Wednesday, July 30, 2014

Is Dark Matter Just The Self-Interaction Of Gravitons?

We discuss the correlation between the dark matter content of elliptical galaxies and their ellipticities. We then explore a mechanism for which the correlation would emerge naturally. Such mechanism leads to identifying the dark matter particles to gravitons. A similar mechanism is known in Quantum Chromodynamics (QCD) and is essential to our understanding of the mass and structure of baryonic matter.

Alexandre Deur, "A correlation between the amount of dark matter in elliptical galaxies and their shape" (28 Jul 2014).

Every now and then there is a paper that suggests that the phenomena described as dark matter is really implied by General Relativity, or a trivial variant of General Relativity, and that the discrepancy between theory and observation that astronomers observe is because they inappropriately conclude that in systems like galaxies and galactic clusters that Newtonian gravity is a reasonably accurate approximation of General Relativity.

This is one of the stronger arguments that I have seen for that position.

Most of those papers argue basically that what a Newtonian gravitational approximation (mediated through a stylized approximation of a galaxy's structure or a numerical many body simulation much like those of lattice QCD) is missing is the gravitational effects of the coherent angular momentum of the many bodies in a galaxy as they rotate around a central black hole.  On balance, I've found that the argument that this contribution is tiny is stronger than the argument that this is the main source of dark matter phenomena.

Put another way, they focus on the additional degrees of freedom (i.e. additional amount of information necessary to describe) the behavior of General Relativity (which requires a spin-2 tensor field graviton), rather than the spin-0 scalar field gravitons of Newtonian gravity.

This paper focuses on a different distinction between Newtonian gravitons and General Relativistic ones.  Newtonian gravitons, like real world photons, don't interact with each other.  They couple only to mass and electric charge, respectively.  In contrast, in General Relativity, gravity is a function of both mass and energy.  Thus, gravity can influence particles that have zero rest mass (i.e. photons and gluons), and in principle, gravity may even influence other gravitons.  In this respect gravitons are more like gluons, which interact both with quarks and with each other, and less like photons.

Focusing on this self-interaction of gravitons suggests an effect of approximately the right order of magnitude to account for differences in the amount of "dark matter" effects seen in ellipical galaxies of different sizes.  A previous paper from 2009 argues that the effect is on the right order of magnitude to account for the rotation curves of galaxies and the Tully-Fisher relation, and may also explain, or at least help to explain, dark matter phenomena in galaxy clusters.

I haven't had the time to rigorously review the accuracy of the analysis, but the approach does look like a fruitful one to explore that is understudied. Deur basically claims to have derived a versions of MOND (Milgrom's modified gravity theory) from first principles using a toy model approximation of GR that preserves the essentials of General Relativity in a symmetric and homogeneous case, while proving much more accurate for galactic clusters and that can explain the Bullet Cluster.

An interesting analysis of what would distinguish MOND from MOND-like theories is found in another 2009 paper by another author.

If he's right, he deserves the Nobel prize for physics. If this approach really works, of course, it would also eliminate the phenomenological need for any beyond the Standard Model particles other than the graviton, which is by far the best empirically supported hypothetical particle. Given our failure to discovery any other dark matter candidates, this increasingly looks like a feature rather than a bug.

Deur's approach was also discussed recently at the Physics Forums.

Monday, July 14, 2014

Bubonic Plague In China

Summary

Modern genetic techniques and assimilation of global historical accounts have revealed that the Black Plague that devastated Europe in the Middle Ages, originated in Asia and arrived via the Silk Road trade route.  To recap some key dates (all in the current era):

* Yuan Empire, China 1331
* Hebei Province, China 1334 (the province that surround the Beijing province in northern coastal China)
* Persia 1335
* Kyrgyzstan 1338-1339
* Kaffa, Crimea 1344
* Syria 1345-1348
* Mecca, Saudi Arabia 1349

Kublai Khan's Yuan Empire with a capitol in Beijing, included by 1294 CE, all of modern China except the Tarim Basin, all of Mongolia and Korea, and some of the adjacent territory in what is now Russia to the North of Mongolia and Manchuria. The Yuan Dynasty extended from 1271 CE to 1368 CE, not quite a century, and not long after the plague ravaged China. (The long lived Ming Dynasty originating in Southern China followed the Yuan Dynasty).

Analysis

The role of the expanding plague and the depopulation and chaos left behind in its wake also makes the rapid expansion of the Mongolian Empire resemble much more closely than is commonly realized, the expansion of European conquest in the New World following the arrival of Columbus, and small pox, in 1492 CE. The Golden Horde may have accumulated plague immunity by the time it reached Europe.

The backdrop of the plague also helps explain the population genetic impact of the Mongol Dynasty in Asia, since it swept into a population bottleneck that subsequently expanded. And, it explains the fall of the Yuan Dynasty in China itself.

Sources

According to one not particularly reliable source, which I nonetheless have no particular reason to doubt:
Many scholars believe that the Black Death began in north-western China, while others cite south-western China or the steppes of Central Asia. We do know that in 1331, an outbreak erupted in the Yuan Empire; it may have hastened the end of Mongol rule over China. In 1334, this disease killed 5 million people in Hebei Province - about 90% of the population.  As of 1200, China had a total population of more than 120 million, but a 1393 census found only 65 million Chinese surviving. Some of that missing population was killed by famine and upheaval in the transition from Yuan to Ming rule, but many millions died of bubonic plague.

From its origin at the eastern end of the Silk Road, the Black Death rode trade routes west. At Central Asian caravanseries and Middle Eastern trade centers, it infected people all across Asia. . . .

Ibn al-Wardi, a Syrian writer who would later die of the plague himself in 1348, recorded that the Black Death came out of "The land of Darkness" (Central Asia). From there, it spread to China, India, the Caspian Sea and "land of the Uzbeks," and thence to Persia and the Mediterranean.  The Egyptian scholar Al-Mazriqi noted that "more than three hundred tribes all perished without apparent reason in their summer and winter encampments, in the course of pasturing their flocks and during their seasonal migration." He claimed that all of Asia was depopulated, as far as the Korean Peninsula. (Orent, 106)

The Central Asian scourge struck Persia just a few years after it appeared in China - proof, if any is needed, that the Silk Road was a convenient route of transmission for the deadly bacterium.  In 1335, the Il-Khan (Mongol) ruler of Persia and the Middle East, Abu Said, died of bubonic plague during a war with his northern cousins, the Golden Horde. This signaled the beginning of the end for Mongol rule in the region.  An estimated 30% of Persia's people died of the plague in the mid-14th century. The region's population was slow to recover, in part due to the political disruptions caused by the fall of Mongol rule and the later invasions of Timur (Tamerlane).
Archaeological excavations on the shores of Issyk Kul, a lake in what is now Kyrgyzstan, reveal that the Nestorian Christian trading community there was ravaged by bubonic plague in 1338-39. Issyk Kul was a major Silk Road depot, and has sometimes been cited as the origin point for the Black Death. It certainly is prime habitat for marmots, which are known to carry a virulent form of the plague.  It seems more likely, however, that traders from further east brought diseased fleas with them to the shores of Issyk Kul. Whatever the case, this tiny settlement's death rate shot up from a 150-year average of about 4 people per year, to more than 100 dead in 1338-39. 
Although specific numbers and anecdotes are hard to come by, different chronicles note that Central Asian cities like Talas, in modern-day Kyrgyzstan; Sarai, the capital of the Golden Horde in Russia; and Samarkand, now in Uzbekistan, all suffered outbreaks of the Black Death. It is likely that each population center would have lost at least 40% of its citizens, with some areas reaching death tolls as high as 70%. 
In 1344, the Golden Horde decided to recapture the Crimean port city of Kaffa from the Genoese, Italian traders who had taken the town in the late 1200s. The Mongols under Jani Beg instituted a siege, which lasted until 1347, when reinforcements from further east brought the plague to the Mongol lines.

An Italian lawyer, Gabriele de Mussis, recorded what happened next: "The whole army was affected by a disease which overran the Tartars [Mongols] and killed thousands upon thousands every day." He goes on to charge that the Mongol leader "ordered corpses to be placed in catapults and lobbed into the city in hopes that the intolerable stench would kill everyone inside."

This incident is often cited as the first instance of biological warfare in history. However, other contemporary chroniclers make no mention of the putative Black Death catapults. A French churchman, Gilles li Muisis, notes that a "calamitous disease befell the Tartar army, and the mortality was so great and widespread that scarcely one in twenty of them remained alive." However, he depicts the Mongol survivors as surprised when the Christians in Kaffa also came down with the disease.

Regardless of how it played out in fact, the Golden Horde's siege of Kaffa certainly did drive refugees to flee on ships, bound for Genoa. These refugees likely were a primary source of the Black Death that went on to decimate Europe.

European observers were fascinated but not too worried when the Black Death struck the western rim of Central Asia and the Middle East. One recorded that "India was depopulated; Tartary, Mesopotamia, Syria, Armenia were covered with dead bodies; the Kurds fled in vain to the mountains." However, they would soon become participants rather than observers in the world's worst pandemic.

In "The Travels of Ibn Battuta," the great traveler noted that as of 1345, "the number that died daily in Damascus [Syria] had been two thousand," but the people were able to defeat the plague through prayer. In 1349, the holy city of Mecca was hit by the plague, likely brought in by infected pilgrims on the hajj.

The Moroccan historian Ibn Khaldun, whose parents died of the plague, wrote about the outbreak this way: 
Civilization both in the East and the West was visited by a destructive plague which devastated nations and caused populations to vanish. It swallowed up many of the good things of civilization and wiped them out... Civilization decreased with the decrease of mankind. Cities and buildings were laid waste, roads and way signs were obliterated, settlements and mansions became empty, dynasties and tribes grew weak. The entire inhabited world changed.
This source cites, as its sources:

* Benedictow, Ole Jorgen. The Black Death, 1346-1353: The Complete History (2004).
* Marshall, Robert. Storm from the East: From Ghengis Khan to Khubilai Khan (1993).
* McNeill, William Hardy. Plagues and People (1976).
* Orent, Wendy. Plague: The Mysterious Past and Terrifying Future of the World's Most Dangerous Disease (2004).

Wikipedia cites:

* As evidence of the genetic origin of the plague in China, Nicholas Wade (31 October 2010). "Europe’s Plagues Came From China, Study Finds". The New York Times. Retrieved 1 November 2010. This report was actually based upon: G. Morelli et al. (2010) Nature Genetics. December; 42 (12):1140-1143. doi:10.1038/ng.705.

* Regarding the Lake Issyk Kul outbreak in Kyrgizstan, Raoult; Drancourt (2008). Paleomicrobiology: Past Human Infections. Springer. p. 152.

* The Cambridge History of China: Alien regimes and border states, 907–1368, p.585 for the proposition ("In the 1330s a large number of natural disasters and plagues led to widespread famine, starting in 1331, with a deadly plague arriving soon after."), and

* Kohn, George C. (2008). Encyclopedia of plague and pestilence: from ancient times to the present. Infobase Publishing. p. 31. ISBN 0-8160-6935-2, as evidence of an early Chinese outbreak ("The 14th-century plague killed an estimated 25 million Chinese and other Asians during the 15 years before it reached Constantinople in 1347.", i.e. from 1332-1347 CE).

* But, Wikipedia notes the lack of a clear medical description of the bubonic plague in China until 1644, relying on Sussman GD (2011). "Was the black death in India and China?". Bulletin of the history of medicine 85 (3): 319–55. doi:10.1353/bhm.2011.0054. PMID 22080795.

There is another Wikipedia article here that specifically addresses the migration of the Black Plague.  It cites regarding the Chinese Outbreak:

* McNeill, William H. (1976). Plagues and People. New York: Anchor Books. ISBN 0-385-12122-9.