MEASUREMENT OF ASTRONOMICAL DISTANCES

When scientists wish to study the universe and develop theories about its nature and structure, they turn to galaxies because these are to be found throughout space. Indeed they seem to be the building bricks of our mysterious universe. Their behaviour would give us clues about the past history of the cosmos and also about its present state. Yet there are difficulties when we come to study the motion of galaxies because of the great difficulty in determining astronomical distances with accuracy. Astronomers have at their command two powerful tools to help them go about their task—these are the “Theory of Red shift” and the “Value of Hubble’s constant”.

None of us would fail to notice a distinct change in the sound of the siren of a police-van or an ambulance car as it approaches us and then moves away. As the vehicle comes near, the siren sounds at a particular pitch (high) and as it moves away it wails at a completely different pitch (low). However, what is strange is that the siren is wailing at the same pitch and to the driver of the vehicle itself the sound of the siren never changes; nor does he notice any change in the pitch. The change in frequency is observed only by us who are not on the vehicle. Naturally one wonders why this happens.

The reason the siren’s pitch seems to change to someone who is outside the vehicle is because the sound waves emitted change in frequency. Every sound the ear picks up is due to vibrations travelling through the air and the pitch depends on how frequently these vibrations reach the ears. If the sounds come from a moving vehicle the frequency with which they reach the ears will alter according to the direction in which the vehicle is moving. When approaching us each vibration starts off a little closer than the one before. As each has a shorter distance to cover, we receive it much earlier than we should have, had the vehicle not been moving. So the vibrations will arrive more frequently from an approaching vehicle and as higher frequency means higher pitch, we hear a higher pitched note from the siren than does the driver. Exactly the reverse happens when the vehicle is speeding away from us. Then the vibrations have a farther distance to travel and the interval between them is lengthened. So the fre­quency drops and we hear a lower pitched sound.

Even though we cannot hear the sound from the stars because of the lack of air to carry them across space, the basic principle of change in frequency of a wave due to the source emitting it moving away from us or toward us remains the same i.e. frequency increases if the source is coming towards us and drops if the source is receding into the distance.

The first person to point out that sound as well as light frequencies should alter when bodies are moving towards us or away from us was the Austrian scientist Johan Christian Doppler who in 1842 observed that in the case of light, motion away from us would make the light being emitted redder in colour (due to the fact that red-light has lower frequency than light of others colours). Similarly, motion towards us would raise the frequency and shorten the wavelength making light seem bluer (since blue-light has high frequency). However, Doppler committed an error in respect of light from the stars because he believed that stars were all white in colour: they radiated equally in every colour of the spectrum. This in turn made him claim that all the colours of the stars are an effect due to their motions, towards us or away from us.

On the other hand the French scientist Armand Hippolyte Fizeau in 1848 realised that because stars also shine at frequen­cies beyond the range of visible coloured spectrum—in ultra­violet and infra-red rays for example—what we observe would not be a change in colour but a shift of the lines in the spectrum. A motion towards us would cause the shift towards the blue end: motion away from us would lead to a red shift of the lines.

Both the effects, the red shift effect as well as the blue shift effect, can be seen in action on the sun. Because the sun is rotating (though at a much slower pace than the Earth), one side is moving towards us and other side moving away. If the sun’s sides are studied with a spectroscope, the one moving towards us would display a blue shift whilst the other side it is moving away show a red shift. So Fizeau was certainly correct.

Red shifts indicate motion away from us. The question is whether they can be used to measure distances in space. The answer to this question lies in two basic facts. The first is that the amount of the shift—how far into the red the lines are moved—is a measure of the speed at which the body is moving away from us. The greater the speed the larger the shift. The second fact is that all distant galaxies are moving away from us. It is clear that the farther off a galaxy is, the faster it is receding and so the greater its red shift. So by measuring the amount of red shift Scientists can obtain a measure of distance.

This red shift measure of distance can be used for any far off stellar object, which sends enough radiation for us to spread it out into spectrum and it is a powerful method probing deep into the depth of space. But there is still some doubt about precisely how the red shift increases with distance and so the method while good for comparing distance cannot perhaps be used to give measurements absolutely correct to within a few light years. All the same we can get a good idea of distance of galaxies by this method and it is found that they extend out to at least to some three thousand million (3 x 109) parsecs, almost 10 thousand million light years), while it may well be that the recent red shift measurements of quasars (Quasi Stellar Sources) have taken Astronomers out to still greater distances than this .The parsec is a unit of length used to measure the large distances to astronomical objects outside the Solar System and is approximately equal to 3.2 light-years i.e19.2 trillion miles.

The difficulty experienced in estimating distances of galax­ies or quasars accurately lies in fixing precisely what is described as Hubble’s constant-named after the American Astronomer who in 1929 found that distant galaxies had larger redshift than near ones. In 1935 Hubble felt that for every million parsecs the velocities of galaxies increased by 530 km per second but this figure is now thought to be inaccurate. Now evidence has reduced the value of the Hubble’s constant from 530 to between 55 and 100 kms per second. For example using a value of 530 if we work out the distance of the one particular galaxy the figure comes to 340 million parsecs. Using a value of 55 would reduce this figure to 3273 million parsecs: the most distant quasars would then be 4364 million parsecs away.

The Red shift has once again attracted the attention of astrophysicists .The Big Bang theory of the creation of the Universe has been based on observation of the ‘red shift’ in the light spectrum (according to Doppler, light wavelength emitted from an astronomical object that is moving away from us exhibits a shift in the light spectrum towards the red end). The Big Bang theory is supported by the understanding that the shift of light towards the red band in the spectrum is continuous and uniform in nature — an indication of all matter (galaxies and all cosmic matter) moving outwards steadily, but at great speed. This would mean that it all began from a single starting point in the centre of the Universe (singularity), which is how the Big Bang theory was popularised among astrophysicists.

Recently four Indian astrophysicists, challenged the Big Bang Theory by studying the Red Shift, using a technique called singular value decomposition. They concluded that the red shift does not occur in a continuous and uniform manner, as indicated above but in recurring stages, in what they refer to as “periodicity in red shift”. They claimed that the superiority of the singular value decomposition method in analysing red shift periodicity, lay in the fact that they analysed very large data samples and used a method, which is more robust than the method usually dealt with by astronomers.