[BPSDB]It seems to me that Young Earth Creationists (YECs) worship a very strange god who likes to play tricks on his worshippers – making the Universe seem to be billions of years old even though – according to them – it is only about six thousand years old.
If that were true the visible Universe would be a sphere six thousand lightyears in radius centred on Earth because light from more distant objects would not yet have reached us. Yet astronomers can see objects fifteen billion lightyears away and even with the unaided eye, we can see the Andromeda Galaxy, over two million light years away. The YEC might answer:- “How do you know how fast light travels and how do you measure the distance of these remote objects?” These are valid scientific questions.
The Danish astonomer Olaus Roemer noticed that the observed eclipses of Jupiter’s satellites were earlier or later than predicted depending on where Eart and Jupiter were in their orbits. He realised that the delay was dependant on the distance light had to travel and thus was able to use the data to calculate the speed of light. More refined techniques have given a value for the speed of light of a little under 300,000 kilometres per second.
The answer to the second question rests on a sequence of techniques. The first is parallax; if you look at an object from two different viewpoints it seems to move against a background of more distant objects. If you measure the distance between the viewpoints and the angle by which the object is displaced, you can calculate the distance using secondary school trigonometry.
The actual measurements of angular displacement of even bodies in our own Solar System against background stars turns out to be very difficult. The angles are tiny, even using the maximum possible baseline, the diamiter of Earth (ie 12,750 kilometers). It turns out that these bodies are tens, hundreds, even thousands of millions of kilometers away.
The parallax of any star is to small to measure using even the diamiter of Earth as a baseline. However, the diamiter of Earth’s orbit gives a baseline of almost 300 million kilometers. Astronomers had to make educated guesses as to which stars were nearby and which were more distant for the purposes of measuring parallexes. They assumed that bright stars such as Sirius and Alpha Centauri, and rapidly moving ones such as 61 Cygni, are nearby while dimmer and apparantly slower moving and stationary stars are more distant. These stars proved to be 4.3, 8.6 and 11 lightyears away respectively.
This method is good for about 150 lightyears. Beyond that, the parallax is too small to measure.
However, in studying the spectra of stars for which they did know the distance astonomers noticed that stars with similar spectra had similar absolute magnitudes. Furthermore they were able to obtain spectra from many stars for which they could not measure the distance by parallex. From the spectra they knew roughly how bight these stars really are and thus could work out how far away they would have to be to appear as bright as they actually are.
Some of these stars had companion stars that were variable. Some of these variable stars had very regular cycles and it was eventually noticed that there was a simple relationship between brightness and cycle length. So if you found such a star (called a Cepheid variable) in a group of stars by measuring its periodic cycle you could determine its actual brightness and thus work out how far away it is. By this means distances to the nearer galaxies could be measured. By this means the scale of the Universe could be seen to be of the order of millions of light years.
In most galaxies individual stars could not be made out – except for those rare events called supernovae. It is known how bright supernovae in our galaxy have been so by assuming the supernovae in other galaxies were of similar brightness, their distance could be calculated.
It was noticed that the spectral lines in the light of most galaxies were shifted towards the red end of the spectrum – and that this red shift was greater for more distant galaxies. By measuring the red-shift of galaxies where there had not been a supernovae recorded, their distance too could be measured.
By this means, the Universe was measured as being some 13 billion light years in radius – so the light from these most distant objects must have left them 13 billion years ago. For astronomers to see them, therefore, the Universe must be at least 13 billion years old.
More information on the “Cosmic Distance Ladder” can be found here.
The YECs, however, insist that it is only 6,000 years old. They believe that god created this vast universe a few thousand years ago with photons from most of the Universe already on their way just to fool us into thinking the Universe is ancient. As I say, it is a strange god the that the YECs worship.
This post also appears here.