I don't see where this explanation is strange, it's straightforward GR (General Relativity), as much as one can call GR "straightforward." It's a simpler explanation than inventing Dark Energy, for which we have no direct observational evidence.
> It's a simpler explanation than inventing Dark Energy
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.
I don't think it's accurate to compare it a theocratic phenomenon. Ask any physicist, they will say that science requires checking our theories against the evidence and changing our theories as new evidence comes to light. It's not like they issue scriptures to all grad students about what you must believe.
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.
Dark energy isn't really a theory, it's more of a placeholder for the observation that the universe's expansion is accelerating. There are a variety of ways you can potentially explain this observation. The simplest theory is the standard lambda-CDM model for dark energy where it is taken to be a cosmological constant. This model has worked well for about 20 years or so, but it is starting to come in tension with newer high precision observations.
Lambda-CDM is a model, yes, but DE sure is a theory, or hypothesis if you prefer. Look at https://en.wikipedia.org/wiki/Dark_energy#Nature and reference [23] and note that there's a proposed mechanism for the acceleration of the expansion of the universe.
That seems reasonable; I guess I'd rather hear "for reasons we don't understand the universe appears to be expanding and the expansion is accelerating by a constant factor we're calling 'lambda'." But I guess "dark energy" is fewer words, if that's what it means.
As I remember it (from 40+ years ago, and well before dark energy was discussed), lambda naturally arises as a constant of integration in GR rather than being "invented" as in the usual story. Assuming this to be true, I'd suggest that any model should either allow for a non-zero lambda, or give a reason for it to be zero. I.e. even if an alternative for dark energy is found, we still have to deal with lambda.
Yes, given the equations of GR, there's no particular reason to assume that the cosmological constant should be zero. (Though one of the major problems in theoretical physics is that back-of-the-envelope estimates as to what it "should" be give numbers that are ~120 orders of magnitude larger than its observed value. So if dark energy is a cosmological constant, it's not really understood why it is non-zero, but extremely small.)
No, dark matter is quite different. There are a number of different lines of evidence for dark matter. One of these is sort of similar to dark energy, in that it is needed to explain the slower-than-expected expansion of the universe at early times. But the most compelling line of evidence for dark matter (IMO) is baryon acoustic oscillations in the CMB.
Idk. My son once used standard knowledge of the quantum froth / particle / anti-particle pair production in vacuum to estimate the light pressure due to that production in great voids and came up with an answer that's off from standard cosmology by just a factor of two (i.e., this explained half the acceleration of the expansion of the universe). It's pretty easy to take assumptions like this and extrapolate. It's not clear that dark energy is obviously wrong, but it's weird because the universe presumably needs energy in order to accelerate its expansion, and... where's that coming from? The standard answer is: from the light traversing the universe (especially the great voids) which loses energy as the universe accelerates its expansion. But what is the mechanism by which that happens? I've yet to see such a mechanism proposed, and that is what makes Dark Energy weird.
I 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.
I've always known that Dark energy/matter is bunk the minute I read about it. It uses <something> variables. More simply put they make up numbers so the math works. Sounds like something a child would do to try to fool their parents. I'll finish the homework later mom see I made IOU's in the answer fields. I've never understood how this theory was not laughed out of the field rather than strongly supported.
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.
Good call. I remember pretty clearly fighting with the spell checker on that one and I think it managed to change it when I wasn't looking. It's fixed!
This is referring to timescape cosmology and the recent paper (Supernovae Evidence for Foundational Change to Cosmological Models) that was discussed 11 days ago on HN:
Standard cosmology is a consensus based on interpretations of spotty data and imperfect methods, therefore standard cosmology itself is a bit "fringe". It natural given the context. Alternatives to standard cosmology will also be "fringy" for the same reasons, and possibly more reasons too.
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.
The vast majority of “fringe” stuff comes from people who don’t really understand the standard model or modern consensus. It consists of half-baked ideas that don’t hold up to passing scrutiny by those who understand the field. Not because of any bias… but because the ideas have glaring flaws, don’t explain existing observations well, or are mostly incoherent.
Science is hard, and it takes a lot of work.
I’m guessing this is what the parent comment was referencing.
The problem is all the easy problems have been solved, and even all the hard problems have been solved in science. We're down to the really hard problems, and since our otherwise astoundingly accurate observations of how the universe acts are thrown off by them, everything looks weird and fringe at the very hard edges.
Sure, we have to think outside the box to solve some of those problems. Or we need more data (and tech to gather it with). Or both. Thinking outside the box means differing from the standard consensus. Differing from the standard consensus causes controversy. Some wildly outside the box thinking will be not even wrong.
Just a thought – could the existence of something like dark matter actually be causing the differences in the rate of time in different parts of the universe? If dark matter affects gravity in ways we don't fully understand, maybe it's also influencing time dilation across the cosmic web. That might reconcile these two ideas – with dark matter being the underlying cause of the time variations that timescape cosmology talks about. I'm curious if anyone has explored this connection or if it's completely off base?
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!
The idea here (timescapes) is specifically about dark energy, not dark matter.
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?
Thanks for clarifying that timescape cosmology focuses on dark energy, not dark matter—makes sense they’re addressing different puzzles. That said, I’m curious: could there be any indirect connection between the two? For instance, if dark matter’s influence on gravity shapes the 'lumpiness' of the universe, could that, in turn, amplify the time dilation effects discussed in timescapes? I realize the authors didn’t delve into dark matter, but wondering if anyone has explored how these two phenomena might interact. Definitely out of my depth here, but this potential connection is what sprung to mind and on rare occasions my ill-informed intuitive curiosities turn out to have unexpected merit, so I’m asking if there could be anything to it?
> could there be any indirect connection between the two?
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.
Anyone happen to know what the time dilation factor would be for the Milky Way at somewhere near the outside edge and somewhere nearish to the center (say at 5% of the radius, so that we're not inside the central black hole)? The article says:
"...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?
Simulations require supercomputers for doing large scale, detailed calculations, but simple situations can be solved completely analytically. For example, gravitational time dilation can be calculated somewhat simply for a central gravitational potential: https://en.wikipedia.org/wiki/Gravitational_time_dilation#Ou...
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.
...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"?
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.
Yes, the formula is the Einstein's Field Equation. Alas, I don't think it's generally considered as nice and simple. But it's the tool for that: you would basically set an energy density distribution and the software would calculate the metric of spacetime (and that would include time distortion)
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.
Given an estimate of the mass and mass distribution of the Milky Way we can compute the time dilation factor we expect assuming that the intergalactic space beyond the galaxy has zero time dilation. However, I think we cannot easily measure or infer the absolute time dilation we experience due to the rest of the universe. But relative time dilation factors are enough for the sort of analysis that is done in timescapes.
Yes: Schwarszchild time dilation, while a solution to a one-massive body model, is a decent approximation if you think of M/r as an integration of the effects of all matter around you: \sqrt{1-\frac{2GM}_{rc^2}}
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.
JWST seems to be showing that more than a billion years of star and black hole evolution happened in the "first billion years". For about a year I've been pretty sure that the first billion years might have been more like ten billion years, maybe Timescape explains that.
It's interesting that in the beginning the universe was incredibly hot, everything moved at the speed of light, but was it incredibly dense and so time moved incredibly slowly. Yet it expanded incredibly quickly.
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.
This article really wants to sound sensationalist vs: "So all the stuff we used to ignore in our calculations, now that we are able to not ignore them it turns out that time dilation is acting in perfectly expected ways."
After hearing the explanation of timescale cosmology, it seems blindingly obvious. Of course mass can cause time dilation on large scales. How could it possibly be otherwise? Given a heterogenous universe, time must also be heterogeneous with respect to mass concentration.
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
My son happens to be working on a flat spacetime variation of GR and that makes it much easier to think about these sorts of things.
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.
I’m no real physicist, and know that light and sound waves are largely unrelated, but I like that this approach makes light behave more like sound.
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?
Oh, that's another thing, in flat spacetime gravitational lensing is really just gravitational refraction. In fact, my son uses refraction laws to model gravitational lensing given his formulas for the slowdown of the speed of light in a gravitational field.
Great question. Basically in my son's flat spacetime variant of GR massive bodies produce a gravitational attractive force whose field also slows down light -- the force produces orbits. If you think of massive particles as standing waves then gravity only slows down light and it happens that you can refract both, light and massive particles, with refraction of standing waves (massive particles) yielding behavior that resembles Newtonian gravity.
I find it hard to believe that people who are working with the kind of the theories that require Dark Matter don't already account for time dilation due to general relativity.
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.
Obviously this is a completely buffoonish question made by someone who has extremely limited knowledge but...what on earth do we mean by "homogeneous at high scales"? Doesn't the existence of stars, galaxies, and galactic clusters with enormous voids in between clearly show the universe is not homogeneous in any meaningful sense? I know the phrase "high scale" is in there but, I mean, if you zoom out far enough, it seems like you could claim anything is homogeneous at a sufficiently high scale, although I don't know what that would mean on a universal level anyway, since you cannot zoom out of the universe.
In the early 20th century, before we knew about galaxies, we thought what we see in what we now know is our galaxy continued that way throughout the whole universe. There were some places with higher or lower stellar density but if you stepped back a ways you'd find that larger volumes had about the same density.
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.
I guess this is still puzzling to me. From what I can gather, it sounds like our idea of cosmological homogeneity and isomorphism are roughly equivalent to my layman's conception of "things are evenly distributed and the universe, could it be viewed as a whole, is symmetrical."
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?
I don't think there was an obvious lack of symmetry. In any random direction you look with a powerful telescope you see about the same number of galaxies, the same mix of types of galaxies, the same kind of variation in brightness.
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.
Not to mention that “this isn’t how optics works!”
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.
DM is an orthodoxy. Going against, or even just noticeably straying from, an orthodoxy isn't a happy path and isn't taken by many. DM proponents with their rotational speed curve argument even failed to account for difference between real gravitational potential inside the disk, and they use a simple model of a centrally located spherical mass instead. Time dilation is orders of magnitude more complex to account for.
The explanation made me think the universe is like one big zip file being extracted and in areas of lower entropy or complexity, it's able to be extracted faster. Maybe it's not an apt analogy but seems fitting as someone who analogizes tech to real world applications often.
Great question! I think it will alter those a fair bit. As well timescapes seems to imply that the visible universe is not a sphere but a lumpy sphere. I think too that we can probably interpret variations of the CMB as variations in the amount of great void space in any one direction. There are lots of implications.
Amazing, isn't it? All some scientist had to do was take a stroll to the nearest bubble, scoop up some of the local vacuum, and bring it back home to weigh it.
I mean, it does not sound like a new theory. Some non-uniform numerical model could have predicted it. PhD Students test various things at lab computers.
You're right, this should have been thought of earlier. The thing is that cosmology is hard, and science is driven by personalities and consensus. It can be hard to break through. Timescapes has been around for a decade but only now it's getting attention -- why? because there is a sea of stuff to pay attention to, a lot of not-even-wrong stuff, and a lot of inertia in the current consensus. In particular very few people really know GR, and it's hard to apply it creatively, especially if that means you need new solutions to the Einstein field equations -- that can be a real pain to deal with.
