8. The Age of All that is
Morsel:
Meal:
There are several methods that are used and combined to determine the
age of the universe so accurately. In simple terms: by studying the
oldest celestial objects within the universe and by measuring how
fast the universe is “expanding”.
By finding the oldest stars, astronomers can determine the lower
limit: the age of the universe must be at least as old as the oldest
star. How long a star lives is determined by its mass: the greater
the mass, the brighter its flame burns, but the quicker it expends
its hydrogen fuel. The Sun, for example, can continue casting its
current luminosity for around another 9 billion years. A star half
its size, however, would burn through its supply in around 800
million years.
A globular cluster is an extremely dense collection of approximately
a million stars, which often contains the most ancient stars in the
observable universe. These stars were born at roughly the same time
and dwell tightly together at the same distance from us. If a
globular cluster is more than 20 million years old, then all of its
stars will be less massive than 10 solar masses, where a solar mass
equals the mass of the Sun. The oldest globular clusters contain
stars less than 0.7 solar masses in size and shine dimly. Such
clusters have been calculated to be around 12 billion years old.
However, this does not definitively tell us the age of the universe
due to the fact that we do not know for certain how long it takes for
these clusters to form nor the exact distance to these clusters from
us. If the cluster is farther away than initially thought, the stars
would be brighter, more massive, and thus younger.
Another useful ancient “candle” is a white dwarf, the stellar
core left behind by a supernova from stars whose mass is relatively
small. White dwarfs are extremely compact: they have mass comparable
to the Sun but comprised in a volume similar to that of the Earth's.
They are also extremely cool and so shine very dimly. By finding
white dwarfs that are especially cool, astronomers can estimate the
length of time it took to become so cool. Such estimations produce a
value of around 12.7 billion years.
By measuring the “Hubble constant”, which indicates the current
expansion rate of the universe. This relates to how quickly points in
space are “moving away” from each other. A common analogy is to
imagine the universe as a balloon with white pen marks on it
representing the galaxies. As the balloon expands, the marks move
farther away from each other but they do not move themselves. Like in
the picture below:
In the 1920s, Edwin Hubble discovered that the distant galaxies were
moving away from us and the more distant these galaxies, the faster
they moved, and that this was based on the eponymous constant. The
Hubble constant is determined by accurately measuring distances to
celestial objects via “cosmic yardsticks”. These yardsticks rely
on a class of stars called Celpheid Variables, which pulsate at a
predictable rate, and once astronomers “calibrate the Cerpheids'
true brightness, they can use them as cosmic yardsticks to measure
distances to galaxies much farther away” than with the normal
“parallax” technique, which surveyors on Earth use to measure
distances.
By knowing the Hubble constant and extrapolating backwards to the Big
Bang, it reveals an age of around 13.8 billion years for the
universe. This is based on one of the most popular models; the Hubble
constant has not been totally constant throughout the lifetime of the
universe – the expansion rate of the universe is currently
accelerating, for example – and it has yet unclear how the Hubble
constant has changed in the past. Despite this, this method is
believed to be the best determiner for the age of the universe.
Recipe:
Anon. n.d. “How
Old Is the Universe?” NASA. Retrieved December 8, 2017.
Anon. 2013. “Hubble
Views an Old and Mysterious Cluster.” Space Telescope.
Retrieved December 8, 2017.
Stierwalt, Sabrina. n.d. “How
Do We Determine the Age of the Universe? (Advanced)” Astronomy
Department at Cornell University. Retrieved December 8, 2017.
Anon. 2016. “Three
Steps to Measuring the Hubble Constant.” NASA. Retrieved
December 8, 2017.
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