Expansion or contraction -- how do we know which?

[Greenlight]

TL;DR Summary

Space/time is relative. What is the expansion relative to?

Space/time is relative. What is the expansion relative to? Is there a reference point of no expansion to make a comparison? Accelerated contraction would appear to be expansion by an observer looking back in time so how can we be certain that the universe is expanding instead of collapsing at an accelerated rate?

like taking a shrinking potion will make the room look like its getting bigger.

Like Two cars that accelerate at the exact same rate
but where one waits some time before starting off.
the car that waits will never catch the first car. The first car will continue to gain distance (expansion) on the second car even though they are accelerating the same rate.

When we look at distance stars, we look into the past, we see the old speed which would be much slower than our current speed.

 

[kimbyd]

The expansion represents distances between objects increasing in proportion to the distance between them. In practice this is observed by noting that redshift increases with distance.

 

[Greenlight]

The expansion represents distances between objects increasing in proportion to the distance between them. In practice this is observed by noting that redshift increases with distance.

that is a nice definition, but i don't think you understood the question because it doesn't address the relativity of expansion.

 

[kimbyd]

that is a nice definition, but i don't think you understood the question because it doesn't address the relativity of expansion.

Yes it does, because the definition I gave is independent of reference frame*. Every observer sees every other observer as receding at a speed proportional to distance (ignoring local variation for the sake of illustration).

* Well, there is an implicit time slicing inherent in this definition, and the observers are assumed to be moving with the expansion flow. But those are relatively pedantic concerns that don't change the essence of the statement.

 

[Greenlight]

Yes it does, because the definition I gave is independent of reference frame*. Every observer sees every other observer as receding at a speed proportional to distance (ignoring local variation for the sake of illustration).

* Well, there is an implicit time slicing inherent in this definition, and the observers are assumed to be moving with the expansion flow. But those are relatively pedantic concerns that don't change the essence of the statement.

This is exactly my question. How do you determine if you are contracting or expanding without a reference frame.

every observer in the now will be looking at an observer in the past. Thus if there is accelerated contracting then the person in the present doing the observation will be getting smaller faster than whatever they are looking at. this makes the universe look like its expanding.

if your ruler is getting smaller how do you know distances are getting bigger and not just getting smaller at a slower rate?

 

[Orodruin]

that is a nice definition, but i don't think you understood the question because it doesn't address the relativity of expansion.

On the contrary, I do not think you have understood what metric expansion means. There are no relative measures here. Expansion (regardless of whether it is accelerated or not) means that distances get larger with a speed that is proportional to the distance itself. In essence, the distance $d(t)$ between any two objects will be given by $d(t) = a(t) d_0$, where $a(t)$ is a growing function with $a(0) = 1$ and $d_0$ is the distance between the objects at $t = 0$.

This is exactly my question. How do you determine if you are contracting or expanding without a reference frame.

Whether you are getting closer or further away from other objects is not a matter of choice of reference frame.

if your ruler is getting smaller how do you know distances are getting bigger and not just getting smaller at a slower rate?

Your ruler is not getting smaller. A meter is still a meter.

 

[PeterDonis]

Space/time is relative

No, it isn't. "Space" and "time" are, but spacetime is not. Spacetime is an invariant 4-dimensional geometric object.

What is the expansion relative to?

When we say the universe is expanding, what we mean is that the congruence of comoving worldlines has a positive expansion scalar. That is the precise, frame-invariant way of saying what @kimbyd said here:

The expansion represents distances between objects increasing in proportion to the distance between them

The only way in which this is "relative" is that you pick a specific family of observers, the comoving observers, whose worldlines have a positive expansion scalar. But these observers also have an invariant definition: they are the ones who always see the universe as homogeneous and isotropic. So there is actually nothing relative at all about the statement that the universe is expanding.

if there is accelerated contracting then the person in the present doing the observation will be getting smaller faster than whatever they are looking at

You appear to have the misconception that relative motion makes objects get smaller. It doesn't. Time dilation and length contraction are not actual physical things that happen to objects. They are "perspective" effects of the spacetime geometry, just as looking at an object in ordinary 3-space sideways makes its profile appear different than looking at it face on. Looking at an object from a different direction doesn't change the object. Similarly, looking at an object in spacetime from a different state of relative motion doesn't change the object.

 

[Greenlight]

No, it isn't. "Space" and "time" are, but spacetime is not. Spacetime is an invariant 4-dimensional geometric object.

i thought Einstein showed us that space and time are somehow facets of the same thing.

When we say the universe is expanding, what we mean is that the congruence of comoving worldlines has a positive expansion scalar. That is the precise, frame-invariant way of saying what @kimbyd said here:

can expansion scalar be negative?

You appear to have the misconception that relative motion makes objects get smaller. It doesn't. Time dilation and length contraction are not actual physical things that happen to objects. They are "perspective" effects of the spacetime geometry, just as looking at an object in ordinary 3-space sideways makes its profile appear different than looking at it face on. Looking at an object from a different direction doesn't change the object. Similarly, looking at an object in spacetime from a different state of relative motion doesn't change the object.

im not even talking about motion at all. I am talking about the contraction/expansion of spacetime.

 

[PeterDonis]

i thought Einstein showed us that space and time are somehow facets of the same thing.

Yes, and that same thing is spacetime. Which is why saying that "spacetime is relative" is incorrect. Spacetime is the invariant thing that "space" and "time" are facets of.

can expansion scalar be negative?

It is possible to have a congruence of worldlines with a negative expansion scalar, yes. But the congruence of worldlines of comoving observers in our actual universe has a positive expansion scalar. That is what the evidence says. And that is why we say our universe is expanding.

im talking about the contraction/expansion of spacetime.

Spacetime does not contract or expand. Spacetime is a 4-dimensional geometry. "Expansion" is a property of a particular congruence of worldlines within that geometry. When we say the universe is expanding, we do not mean spacetime is expanding. We mean exactly what I have said previously.

 

[Greenlight]

Whether you are getting closer or further away from other objects is not a matter of choice of reference frame.

The apparent distance to an object does not determine whether space/time is expanding or not.

Your ruler is not getting smaller. A meter is still a meter.

are you certain that the ruler is not getting smaller or bigger? A meter is defined by constraints in space/time. When space/time changes (expands or contracts), wouldn't those constraints and thus the meter change as well?

 

[PeterDonis]

A meter is defined by constraints in space/time.

No, it isn't. It's defined in terms of the second, by setting the speed of light to a constant value, and then relying on the definition of the second in terms of the frequency of a particular hyperfine transition in the cesium atom. Nothing in this definition has anything to do with the geometry of spacetime (except for the fact that it is locally Lorentz invariant, which is a local property and is unaffected by the global geometry).

When space/time changes (expands or contracts)

Spacetime does not expand or contract. See my post #9.

 

[Orodruin]

The apparent distance to an object does not determine whether space/time is expanding or not.

Not the apparent distance. The proper distance.

are you certain that the ruler is not getting smaller or bigger? A meter is defined by constraints in space/time. When space/time changes (expands or contracts), wouldn't those constraints and thus the meter change as well?

Yes. No. The meter has a fixed definition. Regardless, bound objects do not expand or contract.

 

[Greenlight]

Spacetime does not expand or contract. See my post #9.

so the theory of black hole warping spacetime is a fallacy?

 

[Orodruin]

so the theory of black hole warping spacetime is a fallacy?

A black hole is a feature in spacetime. Mass (or more generally stress-energy) curves spacetime. Nothing said in this thread contradicts that.

Your understanding seems to be based off of popular scientific descriptions. This is not an appropriate way to approach science if yoy want to understand what is actually going on. In order to do that you will need to put in more work.

 

[Ibix]

so the theory of black hole warping spacetime is a fallacy?

That's not really what relativity says about gravity, despite that being a fairly common pop-sci way of phrasing things. It is true that gravity is modeled as curvature of spacetime, but the point is that time is a part of spacetime. It can't change with time.

When we say "space is expanding" what we are doing, in slightly less technical language than Peter used, is imagining slicing 4d spacetime into a stack of 3d slices, each one being "the universe at a particular time". In each slice (if you slice in a particular way), objects are slightly further apart than in the previous slice.

 

[Greenlight]

Not the apparent distance. The proper distance.

How is proper distance determined? aren't we delayed in all of our observations giving us an apparent distance?

 

[Greenlight]

That's not really what relativity says about gravity, despite that being a fairly common pop-sci way of phrasing things. It is true that gravity is modeled as curvature of spacetime, but the point is that time is a part of spacetime. It can't change with time.

When we say "space is expanding" what we are doing, in slightly less technical language than Peter used, is imagining slicing 4d spacetime into a stack of 3d slices, each one being "the universe at a particular time". In each slice (if you slice in a particular way), objects are slightly further apart than in the previous slice.

if spacetime itself is not expanding then how are distant objects capable of moving away from us faster than light?

 

[Greenlight]

Spacetime does not expand or contract. See my post #9.

if spacetime itself is not expanding then how are distant objects capable of moving away from us faster than light?

 

[Orodruin]

How is proper distance determined? aren't we delayed in all of our observations giving us an apparent distance?

You do not determine proper distance. You assume it behaves in a particular way and work out the observational consequences
- such as cosmological redshift.

if spacetime itself is not expanding then how are distant objects capable of moving away from us faster than light?

Space is expanding, not spacetime. You need to take two steps back and get the difference between those two right before you can stand a reasonable chance of reaching understanding.

 

[Ibix]

if spacetime itself is not expanding then how are distant objects capable of moving away from us faster than light?

As Orodruin says, space is expanding (for reasonable definitions of "space"). "Spacetime is expanding" doesn't even really make sense. Time is a direction in spacetime, so the whole thing can't change with time any more than a field can get bigger as you walk along it. It might get wider, but the whole field doesn't grow or change.

Any source that tells you that spacetime is expanding should be disregarded. It is space that is expanding.

 

[kimbyd]

How is proper distance determined? aren't we delayed in all of our observations giving us an apparent distance?

The proper distance is just a definition of distance. It's sufficient when explaining what is occurring. When doing actual observations, whatever distance measure makes sense for the observation is used. For instance, for supernovae, the luminosity distance is used because the measurements are all in luminosity. For Baryon Acoustic Oscillations, we might use the angular diameter distance.

All of these distance measures can be easily converted to one another, typically by multiplying or dividing by the scale factor. So we might measure a luminosity distance and then infer the proper distance from that measurement. If you're curious as to what distance measures are actually used, this is a good in-depth overview:
https://arxiv.org/abs/astro-ph/9905116

 

[Greenlight]

Space is expanding, not spacetime. You need to take two steps back and get the difference between those two right before you can stand a reasonable chance of reaching understanding.

I guess this is where one of us is hung up. I don’t understand how you can have one without the other.

But really this is not of concern.
Space expands.
Does it contract as well?

 

[kimbyd]

I guess this is where one of us is hung up. I don’t understand how you can have one without the other.

But really this is not of concern.
Space expands.
Does it contract as well?

Basically, there's no "supertime".

Space-time describes the entire structure, from the "big bang" to the far future. It's a static structure in that it has specific properties at every spacetime point, and those properties never change. This structure is a 4-dimensional volume.

That space-time can be divided up into a bunch of spacelike slices. Talk of "expanding space" is describing the behavior of those spacelike slices, specifically describing what it looks like if you are an observer within the space-time that experiences the passage of time.

 

[PeterDonis]

so the theory of black hole warping spacetime is a fallacy?

No. Warping (curving) spacetime is not the same as spacetime expanding or contracting. The former is perfectly fine. The latter is wrong.

This thread is basically a repeating cycle of: you state a misconception; someone corrects it. That's not a useful way to have a discussion. Please take some time to learn what our best current model of cosmology actually says, and how GR actually works. Sean Carroll's online lecture notes on GR are a good starting point:

https://arxiv.org/abs/gr-qc/9712019

 

[PeterDonis]

This thread has run its course and is now closed.