Maybe Luke went back in time to save his mom

Yes and yes to the above...

First of all, when the Force Portal was introduced in Rebels that allowed Ezra to save Ahsoka I was quite happy about it, BECAUSE it didn't necessarily alter a timeline per se, it only opened a doorway to another place, AND when Palpatine showed Ezra his parents, that was only a ruse to trap Ezra so Palpatine could gain access to, and power within the portal, and Ezra declined, therefore closing that opportunity for Palpatine.

If SW actually introduces time travel as a "thing" that would be used, it'd be the worst thing they could do, and I think that they know it, because it would create a massive paradox in the SW universe where EVERYTHING could be retconned...and I don't think they'll do it...they're not that stupid and/or blind.

IF they introduced it and actually used and portrayed it, I believe that it would only be utilized as a window for observation and not much more or better than a recording of the past that could be viewed, because otherwise it becomes a huge can of worms, and the SW universe (in the eyes of the fans) would potentially become less credible as a result.

With THAT being said, I think the opening to time and space COULD be utilized going forward as a way to move from place to place in the current timeline but only once or twice under VERY SPECIAL circumstances...as a (please don't hate me) transporter of sorts from Trek...to move to other worlds but ONLY in the current timeline.

As to what was said above, I do also think that due to the time travel paradox, time travel for the sake of changing something in the past would indeed only create a new timeline, as the one that's to be changed is impossible as those following events have already happened and cannot be undone...so therefore a new timeline would spring into existence in a parallel universe, but the current one would remain here, unaltered.

Now, as to the OP's speculation; I've said a hundred times already that I'm not entirely sold on the "fact" that Luke has died, as there are just too many things in my mind that don't ring true...

1. When one ascends into the Force, it is only his (or her) PHYSICAL state, and everything else is left behind....and we DID NOT see Luke's robotic hand fall to the ground when he disappeared, because that hand is a MAJOR thing in this saga, and in my mind, it's too important to be overlooked....and I noticed it right away..."Where's the hand??" I just cannot believe that THAT would've been overlooked...the canonical story group would've had to hard stop that scene and have it added in...or not, for a reason. This alone, is my biggest "pause point" about the disappearance of Luke, because IF it was missed/overlooked, someone (or many someones) blatantly dropped the ball, and if in fact they DID drop that ball and try to lazily explain it away later on, I'll be pretty disappointed in the story groups "Ability" to do their job....especially with an article of that significance. I'm not sold...because they haven't tried to definitively or convincingly (with any real effort) tried to explain it away.
2. We've seen Luke become so adept at his understanding of the Force, that he has been shown to have the ability to project his likeness to other places in the galaxy and interact directly and physically with that environment (he kissed Leia on Crait), yet at the same time, we now know that there is a Force portal where one with enough understanding of the Force can utilize it and physically travel to other places. Remember, Luke is the most powerful Jedi, and he should have that understanding. Did I mention that robotic hand that didn't fall to the ground? Yeah, there's that. Still not sold...
3. When Luke decided to "Disappear" to Ahch-To....why did he leave a map?? THAT in and of itself seems to me to be the biggest blunder that one could make in the act of trying to disappear. It's like running from the law but leaving a forwarding address to your new life....somebody is going to figure it out. With that, I think it's feasible that Luke entered that very portal in order to truly disappear and to not be pursued or sought out...no map, and no forwarding address...and could show his face later when it was truly time.
4. Despite what Mark recently tweeted about Luke dying; I don't trust Hamill when it comes to that, not that I think he's lying, but only perhaps speaking metaphorically, and knowing that as we've seen his trolling tweets often, and as a result, I go back to the working title of Episode 9......."trIXie", and Hamill IS the Trickster, as could be Luke. And ALSO remember that the novel also DOES NOT say that Luke died, only "passed into the Force" which I think could've been VERY craftily and deliberately worded. Nope, still not sold...

In the end, I (without resistance) WILL concede that Luke could very well have died...he could be dead, but in my mind there are just too damned many incidentals that point away from it...too many things that if have been missed by the story group, I will say they've done a piss-poor job of "closure", but I don't imagine that's the case....I just cant believe it, and again because of it...I'm not sold that Luke's dead, and I think they've provided themselves with a discreet avenue to keep him alive...at least through the end of IX, in order to say "We didn't SAY he was dead, only into the Force!!".

In THAT event, I don't think that he went back in time, but potentially (and deliberately) entered the Force to fully understand it...to soak it up...all of it...as his last lesson in the Jedi ways, and will step out of that elevator elsewhere to reemerge as the greatest Jedi ever and close the arc in the most fitting way for Luke, before we see him die in order to relinquish the reigns to the newest generation in the next trilogy with Rey and Ben....and yes, they'll both survive IX.

Could all of this be wishful thinking based on a small handful of facts that seemingly contradict Luke's death?? Yes, it very well could...

Do I think it's "fan wishful thinking"?? No, because I'm still not sold...he's only gone directly into the Force itself for his final lesson as a Jedi, and will re-emerge...otherwise I wouldn't blathered on like this.

 

They may have, but the "sound" would've/could've been easily been covered by the musical score, which (as I said above) would be a weak excuse in my mind.