# Category Archives: Early Universe

## Hexaquark Dark Matter: Bosons, but not WIMPy at all

Dibaryons

Imagine you smash a proton and neutron together. What do you get? Typically you get a deuteron which is the nucleus of deuterium, heavy hydrogen. Deuterium has one electron in its neutral atomic state. And it has two baryons, the proton and neutron, so it is known as a dibaryon.

Now as you have heard, protons and neutrons are really quark triplets, held together by gluons in bound configurations. A proton has two up quarks (electric charge +2/3) and a down quark (charge -1/3) for a net charge of +1 and a neutron has two down quarks and an up quark for a net charge of 0.

These are the two lightest quarks and protons and neutrons are by far the dominant components in the ordinary matter in the universe, mostly as hydrogen and helium.

Quarks, protons, and neutrons are all fermions, particles with half-integer spins (1/2, 3/2, -1/2, etc.).

The other main class of particles is called bosons, and that class includes photons, gluons, the W and Z of the weak interaction, and the never directly observed graviton. They all have integer spins (typically 1, but 0 for the Higgs boson, and 2 for the graviton).

Figure 1: The Standard Model major particles: quarks (purple), leptons (green), force carrier bosons (orange), Higgs boson (yellow) with mass, charge, spin indicated.

Six quarks in a Bag

Suppose you collided a proton and neutron together, each with three quarks, and you ended up with a single six quark particle that was stable. It would be a more exotic type of dibaryon. It would have three up quarks, three down quarks, and it would not be a fermion. It would be a boson, with integer spin, spin 0 or 1, in this case. It would be six quarks in a bag, a bound state held together by gluons.

Figure 2. Six quarks in a bag, a hexaquark

Figure 3. The d* resonance at 2.38 GeV, observed at the Cooler Synchrotron in Julich, Germany

Such a particle has been discovered in the past decade, and is named the d* hexaquark. It is seen as the resonance in Figure 3 above, found in proton-neutron collisions, and has a mass of 2.38 GeV (for reference the proton mass is 0.935 GeV and the neutron mass is 0.938 GeV). It decays to a deuteron and two pions, either neutral as shown in the figure, or charged pions.

It is also possible to produce a d* by irradiating a deuteron with a gamma ray.

The d* was already predicted by the famed mathematician and physicist Freeman Dyson in 1964, working with his collaborator Xuong. Their mass estimate was quite close at 2.35 GeV, using a simple quark model.

Dyson just passed away recently; you may have heard of his Dyson sphere concept. The idea is that an advanced civilization would build a sphere of solid material surrounding its star in order to hold an extremely large population and absorb virtually all of the star’s energy. Larry Niven modified this to a ring in his 1970 sci-fi novel Ringworld.

Hexaquark dark matter

Azizi, Ageav, and Sundu have recently suggested a hexaquark of the form uuddss, that is, two up, two down, and two strange quarks. Their mass estimate is around 1.2 GeV, half that of the d* composed of only up and down quarks. It is expected to be stable with long lifetime.

And also recently, Bashkanov and Watts at the University of York have made a nice proposal that d* could be the dark matter particle. The d* particle is itself unstable, but they propose that stable condensates with many d* particles could form. Their paper,  “A New Possibility for Light-Quark Dark Matter” is here:

https://iopscience.iop.org/article/10.1088/1361-6471/ab67e8/pdf

The d* has one great advantage over the other proposed particles, it has actually been discovered! The d* has a good sized mass for a dark matter candidate, at about 2.5 times the mass of the proton.

The authors find that the d* could form lengthy chains or spherical condensates with thousands to millions of d* particles. Unlike individual d* particles, the condensates could be stable ‘super atoms’ lasting for billions of years.

However to make this work the binding energy would have to exceed the difference between the 2.38 GeV d* mass and the deuteron mass of 2.014mGeV, thus would have to be greater than about 0.4 GeV.

The d* would be produced thermally when the universe was at temperatures in the range from 1 to 3 trillion Kelvins. The condensates would need to form quickly before individual d* particles of short lifetimes decayed away.

The favored candidates for dark matter have been WIMPs, supersymmetric particles. But no supersymmetric particle has ever been detected at the Large Hadron Collider or elsewhere, which is incredibly disappointing for many particle physicists. The other main candidates have been the axion and sterile neutrino, both quite low in mass. These have never been directly detected either; they remain hypothetical.

The d* particle is a boson, and the authors’ theoretical approach is that in the early universe as it cooled, both baryons and dibaryonic matter froze out. The baryons ended up, after the cosmic nucleosynthesis phase as protons, deuterium dibaryons, and helium nuclei (alpha particles, that are composed essentially of two deuterons), the main constituents of ordinary matter.

What would happen to d* under the early conditions of the Big Bang? Bosons like to clump together, into something called Bose-Einstein condensates. Yes, that Einstein. And that Boson. Bose-Einstein statistics were developed in the 1920s and govern the statistics of bosons (integer spin particles), and differ from that of fermions.

To confirm this model would require astronomical observations or cosmic ray observations. Decays of d* particles could result in gamma ray production with energies up to 0.5 GeV. Their decay products might also be seen as upward moving cosmic rays, in Earth-bound cosmic ray experiments. These would be seen coming up through the Earth, unlike normal cosmic rays that cannot penetrate so much ordinary matter, and the decay events would result in gamma rays, nucleons and deuterons, as well as pions as the decay products.

## Does Dark Energy Vary with Time?

Einstein introduced the concept of dark energy 100 years ago.

The Concordance Lambda-Cold Dark Matter cosmology appears to fit observations of the cosmic microwave background and other cosmological observations including surveys of large-scale galaxy grouping exceedingly well.

In this model, Lambda is shorthand for the dark energy in the universe. It was introduced as the greek letter Λ into the equations of general relativity, by Albert Einstein, as an unvarying cosmological constant.

Measurements of Λ indicate that dark energy accounts for about 70% of the total energy content of the universe. The remainder is found in dark matter and ordinary matter, and about 5/6 of that is in the form of dark matter.

Alternative models of gravity, with extra gravity in very low acceleration environments, may replace apparent dark matter with this extra gravity, perhaps due to interaction between dark energy and ordinary matter.

The key point about dark energy is that while it has a positive energy, it rather strangely has a negative pressure. In the tensor equations of general relativity the pressure terms act as a negative gravity, driving an accelerated expansion of the universe.

In fact our universe is headed toward a state of doubling in scale in each dimension every 11 or 12 billion years. In the next trillion years we are looking at 80 or 90 such repeated doublings.

That assumes that dark energy is constant per volume over time, with a value equivalent to two proton – antiproton pair annihilations per cubic meter (4 GeV / m³).

But is it?

The Dark Energy Survey results seem to say so. This experiment looked at 26 million galaxies for the clustering patterns, and also gravitational lensing (Einstein taught us that mass bends light paths).

They determined the parameter w for dark energy and found it to be consistent with -1.0 as expected for the cosmological constant model of unvarying dark energy. See this blog for details:

https://darkmatterdarkenergy.com/2017/08/10/dark-energy-survey-first-results-canonical-cosmology-supported/

The pressure – energy density relation is:

$P = w \cdot \rho \cdot c^2$

The parameter w elucidates the relation between the energy density given by ρ and the pressure P. This is called the equation of state. Matter and radiation have w >= 0. In order to have dark energy with a negative pressure dominating, then w should be < -1/3. And w = -1 gives us the cosmological constant form.

Image credit: www.scholarpedia.org

Cosmologists seek to determine w, and whether it varies over time scales of billions of years.

The Concordance model is not very well tested at high redshifts with z > 1 (corresponding to epochs of the universe less than half the current age) other than with the cosmic microwave background data. Recently two Italian researchers, Risaliti and Lusso have examined datasets of high-redshift quasars to investigate whether the Concordance model fits.

Typically supernovae are employed for the redshift-distance relation, and cosmological models are tested against the observed relationship, known as the Hubble diagram. The authors use X-ray and ultraviolet fluxes of quasars to extend the diagram to high redshifts (greater distances, earlier epochs), and calibrate observed quasar luminosities with the supernovae data sets.

Their analysis drew from a sample of 1600 quasars with redshifts up to 5 and including a new sample of 30 high redshift z ~ 3 quasars, observed with the European XMM-Newton satellite.

They claim a 4 standard deviation variance for z > 2, a reasonably high significance.

Models with a varying w include quintessence models, with time-varying scalar fields. If w decreases below -1, it is known as phantom energy. Their results are suggestive of a value of w < -1, corresponding to a dark or phantom energy increasing with time.

For convenience cosmologists introduce a second parameter for possible evolution in w, writing as:

w = w0 + wa*(1-a)   ,where a, the scale factor equals 1/(1+z) and a = 1 for present day.

The best fit results for their analysis are with w0 = -1.4 and wa ~ 1, but these results have large errors, as shown in Figure 4 above, from their paper. Their results are within the red (2 standard deviation, or σ) and orange (3σ) contours. The outer 3σ contours almost touch the cosmological constant point that has w0 = -1 and wa = 0.

These are intriguing results that require further investigation. They are antithetical to quintessence models, and apparently in tension with a simple cosmological constant.

The researchers plan on further analysis in future work by including Baryon Acoustic Oscillation (large scale galaxy clustering) measurements at z > 2.

References

https://darkmatterdarkenergy.com/2017/08/10/dark-energy-survey-first-results-canonical-cosmology-supported/ – Results from Dark Energy Survey of galaxies

Risaliti, G. and Lusso, E. 2018 Cosmological constraints from the Hubble diagram of quasars at high redshifts https://arxiv.org/abs/1811.02590

## Dark Ages, Dark Matter

Cosmologists call the first couple of hundred million years of the universe’s history the Dark Ages. This is the period until the first stars formed. The Cosmic Dawn is the name given to the epoch during which these first stars formed.

Now there has been a stunning detection of the 21 centimeter line from neutral hydrogen gas in that era. Because the first stars are beginning to form, their radiation induces the hyperfine transition for electrons in the ground state orbitals of hydrogen. This radiation undergoes a cosmological expansion of around a factor of 18 since the era of the Cosmic Dawn. By the time it reaches us, instead of being at the laboratory frequency of 1420 MHz, it is at around 78 MHz.

This is a difficult frequency at which to observe, since the region of spectrum is between the TV and FM bands in the U.S. and instrumentation itself is a source of radio noise. Very remote, radio quiet, sites are necessary to minimize interference from terrestrial sources, and the signal must be picked out from a much stronger cosmic background.

Image credit: CSIRO-Australia and EDGES collaboration, MIT and Arizona State University. EDGES is funded by the National Science Foundation.

This detection was made in Western Australia with a radio detector known as EDGES, that is sensitive in the 50 to 100 MHz range. It is surprisingly small, roughly the size of a large desk. The EDGES program is a collaboration between MIT and Arizona State University.

The researchers detected an absorption feature beginning at 78 MHz, corresponding to a redshift of 17.2 (1420/78 = 18.2 = 1 + z, where z is redshift) and for  the canonical cosmological model it corresponds to an age of the universe of 180 million years.

The absorption feature is much stronger than expected from models, implying a lower gas temperature than expected.

At that redshift the cosmic microwave background temperature is at 50 Kelvins (at the present era it is only 2.7 Kelvins). The neutral hydrogen feature is seen in absorption against the warmer cosmic microwave background, and is much cooler (both its ‘spin’ and ‘kinetic’ temperatures).

This neutral hydrogen appears to be at only 3 Kelvins. Existing models had the expectation that it would be at around 7 Kelvins or even higher. (A Kelvin degree equals a Celsius degree, but has its zero point at absolute zero rather than water’s freezing temperature).

In a companion paper, it has been proposed that interactions with dark matter kept the hydrogen gas cooler than expected. This would require an interaction cross section between dark matter and ordinary matter (non- gravitational interaction, perhaps due to the weak force) and low velocities and low masses for dark matter particles. The mass should be only a few GeV (a proton rest mass is .94 GeV). Most WIMP searches in Earth-based labs have been above 10 GeV.

These results need to be confirmed by other experiments. And the dark matter explanation is speculative. But the door has been opened for Cosmic Dawn observations of neutral hydrogen as a new way to hunt for dark matter.

References:

“A Surprising Chill before the Cosmic Dawn” https://www.nature.com/articles/d41586-018-02310-9

EDGES science: http://loco.lab.asu.edu/edges/edges-science/

EDGES array and program: https://www.haystack.mit.edu/ast/arrays/Edges/

R. Barkana 2018, “Possible Interactions between Baryons and Dark Matter Particles Revealed by the First Stars” http://www.nature.com/articles/nature25791

## Distant Galaxy Rotation Curves Appear Newtonian

One of the main ways in which dark matter was postulated, primarily in the 1970s, by Vera Rubin (recently deceased) and others, was by looking at the rotation curves for spiral galaxies in their outer regions. Although that was not the first apparent dark matter discovery, which was by Fritz Zwicky from observations of galaxy motion in the Coma cluster of galaxies during the 1930s.

Most investigations of spiral galaxies and star-forming galaxies have been relatively nearby, at low redshift, because of the difficulty in measuring these accurately at high redshift. For what is now a very large sample of hundreds of nearby galaxies, there is a consistent pattern. Galaxy rotation curves flatten out.

M64, image credit: NASA, ESA, and the Hubble Heritage Team (AURA/STScI)

If there were only ordinary matter one would expect the velocities to drop off as one observes the curve far from a galaxy’s center. This is virtually never seen at low redshifts, the rotation curves consistently flatten out. There are only two possible explanations: dark matter, or modification to the law of gravity at very low accelerations (dark gravity).

Dark matter, unseen matter, would case rotational velocities to be higher than otherwise expected. Dark, or modified gravity, additional gravity beyond Newtonian (or general relativity) would do the same.

Now a team of astronomers (Genzel et al. 2017) have measured the rotation curves of six individual galaxies at moderately high redshifts ranging from about 0.9 to 2.4.

Furthermore, as presented in a companion paper, they have stacked a sample of 97 galaxies with redshifts from 0.6 to 2.6  to derive an average high-redshift rotation curve (P. Lang et al. 2017). While individually they cannot produce sufficiently high quality rotation curves, they are able to produce a mean normalized curve for the sample as a whole with sufficiently good statistics.

In both cases the results show rotation curves that fall off with increasing distance from the galaxy center, and in a manner consistent with little or no dark matter contribution (Keplerian or Newtonian style behavior).

In the paper with rotation curves of 6 galaxies they go on to explain their falling rotation curves as due to “first, a large fraction of the massive high-redshift galaxy population was strongly baryon-dominated, with dark matter playing a smaller part than in the local Universe; and second, the large velocity dispersion in high-redshift disks introduces a substantial pressure term that leads to a decrease in rotation velocity with increasing radius.”

So in essence they are saying that the central regions of galaxies were relatively more dominated in the past by baryons (ordinary matter), and that since they are measuring Hydrogen alpha emission from gas clouds in this study that they must also take into account the turbulent gas cloud behavior, and this is generally seen to be larger at higher redshifts.

Stacy McGaugh, a Modified Newtonian Dynamics (MOND) proponent, criticizes their work saying that their rotation curves just don’t go far enough out from the galaxy centers to be meaningful. But his criticism of their submission of their first paper to Nature (sometimes considered ‘lightweight’ for astronomy research results) is unfounded since the second paper with the sample of 97 galaxies has been sent to the Astrophysical Journal and is highly detailed in its observational analysis.

The father of MOND, Mordehai Milgrom, takes a more pragmatic view in his commentary. Milgrom calculates that the observed accelerations at the edge of these galaxies are several times higher than the value at which rotation curves should flatten. In addition to this criticism he notes that half of the galaxies have low inclinations, which make the observations less certain, and that the velocity dispersion of gas in galaxies that provides pressure support and allows for lower rotational velocities, is difficult to correct for.

As in MOND, in Erik Verlinde’s emergent gravity there is an extra acceleration (only apparent when the ordinary Newtonian acceleration is very low) of order. This spoofs the behavior of dark matter, but there is no dark matter. The extra ‘dark gravity’ is given by:

$g _D = sqrt {(a_0 \cdot g_B / 6 )}$

In this equation a0 = c*H, where H is the Hubble parameter and gB is the usual Newtonian acceleration from the ordinary matter (baryons). Fundamentally, though, Verlinde derives this as the interaction between dark energy, which is an elastic, unequilibrated medium, and baryonic matter.

One could consider that this dark gravity effect might be weaker at high redshifts. One possibility is that density of dark energy evolves with time, although at present no such evolution is observed.

Verlinde assumes a dark energy dominated de Sitter model universe for which the cosmological constant is much larger than the matter contribution and approaches unity, Λ = 1 in units of the critical density. Our universe does not yet fully meet that criteria, but has Λ about 0.68, so it is a reasonable approximation.

At redshifts around z = 1 and 2 this approximation would be much less appropriate. We do not yet have a Verlindean cosmology, so it is not clear how to compute the expected dark gravity in such a case, but it may be less than today, or greater than today. Verlinde’s extra acceleration goes as the square root of the Hubble parameter. That was greater in the past and would imply more dark gravity. But  in reality the effect is due to dark energy, so it may go with the one-fourth power  of an unvarying cosmological constant and not change with time (there is a relationship that goes as H² ∝ Λ in the de Sitter model) or change very slowly.

At very large redshifts matter would completely dominate over the dark energy and the dark gravity effect might be of no consequence, unlike today. As usual we await more observations, both at higher redshifts, and further out from the galaxy centers at moderate redshifts.

References:

R. Genzel et al. 2017, “Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago”, Nature 543, 397–401, http://www.nature.com/nature/journal/v543/n7645/full/nature21685.html

P. Lang et al. 2017, “Falling outer rotation curves of star-forming galaxies at 0.6 < z < 2.6 probed with KMOS^3D and SINS/ZC-SINF” https://arxiv.org/abs/1703.05491

Mordehai Milgrom 2017, “High redshift rotation curves and MOND” https://arxiv.org/abs/1703.06110v2

Erik Verlinde 2016, “Emergent Gravity and the Dark Universe” https;//arXiv.org/abs/1611.02269v1

## 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, http://www.nasa.gov/jpl/nustar/pia18842

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.

References:

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”

http://www.space.com/26857-medium-size-black-hole-discovery-m82.html – “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): https://youtu.be/KwbXxzgAObU

NEW BOOK just released:

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

## Most Distant Galaxy Known: over 95% of the way back to the origin

Recently, a team of astronomers from the U.S., U.K. and The Netherlands have confirmed the most distant galaxy known. This galaxy had previously been estimated to have a redshift of z = 8.57, from photometric methods, that is, from the general shape of the spectrum.

Image: Hubble Space Telescope, NASA/STScI

More accurate redshifts are obtained by measuring particular emission or absorption lines, which have precisely known laboratory (z = 0) wavelengths.

The team measured Lyman alpha line emission, and have determined the redshift to be z = 8.68, in good agreement with the photometric redshift. The Lyman alpha line is a main transition line in neutral hydrogen that occurs at 1216 Angstroms (.1216 microns) in the rest frame. The authors observed the line in the infrared and centered at 11,776 Angstroms (1.1776 microns) on 2 separate observing nights, detecting the Lyman alpha line each night. The redshift is given by 1 + z = 11,776/1216 = 9.68, thus z for this galaxy is 8.68.

The galaxy image is thought to be somewhat magnified by intervening dark matter gravitational lensing, but less than a factor of 2, and perhaps only around 20%.

The significance here is in the detection of Lyman alpha at such a high redshift, corresponding to a time when the universe was only 600 million years old, less than 5% of its current age. Not only does this result determine the age of this earliest known galaxy, but it also provides insight into the nature of the intergalactic medium.

The cosmic microwave background radiation is the most distant source we can see. It comes from all directions, filling the universe and reflects a time when the universe was only 380,000 years old and transitioned from ionized plasma to neutral hydrogen and helium.

Later on in the universe’s evolution, as the first galaxies and stars form, hot blue stars produce ionizing ultraviolet radiation, and the neutral gas is reionized – electrons are stripped from their atoms. This process has generally thought to have completed by redshift ~ 6, at a time when the universe was around 1 billion years old.

Lyman alpha emission is not expected in a region which is still neutral, that has not yet undergone the reionization process. So the implication here is that the surrounding intergalactic medium in the neighborhood of EGSY8p7 has already been reionized at a significantly higher redshift.

The universe does not become reionized in a uniform way, rather the process would be expected to happen in “bubbles” or regions surrounding energetic galaxies with hot blue stellar populations. Eventually all the ionized regions overlap and the intergalactic medium becomes fully ionized.

This detection helps astronomers to better understand how reionization occurred.

The team’s paper is submitted to the Astrophysical Journal Letters and can be found here:

http://arxiv.org/pdf/1507.02679v2.pdf