Recalibrating the Cosmic Clock

Author:  Cristina Rabaza
Institution:  University of Florida
Date:  November 2010

Astrophysicists are one step closer to estimating the age of our galaxy by shrinking their margin of error to less than a billion years, according to a trio of papers released mid-July in the journal Physical Review C.

The Milky Way is currently said to be 14 billion years old, but the papers explain how scientists have gained ground , and precision , in refining this number, particularly by using neutron-beaming experiments to measure the abundance of the element Osmium-187.

But why is this member of the periodic table so crucial to pinpointing the galaxy's age? Osmium-187's significance is largely due to its relationship with its cosmic counterpart: Rhenium-187.

Rhenium-187 was born amid the first stellar explosions of the Milky Way. Since Rhenium-187 eventually decays into Osmium-187, the ratio relationship they exhibit "is not much more complicated than sand running through an hour-glass," according to the Darmstadt Heavy-Ion Laboratory in Germany. "If the rate of flow of the sand through the narrow glass waist is known, then a time scale can be assigned to the pile of sand below."

If the half-life of Rhenium-187 is known (42 billion years), and the relative abundances of the nuclei are also known at a particular point in time, the ratio between the two elements reveals the time elapsed, or the galaxy's age. This method of calculation can be likened to a "cosmic clock."

And as the clock ticks, trouble is on the rise. Scientists have been stumbling over obstacles to measuring Osmium-187 for quite some time now. This is because other processes , besides the decay of Rhenium-187 , contribute to Osmium-187's abundance (as well as its depletion). Consequently, the Rhenium-Osmium ratio used to calibrate the "cosmic clock" has been obscured, cutting down on its accuracy to more than a billion-year margin of error.

This is why scientists have set out to perform the CERN and Karlsruhe experiments (Riordon, J., 2010) as an attempt to increase the accuracy. In the experiments, scientists fired pulses of neutrons into an osmium target to see how often it converts into another material. The experiments have now successfully narrowed down the rhenium-osmium cosmic clock accuracy to less than a billion-year margin of error.


1. Alden, Andrew. (n.d.). Rhenium-osmium studies. Retrieved from

2. Riordon, J. (2010). Refining a cosmic clock. EurekAlert, Retrieved from Riordon, J. (2010, July 15). Refining a cosmic clock: particle accelerator research helps narrow down the age of our galaxy. EurekAlert!, Retrieved from releases/2010-07/aps-rac071510.php

3. PDF Article from the Darmstadt Heavy-Ion Laboratory: Hubble, Dark Matter and Rhenium-187 or: How Old is the Universe?

Author: Cristina Rabaza

Reviewed by: Natasha Hochlowski, Selby Cull, and Yangguang Ou

Published by: Maria Huang