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Comment by molticrystal | Hacker News Reader

This paper argues that the Timescape model [0] provides a better fit than the cold dark matter model when examining Type Ia Supernovae. According to the Timescape model, clocks run faster in voids where the gravitational field is less, and significant differences exist between a galaxy floating in a void and one like the Milky Way Galaxy. The Timescape model suggests that other models, which fail to account for these differences, lead to less accurate calculations and less plausible solutions.

[0] https://en.wikipedia.org/wiki/Inhomogeneous_cosmology?useski...

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> a better fit than the cold dark matter model

than the Lambda Cold Dark Matter model.

Lambda, i.e. the cosmological constant, a.k.a. dark energy, is what they do away with, not dark matter.

Thanks for saving me time in dismissing this paper lol. Any time somebody wants to get rid of dark energy, i run into some garbage. Reminds me of the mond nuts

Just reading the rest of the comment section is enough to help me verify that.

For some reason, hackernews always gets kooky when it comes to this stuff.

I don't know, the evidence for dark energy has always seemed a lot sketchier than the evidence for dark matter. Dark matter has lots of interlocking lines of evidence. Isn't dark energy pretty much entirely based on various cosmic distance measures that all have huge stacks of assumptions embedded?

I agree. Until i see better evidence for 1a, wmap, and cluster formation in another theory, i really want all the charlatans to be quiet. We dont know what dark energy is, but we have decent evidence to say it is there and also decent theory.

I am not saying this paper is made by charlatan btw. This type of work attracts those people though.

Can't censor it, so you gaslight it?

If clocks run slower in the presence of gravity, wouldn’t it stand to reason it runs more quickly in a void where there’s less gravity? Or is the model saying that clocks run even faster in a void than Einstein’s theory predicts?

Clocks run at "normal" speed (i.e. "1x" speed) in the absence of a gravitational field. The stronger the gravity, the slower they run (i.e. less than "1x" speed).

This has always felt to me like evidence of a sort of computationalism. I am not a computationalist, but the thought is the "universal CPU" needs cycles for each particle. Mass is what takes time to process, so the voids experience no/less computational delay. This reads like the simulation author is messy and constrained, not godlike.

To me it's not about mass, but more like "maximum information density". There's a limit on the information density (rate of happening?), so when a parameter X changes too much, it affects other parameters -- they become constrained so that the total information density stayed within the maximum limit. That would indeed sound like some kind of computational limit if the universe was a massive CPU with constrained resources...

But I'm a layperson and I have no idea what I'm talking about :)

Information density is certainly on the right track, I'd put it as causality combined with a locality constraint.

Photons are massless and have way more interactions that matter probably does.

Right so is the paper saying that lambda CM completely ignored clock differences due to heterogeneity in mass distribution in the universe where isolated galaxies would be experiencing less time slowing than galaxies near other galaxies which would experience more time dilation?

In the standard cosmology the Integrated Sachs-Wolfe effect captures the redshift/blueshift of distant light sources (up to the Cosmic Microwave Background) as it traverses relatively dense regions and relative voids.

https://en.wikipedia.org/wiki/Sachs%E2%80%93Wolfe_effect

Note that in the next paragraph I depart significantly from the vocabulary that the Timescapes programme proponents have been using for the past twenty years.

ISW and comparable spectroscopy is easy enough to think about in terms of an accelerating cosmic expansion, i.e., relative voids are becoming spatially bigger with the expansion. It becomes much less intuitive how to fit the data if one keeps relative voids at roughly constant volume instead implying that there is a significant false vacuum above the ground state and in voids the false vacuum is slowly decaying to that state. (Outside the supervoids, near matter, this false vacuum decays much more slowly still). Because "vacuum" in the voids isn't really vacuum, one is stuck with a running function on the constant c (it gets faster with time from the formation of the CMB; this is because the false vacuum evolves towards a real vacuum) or adapting lightlike geodesics by imposing refraction (since the false vacuum is a medium).

The usual terminology is reasonably capture in the first paragraph here at <https://en.wikipedia.org/wiki/Inhomogeneous_cosmology#Inhomo...> ("Inhomogeneous universe"). The following short section ("Perturbative approach") is what is done in the standard cosmology when one wants to do detailed studies of filamentary distributions and other structures that are lumpy at some (larrrrrge) length scale of interest: the perturbed homogenous background is practically always the standard FLRW.

The justification for perturbation theory on FLRW is that even though there are dense spots (notably most galaxies' central black holes), principles like the Birkhoff theorem capture the idea that as you get far enough away from a galaxy it behaves more and more like a small shell, and this happens at intragalactic scales for these SMBHs: gravitationally, even to its arms' structure, it makes practically no difference whether Andromeda's central bulge has a lot more stars/gas/dust or whether it has one, two, or six central SMBHs (at enough spatial separation that they're not mutually orbiting in a way that would generate gravitational radiation our observatories are sensitive to).

The same idea applies to galaxies->galaxy clusters->filamentary structures: as you "zoom out" the density variations become less important: filaments are pretty sparse on average.

The Timescapes programe wants a sharper difference in matter sparseness between voids and filaments, and proposes that gravitational backreaction by the matter is responsible for generating that: the presence of matter steepens the density of matter over time (without the visible matter clearly becoming denser). I don't personally see how that's much different from a false-vacuum decay in the voids, conceptually. (ETA: well, it depends somewhat on how the Timescape void fraction evolves, but the local universe VF doesn't run void clocks fast enough, unless we do violence to the Copernican principle.)

(Also ETA, mostly a note-to-self: I also don't understand how they capture the angular diameter turnover point in their dressed geometry <https://journals.aps.org/prd/abstract/10.1103/PhysRevD.80.12...> PDF available from institution at <https://ir.canterbury.ac.nz/items/36fe829a-0e7a-45d6-8db6-c2...> (cf <https://astronomy.stackexchange.com/questions/21006/understa...>.))

Finally, I think the most important result of this latest Timescapes paper is a reminder to everyone that supernova data are a mess. A good X-mas present would be a couple readily visible Milky Way supernovae.

The T Corona Borealis supernova will be closely monitored, possibly before long https://www.bbc.com/news/articles/c140exemgk7o.amp

T CrB is a recurrent nova (RN), not a supernova (SN).

There is only a microscopic chance of the white dwarf member undergoing runaway fusion becoming a Type Ia SN. So microscopic it would be truly surprising astrophysically.

Pet theory is that our universe is run on some external computational substrate. A lot of the strangeness we see in quantum physics are side effects of how that computation is executed efficiently.

The inability to reconcile quantum field theory and general relativity is the that gravity is a fundamentally different thing to matter: matter is an information system that's run to execute the laws of physics, gravity is a side effect of the underlying architecture being parallelized across many compute nodes.

The speed of light limitation is the side-effect of it taking a finite time for information to propagate in the underlying computational substrate.

The top-level calculation the universe is running is constantly trying to balance computation efficiently among the compute nodes in the substrate: e.g. the universe is trying to maintain a constant complexity density across all compute nodes.

Black holes act as complexity sinks, effectively "garbage collection." The matter than falls below the event horizon is effectively removed from the computation needs of the substrate. The cosmological constant can be explained by more compute power being available as more and more matter is consumed by black holes.

This can be introduced into GR by adding a new scalar field whose distribution encodes "complexity density." e.g. some metric of complexity like counting micro-states, etc. This scalar field attempts to remain spatially uniform in order to best "smooth" computation across the computational substrate. If you apply this to a galaxy with a large central supermassive black hole, you end up with almost a point sink of complexity at the center, then a large area of high complexity in the accretion disk, and then a gradient of complexity away towards the edges of the galaxy. That is, the scalar field has strong gradients along the radius of the galaxy, and this gives rise to varying gravitational effects over the radius (very MOND-like).

Some back of the napkin calculations show that adding this complexity density scalar field to GR does replicate observed rotation curves of galaxies. Would love to formalize this and run some numerical simulations.

Would hope that fitting the free parameters of GR with this complexity density scalar field would yield some testable predictions that differ from current naive assumptions around dark matter and dark energy.

”External computation susbtrate” is a useful idea if it leads to falsifiable theories. As a ”theory of everything” it sucks because it’s clearly not motivated by any specific maths or observations, but by the human need to map nature into some comprehensible analogue. Ie. taking some simpler subset of nature and trying to pretend the rest of it is like that as well. Usually nature so far has become more incomprehensible the deeper we’ve looked at it.

Newtonian mechanics & mechanical clocks being hottest precision technique led scientists at the time to viewing nature as a clockwork. Now we have computers, we think ”nature is like computers” because it’s an appealing analogue.

But it’s a false analogue imo. Just like clocks are a thing enabled by nature (a subset, in every meaning of the word) similarly computers are a subset of nature. So yes, nature can think (with human brains) and nature can run computations (with cpu:s impregnated with programs) but that also is just a subset of nature.

Now: games of the mind and helpfull analogues rock. And asking ”how is nature analogous to a turing machine” is interesting for sure. But just because a game is fun or analogue appealing, should not one let forget in the philosophical sense that one is playing only with a limited subset of a thing.

There's a Danny Hillis talk on this but I couldn't find it.

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