Interpretation of the ending.

A lot of people have been complaining about the last act of the film (Pinbacker) and to a large extent I can understand their frustrations, but I'd like to throw in this interpretation of the ending that may just change your opinion of it, and the underlying meaning of the whole movie.

Several people have commented on the religious allegory of the film (sun = god) and much of it's spiritual imagery and I have to kind of agree with them on it, but I believe the film's themes go beyond religion and spirituality and into existentialism in a broader sense and here's why:

Pinbacker is Capa's Tyler Durden. Like Tyler Durden in Fight Club, Pinbacker is Capa's imagination materialized. If you notice, each of the crew's deaths is symbolically foreshadowed. Searle enjoys staring into the sun; he is eventually burned by it in essentially the same room, Corazon is the one who suggests killing Trey, eventually she is killed in the oxygen garden, the very place she cared about so much. Harvey is the coldest and most selfish of the group; he literally freezes to ice (I even think it's meant to contrast with Searle). Mace is killed in the ship's mainframe cooler, which he repeatedly takes protective care of even though it is shown earlier to endanger his life. He dies in a similar way to Harvey, only his death seems less visually dehumanizing. It should benoted that Mace is also portrayed as cold and stoic but eventually reveals to have good intentions while Harvey on the other hand is merely selfish and insecure. Similar to Searle, Kaneda also shows a fascination with the sun and dies staring into it; Searle asks him right before he is vaporised "What do you see".
Cassie dies by falling in the cube/bomb's multiple conflicting gravitational fields. If I recall, there was a scene where she and Capa talked about dreams and we see a dream of Capa falling.

Notice there is a motif of seeing and perceiving. throughout the film (dreams, 'what do you see', Pinbacker's dialogue and Mace's therapy scene, where we are shown a fake reality (stimulation) of waves. The scene where Harvey detects the Icarus I has him in darkness, suddenly staring at and sticking his head into a neon green light. In a film where the dominant colours are a contrast of dull grey and blue versus bright orange-red-yellow, the colour green inevitably sticks out to us. Green represents not just nature, but home, or at least the (almost false?) notion of home. The symbolic use of green parallels the very few instances of green we see; the oxygen garden (as well as it's overgrown counterpart in the Icarus I) as well as the background colour message broadcasting system Capa stares into as he sends his final message. The film however also makes it a point to show us how what is seen may not always be real or what is real. The neon tint to the green in the lead up to the discovery of Icarus 1 has ominious undertones to it, and even though the overgrown oxygen garden in Icarus 1 is visually portrayed as a thing of wonder, it turns out to be a trap. Possibly set up by Pinbacker, but then again, is he even real? Did he kill Trey?

Ambiguities like these feed into my overall interpretation that the film is about reality breaking down as mankind approaches god (represented by the sun) and their dreams, memories and fears (especially fears) slowly becoming real. Early on in the film Capa discusses with Kaneda the physics of the bomb's deployment into the sun and the stimulation itself warns Capa that showing the bomb as it gets too close to the sun might be difficult because the physical and temporal dimensions might become unstable (at least, it said something along those lines). This is the first piece of obvious foreshadowing of what is to come.
The next huge piece of foreshadowing of the complete breakdown of physical reality comes in the form of Pinbacker's creepy recorded message to the Icarus 2. If you pay attention to the way his face is portrayed, constantly fizzling in static, it parallels the film's very interesting visual style that becomes more and more prominent towards the end which climaxes in his big reveal. The way the film constantly cuts to disorienting closeups that are filled with (intentional) continuity errors, as if to suggest he was there, and yet in another sense of reality, he wasn't. A scene where Capa is trapped in a room and he and Pinbacker stare at each other through a small glass window and we see his figure blur and almost fizzle, paralleling the way he was foreshadowed in his video warning. Is he real? The only ones who see him are Cassie and Capa. Corazon is killed by him yet she doesn't see him. The scene were Cassie tears Pinbacker's skin of as he strangles Capa is also extremely disorienting, filled with jarring closeups that don't match. In fact, how did Pinbacker find Capa and Cassie in the cube without either of them seeing him coming? It's a huge open space! Did he literally just appear out of nowhere? Given that the Icarus computer warned Capa early on reality breaking down as they approached the sun, this doesn't seem impossible.

I'm not saying that none of this is real, nor am I trying to offer up interpretations of what is 'real' and what isn't per se. Rather, I'm trying to draw attention to how the film deliberately uses a blend of story and (gradually) disorienting visual style to explore what the film's central themes and ideas are. Perhaps the very ending in which Capa stares into the sun isn't real either, but regardless of whether you look at it from a scientific perspective (huge magnetic fields breaking down time and physics), or from a spiritual perspective (that the sun is really god), it doesn't matter because the outcome is still the same. All that matters is that we see exactly what he sees, the completion of a journey that tested him and us physically, emotionally and spiritually. And shouldn't that be all that matters?