3 ms·

I don't see why there is such an agitated response to the term "fudge factor" (I see downvoted comments, several knee-jerk defensive replies, etc) It's importa

I don't see why there is such an agitated response to the term "fudge factor" (I see downvoted comments, several knee-jerk defensive replies, etc) It's important to embrace one's ignorance -- especially when we're at the absolute frontier of human knowledge. If you've seen some of the calculations involving dark matter's effects, calling it a "fudge factor" is a fair characterization, and one that is acceptable among physicists. We postulate a component with certain properties that fudges our calculations in just the right way to match some indirect observations (circumstantial evidence). We still don't know what is the thing which produces the behavior we expect, and what its properties might be. We're still seeking evidence that would seal the deal, so to speak. It's true that several such fudges historically turned out to be profound stepping stones, but till we get there, "fudge factor" is absolutely honest. I'll leave some Richard Feynman quotes here: * "The first principle is that you must not fool yourself — and you are the easiest person to fool." * "I can live with doubt and uncertainty and not knowing. I think it is much more interesting to live not knowing than to have answers that might be wrong. If we will only allow that, as we progress, we remain unsure, we will leave opportunities for alternatives. We will not become enthusiastic for the fact, the knowledge, the absolute truth of the day, but remain always uncertain … In order to make progress, one must leave the door to the unknown ajar."

raattgift 9y ago
> We postulate a component with certain properties versus > We still don't know ... what its properties might be. Which is it? > stepping stones Observation of missing momentum-energy in Lithium-6 decays (Pauli, 1930 [1]) until direct detection of the (electron anti-)neutrino (Reines and Cowan, 1956) took 26 years and a number of false starts including failures to detect neutrinos in detectors at nuclear weapon tests and nuclear reactors, where the production of neutrinos was expected. Observation of solar neutrinos took a further thirteen years (Davis, HOMESTAKE, 1969). Neutrinos are in the strictest sense dark matter: they do not feel or participate in electromagnetism. However, they also have a tiny rest mass, so it is hard to keep them from carrying their momentum-energy away at relativistic speeds. Their fast motion makes them too "hot" to avoid smearing out visible matter during structure formation, so they are not a candidate for the momentum-energy missing in galaxy cluster interactions. On the other hand their fast motion makes it relatively easy to spot atomic recoils (or really the latter's ionization energy) when neutrinos interact with the matter in a detector. That's why detectors were placed at extremely violent neutrino-producing events: the hotter the neutrino, the easier to detect it. The standard cosmology predicts a relic neutrino field comparable to the relic photon field called the Cosmic Microwave Background. These relic neutrinos were relativistic ("hot") in the early universe, but have cooled down adiabatically to about 2 kelvins, just as the hot photons in the CMB have cooled down to a bit under 3 kelvins. The Cosmic Neutrino Background is vital at high redshift (z > 3000) and there is good indirect evidence for it (BBN element abundance, suppressed early small-scale structure formation, damped acoustic oscillations of the CMB). However, it is extremely difficult to detect relic neutrinos with our current level of technology (although experiments like KATRIN will try: https://www.katrin.kit.edu/ https://www.katrin.kit.edu/ and https://arxiv.org/abs/1304.5632 https://arxiv.org/abs/1304.5632 ), because the recoils from slow-moving neutrinos will be smaller than those from relativistic neutrinos. These relic neutrinos are literally Cold Dark Matter. So there have been stepping stones on the road towards direct detection of a form of cold dark matter. It is not the cold dark matter working cosmologists or galaxy astrophysicists are looking for to explain galaxy-and-higher-scale momentum-energy anomalies (notably because its rest mass is too low), but it's suggestive of something more than "fudge". There are options other than a heavy neutrino-like particle or even particle dark matter for galaxy-scale and even cluster-scale problems; indeed, a relativistic "left hand side" term in the Einstein Field Equations is plausible -- one would tend to treat that as a modification of gravity. However cold dark matter is needed to explain several fine features in the cosmic microwave background, and those features are ever sharper in the results of each generation of CMB observatory since BOOMERanG. Finally, returning to the ~ 40 years between observation of anomalous results to direct detection for neutrinos: Vera Rubin et al.'s galaxy rotation curve paper was published in The Astrophysical Journal in 1980. So the long gap between the apparent need for dark matter and today does not seem so long compared to the long gap between the apparent need for a light spin-1/2 particle [1] and direct detection. > "fudge factor" is absolutely honest How exactly does this honesty scale work? Apropos honesty and fudging things, who exactly is in each of the six instances of "we" (excluding the ones in the Feynman quotes) in your comment? - -- [1] I think Pauli's letter is interesting in itself. An English translation follows on the second page: http://microboone-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=953;filename=pauli%20letter1930.pdf http://microboone-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=...
- ssivark 9y ago
  IMHO, we seek models explaining certain observed cosmological behavior. As long as plausible models for dark matter range from WIMPs to primordial black holes or axions and what not, I consider it unreasonable to claim that we know what dark matter is, and all it’s properties. Eg: if all dark matter experiments (direct and indirect detection) returned null results over the next several decades, then dark matter would still be an open puzzle. With regards to “we”, I think it applies equally well to any group as small as physicists working on DM, to all scientists, or everyone discussing the subject. I expect most particle/astro physicists I’ve interacted with to find those statements very reasonable.

raattgift 9y ago

> We postulate a component with certain properties versus > We still don't know ... what its properties might be. Which is it? > stepping stones Observation of missing momentum-energy in Lithium-6 decays (Pauli, 1930 [1]) until direct detection of the (electron anti-)neutrino (Reines and Cowan, 1956) took 26 years and a number of false starts including failures to detect neutrinos in detectors at nuclear weapon tests and nuclear reactors, where the production of neutrinos was expected. Observation of solar neutrinos took a further thirteen years (Davis, HOMESTAKE, 1969). Neutrinos are in the strictest sense dark matter: they do not feel or participate in electromagnetism. However, they also have a tiny rest mass, so it is hard to keep them from carrying their momentum-energy away at relativistic speeds. Their fast motion makes them too "hot" to avoid smearing out visible matter during structure formation, so they are not a candidate for the momentum-energy missing in galaxy cluster interactions. On the other hand their fast motion makes it relatively easy to spot atomic recoils (or really the latter's ionization energy) when neutrinos interact with the matter in a detector. That's why detectors were placed at extremely violent neutrino-producing events: the hotter the neutrino, the easier to detect it. The standard cosmology predicts a relic neutrino field comparable to the relic photon field called the Cosmic Microwave Background. These relic neutrinos were relativistic ("hot") in the early universe, but have cooled down adiabatically to about 2 kelvins, just as the hot photons in the CMB have cooled down to a bit under 3 kelvins. The Cosmic Neutrino Background is vital at high redshift (z > 3000) and there is good indirect evidence for it (BBN element abundance, suppressed early small-scale structure formation, damped acoustic oscillations of the CMB). However, it is extremely difficult to detect relic neutrinos with our current level of technology (although experiments like KATRIN will try: https://www.katrin.kit.edu/ https://www.katrin.kit.edu/ and https://arxiv.org/abs/1304.5632 https://arxiv.org/abs/1304.5632 ), because the recoils from slow-moving neutrinos will be smaller than those from relativistic neutrinos. These relic neutrinos are literally Cold Dark Matter. So there have been stepping stones on the road towards direct detection of a form of cold dark matter. It is not the cold dark matter working cosmologists or galaxy astrophysicists are looking for to explain galaxy-and-higher-scale momentum-energy anomalies (notably because its rest mass is too low), but it's suggestive of something more than "fudge". There are options other than a heavy neutrino-like particle or even particle dark matter for galaxy-scale and even cluster-scale problems; indeed, a relativistic "left hand side" term in the Einstein Field Equations is plausible -- one would tend to treat that as a modification of gravity. However cold dark matter is needed to explain several fine features in the cosmic microwave background, and those features are ever sharper in the results of each generation of CMB observatory since BOOMERanG. Finally, returning to the ~ 40 years between observation of anomalous results to direct detection for neutrinos: Vera Rubin et al.'s galaxy rotation curve paper was published in The Astrophysical Journal in 1980. So the long gap between the apparent need for dark matter and today does not seem so long compared to the long gap between the apparent need for a light spin-1/2 particle [1] and direct detection. > "fudge factor" is absolutely honest How exactly does this honesty scale work? Apropos honesty and fudging things, who exactly is in each of the six instances of "we" (excluding the ones in the Feynman quotes) in your comment? - -- [1] I think Pauli's letter is interesting in itself. An English translation follows on the second page: http://microboone-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=953;filename=pauli%20letter1930.pdf http://microboone-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=...

ssivark 9y ago
IMHO, we seek models explaining certain observed cosmological behavior. As long as plausible models for dark matter range from WIMPs to primordial black holes or axions and what not, I consider it unreasonable to claim that we know what dark matter is, and all it’s properties. Eg: if all dark matter experiments (direct and indirect detection) returned null results over the next several decades, then dark matter would still be an open puzzle. With regards to “we”, I think it applies equally well to any group as small as physicists working on DM, to all scientists, or everyone discussing the subject. I expect most particle/astro physicists I’ve interacted with to find those statements very reasonable.