Let's suppose the amount of physical goods produced increases 1% each year. Let's add that as physical objects they take up some space minimum amount of time. Now let's add that humanity is spreading out in all directions at the speed of light. If humans cover 1 light year of space they can expand faster than the new amount of stuff. However Volume = 4/3 pi * radius ^ 3 so what happens when humanity covers 1,000 light years. Well, next year they need to put 1% more stuff into 1002^3 / 1000^3 = 0.6% more space which means they need to increase the density which at some point means you have a black hole.
PS: 1% growth per year seems slow and sustainable but in 20,000 years your talking about 2.7 with 86 zeros after it. Start with 1 atom for one second and compare it with every atom in the earth for a year and your only at 4.2 with 57 zeros. So if the total economic output of humanity was 1 atom we could not sustain 1% economic growth per year for 30,000 years.
Edit: This assumes that the economy is based on physical goods, if we assume intangibles can take up 99.99...9% of the economy then 1% growth is possible for long periods of time even if it's somewhat meaningless.
PS: 1% growth per year seems slow and sustainable but in 20,000 years your talking about 2.7 with 86 zeros after it. Start with 1 atom for one second and compare it with every atom in the earth for a year and your only at 4.2 with 57 zeros. So if the total economic output of humanity was 1 atom we could not sustain 1% economic growth per year for 30,000 years.
Edit: This assumes that the economy is based on physical goods, if we assume intangibles can take up 99.99...9% of the economy then 1% growth is possible for long periods of time even if it's somewhat meaningless.