I suddenly started wondering if the universe would exist if there was no life to notice it. Any thoughts?

Perception is distinct from reality. So yes.

This is a more elegant "tree falls in the woods" question.
The entire universe exists in your mind.

If the universe is in your mind, why can't we change it at will?

It's completely possible. It's just not easy.

well, you can change you at will :) that is pretty easy.

I am sitting alone in my room with no life to observe me. Do I exist?

See this thread.

You exist because you are observing yourself. ^__^

You die in deep space with no chance of observation. Do you exist?

This question can have a lot of different answers depending on how you decide to interpret your tense usage.

Essentially (I believe) it boils down to consciousness.

your gravity will still effect the rest of the universe.

The concept you are referring to would not exist, since there is no one there to define it as such.

If a tree falls down and no one is near enough to hear it, then it makes no sound, because there is no one present to perceive the longitudinal waves emitted from the tree as sound.

Is this tree still living at the time it falls? Can we prove, without a doubt, that trees themselves cannot interpret the waves as sound?

The tree fell because it was infested with termites, which can perceive sound.