Dark Energy May Not Exist: Something Stranger Might Explain the Universe
sciencealert.com
sciencealert.com
https://en.wikipedia.org/wiki/Friedmann_equations
It's only recently that people have been able to handle the more complex case of an inhomogeneous universe. Scientists are often like the drunk who is looking for his keys under the lightpost.
s/homogeneous/inhomogeneous/ ?
Intuitively I never liked the Dark Energy theory. Doesn't seem valid to just make up an answer supported by no direct evidence for something you can't explain. It sounds a lot like religion in fact.
But I never took physics after high school, so I always assumed I just didn't understand and had faith that these wise men knew what they were talking about.
In practice, science requires balancing multiple criteria. Does a model have the right level of simplicity and complexity? Can I convince my peers that it's correct? Should I trust the results of a crank over the results of someone who has a long history of excellent work, and how much onus is there on my me and my research group to reproduce every single published result?
Keep in mind, no one expects to be responsible for a revolution, so you get an effect similar to "poll herding" -- it is desirable to have a consensus, and as consensus emerges around a theory for how to describe some new observations, you get people going along with it even if they maintain personal reservations. In fact, it has been clear to many physicists that dark energy is just an ad hoc explanation, and there has been serious contention about what better model could exist, and there have been several competing models. But every model had its flaws. It's possible that the timescape model will also have flaws that cause it to be re-evaluated.
I recommend a book called "What is this thing called science?" by Chalmers [1]. When I was an undergrad it gave me a more nuanced understanding of the Philosophy of Science.
[1] https://en.wikipedia.org/wiki/What_Is_This_Thing_Called_Scie...
Well, it sounds like the Aether, really.
Rμν − (1/2)*gμν*R + Λ*gμν = (8*π*G / c^4) * TμνI would not compare DE to religion though. It's DM that's a bit of a bridge too far for me, though even there it's hard to know yet.
Also note that timescapes is only about DE not DM. I don't think you'll find an explanation for the galaxy rotation curves anomalies in GR.
Then again I'm not a physicist.
Dismissing all alternatives to standard cosmology as "fringe" is not helpful when standard cosmonlogy itself is "fringe".
Granted, there will be really far out there alt theories. The uncertainty of the context will bring out wild hypotheses. Figuring out who's a charlatan and who is in the ballpark can be difficult under these circumstances, but we have to.
Science is hard, and it takes a lot of work.
I’m guessing this is what the parent comment was referencing.
Come to think of it, another angle to consider – could it work the other way around? Instead of dark matter causing time dilation, could time dilation itself be a root cause that leads to the accumulation or creation of something like dark matter? If time moves differently in certain regions, maybe it affects how matter and energy interact over cosmic timescales, creating the conditions we interpret as dark matter. Just wondering if this has been explored or if it’s way out there as an idea? I’m definitely not a cosmologist!
EDIT: But, yes, if there's lots of dark matter not uniformly distributed in the universe, then that dark matter would have differing effects on time dilation in different parts of the universe. Questions: Do the great voids contain much dark matter? What about non-void areas that also have very little DM somehow?
Unlikely. DE and DM are very different problems, sharing only a word in their casual names: dark :)
The galaxy rotation problem requires either modified gravity (MOND, etc.), actual dark matter, finding that the measurements and/or their interpretation are wrong, or something else. There are some GR effects to consider, mainly frame dragging, but frame dragging doesn't seem to be anywhere near enough because the frame dragging effect diminishes with the same proportion as gravity: inverse square distance.
"...in fact, an atomic clock located in a galaxy could tick up to a third slower than the same clock in the middle of a void."
Maybe there is an nice simple formula assuming a uniform disk relating the time dilation to radius? I've always been under the impression that general relativity simulations needed supercomputers, but maybe current desktop machines are sufficiently "super" compared to 20 years ago that we could do simple calculations? Anyone have a recommendation on where to start learning enough general relativity to make a computer program? Assuming you've have a couple semesters of calculus-based college physics 101 under your belt as the prerequisite?
I guess you can find repos on Github (first Google result for me was this: https://github.com/spacetimeengineer/spacetimeengine) and try your hand. But the theoretical aspect is intimidating, for sure.
As for getting up to speed in the theory, I cannot but recommend Leonard Susskind lectures on GR that are available on YouTube. Susskind may be best known as one of the founding fathers of String Theory but the man is an exceptionally great teacher on this subject.
The main problem is that you need to have some intuition about what the formula is supposed do in order to test your code to make sure you've not made a silly mistake, and that relativity is very unintuitive.
There's probably some existing python library that already does all of it, may be best to start with one of them.
Precession of Mercury around the Sun?
General Relativity is incredibly math heavy but fundamentally the numerical methods involved are standard methods for differential equations. The hard part is going from the math to a solvable form. See https://arxiv.org/pdf/2008.12931 for a broad overview. This will of course probably not make sense without an introduction to differential geometry, a beast of a topic itself. See some big textbook like https://arxiv.org/pdf/2412.08026 or find yourself a copy of Gravitation by Misner, Thorne and Wheeler.
I did find "Functional Differential Geometry":
https://mitp-content-server.mit.edu/books/content/sectbyfn/b...
...which uses Scheme to teach differential geometry. I would need to learn quite a bit more before tackling that book. Maybe something like: "Structure and Interpretation of Classical Mechanics"?
https://mitp-content-server.mit.edu/books/content/sectbyfn/b...
...but even there, it looks like I would need to start with something more basic.
Of course you can just use the Einstein field equations, but that's more work, though not an approximation. You'll need a good model of the mass distribution in the universe in order to actually get good answers.
Abstract: https://arxiv.org/abs/0912.4563
#1: Supernovae evidence for foundational change to cosmological models:
https://academic.oup.com/mnrasl/article/537/1/L55/7926647
https://arxiv.org/abs/2412.15143
#2: Cosmological foundations revisited with Pantheon+:
Apparently so quickly (and time moved so slowly), that parts of the universe became disconnected it becomes the explanation for the current lumpiness.
The current lumpiness is apparently now alternate explanation for dark energy, which is an explanation for where the energy comes from to power the expanding universe.
Colour me skeptical, but I think we are still missing something.
It's just pure relativity, I can't believe we haven't already reached consensus on timescape over dark matter.
I really hope the theory stands up to scrutiny and that JWST and friends keep sending us new evidence. Timescape cosmology is just so neat that it must be true. Hopefully this will lead us in some new interesting directions
Dark energy. This is about dark energy not dark matter.
To have flat spacetime you have to distort things other than spacetime -- other than space and time. Which things? Mainly the speed of light, but also there's length contraction (why? because the effects of gravity in GR are anisotropic). Thinking that the speed of light might be variable is very strange, but even in flat spacetime the speed of light is always the same locally (in all directions), just not globally (nor in all directions). So now think of the path a photon in the CMB took to get to us, and now trace it backwards: as you trace it backwards through a great void it speeds up! And as you trace its exit from a great void it slows down, but it slows down extra because now the density is higher than where we are (because the universe was denser in the past, and less dense now due to its expansion). This also means that there's different amounts of red-shifting when traversing different regions of the universe at wildly different times and differing wildly in density. There is even some blue-shifting due to traversal of great voids.
We use differences in attenuation of brightness of faraway standard type 1A supernovae candles (which we assume always have the same brightness locally at the time of the event) and their red-shift to infer acceleration of the expansion of the universe. (Assuming not much gravitational lensing we can expect attenuation of brightness of standard candles to be a simple function of distance, and then we'd expect the red-shift to agree, but if it doesn't then... if the red-shift is stronger we assume that extra red-shift to be due to the acceleration of the expansion of the universe.) But the differences in red-shift might be artifacts of the great voids that the light traversed in order to get to us. That's what this timescapes hypothesis is about! The difference in z might be explainable by great voids rather than by acceleration of the expansion of the universe -- or perhaps a combination of both even.
Flat spacetime is not actually incorrect. Einstein's field equations and its solutions (e.g., Schwarzchild's and Kerr's) are mappings between flat spacetime and curved spacetime. For example, the Schwarszchild and Kerr solutions for a single massive body (non-rotating, in the Schwarszchild case, rotating in the Kerr case) are expressed as curved spacetime second derivatives of time, space, and angles where there is an `r` variable which represents the flat spacetime distance to the center of the massive body. This is so much so that one could be forgiven for wondering if curved spacetime is not just a mathematical crutch, or a projection much like -say- a Mercator projection of a 3D map onto 2D. It sure does seem like flat spacetime is more fundamental than curved spacetime given that we have a flat spacetime `r` in the solutions.
We’ve long accepted that the speed of sound is variable based on atmospheric conditions. So why should the speed of light not be variable based on gravitational (or other) conditions?
At the end it depends on the homogeneity of the Universe. Cosmological models assume that the Universe is isotropic and homogeneous at high enough scales. So that would cover your assumption: if it's homogeneous, the voids are included in the average density. What this study is challenging is precisely that homogeneity.
You might have to take into account those density differences if doing calculations about things going on in those regions, but when you were doing calculations about the whole universe treating it as homogenous would still be OK.
Then we learned about the Milky Way galaxy and that there were other galaxies. But it looked like galaxies were fairly evenly spread out. Like with stars within a galaxy, there were places in the universe where the galactic density was higher or lower but if you stepped out to get the bigger picture you'd still find that large volumes all had about the the same density.
So again you might have to take that into account when doing calculations in specific large regions of the universe, but what doing calculations about the whole thing it still appeared uniform enough that treating it as homogenous should still work.
Then we found that galaxies cluster. But step back and the clusters seemed to be distributed pretty evenly. So we still had a homogenous universe as far as calculations on the whole universe were concerned.
There were more rounds of this, finding larger things, but each time those larger things seemed to be pretty uniformly distributed so you could still assume homogeneity when trying to calculate universe-wide stuff.
But now with more recent mapping of these large scale things, it appears that there is some significant density variation where the regions of high or low density are large enough that if you try to step back to see if those regions are distributed uniformly at a large scale you run out of observable universe.
And so now it may be that we finally have to drop the homogeneity assumption when doing things the involve the whole universe, like trying to understand its expansion.
But what I don't understand is why we would expect that to be true. Once we learned about galaxies, what would imply their even distribution on a galactic scale? The very existence of nonidentical galaxies / lack of obvious symmetry around us already suggests the existence of early perturbations that cause an uneven distribution and asymmetry, no?
It was only after decades of developing better and better ways to measure distances that we started getting good enough 3D galaxy position to recognize things like superclusters and filaments. It is only relatively recently that good enough 3D universe maps have been made to show the large voids that might might be distributed sufficiently non-uniformly that they must be taken into account.
Throughout most of that time the models based on a reasonably homogeneous universe worked. It was only relatively recently that they ran into problems. For example in the last 10 or so years a couple different ways to measure the Hubble constant were refined enough to reduce their error bars to the point that the error bars no longer overlapped.
The optical properties of a material caused by its inhomogeneity doesn’t change if you look at it from further away!
I.e.: frosted glass will look frosted from any distance other than “extremely close”. The same applies to wavey or otherwise non-flat transparent or translucent materials.
The same ought to apply to the lumpiness of spacetime caused by galaxies — it shouldn’t matter if you “zoom out”, the bumps and troughs are still there, bending light the same way!
I thought this was obvious, it’s something that occurred to me at least a decade ago!
I can’t believe astrophysicists just hand-waved this away, when it clearly can’t be ignored.
1. The more stuff there is in an area, the more expensive collision detection is for each of those things.
2. The faster two objects move relative to each other, the more expensive collision detection is between those objects.
I do not deserve your irony