The ages of the Earth and the Universe have been estimated by many people over the centuries, and one can construct additional estimates from various historical and cosmological works. But in recent centuries, scientists have developed a variety of techniques for estimating the age of the Earth and the Universe, and to the displeasure of young-earth creationists, those ages turn out to be much greater than the range of ages of he Universe that they prefer to believe.
- Mayan: according to the Long Count Calendar, the world of human beings was created in August 11, 3114 BCE.
- Chinese: according to Xu Zheng (220–265 CE; the Three-Five Historic Records) ~ 39,000 BCE
- Hindu: according to various Puranas, the Universe is cyclic, with various works giving a variety of cycle periods. The number of cycles is very large or infinite. A hymn in the Rig Veda, 10:129, famously asks if the gods know how the Universe came into existence.
- Jain: the Universe is cyclic, with an infinite number of cycles.
- Buddhist: the Buddha, in his Parable of the Poisoned Arrow, argued that there are better things to do than to try to solve insoluble problems like the age of the Universe.
- Sumerian: from the Sumerian King List, "kingship descended from heaven" around 244,100 BCE.
- Babylonian: various estimates, ~ 400,000 – 200,000 BCE.
- Zoroastrian: ~ 9600 - 9500 BCE.
- Egyptian: Turin King List: 39,670 BCE, Manetho: 39,575 BCE and some other values, Diogenes Laertius: 49,219 BCE.
- Greco-Roman historians: the first datable event is usually the flood of Ogyges, around 2000 BCE. Anything before that is too obscure.
- Greco-Roman philosophers: the Atomists and Epicureans, Aristotle, and the Stoics believed that the Universe is eternal, with the Stoics also believing that the Universe is cyclic, periodically destroying itself in a cosmic fire. Democritus, one of the Atomists, believed that "worlds" form and get destroyed inside a larger Universe, suggesting that the Earth has a finite age in an infinitely-old Universe.
There have been numerous attempts to calculate the age of the Universe from the Bible, by adding up its numerous begots, and using various hypotheses to fill in the gaps. Most of the dates cluster in two ranges of values, depending on what version of the Old Testament that the calculators use.
- Masoretic: ~ 4000 BCE
- Septuagint: ~ 5500 BCE
Young Earth Creationism was taken for granted by most believers in Christianity and Judaism until the discovery of convincing geological evidence to the contrary. However, some practitioners of Kabbalah, Jewish mystical lore, had come up with imaginative interpretations of the Bible that make the Universe as much as 15 billion years old (Rabbi Aryeh Kaplan, Kabbalah and the Age of the Universe).
Starting in the late eighteenth century, geologists like John Phillips and physicists like Lord Kelvin used various techniques to estimate the age of the Earth. Techniques like the time for the Earth to cool down from a molten state, and the time necessary to build up the numerous layers of sediment that formed the strata of sedimentary rocks. They usually obtained numbers between 20 and 100 million years.
But the discovery of radioactivity in 1896 opened the question again by offering an additional heat source. It also provided a way of dating rocks that did not depend on sedimentation-rate estimates: radiometric dating. It took a few decades to work out the details of suitable radioactive-decay series like uranium-to-lead, and to learn how to use them, but by the 1930's, it was evident that the Earth was at least 3 billion years old.
The Earth's age is difficult to determine from its rocks, because of its geological activity and erosion. But some very old rocks have been found. The oldest known rock formations are about 3.5 billion years old, the oldest known rocks about 4.1 billion years old, and the oldest known mineral crystals are 4.4-billion-year-old zircon crystals from the Jack Hills in Australia, themselves 3 billion years old. One can determine an overall age from overall lead-isotope fractions and some hypotheses about their initial values. One finds about 4.5 billion years.
This agrees with ages of meteorites and estimates of the ages of the Moon and Mars, and even with the age of the Sun as inferred from helioseismology. That method uses sunquakes to probe the interior of the Sun, providing inputs for estimating how much helium the Sun has made from hydrogen in its core.
Turning from the Earth to the Universe as a whole, Edwin Hubble discovered an important clue in 1929, the expansion of the Universe. He and his successors next tried to find out how fast it is expanding, since that can be extrapolated backward to when it started. The age of the Universe is now estimated at 13.75 +- 0.11 billion years.
But is this a restart of expansion after a previous era that ended in collapse? Is our Universe a bubble in a multiverse? Such possibilities can yield infinite age, though in a multiverse, an overall time may not be very meaningful.