Vitiligo schreef:Je zult toch met me eens zijn, dat de ruimte die ontstaat tussen de sterrenstelsels tijdens de uitdijing van het universum ergens vandaan komt.
Ruimte is op dezelfde manier ontstaan als materie. Er was niet al 'ruimte' waarbinnenin via een Big Bang materie ontstond; materie
en ruimte werden in die helse vuurbal gemaakt. Sterker nog, je kunt via vrij simpele argumenten aantonen dat bij voldoende hoge energie en voldoende kleine lengteschalen je geen onderscheid meer tussen die twee kan maken. Daarom kan je ook niet spreken van 'ruimte buiten het universum' omdat het begrip ruimte daar niet is gedefiniëerd.
Ik quote even een stukje uit Bernard Schulz's boek
Gravity From The Ground Up (pagina 365):
What is the cosmological expansion: does space itself expand?
Because of the cosmological redshift, photons behave as if they were being stretched between the galaxies: their wavelenghts increase exactly as the distances between galaxies increase. It might be tempting to conclude from this that the cosmological expansion stretches space itself, just as our rubber-band universe was built out of stretching material. But this could be quite misleading from a physical point of view.
In particular, one must not thing that space itself is everywhere enlarging, as if extra 'points'were somehow being created among the old ones all the time, and everything was getting bigger in proportion to the Universe: wavelengths, sizes of atoms, sizes of people. If the sizes of atoms and the spacings between atoms in molecules were getting bigger, then the expansion of the Universe would be unmeasurable. If our rulers were enlarging at the same rate as the wavelengths of photons from distant quasars, then we would not notice the redshift, since the incoming light would occupy the same fraction of the standard meter as it did when it left the quasar billions of years ago. The expansion of the Universe is an observable fact precisely because ordinary matter does not expand with it.
It is simpler from a physical point of view to think of the expanding Universe as a simple collection of particles (called galaxies) that are rushing away from one another. The redshift of light is a Doppler shift caused by the motion of the source galaxy away from us.
Notice how this looks from the point of view from a photon. Let us take its source galaxy as the standard of rest, the (arbitrary) center of the Universe. As it moves away from the source, it passes other galaxies. They are all moving away, but not as fast as the photon, which moves at the speed of light. The further the photon travels through the Universe, the faster is the speed of the galaxies it passes, since the faster galaxies have travelled further from the center since the Big Bang. Suppose that it happens to be detected by an astronomer in one of these galaxies. Then, the longer it has travelled, the faster will be the speed of the detecting galaxy relative to the source galaxy, and the bigger will be the detected redshift. The redshift increases with time, but this increase has nothing to do with the metaphysical stretching of space: it is simply the way the Doppler shift works in a homogeneous expanding Universe.
Why does ordinary matter nog expand with the Universe? After all, each proton and electron starts out from the Big Bang participating in the cosmological expansion. The answer is that the particles would continue to expand if they remained free particles, influenced only by the smooth comological gravitational field. But they don't remain free.
The other forces of Nature, such as electromagnetism, disturb the cosmological expansion in small regions, binding individual particles to one another, wiping out the inirial relative velocity between them. Once the 'memory' of the initial expansion is lost, atoms are goverend by forces in their neighborhood, not by cosmological gravity.
This remark even applies to irregularities in gravity. Galaxies form from the expanding gas of the Universe because of some random irregularity in the density of the expanding gas, which causes a local increase in gravity that slows the expansion of the nearby gas. Eventually, if the initial irregularity is big enough, the local selfgravity is strong enough to reverse the cosmological expansion in the gas, and the gas becomes a gravitationally bound object, perhaps a galaxy or a cluster of galaxies. After that, it can be described perfectly adequately by Newtonian gravity, ignoring the rest of the Universe. The average motion of the cluster of galaxies can't be wiped out by the forces between the particles in the cluster, so it still participates in the cosmological expansion, as a whole cluster, relative to other clusters and galaxies. But within the cluster, the expansion has been forgotten.
En waar dan die initiële snelheid vandaan komt... Tjaaaaa. Dan krijg je discussies over de kosmologische constante ('Einsteins blunder'), spontane symmetriesplijting, een vorm van materie met een negatieve druk, enzovoort. Veel te ingewikkelds allemaal, zeker op het niveau waarop wij deze discussie voeren.