# How Old Is the Universe? How Do We Know?

Scientists are homing in on the answer to the question, “How old is the universe?” The age of the universe is around 13.8 billion years old, with an error in the estimate of 1%. The high degree of certainty comes from comparing estimates made using different methods.

• The universe is around 13.8 billion years old, with an error of 1% or about ±100 million years.
• Estimates of its age from comparing the age of the oldest stars and the expansion of the universe since the Big Bang.
• The rate of expansion is the Hubble constant. As scientists refine its value, we get closer to knowing the exact age of the universe.

### How Do We Know How Old the Universe Is?

There are two main ways of finding the age of the universe. The first is finding the oldest stars and working backwards on what we know about star formation to estimate an age. The second method involves tracing back the growth of the universe from the Big Bang, based on cosmic expansion.

#### The Oldest Stars

Both methods are complicated. Finding the oldest stars is tricky business. The first stars formed from only hydrogen and helium, making new elements through fusion. Because they were massive, they burned very brightly, but burned out quickly. So, scientists look at globular clusters that no long have these bright blue stars. The oldest globular clusters contain stars that are between 11 and 14 billion years old. There is some error in the estimate because it’s difficult pinpointing the distance to the clusters. Distance, in turn, affects apparent brightness, which is a key factor in calculating mass and age. Regardless, these measurements offer a minimum age for the universe, since it cannot be younger than its oldest stars.

#### The Expansion of the Universe

Scientists estimate the age of the universe using its rate of expansion, which is called the Hubble constant. The Hubble constant takes its name for astronomer Edwin Hubble. Hubble’s law states that there is a correlation between how far away an object is and the speed at which it is receding. So, if we know the distance an object travels and how far it is from the Big Bang origin, we know the age of the universe.

Astronomers determine the Hubble constant using two different methods: cosmic microwave background (CMB) measurements and local distance measurements. The CMB is the afterglow of the Big Bang, which provides a snapshot of the universe when it was just 380,000 years old. By analyzing the CMB, scientists infer the expansion rate of the universe, which is a more global measurement.

Local measurements, on the other hand, involve observing celestial objects like supernovae and Cepheid variable stars. These objects act as cosmic distance markers. Local measurements provide a direct estimate of the expansion rate, but they are limited to the nearby universe. As it turns out, the rate of cosmic expansion is not constant, so researchers combine CMB and local measurements for estimating the age of the universe.

### Refining the Age of the Universe

Scientists now know the age of the universe with a high degree of certainty. The Wilkinson Microwave Anisotropy Probe (WMAP) project, Planck space observatory, and Atacama Cosmology Telescope (ACT) have all played significant roles in determining the age of the universe. WMAP, launched in 2001, provided high-resolution measurements of the CMB temperature fluctuations, allowing scientists to estimate the age of the universe at 13.77 billion years.

The Planck space observatory, launched in 2009, built upon WMAP’s success by providing even more precise measurements of the CMB. Planck’s data led to a revised estimate of the universe’s age, placing it at 13.82 billion years old.

The Atacama Cosmology Telescope, located in the Chilean Andes, has been instrumental in studying the polarization of the CMB. The Atacama data corroborates the WMAP and Planck missions, making the universe around 13.8 billion years old.

### What Came Before the Big Bang?

Dating the age of the universe answers the question of how long it has been since the Big Bang. However, the universe might have expanded and contracted to a singularity, forming the Big Bang, as part of an endless cycle. Or, there might be other universes distant from our own, like giant bubbles in space. If either theory is true, then the “beginning of time” (if it exists) massively pre-dates the age of the universe.

### References

• Aghanim, N., Akrami, Y., et al. (2020). “Planck 2018 Results”. Astronomy & Astrophysics. 641. doi:10.1051/0004-6361/201833910
• Bennett, C.L.; et al. (2013). “Nine-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Final maps and results”. The Astrophysical Journal Supplement Series. 208 (2): 20. doi:10.1088/0067-0049/208/2/20
• Choi, Steve K.; et al. (2020). “The Atacama Cosmology Telescope: A measurement of the Cosmic Microwave Background power spectra at 98 and 150 GHz.” J. Cosmology and Astroparticle Physics. doi:10.1088/1475-7516/2020/12/045
• Hubble, E. (1929). “A relation between distance and radial velocity among extra-galactic nebulae”. Proceedings of the National Academy of Sciences. 15 (3): 168–173. doi:10.1073/pnas.15.3.168
• Riess, Adam G.; Casertano, Stefano; et al. (2018). “Milky Way cepheid standards for measuring cosmic distances and application to Gaia DR2: Implications for the Hubble constant”. The Astrophysical Journal. 861 (2): 126. doi:10.3847/1538-4357/aac82e