Tag Archives: primordial black holes

Yet Another Intermediate Black Hole Merger

Another merger of two intermediate mass black holes has been observed by the LIGO gravitational wave observatories.

There are now three confirmed black hole pair mergers, along with a previously known fourth possible, that lacks sufficient statistical confidence.

These three mergers have all been detected in the past two years and are the only observations ever made of gravitational waves.

They are extremely powerful events. The lastest event is known as GW170104 (gravitational wave discovery of January 4, 2017).

It all happened in the wink of an eye. In a fifth of a second, a black hole of 30 solar masses approximately merged with a black hole of about 20 solar masses. It is estimated that the two orbited around one another six times (!) during that 0.2 seconds of their final existence as independent objects.

The gravitational wave generation was so great that an entire solar mass of gravitational energy was liberated in the form of gravitational waves.

This works out to something like 2 \cdot 10^{47} Joules of energy, released in 0.2 seconds, or an average of 10^{48} Watts during that interval. You know, a Tera Tera Tera Terawatt.

Researchers have now discovered a whole new class of black holes with masses ranging from about 10 solar masses (unmerged) to 60 solar masses (merged). If they keep finding these we might have to give serious consideration to intermediate mass black holes as contributors to dark matter.  See this prior blog for a discussion of primordial black holes as a possible dark matter contributor:


Image credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)


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

WIMPs or MACHOs or Primordial Black Holes

A decade or more ago, the debate about dark matter was, is it due to WIMPs (weakly interacting massive particles) or MACHOs (massive compact halo objects)? WIMPs would be new exotic particles, while MACHOs are objects formed from ordinary matter but very hard to detect due to their limited electromagnetic radiation emission.


Schwarzenegger (MACHO), not Schwarzschild (Black Holes)

Image credit: Georges Biard, CC BY-SA 3.0

Candidates in the MACHO category such as white dwarf or brown dwarf stars have been ruled out by observational constraints. Black holes formed in the very early universe, dubbed primordial black holes, were thought by many to have been ruled out as well, at least across many mass ranges, such as between the mass of the Moon and the mass of the Sun.

The focus during recent years, and most of the experimental searches, has shifted to WIMPs or other exotic particles (axions or sterile neutrinos primarily). But the WIMPs, which were motivated by supersymmetric extensions to the Standard Model of particle physics, have remained elusive. Most experiments have only placed stricter and stricter limits on their possible abundance and interaction cross-sections. The Large Hadron Collider has not yet found any evidence for supersymmetric particles.

Have primordial black holes (PBHs) as the explanation for dark matter been given short shrift? The recent detections by the LIGO instruments of two gravitational wave events, well explained by black hole mergers, have sparked new interest. A previous blog entry addressed this possibility:

The black holes observed in these events have masses in a range from about 8 to about 36 solar masses, and they could well be primordial.

There are a number of mechanisms to create PBHs in the early universe, prior to the very first second and the beginning of Big Bang nucleosynthesis. At any era, if there is a total mass M confined within a radius R, such that

2*GM/R > c^2 ,

then a black hole will form. The above equation defines the Schwarzschild limit (G is the gravitational constant and c the speed of light). A PBH doesn’t even have to be formed from matter whether ordinary or exotic; if the energy and radiation density is high enough in a region, it can also result in collapse to a black hole.


Cosmic Strings

Image credit: David Daverio, Université de Genève, CSCS supercomputer simulation data

The mechanisms for PBH creation include:

  1. Cosmic string loops – If string theory is correct the very early universe had very long strings and many short loops of strings. These topological defects intersect and form black holes due to the very high density at their intersection points. The black holes could have a broad range of masses.
  2. Bubble collisions from symmetry breaking – As the very early universe expanded and cooled, the strong force, weak force and electromagnetic force separated out. Bubbles would nucleate at the time of symmetry breaking as the phase of the universe changed, just as bubbles form in water as it boils to the surface. Collisions of bubbles could lead to high density regions and black hole formation. Symmetry breaking at the GUT scale (for the strong force separation) would yield BHs of mass around 100 kilograms. Symmetry breaking of the weak force from the electromagnetic force would yield BHs with a mass of around our Moon’s mass ~ 10^25 kilograms.
  3. Density perturbations – These would be a natural result of the mechanisms in #1 and #2, in any case. When observing the cosmic microwave background radiation, which dates from a time when the universe was only 380,000 years old, we see density perturbations at various scales, with amplitudes of only a few parts in a million. Nevertheless these serve as the seeds for the formation of the first galaxies when the universe was only a few hundred million years old. Some perturbations could be large enough on smaller distance scales to form PBHs ranging from above a solar mass to as high as 100,000 solar masses.

For a PBH to be an effective dark matter contributor, it must have a lifetime longer than the age of the universe. BHs radiate due to Hawking radiation, and thus have finite lifetimes. For stellar mass BHs, the lifetimes are incredibly long, but for smaller BHs the lifetimes are much shorter since the lifetime is proportional to the cube of the BH mass. Thus a minimum mass for PBHs surviving to the present epoch is around a trillion kilograms (a billion tons).

Carr et al. (paper referenced below) summarized the constraints on what fraction of the matter content of the universe could be in the form of black holes. Traditional black holes, of several solar masses, created by stellar collapse and detectable due to their accretion disks, do not provide enough matter density. Neither do supermassive black holes of over a million solar masses found at the centers of most galaxies. PBHs may be important in seeding the formation of the supermassive black holes, however.

Limits on the PBH abundance in our galaxy and its halo (which is primarily composed of dark matter) are obtained from:

  1. Cosmic microwave background measurements
  2. Microlensing measurements (gravitational lensing)
  3. Gamma-ray background limits
  4. Neutral hydrogen clouds in the early universe
  5. Wide binaries (disruption limits)

Microlensing surveys such as MACHO and EROS have searched for objects in our galactic halo that act as gravitational lenses for light originating from background stars in the Magellanic Clouds or the Andromeda galaxy. The galactic halo is composed primarily of dark matter.

A couple of dozen of objects with less than a solar mass have been detected.  Based on these surveys the fraction of dark matter which can be PBHs with less than a solar mass is 10% at most. The constraints from 1 solar mass up to 30 solar masses are weaker, and a PBH explanation for most of the galactic halo mass remains possible.

Similar studies conducted toward distant quasars and compact radio sources address the constraint in the supermassive black hole domain, apparently ruling out an explanation due to PBHs with from 1 million to 100 million solar masses.

Lyman-alpha clouds are neutral hydrogen clouds (Lyman-alpha is an important ultraviolet absorption line for hydrogen) that are found in the early universe at redshifts above 4. Simulations of the effect of PBH number density fluctuations on the distribution of Lyman-alpha clouds appear to limit the PBH contribution to dark matter for a characteristic PBH mass above 10,000 solar masses.

Distortions in the cosmic microwave background are expected if PBHs above 10 solar masses contributed substantially to the dark matter component. However these limits assume that PBH masses do not change. Merging and accretion events after the recombination era, when the cosmic microwave background was emitted, can allow a spectrum of PBH masses that were initially less than a solar mass before recombination evolve to one dominated by PBHs of tens, hundreds and thousands of solar masses today. This could be a way around some of the limits that appear to be placed by the cosmic microwave background temperature fluctuations.

Thus it appears could be a window in the region 30 to several thousand solar masses for PBHs as an explanation of cold dark matter.

As the Advanced LIGO gravitational wave detectors come on line, we expect many more black hole merger discoveries that will help to elucidate the nature of primordial black holes and the possibility that they contribute substantially to the dark matter component of our Milky Way galaxy and the universe.


B. Carr, K. Kohri, Y. Sendouda, J. Yokoyama, 2010 “New cosmological constraints on primordial black holes”

S. Cleese and J. Garcia-Bellido, 2015 “Massive Primordial Black Holes from Hybrid Inflation as Dark Matter and the Seeds of Galaxies”

P. Frampton, 2015 “The Primordial Black Hole Mass Range”

P. Frampton, 2016 “Searching for Dark Matter Constituents with Many Solar Masses”

Green, A., 2011 “Primordial Black Hole Formation”

P. Pani, and A. Loeb, 2014 “Exclusion of the remaining mass window for primordial black holes as the dominant constituent of dark matter”

S. Perrenod, 2016

NEW BOOK just released:

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


Primordial Black Holes as Dark Matter?

LIGO Gravitational Wave Detection Postulated to be Due to Primordial Black Holes

Dark matter remains elusive, with overwhelming evidence for its gravitational effects, but no confirmed direct detection of exotic dark matter particles.

Another possibility which is being re-examined as an explanation for dark matter is that of black holes that formed in the very early universe, which in principle could be of very small mass, or quite large mass. And they may have initially formed at smaller masses and then aggregated gravitationally to form larger black holes.

Recently gravitational waves were discovered for the first time, by both of the LIGO instruments, located in Louisiana and in Washington State. The gravitational wave signal (GW150914) indicates that the source was a pair of black holes, of about 29 and 36 solar masses respectively, spiraling together into a single black hole of about 62 solar masses. A full 3 solar masses’ worth of gravitational energy was radiated way in the merger. Breaking news: LIGO has just this month announced gravitational waves from a second black hole binary of 22 solar masses total. One solar mass of energy was radiated away in the merger.


Image credit: NASA/JPL,

Most of the black holes that we detect (indirectly, from their accretion disks) are stellar-sized in the range of 10 to 100 solar masses and are believed to be the evolutionary endpoints of massive stars. We detect them when they are surrounded by accretion disks of hot luminous matter outside of their event horizons. The other main category of black holes exceeds a million solar masses and can even be more than a billion solar masses, and are known as supermassive black holes.

It is possible that some of the stellar-sized and even elusive intermediate black holes were formed in the Big Bang. Such black holes are referred to as primordial black holes. There are a variety of theoretical formation mechanisms, such as cosmic strings whose loops in all dimensions are contained within the event horizon radius (Schwarzschild radius). In general such primordial black holes (PBHs) would be distributed in a galaxy’s halo, would interact rarely and not have accretion disks and thus would not be detectable due to electromagnetic radiation. That is, they would behave as dark matter.

Dr. Simon Bird and coauthors have recently proposed that the gravitational wave event (GW150914) could be due to two primordial black holes encountering each other in a galactic halo, radiate enough of their kinetic energy away in gravity waves to become bound to each other and inspiral to a single black hole with a final burst of gravitational radiation. The frequency of events is estimated to be of order a few per year per cubic Gigaparsec (a Gigaparsec is 3.26 billion light years), if the dark matter abundance is dominated by PBHs.

While low-mass PBHs have been ruled out for the most part, except of a window around one one-hundred millionth of a solar mass, the authors suggest a window also remains for PBHs in the range from 20 to 100 solar masses.

Dr. A. Kashlinsky has gone further to suggest that the cosmic infrared background (CIB) of unresolved 2 to 5 micron near-infrared sources is due to PBHs. In this case the PBHs would be the dominant dark matter component in galactic halos and would mediate early star and galaxy formation. Furthermore there is an unresolved soft cosmic X-ray background which appears to be correlated with the CIB.

This would be a trifecta, with PBHs explaining much or most of the dark matter, the CIB and the soft-X-Ray CXB! But at this point it’s all rather speculative.

The LIGO instruments are now upgraded to Advanced LIGO and as more gravitational wave events are detected due to black holes, we can gain further insight into this possible explanation for dark matter, in whole or in part. Improved satellite born experiments to further resolve the CIB and CXB will also help to explore this possibility of PBHs as a major component to dark matter.


S. Bird et al. arXiv:1603.00464v2 “Did LIGO detect Dark Matter”

A. Kashlinksy arXiv:1605.04023v1 “LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies” – “It’s Confirmed! Black Holes Do Come in Medium Sizes”

Video (artist’s representation) of inspiral and merger of binary black hole GW151226 (second gravitational wave detection):

NEW BOOK just released:

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