HI,
As per your concern take these profiles one by one:
For Cosmologist: You need basic qualification in PCM as main subjects.
Astronomer:
Those who opt for this field, should pass their 12th under the science stream, specially in physics and math's.
Scientist:
A background from specific field which is PCM/B. geology. Astronomy or social sciences is required.

So concluding all, you require a common degree.
Hope this helps.
Good Luck.