This preview shows page 1. Sign up to view the full content.
Unformatted text preview: Cosmology: From an Unchanging Universe to the Big Bang
Biology 190A Bi l 190A Lecture 9 Prof. Steve Schneider Dept. of Astronomy What is "Cosmology"? Lit ll it i th t d f th " Literally, it is the study of the "cosmos," which " hi h means "order" or "orderly arrangement." Within the last century scientific discoveries have produced a profound evolution of our ideas. Cosmology attempts to answer such basic questions as: Did the universe have a beginning? If so, when? Is the universe finite? Is there anything beyond it? y g y Will the universe last forever? What rules govern it? Some of these questions are addressed by Some of these questions are addressed by religions as well, so some people are conflicted. Ad from Chicago Paper: Cosmologists are also not only focused on hair. They also work with the skin and nails. Although they are not as in-depth in depth as a beauty specialist, who works on the deeper layers of the skin and body, they can offer a number of services to Clarification: Cosmologists study the makeup of their customers in the universe Northbrook, Illinois. N thb k Illi i Cosmetologists study the makeup of Miss Universe The Beginnings of Modern Cosmology M d C l
1543 Nicolaus Copernicus publishes On the p p Revolutions of the Celestial Orbs, theorizing that Earth is not the center of the universe. 1584 Giordano Bruno publishes tracts arguing that the Sun is just one of many stars with planets circling it. 1610 Johannes Kepler shows that stars cannot extend to infinite distances, because then the sky would be infinitely bright (later known as Olbers' paradox) infinitely bright (later known as Olbers' paradox) 1696 Isaac Newton argues that the universe must be infinite because otherwise gravity would make it infinite because otherwise gravity would make it collapse toward its center. Olbers' (really Kepler s) Paradox Kepler's) Paradox Draw shells around the Earth, each of thickness T. E h h f hi k T At a distance R, the number of stars in a shell goes as R2 The brightness of each star g declines as 1/R2 (inverse q ) square law) Each shell adds equally! ==> infinite amount of light?? ==> infinite amount of light?? NIGHT SHOULD BE BRIGHT! Not Seeing the Forest for the Trees f h It is more accurate to think in terms of lines of sight. In every possible direction yp there must be a star. It's like looking out from g inside a largeenough forestevery line of sight hits a tree trunk. h k Therefore Kepler (& Olbers) concluded that the universe l d d h h i must be finite in extent. The Problem with Gravity i hG i Unlike other forces, gravity only pulls objects toward each only pulls objects toward each other--it doesn't repel. If the universe is finite, stars near the edge will only feel a pull in one direction and will begin falling inward. In an infinite universe, every star feels a pull in every f l ll i direction, balancing the forces. Newton concluded that the Newton concluded that the universe must be infinite. Einstein's Gravity y Albert Einstein developed his General Theory of Relativity in 1915. Th f R l i i i 1915 It was confirmed by measurements of the bending of light during a solar eclipse in 1919. Einstein's laws show that space itself can have p motion, and he found that even in an infinite , p universe, space could not remain static. He added a "cosmological constant" to his equations that could counteract the pull of equations that could counteract the pull of gravity to provide a static solution. Our "Island Universe" An allsky picture reveals that the faint band of light we call the Milky Way completely surrounds us. A telescope reveals that the faint light comes from hundreds of billions of distant stars. It was concluded (before 1920) that we live inside a diskshaped "island" of stars surrounded by a vast void. We also see scattered about the sky various dimlyglowing objects called "nebulae." ll d " b l " Some of these have an unusual spiral pattern. l l Philosophers as early as Immanuel Kant in 1755 l i speculated that the spiral nebulae might be spiral nebulae might be other "island universes" like the Milky Way. like the Milky Way Spiral Nebulae The Location of Spiral Nebulae The spiral nebulae appear to "avoid" the plane of the Milky Way. Does this mean that the spiral nebulae are part of the Milky Way since they seem to "know" about its location? Milk W i h "k " b i l i ? The Great Great Debate The Great Nebula in Andromeda Are the spiral nebulae part of the Milky Way, or are they separate "island universes" or "galaxies." Shapley & Curtis held a public debate in 1920, but the data were generally considered inconclusive. Individual stars could not be discerned in the spiral nebulae, and why did they "avoid" the plane of the Milky Way if they weren't interacting with it? Other Island Universes I l dU i In 1925, using the then In 1925, using the then largest telescope, Edwin Hubble could observe individual stars in the Andromeda galaxy and proved it was distant. di di At its distance of about 2 million light years, it is the illi li h i i h most distant object visible to the naked eye, a galaxy to the naked eye a galaxy bigger than the Milky Way. Other Galaxies
150,000 light-ye ears By 1926 Hubble identified a variety of galaxy types that ranged from yellow galaxy types that ranged from yellow Ellipticals through a sequence of Spirals to blue Irregulars Spirals to blue "Irregulars" S0 Sa Sb Sc Sd Irr Redshifts of Galaxies Redshifts of Galaxies Astronomers in the early 1900s had discovered that galaxies show "redshifts" implying that the redshifts galaxies are moving away from us at high speeds. The Cosmological Redshift The Cosmological Redshift The more distant a galaxy is, the more its spectrum The more distant a galaxy is the more its spectrum is found to be shifted to longer wavelengths. Hubble's Diagram g
Rece ession Veloci ity
1000 km/s 500 km/s 0 0 1 Mpc 2 Mpc Distance In 1929 Hubble showed that the recession velocity increases in proportion to the distance. Hubble's Law Since galaxies have increasingly larger redshifts with distance, they are not only flying away from with distance they are not only flying away from us, but from each other as well. There's nothin special abo t the Milk Wa 's There's nothing special about the Milky Way's position in the universe. Einstein showed that it isn't that galaxies are moving through space--space itself is expanding. The redshift that we see for a galaxy is caused by the stretching of light waves as they travel through expanding space. Theories of Expansion Constant Expansion Hypothesis: Willem de Sitter found a solution to Einstein's equations in which the cosmological constant makes space accelerate perpetually. Primordial Egg Hypothesis: g Georges Lematre's solution required no cosmological constant, but begins the , g universe with an explosion. SteadyState Model As space expands, everything should get farther apart, but there was no evidence of this in the early 1900s evidence of this in the early 1900s After seeing the movie "Dead of Night," Bondi, Gold, and Hoyle proposed the steadystate model in 1948. In their model, the universe expands forever, but remains unchanging because new b t i h i b matter is created, and forms new galaxies. Toward the Beginning As we look outward, we look back to earlier times. t If there was a Big Bang, at greater earlier times all matter was distance packed together more densely. packed together more densely As the universe expanded, gas in it would have cooled down. in it would have cooled down In every direction we look, we should see back to the time when should see back to the time when gas everywhere was hot and dense. The gas was too hot and dense before The gas was too hot and dense before 400,000 years to see further. longer l ago Echo of a Big Bang In 1964 Arno Penzias and Robert Wilson made radio wavelength observations of the Milky Way. Efforts to reduce noise in the detector left them with a small d l f h ih ll amount of residual radiation that they couldn t explain or get rid of. they couldn't explain or get rid of This radiation had a temperature of a few degrees above absolute of a few degrees above absolute zero--just what George Gamow in 1948 had predicted from a Big Bang, disproving the Steady State model. Wilson Penzias Gamow Modern satellite measurements show that the CMB radiation di ti is extremely uniform in all directions. The Cosmic Microwave (CMB) Background Radiation Background Radiation The radiation comes from a time about 400,000 years after the Big Bang, when the universe's temperature had cooled to about 3000 K. If we look carefully, there are tiny fluctuations in the carefully CMB (1 part in 100,000)--the beginnings of galaxies. Upcoming Lectures Upcoming Lectures
Next time... Trying to understand how the Big Next time Trying to understand how the Big Bang might have been launched, and the discovery of Dark Matter & Dark Energy. discovery of Dark Matter & Dark Energy Next week... How do galaxies and stars form, evolve, and give rise to the conditions of the modern universe. ...
View Full Document
This note was uploaded on 11/22/2011 for the course BIOLOGY 190a taught by Professor Staff during the Fall '11 term at UMass (Amherst).
- Fall '11