9 thoughts on “Hidden in plain sight — dark matter”

1. Space.com followed their June 15 article with another one on July 17, 2017, featuring an “Expert Voice” summary of the topic by Don Lincoln (Senior Scientist, Fermi National Accelerator Laboratory; Adjunct Professor of Physics, University of Notre Dame): Is Dark Matter Real?

   However, the reality is that dark matter’s existence has not yet been proved. Dark matter is still a hypothesis, albeit a rather well-supported one. Any scientific theory has to make predictions, and if it’s right, then the measurements you do should line up with the predictions. The same goes for dark matter. For instance, dark matter theories make predictions for how fast galaxies are rotating. But, until now, measurements made of the detailed dark matter distribution at the center of low mass galaxies didn’t line up with those predictions.

   A recent calculation has changed that. The calculation helps resolve the conundrum of the Tully-Fisher relation, which compares the visible, or ordinary, matter of a galaxy to its rotational velocity. In very simplified terms, scientists have found that the more massive (and therefore brighter) a spiral galaxy is, the faster it spins.

2. Space.com published another article on Dark Matter today (August 14, 2017), again featuring an “Expert Voice” summary by Don Lincoln (Senior Scientist, Fermi National Accelerator Laboratory; Adjunct Professor of Physics, University of Notre Dame): Is Dark Matter Less ‘Lumpy’ Than Predicted?

   The Dark Energy Survey (DES) is a collaboration of about 400 scientists who have embarked on a five-year mission to study distant galaxies to answer questions about the history of the universe. It uses the Dark Energy Camera (DEC) attached to the Victor M. Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory in the Chilean Andes. DEC was assembled in the U.S. at Fermilab near Batavia, Illinois, and is a 570-megapixel camera able to image galaxies so far away that their light is a millionth as bright as the dimmest visible stars.

   Gravitational lensing … Because the mass of the lensing galaxy is a combination of visible matter and dark matter, gravitational lensing allows scientists to directly observe the existence and distribution of dark matter on scales as large as the universe itself. This technique also works when a large cluster of foreground galaxies distorts the images of clusters of even more distant galaxies, which is the technique employed for this measurement.

   The DES collaboration recently released an analysis using exactly this technique. The team looked at a sample of 26 million galaxies at four different distances from Earth. … As expected, they found that the dark matter of the universe was “lumpy.” However, there was a surprise — it was a little less lumpy than previous measurements had predicted.

   Is that a big deal? Maybe. The uncertainty, or error, in the two measurements is big enough that it means they don’t disagree in a statistically significant way. What that simply means is that no one can be sure that the two measurements really disagree. It could be that the discrepancies arise by chance from statistical fluctuations in the data or small instrumental effects that were not considered.

   Even the study’s authors would suggest caution here. The DES measurements have not yet been peer-reviewed. The papers were submitted for publication and the results were presented at conferences, but firm conclusions should wait until the referee reports come in.

3. Space.com published another article on Dark Matter today (August 25, 2017), this time featuring Paul Sutter, host of Ask a Spaceman, We Don’t Planet and COSI Science Now. The article is titled “The Matter with Dark Matter” and discusses the major evidence for its existence: too hot, too fast, too bumpy, too wide.

   As much as we would prefer to live in a simpler universe, dark matter is not the product of some astronomer’s fever dream after a late-night observing session. It’s only after decades of careful observations that cosmologists have come to the inescapable conclusion that most of the matter in our universe is simply invisible.

4. Space.com published another article on Dark Matter yesterday (November 2, 2017), this time featuring Adam Hadhazy, writer and editor for The Kavli Foundation, as part of Space.com’s Expert Voices: Op-Ed & Insights. The article is titled “New Map of Dark Matter Puts the Big Bang Theory on Trial” discusses a new cosmic map unveiled in August, plotting where the mysterious substance called dark matter is clumped across the universe.

   A new cosmic map was unveiled in August, plotting where the mysterious substance called dark matter is clumped across the universe. To immense relief — and frustration — the map is just what scientists had expected. The distribution of dark matter agrees with our current understanding of a universe born with certain properties in a Big Bang, 13.8 billion years ago.

   TKF: If you had to place a bet on what dark matter and dark energy actually are, where would you put your chips?

   DODELSON: We’re living in an era of cognitive dissonance. There is all this cosmological evidence for the existence of dark matter, but over the last 30 years, we’ve run all these experiments and haven’t found it. My bet is that we’re looking at things all wrong. Someone who’s 8 years old today is going to come around and figure out how to make sense of all the data without evoking mysterious new substances.

   EFSTATHIOU: What odds are you giving on that, Scott?

   DODELSON: I’m betting $2,000 of George’s money. [Laughter]

   EFSTATHIOU: I wouldn’t put a bet on any specific candidate for the dark matter. But I bet that dark energy is the cosmological constant, a fudge factor invented by Einstein describing the density of energy in a vacuum.

   WECHSLER: I’m basically with George on this one. I think if Scott’s right, that’ll be wonderful — but that definitely isn’t where I would place my money.

5. Space.com today posted another article on dark matter “The Worst Theoretical Prediction in the History of Physics,” echoing the sentiment of the prior comment on this post.

   When astronomers attempt to measure dark energy’s density in space, they come up with roughly 10^−9 joules per cubic meter, a microscopic but influential amount. However, this observed value, known as the cosmological constant, isn’t remotely close to that which is predicted by the time-tested quantum field theory.

   And so, quantum physicists are in a bit of a pickle. Making slight tweaks to theoretical estimates narrows the ginormous gap considerably, but our best theories from quantum mechanics still overestimate the influence of dark energy by sixty orders of magnitude.

   Cosmological constant is … Holy zero-point energies and quantum foam, Batman!

6. A useful recap of dark matter research … early evidence … WIMPs … axions … sterile neutrinos … primordial black holes … hidden/dark-sector (with dynamics independent of normal matter vs. interacting with normal matter through the known weak force) … SuperCDMS (Super Cryogenic Dark Matter Search) experiment … mass range … tabletop experiments … computer simulations …

   • Caltech Magazine > “Where is Dark Matter Hiding?” (Fall 2020) – Scientists turn to new ideas and experiments in the search for dark matter particles.

   … if we cannot see dark matter, how do scientists know it is there? The answer is gravity. Astronomers indirectly detect dark matter through its gravitational influences on stars and galaxies. Wherever normal matter resides, dark matter can be found lurking unseen by its side.

   The first real evidence for dark matter came in 1933, when Caltech’s Fritz Zwicky used the Mount Wilson Observatory to measure the visible mass of a cluster of galaxies and found that it was much too small to prevent the galaxies from escaping the gravitational pull of the cluster.

   Yet, despite its preponderance, scientists have not been able to identify the particles that make up dark matter.

7. This article (below) is a useful reminder about the origin of “dark matter” models. And why that term (rather than “invisible matter” or “undetectable matter” or “missing mass”) has become so exotic. And ongoing research to detect it (directly or indirectly).

   • APS Physics > “Dark Matter Alternative Passes Big Test” (October 15, 2021) – A cosmological model that doesn’t require dark matter has overcome a major hurdle in matching observations from the cosmic microwave background.[1]

   (quote) This dark-matter-free model is an extension of the so-called MOND (modified Newtonian dynamics) theory, which assumes that the gravitational force on galaxy scales is different from the standard Newtonian force. Previous MOND-based models could not reproduce the CMB. The researchers say that their model can be further tested with observations of galaxy clusters and gravitational waves.

   Skordis and Czech Academy colleague Tom Złośnik have now created a MOND-inspired model that accounts for the CMB while also being consistent with gravitational lensing observations and gravitational-wave speed measurements. The model follows recent MOND efforts in postulating the existence of two fields that permeate all of space [and evolve over cosmic time] and together act like an extra gravitational force. One of these fields is a scalar field – similar to the Higgs field that is associated with the Higgs boson. The other is a vector field, which has a direction at each point in space, somewhat like a magnetic field.

   Cosmologist Katherine Freese from the University of Texas congratulates the researchers for their accomplishment. “It is a big deal to construct a relativistic version of MOND that is able to match all existing data, especially fitting CMB data along with the MOND phenomenology in galaxies,” she says. “However, the model has a lot of ingredients,” she says. “I myself would still vote for dark matter as a simpler explanation.”

   Tessa Baker, an expert in alternative gravity models from Queen Mary University of London says that if dark matter detectors continue to come up empty, “then we may see increased interest in this family of modified gravity models.”

   This research was briefly reported also by Phys.org.

   • Phys.org > “New MOND theory able to account for cosmic microwave background” by Bob Yirka (October 21, 2021)

   Notes

   [1]

   • Cf. Sloan Digital Sky Survey

   • Wiki

   Several independent observations point to the fact that the visible mass in galaxies and galaxy clusters is insufficient to account for their dynamics, when analyzed using Newton’s laws. This discrepancy – known as the “missing mass problem” …

   The majority of astronomers, astrophysicists, and cosmologists accept dark matter as the explanation for galactic rotation curves (based on general relativity, and hence Newtonian mechanics), and are committed to a dark matter solution of the missing-mass problem. MOND, by contrast, is actively studied by only a handful of researchers.

   The dependence in MOND of the internal dynamics of a system on its external environment (in principle, the rest of the universe) is strongly reminiscent of Mach’s principle, and may hint towards a more fundamental structure underlying Milgrom’s law. In this regard, Milgrom has commented:

   It has been long suspected that local dynamics is strongly influenced by the universe at large, a-la Mach’s principle, but MOND seems to be the first to supply concrete evidence for such a connection. This may turn out to be the most fundamental implication of MOND, beyond its implied modification of Newtonian dynamics and general relativity, and beyond the elimination of dark matter.

   See the reference to Mach’s principle in Wiki’s article for frame dragging (regarding relativistic effects in the vicinity of rotating black holes).

8. 

   Making sense of all the data: Another galaxy cluster poses the question of invisible, undetectable matter – the missing mass called ‘dark’ matter.

   • Space.com > “Hubble telescope spies a cosmic ‘spider web’ containing clues to dark secret” by Andrew Jones (October 31, 2022) – a huge cluster of galaxies exhibiting gravitational lensing hangs together despite an apparent mass shortfall.

   (quote) [Galaxy cluster] Abell 611 is located roughly 3.2 billion light years from Earth and confuses astronomers, officials noted, as the whirling galaxies do not contain enough visible mass “to prevent the cluster from flying apart.” The galaxy cluster likely is held together by dark matter, which scientists are struggling to understand because the substance cannot be sensed with conventional instruments.

   Dark matter theories cluster into one of two groups, ESA officials stated. It is either, they wrote, “some type of particle that exists in vast quantities throughout the universe — but for some reason does not interact with light [across the entire electromagnetic spectrum] as other particles do — or some type of massive object that […] does not lend itself to detection using current telescope technology.”

9. 

   This article references a useful NASA visualization of galactic motion with and without (so-called) dark matter.

   Terms: (in)elastic collision, lower mass limit

   • Space.com > “We still don’t know what dark matter is, but here’s what it’s not” by Monisha Ravisetti (August 21, 2023) – “It’s all about mindset in science, where a null result can be as impactful as a positive result.” – Daniel Jardin (Northwestern University), member of Super Cryogenic Dark Matter Search (SuperCDMS) collaboration.

   “There are roughly 1 billion dark matter particles passing through you every second, but they interact so rarely that you can’t tell,” Jardin said. “We’re looking for a 1 in a billion billion billion billion chance of interaction.”

   • YouTube > NASA Goddard > “Mystery of Galaxy’s Missing Dark Matter Deepens” (Jun 17, 2021) – The mystery of why NGC 1052-DF2 is missing most of its dark matter still persists.

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