Cosmology: Why do we think that the universe is expanding right now?

To assume the universe is expanding, is to ascribe limitations to the universe.

True, we can only observe that the universe was expanding, but it is hugely unlikely that this will ever reverse. Recent discoveries have shown us that there is nowhere near enough matter in the universe to exert sufficient gravitational force to reverse the expansion of the universe. We have also found that the rate of expansion is accelerating (see Type Ia supernovae). Combine these two pieces of knowledge, and we can be certain that the universe will never do anything but expand, at least until the point where we can no longer observe expansion, i.e. when every observable object moving away from us is beyond our event horizon. That should happen in about 1.5 trillion years, if my memory serves me well.

Why cant the rate of expansion be like a sine wave-
zero, then increasing(as we observe now) till it reaches a maximum, then decreasing(slowing down of expansion) till the rate becomes zero again- universe was expanding till now, only the rate of expansion was varying- so that now we have the largest size of universe at this stage
Then, as the rate of expansion dips to negative, universe starts contracting, and rate of contraction accelerates increasingly till it reaches the maximum negative rate- then the rate starts slowing down until it becomes zero again so that we have our singularity again, the universe has compressed to the minimum possible size

@krrazypassions I believe you are talking about an oscillating universe. Strictly speaking it is still an option, but from the evidence I’ve seen it is unlikely because of all that missing mass. We don’t have any plausible hypothesis of a force that could draw the universe back together, so at the moment it seems that the universe is destined for a cold death.

@FireMadeFlesh I’ve got a very vague idea about super-symmetry- I know its proposed in the super-string theory- i guess its also related to the standard model of sub-atomic particles where all particles have a symmetrical opposite particle-
any chances that such super-symmetry concepts predict or need repulsive gravity? ( i would have used the word anti-gravity instead of repulsive gravity but a Wikipedia entry about anti-gravity is a term already in used for some other kind of concept)

@krrazypassions Excellent question and provocative thoughts. We have quite a bit more evidence that simple red shift of the most distant objects. True, they are billions of light years away, but all objects outside our own galactic cluster (the Local Group consisting of 50 closely bound galaxies) are speeding away from us. The closest object outside the local group is KUG 1210+301B at just 4.89 million light years away. It is moving away from us. At greater and greater distances, objects exhibit grater and greater red shipt, indicating they are moving away faster. It is therefore little surprise they are further away. So we aren’t dealing with only things whose light originated many billions of years ago., We see a smooth transition of increasing red shift from objects just a few million light years away to those over 10 billion light years away. This is entirely inconsistent with a sine wave hypothesis.

@ETpro We cannot say the present evidence is entirely inconsistent with this sine wave hypothesis.
What we are observing could be the first quarter wavelength of the sine wave, in which the value of expansion rate goes on increasing with time.

To prove this hypothesis, we need an evidence that some galaxies ‘B” much further away from the furthest detected so far ‘A’ are moving away at a rate which is lesser than rate of ‘A’, which will take us to the second quarter of the sine wave, where expansion is still happening but it is decelerating.

But disprove this hypothesis, we really cant! But sure, until there is no evidence, then we will have to ignore the existence of such a sine-wave function in the expansion of universe. But that doesn’t mean we have disproved it.

Therefore, as FireMadeFlesh wrote, “it is still an option, but from the evidence I’ve seen it is unlikely…”- Yes, i agree that from the available evidence as of now, it is only expanding faster and faster.

@krrazypassions If observed evidence is set aside in place of what might be, we can’t say anything and science is pretty useless to us. What we can say (what we do have evidence for) is that if there is a sine wave of expansion/contraction, it’s period is greater than 27.5 billion years, and since expansion is currently accelerating and that acceleration rate is directly proportional to the distance of the object, the sine wave would need to be considerably larger than 27.5 billion years in period. What we can further say from observed evidence is that we know of no gravitational force capable of reversing such an expansion. Even hypotheses need to be based on some observable data and attempt to explain the observation. To make, in the absence of any observed data supporting the notion, guesses about what might happen in the future is the province of the fortune teller, not the cosmologist.

@ETpro I agree with whatever you have said about science and evidence and hypotheses :)

However, i would call upon you to keep the value of period of this sine function of universal expansion to a unresolved number rather than something greater than 27.5 billion years. ( as i have some undecided thoughts on this issue that i have not expressed yet- i need to ask a few more questions before i can come back to the period of this sine function)-

How do we calculate and interpret large distances in universe?
I think after i get answers to it, things will be clearer to get back to this one.

@ETpro , @all Isn’t this universal expansion unexplainable in terms of our existing knowledge?
See, we all know that gravity is attractive. So, all galaxies are actually attracting and getting attracted to all other galaxies! And they are indeed. The problem is that space-time in which they exist is expanding.
The only reason why we are going further apart is that the universe is expanding. The space-time itself is expanding. At greater and greater distances, objects exhibit grater and greater red shift, indicating they are moving away faster. In some cases, we have even found that they are moving away faster than light speed! Now, this universal expansion gets out of hand of relativistic physics! The law that rules this expansion is not in our books yet. isn’t it?

If so, then this sine-wave hypothesis of universal expansion is also not dependent on relativistic physics, since it basically deals with universal expansion which itself isn’t dependent on relativistic physics!

red shift explained by The Idler

@krrazypassions Stellar objects moving away from us as a result of the expansion of the universe do not violate Relativity. Because this apparent motion is due to space being created between us and it, rather than motion through a finite quantity of space, it is not actually moving faster than light even though it is receding from us faster than light. Each of these points is probably quite like our galaxy is today, moving at sub-relativistic speeds within their local group. The fact that we are moving apart at such a rapid rate is actually not reflected in our or their space-time coordinates.

@FireMadeFlesh Hey! Is it true that expansion of space-time is expanding light waves, causing red-shifts in this manner? Then, even if the space-time expands faster than light speed, it would only result in larger red-shifts, while the time taken for light of a particular galaxy to reach us would still remain the same??

@FireMadeFlesh also, what you are saying is fine- but what do we consider when we sit down to model this space-time expansion itself? Surely we need something outside relativistic science then, since this space-time is expanding faster than light speed?

@all Something to thing about guys

I have something to say on expansion theory and Hubble’s evidence:
Go through this part of the Wikipedia article which is titled “Understanding_the_expansion_of_Universe”.

Now, this is the observational evidence:
“Hubble demonstrated that all galaxies and distant astronomical objects were moving away from us, as predicted by a universal expansion. Using the redshift of their electromagnetic spectra to determine the distance and speed of remote objects in space, he showed that all objects are moving away from us, and that their speed is proportional to their distance, a feature of metric expansion. ”

Now bear both these things in mind and have a look at my thoughts:
” The deeper we look into space, the deeper we are looking at the past. So, the speed of expansion being proportional to distance actually indicates that the expansion rate was higher in the past!”