Cosmological Speed of Light
The universe is 13.7 bln years old, however in every direction we look we see objects 47 bln light years away, that is, light should have needed 47 bln years to reach us. So you might ask how could light travel 47 bln light years in just 13.7 bln years? Well this is just a consequence of the expansion of the universe: Light gets dragged away from us. This makes observers to see distant inbound light at effective speeds less than 299792.458 km/sec and to see distant outbound light at effective speeds greater than 299792.458 km/sec. This does not mean that photons accelerate or decelerate; this is just Dark Energy dragging inertial frames apart.
Scientists just confirmed the existence of "Dark Energy", a mysterious repulsive force that acts in opposite to gravity. As the distance increases, the attractive gravitational force decreases but this mysterious repulsive force increases. This repulsive force is pushing galaxies apart; the greater the distance the greater the repulsion. Scientists today do not know what this "Dark Energy" is, but they know that it is causing the entire universe to expand at an increasing rate. For the first 7 bln years after the Big Bang the expansion of the universe slowed down because the attractive gravitational forces were stronger than this repulsive force. However as the distances between the galaxies increased the attractive gravitational forces weakened while this repulsive force became dominant. This made the expansion of the universe to enter an accelerating phase:
Do you remember how a rose opens up? That is, the outer petals move outwards more than the inner petals? Imagine that those petals have galaxies on them and that we are at the center of the rose. Now imagine this rose opening up; the farther out the petals are the faster their recession away from the center (where we are). Well this is exactly how the universe expands around us; the farther galaxies are from us the faster their recession away from us.
Do you remember how an ambulance siren sounds like when it approaches you? And how it sounds like when it recedes from you? The sound pitch changes, right? Similarly when a light source approaches you or recedes from you its colors change. If it is approaching you the colors shift towards the blue, and if it is receding from you its colors shift towards the red. Today we know that galaxies are rushing away from us from redshifting of their light. The more distant galaxies are the more reddish their colors appear to us.
All observers anywhere in the universe also see the universe expanding away from them the same way (like a rose). Hence everyone in the universe thinks that he is at the center of the Big Bang!!! This is because the Big Bang did not have a center. See, if you continue walking on Earth in the same direction you will circle Earth and eventually come back to where you started, right? Similarly if you continue traveling through the universe in the same direction you might come back to where you started; we still don't know this for sure but what we are sure of is that every observer in the universe thinks that he is at the center of the Big Bang; every observer in the universe sees the universe expanding away from him like a rose.
The future of the universe has three possible scenarios depending on this dark energy: The first scenario is if this dark energy is constant over time; then the expansion of the universe would continue accelerating forever. After a hundred billion years or so from now most of today's observable galaxies will not be visible (they will disappear from our sight but they will continue to exist). This is called the Big Chill (cold & lonely):
The second scenario is if this dark energy increases with time then the universe will experience a catastrophic runaway expansion. Within 100 billion years or so from now every galaxy, star and atom in the universe will be ripped apart!!! This is called the "Big Rip":
The third scenario is if this dark energy decreases with time. This eventually leads to a slowing of the expansion of the universe followed by a recollapse. This is called the "Big Crunch". In some ways this scenario resembles the Big Bang in reverse: