Why did Carlsen play 7. dxc5 in Game 7 of the 2018 World Championship? As a class A player, I have some understanding of opening principles. Relevant to the move played in the game, I know that it's advantageous to control the center, to have central pawns controlling the center, and to maintain tension for as long as possible. It's for these reasons that I was confused by 7. dxc5 in game 7 of the world championship.

I would understand this move if Carlsen followed it with 8. cxd5, giving black an isolated queen pawn and trying to play against this for an advantage. But Carlsen did not follow up with this idea, and I was unable to understand the strategy of his next moves.

What's the idea behind 7. dxc5 in this game? Why give up a central pawn and break tension? Here's a link to the game where it can be replayed: Chessbase.
The same move order also occurred in game 2 (at least up to 9...Qa5). It's a good question that you're asking, though I'm afraid there's no intuitive answer in the way that you're looking for. Instead, these are very concrete lines well polished by computer analysis, which means the best way to understand a decision such as 7.dxc5 is to simply play through the lines a bit, and consider a couple of alternatives in order to draw close analogy between these QGD lines and pinpoint the main differences with and without dxc5.

That said, part of the answer also simply lies in the strategies that the players have adopted in this match (which ultimately stems from their exact preparations), e.g., Caruana seems to favour these rather more solid lines with 4...Be7 as opposed to the somewhat sharper variations such as the Vienna (4...dxc4), Tarrasch (4...c5) or Ragozin (4...Bb4) to name a few, most of which by the way Caruana has himself played previously. But then 4...Be7 essentially allows Carlsen to stay in the mainline theory of QGD with Bf4 and e3..., and from Caruana's point of view, it's the 6...c5 variation that he seems to have chosen to prepare for and play in this match against this line (so you should remember the line as Be7... c5, as the key defining moves of this variation), which he has followed similarly in both games with this Nc6 and Qa5-Rd8 or Qa5-Qd8 ideas. 

Now here's my attempt to offer you a bit of intuition for the specific 7.dxc5 variation, but do take it with a grain of salt as these (like most other Queens pawn openings) are inherently very concrete lines after all: (annotations added to all variations and sub-variations)

 [title "London WCC2018 7.dxc5 in QGD games 2&amp;7"]
 [fen "rnbq1rk1/pp2bppp/4pn2/2pp4/2PP1B2/2N1PN2/PP3PPP/R2QKB1R w KQ - 0 7"]

 7. dxc5 {What it achieves in short: i. leaves the bishop slightly misplaced on c5, often it has to return to e7 again e.g. to reroute to f6, or to side-step a3-b4 ideas. ii. prevents black from taking on d4 which would further enable black's development as white is essentially forced to recapture with a piece. On the same token, it keeps the knight on f3 where it is well placed controlling e5 and ready to re-route via d2 if needed. iii. Semi-opens the d-file with ideas to quickly play Qc2 Rd1 with immediate pressure along d-file, where d5 pawn still stands.} Bxc5 (7. Be2 {Let's make a quiet move and see what happens if we let black take on d4} cxd4 8. Nxd4 (8. exd4 dxc4 9. Bxc4 b6 {One line among others after exd4. But it already exemplifies why exd4 is bad for white: to play for any edge white has to maintain their IQP but black gets then an easy development with early b6 Bb7, target on d4 and a somewhat misplaced bishop on c4 which can be exploited to gain tempo. If white decided to resolve the IQP immediately it would lead to a completely liquidation of most pieces ending in a completely equal game.}) Nc6 {Thanks to the knight on d4 the Nc6 move comes somewhat with a bit of tempo as it forces white to react:} 9. Nxc6 bxc6 {and black has a solid grip over the centre, and partly solved their bad c8 bishop problem as now there are ideas to trade via a6, that is, Qa5 Ba6 ...} (9. Nf3 Qa5 {or dxc4, and black even has a slight initiative, Nf3 back is clear tempo given up by white.})) 8. Qc2 {As opposed to the more fashionable lines, namely cxd5. Qc2 side-steps the d-file to avoid queen trades and free the file for a quick rook move. Prophylactically stops Bf5 in certain lines because white will be ready to face it with Bd3, e.g. after Qa5 Re8 e5 Nd4 Bf5. It also controls e4 stopping any possible early Ne4 by black, and avoids immediate material trade that is common to cxd5. Finally, it also x-rays the bishop on c5 which is unprotected, forcing black to react to the potential discovery attack.} (8. cxd5 {still rather famous line, previously played by many including Carlsen, MVL, Grischuk, Caruana.} Nxd5 9. Nxd5 exd5 10. Bd3 Bb4+ 11. Nd2 {10. a3 also possible. And black is fine structurally because even the IQP is not permanent as d4 is playable in many lines that follow. But black has a somewhat slow play, because bishop is misplaced on b4, often it reroutes to f6-b2 diagonal via e7, and black still has to play Nc6 and develop c8 bishop. So it's decent for white but more committal which is maybe why Carlsen is not opting for it.}) Nc6 9. a3 {Still a highly theoretical line, still evolving as Carlsen and Caruana have been showing in the match so far, but it's one that suits both their styles of having a closed positional QGD game, with various continuation choices left for both sides.}


Hopefully, the above analysis (by no means exhaustive) offers you a glimpse of the kinds of ideas that underlie the dxc5 line. But bear in mind that, in general, to answer such questions for yourself, try to uncover not only the concrete aspects of what the move achieves, but also the sorts of ideas it prevents or side-steps.