We can have growth, but the growth rate must decline over time. For example the current growth rate of world population is around 1.1% per year. It is easy to see that it cannot remain above 1% for more than 12101 years (assuming FTL travel is not discovered).

Proof: Right now every living human is either on Earth or close to Earth. Without FTL it follows that 1 year from now every living human must be within 1 light year of Earth, 2 years from now every living human must be within 2 light years of Earth, and so on. 12101 years from now every living human must be within 12101 years of Earth.

If you calculate the volume of a sphere of radius 12101 light years centered on Earth and divide that volume by what the population would be after 12101 years of 1% annual growth you get 0.04 m^3 per person.

But the volume of an average human is ~0.06 m^3, which is greater than 0.04 m^3.

At 0.1% annual growth it would take a little over 127000 years to run out of room.

At 0.01% annual growth we've got 1.34 million years.

Of course the actual limits are much lower because the above is assuming that we can pack humans so that there is no space that is not occupied by humans which we cannot do because, among other things, (1) our shapes don't fit together perfectly to allow such tight packing, (2) there isn't enough mass in the 12101 light year sphere to make that many humans, and (3) we would need to use much of that space for the infrastructure needed to support humans.

> We can have population growth and still solve the impending environmental disaster.

But will we?

Clearly not. We aren't going to solve it.

We probably will continue to expand though.

We could always allow more legal immigration.

Some countries don't have unfunded liabilities that can share some of their population so we can keep the Ponzi scheme going.