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Dark Energy May Not Exist: Something Stranger Might Explain the Universe
- asplake 2y agoWhat does timescape do to estimates of the age of the universe?
- cryptonector 2y agoGreat 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.
- floxy 2y agoAnyone 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?
- pantulis 2y agoYes, 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) I guess you can find repos on Github (first Google result for me was this: https://github.com/spacetimeengineer/spacetimeengine 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.
- ben_w 2y agoThe formula themselves aren't that complicated, as software goes: https://en.wikipedia.org/wiki/Einstein_field_equations#Mathematical_form https://en.wikipedia.org/wiki/Einstein_field_equations#Mathe... 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.
- floxy 2y ago>in order to test your code to make sure you've not made a silly mistake Precession of Mercury around the Sun?
- zifk 2y agoSimulations 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#Outside_a_non-rotating_sphere 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 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 https://arxiv.org/pdf/2412.08026 or find yourself a copy of Gravitation by Misner, Thorne and Wheeler.
- floxy 2y ago>an introduction to differential geometry I did find "Functional Differential Geometry": https://mitp-content-server.mit.edu/books/content/sectbyfn/books_pres_0/9580/9580.pdf?dl=1 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/books_pres_0/9579/sicm_edition_2.zip/book.html 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.
- cryptonector 2y agoGiven 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.
- cryptonector 2y agoYes: 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.
- taylodl 2y agoI 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.
- krunck 2y agoI was wondering the same. It's just an application of GR in a way I thought people would have been doing already, right?
- PaulHoule 2y agoThe equations are really simple for a homogeneous universe https://en.wikipedia.org/wiki/Friedmann_equations 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.
- cryptonector 2y ago> the more complex case of an homogeneous universe s/homogeneous/inhomogeneous/ ?
- PaulHoule 2y agoGood 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!
- SoftTalker 2y ago> 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.
- jandrese 2y agoI 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.
- pantulis 2y agoI guess you meant Dark Energy ;) 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.
- Amezarak 2y agoObviously 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.
- T-A 2y agohttps://en.wikipedia.org/wiki/Cosmological_principle https://en.wikipedia.org/wiki/Cosmological_principle
- tzs 2y agoIn 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.
- king_magic 2y agoi'll wait to see what sean carroll thinks.
- vwkd 2y agohttps://bsky.app/profile/seanmcarroll.bsky.social/post/3lec7s6v6wc2k https://bsky.app/profile/seanmcarroll.bsky.social/post/3lec7...
- king_magic 2y agoexactly what I expected, and thank you!
- fithisux 2y agohard to believe no one found it before.
- tremon 2y agoAmazing, 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.
- fithisux 2y agoI 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. I do not deserve your irony
- cryptonector 2y agoYou'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.
- fithisux 2y agoWith Julia, Octave, Scilab and other FOSS systems I think it is time for this work to be open sourced.
- ckcheng 2y agoThis 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: https://news.ycombinator.com/item?id=42495703 https://news.ycombinator.com/item?id=42495703
- SpaceManNabs 2y agoechoing same comments i made then. HN gets really fringe when this comes up.
- br41nr07 2y ago[dead]
- cryptonector 2y agoStandard 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.
- deepthaw 2y agoThe 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.
- cryptonector 2y agoSure, 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.
- ghjfrdghibt 2y agoI hope this is true, it seems to make sense even to someone with absolutely no knowledge of this stuff, like me. It seems simple. I like simple.
- ninth_ant 2y agoStick with Newton then. Unless you're a specialist, this model explains almost everything you'll ever need to explain, predict, or encounter.
- hprotagonist 2y agoOr do anything with GPS.
- PaulHoule 2y agoJWST 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.
- rstuart4133 2y agoIt'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.
- cryptonector 2y agoMy 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.
- mystified5016 2y agoAfter 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
- cryptonector 2y ago> It's just pure relativity, I can't believe we haven't already reached consensus on timescape over dark matter. Dark energy. This is about dark energy not dark matter.
- deepthaw 2y agoThis 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."
- jahnu 2y agoRandom thought: does the idea that entanglement, time, and space being intimately linked have anything to do with this?
- 0134340 2y agoThe 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.
- furyofantares 2y agoAn idea I've never been able to get out of my head is an observation about collision detection. 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.
- dazzaji 2y agoJust 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!
- cryptonector 2y agoThe 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?
- schiffern 2y agoTo me it sounds like they understood that, but they're asking how dark matter (and its gravity) might interact with this model.
- cryptonector 2y agoI see no way in which timescapes deals with DM, and the authors don't seem to think so either.
- deleted 2y ago[deleted]
- OutOfHere 2y agoFree preprint fulltext: Abstract: https://arxiv.org/abs/0912.4563 https://arxiv.org/abs/0912.4563 PDF: https://arxiv.org/pdf/0912.4563 https://arxiv.org/pdf/0912.4563
- OutOfHere 2y agoThat preprint is from 2009! Here are the two newer papers from 2024: #1: Supernovae evidence for foundational change to cosmological models: https://academic.oup.com/mnrasl/article/537/1/L55/7926647 https://academic.oup.com/mnrasl/article/537/1/L55/7926647 https://arxiv.org/abs/2412.15143 https://arxiv.org/abs/2412.15143 #2: Cosmological foundations revisited with Pantheon+: https://academic.oup.com/mnras/article/536/2/1752/7890815 https://academic.oup.com/mnras/article/536/2/1752/7890815 https://arxiv.org/abs/2311.01438 https://arxiv.org/abs/2311.01438