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Axions, Inflation and Baryogenesis: It’s a SMASH (pi)

Searches for direct detection of dark matter have focused primarily on WIMPs (weakly interacting massive particles) and more precisely on LSPs (the lightest supersymmetric particle). These are hypothetical particles such as neutralinos that are least massive members of the hypothesized family of supersymmetric partner particles.

But supersymmetry may be dead. There have been no supersymmetric particles detected at the Large Hadron Collider at CERN as of yet, leading many to say that this is a crisis in physics.

At the same time as CERN has not been finding evidence for supersymmetry, WIMP dark matter searches have been coming up empty as well. These searches keep increasing in sensitivity with larger and better detectors and the parameter space for supersymmetric WIMPs is becoming increasingly constrained. Enthusiasm unabated, the WIMP dark matter searchers continue to refine their experiments.


LUX dark matter detector in a mine in Lead, South Dakota is not yet detecting WIMPs. Credit: Matt Kapust/ Sanford Underground Research Facility

What if there is no supersymmetry? Supersymmetry adds a huge number of particles to the particle zoo. Is there a simpler explanation for dark matter?

Alternative candidates under consideration for dark matter, including sterile neutrinos, axions, and primordial black holes, and are now getting more attention.

From a prior blog I wrote about axions as dark matter candidates:

Axions do not require the existence of supersymmetry. They have a strong theoretical basis in the Standard Model as an outgrowth of the necessity to have charge conjugation plus parity conserved in the strong nuclear force (quantum chromodynamics of quarks, gluons). This conservation property is known as CP-invariance. (While CP-invariance holds for the strong force, the weak force is CP violating).

In addition to the dark matter problem, there are two more outstanding problems at the intersection of cosmology and particle physics. These are baryogenesis, the mechanism by which matter won out over antimatter (as a result of CP violation of Charge and Parity), and inflation. A period of inflation very early on in the universe’s history is necessary to explain the high degree of homogeneity (uniformity) we see on large scales and the near flatness of the universe’s topology. The cosmic microwave background is at a uniform temperature of 2.73 Kelvins to better than one part in a hundred thousand across the sky, and yet, without inflation, those different regions could never have been in causal contact.

A team of European physicists have proposed a model SMASH that does not require supersymmetry and instead adds a few particles to the Standard Model zoo, one of which is the axion and is already highly motivated from observed CP violation. SMASH (Standard Model Axion Seesaw Higgs portal inflation) also adds three right-handed heavy neutrinos (the three known light neutrinos are all left-handed). And it adds a complex singlet scalar field which is the primary driver of inflation although the Higgs field can play a role as well.

The SMASH model is of interest for new physics at around 10^11 GeV or 100 billion times the rest mass of the proton. For comparison, the Planck scale is near 10^19 GeV and the LHC is exploring up to around 10^4 GeV (the proton rest mass is just under 1 GeV and in this context GeV is short hand for GeV divided by the speed of light squared).


Figure 1 from Ballesteros G. et al. 2016. The colored contours represent observational limits from the Planck satellite and other sources regarding the tensor-to-scalar power ratio of primordial density fluctuations (r, y-axis) and the spectral index of these fluctuations (ns, x-axis). These constraints on primordial density fluctuations in turn constrain the inflation models. The dashed lines ξ = 1, .1, .01, .001 represent a key parameter in the assumed slow-roll inflation potential function. The near vertical lines labelled 50, 60, 70, 80 indicate the number N of e-folds to the end of inflation, i.e. the universe inflates by a factor of e^N in each of 3 spatial dimensions during the inflation phase.

So with a single model, with a few extensions to the Standard Model, including heavy right-handed (sterile) neutrinos, an inflation field, and an axion, the dark matter, baryogenesis and inflation issues are all addressed. There is no need for supersymmetry in the SMASH model and the axion and heavy neutrinos are already well motivated from particle physics considerations and should be detectable at low energies. Baryogenesis in the SMASH model is a result of decay of the massive right-handed neutrinos.

Now the mass of the axion is extremely low, of order 50 to 200 μeV (millionths of an eV) in their model (by comparison, neutrino mass limits are of order 1 eV), and detection is a difficult undertaking.

There is currently only one active terrestrial axion experiment for direct detection, ADMX. It has its primary detection region at lower masses than the SMASH model is suggesting, and has placed interesting limits in the 1 to 10 μeV range. It is expected to push its range up to around 30 μeV in a couple of years. But other experiments such as MADMAX and ORPHEUS are coming on line in the next few years that will explore the region around 100 μeV, which is more interesting for the SMASH model.

Not sure why the researchers didn’t call this the SMASHpie model (Standard Model Axion Seesaw Higgs portal inflation), because it’s a pie in the face to Supersymmetry!


It would be wonderfully economical to explain baryogenesis, inflation, and dark matter with a handful of new particles, and to finally detect dark matter particles directly.


“Unifying inflation with the axion, dark matter, baryogenesis and the seesaw mechanism” Ballesteros G., Redondo J., Ringwald A., and Tamarit C. 2016


Gamma Rays from Dark Matter at the Center of the Galaxy: Stronger Evidence

Evidence has been growing for the detection of dark matter more directly – at the center of the Milky Way Galaxy. Normally, we detect dark matter through its gravitational effects only, although there have been many attempts to detect it more directly, both through laboratory experiments here on Earth and from astronomical measurements. The Earthbound experiments are inconclusive at best, with some claims of detection being contradicted by other experiments.

But the evidence for astronomical detection of dark matter is growing. Expected sources include dwarf galaxies that are found near our Milky Way. The low luminosity of dwarf galaxies due to stars and supernovae can make it easier to extract evidence of dark matter due to its self annihilation.

Our own Milky Way Galaxy has a higher concentration of normal matter at the center, and is expected to have a higher concentration of dark matter as well. For the past 5 years or so, there has been evidence for possible dark matter annihilation at the Galactic Center. See

The mechanism is dark matter self-annihilation, resulting in the creation of decay products of ordinary matter and gamma rays (highly energetic photons). See one of my prior blogs at:

The leading dark matter candidate is some sort of WIMP (weakly interacting massive particle). WIMPs interact only via gravity and perhaps the weak nuclear force. WIMP self-annihilations can produce quarks, neutrinos, gamma rays and other ordinary matter particles.

There is a known gamma ray signal in the Galactic Center (the center of our Milky Way) that extends to 5 degrees away from the center, corresponding to roughly a kiloparsec in extent (a kiloparsec is 3260 light-years, and our Sun is 8 kiloparsecs from the Center). The major alternatives for this signal appear to be dark matter annihilation, cosmic ray interactions with interstellar gas, or emission from rapidly rotating neutron stars (millisecond pulsars).

A recent paper from T. Daylan and co-authors from Harvard, MIT, Princeton, the University of Chicago and the Fermi Laboratory is titled “The Characterization of the Gamma-Ray Signal from the Central Milky Way: A Compelling Case for Annihilating Dark Matter”. They have reanalyzed observations from the Fermi Gamma Ray Space Telescope and confirmed that the distribution of gamma rays in the Galactic Center (GC) is largely spherically symmetric and extended. This spatial distribution likely rules out neutron stars as the source, since these are preferentially found in the galactic disk. 


1-3 GeV residual gamma ray image. From Fig. 10 of T. Daylan et al., this is corrected for galactic diffuse emission and has point sources subtracted. The image extends over a 10◦ by 10◦ region.

Dark matter, on the other hand, would be expected to have a roughly spherical distribution around the GC. Interstellar gas is also largely confined to the galactic disk, so this explanation is disfavored. Their study also confirms that the emission extends beyond the GC to what is known as the Inner Galaxy, further ruling out the two alternatives other than dark matter annihilation. The emission falls off in intensity away from the GC, in a manner consistent with a spherically symmetric dark matter density distribution that is in accordance with a Navarro-Frenk-White profile often used successfully in modeling dark matter halos. No evidence is found for any significant deviation from spherical symmetry for the GC and Inner Galaxy components, the latter extending out to around 2 kiloparsecs.

There are various possible annihilation channels for dark matter and the authors’ analysis appears to favor a dominant channel to primarily b quarks (and b antiquarks). In this scenario the WIMP mass appears to lie in the range of 36 to 51 GeV (by comparison a proton or neutron mass is about .94 GeV). Recall that there are 6 types of quarks, and protons and neutrons are composed of u and d (up and down) quarks. The others are b, t, c, s (bottom, top, charm and strange). The quarks other than u and d are unstable and will decay to u and d.

The spectral (energy) distribution peaks at gamma ray energies of around 1 to 3 GeV and is a good fit to the predictions for annihilation to a b quark pair (b and anti-b). In addition, the cross-section for annihilation calculated from the gamma ray intensity is consistent with that expected from the required rate of thermal production of dark matter particles in the early universe, of order 10^-26 cm^3/sec (actually a value of the cross-section multiplied by the average velocity). The observed dark matter abundance freezes out from thermal equilibrium in the early universe as it expanded and cooled, and implies a cross section of that order.

There is also the possibility for other decay channels, including decays to u, d, c, s and t quarks and to tau lepton particles. The spectral shapes disfavor decays to tau leptons and u, d quarks in particular. After decays to b quarks, the c (charm) and s (strange) quark channels are the most likely.  Either a c or s quark channel implies somewhat lower WIMP masses, around the 20 to 40 GeV range. Annihilations to other fermions appear less likely.

In summary, quoting from their paper:

“This signal consists of a very large number of events, and has been detected with overwhelming statistical significance. The the excess consists of ∼ 10,000 gamma rays per square meter, per year above 1 GeV (from within 10◦ of the Galactic Center). Not only does this large number of events enable us to conclude with confidence that the signal is present, but it also allows us to determine its spectrum and morphology in some detail. And as shown, the measured spectrum, angular distribution, and normalization of this emission does indeed match well with that expected from annihilating dark matter particles.”

“There is no reason to expect that any diffuse astrophysical emission processes would exhibit either the spectrum or the morphology of the observed signal. In particular, the spherical symmetry of the observed emission with respect to the Galactic Center does not trace any combination of astrophysical components (i.e. radiation, gas, dust, star formation, etc.), but does follow the square of the anticipated dark matter density.”

There are also possible detections, marginally significant, of gamma ray emission due to dark matter in nearby dwarf galaxies, and in the direction of the Virgo cluster. We look forward to additional observations and theoretical work to confirm dark matter annihilation signals in our own galaxy and nearby galaxies.

NEW BOOK just released:

S. Perrenod, 2016, 72 Beautiful Galaxies (especially designed for iPad, iOS; ages 12 and up)


Earth’s Dark Matter Sabers

hubble_lightsaber_image2.jpgOrion B Molecular Cloud Complex, a stellar nursery. Credits: NASA/ESA

This week NASA released a celestial light saber photo. It’s a stunning visualization of a ‘light saber’ apparition in the Orion B Molecular Cloud Complex. This is the universe of ordinary matter.

But in the world of dark matter which cohabits with ordinary matter in our universe, there are saber-like objects as well. In an earlier blog we talked about dark matter filaments and walls on the very large scale, at the scale of superclusters of galaxies and above. On the small scale, dark matter particles also align into structures as well.

Dark matter is a ‘collisionless’ fluid. That is, it interacts so rarely with either ordinary matter or itself, that it doesn’t thermalize and dissipate energy via radiation as does ordinary matter. It interacts through gravity alone, both with itself and with ordinary matter, but that can lead to structure as well, even at the very small scale. Regions of very enhanced density (gravitational clumping) are known as caustics.

If there were to be caustics in our solar system – regions of enhanced dark matter density – then it could ease the direct detection search for dark matter. There are many direct detection experiments underway on Earth, mostly with negative results, although for a number of years the DAMA/LIBRA experimenters have claimed direct detection; these results are highly disputed.

Our best understanding of dark matter is that it forms “fine-grained streams” or clumps that move together at the same velocity. These streams can be quite large and many streams should be found in our galactic neighborhood.

Now Gary Prézeau of NASA-JPL has noted that compact bodies, such as the Sun and planets, should act as lenses of sorts, focusing dark matter streams into strands of higher density. He refers to these as ‘dark matter hairs’ and calculates where the roots should lie relative to the center of the Sun or the center of the Earth or other planets.

In the case of the Earth the root of the ‘hair’ would be around 1 million kilometers behind the Earth – behind in this case meaning relative to our orbital motion around the galaxy’s center. The density enhancement might be as large as 1 billion times the normal density. In the solar neighborhood that average normal density is estimated to be 1/3 of a proton mass per cubic centimeter, so it’s not surprising that direct detection of dark matter is so difficult. But at hundreds of millions of proton masses per cc, or some tens of millions of dark matter particles per cc (depending on the dark matter mass) it could be much more feasible.

PIA20176_ipCredit: NASA/JPL – CalTech

I prefer to think of these as dark matter sabers emanating from the Earth.

It turns out that the James Webb Space Telescope, the successor to Hubble, will be placed at one of the Lagrange points (L2) in the Sun-Earth gravitational system, about 1.5 million kilometers out. Perhaps this would be a good place to put a dark matter detection experiment. The earthbound ones are getting to be quite massive, but with the potentially much greater sensitivity, a small scale detector could be quite effective. At the L2 point it would be aligned with the Sun’s orbit around our galaxy each year around the first of June. That could be the best time for detection, but there could be other hairs as well with density enhancements in the millions, and detectable at other times of the year.

References: – Ethan Siegel article – NASA release on possible ‘dark matter hairs’ around Earth – Gary Prezeau, 2015. “Dense Dark Matter Hairs Spreading out from Earth,  Jupiter, and other Compact Bodies” – dark matter structure at the very large scale

Dark Matter Filaments (and Walls)

Dark matter is about 5 times as abundant as ordinary matter in the universe. We see its gravitational affects on large scales – within galaxies, in groups and clusters of galaxies, and in the overall way in which galaxies are spatially distributed on the largest scales.

Galaxies on the large scale are observed to be distributed in filaments and walls of excess galaxy density, along with voids of lower galaxy densities. These filaments and walls have been discovered during the past 30 years or so.

Since galaxies themselves are primarily composed of dark matter, it makes sense that the filaments and walls and voids are reflecting the large scale distribution of matter as well.

Basically, we can think of dark matter as the scaffolding upon which the ordinary matter clumps – the luminous matter seen from galaxies’ light emission and absorption and due to stars, gas, and dust within galaxies. And when we say light, we mean radiation at all frequencies including radio waves, infrared, visible, ultraviolet, X-rays and gamma rays. Radiation sources are due to ordinary matter, since dark matter couples very poorly to electromagnetic fields – that’s why it’s dark!


Map of our Neighborhood (you are in the center) Powell, CC-BY-SA-2.5 

So we know that filaments and walls that we see in the distribution of galaxies reflect (deliberate pun) the underlying distribution of the gravitationally dominant dark matter. It is the gravitational field of the dark matter which has controlled the clumping of ordinary matter into galaxies and clusters and superclusters of galaxies.

Here are some of the most important known filaments discovered over the past 3 decades:

  • the Coma Filament
  • the Perseus-Pegasus Filament
  • the Ursa Major Filament
  • the Lynx-Ursa Major Filament (LUM)
  • the CIG J2143-4423 Filament

Now these are mostly named after constellations, but of course they are behind the constellations and external to our Milky Way galaxy, and at large distances. The most distant known appears to be at redshift z = 2.38, which means that the light comes from a time when the universe was less than 3 billion years old. It has a linear scale of 350 million light-years.

Galaxy filaments include also structures known as walls. And here are the major walls that have been discovered:

  • the CfA2 Great Wall
  • the Sloan Great Wall
  • the Sculptor Wall
  • the Grus Wall
  • the Fornax Wall
  • the Hercules-Corona Borealis Great Wall

The 3-D perspective map above shows superclusters and voids in our neighborhood. It is centered on the Milky Way and extends out to 500 million light-years. Superclusters are highlighted in blue, and 3 walls are highlighted in yellow: the Coma Wall, the Centaurus Wall, and the Sculptor Wall. A number of Void regions are highlighted in red.

The CfA2 Great Wall (also known as the Coma Wall) extends from the Hercules Supercluster to the Coma Supercluster to the Leo Supercluster on the right hand side of the map. It was the first wall discovered, in 1989, by Margaret Geller and John Huchra of the Havard-Smithsonian Center for Astrophysics (CfA).

This Hercules-Corona Borealis Great Wall is much further away, at a redshift of around z = 2, which means that we are seeing it from a time when the universe was a little over 3 billion years old (more than 10 billion years ago). It has an enormous length of around 10 billion light-years. Which is incredible, since the comoving distance  to the Hercules-Corona Borealis Great Wall is 17 billion light-years.

Remember, when you look at these maps, you are also in effect seeing the distribution of the underlying dark matter.

In the next blog, we will talk about possible dark matter filaments on a much, much smaller scale, on the scale of the Sun and the planets within our Solar System, including Earth.


Multi-Billion Light-Year Map: Dark Energy Survey first results

Perhaps the most amazing map ever created by mankind has just been published by astronomers from the Dark Energy Survey collaboration. It’s either this map below or the latest Cosmic Microwave Background map from the Planck satellite. Personally, I find this new map of our “neighborhood” more exciting and esthetic, as it shows the explicit locations of several hundred clusters of galaxies together with the distribution of dark matter across a very large scale.

The Dark Energy Camera (DECam) is a 570 Megapixel system designed by the DES collaboration for deep galaxy survey work. It sits at the focal plane of the 4 meter Victor M. Blanco telescope at the National Optical Astronomy Observatory’s Cerro Tololo Inter-American Observatory in Chile. The Dark Energy Survey team will acquire data over a 5-year period. This is the first map released, based on a small percentage of the data that will ultimately be collected.

Over 2 million galaxies were observed with DECam to create this map, and a supercomputer was required to analyze the data and compare it to theoretical expectations for dark matter and galaxy distributions. The map shows the distribution of dark matter, clusters of galaxies, and matter filaments and voids in a field of 139 square degrees. That’s roughly 1/2 billion light-years or so in the horizontal direction and in the vertical direction. 

The redshifts, and thus distances, of the galaxies in the survey sample have been determined photometrically, by comparing the luminosity of a given galaxy in various wave bands in the optical and infrared regions of the spectrum. About 1 million of the galaxies in the survey are background galaxies with redshifts above 0.6, and another million plus are foreground galaxies with redshifts below 0.5. The higher the redshift, the more distant the galaxy, since the universe is expanding in a uniform fashion. The redshift measures the shift of light toward the red end of the spectrum as it is “stretched out” along with this expansion.

The light from background galaxies is distorted as it passes through concentrations of dark matter on its way to the Earth and the DECam. This is due to gravitational lensing, a consequence of Einstein’s general relativity. A background galaxy’s image will shear in proportion to the strength of the gravitational field its light encounters during its journey to the Earth. 

The amount of shear seen by each of the million or so background galaxies is used to infer the map of the distribution of dark matter at redshift 0.5 and below. This corresponds to a lookback time of up to 5 billion years. Coincidentally, the redshift of 0.5 is around the time when dark energy began to be more important than dark matter, and the expansion of the universe began to accelerate, rather than decelerate. 

The dots on the map are not individual galaxies; rather they are rich clusters of galaxies. Each  cluster is a gravitationally bound system, dominated by dark matter, and containing 100s to 1000s of galaxies. Typically the galaxies themselves contribute only about 1% of a cluster’s mass, and the dark matter can contribute up to 90% of the total. The remainder is found as hot X-ray emitting gas residing between galaxies, but still bound to the cluster due to the predominant gravitational potential contribution from the dark matter.

DES map of dark matter density

The red areas on the map represent the highest concentrations of dark matter, the orange and yellow areas the next highest, and the blue areas the lowest. These blue areas are called voids because of their low density of dark matter and clusters of galaxies. One clearly sees the existence of dark matter filamentary structures in the red, orange, yellow colors. 

Clusters of galaxies are represented by the gray dots, and the larger the dot, the “richer” or larger the cluster. It’s evident to the eye that the clusters are preferentially found in the same locations as the reddish areas and yellow filaments. They are scarcely to be found in the blue-colored voids. The point is that the ordinary matter, which is most concentrated in galaxy clusters, follows the dark matter density, as expected. 

Peruse the map, and let your mind wander out into the vast reaches of intergalactic space.

This is just the beginning; this map was created from the very preliminary observations made in the DES. The survey team plans to eventually acquire 35 times as much coverage of the sky as this map provides. The full survey is expected to advance our knowledge of the nature and distribution of both dark matter and dark energy substantially.

The press release of April 13th announcing this truly amazing map can be found here:

“The Dark Energy Survey is a collaboration of more than 300 scientists from 25 institutions in six countries. Its primary instrument, the Dark Energy Camera, is mounted on the 4-meter Blanco telescope at the National Optical Astronomy Observatory’s Cerro Tololo Inter-American Observatory in Chile, and its data is processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.”

2014 in review

The stats helper monkeys prepared a 2014 annual report for this blog.

Here’s an excerpt:

A New York City subway train holds 1,200 people. This blog was viewed about 7,900 times in 2014. If it were a NYC subway train, it would take about 7 trips to carry that many people.

Click here to see the complete report.

2012 in review

The stats helper monkeys prepared a 2012 annual report for this blog.

Here’s an excerpt:

600 people reached the top of Mt. Everest in 2012. This blog got about 6,200 views in 2012. If every person who reached the top of Mt. Everest viewed this blog, it would have taken 10 years to get that many views.

Click here to see the complete report.

Dark Matter, Dark Energy, Dark Gravity

Enabling a Universe that Supports Intelligent Life

Author: Stephen Perrenod

An e-book now available through:

We are immersed in a sea of light emanating from ordinary matter that is floating, as it were, on an ocean of dark matter. The dark matter itself floats on the dark energy of the particle vacuum that in turn is in embedded within the scaffolding of space-time – which is shaped by the dark gravity effects from all matter and energy.

Table of Contents

  • Dedication
  • Foreword (by Rich Brueckner)
  • Preface and Acknowledgements
  1. Scale of the Universe
  2. The Big Bang Model
  3. Inflation
  4. Dark Matter
  5. Dark Energy
  6. Dark Gravity
  7. Future of the Universe
  • Glossary
  • References, Suggested Reading and Viewing
  • About the Author