How do scientists determine the ages of the Earth and the Universe? -Peyami

Earth

Age of Earth

Planet Earth is approximately 4.54 billion years old. But how did scientists determine this? With the radiometric dating of meteorites and the ages of the oldest known minerals. While the oldest meteorites found on Earth are approximately 4.5 billion years old, the oldest known mineral, zircon, discovered by Jack Hills in Australia is at least 4.4 billion years old. One meteorite used was the Canyon Diablo meteorite (4.55 billion years old) aged by C. C. Patterson. Since most of Earth’s minerals have undergone change in the core, mantle, and crust by plate tectonics, weathering, and hydrothermal circulation (circulation of hot water), scientists usually cannot use them in dating Earth. However, scientists used ancient Archaean lead ores of galena (natural mineral form of lead II sulfide), the earliest formed homogenous lead isotope, which very precisely dated Earth at 4.54 billion years. Furthermore, inclusions rich in calcium and aluminum in meteorites were formed within the solar system about 4.567 billion years age. As the oldest known solid component of meteorites, these Ca-Al inclusions determine the age of the solar system and set the upper limit of the age of Earth. Scientists do not known the time of Earth’s accretion (growth by gravitationally attracting more matter), but believe it started some after the Ca-Al inclusions formed.

In fact, scientists have long debated over and calculated the age of Earth. People had estimated Earth at just hundred of thousands of years! Later, scientists extended their estimates with more evidence. However, it wasn’t until Charles Darwin, who proposed the theory of natural evolution, that scientists began to make closer estimates. Using the molecular clock and the rate of genetic divergence, scientists estimate the last universal ancestor of all organisms at 3.5-3.8 billion years old.

Expansion of the Universe

Age of the Universe

The age of the Universe is 13.75 billion years old. People long thought the Universe as much younger— millions, let alone billion of years old. Edwin Hubble’s observations in the 1920s showed that the Universe has a finite age. Using Doppler Shift, Hubble discovered that the Universe was expanding. Every galaxy seemed to be moving away from each other, showing red shifts in their spectral lines. In 1958, Allan Sandage made the first calculation of a value called the Hubble’s constant, which determines the rate of the Universe’s expansion. With the Hubble’s constant, Sandage made the first accurate (closer than before) estimate of the age of the Universe at ~20 billion years. Discovered in 1965, the microwave cosmic background radiation, a remnant of the Big Bang, confirmed the expanding Universe theory. As the Universe expanded, it gradually cooled. The CBR shows the Universe at 2.7 K. In fact, scientists have recently discovered dark energy. Dark energy accelerates the expansion of the Universe, reducing earlier estimates of >14 billion years to 13.75 billion years.

Graph of Expansion: Expansion is Accelerating!