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

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

Some Half-lives Parent isotope Daughter isotope Half-life Rubidium-87Strontium-8749 billion years Uranium-238Lead billion years Potassium-40Argon billion years Chlorine-36Argon-36300,000 years Carbon-14Nitrogen years Nitrogen-13Carbon-1310 minutes

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?

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?

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.

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.

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.

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

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

(SN1987a pictures and brightness- luminosity diagram shown here)

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

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.

Hubble’s original data:

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

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

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

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

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 astro-ph/ )

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

Globular Cluster: 47 Tuc Keel et al. APOD

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)

(H-R Diagram would be shown here)

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

An old star cluster

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

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

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 , 155

5 Independent Age Measurements 1.Radioactive dating of moon rocks and meteorites: 4.6 billion years old 2.Light travel time to distant galaxies: ~6 billion years old 3.Continuous expansion of universe: 13.6±0.2 billion years old 4.Stellar life cycle: 13±1.5 billion years old 5.White dwarf cooling time: 12.7±0.7 billion years old

To learn more: “Radiometric Dating: A Christian Perspective”, R. C. Weins ns.html ns.html An Ancient Universe: Special Edition of “The Universe in the Classroom” ublications/tnl/56/index.html