Is it correct that time relates to gravity?

Gravity *does* exist in space. Otherwise, the moon would stop circling the Earth, and move off in a straight line. Time (actually space-time) is *affected* by the presence of a gravitational field, slowing it down, but even in the furthest reaches of our universe, time exists.

Now, if the astronaut you're talking about is traveling at speeds near that of light, then yes, time will slow down for him - but, his speed through space, and the lack of gravity near him have nothing to do with each other. He's being affected by the time dilation effect Einstein has postulated.

Actually, gravity does have some effect on time. (And GeoffG, you know this.)

But (1) there is gravity is space. (2) Even if there was no gravity, that just means that time flows at it's "normal" pace.

As an example, on the surface of the Earth, where there is certainly gravity, time flows about 0.0219 seconds per year slower than time in deep space where gravity is very weak. (This is ignoring the effect of the Earth spinning and moving around the Sun.)

Actually, time always exists.

What I think you're getting at is that relative time is adjusted by increases in gravity or acceleration.

Relativity tells us that a strong gravitational field will appear to slow down time relative to an outside observer (one outside the field). Time will run slower for the person in the strong grav field...they will age slower.

The same happens when you accelerate. When you accelerate, time slows down for you relative to those on the outside that are not accelerating.

So, if you have two people in space, near zero gravity, and you synchronize watches, and you have one person move toward a planet, orbiting it for a while (experiencing its gravity), and then you both return back to the starting point again, the person that orbited the planet will have less time elapsed on his watch...because time slowed for him as he was experiencing the gravitational field.

Likewise, if two people are in space (zero gravity), and one person flies away at extremely high speed in a rocket, and then returns back to the starting point (and the other person didn't move), the person that accelerated away and came back will have less time elapsed on his watch...because relative time slowed for that person.

1. Gravity has nothing to do with time. It is a function of the masses of the two bodies and their distance apart.

2. There IS gravity in space. Gravity is everywhere there is mass.

from what I've learned and read over the years, time relates to the expansion of space. If space expansion slows, then our time would slow, but we wouldn't know.

If space stopped expanding and did the opposite (compacting), then time would go backwards, or rather everything's property would reverse:

Water would unmelt into ice, heavy things would become light, the earth might come apart as gravity reverses.

Thee is gravity and time thoughout the universe, time is relative.