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Astronomical Evidence that the Universe is Billions of Years Old

Explore ages determined by radioactive dating, light travel time, & expansion rates in understanding the universe's past. Delve into supernova distances, Hubble's expansion discovery, and the lifetimes of stars.

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Astronomical Evidence that the Universe is Billions of Years Old

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  1. Astronomical Evidence that the Universe is Billions of Years Old Dr. Deborah Haarsma, Calvin College NWCSI-CTABC October 10, 2002

  2. Age from Radioactive Dating • Radioactive isotopes are atoms with an unstable arrangement of protons and neutrons • Half-life is the length of time for half of the radioactive sample to decay. Very stable decay rate. • Find age from relative amounts of parent and daughter isotopes

  3. Some Half-lives

  4. Example: Nitrogen-13 decay • Start with 4000 atoms of Nitrogen-13 • Half life is 10 minutes • How much left after 10 minutes? After 20 minutes? After 30 minutes?

  5. Example: Potassium-40 decay • Half-life is 1.3 billion years • Decays to Argon-40 in gas form • As long as rock is molten, Argon gas can bubble out. When rock hardens, gas is trapped. • Let’s say you find a rock with equal amounts Argon-40 and Potassium-40. How old is it? • What if the rock has 3 times more Argon-40 than Potassium-40?

  6. Careful Radioactive Methods • Some rocks contain fragments of different ages - don’t use them • Make sure sample not contaminated • Measure long half-lives by comparing different isotopes in same rock. • Know the initial amount of the parent isotope.

  7. Age from Radioactive dating: conclusion • Oldest moon rocks and meteorites are 4.6 billion years old. • Oldest Earth rocks are 3.6 billion years old.

  8. Age from Light Travel Time • Light travels at 299,792,458 meters/second This speed is constant everywhere in space (if it weren’t, all physical processes would look different in other galaxies). • Measure the distance to an object (this is the hard part). • Use light travel time to calculate how long ago the light was emitted. • The Universe is a “time machine” – more distant galaxies are younger than nearby ones.

  9. Measuring distances • One of the hardest tasks in astronomy • Many methods in use, can check each other • Ladder: Find distance to nearby objects, than compare properties of nearby and far-away objects to find distance of far-away objects

  10. Distances to Supernovae • Each supernova are extremely luminous, can be detected in distant galaxies • Type Ia supernovae all have same luminosity. (collapse & explosion of white dwarf star, luminosity measured for local objects of known distance) • Measure brightness, know luminosity, calculate distance

  11. (SN1987a pictures and brightness-luminosity diagram shown here)

  12. Age from light travel time: Conclusion • Most distant Type Ia Supernova detected is 6 billion light years away • Universe must be at least 6 billion years old

  13. Age of Universe from Expansion • Edwin Hubble discovered in 1929 that galaxies are moving away at a speed proportional to their distance. v = Hd • The relationship between speed and distance means that space itself is expanding. • Every galaxy experiences the same effect, so we are not at a special place in the universe.

  14. Hubble’s original data:

  15. As dough for raisin bread rises, all of the raisins move apart at a speed proportional to their separation.

  16. Determining the age from expansion • Measure the expansion rate carefully. • “Rewind” the expansion and find when the size was zero • Either assume a constant expansion rate, or determine how expansion rate has changed

  17. (shown here: diagram of redshift vs. distance for several methods, diagram of scale factor vs. time)

  18. Matter slows expansion • Gravity pulls matter in the universe together, slowing the expansion • “Critical density” of universe = density that slows the expansion to a stop without causing collapse

  19. Age from Expansion rate: conclusion • If universe has expanded at a constant rate of 71 km/s per Mpc since the beginning, the current age is 14.0 billion years • If the expansion has been slowed by mass but accelerated by “dark energy”, the current age is 13.6±0.2 billion years (Sievers et. al. 2002 astro-ph/0205387)

  20. Age of Globular Star Clusters • All stars in cluster formed at the same time out of the same cloud of gas & dust • Up to 1 million stars • Diameter up to 300 lightyears • Gravity easily holds cluster together

  21. Globular Cluster: 47 Tuc Keel et al. APOD 981107

  22. Temperature and Luminosity:The H-R Diagram • Temperature, mass, and luminosity of stars are related. • For “Main Sequence” stars, temp and luminosity increase with mass • Stars spend most of their lives on the main sequence, powered by hydrogen fusion • Then they become cooler and more luminous (red giants)

  23. (H-R Diagram would be shown here)

  24. Lifetime of stars • Fuel is hydrogen in the core. Amount is proportional to total mass of star. • Fuel consumption rate is luminosity of star • Lifetime is amount of fuel divided by fuel consumption rate = mass / luminosity • Thus, we can calculate the time a star spends on the main sequence • High-mass stars burn up fast (flash in the pan), low-mass stars last longer

  25. An old star cluster

  26. (H-R Diagrams of star clusters of various ages would be shown here)

  27. Age of Star Clusters: conclusion • “Turn-off” point of cluster H-R diagram shows which main sequence stars are coming to the end of their lives • We know the lifetime of stars on main sequence (gravity, pressure, fusion, etc.) • Lifetime of these stars is age of cluster • The oldest star clusters are 13±1.5 billion years old

  28. Age from White Dwarf Cooling • Initial temperature of white dwarfs known from recent planetary nebulae • Cooling rate easily calculated from energy radiated • Temperature of white dwarfs in globular cluster M4 leads to age of 12.7±0.7 billion years • Hansen et al 2001 Astrophysical Journal 2002 574, 155

  29. 5 Independent Age Measurements • Radioactive dating of moon rocks and meteorites: 4.6 billion years old • Light travel time to distant galaxies: ~6 billion years old • Continuous expansion of universe: 13.6±0.2 billion years old • Stellar life cycle: 13±1.5 billion years old • White dwarf cooling time: 12.7±0.7 billion years old

  30. To learn more: • “Radiometric Dating: A Christian Perspective”, R. C. Weins http://www.asa3.org/ASA/resources/wiens.html • An Ancient Universe: Special Edition of “The Universe in the Classroom” http://www.astrosociety.org/education/publications/tnl/56/index.html

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