Einstein developed his theory or Relativity assuming only one Force is at play at the astronomic scale, that is, Gravity. Gravity, technically speaking, is not a Fundamental Force like the Electromagnetism, Weak and Strong nuclear forces, because no particle (hypothetical gravitons) has been found, associated with Gravity, yet. Furthermore, as Einstein himself found, Gravity is actually an effect of the presence of matter in a location of space, which warps the space-time continuum, causing objects to accelerate toward each others, depending on their distinct mass and the distance that separates them. We still know very little about Gravity and the space-time continuum.
Since Einstein, it was found there are others Forces at play in the universe. The expansion is believed to be accelerating, due to a "dark energy" which is yet to be observed but it has to be there to explain what is actually observed. This is a second force that Einstein was not aware of. However, the red shift used to measure the speed of expansion, has recently started to be questioned.
Gigantic flow of electrons between galaxies and nearby cosmic clouds, has been observed. Electromagnetism may be at play at the astronomical scale, as well.
Recently, it has been observed that a small cosmic cloud orbiting a galaxy is actually showing a red shift that is very different than the galaxy it is orbiting around. Others red shift anomalies are also found elsewhere in the universe. Something else than the speed of expansion is at play to explain the spectrum shift. The Big Bang theory was elaborated solely from the observation of the red shift. If the universe is expanding, if we playback the movie in reverse, then somewhere in the past, it was very very dense and hot, and expanded rapidly. But if the red shift that we observe is not the result of the speed at which galaxies are flying away from us or if there is also another yet to be understood phenomenon that affects the light spectrum that reaches our instruments, then, the universe may be expanding but at a very different rate than believed, there may or may not be an acceleration of that expansion, or even, the universe may not be expanding at all. If so, then, hasta la vista, Big Bang. Astronomers would have to find other explanations for the observed background noise that is believed to be the remains of the gigantic explosion that was the Big Bang.
A theoretical black matter that is believed to be the major constituent of the universe, is still not observed yet. The giant CERN particle accelerator has recently reached energy levels which were supposed to give evidences of the existence of the black matter. But none has been observed yet. Also, still no Higgs boson found yet. Scientists are starting to fear that their beloved Standard Model will have to be remade. Furthermore, recently, other scientists and astronomers have made observations that seems to refute the very existence of the black matter.
In quantum mechanics, it has been observed that information travels between intricated particles, at a speed that far exceed the speed of light. Although this is not a violation of Einstein's theory of Relativity because information does not have any mass, it is still very intriguing plus it is a possible method of communication at the astronomic scale.
In conclusion, I'd say there is still a lot we don't know about the universe and the way it works. Though, scientists and astronomers are making astonishing observations and new discoveries every once in a while and I am confident that one day, we'll have a far better understanding of what's happening up there. I find that subject extremely fascinating.
Daniel
Whether or not it is possible, I think that IS the beauty of Science. Perhaps Einstein was correct to a certain approximation but as evidence and experimentation can prove, disprove or modify proof, scientists gladly accept and change their knowledge of reality.
