Extragalactic Astronomy & Cosmology Lecture 2

[4246] Physics 316 Extragalactic Astronomy & CosmologyLecture 2 Jane Turner Joint Center for Astrophysics UMBC & NASA/GSFC 2003 Spring

Other Early Cosmologies (?) It should be remembered that our knowledge of history is solely dependent us on having written records Also few (if any) of the original works survive, so we must rely on later works (true & complete reporting ?) Who knows what ideas have been lost... One (radical) idea that was not developed (apparently ignored) is due to Aristarchus (c280BCE - between Aristotle & Ptolemy) a Heliocentric universe - the Earth orbiting the Sun (!) Lecture 2

Recap The following should be remembered: Cosmology one of the oldest philosophies/sciences Many ancient cosmologies grappled with some of the same deep philosophical questions we still ponder with today. The Greeks first (we think) reasoned that Universe was formed by natural processes which could be observed, understood/explained by mathematics Developed the Empirical Scientific Method Developed a geocentric system (Pythagoras of Samos, c.550BCE; Aristotle,c.350BCE) culminating with that of Ptolemy,c.150 involving a complex arrangement of spheres & epicycles. Reason and beauty/perfection were a strong influence of their thoughts. The universe was reasoned to be finite but eternal/unchanging Lecture 2

Recap Greeks thought Earth was stationary, if it were moving, wouldn’t we feel a sense of motion (great winds, loose objects whizzing by us etc) Lecture 2

Recap (cont) Mathematics (ie. the Ptolemaic system) seen as a parameterization, By c.1400, the Ptolemaic (geocentric) system had remained essentially unchallenged as the cosmology for 1300 years You should be familiar with the concept of Parallax the basics of how the Ptolemaic system works (how epicycles, deferent etc account for retrograde motion). the concept of Ockham's Razor Again, a detailed knowledge of names, dates and places is not required However, you should be familiar with at least the names & approximate dates of Atristotle (c.350BCE) and Ptolemy (c.150). Lecture 2

Early Summary of Developments Thales (c.585BCE) universe run by natural processes Pythagoras (c.530BCE) spherical Earth Anaxagoras(c.430BCE) “Heaven” is knowable Plato (c.420BCE) Geocentric, planets-circular orbits, stationary Earth Democritus (c.400BCE) Universe is a mechanical system Eudoxus (c.340BCE) Geocentric cosmo, 33 spheres, stationary Earth Aristotle (c.350BCE) Geocentric cosmo, 55 spheres, stationary Earth Aristarchus (c.280BCE) Heliocentric cosmo ! (ignored) Hipparchus (c.125BCE) Distance of Moon (scale on the cosmos) Ptolemy (c.150) Geocentric cosmo, Epicycles Lecture 2

Foundatn of Modern Cosmo Topics: The Earth moves from Center Stage And Then the Apple Dropped… Summary at the beginning of the C20th Lecture 2

State of the Universe, 1400 By 1400, the geocentric cosmology of Aristotle & Ptolemy (based on concentric spheres, epicycles etc) had been essentially unchallenged for well over a thousand years. However the Greek Empire, under which the studies of Aristotle,Ptolemy had flourished (+ Pythagoras, Plato, Aristarchus, Hipparchus ... etc !) long-since collapsed (c.410). An Islamic Renaissance (refinement of Ptolemic model, but radical new ideas) had come and gone by c.1100. However, in the 15th & 16th centuries, following the years of the "Black Death" & centuries of strife, the start of the Renaissance in W.Europe finally allowed scientific & technological progress. Lecture 2

Rumblings of Discontent In c.1430, Nicholas de Cusa published On Learned Ignorance In which he suggested the universe is infinite (also Anaxagoras of Clazomenae (c.430BCE) and in a poem by Lucreutis (c.100BCE) but generally ignored), the universe does not have a center, the pattern of stars would look the same at all locations. all motion is relative, & that the Earth might not be stationary Homogeneity & Relativity Lecture 2

Earth moves from Center Stage... The suggestion by Nicholas de Cusa (c.1430) that the Earth might not be stationary, was supported by Leonardo da Vinci (c1490), who amongst many (!) other things also suggested the Earth moves (rather than the Sun). However it was not until 1543 when Nicholas Copernicus publishes his Revolutionibus Orbium Coelestium (The Revolution of the Celestial Spheres) that this idea was put of a more rigorous footing. Lecture 2

Heliocentric Cosmology Copernicus suggested the planets rotate (on circles) around a central Sun ….with "slower" planets being further from the Sun. Heliocentric Cosmology Copernicus also acknowledged the Earth rotates on its axis Lecture 2

A “Good” (Simpler) Model The heliocentric model of Copernicus obviously could be used to make predictions, that could be compared to observations. It was simpler than the model of Ptolemy that it replaced. However, it’s predictions were not any better than those of Ptolemy’s model unless (much smaller) epicycles were added to quote Alan W. Hirshfeld, in "Parallax - The Race to Measure the Cosmos" "Having donned the Artistolelian straightjacket of uniform circular motion....Copernicus has each planet circle at constant speed in a small epicycle whose center moves uniformly around the Sun." Lecture 2

Copernicus gets the credit, but.. to quote Alan W. Hirshfeld again, in "Parallax - The Race to Measure the Cosmos” "De Revolutionibus may have triggered the so-called Copernican Revolution, but it was not the revolution itself” Lecture 2

The Cosmological Principles Cosmological principles are the assumptions which allow us to deduce the whole of nature on the basis of the observable to the unobservable. Not surprisingly, any study of cosmological principles must combine elements of astronomy, physics and philosophy. Lecture 2

The Cosmological Principles One of the most important aspects of Copernicus’ work - took his heliocentric model, went further and made a model for the cosmos, by saying, lets assume several things, then use observations to test whether this is a good model Lecture 2

The Cosmological Principles There are several flavours of Cosmological Principle, all of which are essentially metaphysical in nature: The Copernican Cosmological Principle This is sometimes simply referred to as simply “The Cosmological Principle” The other cosmological principles are extensions of this principle. The Perfect Cosmological Principle The Anthropic Cosmological Principle This principle comes in two "flavours" Weak and Strong. Lecture 2

The Copernican Cosmo Principle The Copernican Cosmological Principle is a logical extension of the the Copernican theory that the Earth is not the center of the universe. Thus the Earth is not "special", thus the "laws of nature" on (or around) Earth are not special. It is essentially a philosophical requirement/simplification necessary/assumed for all modern cosmologies: - our laws of physics are otherwise "irrelevant" - "justified" by Ockham's Razor Lecture 2

The CCP itself The Copernican Cosmological Principle is that On a large scale, the universe is both homogeneous and isotropic(in 3-D space), and has/will always be so. Note that the statement "has/will always be so" refers to the universe continuing to display the properties of homogeneity & isotropy. The CCP does not imply that any actual observable parameter (e.g. the density of matter in the universe) will remain constant with time. Indeed, the CCP allows the properties of the universe to evolve with time, but states that at any given time the universe will be both homogeneous and isotropic (in 3-D space). Lecture 2

The CCP again Another way of expressing the Copernican Cosmological Principle is that ... all observers (in inertial frames) will see identical properties & laws - homogeneity will NOT see any preferred direction - isotropy We do not occupy a special place in the universe Lecture 2

Homogeneity/Isotropy homogeneous - same properties everywhere isotropic - no special direction, uniform in all dirns homogeneous but not isotropic isotropic but not homogeneous

The CCP - an analogy A (small) sentient being living in the center of a "perfect" loaf of bread…! There may be obvious structure on small scales (air bubbles etc), but on the large scale the loaf can be considered uniform and isotropic The laws of physics (e.g. which caused the dough to rise) are the same throughout the loaf. The loaf might still be rising - but (in this perfect loaf) this happens uniformly & following then same lawsthroughout the loaf Lecture 2

The CCP Evidence for & against The best support for the Copernican Cosmological Principle is the Cosmic Microwave Background (CMB), which is isotropic to 1 part in 105 The obvious observational evidence against the Copernican Cosmological Principle seems to be the structure seen in the universe on a variety of scales (stars, galaxies, clusters, super-clusters..) This is why the qualifier "On a large scale.." is required to be added to the principle. The question them becomes a question of scale (now large is "large" ?), and whether the observed structures on large scales are indeed representative of the universe on these scales (or are "perturbations" which "happen” to be visible to us). Lecture 3

The Perfect Cosmo Principle The Perfect Cosmological Principle is an extension of the Copernican Cosmological Principle and is that not only on a large scale, the universe is both homogeneous and isotropic (in 3-D space), and has/will always be so but also that it is also true for all times This principle is a fundamental assumption of the Steady-State Cosmological Theory: the universe should present a similar aspect when viewed from any point in space AND time. Lecture 2

The PCP - an analogy A (small) sentient being living in the center of a "perfect" loaf of bread…! There may be obvious structure on small scales (air bubbles etc), but on the large scale the loaf can be considered uniform and isotropic The laws of physics (e.g. which caused the dough to rise) are the same throughout the loaf. However (contrary to the case for the CCP loaf) the loaf has always (& will always) exist with the same characteristics as a function of time. So if the loaf appears still be rising, (which in this perfect loaf) this happens uniformly & following the same laws throughout the loaf), then the density of the loaf must remain constant, thus bread-particles must spontaneously appear to compensate for the expansion. Lecture 2

Recap We are up to 1400 We briefly mentioned the ideas of Nicholas de Cusa Homogeneity & Relativity We discussed the Heliocentric system of Nicholas Copernicus We then discussed the Copernican Cosmological Principle On a large scale, the universe is both homogeneous and isotropic (in 3-D space) and the Perfect Cosmological Principle On a large scale, the universe is both homogeneous and isotropic (in space AND time) Lecture 2

The Anthropic Cosmo Principle The Anthropic Cosmological Principle is an extension of the Copernican Cosmological Principle and is that not only on a large scale, the universe is both homogeneous and isotropic (in 3-D space), and has/will always be so but also that by our very being here, we are viewing "our universe" at a "privileged" location in spacetime The rationale behind the first part is as for the CCP. The implication is that the same laws of physics hold throughout the universe. The rationale behind the second part is as an explanation as to why the laws of physics (& the universe itself) are the way they are (at least as seen by us). Lecture 2

The Weak ACP The Weak Anthropic Cosmological Principle states that the conditions necessary for the development of sentient beings (capable of asking the question why is the universe the way it is ?) will only exist in a universe where the laws of physics are the way they are as they are experienced by us. i.e. sentient beings can only evolve and exist in a universe -that "happens" to have a density close to that observed (by us), -that "happens" to be about as old as ours, -where the charge of an electron "happens" to have the value observed (by us). ... etc. Lecture 4

The Weak ACP - an analogy Back to our sentient being in the center of a "perfect" loaf of bread As before, on the large scale the loaf is uniform and isotropic The laws of physics are the same throughout the loaf. However (as an extension to the CCP loaf) the sentient being reasons that out of all the possible loaves (universes) (ingredients, proportions, open temperatures, baking times etc), they exist in the loaf/universe they do since the conditions were just right to bake such a loaf. Had they not been (no yeast added, baking time not long enough etc), then it would not have been possible for them to reach the level they have (if they could even exist at all). This is the Weak flavor of the ACP Lecture 2

The Strong ACP The Strong Anthropic Cosmological Principle takes the Weak ACP one step further to state that there could be many different universes (or regions in a single universe) where the laws of physics are different There are yet more flavors of the Strong ACP dealing with whether or not sentient beings might be able to have evolved in these other "universes". But such metaphysical questions are beyond the scope of the current course Lecture 2

The Strong ACP - yet more loaves In the Strong flavor of the ACP, the sentient beings go on to allow for the possibility of the existence of other loaves having been in the oven (or other ovens), withdifferent mixtures of ingredients etc. Whether the conditions may have been right for other sentient beings to evolve in (a very small number of) these other loaves (most likely in a form v.different to themselves) is a matter of debate. However (most of) the sentient beings see no possiblity of loaf-to-loaf travel in any case... Lecture 2

The ACPs - just games in logic ?! Not really ! There appear to have to have been a large number of “coincidences” for life (as we know it) to exist.. (you will NOT be tested on these) for those interested, see... http://userweb.nashville.com/~al.schroeder/anthcoi.htm Examples… THE BERYLLIUM BOTTLENECK IN THE MAKING OF ELEMENTS NEUTRINOES, THE WEAK INTERACTION, AND SUPERNOVAS ELECTRON/PROTON MASS RATIO ELECTRON-PROTON CHARGE BALANCE ELECTRON/NEUTRON MASS RATIO NEUTRON/PROTON MASS DIFFERENTIAL ELECTRON EXCITATION AND STELLAR TYPES Lecture 2

Tycho Brahe (c.1570)) Also famous for having lost his nose in a swordfight Tycho Brahe (c.1570) did accept that the (other) Planets move around the Sun but did not accept that the Earth & Stars move around the Sun Why ? … Falling bodies fall towards the Earth ... The lack of Stellar Parallax Lecture 2

What was his problem ?) Falling Bodies fall towards the Earth Indeed if you throw something vertically upwards, it falls vertically downwards (to the same spot) Tycho Brahe reasoned this surely meant the Earth was the center of the universe Tycho Brahe was unable to detect (by naked-eye) Stellar Parallax and reasoned that in a Copernican system this would require the Stars to be so far away they would have to be "unreasonably" large/bright. Lecture 2

Tycho Brahe the observer Tycho Brahe was primarily an observer & made observations that strengthened the rejection of the cosmology of Aristotle & Ptolemy a Supernova position did not change (so it was not a comet or meteor), - lack of Parallax must be in one of the outer spheres brightness changed - outer sphere of star does change! a Comet position did not change significantly throughout the night. - lack of Parallax, must lay beyond the orbit of the Venus positions of Mars twice-daily which implied its orbit intersects that of the Sun. - apparent crossing/smashing of the “crystalline spheres” “there are no solid spheres "holding" the celestial bodies” Lecture 2

Tycho Brahe - his contribution So, even though Tycho Brahe’s (geocentric) cosmological model was wrong… … his observations did play a major role in the final rejection of the notions of Aristotle/Ptolemy that the celestial bodies are carried by crystalline spheres, with everything beyond the Moon eternal & unchanging. In addition Tycho Brahe also actually published his data ! In particular his twice-daily measurements of the position of Mars provided Johannes Kepler with a crucial database a few years later. Lecture 2

Johannes Kepler - Laws 1 & 2 In 1609 Johaness Kepler publishes Astronomia Nova (New Astronomy) 1st two laws: Planets move in ellipses with the Sun at one focus (not the perfect circular orbits previously assumed) Planets "sweep out" equal "areas" in equal times during their motion around the Sun Lecture 2

Johannes Kepler - Law 3 In 1619 Johaness Kepler publishes Harmonices Mundi (Harmonies of the World) 3rd law: The period P (of a Planet's orbit) squared is proportional to the (average) distance R cubed P2 = K x R3(where K is a constant) Cosmology finally escapes the "Artistolelian straightjacket of uniform circular motion” Kepler’s laws are "parameterizations" of motion (rather than how/why …) e.g. the physics "hidden" in the constant of proportionality K was not understood but for the 1st time, laws provide the ability to measure the relative size of the Solar System. Lecture 2

Sun at the center - still somewhat “radical” Following the publication of Revolutionibus Orbium Coelestium by Nicholas Copernicus (1543), a heliocentric solar system slowly slipped into the scientific mainstream. Many still considered the Copernican system radical ... Others took the ideas even further. Thomas Digges (1572) - universe is infinite (in space) and populated by innumerable suns & worlds. Giordane Bruno (1584) - universe is infinite in both time & space and contains many worlds with intelligent beings. Lecture 2

Kepler Orbits and Keplers Laws ht For the Earth, we know that: • P=1 year • R=150 million km (1 Astronomical Unit, A.U.)

Kepler Orbits and Keplers Laws ht

Homework Read Chapters 1+2 of Hawley & Holcomb Answer chapter 2, question 10 (should take ~10 mins), email response to turner@lucretia.gsfc.nasa.gov work through web-tutorial www.aw.com/astronomyplace #1 “Scales of the Universe”

Extragalactic Astronomy & Cosmology Lecture 2