Imagine if our world had a diary and on the interesting days, someone pulled out a big book and wrote about it.
In our puny life times, we think of a couple of thousand years as a long time and that is not so surprising given the short span of our lives, a mere 7 or 8 dozen passes around the sun if you’re lucky and that’s it, you’re done.
Look at our history, a mere 500 generations back our grandfathers were just learning the art of growing food. Ten thousand generations back and we had only just become a clearly defined species in our own right, so how could we appreciate the passage of say, a million years?
If we struggle with a million, then what of this day in our diary when the Sun and the Earth were still six thousand million years in the future? Let’s just think about that again, not one million or a hundred million, but six thousand million years in the future. That’s when our Sun and Earth will come into existence.
The universe has already been doing its thing for so long, entire galaxies of stars were already 3 billion years old. Not only stars but planets too and yes, probably even life in some form. To get just a glimpse of this time scale, say a million years equals a week.
From this point in the life of the universe, you would still have to wait another 115 years for the Sun and the Earth to form.
Then you’d have to wait another 87 years before reaching the dawn of homo sapiens. All this time at the rate of a million years every week, is a very long time to wait, time scales that are hard to comprehend.
The difficulty of grasping these concepts is the reason why we find it so hard to see how astronomers in particular can be so sure their time and distance measurements and estimates are realistic.
It is one of the most puzzling matters for non-scientists, the certainty of the incredible distances involved in astronomy. Not only that, but what the stars are made of, how hot they are etc,. How do they know?
Back at the beginning of the 20th Century, well before the First World War, a Dane and an American discovered the brightness (luminosity) of many stars was in step with the composition. They already had the means to determine the content of a star, the mixture of gas and metals by analysing the light. They also knew that large objects cool at a faster rate than small ones because of surface area so these two clever chaps were the names behind the Hertzsprung-Russell Diagram. They now had a method to measure the diameter of a star, the composition and the temperature.
By cross checking size temperature and brightness, many types of stars became apparent. For example, they can tell from the light a given star is generally converting hydrogen in helium but if the light is both very bright and mainly red, it will be a star well past its prime, running out of hydrogen and expanding, much like what will happen to our own Sun in the distant future. If the star is still very hot but small (measuring its luminosity) it will be a white dwarf, having already used up all its hydrogen and the remaining helium.
Important point, there is some controversy relating to the Big Bang itself, not least from those who prefer a ‘creator’ explanation, however regardless of the accuracy of the description of the event itself, all the measurements relating to the age of the universe are taken from this point. In other words, doubts about the origin of the universe do not affect the confirmed age, so far, of 13.8 billion years. It may well be even older. We have yet to find out.What we need to know however, is how these measurements are done.
The most basic tool in the astronomer’s kit is the parsec, described earlier, which is a simple calculation you can do in the back yard at home for calculating the distance between two objects, one behind and slightly to the side of the other. We know the diameter of the earth so we know how far we have moved left to right during half a year’s turn around the sun. By measuring the angle of change between the two objects we can easily work out the distance between them.
If you know the distance to the first, you just add the difference to get the second.
Another calculation we can do here on Earth is put a light beam through a prism to separate it into its colours. We figured out a long time ago the different bands in the spectrum matched the composition of the source of the light so we can tell what the object is made from and in what proportions.
A third calculation we can do is measure the colour change when an object is moving toward the observer and away from the observer. This change occurs in all reflective surfaces and works much the same way as the famous Doppler Effect on sound, changing pitch as it passes.
All these tools are simple measurements that can be done now at the basic science level so they are beyond dispute.
In the early part of the 20th Century the famous Edwin Hubble (after whom the Hubble Telescope is named) made the rather startling discovery that distant galaxies were moving away from us (and each other) by carefully measuring the red light shift. This was cutting edge stuff in 1905 but routine work now. That meant the universe was expanding and this was not what scientists expected. To top it off, he also discovered the further out they were, the faster they were moving away from each other. This meant that if they were all moving away from each other now, they were closer together in the past and so at some point, they were at the same place.
In the decades since, many cosmologists have done the calculations measuring the distance between any two galaxies and working back. Pick any two galaxies out of the estimated 170,000,000,000 and measure the distance between them. No matter which pair you choose, the answer always comes back the same, they were together 13.7 billion years ago.
Especially, just after this discovery, there were a lot of scientists trying to prove the Big Bang model to be incorrect, which is science’s most important role, second only to discovery itself. If it was true, we should be able to find traces of the explosion, some background radiation would be inevitable.
The discovery of the cosmic microwave background radiation validated the model almost beyond doubt, as near to absolute as scientists will ever claim.
Skeptical as ever, scientists continued to question the time scale and came up with a reason why the universe was not that old. Gravity comes from mass and in the early universe there was no mass only energy, so gravity, which would slow down expansion, would not have been as strong, therefore the Big Bang may have only been 9 billion years ago.
Since then models have demonstrated that much dark matter exists that does not reflect light and the existence of this would have added to gravity in the early stages so the age was re-evaluated to 11 billion and now the understanding of a related force, dark energy, has brought the date out slightly further, just past the first calculations to now settle on about 13.8 billion.
Every possible cross checking of conclusions must be done to convince scientists that a given explanation can be relied upon. After all, they don’t want to use data in their work that is founded in speculation which might torpedo years of their own work and the best corroboration would come from independent sources, as different as possible.
For simplicity’s sake we can take just two methods, which have several sub-methods within each method of corroborating the age of the universe.
The first is to take an object whose age we are really confident about and use it as a yardstick for calculations. After all, it would be hard to see how an object that we know about can be older than the universe, so determining the age of the oldest structure we can find, sets at least a minimum age for the universe.
To do this measurements are taken to determine the expiry date of some stars, ie ones that have used up their fuel from a known starting quantity. We know how much they had by their size which we knew from their luminosity which is proportional to size. The oldest stars were at least 9-11 billion years old, so this was great corroboration but from the same field.
Another form of measurement, radioactive nuclei, a cosmic clock if you will, comes from an entirely different source and so carries more gravitas. Any radioactive atom will give off a particle eventually and change into a different element of isotope. This simply means the number of protons and neutrons that make up the atom diminish over time and by determining the state of the atom, one can tell how long is has been doing the changing. For the sake of accuracy a large number of atoms of the one type must be sampled to confirm the average degradation. On earth much of the appropriate material is contained in rock so the particles given off are still present to add to the total and so give a very accurate indicator of the length of time the radioactive material has been in the rock.
By this method alone is has been possible to date the life of the earth and by using the same methods on meteorites, it has been possible to date them back much further, far enough indeed to corroborate the other methods of measuring the age of the universe.
There are many more types of studies done in other fields, each reaching the same conclusion, so while we may for now continue to speculate about the nature of the first few minutes, days or years, when that occurred is now beyond all reasonable doubt.
(You can see ALL the interesting ‘days’ in the magnificent science poster which you can download and print.
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