If our universe is inflating, which means it is getting bigger and bigger in size, and every thing it consists of is also enlarging, in a special way such that the mass of the universe remains constant and only the volume changes.

In that case, how will we observe the changes in the physical layout of the universe? In particular, are these guesses correct?

  1. We will observe that the speed of light is decreasing, as light usually travels $3*10^8$m, in a second, but now since the universe is inflating, it will travel something lesser then that for instance $1.5*10^8$

  2. As far as observation is considered, I dont think we will see any changes, since the retina of our eyes would then also have enlarged,

  3. As far as physics of the universe is considered I think the planets will keep to their orbits as their mass has not changed a bit and they maintain same distance with other celestial bodies as they used to,

  • $\begingroup$ Is soft question the right tag for this? $\endgroup$ – Tomarinator Apr 27 '12 at 7:16
  • $\begingroup$ Your edit basically makes this a science fiction question not a science question. As your question stands at the moment I'd guess a moderator will be along to close it any moment now ... $\endgroup$ – John Rennie Apr 27 '12 at 8:32
  • $\begingroup$ Yeah, this may well be an inappropriate question here, although I'll try just reverting it rather than closing it for now because I'd rather not close it unilaterally. (And no, "soft-question" is not the right tag for this. That is for questions which aren't really about physics, but if you're asking such a question, it's probably off topic here anyway.) $\endgroup$ – David Z Apr 27 '12 at 8:41

See spacetime expansion and universe expansion?, which addresses some of your questions. In particular the planetary orbits won't change because on the scale of the solar system the gravitational force between the planets far outweighs the expansion of the universe. The retinas in our eyes also don't expand, again because the intermolecular forces holding them together are far stronger than the forces caused by the expansion.

Actually a quick note on terminology: the term "inflation" applies only to an initial rapid expansion of the universe shortly after the Big Bang. The term used for the current growth of the universe is simple "expansion".

To answer the first part of your question, the speed of light is (locally) constant i.e. it's 3 x 10$^8ms^{-1}$ and doesn't change as the universe expands. On very large scales, i.e. gravity cluster scales, the expansion does overcame gravity so on large scales we see the expansion as the red shifts of distant galaxies.

The matter in the universe remains (approximately) constant, however an important contribution to the total energy of the universe is dark energy. Unlike mass, dark energy is a property of spacetime so it increases at the same rate as the universe expands. That means in the future, as matter is diluted by the exansion the universe will increasingly be dominated by dark energy.

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  • $\begingroup$ But,my question was to imagine a different universe in which such a phenomenon of inflation (not mere expansion) was taking place,:-) $\endgroup$ – Tomarinator Apr 27 '12 at 7:30
  • $\begingroup$ I reckon it was my fault in framing the question, so i edited it. :-) $\endgroup$ – Tomarinator Apr 27 '12 at 7:32
  • $\begingroup$ If you accept General Relativity you will always get expansion/inflation that is basically the same as in our universe. Incidentally the inflation immediately after the Big Bang was so fast it would have torn your retinas apart along with the rest of you and indeed the atoms making you up (so it's a good thing it ended after $10^{-32}$ seconds). $\endgroup$ – John Rennie Apr 27 '12 at 7:36
  • $\begingroup$ Re your edit: you couldn't have everything expanding along with the universe unless the scale of the strong, weak and EM forces was increasing at the same rate. But this can't happen unless the mass of the fundamental particles decreases, and you've already said you want mass to remain constant. $\endgroup$ – John Rennie Apr 27 '12 at 7:38
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    $\begingroup$ Because it's the balance of these forces that determines the size of atoms and molecules. If you want objects to expand along with the universe this requires the size of atoms to change or the distances between atoms or both. This won't happen unless the strength of the fundamental forces changes. $\endgroup$ – John Rennie Apr 27 '12 at 17:05

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