Astro.101 Sept. 9, 2008 Announcements A few slots are still open in the class; see prof. to sign up Web-page computer has been down; o.k. now Turn in your student contract Don t forget to do the OWL tutorial and register your PRS clicker PRS clickers sold out, so we ll start that on Thurs. Your clicker should look like this: The Age of the Universe If the entire age of the Universe were 1 calendar year, then 1 month would be equivalent to roughly 1 billion years Jan. 1: The Big Bang Feb: Our galaxy formed Sept. 3: Earth formed Sept. 22: Earliest life on Earth Dec. 26: Rise of dinosaurs Dec. 30: Dinosaurs disappear 25 seconds before midnight: first agriculture 11 seconds before midnight: pyramids built 1 second before midnight: Kepler and Galileo obtain first correct understanding of the Solar System The implications of the finite speed of light Light moves rapidly (!), but it does not move infinitely fast. If we observe a galaxy that is 11 billion light years away, then the light started moving towards us 11 billion years ago and is only arriving at our eyes right now. What happened to that galaxy 1000 years later? We won t know until 3008 However, we can compare it to a galaxy that is only, say, 5 billion light years away to study galaxy evolution. Our Solar System formed roughly 5 billion years ago, and we similarly study how the old Solar System compares to the current Solar System. For example, comets are leftover junk from when the planets first formed, i.e., effectively fossils The Funky Discoveries of Edwin Hubble and Albert Einstein In the 1920s, Edwin Hubble observed the following: There are many galaxies in the universe external to our Milky Way. They exhibit two odd behaviors: 1. The vast majority of galaxies are moving away from us. 2. The more distant the galaxy, the faster it is moving away from us! PRS question. True or false: this means that there was a huge explosion, and since all galaxies are moving away, we are close to the center of the explosion. 1. TRUE 2. FALSE (Survey Question: Automatic Full Credit) 1
There are at least 100,000,000,000 galaxies in the universe. They exhibit two odd behaviors: 1. The vast majority of galaxies are moving away from us. 2. The more distant the galaxy, the faster it is moving away from us! PRS question. True or false: this means that there was a huge explosion, and since all galaxies are moving away, we are close to the center of the explosion. 1. TRUE 2. FALSE Consider the idea of a huge cosmic explosion: Suppose that objects move more rapidly close to the explosion source and slow down as they move away A B C Galaxy A moves more rapidly than B, so it overtakes B. Likewise, B overtakes C. If we live in Galaxy B, A In the center, all galaxies would be moving away, but more distant ones would move more slowly would appear to be moving toward us, and C would be moving toward us too. Consider the idea of a huge cosmic explosion: We should see evidence of the explosion in the spatial A B C distribution of the galaxies. This idea predicts: no galaxies near the center; many galaxies in a shell around the periphery. This is not observed; we see comparable numbers of galaxies in all directions. Placing ourselves in the center violates the Copernican principle we are likely to be a random place, not the special location of the very center (also known as the cosmological principle ) An Einstein-style thought experiment Suppose that the universe is a raisin cake with equally spaced raisins. As the cake bakes, it expands at the same rate in all directions. Let s measure the distances between the raisins at the beginning, and then again once hour later. Suppose that we live on a raisin that we (fondly) call the local raisin. Raisin Number Distance before baking Distance after baking Speed The 1 funkiest bit 1 of cm all: the expansion 3 cm of the 2 cm/hr 2 2 cm 6 cm 4 cm/hr 3 3 cm Universe! 9 cm 6 cm/hr Speed = Distance All of the raisins appear to be moving away from us. The farther away they are, the faster they are moving. Just like raisins in a raisin cake, galaxies all move apart from each other as the dough (space itself) expands. The Hubble Law and the Age of the Universe Edwin Hubble noticed that galaxy velocities all follow a simple relation: Velocity = Constant x Distance The Hubble Constant But we also know that Velocity = Distance For a particular galaxy Velocity = Constant x Distance = Distance Therefore This can be measured!! Hubble Constant = 1 This is the time that has passed since the expansion started!!! 2
The Observable Universe Einstein s General Theory of Relativity Mass affects space itself. Sufficiently massive objects curve the fabric of space. Space can move. Near a black hole, space itself is falling into the black hole. If space can move, the universe can be contracting or expanding. Einstein thought that this was ridiculous and fudged the theory to make it go away. My greatest blunder A comment about our origins Nearby stuff, however, is Contains significant amounts Of carbon, oxygen, etc. The most distant stuff That we can observe Contains only hydrogen and helium. No carbon, nitrogen, oxygen, etc. Several lines of evidence indicate that the universe is roughly 14 billion years old: - The Hubble Law - The ages of stars This means that the greatest distance we can observe is 14 billion light years. This is the limit of the observable universe. Your ancestors may surprise you In the early history of the Universe, there was nothing but hydrogen and helium. All other elements were manufactured deep in the cores of stars by nuclear fusion reactions, or in nuclear reactions that occur when a massive star ends its life in a violent explosion called a supernova. Seek to be one with the cosmos? You already are! Gravitational lensing. The force of gravity affects light as well as mass. Gravity changes the path light follows just like an optical lens! 3
Another funky Einstein thing: Gravitational lensing Below: arcs due to gravitational lensing Gravitational lensing comes in various flavors Above: lensing can distort an image and produce Einstein rings (see examples on the following page) This is a gravitational lens. Einstein s Theory of Relativity states that the force of gravity affects light as well as mass. a massive galaxy cluster will bend the path of light which approaches it (like a lens) the blue arcs are the lensed images of a galaxy which is behind the cluster Left: four images of a single quasar lensed by a foreground galaxy (blob in the middle is the galaxy) Gravitational lensing comes in various flavors This page shows Einstein rings The blue rings are distorted images of background galaxies magnified by gravitational lensing due to the mass in the foreground (yellow) galaxies The most famous open star cluster: The Pleiades The Seven Sisters. Several thousand stars; 6 are easily seen with the naked eye. Japanese name: Subaru The Solar System: 9 planets and 1 star Nearby stars (alpha Centauri, Sirius) Loose star clusters, open clusters : several thousand stars spread over a region ~30 light years across Globular star clusters: 20,000 -- several million stars spread over 60 -- 150 light years Galaxies: Vast islands of stars, 106 -- 1012 stars, 100,000 light years across PRS question. True or false: if you sit still in your chair, you are not moving. 1. TRUE 2. FALSE 4
How can we sleep when the earth is turning? The many ways the Earth moves Contrary to our perception, we are not sitting still. We are moving with the Earth. and not just in one direction The Earth orbits around the Sun once every year The Earth rotates around it s axis once every day The Earth s axis is tilted by 23.5º Our Sun and the stars of the local Solar neighborhood orbit around the center of the Milky Way Galaxy every 230 million years Our Sun moves relative to the other stars in the local Solar neighborhood The Scientific Method 1 Question 2 Hypothesis a tentative explanation 3 Prediction 4 Test 5 Result confirm, reject, or modify should be the same no matter who conducts the test Did Mars have flowing water on its surface in the past? The Scientific Method 1 Question: why are river-like features present on the surface of Mars? 2 Hypothesis: water was present on the surface in the past 3 Prediction: hematite should be found in these areas 4 Test: send rover to Mars to search for and study hematite 5 Result confirm, reject, or modify Viking Orbiter image, mid 1970s 5