Here are a few tips for combining your keywords into powerful searches:

Put quotation marks around a phrase

"cosmic microwave background"

Without the quotation marks you could get an article that mentions microwaves but not CMBR specifically.

Use an asterisk * after the word stem

gravit*

This returns articles that mention gravity as well as articles that use the word gravitational.

Combine synonyms with OR

"cosmic microwave background" OR CMB OR CMBR

Look for a + sign button to add more topics to your search

If you want both topics included in your search, make sure select the AND operator to connect them.

Use parentheses and AND to combine topics in a single search bar

("cosmic microwave background" OR CMB OR CMBR) AND "gravitational waves"