Cosmology Overview (so far): The Universe: Everything Observable Universe: Everything we can The Universe has no special locations No If no, then no Cosmology Overview (so far): Oblers s Paradox: The sky is dark at night, so the Universe must have a Hubble Expansion: All galaxies are moving away from us, so they must have all started out at Beginning + Expansion =! The Big Bang Galaxies are moving from us now. If we play the movie backward, we would see them getting closer This initial of space outward is called The Big Bang Not an explosion which occurred in place: it was! The Universe was just smaller back then. The Age of the Universe The Big Bang gives us a : a point when we can start keeping track of time This tells us that the Universe isn t old; it has an age! The Hubble Constant H 0 measures how the Universe is expanding The Age of the Universe Using various techniques, astronomers have determined the Hubble Constant: H 0 = km/s / Mpc Using this value of the Hubble Constant it is possible to determine the age of the Universe: Predictions of the Big Bang Theory Like any good theory, the Big Bang Theory must make predictions which are confirmed by observation. 1. It predicts a (CMB) 2. It predicts small in this CMB 3. It predicts the of light elements (Hydrogen, Helium ) The Universe is about: years old. 1
Cosmic Microwave Background The early Universe was very, ionized, and opaque. As the Universe expanded, it. 400,000 years after the Big Bang, it had cooled to 3000 K Electrons joined protons to form the first The universe was no longer opaque, and the light could. The Big Bang Theory says we should still see this light today. Ionized H Neutral H Look-back Times We say a galaxy is 10 million light years away The light from this galaxy has been traveling for years to reach our eyes! Its look-back time is 10 million years: we are seeing a 10-million-yearold picture of that galaxy Look-back Times By observing more and more distant objects, we can look further and further into the This is because the speed of light is Also means we can observe from the Big Bang: The Cosmic Microwave Background (CMB) Looking Back Towards the Early Universe The more distant the objects we observe, the further back into the past of the universe we are looking. CMB Looking at Distant Objects Lecture-Tutorial: Page 131 Cosmic Microwave Radiation Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Discuss each question and come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. The wavelength of this light from the Big Bang would have stretched with the expansion of the universe. It should have a temperature of about 3º K. It would now have a wavelength in the region 2
The Fate of the Universe It s expanding now will it keep going? The universe has three possible fates: The current expansion could continue forever (Big ) The expansion could halt, and reverse (Big ) Or, it could stop expanding and become The Fate of The Universe The true density divided by the critical density is called Omega Ω = ρ true / ρ crit. If Ω >1, the universe will collapse. ( Universe) If Ω <1, the universe will expand forever. ( Universe) If Ω =1, the universe s expansion will gradually slow ( Universe) Shape of the Universe Open, Closed and Flat universes have different shapes. How do we learn which one we re living in? The key is! CMB and Density The CMB is mostly smooth, but has small variations about 1 degree in size Spots are the right size for a universe Open universe -> smaller spots Closed universe -> larger spots Closed surface Flat surface Open surface (positive curvature) (zero curvature) (negative curvature) Cosmology Review How do we know the universe has a finite age? How do we know there must be a Big Bang? What is the Cosmic Microwave Background? What are its basic properties? What predictions are made by Big Bang theory? What is the shape and fate of the Universe? The Solar System One star Planets Asteroids Comets Kuiper Belt Objects (KBOs) 3
Planets The planets of the solar system fall into three categories: Terrestrial Planets (those like ) Mercury Venus Earth & Moon Mars Jovian Planets (those like ) Jupiter Saturn Uranus Neptune Planets, including Pluto, and Eris & Ceres Terrestrial Planets Terrestrial comes from Latin terra meaning earth Mercury, Venus, Earth, and Mars have similar features: Rocky Thin (or nonexistent) Dense metal cores Craters moons Jovian Planets (not gas giants!) Better name: giants (Jupiter & Saturn) or giants (Uranus & Neptune) mass (about 15x-300x mass of Earth) density (1/3rd to 1/7th of Earth) Lots of moons, ring systems What exactly is a planet? 1. It orbits the Sun 2. Has enough mass so that it is round 3. It has Pluto & Ceres satisfy #1 and #2, but not #3! Pluto: other Kuiper Belt Objects Ceres: other Asteroids Solar System Today (Not to Scale) Inner Planets, Orbits to Scale 4
Inner Planets, Sizes to Scale Top Row: Earth, Venus Bottom Row: Mars, Mercury, the Moon Planets, Sizes to Scale Top Row: Jupiter & Saturn Middle Row: Uranus & Neptune Bottom Row: Earth, Venus, Mars, Mercury, & the Moon Planets with the Sun, Sizes to Scale Early Solar System The young Sun probably had a disk of gas & dust: the Planets formed out of this disk: small cores grow through accretion Temperature is as you get closer to the center (where the Sun is!) Differentiation in the Solar Nebula Material forms clumps according to Only high-density elements can form clumps at temperatures Cooler Temperatures 5
Step 1: Condensation Gas Dust Grains or Particles Small stuff grows fast this way! At high temperatures, only elements can condense Step 2: Accretion Particles sticking together Planetesimals (usually about 1 km wide) Different composition depending on where they formed in the solar nebula Step 3: From Planetesimals to Protoplanets Head-on collisions or same-direction collisions? Largest planetesimals grow fastest Can grow via gravitational collapse when it reaches 10-15 times the Earth s mass Step 4: Clearing the Nebula Sun turns on:! Two effects: Radiation pressure from fusion Solar wind Small dust grains and gas atoms get pushed out Predictions from the Solar Nebula Hypothesis Planet orbits should fall roughly in one plane Orbit and spin directions should be mostly the same Planets should have roughly the same age as their star Evidence of collisions! Temperature and Formation of Our Solar System Lecture Tutorial: Pages 103-104 Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Discuss each question and come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. 6
Extrasolar Planets The first Extrasolar planets were discovered in the 1990s. These planets were very different from the planets in our solar system orbital periods Highly orbits Most are about the size of M Jupiter = 300 M Earth 51 Peg. Orbit Temp: 1,500 K 51 Pegasi: A Hot Jupiter Planet Exoplanet Discoveries Over 450 Planets Discovered (7% of stars) About 40 Multiple-Planet Systems 65 Transiting Planets 1 Possibly habitable planet planets are more numerous. 55 Cancri 1 Jupiter-like planet at 5 AU (d) 1 Jupiter-like planet at 0.1 AU (b) 2 Saturn-like planets at 0.25 and 0.8 AU (c & f) 1 Neptune-like planet at 0.03 AU (e) Saturn-like planet at 0.8 AU (f) is in the habitable or Goldilocks zone! Life on moons? Planet Search Methods Doppler Effect Detect wobble toward or away from us Astrometry Detect side-to-side wobble Transits Search for eclipses as the planet passes in front of the star. Doppler Method Planets orbiting other stars tug on their star a bit The star wobbles when the planet tugs on it We can detect that wobble by measuring Doppler shifts We ve never actually the planets! 7
Doppler Method Measuring light from the, not the! Use the motion of the star to infer the presence of a planet Astrometry Like the Doppler method, astrometry searches for wobbling stars. The Doppler Effect measures stars coming toward or moving away from us. Astrometry measures stars wobbling side to side. Wobble of our own Sun --> Eclipsing Planets Transit Method A few planets have now been discovered using the Transit Method. If a planet happens to transit its host star: A planet might pass right in front of its star, Making a small, or transit. We can learn the size of the planet We might see the planet s. Selection Effects: Stars with the most massive planets will wobble more Their Doppler shifts will be larger, easier to measure Our method selects the most massive planets Chapter 20: Life on Other Worlds 8
Searching for Life: What does life look like here? How did Earth get life? Is Earth ordinary or extraordinary? If Earth is ordinary, where is everyone else? Life in the Universe The Earth formed about 4.5 billion years ago The oceans formed about 4.1 billion years ago It appears that life arose very quickly on Earth: 3.8 billion years ago Perhaps life could easily form on other planets as well. Earliest Fossils Oldest fossils show that bacteria-like organisms were present over 3.5 billion years ago Carbon isotope evidence pushes origin of life to more than 3.85 billion years ago Laboratory Experiments Miller-Urey experiment (and more recent experiments) shows that building blocks of life form easily and spontaneously under conditions of early Earth. Building blocks of life but no life yet! Extremophiles We find life almost everywhere on Earth! Organisms that thrive in extreme environments: Volcanoes (high temperatures) Ice Caps (low temperatures) Acidic environments Salty environments Dry environments Is Life Possible Elsewhere? Life arose early and quickly on Earth Complex organic molecules form easily from ingredients and conditions on the young Earth Living organisms exist is extreme environments on Earth 9
Searches for Life on Mars Could there be life on Europa or other jovian moons? Mars had liquid water in the distant past Still has subsurface ice; possibly subsurface water near sources of volcanic heat. Titan Are habitable planets common? Surface too cold for liquid water (but deep underground?) Liquid ethane/methane on surface Definition: A habitable world contains the basic necessities for life as we know it, including liquid water. It does not necessarily have life. Constraints on star systems: 1) Old enough to allow time for evolution (rules out high-mass stars - 1%) 2) Need to have stable orbits (might rule out binary/multiple star systems - 50%) 3) Size of habitable zone : region in which a planet of the right size could have liquid water on its surface. Even so billions of stars in the Milky Way seem at least to offer the possibility of habitable worlds. The more massive the star, the larger the habitable zone higher probability of a planet in this zone. 10
Elements and Habitability Do we require heavy elements (Carbon, Iron, Calcium, Oxygen) in precise proportions? Heavy elements are more common in stars in the disk of the Milky Way A galactic habitable zone? Impacts and Habitability Are large planets (like Jupiter and Saturn) necessary to reduce rate of impacts? If so, then Earth-like planets are restricted to star systems with Jupiter-like planets Climate and Habitability Are plate tectonics necessary to keep the climate of an Earth-like planet stable? The Bottom Line We don t yet know how important or negligible these concerns are. Any life: bacteria, microbes, bluegreen algae Intelligent life: beings that can build telescopes and are interested in talking to us Looking for Life: Probes to other planets are very expensive, but radio signals are cheap! SETI experiments look for deliberate signals from E.T. 11
The Drake Equation In 1961 Astronomer Frank Drake tried to calculate the number of ET civilizations in our galaxy, N His calculation is known as the Drake Equation The Drake Equation Number of civilizations with whom we could potentially communicate N C = N* f P n p f HZ f L f I F S N * = # of stars in galaxy f p = fraction with planets n p = number of planets per system f HZ fraction with good conditions for life f L = fraction with life f I = fraction with intelligent life F S = Fraction of star s (or galaxy s) lifetime for which a civilization exists We do not know the values for the Drake Equation! The Drake Equation The final factor in the Drake Equation could be the most important: (L/L MW ) L MW = lifetime of the Milky Way Galaxy (10 billion years) L = lifetime of a technological civilization (50 years? 1000 years? 1 million years?) What will it take for us to survive? SETI We can search for ETI several ways Spaceships: Flying spaceships to other stars would take a long time, and be very expensive. Broadcasting: We could easily send signals to other stars using a radio telescope. Communicating with ET November 16, 1974, astronomers sent a message about people on Earth toward the globular star cluster M13 in the constellation Hercules. The message was intended to be easy to decode. M13 is 25,000 light years away These signals travel at the speed of light We won t get an answer for at least 50,000 years! M 13 Globular Cluster 12
SETI Listening: We can easily listen for signals from ET If the ET s are much more advanced than us, they might share a wealth of knowledge. Because radio waves have low energy, people have searched using radio telescopes. The first search was in 1960. Now, several searches are underway including one at Arecibo in Puerto Rico Tuning in to a Signal There are millions of radio frequencies Which station would they transmit on? To be safe we should observe all frequencies. We should also observe all parts of the sky To do this we would have to process a huge amount of data. Fermi s Paradox Plausible arguments suggest that civilizations should be common, for example: Possible solutions to the paradox Even if only 1 in 1 million stars gets a civilization at some time 100,000 civilizations So why we haven t we detected them? Earth as seen from the edge of the Solar System Voyager 1 photo 13