km/sec299792.458

Cosmological Speed of Light

Effect of Dark Energy on the measured 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:





NASA recently ruled out the second scenario (No Big Rip; see also: Universe Today). This leaves the universe with only two possible endings: Big Chill or Big Crunch (first or third scenarios), depending on what this Dark Energy turns out to be.

Today we can observe galaxies 13 billion years old, that is, light already traveled 13 billion years before it reached us. If the universe were not expanding then light from those galaxies would have only needed 13 million years to reach us (ratio of distances might be 1000 times). If the expansion of the universe suddenly stops today then light emitted today from those galaxies would need 47 billion years to reach us. This is because the expansion of the universe is causing inbound light to be dragged away from us en route. Hence for an observer on Earth the effective displacement of light is different than the total distance traveled by light (different than 13 billion years multiplied by 299792.458 km/sec).

When those distant galaxies (13 billion light years away) first emitted this light they were receding from us at speeds greater than 299792.458 km/sec (faster than our local speed of light). However the expansion of the universe slowed down for the first 7 billion years. This gave light a chance to approach Earth. However the expansion of the universe has been accelerating for the last 7 billion years. This made those galaxies to recede away from us today at speeds again greater than 299792.458 km/sec.

We are sure that galaxies 13 billion light years away today have recessional speeds greater than our local speed of light. If the universe continues to expand forever (first two scenarios) then light emitted today from those galaxies will not reach us in the future anymore (they will disappear from our sight). No observer anywhere in the universe feels any acceleration because this Dark Energy is causing spacetime itself to expand. This Dark Energy is creating space everywhere each second. If this created space each second between a certain point and Earth measures more than 299792.458 km then light from that point traveling at 299792.458 km/sec can never reach Earth. An observer today 13 billion light years away sees light locally (at his location) to travel towards Earth at 299792.458 km/sec, however this light will never reach Earth. This distant light heading towards us is actually being displaced away from us (negative effective speed).

The expansion of the universe causes 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. However every non-accelerating observer measures the speed of light locally (in his location) at 299792.458 km/sec in any direction.

Since the effective speed of light varies with distance and direction then this means that if the local speed of light (or the local speed of any object) were defined in km/sec then this definition will be wrong for distant observers. By using classical orbital mechanics we discovered that outside the gravitational field of the sun 12000 Lunar Orbits/Earth Day becomes equivalent to our local speed of light. This definition will never be wrong to any distant observer because 12000 Lunar Orbits/Earth Day is independent of direction, co-moving with the expanding universe and common to all observers. It also turned out to be a constant forever. Learn more...