This is a common belief but it is untrue–that not-yet-ripe tomatoes can be hurried to juicy perfection by a few days in the sun. Sunlight does not help and can in fact be harmful if the tomato gets too warm or suffers from sun scald. Sure, those pretty tomatoes look nice sitting on a windowsill, but there is a way to hurry ripening that actually works. Put the ‘maters in a sealed paper bag for a few days, ideally with a ripe banana.
Say what? You see, fruit (tomato is, technically, a fruit) naturally gives off ethylene gas as part of the ripening process. It’s completely natural and perfectly harmless and by keeping the tomatoes in a bag you concentrate the ethylene and hasten the ripening. And further, bananas are the champs at giving off ethylene so including one speeds things up even more.
Would apples be a better ripener than bananas in the bag? This source claims that “Bananas actually only produce moderate levels of ethylene”:
https://www.sciencefocus.com/nature/why-do-bananas-make-fruit-ripen-faster/
I haven’t found a direct comparison of apples vs. bananas as ethylene producers, but this page classifies apples as Very High ethylene producers:
https://www.bryair.com/technical-articles/control-of-ethylene-in-fruits-vegetables-warehouses-and-cold-stores/
As a side note, in bananas it’s apparently their stems which emit much of the ethylene; wrapping the stems in plastic wrap can inhibit that ethylene from ripening the bananas (and anything else nearby). I wonder if cutting the stems off would work as well to delay ripening, and if saving fresh banana stems to use in a paper bag ripener would work well (many stems would fit, and you don’t have to imprison a good banana!).