The multiverse theory proposes that there are beyond our observable universe there, other realities – and timelines – exist as well.
Academic Tom Siegfried told the National Geographic in 2022 of the possibility that "we’re just one of many universes that are different, and we live in the one that’s nice and comfortable."
Considering their fortunes over the past decade, Manchester United fans may not entirely agree with that description.
Since Sir Alex Ferguson departed in 2013 the Red Devils have not come close to lifting another Premier League trophy, and there's only been a smattering of other silverware to satisfy their supporters' appetite for glory.
There has been a series of sliding-doors moments in that time, in the form of failed transfers, baffling managerial appointments and off-field controversy. One of the biggest 'What if?' moments has to be United's failed pursuit of Robert Lewandowski in 2012.
Fresh from losing out on the Premier League title to bitter rivals Manchester City on the final day of the season, United were on the lookout for a new striker and eventually settled on Arsenal's Robin van Persie.
However, before acquiring the Gunners' then-talisman, United had all-but-secured the signing of Lewandowski. The 24-year-old had seriously impressed at Borussia Dortmund the previous season, netting 22 times in the Bundesliga as Jurgen Klopp's side secured a domestic double.
“After my second year in Dortmund, I had a conversation with Sir Alex Ferguson,” Lewandowski told France Football in 2020. “He wanted me to come to Manchester.
“I was very interested. I can even say I was ready. But Dortmund didn’t want to let me go. It didn’t upset me more than that because things were going well with Borussia.”
How different would things have been if United and Lewandowski had teamed up? Below, GOAL walks you through that alternative history...