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I'm surprised to read an article with so much assumed knowledge about dark matter - about how it heats, etc. Is there a good place to read about the current be

by Pxtl 3y ago

I'm surprised to read an article with so much assumed knowledge about dark matter - about how it heats, etc. Is there a good place to read about the current best-guesses of the properties of dark matter?

dylan604 3y ago
Isn't the concept of dark matter just an assumption in itself? Seems to follow that any knowledge about it would be an assumption. In fact, that seems like good science, on instead of the word assumption, one might use theory. Once the theory is built up to test, it might actually one day become fact (or proven wrong).
- phkahler 3y ago
  The word they should be using is hypothesis, not theory. A theory stands up to scrutiny.
  - dylan604 3y ago
    there you go, an even better word. good thing i don't call myself a scientist
  - mr_mitm 3y ago
    That's not how the term "theory" is used in science. Only hobbyists care about the distinction, actual scientists infer the implied certainty from context. There is Born's rule, Noether's theorem, Newton's law (which we know is wrong), Einstein's theory of GR, the standard model (which is the best thing we have), ... sometimes we use the word "a quantum theory" to mean a certain Lagrangian, even one which we know does not describe reality at all.
    - consilient 3y ago
      > Noether's theorem is not like the other entries on your list. It's a full-blown mathematically rigorous theorem (that incidentally also happens to be of central importance in physics), not a physical model.
- roywiggins 3y ago
  This paper predicts that dark stars would have a distinct spectrum that would distinguish them from ordinary galaxies.
- deleted 3y ago
  [deleted]
- nwallin 3y ago
  > Isn't the concept of dark matter just an assumption in itself? Dark matter is not an assumption. Dark matter is not a hypothesis. Dark matter is not a theory. Dark matter is a series of observations of the universe. Galaxies spin are observed to spin differently than our models and estimates of their mass say they should. Velocities of galaxies in galaxy clusters are much faster than the sum total of the gravitational effects of the cluster can account for. The bullet cluster lenses gravity in a distribution that is not in accordance with the matter that we see. The CMB (cosmic microwave background) is lumpy, too lumpy for our models. This is dark matter; dark matter is a series of observations where the stuff we see does not line up with what our models predict. Dark matter is not an assumption. Dark matter is opening our eyes and looking at the sky. Now, we can have different theories of dark matter. Hypotheses or theories that attempt to explain the observations. Currently the leading theory is WIMPS, but MACHO and MOND were in the running for a while. By way of analogy, we have known that light was a thing for thousands of years. Light is not an assumption. Light is not a hypothesis. Light is not a theory. Light is the observation that we are able to see. Light is the observation that we see better when the Sun is up than when a full Moon is up, and sometimes barely at all if there's a new moon or if we're in a cave. Light is the observation that the Sun is brighter than the Moon. Light is the observation that we can make a fire, perhaps a campfire, or a candle, or a torch, that can enable us to see in the dark. Light is the observation that if we put the fire out, we can't see anymore. There were several theories that tried to explain what light is; the Greek theory about our eyes sending out feelers, or waves in the luminiferous aether, or a stream of billiard ball-like particles, or waves in the electromagnetic field. We can have a meaningful discussion about which of these theories is the best one, but we can't have a meaningful discussion about whether the phenomena known as "light" is an assumption: We can see. Therefore light, whatever it happens to be, does exist. Dark matter is no different.
  - formulathree 3y ago
    To translate more what he means. I think the person you replied to is asking: "Is what we see the result of a form of invisible matter?" Invisible matter aka dark matter is the "theory" he is referring to.
phkahler 3y ago
>> I'm surprised to read an article with so much assumed knowledge about dark matter - about how it heats, etc. Exactly! I'm astounded at the amount of - quite literally - made up unsubstantiated assumptions about DM. My favorite is to "explain" a galaxy rotation curve by assuming a spherical distribution of DM around the galaxy, but never explaining why or how it would take on such a distribution. Just don't ask questions...
- roywiggins 3y ago
  The paper is trying to explain some unexpected observations by the JWST. It starts with the idea that maybe these observations are dark stars, works backwards to show that there are a couple of models of dark matter where this would work, and then predicts that, if these really are dark stars, they'll have a measurably different spectrum than regular stars. Measuring the spectrum would then substantiate (or not) a model of dark matter. This sort of thing is how they might nail down a more likely model of dark matter. This is not unlike trying to explain those observations with ordinary matter, working backwards to try and work out what distribution of ordinary matter could produce the observations and what sort of physics would create such a distribution.
- mr_mitm 3y ago
  > Just don't ask questions... The insinuation packed in this statement is beyond ridiculous. As if there was a giant conspiracy by Big Cosmology. What makes you think asking questions isn't welcome? Go visit your local college, they probably have a weekly seminar open for everybody. Observe how they interact. Scientists and students ask each other questions (and I mean hard questions) all the time. Pointing out failures in each other theories is the scientists' favorite past time. It's literally how science works. The structure formation of dark matter is extensively studied and simulations are in good agreement with observations (not perfect though, look up the dwarf galaxy problem). > I'm astounded at the amount of - quite literally - made up unsubstantiated assumptions about DM. This is done all the time in cosmology: let's assume X is true just for the heck of it, what does that mean for Y? Could we observe it? Would it perhaps explain several observations at once? Even toy worlds or toy models are explored all the time, by which I mean worlds or models that we know do not describe our world. Valuable insights can still be gained. And why wouldn't they make some assumptions? Would you prefer it if certain ideas are forbidden to be explored?
Sharlin 3y ago
So-called cold dark matter [1] is the currently favored hypothesis. The Dark Star hypothesis additionally assumes that the CDM particles interact with each other even though they do not interact with baryonic ("normal") matter (other than via gravity). If they do, they'd be their own antiparticles and would annihilate on collisions. This process would produce the energy, in the form of ordinary photons, that powers the Dark Stars. Like all proper scientific hypotheses, the DS hypothesis makes testable predictions, and now it appears that some relevant JWST observations support, or at least do not conflict with, the DS hypothesis. At the same time, the more mainstream model of protogalaxies and Population III stars [2] has some difficulties explaining the same observations. Of course, this is only very slight evidence in favor of the DS model, but that's science for you. Small steps. [1] https://en.wikipedia.org/wiki/Cold_dark_matter https://en.wikipedia.org/wiki/Cold_dark_matter [2] https://en.wikipedia.org/wiki/Stellar_population#Population_III_stars https://en.wikipedia.org/wiki/Stellar_population#Population_...
- Pxtl 3y ago
  So wait, the hypothesis is that the reason Dark Stars are so diffuse that they look like galaxies is that maybe dark matter self-annihilates, so it can't coalesce into true stars? But what keeps it from just all annihilating quickly? Can't ever reach the critical mass of gravity needed for rapid annihilation because it's destroyed as fast as it accumulates? The annihilation provides repulsive force keeping them from collapsing too fast? Or is there no repulsive force?
  - gjhan 3y ago
    In the context of this paper, the DSs are significantly smaller than galaxies, but JWST doesn't have enough resolution to distinguish a galaxy from the much smaller DS. In their DS model, what seems to limit the rate at which the DM annihilates is that any interaction between DS particles has a low probability of happening (a cross section). We can imagine this as particles just whizzing around each other, gravitationally bound (i.e. confined to a nearby volume) but so small that they have a low probability of actually interacting.
  - Sharlin 3y ago
deleted 3y ago
[deleted]
nonameiguess 3y ago
This is the Proceedings of the National Academies of Science. It's getting shared here, but the target audience is other physicists, who would have this background knowledge.
sesm 3y ago
Basically, there are 2 hypothesis: WIMPs and MACHOs. WIMPs stands for Weakly Interacting Massive Particles, and proposes that DM consist of yet unknown massive particles that don’t interact much with regular matter. The problem with that hypothesis is that no such particles are predicted by Standard Model and decades of searching for any traces of those particles didn’t yield anything. MACHOs stands for Massive Compact Halo Objects and assumes that DM in galactic halos consists of some known dark objects, most likely Black Holes. The question is: where those black holes come from. There is a model of cyclical universe by Gorkavyi, Mathers et al, where those are primordial black holes left from the previous cycles of the universe. It also explains galaxy formation in early universe (observed by JWST) and predicted gravitational wave background recently discovered by NANOGrav.

by Pxtl 3y ago

dylan604 3y ago

Isn't the concept of dark matter just an assumption in itself? Seems to follow that any knowledge about it would be an assumption. In fact, that seems like good science, on instead of the word assumption, one might use theory. Once the theory is built up to test, it might actually one day become fact (or proven wrong).

phkahler 3y ago
The word they should be using is hypothesis, not theory. A theory stands up to scrutiny.
- dylan604 3y ago
  there you go, an even better word. good thing i don't call myself a scientist
- mr_mitm 3y ago
  That's not how the term "theory" is used in science. Only hobbyists care about the distinction, actual scientists infer the implied certainty from context. There is Born's rule, Noether's theorem, Newton's law (which we know is wrong), Einstein's theory of GR, the standard model (which is the best thing we have), ... sometimes we use the word "a quantum theory" to mean a certain Lagrangian, even one which we know does not describe reality at all.
  - consilient 3y ago
    > Noether's theorem is not like the other entries on your list. It's a full-blown mathematically rigorous theorem (that incidentally also happens to be of central importance in physics), not a physical model.
roywiggins 3y ago
This paper predicts that dark stars would have a distinct spectrum that would distinguish them from ordinary galaxies.
deleted 3y ago
[deleted]
nwallin 3y ago
> Isn't the concept of dark matter just an assumption in itself? Dark matter is not an assumption. Dark matter is not a hypothesis. Dark matter is not a theory. Dark matter is a series of observations of the universe. Galaxies spin are observed to spin differently than our models and estimates of their mass say they should. Velocities of galaxies in galaxy clusters are much faster than the sum total of the gravitational effects of the cluster can account for. The bullet cluster lenses gravity in a distribution that is not in accordance with the matter that we see. The CMB (cosmic microwave background) is lumpy, too lumpy for our models. This is dark matter; dark matter is a series of observations where the stuff we see does not line up with what our models predict. Dark matter is not an assumption. Dark matter is opening our eyes and looking at the sky. Now, we can have different theories of dark matter. Hypotheses or theories that attempt to explain the observations. Currently the leading theory is WIMPS, but MACHO and MOND were in the running for a while. By way of analogy, we have known that light was a thing for thousands of years. Light is not an assumption. Light is not a hypothesis. Light is not a theory. Light is the observation that we are able to see. Light is the observation that we see better when the Sun is up than when a full Moon is up, and sometimes barely at all if there's a new moon or if we're in a cave. Light is the observation that the Sun is brighter than the Moon. Light is the observation that we can make a fire, perhaps a campfire, or a candle, or a torch, that can enable us to see in the dark. Light is the observation that if we put the fire out, we can't see anymore. There were several theories that tried to explain what light is; the Greek theory about our eyes sending out feelers, or waves in the luminiferous aether, or a stream of billiard ball-like particles, or waves in the electromagnetic field. We can have a meaningful discussion about which of these theories is the best one, but we can't have a meaningful discussion about whether the phenomena known as "light" is an assumption: We can see. Therefore light, whatever it happens to be, does exist. Dark matter is no different.
- formulathree 3y ago
  To translate more what he means. I think the person you replied to is asking: "Is what we see the result of a form of invisible matter?" Invisible matter aka dark matter is the "theory" he is referring to.

phkahler 3y ago

>> I'm surprised to read an article with so much assumed knowledge about dark matter - about how it heats, etc. Exactly! I'm astounded at the amount of - quite literally - made up unsubstantiated assumptions about DM. My favorite is to "explain" a galaxy rotation curve by assuming a spherical distribution of DM around the galaxy, but never explaining why or how it would take on such a distribution. Just don't ask questions...

roywiggins 3y ago
The paper is trying to explain some unexpected observations by the JWST. It starts with the idea that maybe these observations are dark stars, works backwards to show that there are a couple of models of dark matter where this would work, and then predicts that, if these really are dark stars, they'll have a measurably different spectrum than regular stars. Measuring the spectrum would then substantiate (or not) a model of dark matter. This sort of thing is how they might nail down a more likely model of dark matter. This is not unlike trying to explain those observations with ordinary matter, working backwards to try and work out what distribution of ordinary matter could produce the observations and what sort of physics would create such a distribution.
mr_mitm 3y ago
> Just don't ask questions... The insinuation packed in this statement is beyond ridiculous. As if there was a giant conspiracy by Big Cosmology. What makes you think asking questions isn't welcome? Go visit your local college, they probably have a weekly seminar open for everybody. Observe how they interact. Scientists and students ask each other questions (and I mean hard questions) all the time. Pointing out failures in each other theories is the scientists' favorite past time. It's literally how science works. The structure formation of dark matter is extensively studied and simulations are in good agreement with observations (not perfect though, look up the dwarf galaxy problem). > I'm astounded at the amount of - quite literally - made up unsubstantiated assumptions about DM. This is done all the time in cosmology: let's assume X is true just for the heck of it, what does that mean for Y? Could we observe it? Would it perhaps explain several observations at once? Even toy worlds or toy models are explored all the time, by which I mean worlds or models that we know do not describe our world. Valuable insights can still be gained. And why wouldn't they make some assumptions? Would you prefer it if certain ideas are forbidden to be explored?

Sharlin 3y ago

So-called cold dark matter [1] is the currently favored hypothesis. The Dark Star hypothesis additionally assumes that the CDM particles interact with each other even though they do not interact with baryonic ("normal") matter (other than via gravity). If they do, they'd be their own antiparticles and would annihilate on collisions. This process would produce the energy, in the form of ordinary photons, that powers the Dark Stars. Like all proper scientific hypotheses, the DS hypothesis makes testable predictions, and now it appears that some relevant JWST observations support, or at least do not conflict with, the DS hypothesis. At the same time, the more mainstream model of protogalaxies and Population III stars [2] has some difficulties explaining the same observations. Of course, this is only very slight evidence in favor of the DS model, but that's science for you. Small steps. [1] https://en.wikipedia.org/wiki/Cold_dark_matter https://en.wikipedia.org/wiki/Cold_dark_matter [2] https://en.wikipedia.org/wiki/Stellar_population#Population_III_stars https://en.wikipedia.org/wiki/Stellar_population#Population_...

Pxtl 3y ago
So wait, the hypothesis is that the reason Dark Stars are so diffuse that they look like galaxies is that maybe dark matter self-annihilates, so it can't coalesce into true stars? But what keeps it from just all annihilating quickly? Can't ever reach the critical mass of gravity needed for rapid annihilation because it's destroyed as fast as it accumulates? The annihilation provides repulsive force keeping them from collapsing too fast? Or is there no repulsive force?
- gjhan 3y ago
  In the context of this paper, the DSs are significantly smaller than galaxies, but JWST doesn't have enough resolution to distinguish a galaxy from the much smaller DS. In their DS model, what seems to limit the rate at which the DM annihilates is that any interaction between DS particles has a low probability of happening (a cross section). We can imagine this as particles just whizzing around each other, gravitationally bound (i.e. confined to a nearby volume) but so small that they have a low probability of actually interacting.
- Sharlin 3y ago

deleted 3y ago

[deleted]

nonameiguess 3y ago

This is the Proceedings of the National Academies of Science. It's getting shared here, but the target audience is other physicists, who would have this background knowledge.

sesm 3y ago

Basically, there are 2 hypothesis: WIMPs and MACHOs. WIMPs stands for Weakly Interacting Massive Particles, and proposes that DM consist of yet unknown massive particles that don’t interact much with regular matter. The problem with that hypothesis is that no such particles are predicted by Standard Model and decades of searching for any traces of those particles didn’t yield anything. MACHOs stands for Massive Compact Halo Objects and assumes that DM in galactic halos consists of some known dark objects, most likely Black Holes. The question is: where those black holes come from. There is a model of cyclical universe by Gorkavyi, Mathers et al, where those are primordial black holes left from the previous cycles of the universe. It also explains galaxy formation in early universe (observed by JWST) and predicted gravitational wave background recently discovered by NANOGrav.