Jack and Rebecca are, in many ways, the heart of This Is Us. As the parents of the other three main characters, they're where the story starts; their relationship has determined the course of their children's entire lives. Jack and Rebecca are also easy to root for because despite any setbacks, it's clear they love each other deeply and want to make their family work. That's why it was so shocking to see Rebecca show up in the present day storyline without Jack, and led fans to believe that only one thing could have broken up a marriage like theirs. Awful as it is to imagine, is Jack dead on This Is Us?
There are a few hints about Jack's fate even though no one mentioned him in the present day storyline or explained where he was. First of all, Rebecca was married to someone else – and not just any old random, but Jack's best friend Miguel. Second of all, she was still wearing the moon necklace Jack gave her when he promised to do a better job as a husband and father. It would be kind of odd for Rebecca to divorce Jack, marry his best friend, and keep on wearing the necklace he gave her. Rebecca swore she would never take it off, but that's not the kind of promise you have to keep when you move on to a new relationship. However, it is the kind of promise someone keeps if their partner dies.
It was also telling that Randall's children referred to Rebecca and Miguel as grandma and grandpa, implying that he's been in the picture as her husband at least as long as they've been alive. That would mean that Rebecca and Jack have been apart for years, and Miguel could be the only grandfather the girls know. Of course, it's totally possible that something happened in Jack and Rebecca's marriage that made them decide to get divorced. Randall's children could call both Jack and Miguel grandpa.
However, the reverence with which Jack's children speak of him – and the emphasis on remembering the lessons he departed to them during their childhood – sounds a lot more like the way people talk about someone who's gone. Jack's death would be a major heart-wrenching event that is totally in line with the pattern the show has already established in such a short time: the death of a family member is something that happens every day, but it's also devastating. That's basically what This Is Us does on a weekly basis.
The show has planted seeds about discord in Jack and Rebecca's marriage, but they're so dedicated to each other that it just doesn't seem likely that they wouldn't try to make it work no matter what. While it's a much more upsetting outcome, Jack's death might also be the one that makes the most sense based on what information the audience has right now.