More Cosmology: Looking Back in Time

ore strange and wonderful theoretical stuff" As we look backwards in time, the universe gets denser and hotter; we know this from the absorption lines in the background radiation, and it fits with our current theory. The densities of galaxies have also evolved as expected.

In the first seconds after the Big Bang, radiation dominated, then matter dominated with formation of galaxies.

Electrons and positrons collided, annihilated one another, and created gamma ray photons. For a brief time, there existed an equilibrium between matter, anti-matter, and radiation. However, there was an excess of 1 matter particle out of every billion particles, so the system was no longer in equilibrium, although this process is not yet well understood.

During the first few minutes, the universe expanded and cooled; protons and neutrons formed a few helium nuclei, the rest of the protons remained as hydrogen nuclei. Free neutrons have a half-life of 15 minutes, so if they get locked up in helium, they're are out of circulation; others wound up decaying to produce more hydrogen. Some of these processes extended beyond first few minutes, but helium is a limiting factor, tying up neutrons.

The radiation dominated era produced hot plasma (ionized gas) and a period of expansion and cooling, but no new synthesis of elements. Photons were incessantly scattered by free electrons, in equilibrium with matter.

Then followed an era of recombination. Protons and electrons recombined to form atoms, in a transition to the matter-dominant era. Temperature fell to a few thousand K, like the surface of a star. Gas was no longer ionized plasma, but neutral hydrogen, which does not react so readily to photons, so the universe became "transparent" to photons. Radiation could penetrate billions of light years and be detected by us. This is where microwave background radiation comes from.

Reionization. After less than 1 billion years, the first stars formed; ultraviolet radiation from the first stars and quasars (emit ionizing radiation) re-ionized the gas in the early universe. Bubbles of ionized gas overlapped, and the universe became opaque again.

Cosmological principle assume: Homogeneity, that the local universe has same physical properties throughout the larger universe, each region has same physical properties (mass density, expansion rate, visible vs. dark matter). Isotropy (no preferred direction); on largest scale, local universe looks the same in any direction. Universality; the laws of physics are the same everywhere in the universe (size scale > 100 million parsecs.)

Space itself seems to posses a "dark energy" or anti-gravity that acts to expand space, an intrinsic energy associated with space/vacuum. We have no clue what this is, but it seems to be the most important energy constituent of the universe today.

Until 6 billion years ago, the gravitational energy of matter was stronger than the acceleration forces, but then dark energy began to dominate energy density of universe. We see an anti-gravity effect that pushes things apart faster and faster. Today, the acceleration due to dark energy dominates, creating an exponential expansion. If the energy density remains constant, we get a future "the big empty." Galaxies will move away faster than the speed of light, so their light will never reach us. Within the Milky Way, however, systems will be held together by gravity, but we won't be able to see beyond our galaxy.

A Medieval Jewish Ethical Will

A discussion over on la_marquise_de's LJ has gotten me thinking about how distance the Middle Ages seem to most Westerners...except Jews. We still study the works of Rashi and Maimonides and so many others, seeing them not as quaint historical curiosities but as vital and profound. Noodling around the 'nets, I found some "ethical wills," instructions to survivors, that were common not only in Jewish communities, but in Muslim and Christian as well.

From The Ethical Will of Judah ibn Tibbon, France, about 1160­-1180 :

You know, my son, ... I have honored you by providing an extensive library for your use, and have thus relieved your of the necessity to borrow books. Most students must bustle about to seek books, often without finding them. But you, thanks be to God, lend and borrow not many books, indeed, you own two or three copies. I have besides made for you books on all sciences, hoping that your hand might find them all as a nest.

Seeing that your Creator had graced you with a wise and understanding heart, I journeyed to the ends of the earth and fetched for you a teacher in secular sciences. I minded neither the expense nor the danger of the ways. Untold evil might have befallen me on those travels, had not the Lord been with us!

Therefore, my son! Stay not your hand ... but devote yourself to the study of the Torah and to the science of medicine. ... Let your countenance shine upon the sons of men; tend their sick and may your advice cure them. Though you take fees from the rich, heal the poor gratuitously; the Lord will requite you. Thereby shall you find favor and good understanding in the sight of God and man. Thus wilt you win the respect of high and low among Jews and non-­Jews, and your good name will go forth far and wide. You wilt rejoice your friends and make your foes envious. For remember what is written in the Choice of Pearls [53:617, of Ibn Gabirol]l: "How shall one take vengeance on an enemy? By increasing one's own good qualities."....

My son! I command your to honor your wife to your utmost capacity. She is intelligent and modest, a daughter of a distinguished and educated family. She is a good housewife and mother, and no spendthrift. Her tastes are simple, whether in food or dress. Remember her assiduous tending of you in your illness, though she had been brought up in elegance and luxury. ... Were she a hired nurse, she would have earned your esteem and forbearance; how much the more, since she is the wife of your bosom, the daughter of the great, art you bound to treat her with consideration and respect. To act otherwise is the way of the contemptible. The Arab philosopher [probably Al­Ghazali, 1058­1112] says of women: "None but the honorable honors them, none but the despicable despises them.".... If you would acquire my love, honor her with all your might.

Let your expenditure be well ordered. It is remarked in the Choice of Pearls [1: 3] "Expenditure properly managed makes half an income." And there is an olden proverb: "Go to bed without supper and rise without debt." Defile not the honor of your countenance by borrowing; may the Creator save your from that habit! ....

Examine your Hebrew books at every New Moon, the Arabic volumes once in two months, and the bound codices once every quarter. [Arabic and Latin were the languages of science in Spain, the Provence, and southern Italy.] Arrange your library in fair order so as to avoid wearying yourself in searching for the book you need. Always know the case and the chest where the book should be. A good plan would be to set in each compartment a written list of the books therein contained. If, then, you art looking for a book, you can see from the list the exact shelf it occupies without disarranging all the books in the search for one. Examine the those leaves in the volumes and bundles, and preserve them. These fragments contain very important matters which I collected and copied out. Do not destroy any writing or letter of all that I have left. And cast your eve frequently over the catalog so as to remember what books are in your library.

... Whatever you have learned from me or from your teachers, impart it again regularly to worthy pupils, so that you may retain it, for by teaching it to others you wilt know it by heart, and their questions will compel your to precision, and remove any doubts from your own mind.

Never refuse to lend books to anyone who has not the means to purchase books for himself, but only act thus to those who can be trusted to return the volumes. Cover the bookcases with rugs of fine quality, and preserve them from damp and mice, and from all manner of injury, for your books are your good treasure. If you lend a volume, make a memorandum before it leaves your house, and when it is returned, draw your pen over the entry. Every Passover and Tabernacles [that is, every six months] call in all books out on loan.

I enjoin on your, my son, to read this, my testament, once daily, at morn or at eve. Apply your heart to the fulfillment of its behests, and to the performance of all therein written. Then wilt you make your ways prosperous, then shall you have good success.