Rob Sheldon responds below.
Eighty-five percent of the universe is thought to be made up of an unknown something “that isn’t atoms”:
Dark matter’s no-show means that many possible explanations for it that people like me favoured just a decade ago have now been ruled out. That is forcing us to radically revisit assumptions not only about the nature of dark matter, but also about the early history of our universe. This is the latest twist in a long-running saga: our failure to detect the particles that make up dark matter suggests that the beginning of the universe may have been very different from what we imagined.Dan Hooper, “Why dark matter’s no-show could mean a big bang rethink” at New Scientist
Our physics color commentator Rob Sheldon, replies:
Dark Matter Definition: Back in 1884 Lord Kelvin opined that stars in our galaxy were moving faster than expected from the mass of other stars, so there must be many “dark stars”. This led to a “dark matter” hypothesis from Poincare in 1906, Kapteyn in 1922, Oort in 32, and in finally, Zwicky in 1933. Despite all this theorizing, the spectral redshift observations that pinned it down were done by Vera Rubin and others in the 1970’s, though Rubin’s name comes up often because she was the first woman involved. What the observations showed, was that stars in all the galaxies observed were spinning faster than the gravity inferred from their observed stars + dust. Thus there must unobserved or “dark” matter providing the extra gravitational pull.
Dark Matter properties: On a different tack, cosmology began with Lemaitre’s recognition that Einstein’s Theory of General Relativity predicted that the universe would collapse into a black hole unless it had some kinetic energy, some momentum countering gravity, corresponding to a primeval explosion. Gamow’s student, Robert Alpher, wrote his PhD thesis in 1946 on how this “Big Bang” explosion would have been hot enough to smash protons and neutrons together and make the elements (leading to the famous Alpher, Bethe, Gamow paper, to which Bethe merely loaned his name.) Fred Hoyle, who wanted the universe to be eternal so as to give evolution a fighting chance, countered with the “Steady State” theory, where all the elements above hydrogen were made inside stars. To distinguish between the models, he wanted to demonstrate that the Big Bang cooled too rapidly to make anything heavier than helium, and collaborated with Wagner on the 1967 “Big Bang Nucleosynthesis” model, with some 80 nuclear cross section (reaction rates) borrowed from Los Alamos bomb codes. The code worked so well that it became a field of its own, and today the European PLANCK satellite consortium has spent the last 10 years refining the model to deliver some six decimal digits of precision to the abundances of hydrogen, helium, lithium, and light elements made in the BBN. And yes, Hoyle was partly right, the BBN doesn’t make the carbon and oxygen that we see on Earth—so presumably it is made in stars.
What is the connection between these two fields?
Glad you asked. When cosmic modellers were messing with the expansion speed and gravity of the Big Bang, it seemed like one could turn up the matter dial as long as one simultaneously turned up the explosion dial. They weren’t independent parameters. But the BBN models showed that if the matter density dial were higher, more Lithium and Carbon would be produced, so they put an upper limit on the matter density. Likewise, the bigger the explosion, the more photons it produced, so studying the Cosmic Microwave Background Radiation gave an upper limit on the explosion energy. It looked as if all the parameters were fixed, and the BB model had no free parameters. (The holy grail of theoretical models.)
But it gave the wrong dynamics. That is, the model produced either an expanding gas universe, or a condensing black hole universe, not the lacy universe sprinkled with galaxies and stars that we actually observe.
By adding a “late expansion” term, (symbol=Lambda) they could get the low-density bubbles (holes in the lace), and by adding a “early mass” term (symbol=CDM=”cold dark matter”), they could get the galaxies to condense (threads in the lace). Rubin’s “dark matter”, since it wasn’t ever observed, was just what the doctor ordered, and they made it an “early mass” term that was NOT made out of protons (or baryons) so as to decouple it from the BBN. The bubbles they attributed to an “anti-gravity” term, one that Einstein briefly considered as a way to get Steady State but removed after Lemaitre and Hubble showed that the universe was actually expanding. Alas, there was no justification for anti-gravity and so the powers-that-be made it clear anyone finding a justification would be awarded a Nobel prize. In 2011, Perlmutter, Schmidt & Reiss (all younger than me) were awarded a Nobel for observations made on Type Ia supernovae that very weakly suggested the presence of anti-gravity or “dark energy”. (The observations on 74 SN/Ia have since weakened considerably as 1000 more SN/Ia were added to the data set and the Planck data was analyzed, which is one reason for the urgency of the award. After all, a Nobel prize makes anything true!)
The fly in the ointment is the missing observations of “dark matter”. Just as we saw for “dark energy” it was a dial needed for the dynamic cosmological models to work, but it couldn’t be normal matter–say, frozen black comets–because that would mess up the BBN model. Mind you, the astronomer’s galaxy rotation curves could easily be due to frozen black comets, because astronomers don’t care what provides the gravity as long as it doesn’t emit photons or disturb the light of background stars. But the cosmology modellers desperately needed a free parameter that was not made of protons/baryons.
The result of this dilemma, was the 20 year search for “exotic” nuclear particles, like heavy neutrinos, or supersymmetric (SUSY) matter, or axions, or Majorana fermions, or magnetic monopoles, and the like. It didn’t hurt that the cancellation of the Texas Supercollider in 1992 meant that 1000 particle physicists had nothing to work on–so they all started writing dark matter proposals /papers. One significant theoretical prediction was that a SUSY particle that was heavy and possessed only the weak force and not the strong force, would provide exactly the right amount of mass without interacting with the BBN (strong force) element production. This prediction was known as the “WIMP miracle” where WIMP = weakly interacting massive particle. It led to 100’s of WIMP-detector experiments: tons of liquid Xe in an underground mine, ultra-pure NaI detectors in a mine, plastic detectors on the Space Shuttle, liquid gallium in the Grand Sasso tunnel, etc.
And the result of 20 years of increasingly sensitive experiments? Nothing. Zippo. Nada. (Which in itself is a profound discovery.)
The recent publication of the Italians+Silk paper has now voiced the unspeakable: there is something wrong with the Lambda-CDM Big Bang model, and by inference, the 2011 Nobel Prize. Neither “dark matter” nor “dark energy” seem to exist in a form that makes the model work. This doesn’t negate the conundrum of galaxy rotation curves, nor the weak significance of extra-faint SN/Ia, but it means that we cannot blithely attribute them to either an “exotic particle” in the BBN nor an anti-gravity term in General Relativity.
The repercussions of this failure are still reverberating, and different physicists have differing solutions. My contention is that no one has yet gone back to square one and rebuilt cosmology models to reflect all the new observations. Instead, they seem intent on patching the old model with “new physics”, e.g. more dials, more epicycles. So, for example, the New Scientist has this line in their introduction:
“Our best measurements indicate that some 85 per cent of all matter in our universe consists of “dark matter” made of something that isn’t atoms.”
The inference that it isn’t atoms was made by the BBN modellers using the Lambda-CDM model universe. If we redo the models, we no longer can conclude that dark matter isn’t made of atoms. This is 95% of the reason theorists are at their wits’ end. They don’t know which observations are permanently model-independent, and which are disposably model-dependent. This is particularly true of the Planck observations, which are all 6-digit fits to a Lambda-CDM model.
So my long jeremiad in the wilderness is now being whispered in the salons—it is time to rebuild the models. This isn’t actually that hard—the original BBN model was one-dimensional, homogeneous, and isotropic because that was all the computing power they had in 1967. If it hadn’t worked as well as it did, we’d be discussing 3D anisotropic non-homogeneous models right now. For the past 10 years I’ve been working on a very simplified model version that adds chaotic magnetic fields to the old model, without changing its 1D, isotropic, homogeneous fine-grained structure. The preliminary results are very promising: elimination of the inflaton-field (magnetic field providing the global coherence), suppression of the early expansion (which brings the two Hubble constants back into alignment and functions like dark energy), increasing production of CNO elements (which add black comets to explain dark matter as well as 7Li abundances and early galaxy formation). Perhaps now the community will permit it to be finally published. But then again, we’ve been expecting the imminent demise of Darwinism for two decades now. Some things change, as a recent paper documents, only on the timescale of funerals.
As I said before, theorists can’t seem to distinguish between empirical facts and theory-laden conclusions, which is why they start inventing new physics to resolve the dilemma. It reminds me of bad scriptwriters who having unnecessarily convoluted the plot, cannot think of a way to end gracefully and after several dismal attempts to kill off all the characters in a desperate bid to close out the story, end up hiring George Lucas to write the ending. Only, it was God who wrote, in the beginning, and it will take a recognition of his Intelligent Design to find a graceful resolution to cosmology.
First off, the author, Dan Hooper, can’t help but plug his own solution. He still has hopes that the gamma-ray signal he discovered from the center of our galaxy is “the signature of dark matter”, but pretty much 99% of the community attribute it to neutron star binaries (right place, right amount, right spectrum, etc.) Likewise, yesterday’s news item on antimatter finds zero evidence that anti-protons behave differently around unseen “dark matter” than normal protons. (Send more money, say the scientists, we’ll find it yet.)
What is more interesting, is the list of things the Hooper “knows” about dark matter, that is, besides the extra gravity it provides to galaxies. His list of dark matter things “we know” is followed by a list of things “we don’t know”. If you ask, “given that we don’t know much, how do we know these items?” you will find a most peculiar mish-mash of real observations mixed with dubious model-based reasoning. Here’s a small example:
“Let’s start with what we do know about this substance—or perhaps substances. Dark matter isn’t familiar atomic matter, or any of the exotic forms of matter created at the Large Hadron Collider buried underground near Geneva, Switzerland, or at other particle accelerators. It doesn’t appreciably interact with itself, or with ordinary matter, except via gravity. It can pass through solid objects like a ghost, and doesn’t emit, absorb or reflect any easily measurable quantities of light. It is invisible, or at least nearly so.”
Let’s break this down into statements.
Why do we know it isn’t made at CERN?
Because we were looking for exotic particles and didn’t find any. So it either it escaped detection, or it isn’t exotic.
Why doesn’t it interact with itself?
Because then it would show angular momentum transfer, or dynamic viscosity, which is what converted clouds of hydrogen gas into spiral galaxies. That is, we should find a concentration at the center of our galaxy. However, based on the gravitational pull, the dark matter is distributed evenly throughout the disk, but excluded from the center. So either it doesn’t interact with itself, or something else is compensating the dynamic viscosity.
Why doesn’t it interact with ordinary matter?
Same reason as above. So either it doesn’t interact with ordinary matter, or something is compensating for viscosity.
Why does it pass through solid objects like a ghost?
Because we have built ginormous detectors made of CCl4, NaI, H20, liquid Xe, Ga — and we never saw it. So either it is ghostlike or it doesn’t pass through our detectors.
Why does it not emit, absorb or reflect light?
Because astronomers look through megaparsecs of space without noticing it. Of course, they don’t see protons or electrons or black comets or asteroids either–some things are either too dark, too few, or too cold for astronomers to see. So either it is invisible, or astronomers can’t see it.
Where does that leave our dark matter theory?
Either, it is a truly exotic particle harder to see than a neutrino, or, it is a black comet that is made of normal matter, emits gas jets to compensate for dynamic viscosity, jets away from hot stars leaving the center of our galaxy empty, doesn’t pass through underground mines, and is too cold and dark for astronomers to see. Guess which one is getting all the press.
Remember, it isn’t what you don’t know that hurts you, it is what you do know that ain’t so.
Why is this failure hitting the press now?
I had hoped it was the effect of Silk’s reanalysis of Planck data, but Dan Hooper doesn’t even mention it, perhaps because he isn’t an astronomer. Though he has written papers with the more senior Silk, and presumably keeps in contact, so perhaps it is just too dangerous a thought for a mid-career scientist.
See also: Discover: Even the best dark matter theories are crumbling
Researcher: The search for dark matter has become a “quagmire of confirmation bias” So many research areas in science today are hitting hard barriers that it is reasonable to think that we are missing something.
Physicists devise test to find out if dark matter really exists
Largest particle detector draws a blank on dark matter
What if dark matter just doesn’t stick to the rules?
A proposed dark matter solution makes gravity an illusion
Proposed dark matter solution: “Gravity is not a fundamental governance of our universe, but a reaction to the makeup of a given environment.”
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