Hi! I’m Everyday Einstein, Sabrina Stierwalt, bringing you Quick and Dirty Tips to help you make sense of science.
Our universe is 13.8 billion years old, a timescale much longer than the more relatable spans of hundreds or thousands of years that impact our lived experiences. So how do astronomers arrive at such an enormous number?
Ancient Stars
The universe, quite simply, must at least be as old as the oldest thing we can find in it. Thus, a direct test of the age of the universe is to go hunting for ancient stars.
Stars in clusters, or agglomerations of stars, all born at the same time, can be most accurately age-dated by looking for what is called the “main sequence turn off” for the cluster. The longest stage of a star’s life is spent burning hydrogen. During this phase, stars follow a relation between their temperature and brightness known as the main sequence. In other words, hotter stars shine brighter.
Once a star runs out of hydrogen to burn, it will begin to cool and thus leave this main sequence relation before becoming a supergiant, a white dwarf or even a black hole. Based on our knowledge of stellar evolution, astronomers can estimate how long certain types of stars will continue burning hydrogen on the main sequence. Our Sun, a relatively low mass star, has been burning hydrogen for nearly 5 billion years and will continue to do so for another 4 to 5 billion more. Even though they have more fuel to burn, more massive stars spend a shorter time on the main sequence because they burn through that fuel much faster.
See also: Will the Sun Explode?
As a star cluster ages, the most massive stars leave the main sequence first, followed by stars of decreasing mass. Observations of relatively young star clusters will thus reveal all types of stars filling out the main sequence. Older clusters will show a less complete main sequence as the most massive stars have already exhausted their hydrogen fuel and “turned off” the main sequence.
The most massive stars still on the main sequence (i.e. still burning hydrogen) place a limit on the age of the cluster. The oldest observed star clusters have ages in the range of 11-13 billion years.
White Dwarfs
In a previous episode, we discussed the end stages of a dying star’s life and how a low mass star like our sun can evolve into white dwarf. White dwarfs are extremely dense objects that pack the equivalent of the Sun’s mass into the size of Earth. A teaspoon of white dwarf material weighs 15 tons!
Since white dwarfs are no longer burning elements through fusion to produce and emit radiation, they are instead left to cool much like the dying embers of a fire. The temperatures of white dwarf stars can thus tell us how long they have spent cooling and place a limit on their age. Observations with the Hubble Space Telescope find the oldest white dwarfs to be in the range of 12-13 billion years old.
The Cosmic Microwave Background
While age-dating the universe through ancient stars is an important check, the most direct determination of its age comes from relic radiation left behind from the Big Bang, called the cosmic microwave background radiation, or CMB for short.
Simply put, our universe is expanding as time goes on, leaving more and more space between us and our extragalactic neighbors. We can turn back the clock however, by rewinding this expansion, through the help of information encoded in the CMB, to determine how long the universe has been expanding.
The CMB is radiation produced during the Big Bang, the singularity that in a fraction of a second began the combination of high densities, temperatures, and pressures that later expanded and cooled into the universe we observe today. The CMB radiation has cooled significantly as it has traveled, but it still encodes information from the Big Bang event. The CMB thus offers us the equivalent of a baby picture of our universe, a snapshot of what occurred in the beginning.
The age of the universe is tied to 3 cosmological parameters that together describe the expansion of the universe:
- the rate of expansion of the universe, known as the Hubble constant
- the density of both baryonic (normal) and dark matter in the universe (i.e. how much matter needs to be expanded)
- the cosmological constant, a parameter tied to the acceleration of that expansion
This method finds the age of the universe to be 13.8 billion years old, plus or minus 37 million years. This uncertainty in the age, which is relatively small compared to a total time of 13.8 billion, comes from the uncertainties associated with measuring each of the three cosmological parameters.
To put this age in perspective, the age of our Solar System is only about 4.5 billion years. Certain isotopes that were created with the Solar System, like potassium and uranium, offer clues as to the age of our Solar System. These isotopes undergo radioactive decay, and thus, offer a very accurate measurement of the time elapsed since their formation.
The fact that the age determined by the CMB is consistent with the minimum ages calculated for the oldest star clusters and white dwarf stars tells us astronomers that we are on the right track. Keep in mind, though, that we are defining the age of the universe as the time that has elapsed since the Big Bang. None of our observational evidence can tell us what may have happened before the Big Bang—a question that may be better answered by a theoretical astrophysicist or even a philosopher, rather than an observational astronomer.
WMAP image of CMB courtesy of nasa.gov.
Hubble Space Telescope Deep Field courtesy of nasa.gov.
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