Read this phrase from Hubblesite and was confused, “In 1929, Edwin Hubble provided the first observational evidence for the universe having a finite age. Using the largest telescope of the time, he discovered that the more distant a galaxy is from us, the faster it appears to be receding into space. This means that the universe is expanding uniformly in all directions.”
If the speed of expansion is uniform, shouldn’t distance not even be a variable?
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You’re confusing *rate* and *amount*.
Think of it like, say the distance between two points doubles in 1 hour, that’s a constant and uniform rate, right?
If the distance starts at 1, in 1 hour it’s now 2. If the distance starts at 1,000, in 1 hour it’s now 2,000. See what happened there?
If the first example were a ‘close galaxy” it would appear to be moving at 1 unit per hour away from us, the latter is a ‘far galaxy’ that appears to be moving 1,000 units per hour, all because of a uniform rate of expansion.
Because they **do** recede faster!
Think about this: if everything around us was moving away at 10km per year, then two of those things next to each other would be “staying still” (relative to each other). This isn’t what actually happens.
By “uniform expansion”, it is more understood as “1 km this year -> 1.00002 km next year”. So if you start 1km away, you hardly notice any difference after a year (only 2cm), but if you start 100 billion km away, you’ll move 2 million km.
Note that this also allows for everything to experience expansion equally (appears the same to all perspectives).
It’s not the speed of expansion that is uniform, but the rate.
Given any two points in space, the space between them is growing. But the amount it grows depends on how big it currently is.
The Hubble not-quite-a Constant is usually written as 90km/s/Mpc, but in SI units that works out to be about 2.27×10^(−18) /s.
This means that given any length, that length increases by a factor of 2.27×10^(−18) every second.
Two things that are 1m apart now, will be about 1 + 2.27×10^(−18)m apart a second later (in theory, this doesn’t happen in practice due to other forces).
But things that are 2m apart will be about 2 + 4.54×10^(−18)m apart a second later. The expansion is proportional to the current distance.
This means that things that are far apart get further apart faster than things that are closer together. The expansion happens at the same rate, but the relative speed is proportional to the distance.
If you have galaxies laid out like this
A-B-C-D-E
and after a billion years they look like this
A–B–C–D–E
the rate of expansion was uniform…each “-” became “–“. B moved one “-” from A. But C moved *two* “–“s away from A! And E moved *four* away! Because there’s more starting space between them, there is more space to expand.
Imagine two points on a rubber band that are nearly touching one another. When you stretch the rubber band as much as possible, those two points are still going to be relatively close together. But the distance between two points that were separated by an inch are now much more than an inch apart.
When things expand or stretch, the space between points increases: more space between starting points means more expansion between them as they are stretched away. On astronomical scales this means that the distance between distant galaxies is increasing faster than the distance between neighboring galaxies.
I recommend getting a rubber band and a yardstick/meter stick. Cut the rubber band so it’s just a line. Put 3 black marks on it, 2 by either end and one in the middle. Now, stick one end down to the yardstick at 0 with one finger, and note the positions of the other 2 dots. (Let’s say at 3 and 6 inches, meaning the rubber band is 6 inches long).
Now stretch the band by it’s free end. Say, double. So the far end is 12 inches away. The middle dot is… still in the middle. 12/2 or 6 inches. Now, both dots traveled over the same amount of time, but one went 3 inches (from 3 to 6) and the other went 6 inches (6 to 12). Let’s say it took 1 second to stretch the rubber band. The closer dot moved 3 in/sec while the further dot traveled 6 in/sec.
The thing about stretching is that it is modeled as a uniform scaling. Scaling means multiplication. Multiply a small number by the same factor as a large number, and the larger number has a larger change. Essentially, a larger distance is made up of more… space. So if all space expands at the same rate, then more space adds up all the little expansions together into one big expansion.
You can see this effect (in one less dimension) with a toy ballon. First, inflate it a little bit – enough so that it’s basically round and a little bit firm. Then use a soft-tipped marker (like a Sharpie) to make dots in various places on the surface of the balloon. These are your objects, and you could imagine they are stars in your experimental universe. Now, blow up the balloon some more. You will see that the ‘stars’ that are close to each other move apart some distance (as measured across the surface of the balloon, not through the inside of it). ‘Stars’ that are farther away from each other move even farther apart as the balloon universe expands.
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