Well, it's only a theory, no?

What do we know about our universe? Why do we believe there was a Big Bang? How can we know the age of our universe? What will the future bring?

I'll try to explain all this in an easy way. I thought about doing this for quite a long time as I'm very much interested in science, and especially astronomy and astrophysics (but less in solving equations). The best way to understand something is by figuring it out, with examples or comparisons. I jumped right into the subject and realized that maybe I would have to start from the beginning, to see the evolution of our vision of the universe.

In my opinion, many are misusing the word "Theory". "Every theory is true until proven wrong." Who did not hear that sentence? Or sometimes you will hear people saying "It's only a theory" as if it's meant to be unproven."

So let's begin with what a theory really is. What has Wikipedia to say about this?

"Theories may be expressed mathematically, symbolically, or in common language, but are generally expected to follow principles of rational thought or logic.

Theory is constructed of a set of sentences which consist entirely of true statements about the subject matter under consideration."

The Free Dictionary gives us the following definition:

"A set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena."

They added or is widely accepted because some theories can not be tested. But if it can explain a group of facts or phenomena and make correct predictions, then it's also a theory.

So, a theory should not be confused as a hypothesis. If there were any doubt, it could not be called a theory. Someone who says "it's only a theory" simply has no idea what he's talking about.

And what about "Every theory is true until proven wrong"? Well, the problem here is that one may think that every theory could be proven wrong, what is not the case. It is true that if at any time an observation is made that contradicts a theory, that theory may be wrong. But before changing the theory, it is the observation which must be proven true, by inquiring, repeating and testing it.

For example during the OPERA (http://operaweb.lngs.infn.it/?lang=en) experiment, neutrinos, coming from CERN and recorded in Gran Sasso, were observed to be traveling faster than the speed of light. Which, according to general relativity, should be impossible. Some papers already published that Einstein might have been wrong, but in fact the problem was solved a few months later : the neutrinos weren't traveling faster than the speed of light, there were errors in the measurements due to an improper attached fiber optic cable, and a clock oscillator ticking too fast. At the end, those elements did not travel faster than light.

(http://en.wikipedia.org/wiki/Faster-than-light_neutrino_anomaly)

For the hardcore people out there, here is a link to the (revised) publication "Measurement of the neutrino velocity with the OPERA detector in the CNGS beam" :
http://arxiv.org/abs/1109.4897 (the download is on the right on that page)

I will conclude this by Sir Karl Popper's own conclusions what a theory should be about (published in Conjectures and Refutations in 1963) :

"These considerations led me in the winter of 1919-20 to conclusions which I may now reformulate as follows.

1. It is easy to obtain confirmations, or verifications, for nearly every theory — if we look for confirmations.
2. Confirmations should count only if they are the result of risky predictions; that is to say, if, unenlightened by the theory in question, we should have expected an event which was incompatible with the theory — an event which would have refuted the theory.
3. Every "good" scientific theory is a prohibition: it forbids certain things to happen. The more a theory forbids, the better it is.
4. A theory which is not refutable by any conceivable event is non-scientific. Irrefutability is not a virtue of a theory (as people often think) but a vice.
5. Every genuine test of a theory is an attempt to falsify it, or to refute it. Testability is falsifiability; but there are degrees of testability: some theories are more testable, more exposed to refutation, than others; they take, as it were, greater risks.
6. Confirming evidence should not count except when it is the result of a genuine test of the theory; and this means that it can be presented as a serious but unsuccessful attempt to falsify the theory. (I now speak in such cases of "corroborating evidence.")
7. Some genuinely testable theories, when found to be false, are still upheld by their admirers — for example by introducing ad hoc some auxiliary assumption, or by reinterpreting the theory ad hoc in such a way that it escapes refutation. Such a procedure is always possible, but it rescues the theory from refutation only at the price of destroying, or at least lowering, its scientific status. (I later described such a rescuing operation as a "conventionalist twist" or a "conventionalist stratagem.")

One can sum up all this by saying that the criterion of the scientific status of a theory is its falsifiability, or refutability, or testability."

(http://www.stephenjaygould.org/ctrl/popper_falsification.html)