What happens to time when spacetime expands

[acepilot84]

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

 

[Godofgamblers]

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.

 

[Dmitry67]

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

galaxies are immune to the 'normal', non-accelerated expansion
they are not immune to the dark-energy-driven-accelerated expansion.

 

[Chalnoth]

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?

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:
$$ds^2 = dt^2 - a(t)\left(dx^2 + dy^2 + dz^2\right)$$

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

$$ds^2 = a(t)\left( dt^2 - dx^2 - dy^2 - dz^2\right)$$

...is identical in every respect to:

$$ds^2 = dt^2 - dx^2 - dy^2 - dz^2$$

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.

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.

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.

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.

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.

 

[sardar]

I am happy to help clarify these concepts for you. Time is a fundamental aspect of spacetime, and it is affected by the expansion of the universe. As the universe expands, the fabric of spacetime is stretched, causing time to also be stretched. This means that time passes differently in regions of space that are expanding compared to regions that are not expanding.

In the regions of space between galaxies, where there is expansion happening, time is also expanding. This means that time is passing slower in these regions compared to regions within galaxies where gravity is dominant and the expansion is not significant.

The big tear or stretch theory, also known as the Big Rip theory, is a hypothetical scenario where the expansion of the universe accelerates to the point where it tears apart all matter, including atoms and subatomic particles. This theory is based on the idea that dark energy, a mysterious force that is causing the expansion of the universe to accelerate, will continue to increase and eventually overcome all other forces, including gravity.

However, it is important to note that the Big Rip theory is just one of many possible scenarios for the future of the universe and is not currently supported by observational evidence. The fact that gravity is a relatively weak force compared to other fundamental forces does not necessarily mean that it will be overcome in the future. In fact, it is possible that the expansion of the universe will eventually slow down and even stop, leading to a "Big Freeze" scenario where the universe reaches a state of maximum entropy and all matter and energy become evenly distributed and cold.

In terms of what force is causing the expansion of space and time, it is currently believed to be due to the presence of dark energy. However, the exact nature of dark energy is still a mystery and is an active area of research in physics and cosmology.

I hope this helps to clarify some of the concepts you were struggling with. Keep asking questions and seeking answers, as that is the heart of science and the path to understanding our universe.