How astronomers measure the age of the universe

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

28 January 2024

Last week we ended with this question: “can we see so great a distance away that the light we see is the one that was emitted at the beginning of the universe?” The answer is yes, but it’s not straightforward. Well, before pursuing that problem, we should first establish whether the triangulation technique can measure such great distances. The answer is no.

The farther away an object is, the smaller the angles to be measured. The limit of measurable angles is reached even before we observe objects outside our own galaxy – the Milky Way. That is about a hundred thousand light-years.

Other methods are required for distances greater than a about 500 light years. A common one is measuring the brightness of the stars. Unfortunately, however, stars do not have the same luminosity. One may appear dim because it is very far away while another could also be dim because it is generating very little light.

Fortunately, the intrinsic brightness of stars can be determined and, when this is compared the luminosity observed from earth, the distance to that star can be evaluated. This method has been used to measure distances up to many millions of light-years.

But it some objects can be too far away for their intrinsic brightness becomes to be accurately measured. That calls for a different method of distance measurement. Over the last 100 years, astronomers have observed that galaxies are moving away from each other. And the farther a galaxy is, the faster it is observed to move. The relationship between distance and speed has been determined fairly accurately (the Hubble constant).

The speed of a galaxy is determined by careful observation of the wavelengths of its light. The faster it moves, the more these wavelengths are shifted towards the red end of spectrum. From this so-called “red-shift”, the speed can be calculated, and, from that speed, the distance is evaluated. This extends the measurable distances to many billions of light-years.

Now, when we observe an object moving away from us and we know its speed and distance, we can work out when it was at the origin of its motion. Speed is distance divided by time taken, so the time taken is distance divided by speed.

For all galaxies, the time taken comes to the same value: 13.5 billion years. Is that the age of the universe? We may not be sure, but it is the time when all galaxies started their motion…