主题：【原创】宇宙学的终结 -- 苏萸

The Universe did not expand faster than light. The Universe was big enough 12 billion years ago that the light from some distant objects is only getting to us now. That doesn't mean that the Universe was more than 12 billion light years wide 12 billion years ago. Because we're moving away from the object, the light has had to catch up to us.

Dr. Eric Christian

To imagine this situation think of the Universe as of a gigantic - maybe even infinitely large - dough of raisin bread. Scale your imagination down to a finite piece of dough, where you are sitting on a raisin in its center. The outer edge of the dough is equivalent to the edge of the "observable universe", from where we see the cosmic microwave background radiation. You have put a lot of yeast into your raisin bread, and you have set up microwave transmitters all around on the outer edge, blasting inward. Imagine that in reaction to this heating the dough rises at such a furious rate that the outer edge recedes from the center at almost the speed of light. Although the microwave transmitters were only at a relatively short distance at the beginning of this "gedankenexperiment" (thought experiment), the microwaves have to cross a much larger (continually growing) distance because of the ongoing expansion.

I would like to add a cautionary note, in case you might be wondering. Don't get confused! This expansion does not violate Einstein's theory of relativity, even though the imaginary dough of the even larger Universe, which we can't see beyond that edge, appears to recede at speeds larger than the speed of light. The dough represents space itself, and in our expanding Universe space itself is expanding, carrying the galaxies (represented by the raisins) along on a ride. Einstein's limit to the speed of light applies only to motion through space, and not to expansion of space itself.

Dr. Eberhard Moebius

Can objects move away from us faster than the speed of light?

Again, this is a question that depends on which of the many distance definitions one uses. However, if we assume that the distance of an object at time t is the distance from our position at time t to the object's position at time t measured by a set of observers moving with the expansion of the Universe, and all making their observations when they see the Universe as having age t, then the velocity (change in D per change in t) can definitely be larger than the speed of light. This is not a contradiction of special relativity because this distance is not the same as the spatial distance used in SR, and the age of the Universe is not the same as the time used in SR. In the special case of the empty Universe, where one can show the model in both special relativistic and cosmological coordinates, the velocity defined by change in cosmological distance per unit cosmic time is given by v = c ln(1+z), where z is the redshift, which clearly goes to infinity as the redshift goes to infinity, and is larger than c for z > 1.718. For the critical density Universe, this velocity is given by v = 2c[1-(1+z)-0.5] which is larger than c for z > 3 .

For the concordance model based on CMB data and the acceleration of the expansion measured using supernovae, a flat Universe with OmegaM = 0.27, the velocity is greater than c for z > 1.407.

Dr. Edward L. Wright

Some of the misunderstandings surrounding this topic might come from confusion over what is meant by the universe "expanding faster than the speed of light." However, for the simplest interpretation of your question, the answer is that the universe does expand faster than the speed of light, and, perhaps more surprisingly, some of the galaxies we can see right now are currently moving away from us faster than the speed of light! As a consequence of their great speeds, these galaxies will likely not be visible to us forever; some of them are right now emitting their last bit of light that will ever be able to make it all the way across space and reach us (billions of years from now). After that, we will observe them to freeze and fade, never to be heard from again.

Dr. Dave Rothstein

What happens to a substance if its speed is more than the speed of light?

We do not know what happens to a substance if it moves faster than the speed of light for the very simple reason that it can never move faster than the speed of light. The speed of light poses a fundamental limit to the speed that an object can take, relative to objects nearby it. In fact, no object with any finite rest mass can move at the speed of light. That is why all the particles that move at the speed of light (e.g. photons) have zero rest mass. As a particle with mass approaches the speed of light, its energy increases and becomes infinite at the speed of light, which is the reason why it can never be accelerated to reach that speed. This has actually been verified by experiments, and it has been shown that nothing moves faster than the speed of light.

However, the above discussion only applies to objects on small scales in the universe -- for example, if you take a baseball or a planet or a star or a galaxy and try to accelerate these objects to the speed of light relative to objects nearby them, it is impossible to do. However, there is nothing which prevents objects that are separated by huge distances from moving relative to each other faster than the speed of light. Over these large distances, the effects of the universe's expansion become important, and the above discussion no longer applies.

Dr. Jagadheep D. Pandian