Research is always reported in the past tense. (Tense is the grammatical term for time.) Whatever was said, done, written, etc. was done earlier and is not still being done. Note the earlier examples and their past tense verbs. Here are some past tense verbs used for writing about research. These words have different meanings, so select carefully for your context:

accepted accounted for advised advocated affirmed
agreed analyzed asserted claimed commented
concurred considered contradicted countered declared
defended demonstrated denied described disavowed
disclaimed discovered disputed dissented emphasized
established examined explained explored expressed
implied indicated informed inquired investigated
maintained mentioned noted observed offered
posited presented probed promoted proposed
questioned recognized recorded recounted refuted
rejected related remarked reported repudiated
revealed stated studied suggested summarized
supported surveyed theorized urged

Although research is reported in the past tense (because the research was done in the past), sometimes present tense is needed to express general truths or facts that exist in the present. Consider the following example:

Galileo realized that the earth revolves the sun.

The realizing took place in the past, but the earth is still revolving around the sun.