Is it true that leaving such a young baby with his grandparents could
create long-term trauma?
Unfortunately, I don't think there is any scientific evidence one way or another. I could not find any study that deals specifically with babies being in non-parental care for a week or more.
However, serious trauma seems very unlikely to me.
- First, there is quite a bit of research into non-parental day care (i.e. care for a day at a time), and the general consensus is that on balance the effect is mostly positive, and at any rate other factors (relationship to parents, contact to other children) are more important. See for example this article: Is Non-Parental Daycare Bad for Children?, and the study The long term effects of non-parental care on children. While your case is a bit different, this does not point at likely harm. In particular, small babies live very much in the "here and now", so it's unclear whether several consecutive days vs. single days makes much difference.
- Secondly, small children are very adaptable, and while they will know that it's not their parents looking after them, as long as they are well looked after (both physically and emotionally), they will be fine. Note that for example in many families one parent (usually the father) is only really present during the weekend - still even small children will accept them as a father, despite them only being present a few days a week. So children can definitely accept someone as a caregiver who they do not see every day.
The most important point, I think, is to make sure that the baby knows the grandparents before being left with them for a week - and equally important the grandparents know the baby, and their peculiarities.
Spend a few afternoons together first (if that has not happened already), then try leaving your baby with granny and grandpa for a night. This gives both sides time to get used to each other. And finally, if your baby should really suffer from being with your grandparents, you will know - you are the parents, after all. Most probably you will get back a happy baby that had a nice time away from home.
Have a good trip! And don't worry too much - typically these kinds of separation are much harder on the parents than on the child.