5 ms·

A naive question (not read the actual paper or know much about the field) - if requiring dark matter to explain galactic physics can be dispensed with by a dist

by sriku 7y ago

A naive question (not read the actual paper or know much about the field) - if requiring dark matter to explain galactic physics can be dispensed with by a distance correction, doesn't "we've been underestimating distances all this time" serve as a simpler fact of galactic physics than needing "there is mysterious dark matter everywhere" to explain galactic physics?

idlewords 7y ago
There are several different features of galaxies that dark matter explains. One of them is the speed with which stars orbit the center of a galaxy—this "rotation curve" is independent of how far the stars are from the galactic center, where if there were no dark matter, you would expect the outermost stars to go slower. That is an effect that is not affected by errors in measuring the distance to remote galaxies. We can see it in our own galaxy! https://www.e-education.psu.edu/astro801/content/l8_p8.html https://www.e-education.psu.edu/astro801/content/l8_p8.html In general, if you can think of an explanation that does away with the need for dark matter, you can also assume experts have thought hard about it and rejected it for good reason. Nobody likes dark matter as a hypothesis, everyone remembers analogous complications in the history of science that proved to be signposts to a better theory, and everyone knows that a Nobel prize waits for anyone who can find the correct explanation.
- trhway 7y ago
  >One of them is the speed with which stars orbit the center of a galaxy do we really orbit it? I mean is it a stable orbit (that is the option which requires DM ) or a just a spiral trajectory (no DM required)? The spiral galaxies look like a fireworks wheel https://depositphotos.com/69366109/stock-video-fireworks-pinwheel-catherine-wheel.html https://depositphotos.com/69366109/stock-video-fireworks-pin... , and without DM our "orbital" speed being higher than otherwise expected Keplerian orbital speed means that we're just moving away on a spiral trajectory from the center the same way like the sparks fly away on a spiral trajectory from the center of the fireworks wheel.
  - zaarn 7y ago
    There are plenty of non-spiral galaxies where we observe the same. Our sun moving away from the galactic center is also unlikely as if it had been much further in during it's creation phase, it would have been unable to create planets that support life. A galactic year is only a few million earth years, so our orbit must have remained in the stable region for the first billion years or so and we continue to be inside it. That points to a stable orbit around the galaxy. If it was a firework wheel our current position would mean that sun would have been traveling to the outside of the galaxy far below escape velocity or would have been unable to produce life-bearing planets.
    - trhway 7y ago
      >A galactic year is only a few million earth years no. It is 250 million years. >so our orbit must have remained in the stable region for the first billion years or so and we continue to be inside it. not really. While moving on that spiral trajectory, we've made only 16 rotations since Sun's formation. When the life on our planet started our neighborhood looked very different, not the boonies we're at right now :)
      - zaarn 7y ago
        >no. It is 250 million years. I don't see how that contradicts is being a few million years? >not really. While moving on that spiral trajectory, we've made only 16 rotations since Sun's formation. When the life on our planet started our neighborhood looked very different, not the boonies we're at right now :) Orbits don't really do spiral patterns in practice. Atleast not at this scale. The star would either be in a fairly stable orbit or on trajectory to leave the galaxy (minus elliptical orbits but those would carry the sun in and out of the dead zone quite often). Moving on a spiral trajectory does happen in orbital mechanics but only in one direction; inwards. Outwards spiraling would require quite a few quirks in the galactic gravity field. The spiral pattern itself isn't the result of a firework wheel, it's quite a quirk in the distribution of stars, though there are various models that explain them, they all involve very stable orbits for most stars.
  - 7y ago
- sriku 7y ago
  Good link! Thanks. > you can also assume experts have thought hard about it and rejected it for good reason. Of course. It seemed more of a reporting anomaly to focus exclusively on the distance correction if other factors also exist that would've required dark matter.
- pmoriarty 7y ago
  "In general, if you can think of an explanation that does away with the need for dark matter, you can also assume experts have thought hard about it and rejected it for good reason." Nobel prizes have been given for explanations which were at first rejected for what seemed like good reasons at the time, but which later proved to be the wrong reasons. The first such example which springs to mind is the explanation of helicobacter causing ulcers. Back to astrophysics, black holes were once considered science fictional phenomena.
  - jmcqk6 7y ago
    Just because something like this has happened in the past does not justify believing in a theory now that is following the same pattern. This is an example of survivorship bias. You have an example where this happened, but there are thousands of theories like this that didn't pan out at all.
privong 7y ago
> if requiring dark matter to explain galactic physics can be dispensed with by a distance correction, doesn't "we've been underestimating distances all this time" serve as a simpler fact of galactic physics than needing "there is mysterious dark matter everywhere" to explain galactic physics? It would be a simpler explanation, but that would require that the distances to _all_ galaxies are overestimated. Presently there's no good evidence that our distance estimates for galaxies are systematically overestimated. Something like this did happen in the 1940s, when it was discovered that a key type of star in the cosmic distance ladder (Cepheid variables) actually came in two types, with different period-brightness relations. Recognizing these two types and correcting distances doubled the distances inferred to nearby galaxies: https://en.wikipedia.org/wiki/Cepheid_variable#History https://en.wikipedia.org/wiki/Cepheid_variable#History
- ajross 7y ago
  To be fair, the cepheid correction was presaged by good data from elsewhere in science. Until the 40's, it was possible to date the earth (via isotope ratios in decay products trapped in rocks) as older than the universe.
- pdonis 7y ago
  > that would require that the distances to _all_ galaxies are overestimated. No, underestimated. The estimate of the distance to this particular galaxy was reduced--that's what made its observed parameters consistent with dark matter. So if it were really true that no galaxies have dark matter, our current distance estimates would all have to be too low--in the case of this particular galaxy, 13 Mpc (consistent with dark matter) vs. 20 Mpc (if there were no dark matter).
  - privong 7y ago
    > No, underestimated. Oops, yes. Wrote that backwards. Thanks for the correction.
ajross 7y ago
> if requiring dark matter to explain galactic physics can be dispensed with by a distance correction It's the other way around. We see dark matter everywhere except these galaxies. And the reason for that isn't that the models for dark matter were wrong, it's that our distance measurement for those few galaxies was wrong. I mean, sure, I guess it's possible that we got the distances to the handful of galaxies right and those to, y'know, the rest of the universe wrong. But...
jvanderbot 7y ago
Yeah, I love this article's sense of relief that indeed, we must indeed use the strange dark matter correction factor. Whereas before, it was a perfectly normal mass of matter that we just couldn't properly measure the distance to. "Oh thank god, we don't understand this galaxy after-all".
- hulahoof 7y ago
  It returns to the known unknown of dark matter on a few galaxies than a new unknown unknown on the remainder of the universe.
  - ngcc_hk 7y ago
    Cannot stop laughing. But sorry it is so sad. I guess the silver lining is — if anyone finally fix it, he/she/AI may get Nobel prize
    - ncmncm 7y ago
      As (usually) Mark Twain said, it's not what you don't know that gets you, it's what you think you know that ain't so. If only Donald Rumsfeld had heard that, we might have stayed out of Iraq.
pdonis 7y ago
> if requiring dark matter to explain galactic physics can be dispensed with by a distance correction It's not that simple. First, as privong pointed out, our estimates of distances to all galaxies would have to be wrong, and we have no evidence that that is the case. Second, dark matter explains more than just galaxy rotation curves and other observed parameters. It also explains why our universe has structure at all (galaxy clusters, galaxies, and stars) even though the data on Big Bang nucleosynthesis puts a strong limit on the amount of ordinary matter (baryons) in the universe, and it's far too small to account for the amount of structure (gravitational clumping) that we see.

by sriku 7y ago

idlewords 7y ago

There are several different features of galaxies that dark matter explains. One of them is the speed with which stars orbit the center of a galaxy—this "rotation curve" is independent of how far the stars are from the galactic center, where if there were no dark matter, you would expect the outermost stars to go slower. That is an effect that is not affected by errors in measuring the distance to remote galaxies. We can see it in our own galaxy! https://www.e-education.psu.edu/astro801/content/l8_p8.html https://www.e-education.psu.edu/astro801/content/l8_p8.html In general, if you can think of an explanation that does away with the need for dark matter, you can also assume experts have thought hard about it and rejected it for good reason. Nobody likes dark matter as a hypothesis, everyone remembers analogous complications in the history of science that proved to be signposts to a better theory, and everyone knows that a Nobel prize waits for anyone who can find the correct explanation.

trhway 7y ago
>One of them is the speed with which stars orbit the center of a galaxy do we really orbit it? I mean is it a stable orbit (that is the option which requires DM ) or a just a spiral trajectory (no DM required)? The spiral galaxies look like a fireworks wheel https://depositphotos.com/69366109/stock-video-fireworks-pinwheel-catherine-wheel.html https://depositphotos.com/69366109/stock-video-fireworks-pin... , and without DM our "orbital" speed being higher than otherwise expected Keplerian orbital speed means that we're just moving away on a spiral trajectory from the center the same way like the sparks fly away on a spiral trajectory from the center of the fireworks wheel.
- zaarn 7y ago
  There are plenty of non-spiral galaxies where we observe the same. Our sun moving away from the galactic center is also unlikely as if it had been much further in during it's creation phase, it would have been unable to create planets that support life. A galactic year is only a few million earth years, so our orbit must have remained in the stable region for the first billion years or so and we continue to be inside it. That points to a stable orbit around the galaxy. If it was a firework wheel our current position would mean that sun would have been traveling to the outside of the galaxy far below escape velocity or would have been unable to produce life-bearing planets.
  - trhway 7y ago
    >A galactic year is only a few million earth years no. It is 250 million years. >so our orbit must have remained in the stable region for the first billion years or so and we continue to be inside it. not really. While moving on that spiral trajectory, we've made only 16 rotations since Sun's formation. When the life on our planet started our neighborhood looked very different, not the boonies we're at right now :)
    - zaarn 7y ago
      >no. It is 250 million years. I don't see how that contradicts is being a few million years? >not really. While moving on that spiral trajectory, we've made only 16 rotations since Sun's formation. When the life on our planet started our neighborhood looked very different, not the boonies we're at right now :) Orbits don't really do spiral patterns in practice. Atleast not at this scale. The star would either be in a fairly stable orbit or on trajectory to leave the galaxy (minus elliptical orbits but those would carry the sun in and out of the dead zone quite often). Moving on a spiral trajectory does happen in orbital mechanics but only in one direction; inwards. Outwards spiraling would require quite a few quirks in the galactic gravity field. The spiral pattern itself isn't the result of a firework wheel, it's quite a quirk in the distribution of stars, though there are various models that explain them, they all involve very stable orbits for most stars.
- 7y ago
sriku 7y ago
Good link! Thanks. > you can also assume experts have thought hard about it and rejected it for good reason. Of course. It seemed more of a reporting anomaly to focus exclusively on the distance correction if other factors also exist that would've required dark matter.
pmoriarty 7y ago
"In general, if you can think of an explanation that does away with the need for dark matter, you can also assume experts have thought hard about it and rejected it for good reason." Nobel prizes have been given for explanations which were at first rejected for what seemed like good reasons at the time, but which later proved to be the wrong reasons. The first such example which springs to mind is the explanation of helicobacter causing ulcers. Back to astrophysics, black holes were once considered science fictional phenomena.
- jmcqk6 7y ago
  Just because something like this has happened in the past does not justify believing in a theory now that is following the same pattern. This is an example of survivorship bias. You have an example where this happened, but there are thousands of theories like this that didn't pan out at all.

privong 7y ago

> if requiring dark matter to explain galactic physics can be dispensed with by a distance correction, doesn't "we've been underestimating distances all this time" serve as a simpler fact of galactic physics than needing "there is mysterious dark matter everywhere" to explain galactic physics? It would be a simpler explanation, but that would require that the distances to _all_ galaxies are overestimated. Presently there's no good evidence that our distance estimates for galaxies are systematically overestimated. Something like this did happen in the 1940s, when it was discovered that a key type of star in the cosmic distance ladder (Cepheid variables) actually came in two types, with different period-brightness relations. Recognizing these two types and correcting distances doubled the distances inferred to nearby galaxies: https://en.wikipedia.org/wiki/Cepheid_variable#History https://en.wikipedia.org/wiki/Cepheid_variable#History

ajross 7y ago
To be fair, the cepheid correction was presaged by good data from elsewhere in science. Until the 40's, it was possible to date the earth (via isotope ratios in decay products trapped in rocks) as older than the universe.
pdonis 7y ago
> that would require that the distances to _all_ galaxies are overestimated. No, underestimated. The estimate of the distance to this particular galaxy was reduced--that's what made its observed parameters consistent with dark matter. So if it were really true that no galaxies have dark matter, our current distance estimates would all have to be too low--in the case of this particular galaxy, 13 Mpc (consistent with dark matter) vs. 20 Mpc (if there were no dark matter).
- privong 7y ago
  > No, underestimated. Oops, yes. Wrote that backwards. Thanks for the correction.

ajross 7y ago

> if requiring dark matter to explain galactic physics can be dispensed with by a distance correction It's the other way around. We see dark matter everywhere except these galaxies. And the reason for that isn't that the models for dark matter were wrong, it's that our distance measurement for those few galaxies was wrong. I mean, sure, I guess it's possible that we got the distances to the handful of galaxies right and those to, y'know, the rest of the universe wrong. But...

jvanderbot 7y ago

Yeah, I love this article's sense of relief that indeed, we must indeed use the strange dark matter correction factor. Whereas before, it was a perfectly normal mass of matter that we just couldn't properly measure the distance to. "Oh thank god, we don't understand this galaxy after-all".

hulahoof 7y ago
It returns to the known unknown of dark matter on a few galaxies than a new unknown unknown on the remainder of the universe.
- ngcc_hk 7y ago
  Cannot stop laughing. But sorry it is so sad. I guess the silver lining is — if anyone finally fix it, he/she/AI may get Nobel prize
  - ncmncm 7y ago
    As (usually) Mark Twain said, it's not what you don't know that gets you, it's what you think you know that ain't so. If only Donald Rumsfeld had heard that, we might have stayed out of Iraq.

pdonis 7y ago

> if requiring dark matter to explain galactic physics can be dispensed with by a distance correction It's not that simple. First, as privong pointed out, our estimates of distances to all galaxies would have to be wrong, and we have no evidence that that is the case. Second, dark matter explains more than just galaxy rotation curves and other observed parameters. It also explains why our universe has structure at all (galaxy clusters, galaxies, and stars) even though the data on Big Bang nucleosynthesis puts a strong limit on the amount of ordinary matter (baryons) in the universe, and it's far too small to account for the amount of structure (gravitational clumping) that we see.