Weeks 11-14 - Cosmology In studying cosmology we try to...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Cosmology In studying cosmology we try to understand the origin of the Universe, its organization, its evolution, and its ultimate fate. To do this we must understand its expansion rate and how fast the expansion is slowing down (deccelerating) or speeding up (accelerating). The Steady State Theory (continuous creation of matter with no beginning or end) put forth by Hoyle in the 1940s is now rejected by most (because the Universe is seen to evolve with redshift, e.g., there were more quasars in the past). The Big Bang Theory, first proposed by LeMaitre in the 1940s, is widely accepted. Georges LeMaitre, a Belgian Catholic priest who was one of the originators of what became the Big Bang Theory. Fred Hoyle proposed the Steady State Theory and coined the term "Big Bang" for the opposing theory. What do current cosmological observations that probe back in time tell us? The Universe had an origin point in Time, not "in space". The Universe began in a hot, dense state The "Big Bang". The Universe experienced a rapid and short lived exponential growth phase known as INFLATION. The Universe is 13.7 Billion Years Old +/ .2 Billion years! The Universe is geometrically "FLAT". The Universe continues to expand and this expansion is accelerating! What do current cosmological observations that probe back in time tell us? Assuming modern gravity theory (i.e., the General Theory of Relativity) is correct, the Total Mass Energy of the Universe consists of approximately: 4% Ordinary Matter and Energy 23% "Dark Matter" 73% "Dark Energy" The dark matter is PROBABLY a relatively exotic subatomic particle not yet observed in particle accelerators. The visible Matter and Energy in the Universe accounts for only 1% of the Total MassEnergy (i.e. 99% of the Universe is "unseen". Dark Energy accounts for the fact that the expansion of the Universe is accelerating! Dark Matter and Dark Energy? Evidence for dark matter dates back to at least the 1960s (e.g., rotation curves of galaxies). However, evidence for dark energy has only surfaced in the last 5 to 10 yrs. The current evidence is compelling, but the nature of the dark energy is so mysterious that additional work is required to search for new evidence for dark energy and to develop a robust model. It is useful to examine the properties of Big Bang models that don't include dark energy. Summary of (Possible) Big Bang Models [Assuming No "Dark Energy"] Open (saddlelike shape). Universe expands forever. Massenergy density of Universe is less than the "critical value" needed to stop expansion. Flat (flat shape). Between an open and closed Universe. Massenergy density of Universe equals the "critical value"' and Universe would stop expanding only after an infinite amount of time passed. Closed (spherical shape). Universe will someday stop expanding and collapse into a big crunch. Massenergy density of Universe is greater than the "critical value" needed to stop expansion. What Big Bang Theories Explain The expansion of the Universe (Hubble Law). The observed evolution of objects (e.g. quasars and galaxies) with lookback time. Cosmic nucleosynthesis, which explains the amounts of helium and some other light elements. The 3 K cosmic microwave background radiation. The Expansion of the Universe Slipher showed that spiral objects often had redshifted spectra. A redshifted spectrum can be caused by motion of an object away from the observer (i.e. the Doppler effect). With the realization that there were external galaxies, in 1929 Hubble announced that galaxies in all directions were moving away (receding) from the MWG. Hubble established a redshiftdistance relation (v = Hod), now know as the Hubble Law. H ubbl e's r edshi ft-di stance r el ati on: due to the expansi on of the Uni ver se, mor e di stant (fai nter ) gal axi es exhi bi t mor e hi ghl y r edshi fted spectr a. Cosmol ogi cal Redshi ft, H ubbl e's L aw, and the Expandi ng Uni ver se Emi tted spectr um Si mi l ar to: The Doppl er Effect Redshi ft z = -0 0 Recei ved spectr um depends on wavel ength shi ft for non-r el ati vi sti c vel oci ti es, wher e v i s expansi on vel oci ty and c i s the speed of l i ght for r el ati vi sti c vel oci ti es z = v/ c z= c+ v -1 c-v Redshi ft, H ubbl e's L aw, and the Expandi ng Uni ver se H ubbl e di scover ed that the mor e di stant a gal axy was, the gr eater i ts r edshi ft, and ther efor e the hi gher i ts vel oci ty. T he U ni ver se i s expa ndi ng. T v = H0 x D v i s vel oci ty D i s di stance H 0 i s H ubbl e's constant H ubbl e's constant gi ves the expansi on r ate of the Uni ver se the age of the Uni ver se Development of the Universe: "Inflation" a period of rapid expansion 2Dimensional Analogy of the Geometry of the Universe sum of angl es of a tr i angl e Cl osed Fl at Open Evolution in the Universe Cosmic Nucleosynthesis Bi g Bang theor y pr edi cts that hel i um and other l i ght el ements for med a few mi nutes after the bi g bang, consi stent wi th obser vati ons. Fluctuations in the CMB are a few parts in a million Steady State Theory unlikely Big Bang Theory has had great success The Limits of Modern Physics Incredibly, the reliability of the laws of modern physics suggest that we can understand the Universe back to a time when it was only 1043 sec old! Then gravity theory and quantum theory clash and firm theoretical predictions are not possible. Modern Quantum SpaceTime Theories Near the quantum Planck scale of 1033 cm, spacetime must change its structure drastically (e.g. might need a theory with 11 dimensions of space). There is some speculation that big bangs happen often, and that one can some day investigate what happened before our particular Big Bang! Grand Unification Theories: GUTs Solar System Formation Orion Nebula (NASA) Proto planetary systems in Orion Supernova Shock Wave Shell of gases ejected from a supernova as a shock wave. Compression of Nebula by Shock Wave Interaction of shock wave front with nebula causes contraction Passage of Shock Wave Shock wave passes leaving proto-planetary system Formation of Solar disk Processes: (a) Electrostatic sticking of grains (b) gravitational contraction (c) rotation (d) ignition of nuclear fusion (e) ejection of remaining nebula A Possible Proto-Planetary System Rings of dust and gas around a young star Growth of protoplanets from solar disk Segregation of Inner an,,,d Outer Solar System Condensation of Elements The End Result Planets to Size Inner and Outer Planets to Size Comets Kuiper Belt Objects KBOs Edge View When did life arise on Earth? Probably before 3.85 billion years ago. Shortly after end of heavy bombardment, 4.23.9 billion years ago. Evidence from fossils, carbon isotopes. 2 billion years... Fossil evidence for microbes 3.5 billion years ago Already fairly complex life (photosynthesis), suggesting much earlier origin. Carbon isotope evidence pushes origin to before 3.85 billion years ago. How did life arise on Earth? Life evolves through time. All life on Earth shares a common ancestry. We may never know exactly how the first organism arose, but laboratory experiments suggest plausible scenarios. Could life have migrated to Earth? Venus, Earth, Mars have exchanged tons of rock (blasted into orbit by impacts) Some microbes can survive years in space... Brief History of Life 4.4 billion years early oceans form 3.5 billion years cyanobacteria start releasing oxygen. 2.0 billion years oxygen begins building up in atmosphere 540500 million years Cambrian Explosion 22565 million years dinosaurs and small mammals (dinosaurs ruled) Few million years earliest hominids What are the necessities of life? Nutrient source Energy (sunlight, chemical reactions, internal heat) Liquid water (or possibly some other liquid) H ar dest to fi nd on other pl anets Could there be life on Mars? Mars had liquid water in the distant past Still has subsurface ice; possibly subsurface water near sources of volcanic heat. In 2004, NASA Spirit and Opportunity Rovers sent home new mineral evidence of past liquid water on Mars. Close-up view of rock apparently formed in water. The Martian Meteorite debate composition indicates origin on Mars. 1984: meteorite ALH84001 found in Antarctica 13,000 years ago: fell to Earth in Antarctica 16 million years ago: blasted from surface of Mars 4.5 billion years ago: rock formed on Mars Does the meteorite contain fossil evidence of life on Mars? Could there be life on Europa or other jovian moons? Ganymede, Callisto also show some evidence for subsurface oceans. Relatively little energy available for life, but still... Intriguing prospect of THREE potential homes for life around Jupiter alone... Ganymede Callisto Titan Surface too cold for liquid water (but deep underground?) Liquid ethane/methane on surface? Huygens probe descent, Jan. 2005 Are habitable planets likely? Definition: A habitable world contains the basic necessities for life as we know it, including liquid water. It does not necessarily have life. Caveat: Telescopically we can search only for planets with habitable surfaces -- not for worlds with Europa-like subsurface oceans. Constraints on star systems: 1) Old enough to allow time for evolution (rules out highmass 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. Finding them will be hard Recall our scale model solar system: Looking for an Earthlike planet around a nearby star is like standing on the East Coast of the United States and looking for a pinhead on the West Coast -- with a VERY bright grapefruit nearby. But new technologies should soon show the way... Detection of Planets Around Nearby Stars Use of: Doppler wobble, astrometric wobble planetary transits Protoplanetary disks around stars. Examples of discovered planets (www.exoplanets.org). Prospects for the future (Drake Equation). This works best for large orbits (which take a long time) and stars that are nearby. Interferometry would allow very small motions to be measured. Wobble Doppler Wobble Shift is 1 part in 100 million HST measurement of HD209458 We can watch for the dimming of the star if the planet crosses in front of it. This is by the ratio of their areas: 1% for Jupiter and 0.008% for the Earth. This has been seen for one case (confirming the radial velocity detections). Transits Habitable Zones (liquid surface water) Kepler (2007 launch) will monitor 100,000 stars for transit events for 4 years. Later: SIM (2009?), TPF (2015?): interferometers to obtain spectra and crude images of Earth-size planets. Galactic "habitable zone": minimum limits on heavy element abundance, distance from galactic center? Jupiter protection from frequent impacts? Climate stabilized by a large Moon and plate tectonics? Are Earthlike planets rare or common? We don't yet know how important or negligible these concerns are. How does SETI work? Looking for deliberate signals from E.T. We've even sent a few signals ourselves... Earth to globular cluster M13: Hoping we'll hear back in about 42,000 years! Your computer can help! SETI @ Home: a screensaver with a purpose. How difficult is interstellar travel? Very! Current spacecraft travel at <1/10,000 c; 100,000 years to the nearest stars. Pioneer plaque Voyager record Real interstellar travel faces huge hurdles: Far more efficient engines Incredible energy requirements Ordinary particles become dangerous cosmic rays Time dilation affects crew Where are the aliens? "Fermi's Paradox" Plausible arguments suggest that civilizations should be common, for example: 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? Possible solutions to the paradox 1) We are alone: life/civilizations much rarer than we might have guessed. Our own planet/civilization looks all the more precious... Possible solutions to the paradox 1) Civilizations are common but interstellar travel is not. Perhaps because: Interstellar travel more difficult than we think. Desire to explore is rare. Civilizations destroy themselves before achieving interstellar travel These are all possibilities, but not very appealing... Possible solutions to the paradox 1) ... and some day we'll meet them... There IS a galactic civilization... The Drake Equation What is the probability that intelligent life exists elsewhere in the Milky Way Galaxy (MWG)? N = R*fPnEfLfIfCL is the number of civilizations in the MWG that could contact each other, where R* = star formation rate in MWG, fP = fraction of stars with planets (solar systems), nE = # of (earthlike) planets per solar system suitable for life, fL = the fraction of these planets on which life arises, fI = the fraction of this life which develops intelligence, fC = the fraction of intelligent civilizations which choose to communicate and develop the technology to do so, and L = lifetime of advanced (able to communicate) civilization. How difficult is interstellar travel? What have we learned? Convenient interstellar travel remains well beyond our technological capabilities, because of the technological requirements for engines, the enormous energy needed to accelerate spacecraft to speeds near the speed of light, and the difficulties of shielding the crew from radiation. Nevertheless, it seems reasonable to think that we will someday achieve interstellar travel if we survive long enough. What have we learned? Where are the aliens? It seems that we should be capable of colonizing the galaxy in a few million years or less, and the galaxy was around for at least 7 billion years before Earth was even born. Thus, it seems that someone should have colonized the galaxy long ago--yet we have no evidence of other civilizations. Every possible category of explanation for this surprising fact has astonishing implications for our species and our place in the universe. ...
View Full Document

Ask a homework question - tutors are online