Islamic Astronomy

Islamic Astronomy

During the period when Western civilization was experiencing the dark ages, between 700-1200 A.D., an Islamic empire stretched from Central Asia to southern Europe. Scholarly learning was highly prized by the people, and they contributed greatly to science and mathematics. Many classical Greek and Roman works were translated into Arabic, and scientists expanded on the ideas. For instance, Ptolemy's model of an earth-centered universe formed the basis of Arab and Islamic astronomy, but several Islamic astronomers made observations and calculations which were considerably more accurate than Ptolemy's. Perhaps the most fascinating aspect of Islamic astronomy is the fact that it built on the sciences of two great cultures, the Greek and the Indian. Blending and expanding these offen different ideas led to a new science which later profoundly influenced Western scientific exploration beginning in the Renaissance.
Purposes of Islamic Astronomy Perhaps the most vital reason that the Muslims studied the sky in so much detail was for the purpose of time-keeping. The Islamic religion requires believers to pray five times a day at specified positions of the sun. Astronomical time-keeping was the most accurate way to determine when to pray, and was also used to pin-point religious festivals. The Muslim holy book, the Koran, makes frequent reference to astronomical patterns visible in the sky, and is a major source of the traditions associated with Islamic astronomy. Another important religious use for astronomy was for the determination of latitude and longitude. Using the stars, particularly the pole star, as guides, several tables were compiled which calculated the latitude and longitude of important cities in the Islamic world. Using this information, Muslims could be assured that they were praying in the direction of Mecca, as specified in the Koran. Aside from religious uses, astronomy was used as a tool for navigation. The astrolabe, an instrument which calculated the positions of certain stars in order to determine direction, was invented by the Greeks and adopted and perfected by the Arabs (see picture below). The sextant was developed by the Arabs to be a more sophisticated version of the astrolabe. This piece of technology ultimately became the cornerstone of navigation for European exploration.
Great Islamic Astronomers Science was considered the ultimate scholarly pursuit in the Islamic world, and it was strongly supported by the nobility. Most scientists worked in the courts of regional leaders, and were financially rewarded for their achievements. In 830, the Khalifah, al-Ma'muun, founded Bayt-al-Hikman, the 'House of Wisdom', as a central gathering place for scholars to translate texts from Greek and Persian into Arabic. These texts formed the basis of Islamic scientific knowledge. One of the greatest Islamic astronomers was al-Khwarizmi (Abu Ja'far Muhammad ibn Musa Al-Khwarizmi), who lived in the 9th century and was the inventor of algebra. He developed this mathematical device completely in words, not mathematical expressions, but based the system on the Indian numbers borrowed by the Arabs (what we today call Arabic numerals). His work was translated into Latin hundreds of years later, and served as the European introduction to the Indian number system, complete with its concept of zero. Al-Khwarizmi performed detailed calculations of the positions of the Sun, Moon, and planets, and did a number of eclipse calculations. He constructed a table of the latitudes and longitudes of 2,402 cities and landmarks, forming the basis of an early world map. Another Islamic astronomer who later had an impact on Western science was al-Farghani (Abu'l-Abbas Ahmad ibn Muhammad ibn Kathir al-Farghani). In the late 9th century, he wrote extensively on the motion of celestial bodies. Like most Islamic astronomers, he accepted the Ptolemaic model of the universe, and was partially responsible for spreading Ptolemaic astronomy not only in the Islamic world but also throughout Europe. In the 12th century, his works were translated into Latin, and it is said that Dante got his astronomical knowledge from al-Farghani's books. In the late 10th century, a huge observatory was built near Tehran, Iran by the astronomer al-Khujandi. He built a large sextant inside the observatory, and was the first astronomer to be capable of measuring to an accuracy of arcseconds. He observed a series of meridian transits of the Sun, which allowed him to calculate the obliquity of the ecliptic, also known as the tilt of the Earth's axis relative to the Sun. As we know today, the Earth's tilt is approximately 23o34', and al-Khujandi measured it as being 23o32'19". Using this information, he also compiled a list of latitudes and longitudes of major cities. Omar Khayyam (Ghiyath al-Din Abu'l-Fath Umar ibn Ibrahim al-Nisaburi al-Khayyami) was a great Persian scientist, philosopher, and poet who lived from 1048-1131. He compiled many astronomical tables and performed a reformation of the calendar which was more accurate than the Julian and came close to the Gregorian. An amazing feat was his calculation of the year to be 365.24219858156 days long, which is accurate to the 6th decimal place! Western science owes a large debt to Islamic and Arab scientists, whose contributions range from the Arabic names of stars which we still use today to the mathematical and astronomical treatices used by Europeans to enter our modern world of science.

History of the Universe

OVERVIEW ~ Big Bang Theory – an introduction to the backbone of cosmology ~ Testing the Big Bang Model – theories and experiments throughout the years which have supported the idea of a Big Bang ~ Cosmic Microwave Background radiation – a snapshot of the early universe which is shedding light on the Big Bang ~ WMAP project - a brand new mission designed to unlock the mysteries of the universe Questions to investigate: What is the content of the universe? What is the universe’s expansion rate? Is it accelerating or decelerating? When did the first stars form? What is the shape of the universe? How old is the universe? What will be the fate of the universe?
BIG BANG THEORY The Big Bang Model is widely accepted as a general description of the formation and evolution of the universe, and is continually tested with observations. 12-14 billion years ago, the diameter of the universe was a few millimeters. It quickly experienced an expansion and cooling which continues today. Remnants of early hot dense matter can still be seen today as cosmic microwave background radiation (CMB). The COBE satellite, launched in 1989, was the first attempt to map Big Bang radiation. The new WMAP satellite, launched in February 2003, has even more resolution and sensitivity, leading to dramatic increases in our understanding of the fundamentals of the early universe.
MISCONCEPTIONS ABOUT BIG BANG THEORY The Big Bang did NOT occur as an explosion at a single point in space! Questions beyond the realm of the Big Bang Model include: ~ What happened before the Big Bang? ~ What ‘caused’ the Big Bang?  ~ What is the universe expanding into? Forces described in table below: G = gravity, EM = electromagnetic, WN = weak nuclear, SN = strong nuclear
FOUNDATIONS OF THE BIG BANG MODEL Big Bang Theory = General Theory of Relativity + Cosmological Principle Einstein's General Theory of Relativity (1916) is a generalization of Newton’s Law of Gravity. Gravity is described as a distortion  of space and time. The Cosmological Principle is an assumption that matter in the universe is uniformly distributed when  averaging over large-scales, and that the distribution of matter is homogeneous and isotropic.
THE COSMOLOGICAL CONSTANT The first version of relativity predicted expansion. Einstein added the cosmological constant lambda to stop the expansion. After the experimental discovery of expansion, Einstein declared that adding lambda was ‘his greatest mistake’. Was lambda really a mistake? Today there is discussion of reviving the cosmological constant as a term associated with the energy density of the vacuum. Dark energy associated with the cosmological constant could help explain the accelerating expansion and the fate of the universe!
GEOMETRY OF THE UNIVERSE What determines the shape of the universe? » Average density of matter Assuming the cosmological principle holds, the universe can have one of three shapes, as shown on the right: closed, open, or flat. Critical density ~ 6 H atoms/m^3.
TYPES OF MATTER IN THE UNIVERSE Radiation » massless and nearly massless particles that move at the speed of light (photons, neutrinos) Baryonic Matter » ordinary matter (protons, neutrons, electrons) Dark Matter » exotic non-baryonic matter that interacts weakly with baryonic matter (never directly observed in laboratory) Dark Energy » mysterious, only type of matter that could cause expansion to accelerate, linked to cosmological constant » How much of each type of matter is there?
TESTING THE BIG BANG MODEL Theoretical and experimental tests of the Big Bang Theory have been performed since 1929. ~ Hubble’s expansion law ~ Cosmic microwave background radiation ~ COBE and WMAP experiments » All indicate reliability of Big Bang Theory!
EXPANSION OF THE UNIVERSE In 1929, Hubble found that galaxies outside our own are moving away from us with a speed proportional to their distance from us. How did Hubble find distances to far-away galaxies? Stars similar to Cepheid variables were used as distance markers. Hubble's Law: velocity = Hubble constant * distance. Recent estimates of the Hubble constant show that it is between 50 km/sec/Mpc < H < 100 km/sec/Mpc.
COSMIC MICROWAVE BACKGROUND Cosmic Microwave Background Radiation (CMB) ~ remnant heat from the Big Bang 1948 : CMB predicted by Gamow 1950 : CMB predicted by Alpher and Herman 1965 : CMB observed as noise in a radio receiver built by Penzias and Wilson 1965 : Paper on observations by Penzias and Wilson, paper on cosmological interpretation by Dicke, Peebles, Roll, and Wilkinson 1978 : Penzias and Wilson receive Nobel prize in physics ~ The CMB has a very uniform temperature across the entire sky of ~2.725 K. ~ CMB maps are snapshots from 380,000 years after the Big Bang, the last time that CMB photons directly scattered off matter. ~ The COBE and WMAP satellites have provided maps of the CMB that show tiny fluctuations in the temperature, which represents fluctuations in the density of matter in the early universe.
CMB RADIATION: COBE VS. WMAP
WMAP: THE SPACECRAFT Goal of WMAP ~ to map the relative CMB temperature over the full sky Technical Specifications: ~ two back-to-back symmetric reflector telescopes focus microwave radiation into receivers ~ angular resolution = 0.3o  ~ sensitivity = 20 mK per 0.3o square pixel  ~ instrumental artifacts limited to 5 mK per pixel
WMAP: THE ORBIT L2 orbit ~ Lissajous orbit about Sun-Earth Lagrange point (position where combined gravitational pull of Earth and Sun equals the centripetal force required to rotate with them), 1.5 million km from Earth. This special orbit provides the following benefits: ~ protection from Earth’s microwave emission and magnetic field  ~ a stable thermal environment ~ the Sun, Earth, and Moon are always behind instrument’s field of view
WMAP: THE SCIENCE The format of a WMAP map is similar to looking at an oval map of the whole earth. Microwave radiometers scan ~30% of the sky each day, and the full sky is scanned every six months. WMAP records five separate frequency bands from 22-90 GHz. The five frequency-dependent maps are compiled into one, and microwave emission from the Milky Way is subtracted out. This procedure is seen on the right.
BEYOND THE BIG BANG MODEL How do we explain the temperature fluctuations in CMB? ~ Go BEYOND the Big Bang Theory! The cosmological principle, an integral part of the Big Bang Model, assumes a uniform distribution of matter on global and local scales. So why are there local structures like galaxies in ‘empty’ space? Big Bang Theory does not answer these questions!
ORIGIN OF STRUCTURE Why did galaxies form? ~ Structure grew from the gravitational pull of small fluctuations in the quasi- uniform density of the early universe. The time sequence at the right shows how galaxies eventually formed beginning with the small clumpings of matter.
INFLATION THEORY This theory was developed by Guth, Linde, Steinhardt, and Albrecht as an extension to the Big Bang Theory. Proposals of Inflation Theory ~ there was a period of extremely rapid expansion just after the Big Bang  ~ during this time period, the energy density of the universe was dominated by a cosmological constant term Predictions of Inflation Theory ~ the density of the universe is close to critical density  ~ the geometry of the universe is flat and infinite  ~ there are equal numbers of hot and cold spots in the CMB radiation WMAP will directly test these predictions!
PUTTING THE PUZZLE PIECES TOGETHER WMAP is working to compile a list of properties and characteristics of the universe: ~ Abundance of different types of matter  ~ Expansion (Hubble constant; accelerating, decelerating?)  ~ Origin of structure  ~ Age ~ Shape (open, closed, flat; finite, infinite?)  ~ Ultimate fate
MATTER IN THE UNIVERSE Mass discrepancy: the mass inferred for most galaxies is 10 times larger than the mass associated with stars, gas, and dust. This has been confirmed by observations of gravitational lensing, the bending of light predicted by relativity. An example of gravitational lensing is shown in the Hubble photograph at the right.
	Dark matter candidates: ~ MACHOs (MAssive Compact Halo Objects)  ~ supermassive black holes  ~ WIMPs (Weakly Interacting Massive Particles), new forms of matter
EXPANSION AND ORIGIN OF STRUCTURE WMAP ~ Hubble constant H0 = 71 km/sec/Mpc (+-5%)  This was measured independently of the usual method using Cepheid variables. WMAP ~ Expansion of the universe is accelerating. ‘Cosmological constant matter’ or ‘dark energy’ is critical and accounts for ~73% of the universe’s matter. WMAP ~ Stars ignited 200 million years after Big Bang. Equivalent to the first baby steps in the lifetime of an 80 year old person.
	AGE OF THE UNIVERSE How can we find the age of the universe? ~ determine the age of the oldest stars by measuring the expansion rate of the universe and extrapolating back to the Big Bang. Globular clusters ~ 11-18 billion years old Measure Hubble constant accurately and extrapolate to find ~ 12-14 billion years old WMAP ~ The universe is 13.7 billion years old.
SHAPE AND FATE OF THE UNIVERSE WMAP ~ The universe is flat! Universal geometry is determined by the struggle between the momentum of expansion and the pull of gravity. WMAP ~ The universe will continue to expand forever. ‘Some say the world will end in fire, others say in ice’ - Robert Frost
SUMMARY OF WMAP RESULTS Big Bang Theory + Inflation Theory + Cosmological Constant Term = New Understanding of the Universe!
CONCLUSIONS Big Bang Theory accurately describes many aspects of the universe’s evolution. Current theoretical and experimental research is attempting to add to the Big Bang Theory in order to explain observable phenomena. The WMAP project has recorded a cosmic fingerprint that sheds light on the origin, structure, and fate of the universe.