special relativity - Is 2.5x speed of light possible between two objects?

These news are in Finnish: http://www.hs.fi/ulkomaat/Kaukaisin+havaittu+galaksi+et%C3%A4%C3%A4ntyy+maasta+valoa+nopeammin/a1305618680897

The main excerpt is: "Distant galaxy moves away from us as much as 2.5 times the speed of light"

I'm not a physicist, I can somehow comprehend the idea that 2x speed of light can be reached, like in the case of 2 light beams shot to opposite directions. But 2.5x? Would 1000000x speed of light possible in theory between 2 objects?

Answer

While it is often good to read mainstream press regarding science with a critical eye, in this case there is an explanation.

First, though, there is a misconception that needs to be cleared up. If you fire two objects in opposite directions, say with speeds $v_1$ and $v_2$, their relative speed (the speed of either one as seen by the other) is not $v_{1+2} = v_1 + v_2$ as far as special relativity is concerned. It is actually $$ v_{1+2} = \frac{v_1+v_2}{1+v_1v_2/c^2}, $$ where $c$ is the speed of light. (See Wikipedia for a derivation.) When $v_1,v_2 \ll c$, the more intuitive formula is a very good approximation. However, as speeds approach $c$, you find things can never move apart faster than $c$. The limit is not $2c$. That's special relativity for you.

The catch is in general relativity, which is absolutely needed for cosmological distances. It turns out the very universe itself is growing larger, the space between "stationary" objects growing larger all the time. (In fact this expansion is accelerating, the discovery of which earned the 2011 Nobel Prize in physics.) The space between two distance objects can expand at any rate; it is not an actual motion of an object through space. There are more details due to lookback time, what your precise definition of "distance" is, etc., but the end result is there is some meaning to saying a galaxy is receding faster than light, or faster than $2c$ even.

Edit: The answer to the question of what is the maximum velocity requires some modeling of the constituents of the universe, and a solid understanding of horizons (I cited a beautiful but technical diagram that comes in handy for this sort of thing in an answer to another question). Long story short: the furthest objects within our past light cone have a comoving distance of about 46 billion light years. Hubble's Law tells us the proper distance to those objects is increasing at a rate of about $10^6\ \mathrm{km}/\mathrm{s}$, which is a little more than $3c$. The answer is not much different if you take into account the fact that pre-recombination light is scattered, so we can't quite see $46\ \mathrm{Gly}$ away.