Can I start a business without her wanting a share or wanting to add it to the divorce?

You need to speak with an attorney so that we can gather more information, such as, you stated you were married for 28 days and then separated, how long have you been separated (and legally married)? Why are you waiting until December to file for divorce? There are pertinent questions that need to be answered.

Technically anything started during your marriage is marital property, so your business would be marital property. Whether it has any value is a different issue. But if you have invented the greatest thing in the world during your "28 day marriage" - and yes it is longer than 28 days now, it is not over until a judge says it is - that invention is marital property. Either get divorced now or hold off on the business until you are divorced - or get your spouse to agree, in writing, that she waives any claim to the business.