Cosmology

The study of the Universe in its totality, and by extension,

humanity's place in it

Learning Objectives

• At the end of this lecture students will be able to explain:

• The generally accepted scientific view of the structure of our Universe and how this was established through observations.

• How different types of telescope help Astronomers observe the heavens.

The Dark Ages

• After the fall of the Roman Empire, Western Europe fell into a period called the dark ages.

• The Romans had previously established the radius of the earth

• Greek philosopher Plato (427 BCE - 347 BCE) taught his students that Earth was a sphere.

• Indian and Islamic mathematicians/astronomers continued to recognize the earth's sphericity.

• The faltering start to the Western European Renaissance period heralded “our” return to more sophisticated views of the universe!

Nicolaus Copernicus

• Nicolaus Copernicus (19 February 1473 – 24 May 1543) was the first astronomer to formulate a comprehensive heliocentric cosmology, which displaced the Earth from the center of the universe. (as opposed to geocentric)

• Copernicus' epochal book, De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres), published just before his death in 1543, is often regarded as the starting point of modern astronomy

Nicolaus Copernicus

Planetary Motion

• A planet (Greek meaning "wanderer")• If observed from one night to the next a planet

normally appears to move from West to East against the background stars most of the time. Occasionally, however, the planet's motion will appear to reverse direction, and the planet will, for a short time, move from East to West against the background constellations. This reversal is known as retrograde motion, and is illustrated in the following composite image.

Solar System

Galileo Galilei

• Galileo Galilei (15 February 1564 –1642)was a gifted Italian physicist, mathematician, astronomer, and philosopher.

• Developed his own refracting telescopes• Galileo was found "vehemently suspect of heresy,"

namely of having held the opinions that the Sun lies motionless at the centre of the universe, that the Earth is not at its centre and moves, and that one may hold and defend an opinion as probable after it has been declared contrary to Holy Scripture.

• He was ridiculed by the church who forced him to spend spent the remainder of his life under house arrest, where he later became blind.

Galileo Galilei

Johannes Kepler

• Johannes Kepler (1571 –1630) was a German mathematician, astronomer and astrologer

• He is best known for his laws of planetary motion

• They provided one of the key foundations for Isaac Newton's theory of universal gravitation.

Johannes Kepler

Kepler mathematically described the elliptical orbits of planets

Isaac Newton

• Isaac Newton proposed that the same gravitational force that caused objects to fall on earth (e.g. apples) was the same force that caused planets to orbit the sun.

• He invented a branch of mathematics called the infinitesimal calculus.

• Using his calculus methods Newton predicted planetary motion (due to gravitational attraction) exactly in accordance with Kepler’s observations.

IsaacNewton

Newtonian Reflector Telescope

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Problems with Earthbound Telescopes

• Recall that convection currents in the atmosphere are responsible for weather systems.

• Different temperatures of air must have differing densities.

• Different densities have different refractive indices. (eg speed of light differs)

• Light deviates as it passes transits between different densities of air.

• Effect a bit like looking through frosted glass

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Flaws in Newton’s Laws

• It took time to find fault with Newton’s predictions.

• Newtonian mechanics is sufficient to navigate a spacecraft to the moon with an accuracy of within a centimetre.

• However it eventually became apparent that Mercury's orbit was not exactly elliptical.

• Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 days.

Perihelion of Mercury

Einstein and General Relativity

• Einstein’s theory of general relativity solved several perplexing physics questions including Mercury’s perihelion shaped orbit.

• The deviation from an elliptical shaped orbit is explained by gravitation being mediated by the curvature of space-time.

• Einstein successfully showed that general relativity predicts exactly the observed amount of shift.

• This was a powerful factor motivating the adoption of general relativity.

Einstein

Beyond our Solar System

• Both Newton and Einstein had difficulty explaining why the universe was not accelerating in on itself.

• Gravitational attraction should cause the matter in the universe to pull in on itself!

• Both Physicists produced “Fudge Factors” to attempt to describe a stable universe.

Edwin Hubble

• In 1929, Edwin Hubble announced that almost all galaxies appeared to be moving away from us.

• In fact, he found that the universe was expanding - with all of the galaxies moving away from each other. This phenomenon was observed as a redshift of a galaxy's spectrum.

• This redshift appeared to be larger for faint, presumably further, galaxies. Hence, the farther a galaxy, the faster it is receding from Earth.

Edwin Hubble

Absorption Spectra Shift

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