Modern cosmology seeks to describe, in as a simple way as possible, the
whole of the universe. There are two main aspects:
- To find the laws that tell us how the universe changes with time.
- To understand and describe its initial configuration.
Some scientists believe that the second problems should be left to metaphysicists, or to religion. However, as the universe changes according to precise laws there must also be other laws that apply to its initial state.
The behaviour of the universe is so complex that it has been impossible to find a general theory that describes all its aspects. Until now the problem has been divided into many parts for which partial theories, predicting a limited number of observations, have been proposed. It is not certain that this is the right approach because if everything in the universe depends on everything else, partial theories are not the answer, and only a general theory would do it.
At the present time scientists describe the universe on the base of two
separate theories:
- The general theory of relativity that deals with the force of gravity
and that describes the macro-structure of the universe.
- The quantum mechanics that is used to deal with problems on a very small
scales such as 10 to the -12th power of an inch.
These two theories are incompatible between them and, as both cannot be correct at the same time, scientists are trying to find a higher theory that combines both (a "quantum theory of gravity!")
What about the Stars?
The brightness of a star depends on:
- How much light it radiates, or it luminosity.
- How far it is from the earth.
For the stars close to us, we can measure their brightness and their distance from us, and we can then calculate their luminosity. On the other hand, if we know the luminosity of a far-away star and their brightness, we can calculate its distance from us.
The American astronomer Edwin Hubble observed in 1924 that some types of stars have always the same luminosity, their nearness allowed him to measure it. He assumed that if he could find the same type of stars in faraway galaxies they should have the same luminosity too. This allowed him to calculate the distance to their galaxy.
We know that the light emitted by a very hot opaque object has a spectrum (known as thermal spectrum) that depends only on the object's temperature. The thermal spectrum of a star allows us to determine its temperature. Experimental scientists found that some colours are sometime missing from a star's spectrum and the missing colours vary from star to star. Moreover, we know that chemical elements absorb specific colours. These missing colours allow determining which elements are present in the star's atmosphere.
Static Universe?
Until the beginning of the 20th century all the theories, including Newton's, stated that the universe was static.
Expanding universe
Most of the stars appear to be fixed in the sky, but some of them are changing slightly their positions relative to each other as the earth orbit the sun. This is due to the fact that these stars are nearer to us than the others. This apparent change in relative position is called "Parallax". This effect allows scientists to measure the distance of these close stars from the earth.
In 1924 The American astronomer Edwin Hubble showed that our galaxy, the Milky Way, is not the only one. On the opposite there are a very large number of galaxies, each separated by large tracks of empty space. To prove his theory he needed to know the distance from the other galaxies to the earth but, unfortunately, the parallax method did not work with these far away stars, as they appear to be in fixed positions in the sky. The brightness of a star is an indirect way to find its distance from us, but the apparent brightness of a star does not depend only how far away it is, but also on its luminosity (how much light it radiates). However, a dim star close to us will outshine a brighter one far away. So to use the apparent brightness of a star to measure its distance from us, we must also know its luminosity.
The luminosity of a star not too far from the earth can be calculated since we can measure their distances (by the parallax method) and their brightness. Hubble found that stars can be classified into various types by the kind of light they radiate; and the stars of one type all have the same luminosity. He assumed that if one could find the same types of stars among those far away, they must have the same luminosity as those stars of the same type closer to us. Based on this assumption, it is possible to calculate the distance from the earth of the stars in far-away galaxies and, if the distance of many stars in the same galaxies is the same, then we can assume that this is also the distance from the galaxy to the earth.
It is known that, with naked eyes, we can only see about 5,000 stars, all in the Milky Way, and this is about 10E-6 of all the stars in this galaxy. Moreover, the Milky Way is only one among about 10E+11 known galaxies, each galaxy contains on average 10E+11 stars. We cannot see the size or shape of faraway stars, but we have seen that they are only a certain number of star types. Moreover, we can tell to which type an individual star belongs by the colour spectrum of its light that can be seen through a ordinary glass prism, or better through a modern telescope. From their light spectrum one can tell the temperature of the star, temperature resulting from the thermal motion of their atoms and called blackbody radiation. In other words, light emitted by a glowing body is like a thermometer. In addition, some colours are missing in the starlight spectrum. These missing colours are not the same for every star and they allow us to find the composition of the star's atmosphere. At the beginning of the 20th century, astronomers found that the pattern of missing colours is the same for nearby and faraway stars and, moreover, they were all shifted towards the infrared end of the spectrum. This is known as the Doppler effect and the shifting towards the infrared is a proof that the stars are all moving away from us, in other words that the universe is expanding.
The expansion of the universe could have been predicted from Newton's theory but the conviction that the universe was static was so strong that any other theory was unthinkable. Even Einstein, in his general theory of relativity of 1915, introduced an arbitrary constant -the cosmological constant- in his equations to make certain that the universe remained static. Without this added constant his theory showed that it was expanding. Later on Einstein described the addition of this constant as his biggest mistake.
When the astronomers started to analyse the light spectra of stars from other galaxies they found that the same colours were missing that in the stars in our own galaxy. In addition, all the colours were shifted by the same amount to the right, that is towards the infrared side of the spectrum.
This means that the stars are moving away from us (see Annex 4, Doppler Effect). Moreover the shift increases the farther the stars are away from us. This shows that the universe is not static, it is expending.
This continuous expansion should already have been obvious before the discovery of the spectrum shift. A static universe would have to contract under the effect of gravity. But, if it were expanding at a velocity above a well-defined critical value, the force of gravity would not be able to stop the expansion that would continue forever. Fortunately for us, this is the case.
At the beginning of the 20th century only one man, the Russian physicist
and mathematician Alexander Friedmann, fully accepted Einstein's General
Theory of Relativity according to which the universe is expanding. In 1922
Friedmann made two assumptions:
- The universe looks the same in all direction.
- This is true from any place we look at it.
From these assumptions Friedmann showed that the universe could not be static.
A few years later Hubble showed experimentally that this was true.
The assumption that the universe looks the same in all direction seems wrong if we observe the sky with our eyes. But, if we take into consideration the faraway galaxies, this hypothesis is reasonably correct on large scales, if we ignore the local differences. Moreover, in 1965, physicists using very sensitive microwave detectors found that the background noise was the same in all directions; it was the same during the night as it was during the day, and it remained identical through the year. The only possible conclusion was that it must come from outside the solar system and also from outside our galaxy. This shows that, on a large scale, the universe is the same in all directions.
The physicist George Gamow suggested that at the beginning of time the universe was very hot. Other physicists believed that it was possible to detect the glow of the early universe by looking at the light of the far away stars as their light only reached us now. However, they realised that due to the expansion of the universe these lights would be strongly shifted towards infrared and appear to us as microwaves.
Among other things, Friedmann's second assumption means that the universe looks the same in any direction from any point, not only in the solar system, but also from any point in the universe. In consequence, all the galaxies are moving away from each other in all directions. In addition, the speed with which they move apart is directly proportional to the distance between them, and this means that their light shift toward infrared is proportional to their distances from us as Hubble found experimentally.
There are in fact three possible models that follow Friedmann's two assumptions:
- In the first the universe expansion slows down after a certain time due
to the gravitational forces and, finally, the universe collapses on itself
(Figure 8.a). This universe is finite in space but it has no boundaries.
Gravity is strong and the space is bent on itself (like a sphere). A person
walking in one direction will not be stopped by a wall, or falls over the
edge, but would come back where it started. Time is also finite; it is like
a line with two ends. However to go around the universe is impossible because
it would take such a long time that the universe would have collapsed on
itself before the person reaches home. To be able to do it requires that
the person travels at a velocity greater that the speed of light, and this
is not possible.
- In the second model the expansion is so fast that the gravitational force
cannot stop it but only slow it a little (Figure 8.b). Here the universe
is infinite being bent like a saddle.
- In the third model, the universe is expanding just fast enough to avoid
it to collapse. The speeds between galaxies are decreasing, but the expansion
never stops. Space here too is infinite and, moreover, flat.
To find out which of these three models represents our universe we must
know its present rate of expansion and its average density. If the density
is below a certain critical value, the gravity will be low and the expansion
will last forever. If the density is above this critical value, gravity
will make it collapse on itself in the far future.
Galaxies are moving away from us with a velocity that can be measured by the Doppler Effect (see Annex 4) but the accuracy of the measurements is poor. All we can say is that the universe expands between 5 and 10% every 10 E+9 years. The average density is known with even less accuracy. If we add the mass of the known galaxies, of the "dark" stars in these galaxies, and that of the invisible matter known to exist between the galaxies, we only get a value of about 1% of the critical mass. In conclusion there must be other unknown masses somewhere in the universe, some dark matter that we cannot see, but which we know exist because of their gravitational effect on the orbits of the stars in the galaxies. And although we know that the amount of dark matter exceeds the amount of ordinary matter in the universe, what we know is only about 10% of what would be required to stop the expansion. Probably there are other kinds of dark matter in the universe that we have not detected yet.
Recent experiments seem to show that the rate of expansion of the universe is not slowing down but, on the opposite, is increasing and Friedmann's models do no foresee this. What force is responsible for this is not known yet, but it could show that Einstein was right in introducing his cosmological constant.
In conclusion, it seems that the universe will expand forever. However
even if the total mass was above the critical level, the collapse of the
universe would not happen before 10 E+10 years since it went on expanding
for at least that many years. By then the sun would have run out of fuel
and be extinguished, and us too!