5 ms·

So our gravitation models weren’t working very well, so we thought maybe dark matter. Then that wasn’t working very well either, so we thought maybe dark energy

by d--b 5y ago

So our gravitation models weren’t working very well, so we thought maybe dark matter. Then that wasn’t working very well either, so we thought maybe dark energy? And now this is showing a lot of shortcomings too? I know nothing of astro physics, but I find it odd that a majority of astro physicians think of dark energy as a near certainty when all mainstream articles are about contradicting its implications.

dilyevsky 5y ago
Expansion of the universe (dark energy) has nothing to do with the dark mater. At least not according to any models known today. We know galaxies are racing away faster because we can see it (from red shift) so that part is certain. Why they are moving away is not certain hence “dark” energy
- ErikVandeWater 5y ago
  Peripheral question: Without dark matter, would there be more red shift?
  - raattgift 5y ago
    This is an excellent question. Light exiting from stars deep in a galaxy (which is in turn deep in a galaxy cluster) is subject to gravitational redshift. The greater the mass of the galaxy and cluster, the greater the redshift. If we add transparent, non-radiating mass to a galaxy and its cluster, the mean redshift of starlight from the galaxy will redshift, and we'll see the result on various absorption and emission lines in the spectrum. Note, we do not need dark matter for this; increasing the central mass of a galaxy cluster will do the trick too. We can do this by adding more stars, or by adding black holes, or by making a central black hole more massive. All of this will cause an increase in gravitational redshift. However, adding more central mass to galaxies will be reflected in the trajectories of clouds of gas and collections of stars within the galaxies; likewise, adding more concentrated mass to a galaxy cluster will alter the trajectory of galaxies within it. In general, these objects will have faster orbits -- fast enough that a non-gravitational redshift will be apparent in the arrangement of spectral lines. (We would also need to figure out how to keep any central masses from being extremely bright anywhere in the electromagnetic spectrum, and how to keep them from eclipsing parts of their home galaxies and clusters that are behind them from our perspective.) If we instead redistribute our increase in galactic mass into a gaseous halo-like structure that extends well beyond the outer stars, we get the increased gravitational redshift without requiring stars and gas to travel faster to avoid falling inwards. The halo still has to be electromagnetically quiet, and non-eclipsing, or we would see it. (This all applies to clusters, too). Stepping away from the "dark matter puzzle" of the spectral lines of molecular and atomic gas clouds in galaxies and clusters of them, we can then look at another aspect of your question. Galaxy clusters, from their point of view, are just floating passively in space. The expansion of the universe follows a model wherein if we treat the astrophysical content of galaxies and clusters as sensitive accelerometers that in aggregate show off the peculiar accelerations experienced by these large scale objects, there is no evidence that these accelerometers should report a non-zero magnitude or point in any particular direction. That is, galaxies and galaxy clusters are themselves in free fall, and we can see this because of lines of evidence including expansion-direction-and-magnitude squashings of https://en.wikipedia.org/wiki/Light_echo https://en.wikipedia.org/wiki/Light_echo lines turn out to be null, likewise there are no radio-loud edge shocks for galaxies that appear to be accelerating away from us, there are no shape distortions of the most highly "accelerated" distant galaxies, and so forth. There is simply a spectral redshift. So, for an isolated galaxy the spectrum it emits depends on a combination of its starshine, any noise from close to its central black hole(s), the noise from any thick gas clouds undergoing gravitational collapse, and the absorption and reemission of diffuse gas clouds heated by these types of emission and absorption-and-reemission sources and the distribution of dark matter within and around the galaxy. The more dark matter, the more redshifted the galaxy's spectral lines will be. Preserving the Copernican Principle, immersing a set of such isolated galaxies into an expanding cosmos leads to the inhabitants of each galaxy observing a cosmological redshift on the gravitatonally-redshifted spectrum emitted by other sufficiently distant galaxies. The gravitational-redshift depends on the mass of the galaxies including their dark matter. The cosmological redshift only depends on a light-travel distance from the galaxy, no matter how light or how massive that galaxy (or its parent cluster) is. Next, if we have an arrangement b-->F-->U, where b is a background galaxy, F is a foreground galaxy, and U is an observer, we will that the light from b is cosmologically redshifted before it interacts with the molecular and atomic gas clouds in F, causing a spreading of absorption and reemission lines rather than a thickening. For the very obvious lines from neutral atomic hydrogen, we see this as the https://en.wikipedia.org/wiki/Lyman-alpha_forest https://en.wikipedia.org/wiki/Lyman-alpha_forest which is an important tool in astrophysics and in checking the soundness of our theory of gravitation. We would get a Lya-forest structure even in the absence of dark matter in b or F or anywhere else; only the fine strucutre would differ. Indeed, we could substitute a diffuse static cloud of atomic hydrogen for F and still see a "forest" structure minus small contributions from the starshine in F and the orbital motions of the gas in F around some central mass. The deep infrared noise from F in that case, however, would not be gravitationally redshifted. Finally the answer to your question: the cosmological redshift does not in general depend on the "late" distribution of dark matter. (Late as in at least millions of years after the hot big bang). But the gravitational redshift on light emitted by stars and galaxies does depend on dark matter and in general the starshine is reddened by it. The reddening is because of the additional mass of the dark-matter-containing structures, and also because the distribution of the dark matter into halos allows for slower orbits of stars and gas clouds, thus suppressing their non-gravitational non-cosmological redshifts. (In an edge-on spiral galaxy without dark matter we would see a greater difference in the blueshift of one extreme tip of the thin disk and the redshift of the opposite extreme tip of the thin disk, compared to the same spiral galaxy with dark matter; the light-and-radio from the central bulge would be less redshifted in the absence of dark matter). So, no, the cosmological redshift does not depend on dark matter. The gravitational redshift on light from galaxies, however, does. So does the non-gravitational, non-cosmological redshift on gas clouds whirling around the edges of rotating (e.g. spiral) galaxies, or rising and sinking on radial orbits within elliptical galaxies.
    - Keysh 5y ago
      One important caveat to all this is that the gravitational redshift for a galaxy or galaxy cluster is an incredibly tiny effect. For example, the mean gravitational redshift for massive galaxy clusters has been measured (e.g., link below), and has a value of ~ 10 or 15 km/s. This can be compared with the typical motions of galaxies within a cluster (dispersions of ~ 1000 km/s) and the cosmological redshift of the clusters (for the sample in the paper I linked to, it's equivalent to an average Doppler shift of ~ 50,000 km/s). In order to make that measurement, the authors had to combine redshift measurements for ~ 20,000 galaxies from ~ 2,500 different clusters; as they note, it's impossible to detect the effect for just a single cluster: "... even assuming that every galaxy in a cluster could be spectroscopically measured, there are simply too few galaxies to allow the statistical detection of a non-zero gravitational redshift." So while it's technically true that adding dark matter to a galaxy would increase its gravitational redshift and thus its total observed redshift, this effect is immeasurably small. Mpetha et al. (2021), "Gravitational redshifting of galaxies in the SPIDERS cluster catalogue" https://arxiv.org/abs/2102.11156 https://arxiv.org/abs/2102.11156
      - raattgift 5y ago
        I can only reply to your "impossible to detect" with respect to clusters with the "technically challenging but apparently real and prospects for better measurements and extension to larger scales is promising" at the end of Kaiser's 2018 slide deck https://chalonge-devega.fr/Kaiser_29mars2018.pdf https://chalonge-devega.fr/Kaiser_29mars2018.pdf which I do not think is contradicted by the (interesting, thank you!) Mpetha et al (2021) paper, which cites at least three of Kaiser's earlier publications. Broadhurst & Scannapieco (2000) https://iopscience.iop.org/article/10.1086/312630 https://iopscience.iop.org/article/10.1086/312630 is also relevant here, as it distinguishes between X-Ray studies and optical ones (SPIDERS being an example of the latter) pointedly at §4.2. I do not disagree that ~ 10-20 km/s is small, but it's the little things that cause "tension" headaches, isn't it?
        Keysh 5y ago
        Kaiser's presentation doesn't suggest you can do the measurement for individual clusters; the example analysis is from Wojtak et al. (2011), which is similar to Mpetha et al. in using thousands of galaxies and thousands of clusters. The Broadhurst & Scannapieco discussion about X-ray studies is interesting and potentially promising, but no one seems to have tried doing for real.
- Twisol 5y ago
  Yes; the "dark" in these terms reflects our lack of understanding about both topics, not that they share some "dark" aspect in common. They're names for holes in our models, not for entities predicted by them.
  - denton-scratch 5y ago
    They're both "dark" because neither interacts with electromagnetic radiation. I share the opinion that they're names for holes, not entities; but I'm not qualified to defend that opinion.
    - 6nf 5y ago
      That's true but even more than that, we're not really sure dark matter is actual particles at all. Modified gravity for example is one alternative explanation that don't require a new particle or field.
    - yawaworht1978 5y ago
      So it is something but we don't know what, right? Is it safe to say it's not something from the periodic table or we simply don't know at all? And, in order to understand a something, do we always need that something to interact with EM radiation?
      - denton-scratch 5y ago
        Neither "dark matter" nor "dark energy" can be made from atoms (i.e. from the periodic table), because atoms interact with photons. I think to say "it is something" is a strong claim, requiring strong evidence. Dark energy, in particular, can't be a kind of "stuff". We could "understand" something that doesn't interact with EM; but we'd have trouble observing it.
  - jquery 5y ago
    I don’t know about dark energy, but dark matter isn’t called such because it’s a “hole in our models”. The theory is it doesn’t react electromagnetically with other matter. So it’s quite literally, “dark” matter.
- tsimionescu 5y ago
  Note that the red shift data has been coming into question lately, as it relies on some assumptions of the uniformity of the universe beyond the scale of galaxy super clusters, which doesn't seem like a safe assumption anymore as far as I understand.
wereHamster 5y ago
Which mainstream articles are you referring to?
spaetzleesser 5y ago
“I know nothing of astro physics,l Before criticizing physicists you should keep this in mind.
arbitrage 5y ago
dark matter is pretty solid of an idea. it's been around for over a hundred years at this point. it isn't a band-aide fix for wonky math in an inaccurate theory. the foundations of the concept of dark matter are fairly reasonable.
- Maursault 5y ago
  > dark matter is pretty solid of an idea Not really. It is a hypothesized form of matter. There are competing hypotheses. Dark matter came to popularity in order to explain observations of galaxy rotation curves in the late 1930s. But these galaxies were observed in isolation, giving the illusion of gravitational isolation, which doesn't exist. Once accounting for the tidal effects of nearby galaxies, [1] dark matter becomes unnecessary to explain galaxy rotation curves, all that is needed is Newtonian physics. But in the period since dark matter was hypothesized, other observations that could not be readily explained were lumped into dark matter. Dark matter is nothing if not an amalgamation of unexplainable observations. So while dark matter is unnecessary to explain galaxy rotation curves, it is still somehow useful to explain unrelated observations. > it isn't a band-aide fix It really sort of seems like that: can't explain an observation, so invent something out of thin air to explain it. Honestly, an hypothesis that elves are causing the observations has just as much validity (or lack thereof). Now, elves have not been proven to be the cause of these unexplained observations, but neither has dark matter. > the foundations of the concept of dark matter are fairly reasonable. See above concerning galaxy rotation curves and endnote. Please understand it is quite possible that dark matter does not exist. The reason why it seems like accepted science has to do with scientific paradigms. Once a notion becomes an accepted part of the science paradigm, it takes years if not decades after disproving it beyond all doubt to dislodge it from the paradigm. If science has a problem, it is paradigms. [1] https://www.youtube.com/watch?v=PL0ewiwqoTw&t=6m11s https://www.youtube.com/watch?v=PL0ewiwqoTw&t=6m11s
  - Keysh 5y ago
    The observations in the 1930s you allude to were of the motions of galaxies in galaxy clusters. The rotation-curve measurements for individual galaxies started in the 1970s and really took off in the 1980s. "Tidal effects" are a non-starter, and do nothing to explain galaxy rotation curves. Nor do they explain the excess motions of individual galaxies in galaxy groups and clusters, or the excess pressure of hot, X-ray emitting gas in groups and clusters -- all of which requires either some kind of dark matter, or some kind of alternate theory of gravity, or both. (I notice that I pointed out the ignorant nonsense of that video when you posted a link to it 7 months ago -- https://news.ycombinator.com/item?id=27085979#27090683 https://news.ycombinator.com/item?id=27085979#27090683)
notfed 5y ago
No... We observe with telescopes that the universe is expanding. We don't know why, so we call the mysterious force dark energy. It's just a name for something that we are objectively observing. (Also this has nothing to do with dark matter, that unrelated to this discussion and you're just getting the names conflated.)

by d--b 5y ago

dilyevsky 5y ago

Expansion of the universe (dark energy) has nothing to do with the dark mater. At least not according to any models known today. We know galaxies are racing away faster because we can see it (from red shift) so that part is certain. Why they are moving away is not certain hence “dark” energy

ErikVandeWater 5y ago
Peripheral question: Without dark matter, would there be more red shift?
- raattgift 5y ago
  This is an excellent question. Light exiting from stars deep in a galaxy (which is in turn deep in a galaxy cluster) is subject to gravitational redshift. The greater the mass of the galaxy and cluster, the greater the redshift. If we add transparent, non-radiating mass to a galaxy and its cluster, the mean redshift of starlight from the galaxy will redshift, and we'll see the result on various absorption and emission lines in the spectrum. Note, we do not need dark matter for this; increasing the central mass of a galaxy cluster will do the trick too. We can do this by adding more stars, or by adding black holes, or by making a central black hole more massive. All of this will cause an increase in gravitational redshift. However, adding more central mass to galaxies will be reflected in the trajectories of clouds of gas and collections of stars within the galaxies; likewise, adding more concentrated mass to a galaxy cluster will alter the trajectory of galaxies within it. In general, these objects will have faster orbits -- fast enough that a non-gravitational redshift will be apparent in the arrangement of spectral lines. (We would also need to figure out how to keep any central masses from being extremely bright anywhere in the electromagnetic spectrum, and how to keep them from eclipsing parts of their home galaxies and clusters that are behind them from our perspective.) If we instead redistribute our increase in galactic mass into a gaseous halo-like structure that extends well beyond the outer stars, we get the increased gravitational redshift without requiring stars and gas to travel faster to avoid falling inwards. The halo still has to be electromagnetically quiet, and non-eclipsing, or we would see it. (This all applies to clusters, too). Stepping away from the "dark matter puzzle" of the spectral lines of molecular and atomic gas clouds in galaxies and clusters of them, we can then look at another aspect of your question. Galaxy clusters, from their point of view, are just floating passively in space. The expansion of the universe follows a model wherein if we treat the astrophysical content of galaxies and clusters as sensitive accelerometers that in aggregate show off the peculiar accelerations experienced by these large scale objects, there is no evidence that these accelerometers should report a non-zero magnitude or point in any particular direction. That is, galaxies and galaxy clusters are themselves in free fall, and we can see this because of lines of evidence including expansion-direction-and-magnitude squashings of https://en.wikipedia.org/wiki/Light_echo https://en.wikipedia.org/wiki/Light_echo lines turn out to be null, likewise there are no radio-loud edge shocks for galaxies that appear to be accelerating away from us, there are no shape distortions of the most highly "accelerated" distant galaxies, and so forth. There is simply a spectral redshift. So, for an isolated galaxy the spectrum it emits depends on a combination of its starshine, any noise from close to its central black hole(s), the noise from any thick gas clouds undergoing gravitational collapse, and the absorption and reemission of diffuse gas clouds heated by these types of emission and absorption-and-reemission sources and the distribution of dark matter within and around the galaxy. The more dark matter, the more redshifted the galaxy's spectral lines will be. Preserving the Copernican Principle, immersing a set of such isolated galaxies into an expanding cosmos leads to the inhabitants of each galaxy observing a cosmological redshift on the gravitatonally-redshifted spectrum emitted by other sufficiently distant galaxies. The gravitational-redshift depends on the mass of the galaxies including their dark matter. The cosmological redshift only depends on a light-travel distance from the galaxy, no matter how light or how massive that galaxy (or its parent cluster) is. Next, if we have an arrangement b-->F-->U, where b is a background galaxy, F is a foreground galaxy, and U is an observer, we will that the light from b is cosmologically redshifted before it interacts with the molecular and atomic gas clouds in F, causing a spreading of absorption and reemission lines rather than a thickening. For the very obvious lines from neutral atomic hydrogen, we see this as the https://en.wikipedia.org/wiki/Lyman-alpha_forest https://en.wikipedia.org/wiki/Lyman-alpha_forest which is an important tool in astrophysics and in checking the soundness of our theory of gravitation. We would get a Lya-forest structure even in the absence of dark matter in b or F or anywhere else; only the fine strucutre would differ. Indeed, we could substitute a diffuse static cloud of atomic hydrogen for F and still see a "forest" structure minus small contributions from the starshine in F and the orbital motions of the gas in F around some central mass. The deep infrared noise from F in that case, however, would not be gravitationally redshifted. Finally the answer to your question: the cosmological redshift does not in general depend on the "late" distribution of dark matter. (Late as in at least millions of years after the hot big bang). But the gravitational redshift on light emitted by stars and galaxies does depend on dark matter and in general the starshine is reddened by it. The reddening is because of the additional mass of the dark-matter-containing structures, and also because the distribution of the dark matter into halos allows for slower orbits of stars and gas clouds, thus suppressing their non-gravitational non-cosmological redshifts. (In an edge-on spiral galaxy without dark matter we would see a greater difference in the blueshift of one extreme tip of the thin disk and the redshift of the opposite extreme tip of the thin disk, compared to the same spiral galaxy with dark matter; the light-and-radio from the central bulge would be less redshifted in the absence of dark matter). So, no, the cosmological redshift does not depend on dark matter. The gravitational redshift on light from galaxies, however, does. So does the non-gravitational, non-cosmological redshift on gas clouds whirling around the edges of rotating (e.g. spiral) galaxies, or rising and sinking on radial orbits within elliptical galaxies.
  - Keysh 5y ago
    One important caveat to all this is that the gravitational redshift for a galaxy or galaxy cluster is an incredibly tiny effect. For example, the mean gravitational redshift for massive galaxy clusters has been measured (e.g., link below), and has a value of ~ 10 or 15 km/s. This can be compared with the typical motions of galaxies within a cluster (dispersions of ~ 1000 km/s) and the cosmological redshift of the clusters (for the sample in the paper I linked to, it's equivalent to an average Doppler shift of ~ 50,000 km/s). In order to make that measurement, the authors had to combine redshift measurements for ~ 20,000 galaxies from ~ 2,500 different clusters; as they note, it's impossible to detect the effect for just a single cluster: "... even assuming that every galaxy in a cluster could be spectroscopically measured, there are simply too few galaxies to allow the statistical detection of a non-zero gravitational redshift." So while it's technically true that adding dark matter to a galaxy would increase its gravitational redshift and thus its total observed redshift, this effect is immeasurably small. Mpetha et al. (2021), "Gravitational redshifting of galaxies in the SPIDERS cluster catalogue" https://arxiv.org/abs/2102.11156 https://arxiv.org/abs/2102.11156
    - raattgift 5y ago
      I can only reply to your "impossible to detect" with respect to clusters with the "technically challenging but apparently real and prospects for better measurements and extension to larger scales is promising" at the end of Kaiser's 2018 slide deck https://chalonge-devega.fr/Kaiser_29mars2018.pdf https://chalonge-devega.fr/Kaiser_29mars2018.pdf which I do not think is contradicted by the (interesting, thank you!) Mpetha et al (2021) paper, which cites at least three of Kaiser's earlier publications. Broadhurst & Scannapieco (2000) https://iopscience.iop.org/article/10.1086/312630 https://iopscience.iop.org/article/10.1086/312630 is also relevant here, as it distinguishes between X-Ray studies and optical ones (SPIDERS being an example of the latter) pointedly at §4.2. I do not disagree that ~ 10-20 km/s is small, but it's the little things that cause "tension" headaches, isn't it?
      - Keysh 5y ago
        Kaiser's presentation doesn't suggest you can do the measurement for individual clusters; the example analysis is from Wojtak et al. (2011), which is similar to Mpetha et al. in using thousands of galaxies and thousands of clusters. The Broadhurst & Scannapieco discussion about X-ray studies is interesting and potentially promising, but no one seems to have tried doing for real.
Twisol 5y ago
Yes; the "dark" in these terms reflects our lack of understanding about both topics, not that they share some "dark" aspect in common. They're names for holes in our models, not for entities predicted by them.
- denton-scratch 5y ago
  They're both "dark" because neither interacts with electromagnetic radiation. I share the opinion that they're names for holes, not entities; but I'm not qualified to defend that opinion.
  - 6nf 5y ago
    That's true but even more than that, we're not really sure dark matter is actual particles at all. Modified gravity for example is one alternative explanation that don't require a new particle or field.
  - yawaworht1978 5y ago
    So it is something but we don't know what, right? Is it safe to say it's not something from the periodic table or we simply don't know at all? And, in order to understand a something, do we always need that something to interact with EM radiation?
    - denton-scratch 5y ago
      Neither "dark matter" nor "dark energy" can be made from atoms (i.e. from the periodic table), because atoms interact with photons. I think to say "it is something" is a strong claim, requiring strong evidence. Dark energy, in particular, can't be a kind of "stuff". We could "understand" something that doesn't interact with EM; but we'd have trouble observing it.
- jquery 5y ago
  I don’t know about dark energy, but dark matter isn’t called such because it’s a “hole in our models”. The theory is it doesn’t react electromagnetically with other matter. So it’s quite literally, “dark” matter.
tsimionescu 5y ago
Note that the red shift data has been coming into question lately, as it relies on some assumptions of the uniformity of the universe beyond the scale of galaxy super clusters, which doesn't seem like a safe assumption anymore as far as I understand.

wereHamster 5y ago

Which mainstream articles are you referring to?

spaetzleesser 5y ago

“I know nothing of astro physics,l Before criticizing physicists you should keep this in mind.

arbitrage 5y ago

dark matter is pretty solid of an idea. it's been around for over a hundred years at this point. it isn't a band-aide fix for wonky math in an inaccurate theory. the foundations of the concept of dark matter are fairly reasonable.

Maursault 5y ago
> dark matter is pretty solid of an idea Not really. It is a hypothesized form of matter. There are competing hypotheses. Dark matter came to popularity in order to explain observations of galaxy rotation curves in the late 1930s. But these galaxies were observed in isolation, giving the illusion of gravitational isolation, which doesn't exist. Once accounting for the tidal effects of nearby galaxies, [1] dark matter becomes unnecessary to explain galaxy rotation curves, all that is needed is Newtonian physics. But in the period since dark matter was hypothesized, other observations that could not be readily explained were lumped into dark matter. Dark matter is nothing if not an amalgamation of unexplainable observations. So while dark matter is unnecessary to explain galaxy rotation curves, it is still somehow useful to explain unrelated observations. > it isn't a band-aide fix It really sort of seems like that: can't explain an observation, so invent something out of thin air to explain it. Honestly, an hypothesis that elves are causing the observations has just as much validity (or lack thereof). Now, elves have not been proven to be the cause of these unexplained observations, but neither has dark matter. > the foundations of the concept of dark matter are fairly reasonable. See above concerning galaxy rotation curves and endnote. Please understand it is quite possible that dark matter does not exist. The reason why it seems like accepted science has to do with scientific paradigms. Once a notion becomes an accepted part of the science paradigm, it takes years if not decades after disproving it beyond all doubt to dislodge it from the paradigm. If science has a problem, it is paradigms. [1] https://www.youtube.com/watch?v=PL0ewiwqoTw&t=6m11s https://www.youtube.com/watch?v=PL0ewiwqoTw&t=6m11s
- Keysh 5y ago
  The observations in the 1930s you allude to were of the motions of galaxies in galaxy clusters. The rotation-curve measurements for individual galaxies started in the 1970s and really took off in the 1980s. "Tidal effects" are a non-starter, and do nothing to explain galaxy rotation curves. Nor do they explain the excess motions of individual galaxies in galaxy groups and clusters, or the excess pressure of hot, X-ray emitting gas in groups and clusters -- all of which requires either some kind of dark matter, or some kind of alternate theory of gravity, or both. (I notice that I pointed out the ignorant nonsense of that video when you posted a link to it 7 months ago -- https://news.ycombinator.com/item?id=27085979#27090683 https://news.ycombinator.com/item?id=27085979#27090683)

notfed 5y ago

No... We observe with telescopes that the universe is expanding. We don't know why, so we call the mysterious force dark energy. It's just a name for something that we are objectively observing. (Also this has nothing to do with dark matter, that unrelated to this discussion and you're just getting the names conflated.)