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Scientists say they may have detected dark energy

150 points by Osiris30 5y ago

paulpauper 5y ago
This is why I don't belive there is scientific stagnation. New discoveries and theories are constant being made and proposed. Its only because the problems have gotten harder that progress seems slower.
- BiteCode_dev 5y ago
  Honestly progress doesn't seems slower at all. I hear news of scientific discovery every week. Science is large, and it's not just physics.
  - 3pt14159 5y ago
    Also what people consider science is a bit of a continuum. There is what is universally objective scientific breakthrough (like discovery of quarks) on the one end, and universally objective non-breakthrough (like the construction of a simple bridge) on the other but where does the progress of iPhone 1 to iPhone 13 fit? So much advancement is going on in or around computing right now I think that in the future we'll sorta look back and consider that closer to science than mere engineering.
    - adtac 5y ago
      No, I disagree. Science is something that's well established and has been around for centuries. Engineering is also well established and has been around for centuries. The iPhone is an engineering advancement and will always be viewed that way; it'll always be in the same category as building the aqueducts. There was nothing scientifically groundbreaking about it, unlike discovering gravity or radiation.
- andrew_eit 5y ago
  This is not how I understood the argument of scientific stagnation though, wasn't it more to do with 'breakthrough' discoveries that somehow radically disrupted / changed our way of life? In that regard I'm inclined to believe there is some sort of stagnation. Though not necessarily at the fault of the community or researchers. It could very well be a mix of what you said about problems being harder that we have, between 1900 - 2000 picked all the 'low hanging fruit' in physics, and that we now need lots of patient, 'baby step' type improvements to get to a new era where the technology is powerful enough to make big leaps again. Kind of like how Deep Learning research stagnated due to lack of data volume and processing power. This is my perspective, but I think it's totally up for debate and I'm keen to hear different opinions.
  - NineStarPoint 5y ago
    Whether its the fault of the community or just natural consequences of the low hanging fruit being picked away will only be possible to know in retrospect, really. Is the issue that we don’t have the technology to figure things out, or that the community was too stuck in a local maximum of explainability to find the summit where everything ti explained? Until the next major breakthrough actually happens, it’s hard to know which is the case. Although in general, I’d say it’s possible a big leap never happens again. Given that physics will ultimately be a finite set of rules, if we can explain the vast majority of phenomenon accurately, slotting the last few pieces into place might not grant us much. It will feel great for humanity to know of course, but it’s entirely plausible that the reason we have so much trouble figuring these these out is that they’re almost completely separate from the human experience. We might figure out quantum gravity, go “that’s nice”, but if it’s only relevant when there are stellar masses involved not be able to use it to change our way of life. Big changes to how humanity lives going forward could be entirely reliant on human invention/ingenuity, not us learning new facts about the universe we live in.
    - skulk 5y ago
      There are still poorly understood physical phenomena that could prove to be tremendously useful, like superconductors. If quantum gravity leads us to room-temperature superconductors (for example), that would be absolutely earth-shattering for humanity.
- BurningFrog 5y ago
  Under my Dark Stagnation theory, scientific stagnation varies a lot between different scientific fields.
- tengbretson 5y ago
  My model for determining the number of scientific discoveries indicates that visible, understandable scientific discoveries only account for about 10% of the scientific discoveries out there. I theorize that there must be a remaining 90% of scientific discoveries that are undetectable, unknowable "dark discoveries."
- deleted 5y ago
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eterevsky 5y ago
I would bet at 10 to 1 rate that this will turn out not to be dark energy. We haven't observed any dark energy effects with scales less than billions light years. I would've been much less skeptical if they announced that they found dark matter.
mabbo 5y ago
This is neat, but I would take it with so many grains of salt. We don't know what dark matter is. We have some ideas. XENON1T was/is testing some of those theories about dark matter. We know next to nothing at all about dark energy. We're pretty sure it's a thing, or else our model of the universe is just completely wrong and it's not anything at all. So if XENON1T finds something that doesn't fit their model of dark matter, going on a limb and saying "I dunno, maybe it's dark energy?" is a fun thought experiment and model, but at best it might lead to further experiments to see how likely that model is to be true. And from what I'm reading, that's the attitude the scientists behind XENON1T are taking here. But alas, we all know that tomorrow we can expect pop-sci articles saying "DARK ENERGY HAS BEEN SOLVED".
- raattgift 5y ago
  Sunny Vagnozzi can't really be said to be suddenly coming up with the idea - there are several obviously related preprints at https://arxiv.org/a/vagnozzi_s_1.html https://arxiv.org/a/vagnozzi_s_1.html I'd like to give some broad background on the link at the top of this discussion. If one substitutes "dark energy" with "quintessence field driving the accelerated expansion" one gets closer to what the press release (and associated paper, found at https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.063023 https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.0... ) discusses. The "quint" in "quintessence" refers to a "fifth fundamental force", which I'll get to below. As early as the 1920s (per https://arxiv.org/abs/1211.6338 https://arxiv.org/abs/1211.6338 which is really good history of science) relativists were discussing whether some long-range force could stabilize a universe filled with dense matter, which would otherwise have a tendency to collapse over surprisingly short timescales (e.g., attempts to measure the age of the Earth by studying radioisotope ratios and coming up with billions of years of planetary age meant Earth should already be squashed into the remains of a collapsed universe). Einstein preferred a geometrical solution, the cosmological constant, to model the obvious non-collapse. With the 1990s discovery of the accelerating expansion ( https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_universe https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_... ) this preference has been vindicated in the effectiveness of the standard cosmology, \Lambda-CDM, where \Lambda is the cosmological constant. It captures in one variable the measured value of the accelerated expansion, even taking into account disputes over the exact value of the Hubble constant. The bulk scatter of redshifting luminous matter in universe is best approximated by the Friedmann-Lemaître-Robertson-Walker equations, with the Robertson-Walker term being a metric describing a universe that, if one considers 2d-plane-slicing of space, is like a stack of plates, where each plate is stacked upon an earlier plate, and in turn supports a stack of later plates. The Robertson-Walker metric has parameters which define how flat each plate is -- typical dinner plates, tea saucers, and so on tend to be curved so that liquids don't roll off the edges. (We observe distant galaxies "rolling out of sight", growing too dim and too red to see). The R-W metric also has parameters which define the evolution in size from one plate to the next. Plates may get bigger towards the future; stay the same size; decrease; or even oscillate; depending on the parameter function. Bigger here means that we consider the radius from the centre of the plate to the edge. When we add the Copernican Principle, which says our model should not require that Earth be at or even very close to the exact centre of the cosmos (if there even is such a centre), we take this to mean the distance from us (or any comparable observer) to the observer-centred cosmological horizon. In our universe, we have exceptionally flat "plates" stacked in such a way that earlier plates are much smaller than larger plates. The "plates" at this age of the universe have a diameter of nearly a hundred billion light-years. The accelerated expansion means that the growth in the future direction is superlinear. And of course, we extend the 2-d planar plates to a 3-d volume. However, we can go the route that Einstein did not prefer, namely that something non-geometrical is at work driving the accelerated expansion of the universe. In that case, we probably retain the ideas from Robertson-Walker and stack up "plates", but choose a function where either all the plates are the same size, or where the plates grow purely linearly -- no acceleration of the expansion. (The latter approach is how cosmology was modelled when the expansion was believed to be inertial, driven by some early-time impulse that went away around the time of the hot big bang). To this non-accelerating universe, we have to add something to make galaxies continue to accelerate apart, rather than drifting apart inertially, or recollapsing in the distant future. We add nonzeros to the tension (aka pressure, but negative) components of the stress-energy tensor. That tensor is really a tensor-field where each point in spacetime has a tensorial value which describes the flow of energy-momentum in and out of a point, including in the timelike-direction. In general the tension term must be very small, or the accelerated expansion would look very different, and galaxies would look even more different. In general the tension must vanish inside stable gravitationally bound structures. Moreover, the tension term must also appear in vacuum in deep inter-galaxy-cluster space. When we study mechanisms that can plausibly generate such stress-energy, we'd tend to be drawn towards our best fundamental theories for known forms of matter, namely the Standard Model of particle physics. We'd want to extend the Standard Model by introducing some particles which give rise to interpretation as carrying and feeling a new fundamental matter force to go with electromagnetism and the nuclear forces. This is usually called a fifth force, but maybe better to be just called yet another long (as in practically infinite) distance force comparable to electromagnetism or Newtonian gravitation. Now, an aside. Manhattan is not undergoing cosmic expansion. The Earth-Moon system is not expanding. Neither is our solar system to the best of our ability to measure. Our galaxy and the cluster that it's in do not appear to be expanding. Distant galaxy clusters appear to be bound together against the cosmological accelerated expansion. In the standard cosmology we are forced to say that the cosmos in bulk is best described by the Friedmann-Lemaître-Robertson-Walker model, but that local systems, like Earth, are best described with exterior Kerr metrics, or a Lemaître-Tolman-Bondi collapsing metric, which we can stitch together through an annoying process called the Israel-Darmois Junction, or alternatively we can use something inhomogeneous that captures a set of these Kerr/Schwarzschild-like metrics all at once. This is no big deal; general-relativists do this all the time at much smaller scales with good results. Cosmologically, for instance, "swiss-cheese" models are reasonably powerful, where the "cheese" is the increasingly sparse expanding space and the "holes" are voids where the matter is collapsing into ever denser arrangements. A fifth force model also has to avoid an expanding solar system. It can do this in at least two ways. Firstly, one can distribute the sources of the fifth force carefully in such a way that they are almost exclusively found outside galaxy clusters. (We take a similar approach with Dark Matter, which is found mainly inside galaxy clusters; the exclusiveness of Dark Matter is less strict than what would have to apply to a fifth force particle family). Alternatively, one could create an interaction with matter which turns the fifth-force charge into something else, like a low-mass particle such as the axion. This process is called "screening" when a large object like the sun (or its magnetic field) encourages this conversion. One could then compare the environment around the sun and Jupiter to see a mass-dependence or magnetic-field-strength dependence on such screening, or even look to more extreme objects like magnetars. This paper by Vagnozzi chases a small deviation from expected results towards this type of fifth-force screening by the sun driven by theoretical particles which [a] seek to solve a problem in the Standard Model of Particle physics, [b] are decent candidates for at least some of the dark matter energy-density, and [c] are still as far as I know theoretical rather than discovered. https://en.wikipedia.org/wiki/Axion https://en.wikipedia.org/wiki/Axion At this stage, this idea is a more complicated way of capturing the results we have from many astrophysical observations in a way that must practically completely reproduce what we get from the FLRW equations of \Lambda-CDM. More complicated because it adds parameters to the energy-density of the standard cosmology. It also almost certainly would require additional parameters in the Standard Model of Particle Physics. These are not impossible demands, but will be difficult enough to do in practice that few people will make realistic attempts without more compelling results than those reported by Vagnozzi et al.
raattgift 5y ago
The original text which the link at the top simply duplicates minus the hyperlinks is at : https://www.cam.ac.uk/research/news/have-we-detected-dark-energy-cambridge-scientists-say-its-a-possibility https://www.cam.ac.uk/research/news/have-we-detected-dark-en... which links to the open access paper at Phys.Rev.D : https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.063023 https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.0...
- carbocation 5y ago
  That's very helpful. Based on the paper, which seems to state that they have a 2σ significance level, this seems very preliminary (I am absolutely not a physicist!) > As an example, we examine whether the electron recoil excess recently reported by the XENON1T collaboration can be explained by chameleon-screened dark energy, and find that such a model is preferred over the background-only hypothesis at the 2.0σ level, in a large range of parameter space not excluded by stellar (or other) probes.
  - jug 5y ago
    Yes - the Phys.org article also states they need to replicate these results and the kind of projects that are underway that can assist with this sort of science.
  - raattgift 5y ago
    > That's very helpful. Thank you. > seems very preliminary Agreed. I made a further comment in reply to another commenter at https://news.ycombinator.com/item?id=28566823 https://news.ycombinator.com/item?id=28566823
    - carbocation 5y ago
      Your other reply feels like it could be packaged up a bit and published somewhere!
verytrivial 5y ago
"detected dark energy" Ooo! This could be really interes-- "(phys.org)" .... Oh. Expectations adjusted accordingly.
- jug 5y ago
  I don’t see anything wrong done here by Phys.org. It’s the world’s most sensitive dark matter detector so I’m happy they’re reporting on its findings. This isn’t some tabloid-level news but making its rounds at universities.
  - cozzyd 5y ago
    To be clear, they're reporting on an interpretation of a not-understood part of Xenon1T results (a low-energy excess in electronic recoil event rate). Here is the original paper by Xenon1T reporting on the excess: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.102.072004 https://journals.aps.org/prd/abstract/10.1103/PhysRevD.102.0... And no, there's nothing wrong with it, but there are many many such interpretation papers any time an experiment reports something unusual and at most one of them (but probably zero!) might be true :).
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mancy00 5y ago
Isn’t dark energy the aura that surrounds politicians?
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tppiotrowski 5y ago
> About 27% is dark matter—the invisible force holding galaxies and the cosmic web together—while 68% is dark energy, which causes the universe to expand at an accelerated rate. I understand that matter slows and energy accelerates the expansion of the universe but how is this ratio calculated? It seems like we would need to know the strength of dark energy to arrive at this reasoning.
- LegitShady 5y ago
  It’s based on actual observations, with some assumptions on the nature of the universe. As you said - dark matter is detected by extra “holds stuff together” force than observed masses would indicate exists from a host of phenomena (find the Wikipedia article on dark matter for more detail), and dark energy which is the energy required to make the universe expand at an accelerated rate is based on on distance-redshift relation, measurements of the cosmic background radiation, and theoretical additional non matter or dark matter energy required to form an ‘observationally flat universe’. The basics for both of these are discussed on the Wikipedia for both subjects. Disclaimer - I am not a physicist but I did stay at the holiday inn express last night and I do watch a lot of PBS Spacetime on youtube.
  - LegitShady 5y ago
    cant edit but "accelerated rate" should be "accelerating rate" and "dark matter energy" should be "non dark matter energy"
dirtbag__dad 5y ago
This article was a breeze to read. (As opposed to the complicated scientific abstracts I usually give up on after a few paragraphs.) Anyone have recommendations for approachable coverage of science journals and news?
- sanxiyn 5y ago
  I actually found both Nature and Science to be excellent at covering science news. They are famous for science journal, but they also have science news department. https://www.nature.com/news https://www.nature.com/news https://www.science.org/news https://www.science.org/news
  - mcguire 5y ago
    Both of which have excellent weekly podcasts covering their articles and other science news of the week. Oh, and speaking of Science News, that is (was?) an excellent weekly with short articles on recent results. I just haven't subscribed in a number of years.
snambi 5y ago
Are the scientists not sure?
- tiborsaas 5y ago
  Only the good ones.
jjtheblunt 5y ago
"may" -> subjunctive mood verb -> usually turns out false when in headlines, it seems
- canjobear 5y ago
  There is no subjunctive mood in the title
  - deleted 5y ago
    [deleted]
  - jjtheblunt 5y ago
    "may have" ?
peter_retief 5y ago
Right, I wouldnt hold my breath about the dark matter theory. It is up there with alien space ships and santa.
mcguire 5y ago
I haven't been following cosmology very closely in a while; could someone update my knowledge a bit? * The only evidence for dark matter is that galaxies and galactic clusters are moving faster than they should be; i.e. they are acting like there is more gravitational mass around. * The only evidence for dark energy is that the redshift of distant objects is higher than it should be; i.e. they seem to be accelerating away rather than decelerating. * All of the other properties of dark matter and dark energy are negative: we have not been able to observe anything, so we know what it's not.
- nerfhammer 5y ago
  I believe another piece of evidence is there is more gravitational lensing than expected in various places
- jasonwatkinspdx 5y ago
  > The only evidence for dark matter is that galaxies and galactic clusters are moving faster than they should be; i.e. they are acting like there is more gravitational mass around. There's more than that: https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... Likewise for dark energy: https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existence https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existe...
  - mcguire 5y ago
    Thanks!
- skohan 5y ago
  I'm pretty far from this stuff, so I am sure the people studying it know far more than I do. However when I read about dark matter/energy, and quantum physics, it sometimes reminds me of stories of the geocentric model, where the scientists of the day derived this incredibly complex clockwork geometry of the solar system, to account for the strange curves the other planets seemed to take to cross the night's sky. I wonder if we're similarly missing a key insight which will make these "strange results" fall into place. On the other hand, it's entirely possible that we've just reached the limits of what can be found intuitive to our primitive ape brains.
aszantu 5y ago
I just thought: if fusion makes heavier elements, it makes sense that the universe expands faster... And I don't even know how I got there... fusion makes elements heavier. Heavier collections of elements make more gravity. (is the creation of matter limited in our universe?) In bucket with limited supply of new matter, through fusion the existing matter makes clumps which get heavier and heavier. But the substrate will thin everywhere else. Supermassive black holes make more gravity. The more compression there is, the more heat is generated, And matter switches state at some point. (solid -> gaseous -> plasma -> another step -> another step?) If the matter "becomes" dark energy as a new sort of state, it could get "flung" out like two electrons with the same poles. Since Energy doesn't get lost, there's no other way than to "collect" at the fringes of the universe If the universe was like a baloon in the universe it would push the boundaries of the bucket indefinitely. Gravity is kinda like our substrate, not sure if fish are aware about their water being "heavier or less heavy", not sure if this analogy will ever hold up... can someone let me know if I got some of it right?
- tsimionescu 5y ago
  First of all, while fusion makes elements heavier, gravity does not increase this way, since the total energy of the system remains constant, and E=mc^2. It could be that the radius the mass/energy is contained in does decrease, but not all stars end up as black holes, while ack holes have no more or less gravity than any other object with the same mass/energy. Then, most of your explanation is carried by the solid -> liquid -> gas -> plasma ->... -> dark energy idea, which makes no sense. Dark energy is not matter - none of the observations are consistent with any kind of matter in intergalactic space. And matter of any kind will always have gravity, so it can't be responsible for the expansion of space time.
  - aszantu 5y ago
    unless it goes from matter to something else (like two particles with oposite poles) when being cooked up inside the black that tries to get away from each other
    - 5y ago

150 points by Osiris30 5y ago

This is why I don't belive there is scientific stagnation. New discoveries and theories are constant being made and proposed. Its only because the problems have gotten harder that progress seems slower.

BiteCode_dev 5y ago
Honestly progress doesn't seems slower at all. I hear news of scientific discovery every week. Science is large, and it's not just physics.
- 3pt14159 5y ago
  Also what people consider science is a bit of a continuum. There is what is universally objective scientific breakthrough (like discovery of quarks) on the one end, and universally objective non-breakthrough (like the construction of a simple bridge) on the other but where does the progress of iPhone 1 to iPhone 13 fit? So much advancement is going on in or around computing right now I think that in the future we'll sorta look back and consider that closer to science than mere engineering.
  - adtac 5y ago
    No, I disagree. Science is something that's well established and has been around for centuries. Engineering is also well established and has been around for centuries. The iPhone is an engineering advancement and will always be viewed that way; it'll always be in the same category as building the aqueducts. There was nothing scientifically groundbreaking about it, unlike discovering gravity or radiation.
andrew_eit 5y ago
This is not how I understood the argument of scientific stagnation though, wasn't it more to do with 'breakthrough' discoveries that somehow radically disrupted / changed our way of life? In that regard I'm inclined to believe there is some sort of stagnation. Though not necessarily at the fault of the community or researchers. It could very well be a mix of what you said about problems being harder that we have, between 1900 - 2000 picked all the 'low hanging fruit' in physics, and that we now need lots of patient, 'baby step' type improvements to get to a new era where the technology is powerful enough to make big leaps again. Kind of like how Deep Learning research stagnated due to lack of data volume and processing power. This is my perspective, but I think it's totally up for debate and I'm keen to hear different opinions.
- NineStarPoint 5y ago
  Whether its the fault of the community or just natural consequences of the low hanging fruit being picked away will only be possible to know in retrospect, really. Is the issue that we don’t have the technology to figure things out, or that the community was too stuck in a local maximum of explainability to find the summit where everything ti explained? Until the next major breakthrough actually happens, it’s hard to know which is the case. Although in general, I’d say it’s possible a big leap never happens again. Given that physics will ultimately be a finite set of rules, if we can explain the vast majority of phenomenon accurately, slotting the last few pieces into place might not grant us much. It will feel great for humanity to know of course, but it’s entirely plausible that the reason we have so much trouble figuring these these out is that they’re almost completely separate from the human experience. We might figure out quantum gravity, go “that’s nice”, but if it’s only relevant when there are stellar masses involved not be able to use it to change our way of life. Big changes to how humanity lives going forward could be entirely reliant on human invention/ingenuity, not us learning new facts about the universe we live in.
  - skulk 5y ago
    There are still poorly understood physical phenomena that could prove to be tremendously useful, like superconductors. If quantum gravity leads us to room-temperature superconductors (for example), that would be absolutely earth-shattering for humanity.
BurningFrog 5y ago
Under my Dark Stagnation theory, scientific stagnation varies a lot between different scientific fields.
tengbretson 5y ago
My model for determining the number of scientific discoveries indicates that visible, understandable scientific discoveries only account for about 10% of the scientific discoveries out there. I theorize that there must be a remaining 90% of scientific discoveries that are undetectable, unknowable "dark discoveries."
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eterevsky 5y ago

I would bet at 10 to 1 rate that this will turn out not to be dark energy. We haven't observed any dark energy effects with scales less than billions light years. I would've been much less skeptical if they announced that they found dark matter.

mabbo 5y ago

This is neat, but I would take it with so many grains of salt. We don't know what dark matter is. We have some ideas. XENON1T was/is testing some of those theories about dark matter. We know next to nothing at all about dark energy. We're pretty sure it's a thing, or else our model of the universe is just completely wrong and it's not anything at all. So if XENON1T finds something that doesn't fit their model of dark matter, going on a limb and saying "I dunno, maybe it's dark energy?" is a fun thought experiment and model, but at best it might lead to further experiments to see how likely that model is to be true. And from what I'm reading, that's the attitude the scientists behind XENON1T are taking here. But alas, we all know that tomorrow we can expect pop-sci articles saying "DARK ENERGY HAS BEEN SOLVED".

raattgift 5y ago
Sunny Vagnozzi can't really be said to be suddenly coming up with the idea - there are several obviously related preprints at https://arxiv.org/a/vagnozzi_s_1.html https://arxiv.org/a/vagnozzi_s_1.html I'd like to give some broad background on the link at the top of this discussion. If one substitutes "dark energy" with "quintessence field driving the accelerated expansion" one gets closer to what the press release (and associated paper, found at https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.063023 https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.0... ) discusses. The "quint" in "quintessence" refers to a "fifth fundamental force", which I'll get to below. As early as the 1920s (per https://arxiv.org/abs/1211.6338 https://arxiv.org/abs/1211.6338 which is really good history of science) relativists were discussing whether some long-range force could stabilize a universe filled with dense matter, which would otherwise have a tendency to collapse over surprisingly short timescales (e.g., attempts to measure the age of the Earth by studying radioisotope ratios and coming up with billions of years of planetary age meant Earth should already be squashed into the remains of a collapsed universe). Einstein preferred a geometrical solution, the cosmological constant, to model the obvious non-collapse. With the 1990s discovery of the accelerating expansion ( https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_universe https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_... ) this preference has been vindicated in the effectiveness of the standard cosmology, \Lambda-CDM, where \Lambda is the cosmological constant. It captures in one variable the measured value of the accelerated expansion, even taking into account disputes over the exact value of the Hubble constant. The bulk scatter of redshifting luminous matter in universe is best approximated by the Friedmann-Lemaître-Robertson-Walker equations, with the Robertson-Walker term being a metric describing a universe that, if one considers 2d-plane-slicing of space, is like a stack of plates, where each plate is stacked upon an earlier plate, and in turn supports a stack of later plates. The Robertson-Walker metric has parameters which define how flat each plate is -- typical dinner plates, tea saucers, and so on tend to be curved so that liquids don't roll off the edges. (We observe distant galaxies "rolling out of sight", growing too dim and too red to see). The R-W metric also has parameters which define the evolution in size from one plate to the next. Plates may get bigger towards the future; stay the same size; decrease; or even oscillate; depending on the parameter function. Bigger here means that we consider the radius from the centre of the plate to the edge. When we add the Copernican Principle, which says our model should not require that Earth be at or even very close to the exact centre of the cosmos (if there even is such a centre), we take this to mean the distance from us (or any comparable observer) to the observer-centred cosmological horizon. In our universe, we have exceptionally flat "plates" stacked in such a way that earlier plates are much smaller than larger plates. The "plates" at this age of the universe have a diameter of nearly a hundred billion light-years. The accelerated expansion means that the growth in the future direction is superlinear. And of course, we extend the 2-d planar plates to a 3-d volume. However, we can go the route that Einstein did not prefer, namely that something non-geometrical is at work driving the accelerated expansion of the universe. In that case, we probably retain the ideas from Robertson-Walker and stack up "plates", but choose a function where either all the plates are the same size, or where the plates grow purely linearly -- no acceleration of the expansion. (The latter approach is how cosmology was modelled when the expansion was believed to be inertial, driven by some early-time impulse that went away around the time of the hot big bang). To this non-accelerating universe, we have to add something to make galaxies continue to accelerate apart, rather than drifting apart inertially, or recollapsing in the distant future. We add nonzeros to the tension (aka pressure, but negative) components of the stress-energy tensor. That tensor is really a tensor-field where each point in spacetime has a tensorial value which describes the flow of energy-momentum in and out of a point, including in the timelike-direction. In general the tension term must be very small, or the accelerated expansion would look very different, and galaxies would look even more different. In general the tension must vanish inside stable gravitationally bound structures. Moreover, the tension term must also appear in vacuum in deep inter-galaxy-cluster space. When we study mechanisms that can plausibly generate such stress-energy, we'd tend to be drawn towards our best fundamental theories for known forms of matter, namely the Standard Model of particle physics. We'd want to extend the Standard Model by introducing some particles which give rise to interpretation as carrying and feeling a new fundamental matter force to go with electromagnetism and the nuclear forces. This is usually called a fifth force, but maybe better to be just called yet another long (as in practically infinite) distance force comparable to electromagnetism or Newtonian gravitation. Now, an aside. Manhattan is not undergoing cosmic expansion. The Earth-Moon system is not expanding. Neither is our solar system to the best of our ability to measure. Our galaxy and the cluster that it's in do not appear to be expanding. Distant galaxy clusters appear to be bound together against the cosmological accelerated expansion. In the standard cosmology we are forced to say that the cosmos in bulk is best described by the Friedmann-Lemaître-Robertson-Walker model, but that local systems, like Earth, are best described with exterior Kerr metrics, or a Lemaître-Tolman-Bondi collapsing metric, which we can stitch together through an annoying process called the Israel-Darmois Junction, or alternatively we can use something inhomogeneous that captures a set of these Kerr/Schwarzschild-like metrics all at once. This is no big deal; general-relativists do this all the time at much smaller scales with good results. Cosmologically, for instance, "swiss-cheese" models are reasonably powerful, where the "cheese" is the increasingly sparse expanding space and the "holes" are voids where the matter is collapsing into ever denser arrangements. A fifth force model also has to avoid an expanding solar system. It can do this in at least two ways. Firstly, one can distribute the sources of the fifth force carefully in such a way that they are almost exclusively found outside galaxy clusters. (We take a similar approach with Dark Matter, which is found mainly inside galaxy clusters; the exclusiveness of Dark Matter is less strict than what would have to apply to a fifth force particle family). Alternatively, one could create an interaction with matter which turns the fifth-force charge into something else, like a low-mass particle such as the axion. This process is called "screening" when a large object like the sun (or its magnetic field) encourages this conversion. One could then compare the environment around the sun and Jupiter to see a mass-dependence or magnetic-field-strength dependence on such screening, or even look to more extreme objects like magnetars. This paper by Vagnozzi chases a small deviation from expected results towards this type of fifth-force screening by the sun driven by theoretical particles which [a] seek to solve a problem in the Standard Model of Particle physics, [b] are decent candidates for at least some of the dark matter energy-density, and [c] are still as far as I know theoretical rather than discovered. https://en.wikipedia.org/wiki/Axion https://en.wikipedia.org/wiki/Axion At this stage, this idea is a more complicated way of capturing the results we have from many astrophysical observations in a way that must practically completely reproduce what we get from the FLRW equations of \Lambda-CDM. More complicated because it adds parameters to the energy-density of the standard cosmology. It also almost certainly would require additional parameters in the Standard Model of Particle Physics. These are not impossible demands, but will be difficult enough to do in practice that few people will make realistic attempts without more compelling results than those reported by Vagnozzi et al.

raattgift 5y ago

The original text which the link at the top simply duplicates minus the hyperlinks is at : https://www.cam.ac.uk/research/news/have-we-detected-dark-energy-cambridge-scientists-say-its-a-possibility https://www.cam.ac.uk/research/news/have-we-detected-dark-en... which links to the open access paper at Phys.Rev.D : https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.063023 https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.0...

carbocation 5y ago
That's very helpful. Based on the paper, which seems to state that they have a 2σ significance level, this seems very preliminary (I am absolutely not a physicist!) > As an example, we examine whether the electron recoil excess recently reported by the XENON1T collaboration can be explained by chameleon-screened dark energy, and find that such a model is preferred over the background-only hypothesis at the 2.0σ level, in a large range of parameter space not excluded by stellar (or other) probes.
- jug 5y ago
  Yes - the Phys.org article also states they need to replicate these results and the kind of projects that are underway that can assist with this sort of science.
- raattgift 5y ago
  > That's very helpful. Thank you. > seems very preliminary Agreed. I made a further comment in reply to another commenter at https://news.ycombinator.com/item?id=28566823 https://news.ycombinator.com/item?id=28566823
  - carbocation 5y ago
    Your other reply feels like it could be packaged up a bit and published somewhere!

verytrivial 5y ago

"detected dark energy" Ooo! This could be really interes-- "(phys.org)" .... Oh. Expectations adjusted accordingly.

jug 5y ago
I don’t see anything wrong done here by Phys.org. It’s the world’s most sensitive dark matter detector so I’m happy they’re reporting on its findings. This isn’t some tabloid-level news but making its rounds at universities.
- cozzyd 5y ago
  To be clear, they're reporting on an interpretation of a not-understood part of Xenon1T results (a low-energy excess in electronic recoil event rate). Here is the original paper by Xenon1T reporting on the excess: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.102.072004 https://journals.aps.org/prd/abstract/10.1103/PhysRevD.102.0... And no, there's nothing wrong with it, but there are many many such interpretation papers any time an experiment reports something unusual and at most one of them (but probably zero!) might be true :).

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mancy00 5y ago

Isn’t dark energy the aura that surrounds politicians?

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tppiotrowski 5y ago

> About 27% is dark matter—the invisible force holding galaxies and the cosmic web together—while 68% is dark energy, which causes the universe to expand at an accelerated rate. I understand that matter slows and energy accelerates the expansion of the universe but how is this ratio calculated? It seems like we would need to know the strength of dark energy to arrive at this reasoning.

LegitShady 5y ago
It’s based on actual observations, with some assumptions on the nature of the universe. As you said - dark matter is detected by extra “holds stuff together” force than observed masses would indicate exists from a host of phenomena (find the Wikipedia article on dark matter for more detail), and dark energy which is the energy required to make the universe expand at an accelerated rate is based on on distance-redshift relation, measurements of the cosmic background radiation, and theoretical additional non matter or dark matter energy required to form an ‘observationally flat universe’. The basics for both of these are discussed on the Wikipedia for both subjects. Disclaimer - I am not a physicist but I did stay at the holiday inn express last night and I do watch a lot of PBS Spacetime on youtube.
- LegitShady 5y ago
  cant edit but "accelerated rate" should be "accelerating rate" and "dark matter energy" should be "non dark matter energy"

dirtbag__dad 5y ago

This article was a breeze to read. (As opposed to the complicated scientific abstracts I usually give up on after a few paragraphs.) Anyone have recommendations for approachable coverage of science journals and news?

sanxiyn 5y ago
I actually found both Nature and Science to be excellent at covering science news. They are famous for science journal, but they also have science news department. https://www.nature.com/news https://www.nature.com/news https://www.science.org/news https://www.science.org/news
- mcguire 5y ago
  Both of which have excellent weekly podcasts covering their articles and other science news of the week. Oh, and speaking of Science News, that is (was?) an excellent weekly with short articles on recent results. I just haven't subscribed in a number of years.

snambi 5y ago

Are the scientists not sure?

tiborsaas 5y ago
Only the good ones.

jjtheblunt 5y ago

"may" -> subjunctive mood verb -> usually turns out false when in headlines, it seems

canjobear 5y ago
There is no subjunctive mood in the title
- deleted 5y ago
  [deleted]
- jjtheblunt 5y ago
  "may have" ?

peter_retief 5y ago

Right, I wouldnt hold my breath about the dark matter theory. It is up there with alien space ships and santa.

mcguire 5y ago

I haven't been following cosmology very closely in a while; could someone update my knowledge a bit? * The only evidence for dark matter is that galaxies and galactic clusters are moving faster than they should be; i.e. they are acting like there is more gravitational mass around. * The only evidence for dark energy is that the redshift of distant objects is higher than it should be; i.e. they seem to be accelerating away rather than decelerating. * All of the other properties of dark matter and dark energy are negative: we have not been able to observe anything, so we know what it's not.

nerfhammer 5y ago
I believe another piece of evidence is there is more gravitational lensing than expected in various places
jasonwatkinspdx 5y ago
> The only evidence for dark matter is that galaxies and galactic clusters are moving faster than they should be; i.e. they are acting like there is more gravitational mass around. There's more than that: https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... Likewise for dark energy: https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existence https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existe...
- mcguire 5y ago
  Thanks!
skohan 5y ago
I'm pretty far from this stuff, so I am sure the people studying it know far more than I do. However when I read about dark matter/energy, and quantum physics, it sometimes reminds me of stories of the geocentric model, where the scientists of the day derived this incredibly complex clockwork geometry of the solar system, to account for the strange curves the other planets seemed to take to cross the night's sky. I wonder if we're similarly missing a key insight which will make these "strange results" fall into place. On the other hand, it's entirely possible that we've just reached the limits of what can be found intuitive to our primitive ape brains.

aszantu 5y ago

I just thought: if fusion makes heavier elements, it makes sense that the universe expands faster... And I don't even know how I got there... fusion makes elements heavier. Heavier collections of elements make more gravity. (is the creation of matter limited in our universe?) In bucket with limited supply of new matter, through fusion the existing matter makes clumps which get heavier and heavier. But the substrate will thin everywhere else. Supermassive black holes make more gravity. The more compression there is, the more heat is generated, And matter switches state at some point. (solid -> gaseous -> plasma -> another step -> another step?) If the matter "becomes" dark energy as a new sort of state, it could get "flung" out like two electrons with the same poles. Since Energy doesn't get lost, there's no other way than to "collect" at the fringes of the universe If the universe was like a baloon in the universe it would push the boundaries of the bucket indefinitely. Gravity is kinda like our substrate, not sure if fish are aware about their water being "heavier or less heavy", not sure if this analogy will ever hold up... can someone let me know if I got some of it right?

tsimionescu 5y ago
First of all, while fusion makes elements heavier, gravity does not increase this way, since the total energy of the system remains constant, and E=mc^2. It could be that the radius the mass/energy is contained in does decrease, but not all stars end up as black holes, while ack holes have no more or less gravity than any other object with the same mass/energy. Then, most of your explanation is carried by the solid -> liquid -> gas -> plasma ->... -> dark energy idea, which makes no sense. Dark energy is not matter - none of the observations are consistent with any kind of matter in intergalactic space. And matter of any kind will always have gravity, so it can't be responsible for the expansion of space time.
- aszantu 5y ago
  unless it goes from matter to something else (like two particles with oposite poles) when being cooked up inside the black that tries to get away from each other
  - 5y ago