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-livesParent isotope

Daughter isotope

Half-life

Rubidium-87 Strontium-87 49 billion years

Uranium-238 Lead-206 4.5 billion years

Potassium-40 Argon-40 1.26 billion years

Chlorine-36 Argon-36 300,000 years

Carbon-14 Nitrogen-14 5715 years

Nitrogen-13 Carbon-13 10 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/secondThis 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. 2002 astro-ph/0205387)

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 981107

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 2002 574, 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 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