Using current time in UTC as default value in PostgreSQL

Wrap it in a function:

create function now_utc() returns timestamp as $$
  select now() at time zone 'utc';
$$ language sql;

create temporary table test(
  id int,
  ts timestamp without time zone default now_utc()
);

Still another solution:

timezone('utc', now())

What about

now()::timestamp

If your other timestamp are without time zone then this cast will yield the matching type "timestamp without time zone" for the current time.

I would like to read what others think about that option, though. I still don't trust in my understanding of this "with/without" time zone stuff.

EDIT: Adding Michael Ekoka's comment here because it clarifies an important point:

Caveat. The question is about generating default timestamp in UTC for a timestamp column that happens to not store the time zone (perhaps because there's no need to store the time zone if you know that all your timestamps share the same). What your solution does is to generate a local timestamp (which for most people will not necessarily be set to UTC) and store it as a naive timestamp (one that does not specify its time zone).

A function is not even needed. Just put parentheses around the default expression:

create temporary table test(
    id int, 
    ts timestamp without time zone default (now() at time zone 'utc')
);