# Expansion of space vs stuff just moving away

#### imsmooth

NOTE: I am not a cosmologist, so if any of my statements are not correct please tell me.

When we observe distance galaxies we can measure how fast they move away using the red-shifting of their light. So how do we know space itself is expanding vs the galaxies are just moving away relative to ours. That is, if the galaxies are cars on a road, how do we know the cars are driving away from each other vs the road is just getting bigger between them?

#### bapowell

We cannot. Both interpretations are equally valid.

#### imsmooth

If that is the case, why are all the recent stories about inflation talk about space expanding and not about objects moving away at near-light speed?

#### phinds

Gold Member
If that is the case, why are all the recent stories about inflation talk about space expanding and not about objects moving away at near-light speed?
Because space IS expanding. Things are not traveling at > c in our reference frame, they are receding from us at > c. I'll leave it to bpowell to discuss his response.

Things at the edge of our observable universe, for example, are receding at about 3c. They most certainly are not actually moving at 3c from any frame of reference. Nothing moves at speeds >c but recession velocities have no limit and break no speed limits.

Let's assume an object X at the edge of our observable universe and an object Y that is half way between X and us and posit that Y is co-moving with us.

To us X is receding at 3c and Y is stationary. According to Y, we are stationary and X is receding at considerably less than 3c because recession is distance dependent. If X were MOVING at 3c relative to us, it would also be moving at 3c according to Y. An object that was comoving with us but positioned out next to X would see X standing still.

I am glossing over very slight proper motions of objects relative to us because the magnitudes are insignificant compared to the recession velocity.

Google "metric expansion" for more discussion.

#### Drakkith

##### Armchair Scientist
Staff Emeritus
2018 Award
Actually, I think the galaxies really are receding from us at a velocity of 3c as viewed from our own frame of reference here on Earth. Also, galaxy Y is co-moving with us, which would require that galaxy X also be moving at 3c relative to it.

#### imsmooth

I appreciate the replies on how fast the edge of space is receeding, but this does not answer the question: how do we know space itself is expanding and the objects are not just moving away really fast? Can the red-shift show a >3c expansion? Can we measure a greater-than-light recession, and therefore conclude space is expanding?

#### phinds

Gold Member
Actually, I think the galaxies really are receding from us at a velocity of 3c as viewed from our own frame of reference here on Earth.
Yes, that's what I said. They just aren't MOVING relative to us (except for a small proper motion)

Also, galaxy Y is co-moving with us, which would require that galaxy X also be moving at 3c relative to it.
No, that can't be right. If Y were NOT commoving with us, but receding, it would be receding at well under 3c AND it would see X receding at well under 3c (and us receding at the same speed in the opposite direction). X is NOT "moving" relative to us it is RECEDING relative to us and it will recede at a different rate for Y because Y is closer to it.

#### phinds

Gold Member
I appreciate the replies on how fast the edge of space is receeding, but this does not answer the question: how do we know space itself is expanding and the objects are not just moving away really fast? Can the red-shift show a >3c expansion? Can we measure a greater-than-light recession, and therefore conclude space is expanding?
All galaxies outside the local cluster are moving away from us. AND they are red shifted. AND the farther away they are the more redshirted they are. Only expansion can explain all that. This is not the only reason. Did you google "metric expansion" ?

#### Mordred

NOTE: I am not a cosmologist, so if any of my statements are not correct please tell me.

When we observe distance galaxies we can measure how fast they move away using the red-shifting of their light. So how do we know space itself is expanding vs the galaxies are just moving away relative to ours?
There is one vital detail being overlooked. The above statement implies a preferred reference frame. All non gravitationally bound move away from each other equally. In simpler words take any number of coordinates. 3 or more, regardless of which coordinate you choose as the reference frame. You will measure the same rate of expansion to any other coordinate. However as Phinds pointed out the further away a coordinate is from the referencd frame you will measure a greater redshift.

Unfortunately I am typing from my phone so I cannot post a specific post. However this related thread has an article "Redshift and Expansion" that will provide further details on the distance dependant recessive velocity Phinds mentioned with his 3c example.

Another key detail is that when one describes an object as moving. You are stating that the object has inertia. In expansion the objects themselves have no inertia. The space between them is simply increasing.

Edit there is one vital detail in regards to the chosen reference frame I forgot to add. The chosen reference frame must be at rest. A relativistic reference frame would not measure the universe as being homogeneous and isotropic. As his own movement would be a preffered loacation and direction

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#### marcus

Gold Member
Dearly Missed
...
When we observe distance galaxies we can measure how fast they move away using the red-shifting of their light. So how do we know space itself is expanding vs the galaxies are just moving away relative to ours. That is, if the galaxies are cars on a road, how do we know the cars are driving away from each other vs the road is just getting bigger between them?
When we observe a redshift, does that tell us how fast the source is moving away NOW? Or does it tell us how fast it WAS moving away back when it emitted the light?
(According to standard model of cosmos, the redshift does not correspond in a straightforward doppler way to either of those two speeds.)

I think the choice of interpretation is partly a matter of making sense of the whole picture. For example we do not only observe galaxies, we observe the Ancient Light ("cosmic microwave background"). The physical explanation of that light is that it has to have been emitted when glowing hot gas filling space had cooled to around 3000 kelvin, because any hotter and ionization of the gas would dazzle and scatter the light. That 3000 kelvin is like a "threshold of transparency" when the fog clears.

We know what mix of wavelengths gas emits at 3000 kelvin. So we measure the wavelengths NOW and we see that the CMB ancient light has been redshifted by a factor of 1000 (more exactly estimated 1090, but roughly 1000). Wavelengths are that much longer.

The temperature and density of the CMB light NOW squares with the model that distances have increased 1000-fold and volumes increased 10003-fold, and temperature decreased 1000 fold, since the moment the hot gas filling space cleared (so light could pass freely).

The simplest way to model the CMB is with an expanding distances model. What could be moving away, and when? to produce just that redshift of 1000? It's hard to think of an imaginary set-up that would reproduce what we observe with MOTION. Expanding distance works better (plus it is a prediction of GR, the version called Friedman equation that cosmologists use, and GR has been tested a lot.)

Individually, just looking at galaxies (not the whole overall picture), you can go thru some mathematical gymnastics (there was a famous paper by Bunn and Hogg that did this) and explain the redshift as a series of a large humber of intervening doppler shifts. But it's simpler to just equate it to the factor by which the distance has grown while the light was in transit.

It's not mathematically WRONG to look at redshift as the cumulative effect of hundreds of intervening little doppler shifts. It is just clumsy and inconvenient. And then you still have the CMB to explain.

So maybe it just comes down to convenience---astronomers wanting the simplest best-fit model.

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#### Drakkith

##### Armchair Scientist
Staff Emeritus
2018 Award
I appreciate the replies on how fast the edge of space is receeding, but this does not answer the question: how do we know space itself is expanding and the objects are not just moving away really fast? Can the red-shift show a >3c expansion? Can we measure a greater-than-light recession, and therefore conclude space is expanding?
There is no single, easy answer. The current model of the universe is known as the ΛCDM (Lambda CDM) Big Bang model. In this model, the universe is described as expanding from a once very dense, very hot state to the cooler, less dense state that it is in now.

Now, it's very important to realize that scientific models are not built upon only one or two pieces of evidence. Such is the case for the ΛCDM Big Bang model. Each piece of evidence, when viewed alone, could have a great many explanations. Only by looking and trying to explain ALL the evidence (or at least as much as possible) can we build the predictive models that science strives for.

For example, a measurement of redshift, by itself, does not let us know whether or not galaxies are moving "through" space or whether space itself is expanding. Instead it is a combination of many different pieces of evidence combined to form a model that predicts and explains what we can and should see to a very, very high degree of accuracy.

Per Ned Wright's Cosmology FAQ: http://www.astro.ucla.edu/~wright/cosmology_faq.html#BBevidence

The evidence for the Big Bang comes from many pieces of observational data that are consistent with the Big Bang. None of these prove the Big Bang, since scientific theories are not proven. Many of these facts are consistent with the Big Bang and some other cosmological models, but taken together these observations show that the Big Bang is the best current model for the Universe. These observations include:

The darkness of the night sky - Olbers' paradox.
The Hubble Law - the linear distance vs redshift law. The data are now very good.
Homogeneity - fair data showing that our location in the Universe is not special.
Isotropy - very strong data showing that the sky looks the same in all directions to 1 part in 100,000.
Time dilation in supernova light curves.

The observations listed above are consistent with the Big Bang or with the Steady State model, but many observations support the Big Bang over the Steady State:

Radio source and quasar counts vs. flux. These show that the Universe has evolved.
Existence of the blackbody CMB. This shows that the Universe has evolved from a dense, isothermal state.
Variation of TCMB with redshift. This is a direct observation of the evolution of the Universe.
Deuterium, 3He, 4He, and 7Li abundances. These light isotopes are all well fit by predicted reactions occurring in the First Three Minutes.
Finally, the angular power spectrum of the CMB anisotropy that does exist at the several parts per million level is consistent with a dark matter dominated Big Bang model that went through the inflationary scenario.
To add to this, I would say that we also have a theory of gravity, General Relativity, which is perfectly able to have an expanding universe in which space is expanding instead of galaxies moving through space. GR is a theory of geometry, and a basic explanation of how this is possible is that on the largest scales of the universe, geometry itself is "dynamic", meaning that it can change. This changing geometry is a very "intuitive" way of explaining expansion, meaning that expanding space falls very naturally right out of GR.

Now, I know this isn't quite a direct answer to your question, but the fact is that the issue is, as far as I know, a little too complicated to explain with just a few pieces of evidence and ignoring the current model as a whole.

Yes, that's what I said. They just aren't MOVING relative to us (except for a small proper motion)
Okay, if you use "recede" and "moving" as two different words, then sure.

No, that can't be right. If Y were NOT commoving with us, but receding, it would be receding at well under 3c AND it would see X receding at well under 3c (and us receding at the same speed in the opposite direction). X is NOT "moving" relative to us it is RECEDING relative to us and it will recede at a different rate for Y because Y is closer to it.
That isn't possible. If Y is co-moving with us then X MUST be receding at 3c relative to both of us. I think the problem here is that it isn't possible for Y to be co-moving with us because of the expansion of space.

#### Mordred

Roflmao... You have no idea how much of a headache it was to include Bunn and Hoggs work into the Redshift and expansion article. Thankfully PAllen's assistance and extreme patience stepped me through it lol.

#### bahamagreen

Another key detail is that when one describes an object as moving. You are stating that the object has inertia. In expansion the objects themselves have no inertia. The space between them is simply increasing.
Could this be clarified?

What is the relationship between the object and the expanding space that compels the object to move with or "stick to" the expanding space? How is inertia of objects no longer independent, but now dependent on the space metric?

You are suggesting that objects subject to expansion offer no inertial resistance to the expansion, which is also a net acceleration, and a local accelerometer would show no indication... very similar to free fall in a gravitational field...?

#### phinds

Gold Member
That isn't possible. If Y is co-moving with us then X MUST be receding at 3c relative to both of us. I think the problem here is that it isn't possible for Y to be co-moving with us because of the expansion of space.
So you think that recession velocity is independent of distance? That is just flat wrong.

As a thought experiment, there is nothing wrong with having Y commoving with us and since recession velocity IS dependent on distance, X's recession velocity from Y will not be the same as X's recession velocity from us.

#### Mordred

Could this be clarified?

What is the relationship between the object and the expanding space that compels the object to move with or "stick to" the expanding space? How is inertia of objects no longer independent, but now dependent on the space metric?

You are suggesting that objects subject to expansion offer no inertial resistance to the expansion, which is also a net acceleration, and a local accelerometer would show no indication... very similar to free fall in a gravitational field...?
Objects at rest require a force to act upon them to get them moving and a force to act upon it to stop that motion. The three laws of inertia apply on this case. Now consider the geometry involved in expansion metrics. All objects are increasing in distance from each other equally. (not gravitationally bound)
This alone tells cannot be explained as the result of a kinetic explosion, you would have a centralized source. It also cannot be explained as a higher pressure/temperature flowing to a lower pressure/temp as this too would give a preferred location.

In order for expansion to be explained by inertia would require some very unusual mechanism to be both homogeneous and isotropic. After all what force could act upon those objects that would fit a homogeneous and isotropic expansion? I certainly can't think of any.

In expansion however the geometric distance between objects is simply put increasing. This increase in volume is both homogeneous and isotrophic. Think of it simply as an increase in geometric volume.

google metric expansion for more detail

#### bahamagreen

The problem is applying the laws to pairs of objects... individual objects may appear to be each locally inertially at rest, but the distances between pairs are not simply increasing; the distances are accelerating. The mutual acceleration of distance between massive objects typically implies force.

Acceleration is absolute, yet with expansion, two objects whose separation distance is accelerating are inertially at rest. What is the mechanism for that?

I'm not seeking to explain expansion by inertia; I'm trying to see how expansion overcomes the inertia of objects. Saying that the objects don't move but rather recede is at the heart of it - that is saying objects resist changes in movement because of their inertia, but objects don't resist recession, which is an acceleration of distance.

How do two objects accelerating their distance apart distinguish movement from recession, and so offer inertial resistance to the former but not the later?

#### Drakkith

##### Armchair Scientist
Staff Emeritus
2018 Award
So you think that recession velocity is independent of distance? That is just flat wrong.
No, that's not what I'm saying.

As a thought experiment, there is nothing wrong with having Y commoving with us and since recession velocity IS dependent on distance, X's recession velocity from Y will not be the same as X's recession velocity from us.
I don't think its possible for a galaxy to be co-moving with us when it's at a distance where the recession velocity due to the expansion of space is greater than c. That would seem to imply that the galaxy would need to be moving "through" space at a speed greater than c in order to stay co-moving with us.

#### russ_watters

Mentor
I appreciate the replies on how fast the edge of space is receeding, but this does not answer the question: how do we know space itself is expanding and the objects are not just moving away really fast? Can the red-shift show a >3c expansion? Can we measure a greater-than-light recession, and therefore conclude space is expanding?

#### Cosmobrain

There are three types of redshift:

Doppler redshift: This is caused by an object moving away from us. It's maximum value is 1.4. For a greater redshift, the object would have to be traveling faster than light or...
Cosmological redshift: The redshift can be caused by the expansion of space itself that stretches the electromagnetic wave. That's how Hubble can measure redshifts higher than 8
Gravitational redshift: This is caused by an object that stretches the wave due to its gravitational pull. This is only significant when the body is very dense, like a neutron star. This doesn't contribute much to the measured redshift of the galaxies

correct me if I'm wrong

#### Drakkith

##### Armchair Scientist
Staff Emeritus
2018 Award

There are three types of redshift:

Doppler redshift: This is caused by an object moving away from us. It's maximum value is 1.4. For a greater redshift, the object would have to be traveling faster than light or...
Doppler shift resulting from relative velocity is indistinguishable from other types of redshift and can have any value. Using the relativistic formula instead of the classical one gives you the correct values.

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