This is a good example to get the idea of expansion across! The image is a good one to focus on. Have you estimated how long it will take the driver to get from Quasar to Milktown?
DaveC426913 said:
I am driving between two towns 60 miles apart, traveling at 60mph - the speed limit.
This road is special - it is not fixed, it is expanding. Every moment I drive, the distance between the towns grows by 100 miles every hour.
At no time am I moving at more than 60mph, nor is any local part of the road expanding at greater than 60 mph.
It is a great example! We are in Milktown and the city of Quasar is 60 miles away.
Hubble parameter is 100 mph/60 miles so Hubble radius is 36 miles.
The Hubble radius is the distance from us (Milktowners) at which a point on the road is receding at 60 mph.
At first the driver is being swept back at about 40 mph but this gradually lessens as the Hubble parameter decreases.
After one hour the driver is LESS THAN 100 MILES FROM MILKTOWN. Because he started at 60 miles and is swept back slower than 40 mph! A fair guess would be that he is actually LESS THAN 90 miles from us, because on average being swept back at considerably less than 40 mph. The figure of 40 was only right at the start of his trip.
Now after the first hour of driving, the Hubble radius is only 96 miles! So the driver is NO LONGER OUTSIDE THE HUBBLE RADIUS! Great, so he is no longer beings swept back at all. He is actually making forwards progress towards Milktown. And he will eventually get there.
Quincy said:
If the Universe is 96 billion light years across and 13.7 billion years old, then doesn't that mean that the Universe has expanded faster than the speed of light? How can that be if speed of light is the speed limit of the Universe?
Quincy the OBSERVED universe is said to have a radius of around 47 billion LY. The socalled "particle horizon" is about 47 billion LY. We are now seeing stuff that is currently about 46 or 47 billion LY away.
That is not the size of the universe. The universe is larger. We really shouldn't say that the universe is 96 billion LY across because it confuses people. If we are talking about what has so far been observed we should say OBSERVABLE universe. Believe me I have seen people get awfully confused just because someone left off the word 'observable' and they got the wrong impression.
Large distances
are permitted by Einstein's relativity to expand faster than the speed of light.
In a large universe like ours, relativity REQUIRES there be distances changing faster than the speed of light. If large distances weren't expanding FTL as they are for us, they would be contracting.
You may have gotten the wrong impression about the LOCAL SPEED LIMIT because somebody talking to you left out the word 'local'. The special relativity speed limit only applies to local motion-----shortrange distances between things in a local, approximately flat patch of space.
