How do we know space is expanding?

[Galactic explosion]

I understand that there are many forums and topics written on the fact that space is expanding. But I've looked and looked, and couldn't find an answer to my specific question.

How do we know space is expanding, rather than galaxies just regularly moving apart?

How do we know galaxies are actually "riding" space like a surfer on a wave in the ocean (or that typical answer you get with a balloon blowing up with dots on it)? Is there even an actual difference? Can't they just be striding along in all directions without including such a weird phenomenon where the geometry of space is increasing in magnitude, and dragging everything with it like some kind of warp drive?

With the balloon analogy, it's quite obvious the balloon is inflating because we can actually differentiate the balloon as a separate object from the dots. But when it comes to actual space, isn't space an emergent consequence of there being many objects relative to each other? In order for space to even exist, you naturally need to have a measurable distance between two objects. No objects? No space. And vise versa.

For example, all that exists are 3 things; two objects, and a hand to push the objects. If the hand pushed one object in the opposite direction of the other, it would at least from some point of view, appear that both objects are moving away from each other equally, depending on the frame of reference. Let's say the hand forgot that it pushed that one object, and wanted to measure both their motions. Would it come to the conclusion that the space between the objects is expanding, or the objects are simply moving away from each other because there was an initial force at some point?

I hope that example made sense and I didn't butcher my question.

 

[PeterDonis]

Moderator's note: Thread moved to cosmology forum.

 

[PeterDonis]

isn't space an emergent consequence of there being many objects relative to each other?

No.

"Space" has to be sliced out of spacetime--basically, "space" is what you get when you take a 4-dimensional spacetime and slice it up into a series of 3-dimensional spacelike slices, with each slice labeled by its own time.

In the spacetimes that are used in cosmology to describe our universe, when you do the slicing in such a way that each 3-dimensional slice is homogeneous and isotropic, then the scale factor of the slices increases with time. That is what cosmologists mean when they say space is expanding.

In general, there are an infinite number of different ways to slice up a spacetime, and the laws of physics by themselves don't pick out any particular slicing as preferred. However, in the case of the spacetimes used in cosmology, picking out slices that are homogeneous and isotropic is "preferred" in the sense that it matches an important symmetry of the spacetime. So it makes sense to use that slicing to define the expansion of space.

 

[Dale]

How do we know space is expanding, rather than galaxies just regularly moving apart?

What is the difference between space expanding homogenously and isotropically and galaxies moving apart homogenously and isotropically? The weird part is the fact that it is homogenous and isotropic, worrying about calling it space expansion or galactic motion is missing the point.

 

[Galactic explosion]

No.

I would better understand your answer if you didn't reject my explanation of what space is. Using my logic, if we break down spacetime to understand what it is, we must first define it. Let's start with time: Time is the continuous motion or sequence of events (add through space). Space is the area and distance in which objects are allowed to exist.

Obviously we can't separate space from time. But for the purpose of my question, I focused just on space because that is what allows objects to occupy a position in the first place, before they can experience time.

But how can there be space, if there are no objects? Cosmologists also say that spacetime didn't exist before the big bang; that the universe was just very hot and dense, in a single point. That's what my understanding is. If there is just a single dense point, and nothing around it, then there is no space to measure the distance from that point, as you would need a distance and a second object/observer to measure it. Basically, spacetime was birthed out of the big bang, after matter/energy started to expand (space), creating the space needed for things to happen (time).

Again, this is how I understand it. If I'm wrong, then I guess there's no use in further trying to explain this to me, because this just twists my brain and I need a break lol.

 

[Galactic explosion]

What is the difference between space expanding homogenously and isotropically and galaxies moving apart homogenously and isotropically? The weird part is the fact that it is homogenous and isotropic, worrying about calling it space expansion or galactic motion is missing the point.

Hmm, I really like that answer. But on the contrary, scientists are saying that it is indeed the space between the galaxies that's expanding, not the galaxies moving away from each other, as stated in the "balloon & dots" reference. If the latter is the case, I'm still very confused.

 

[PeterDonis]

I would better understand your answer if you didn't reject my explanation of what space is.

I had to because your understanding of what "space" is is wrong. You won't be able to understand my answer until you discard your wrong understanding of what "space" is.

for the purpose of my question, I focused just on space because that is what allows objects to occupy a position in the first place, before they can experience time.

This is part of your wrong understanding that you need to discard. In relativity, space is not logically prior to time. Spacetime is logically prior to both space and time. "Position" is not a primitive concept in relativity. Spacetime is.

how can there be space, if there are no objects?

In relativity, spacetime is a separate entity from the objects that occupy it.

Cosmologists also say that spacetime didn't exist before the big bang;

In certain models of the universe, yes. Not all models of the universe have a "beginning" in this sense. In eternal inflation models, for example, our "big bang" is not the beginning of spacetime; it is only the beginning of the particular region of it that we occupy and can observe.

that the universe was just very hot and dense, in a single point.

If you mean the universe was like this before spacetime and the big bang, no, this is not true of any models, even the ones where spacetime does have a beginning.

If I'm wrong

Yes, you are. See above.

 

[weirdoguy]

But on the contrary, scientists are saying that it is indeed the space between the galaxies that's expanding, not the galaxies moving away from each other

No they are not - not in textbook or peer reviewed papers. Pop-sci woo-hoo doesn't count as a source of knowledge.

 

[Galactic explosion]

This is part of your wrong understanding that you need to discard. In relativity, space is not logically prior to time. Spacetime is logically prior to both space and time. "Position" is not a primitive concept in relativity. Spacetime is.

I understand you can't actually, realistically separate space and time. They are mutual necessities that would break the universe if one of them got destroyed. But in order to have spacetime, you quite literally need to combine space and time, of course. That is what the mathematical model of spacetime is: 3-dimensional space woven into 1 dimensional time. I just focused on one in order to make a point "in theory".. I guess that didn't work.

In relativity, spacetime is a separate entity from the objects that occupy it.

So you're saying spacetime is not intrinsic to objects or things that exist within it? Can spacetime exist regardless of any matter/energy? The reason why this confuses me is because in order to have the "time" part in spacetime, you need to have something that can manifest itself in sequences from the past to the future. Using that logic, the entirety of spacetime collapses and ceases to exist if there are no objects to create the "time" part in spacetime, as of course the sole definition of time is the motion of objects through space, which together should create... spacetime?

If you mean the universe was like this before spacetime and the big bang, no, this is not true of any models, even the ones where spacetime does have a beginning.

Strange. I recall reading somewhere that before the "universe" and "time" and "space" existed, there was just a hot dense singularity that one day "exploded", or in other words "expanded", and THEN created the spacetime and matter/energy we know and love today.

 

[Galactic explosion]

No they are not - not in textbook or peer reviewed papers. Pop-sci woo-hoo doesn't count a source of knowledge.

Ahh.. now I'm starting to see where my confusion is coming from again... That darn pop-sci "education"

 

[PeterDonis]

in order to have spacetime, you quite literally need to combine space and time, of course

No, you have it backwards. You don't get spacetime by combining space and time. You get space and time by splitting up spacetime. Spacetime is the fundamental thing in relativity.

That is what the mathematical model of spacetime is: 3-dimensional space woven into 1 dimensional time.

No, you have it backwards. See above.

Can spacetime exist regardless of any matter/energy?

According to relativity, yes. There are spacetimes that contain no matter or energy anywhere, but the spacetime geometry is still there. In more technical language, there are different, distinct vacuum solutions of the Einstein Field Equation.

in order to have the "time" part in spacetime, you need to have something that can manifest itself in sequences from the past to the future.

You are confusing coordinate time, which is what you get when you make a particular choice of how to split up spacetime into space and time, with proper time, which is the time you and I and everyone else experience as we travel through spacetime along our particular worldlines. Proper time is indeed a sequence of events from past to future, as you say. But proper time is a property of a particular curve in spacetime, not a property of spacetime itself. There is no requirement that spacetime as a whole has to have sequences such as you describe.

the sole definition of time is the motion of objects through space

No, it isn't. I don't know where you are getting all this from, but your understanding of relativity is flawed. I strongly suggest taking some time to work through a relativity textbook. Sean Carroll's lecture notes on GR, which are available for free online, would be a good start.

I recall reading somewhere

Please give a specific reference. And be prepared to be told that (as I strongly suspect) the reference you give is pop science and not reliable.

before the "universe" and "time" and "space" existed, there was just a hot dense singularity that one day "exploded", or in other words "expanded", and THEN created the spacetime and matter/energy we know and love today.

Wherever you read this from, either you were misunderstanding it or it was incorrect.

 

[Orodruin]

Ahh.. now I'm starting to see where my confusion is coming from again... That darn pop-sci "education"

Popular science is not written to educate people. It is written to tell people about a subject in a way that captures their imagination and makes them go ”wow”.

If you want to actually be educated you need a textbook in the subject (and in all the prerequisite subjects).

 

[Chiclayo guy]

Popular science is not written to educate people. It is written to tell people about a subject in a way that captures their imagination and makes them go ”wow”.

Perhaps some but not all. Popular science provides a valuable service to people like me who lack the background in math and physics to even begin to understand the science behind the standard model. Yes, popular science oversimplifies and in doing so contains errors and creates wrong impressions, but it does present broad concepts in understandable language that helps to provide a basic understanding of the origin and workings of the universe. It gets a little tiresome to see popular science constantly degraded on this forum.

 

[weirdoguy]

but it does present broad concepts in understandable language that helps to provide a basic understanding

What's the point in understanding things that are not correct or have little to do with how the universe actually works?

 

[Orodruin]

Perhaps some but not all. Popular science provides a valuable service to people like me who lack the background in math and physics to even begin to understand the science behind the standard model.

Understanding the science is understanding the math. What you are getting from popular science is not understanding, it is a general description of the interpretation of the science. In no way can you base scientific argumentation on what you learn from popular science.

Do not get me wrong, it provides an important part in keeping the general public updated on more or less what is going on. It just is not providing any sort of understanding for the physics.

I do not think there is anyone on this forum degrading popular science. What may be confused with this is pointing out that popular science cannot be used as a starting point for making arguments about physics with the impression that you understand something because somebody painted an ”intuitive” image in their mind based on their consumption of popular science.

Popular science definitely has an important role to play - it just is not providing people with actual understanding.

 

[timmdeeg]

How do we know space is expanding, rather than galaxies just regularly moving apart?

You may have a look at Ned Wright's Tutorial

Are galaxies really moving away from us or is space just expanding?

where he explains:

This depends on how you measure things, or your choice of coordinates. In one view, the spatial positions of galaxies are changing, and this causes the redshift. In another view, the galaxies are at fixed coordinates, but the distance between fixed points increases with time, and this causes the redshift. General relativity explains how to transform from one view to the other, and the observable effects like the redshift are the same in both views. Part 3 of the tutorial shows space-time diagrams for the Universe drawn in both ways.

 

[Galactic explosion]

No, you have it backwards. You don't get spacetime by combining space and time. You get space and time by splitting up spacetime. Spacetime is the fundamental thing in relativity.

If general relativity is about "relativity", why can't you both combine space and time and split spacetime equally? I thought there is no "right way" in GR? The sun orbits the Earth just as the Earth orbits the sun. And you can (theoretically) switch spacetime into timespace when dealing with black holes.

According to relativity, yes. There are spacetimes that contain no matter or energy anywhere, but the spacetime geometry is still there. In more technical language, there are different, distinct vacuum solutions of the Einstein Field Equation.

But that's only theoretical. Wormholes also show up in Einstein's equations, but we don't know if they exist or are possible. I'm here to talk about what we actually know about spacetime. And I never disagreed that spacetime is fundamental. My argument was that spacetime is fundamental ALONG with its other counterparts, like matter and energy; you can't have any of them without each other. Why is it such profanity to say otherwise?

You are confusing coordinate time, which is what you get when you make a particular choice of how to split up spacetime into space and time, with proper time, which is the time you and I and everyone else experience as we travel through spacetime along our particular worldlines. Proper time is indeed a sequence of events from past to future, as you say. But proper time is a property of a particular curve in spacetime, not a property of spacetime itself. There is no requirement that spacetime as a whole has to have sequences such as you describe.

But spacetime does have proper sequences from past to future, as stated by causality. I'm pretty sure causality is the only indisputable physical constant. Yes, everything experiences its own time, but each independent time is fixed in a specific direction that can't be changed.

No, it isn't. I don't know where you are getting all this from, but your understanding of relativity is flawed. I strongly suggest taking some time to work through a relativity textbook. Sean Carroll's lecture notes on GR, which are available for free online, would be a good start.

Really? Time isn't the motion of objects through space? Then what is it?

Please give a specific reference. And be prepared to be told that (as I strongly suspect) the reference you give is pop science and not reliable.

Don't scold me for using Wiki, but this was the quickest reference I could find that supports my claim. The first time I heard this was on a YouTube video, but I don't have time to find that, so Wiki should be a sufficient substitute:

"The initial singularity is a gravitational singularity predicted by general relativity to have existed before the Big Bang and thought to have contained all the mass and space-time of the Universe."

Basically, there was no spacetime outside of the singularity, which supports my claim of spacetime not existing before the big bang.

- https://en.wikipedia.org/wiki/Initial_singularity

 

[Galactic explosion]

You may have a look at Ned Wright's Tutorial

Are galaxies really moving away from us or is space just expanding?

where he explains:

This depends on how you measure things, or your choice of coordinates. In one view, the spatial positions of galaxies are changing, and this causes the redshift. In another view, the galaxies are at fixed coordinates, but the distance between fixed points increases with time, and this causes the redshift. General relativity explains how to transform from one view to the other, and the observable effects like the redshift are the same in both views. Part 3 of the tutorial shows space-time diagrams for the Universe drawn in both ways.

Okay, so basically the answer to my original question on whether the space in between galaxies is expanding or the galaxies are simply moving apart, is relative? I understand dark energy is the precursor to all this, but in the end it makes no difference to what actually is being observed, since it's relative.

 

[PeroK]

Okay, so basically the answer to my original question on whether the space in between galaxies is expanding or the galaxies are simply moving apart, is relative? I understand dark energy is the precursor to all this, but in the end it makes no difference to what actually is being observed, since it's relative.

Spatial expansion is not dependent on dark energy. Dark energy is responsible for the accelerating expansion.

 

[Galactic explosion]

Spatial expansion is not dependent on dark energy. Dark energy is responsible for the accelerating expansion.

Ahh, that's right. Disregard my last sentence then. Whoops!

 

[timmdeeg]

Okay, so basically the answer to my original question on whether the space in between galaxies is expanding or the galaxies are simply moving apart, is relative?

Both notions or views are not physical phenomena which in principle are measurable. How can 'nothing' expand? A rubber sheet yes, but nothing? As Ned Wright says these views are coordinate dependent. Therefore expansion of the universe doesn't mean more than increasing distances between comoving objects.

 

[phinds]

Don't scold me for using Wiki,

There is no "scolding" going on. As you have been told repeatedly in this thread, pop-science just doesn't count as a reference source and much of Wikipedia is pop science. Wikipedia is not taken as a reliable source on this site.

 

[phinds]

With the balloon analogy, it's quite obvious the balloon is inflating because we can actually differentiate the balloon as a separate object from the dots.

I think you have a misunderstanding about what's useful and what's not in the balloon analogy. Try the link in my signature.

 

[PeterDonis]

If general relativity is about "relativity", why can't you both combine space and time and split spacetime equally? I thought there is no "right way" in GR? The sun orbits the Earth just as the Earth orbits the sun.

You are confusing the freedom to choose coordinates in GR with what is logically prior to what. You can choose coordinates in GR any way you like. The coordinates don't even have to split spacetime into "space" and "time" the way standard inertial coordinates on Minkowski spacetime do. None of that changes the fact that spacetime is the invariant thing, the thing that is logically prior. "Space" and "time" (more precisely, "coordinate time"--see further comments below) are emergent from particular choices of coordinates on spacetime; they are not logically prior to spacetime.

You really, really, really need to take the time to learn relativity from a textbook. This thread is going in circles because you keep trying to interpret what we are telling you in terms of your mistaken understanding of how relativity works. That will never get us anywhere.

you can (theoretically) switch spacetime into timespace when dealing with black holes.

No, you can't. You can choose coordinates in the interior of black holes in which the coordinate called $t$ is spacelike and the coordinate called $r$ is timelike, but that in no way means you are "switching spacetime into timespace".

You really, really, really need to take the time to learn relativity from a textbook instead of relying on pop science sources.

that's only theoretical.

No, it isn't. A large portion of the universe is vacuum, and vacuum solutions of the Einstein Field Equations are of great practical usefulness in describing those portions of the universe.

You really, really, really need to take the time to learn relativity from a textbook.

My argument was that spacetime is fundamental ALONG with its other counterparts, like matter and energy

Relativity disagrees with you, since, as I have already pointed out, you can have spacetime without any matter or energy, but you cannot have matter or energy without spacetime; matter and energy in relativity is described by the stress-energy tensor, which has no meaning without spacetime.

You really, really, really need to take the time to learn relativity from a textbook.

spacetime does have proper sequences from past to future, as stated by causality

That's not what causality means. Causality in spacetime is described by the light cone structure, not by "proper sequences from past to future".

You really, really, really need to take the time to learn relativity from a textbook.

Time isn't the motion of objects through space? Then what is it?

Proper time is what I already described, the time experienced by an observer following a particular worldline.

Coordinate time is the timelike coordinate in any coordinate chart that has one (many charts do not).

There are no other concepts of "time" in relativity.

You really, really, really need to take the time to learn relativity from a textbook.

Don't scold me for using Wiki, but this was the quickest reference I could find that supports my claim. The first time I heard this was on a YouTube video, but I don't have time to find that, so Wiki should be a sufficient substitute

No, it isn't, since neither Wikipedia nor YouTube are valid sources. Plus, you didn't even provide a link so I have no way of reading what you quoted in context, so even if I were willing in principle to consider that some Wikipedia article or YouTube video might have reliable information in it, you are not giving me the information I would need to make that judgment in this case.

there was no spacetime outside of the singularity, which supports my claim of spacetime not existing before the big bang

Nobody is disputing that, in spacetimes with an initial singularity, spacetime does not exist before the initial singularity. However, that one correct statement of yours in no way means that everything else you have said is valid.

You really, really, really need to take the time to learn relativity from a textbook.

 

[PeterDonis]

This thread is going nowhere and is now closed.