I am trying to brew 5 gal of Dunkelweizen. I used instructions from:
I measured the OG of 1080 after cool down the wort. It should be 1050-1053.
What should I do in order to decrease the OG? What will happen if I don't do something?
You can, of course decrease it by adding water......this calculator will help:
If you don't decrease your gravity a couple of things could happen. First, your yeast might not work, usually different (or more) yeasts are used for high gravity brews. If your yeast does work it might not attenuate fully. Or it might end up attenuating fully and your final ABV will be quite a bit higher than expected. It's really impossible to say what's going to happen with any more certainty based on the information you've given.
If you used extract, and you didn't dramatically change the water volumes or add any additional fermentables, then it's highly unlikely that your gravity is that far off.
The usual cause with this sort of process is incomplete mixing of the boiled portion added to the water in the carboy, leading you to get a sample of the exceptionally-concentrated boiled wort rather than well-mixed wort. Does this seem likely?
Practically speaking, if this had just happened to you, you could dilute out your wort with water, although this would mess up your volume, and dilute out everything, including some other ingredients which may be at the right concentration.
If you were to do that though, let's say your target volume is 5 gallons, and you have 5 gallons of real OG (ROG) greater than target OG (TOG). Then, ignoring all other concerns of overdiluting other ingredients and sanitation concerns, you would want to dump out (1 - (TOG-1)/(ROG-1))*5 gallons of your wort, and replace with the same volume of water. To use your #s as an example, (1 - (.053/.08))*5 = 1.69 gallons!
Apologies for mathiness. The result will be that you have diluted to the amount to reach your TOG, and have maintained the target volume. And you will have sadly dumped out some precious wort!
Short answer: No. That's why it's called "Original Gravity". It's actually the specific gravity of your wort before fermentation starts. There are a couple of things that could have happened. 1) you didn't let the wort cool to 60 degrees F to read your hydrometer or 2) you didn't have 5 gallons of wort.