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If you traveled back in time, the earth would had moved on, not to mention the solar system and galaxy. Wouldn't you have to travel in time and space?

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That brings the question, what would be the spatial reference point for your time machine? Until we figure out what the universal reference point is, there's no way to know where in space your time travel machine would land. You'd also need to somehow match the angle and speed of the place you're landing, or find yourself instantly flying off the Earth's surface or squashing against it like a bug.

Good points, this is getting complicated.

Aren't you also increasing the mass of the Universe? You could add enough mass to halt the expansion.

That's a very good point as well. You probably couldn't move enough matter to make a noticeable difference to the Universe as a whole, but it does point to a violation of some kind of equilibrium.

I think it's just easier to assume time travel is impossible.

I bet that the universal reference point will be at a different location for each person who "views" it, and that it changes in between "views" for the same person.

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