If you shot a gun in space, the bullet would travel forever because the universe is expanding faster than the bullet can move
— UberFacts (@UberFacts) December 28, 2020
Scroll down for the translation.
The if sentence could be considered contrary to reality, it’s unlikely that we are going to shoot a bullet in space. We need:
Si <imperfect subjunctive>, <conditional>
Let’s use tú.
Si (tú) dispararas una pistola en el espacio, la bala viajaría para siempre porque el universo se expande más rápidamente de lo que la bala puede moverse.
dispararas = disparases. You can use either. Both are imperfect subjunctive of disparar, to shoot.
Another possibility. You don’t need to make literal translations always. Comparisons in Spanish are more complex than in some other languages. The universe is expanding technically would be se está expandiendo, but it makes the sentence even longer. Rápido is an adjective, but it’s also an adverb that means rápidamente. We could use se in the if clause to make the sentence impersonal, and it’s OK to use the indicative in this case because this experiment probably has already taken place in reality.
Si se dispara una pistola en el espacio, la bala viajará eternamente porque el universo se expande más rápido que el movimiento de la bala.
The Spanish translation turns out to be longer than the original English text.