Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What happens to time when spacetime expands

  1. Sep 7, 2010 #1
    i recently had trouble understanding some concepts with one of my astronomy assignments. I understand that the universe is expanding, and that galaxies further away from each other are expanding at a faster rate than galaxies closer together, yet the space inside a galaxy remains constant due to gravity. So in the regions of space that is suppose to be expanding between galaxies, what happens to time? If spacetime is an interwoven fabric, then how does time expand?

    also, i have heard of the big tare or stretch theory, where everything keeps expanding until eventually even atoms and electrons with separate from their basic elements and everything will lose heat and be an emptiness of cold black. how does this theory play into the fact that they say space inside galaxies does not expand due to gravity. i understand that gravity is a weak force and never dies, but what force is making this stretch of space and time.

    i asked my instructor these questions, and he told me he had never thought of these questions, and that i should join some forum and ask away. so here i am, confused, and trying to make sense of this. thanks
  2. jcsd
  3. Sep 7, 2010 #2
    Think of time as a distance. Gravity bends the time space continuum and so if you pass by an object of large mass, your internal clock will slow down. You will take longer to go around the object though the lag will be relative (you will not feel it).

    Galaxies are not immune to cosmic expansion and the force of expansion/entropy will eventually overcome their local gravity too.

    Eventually all will be sundered on the lathe of heaven.
  4. Sep 7, 2010 #3
    galaxies are immune to the 'normal', non-accelerated expansion
    they are not immune to the dark-energy-driven-accelerated expansion.
  5. Sep 8, 2010 #4


    User Avatar
    Science Advisor

    Well, GR is a bit sticky in that there is no absolute time dimension: which dimension is time depends upon the observer.

    However, in the coordinates in which we normally take space to be expanding, time is not. One way we write the metric for these coordinates is:
    [tex]ds^2 = dt^2 - a(t)\left(dx^2 + dy^2 + dz^2\right)[/tex]

    It turns out that if you try to make time expand along with space, the result is indistinguishable from flat space-time. That is,

    [tex]ds^2 = a(t)\left( dt^2 - dx^2 - dy^2 - dz^2\right)[/tex]

    ...is identical in every respect to:

    [tex]ds^2 = dt^2 - dx^2 - dy^2 - dz^2[/tex]

    Note that you don't have to use these coordinates, so we can't unequivocally say, "space is expanding but time is not," it's just that in the usual description, this is the case. One can think of ways for time to expand in a different way from space and end up with the same behavior.

    The big rip, you mean? Most physicists consider this to be completely ridiculous. One way to look at it is that this would violate what is known as the dominant energy condition, which is a statement that matter can't travel faster than light.

    The idea here is that you have something that behaves sort of like a cosmological constant, but the energy density grows with time. This provides a repulsive force between objects that also grows with time. So as time passes, this repulsive force starts ripping apart smaller and smaller objects, starting with galaxy clusters and ending up ripping apart protons and neutrons. It turns out that this accelerated expansion approaches a singularity in the finite future with everything torn apart.

    But it requires an inconsistent theory of General Relativity, so it's highly highly unlikely to be accurate.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook