Wolfgang Korsus Dipl.-Ing.NT, Astrophysiker
Website : wolfgang.korsus.net
simple question : Why is the sky dark at night ?
One can ask this question, but I will be careful not to ask „this“ question! Only this topic every scientific observer of the universe and its structure should at least once take to mind….and then maybe smile mischievously
It is an apparent paradox and is named today after Heinrich Olbers, a German astronomer. He was not the first who noticed the thing; because already Kepler had noticed this and had concluded from it that the sequence of the stars cannot be infinite. An example where one can also think of the problem is the following:
Imagine something simple, namely a gigantic dense forest, infinitely extended. (Amazon) Wherever you also look, your view meets just on a tree. Well… !?
In 1823 Olbers clearly summarized the conditions for „his“ paradox:
– The universe is infinite in every direction and has always existed in its present form.
– The stars are distributed in equal density in the universe, have always existed and have a given size and brightness.
A determination quite special way… ! ?
Under these conditions, so my conclusion is, the whole sky should be as bright as a typical star and it should never become dark at night. So I say, there is „something“ not right and not at all. And the before mentioned „something“ brings us damn fast to modern cosmology and also to their ideas about the beginning of the universe….. and that says…..
So be the age of the universe limited, also everything has begun with a „big bang“ before soundsovielen years, then the universe visible for us today is of finite size, because the light has had, after all, only these years to reach us. These distances and times are just enormous, only…. they are not infinite.
Likewise I state that the stars had to originate sometime after the big bang, so that their number is also finite. ….the limited age of the universe gives us then only insight into a finite spatial part of the whole, and in this part can have originated since the big bang just only a finite number of stars.
Therefore I say, the sky is dark at night, how should it be otherwise…….also Mr. Olbers, since we know that the speed of light is finite as well as the „big bang“ also sometime the world beginning was, so these assumptions are quite simply at least continuing !
Now the question must follow, but how can we convince ourselves that these ideas are really the right ones?
So it is about the origin of the world and even the question whether it has an origin – all this is still today much discussed in science, philosophy and „religions“. There are just two main reasons why most scientists today consider the big bang theory to be correct – now I say : not so hastily we approach the subject significantly slowly, step by step and one more step.
I begin with theoretical slow motion and it is the following :
A quite well-known effect of the everyday life physics is e.g. the „sound shift“ with a moving sound source. Surely everyone who has watched a car race knows this. The engine sound of a Formula 1 race car is higher when the vehicle is coming towards us, and then becomes lower when it drives away; there is a…..I’ll call it a „tone flip“ as it passes. In physics, the process is called a Doppler shift, or better „effect.“ Simply named after the Austrian physicist Christian Doppler. What do you hear there ? Yes, the sound we hear is created, as we know, by sound waves that have a certain wavelength, and so when the source of the sound approaches us, the wave acts as if squeezed, because the distance between two successive peaks shortens and that means an increase in the sound. What happens with a sound source moving away …..the opposite. Such a Doppler effect occurs not only with sound waves, but also with light waves.
That means then, one can measure likewise something, i.e. whether a certain star moves seen from us. Stars, as many know emit ( electrons ) . Light of certain wavelengths and namely with so-called spectral lines, and if these appear shifted, the star must move naturally.
So to repeat again.
If the star comes to us the wavelengths become shorter (thus blue shift).
…………. the star moves away………. the wavelengths become longer(i.e. red shift)
The faster it moves, the bigger is the shift !
Now it comes : Already in the 1920s the astronomer „Edwin Hubble“ at the Mount-Wilson-Observatory in California was occupied with light and indeed very distant stars. An extensive exploration started….
He had already measured clear red shifts; of course it was known that these stars are moving away from us. But Hubble now made another surprising discovery. They disappear the faster the further away they are.
It may be mentioned the known Doppler shift and that means also the stellar velocity, were measurable without problems. But there was a difficulty in determining the distance of the observed stars, to measure relatively close celestial bodies, such as planets, was easy. The parallax method had been used previously. Also Cassini and Richer had calculated the distance between Earth and Mars with this method, all respect.
Only now it was time to use another method for very distant stars.
But for the distant stars Hubble was targeting, the parallax angle was much too small for any measurements. Hubble came up with a solution quite quickly in a fairly simple way. He knew the brightness of a light source decreases the further away it is and since the light spreads out spherically from its source, therefore less and less light falls on a given area with increasing distance.
Continuing: I designate the distance to the light source with d, from it follows the spherical surface increases then with d 2. The incident light per unit area as 1/d 2 decreases as a result.
So we can conclude :
If we know the initial brightness of a source and also the brightness measured at a certain (initially unknown) distance, then the difference in brightness is determined by the distance d .
Next we turn to the expression „reference candles“. What is this again ?
Hubble had measured the brightness of quite certain stars. These were the Cepheids, they had just recently been measured. The astronomers made of it the….na what ! „The Reference Candles“…..and this was achieved by measuring their relative brightness in his observatory at Mount Wilson, thus Hubble also got a good estimate of their distance. – He found that their escape velocity v increased with increasing distance d from Earth. (See further details in my short note X2 (a little later).
From all considerations and their calculations, Hubble’s law v = H 0 d , with the scale determination H 0 again called Hubble’s constant, thus arose.
If we look at the value resulting at that time, it can be stated that it is a little bit inaccurate from today’s point of view, but what must not be forgotten, only the idea of Hubble is completely correct and much more important is the fact of the change of our world view.
Hubble did not stop to observe stars. It can be said, wherever Hubble looked, the stars disappear, the view arose: does the whole universe explode ? This view led then to another question: Was this possible? Or is such a thing impossible ?
Today I make the assertion : This was an ideal time, because in the year 1916 already Albert Einstein’s new, yes new general relativity theory appeared.
Hubble’s discovery took place at an ideal time, a little later. Only little before, in the year 1916, Albert Einsteins new general relativity theory had appeared, which connected simply expressed the effects of the gravity with the nature of space and time. In addition a statement which forces to think. I let „an arbitrary round thing“ circle at the tape, whereby the tension of the tape compensates even the centrifugal force. Now we consider only the ball movement, and already I could also come to the conclusion, this round thing, let’s call it a ball, moves freely on a curved track.
Now this way of thinking forces me to think that the role of the force can be replaced by a kind of space curvature. For Einstein’s theory states that in the vicinity of very massive celestial bodies, such as the sun, space is so deformed by gravity that even a ray of light deviates from its straight path. His prediction was tested and confirmed in 1919 in what is now a very famous experiment.
It was the English astronomer Arthur Eddington who, with his collaborators, showed that starlight passing close to the Sun was indeed deflected by exactly the amount calculated by Einstein.
The experiment took place, of course, during a solar eclipse.
Einstein became of course by this observation abruptly world-famous.
It is still to be mentioned absolutely the statement of the general public at the time when Einstein set up his general relativity theory. One thought the universe for static; because one did not think (yet) of the possibility of an expansion or contraction.
Einstein knew this and already he started thinking. He needed just now any force which simply compensated the attracting effect of the gravity. Immediately no candidates came to his mind and even until today the problem still puzzles us „astros“. As already mentioned, Einstein started, but with little enthusiasm. Thus a “ solution“ was created ?! You notice, it is not so easy for me to comment on it. He began to spread out his first proposal and that was the introduction of a mysterious „cosmological liquid“. This should fill the whole space and compensate the gravity approximately by its pressure.
But this is too little, the description of the criteria goes on…… This fluid had to have still quite strange properties …..it could not influence any process in the universe, except gravity, so that it remained unobservable for all other measurements. Its pressure and therefore its density had to be precisely determined one hundred percent in order to compensate gravity exactly.
As actually known, the mentioned resembles in certain sense, however, to that of a new form of ether and thus this statement was particularly undesirable to Einstein and perhaps even „embarrassing“ . So a new designation had to come and so he has called then later the introduction of this size called today as cosmological constant as the biggest blunder of his life. If, would have, bicycle chain, one would like to say there. His original equations without cosmological constant, on the other hand, predicted an expansion of the universe. That „would have“ then still before it was discovered then by Hubble, took place.
Nowadays, especially among cosmologists, one is not quite sure whether this quantity was really a mistake. Because the „dark energy“, to which I will come a little later, is something like a resurrection of Einstein’s cosmological constant.
Still important to mention is the fact that in 1922 the Russian theorist Alexander Friedmann proved that the general solution of Einstein’s equations describes very well expanding universes. But let’s stay with Hubble he finally found his expansion, now everything was ready. The next chapter follows……..