cosmology remains built on a stack of assumptions. 

What do you think of the idea of cosmic voids generating dark energy? Or any of the other developments? Or about the assumptions that cosmology is based on?

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26 thoughts on “Cosmic voids and dark energy”

  2. There’s also been some new studies of binary systems that seems to confirm a variation of MOND. Of course, subject to dispute. But that would likely eliminate the need for dark matter.

    There was also some study that suggested the universe might be twice as old as 13.7 billion years usually quoted.

    https://cosmosmagazine.com/space/astrophysics/universe-27-billion-years-old/

    There’s a lot of evidence of unexpected stellar and galactic structures close in time to the 13.7 billion year mark. So, it seems something’s got to change

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    1. My knowledge of the MOND vs dark matter debate is fairly superficial. Most of what I’ve heard in support of MOND talks about galaxy rotation speeds. But it sounds like the other gravitational effects are pretty lumpy and uneven, and can be used to chart distributions of something with mass, which doesn’t seem conducive to a simple rule change, or at least not one that’s the sole explanation.

      On the Cosmos article, thanks. I think I saw it a while back, or ones like it. It seems like ever since the JWST came online, people have been claiming the whole big bang model is falsified, or in need of heavy revision. As the article mentions, data indicating changes are exactly what cosmologists were hoping for, but it doesn’t sound like we can rule out yet it isn’t just our understanding of how galaxies form that’s an issue.

      If something like tired light, or laws or constants changing over time are involved, our whole understanding might have to be thrown out. But it seems like we’d have other indications if any of those were the case. Red shifting, light doppler effects, show up in local experiments, but I haven’t heard anything about light fatigue showing up, at least not yet. Of course, we can’t rule out something below the precision of current instruments.

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      1. I’ve sometimes wondered if basic constants were changing and they all changed proportionally would we be able to detect it. All we have to go on regarding the physics of 13 billions years ago is light and theories. If light was slowing with time, would we know it?

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        1. I’ve wondered the same thing. But I think if everything is changing proportionally, the question becomes, in what sense is it changing? There has to be something relative to those changes which remains constant, or at least only changes at its own rate. And if we detect that thing changing, how do we know part of what we’re seeing is everything else changing? Is this even a meaningful question?

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          2. This reminds me of something Brian Greene once pointed out, in responding to statements that speaking of a “now” throughout the universe being misguided. He pointed out that we could use the average matter density of any region to compare its age with any other region’s. Of course, he was assuming homogeneity, which is what the recent results have been calling into question.

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      2. If laws or constants are changing over time, then God not only plays dice with the universe, He changes the rules in the middle of the game. Looking at the rest of this bizarre world, maybe that shouldn’t be too surprising.

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        1. Yeah, if the laws change over time, or in different regions, I think it’s fair to say our understanding of the universe would be hopelessly wrong. I think of Vernor Vinge’s A Fire Upon the Deep, one of the few sci-fi novels that explores this kind of possibility.

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  3. @selfawarepatterns.com

    You have the statement "it’s worth remembering how much of our current understanding of cosmology remains built on a stack of assumptions.", which I fully agree with. Are you aware of any formal or at least reasonably complete statement of that set of assumptions? Ideas like MOND could undermine the work of entire astronomy faculties, and that is just one element.

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    1. @grant_h @selfawarepatterns.com

      Good question. The main one I've read about is the Cosmological Principle, which is that stuff far away works the same as it does here.
      https://en.wikipedia.org/wiki/Cosmological_principle

      It worked for Newton in figuring out gravity for the solar system (same thing that makes an apple fall controls planetary orbits). But who can say whether it applies to things billions of light years away. Except that it *looks* like it does.

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      1. @selfawarepatterns @selfawarepatterns.com there are a couple of studies on widely separated binaries at nano acceleration (10^-9 m.s^-2) seeming to imply MOND or 1/r for gravity at that scale. Given that Newton's gravitational relation was given the rare status of scientific "Law", a _lot_ of work is going to be based on that. The science implications are one thing. The social implications for people who's life work is built on that are much larger. One understands why Galileo got in trouble.

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        1. @grant_h @selfawarepatterns.com

          On social implications, definitely. And it happens both within and outside of science. Galileo got caught up in the wider politics associated with the theology of cosmology. But even within science, for several decades, physicists who studied quantum foundations put their careers in jeopardy. The risk only faded as the original Copenhagen camp died off.

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          4. @grant_h @selfawarepatterns.com

            I've had Carroll's book sitting in my Kindle for a while now with the intention of reading it, but the last year was tough so I haven't yet. I know his quantum one is coming soon, so I'll need to catch up.

            There are definitely subjects people in academia are advised not to touch until they at least have tenure, with close to retirement being better. Or move to the philosophy department.

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    1. Thanks Mike! Fascinating stuff. This part in particular, if accurate, blows my mind.

      Thus, from the perspective of the same observer, a clock will move faster in empty space, which possesses low gravitation, than inside a galaxy, which has much more gravity, and he argued that as large as a 38% difference between the time on clocks in the Milky Way and those in a galaxy floating in a void exists.

      It seems like the discoveries of all these inhomogeneities might give these theories new life.

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      1. Well, it makes sense qualitatively, at least. You see, it was previously assumed that space-time geometry in voids simply reverted to near flat, but according to Buchert equations, GR probably causes it to go negative in compensation for positive curvature around mass concentrations. This seems intuitively very plausible and, as I understand it, it makes a sufficient difference to magic away dark energy.

        BTW, Einstein’s lament about the cosmological constant is probably misunderstood. GR is a set of differential equations and hence any solution will feature an arbitrary constant. Einstein’s blunder was to try to give it a value which would result in a static universe.

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        1. Thanks for elaborating on where that percentage comes from. It seemed high, but we’re not talking about the normal differences, but something inflated due to the void’s vast underdensities. (I initially wrote “emptiness” here, but I have to keep reminding myself that there are galaxies in voids, just at a much lower density than the superclusters and filaments.)

          I have to admit I thought the only reason for the cosmological constant was for that static outcome, even if it gained an apparent usefulness later.

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          1. Mathematically speaking, the cosmological constant of some unknown value is a no-brainer. And its value does not affect the structure of the theory, which is why it can be added or removed as desired without tweaking the rest of GR. But GR is a physical theory and hence a non-zero value of the constant would need a physical interpretation. Hence by Occam’s Razor its null-hypothesis value should be zero, meaning there is nothing to explain, unless observational data dictates otherwise.

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