# Is the universe truly expanding?

Bandersnatch
i need to know how can nothing grow in size
You know how distance can grow? You've got a car, or a plane or whatever moving away from you, and this thing called distance grows. Ask yourself what is distance? Is it a physical thing? Does it matter if there's air or vacuum in-between you and the receding object for the distance to grow?

Space is like distance - a measure of separation between objects - only in three dimensions. Where objects are relative to each other. It can grow or shrink in the same way distance can, but it does not imply in any way that it is a material thing.

(you were faster, nitsuj )

Space is vaccum, does this vaccum expand? Where does it come from? if we measure it to be 15 today, and tomorrow it is 20 where did the extra 5 come from???
Space(-time) is a "thing" ... it has dimensions and (from our measurements of the expansion) a negative pressure. The more space-time you have, the more negative pressure. Hence the 'expansion' of space accelerates. This is what we are measuring.

We don't yet have a good handle on the source of the negative pressure other than to say it is intristic to space time. It is what we call dark energy.

What we 'measure' is that more space-time is being created constantly.

Say the universe was 1 dimensional, with a seemingly endless number of objects each perfectly separated by 1 unit of distance strung along the 'line'.

If each object moves away from the other, without getting closer to any other, does that not imply that the 1 dimensional line is expanding?

If so, it seems intuitive then to consider the 'line' to be made of some substance, a substance that is either uniformly increasing its size, or stretching without noticeable changes in the properties of the 'line', or is this a fundamentally flawed intuition?

Say the universe was 1 dimensional, with a seemingly endless number of objects each perfectly separated by 1 unit of distance strung along the 'line'.

If each object moves away from the other, without getting closer to any other, does that not imply that the 1 dimensional line is expanding?

If so, it seems intuitive then to consider the 'line' to be made of some substance, a substance that is either uniformly increasing its size, or stretching without noticeable changes in the properties of the 'line', or is this a fundamentally flawed intuition?
If you consider the "line" to be space-time itself, you have the idea.

You can easily extend this idea to 2D by considering the surface of a balloon. Blow it up a little, use a marker to create dots on the surface... Blow it up more: you will notice that every dot moves away from every other dot on the surface. There is no 2D center of expansion... More surface is "being created" (actually stretched in this case) everywhere simultaneously.

This is what's happening to space-time. At every point in the universe, more space-time is being 'created'. Only the electromagnetic forces between atoms in your body keep you from being 'stretched' apart. Just as gravity keeps the solar system together (overcoming the expansion) ... as it does for our galaxy, etc. Only on the largest cosmological scales do things 'move apart' like the dots on the surface of the balloon.

at ashuchiha and everyone:
no, that doesnt answer my quest. i need to know how can nothing grow in size, is there any matter that is getting taken out of our universe, i just struggle to making sense that how can nothing grow at an accelerating rate.

imagine vacuum in bottle for instance, for there to be more of it (vacuum) in that bottle, more air has to be sucked out. which is why i suspect that there could be some matter large in size, and somehow is disappearing.
space-time is not "nothing" ... space-time is a stage on which matter, energy and time play out... Why it has negative pressure and 'expands' is still unknown: but we can say we observe that this is a characteristic of space-time.

I think that's about as far as physics will take you.

ARe you saying that this boundry is not fixed?
This is no boundary as such.

It's easy to think that way because in a classical sense your brain is hardwired to think that way, but it's not how it happens. The universe is not expanding into anything and as such there can not be a boundary.

Now, if you want to postulate about multiple universes or M-Theory and branes floating in a bulk, that's one thing and it's also a pretty wild theory at that.

There are arguments that the universe has a bounded size or an infinite size. Infinite is one thing as you can imagine the sum mass and energy of the universe spreading out into that infinite abyss.

If the universe is finite in size, then it is a little harder to wrap one's brain around because you naturally want to think it must be getting bigger into something or worse yet, that if you could theoretically travel at a straight vector you would reach a boundary, but you would not because what is straight will ultimately lead you back to where you started.

Finally, another misnomer is that because the universe is expanding then the space between all mass is also expanding. Not so. Gravity plays a role in keeping things together, so the Milky way is the same size, even though space is expanding and the universe is getting larger, just as the distance between your two ears remains constant.

Intriguing... What new physical force is it that "glues" the galaxies to this "space" thing, so they get "dragged along" as the "space thing" expands? It has to be way stronger than the gravity holding the galaxies in place to begin with... A second question that screams for an answer is: Where is all the energy coming from, that must fill the newly created (expanding) "space thing"? And as this energy (the "vacuum energy," a.k.a. the "zero point field") constitutes "information" (of sorts), it cannot possibly fill the newly created (expanding "space thing") faster that light... Intriguing, indeed.

Drakkith
Staff Emeritus
Intriguing... What new physical force is it that "glues" the galaxies to this "space" thing, so they get "dragged along" as the "space thing" expands? It has to be way stronger than the gravity holding the galaxies in place to begin with...
There is no such force holding things in place. In General Relativity, this expansion is a result of the geometry of the universe, not because something is 'pinning' galaxies to space. This is kind of like how if you have two rockets, one heading one way and the other heading 45 degrees in another direction, the distance between them grows over time.

two rockets, one heading one way and the other heading 45 degrees in another direction, the distance between them grows over time
Of course: the rockets ar moving -- just how is that "rocket analogy" relating to galaxies (with (usually) very small "peculiar motion")? It would also be nice if you (or someone) could point me to the relativity equations that "predict" that "expansion at light speed" is taking place to begin with...

For the distance between two objects (in free space or whereever) to increase, one or both objects must be moving. If an object (like a galaxy) is not moving (zero peculiar motion), "expanding space" would maybe expand some imaginary "space bubble" containing that galaxy, but nothing in SR or GR says anything about matter "hanging on" to space.

And what about the much needed (according to QM) ZPF (vacuum energy)?

Drakkith
Staff Emeritus
Of course: the rockets ar moving -- just how is that "rocket analogy" relating to galaxies (with (usually) very small "peculiar motion")?
It's just an example of geometry at work. Perhaps a poor example.

It would also be nice if you (or someone) could point me to the relativity equations that "predict" that "expansion at light speed" is taking place to begin with...
Sure. I believe what you want is the FLRW metic, the Friedmann equations, and the scale factor. You can look at Einstein's Field Equations too if you'd like.

For the distance between two objects (in free space or whereever) to increase, one or both objects must be moving. If an object (like a galaxy) is not moving (zero peculiar motion), "expanding space" would maybe expand some imaginary "space bubble" containing that galaxy, but nothing in SR or GR says anything about matter "hanging on" to space.
They are moving. Indeed, they are actually accelerating over time. But there is no force being applied to accelerate these objects. By force, I mean a force that an accelerometer could measure. Just like how accelerating in free falling under gravity cannot be measured with an accelerometer, neither can expansion. Both are the result of the geometry of spacetime. When we say that 'space is expanding', the actual mathematical effect is that the scale factor of the Friedmann equations is changing, which then affects the rest of the math accordingly.

And what about the much needed (according to QM) ZPF (vacuum energy)?

Well, if space is expanding, energy must be coming from somewhere, in order to keep a supply of "vacuum energy". The question is, from where is this energy coming? Further, if "space expands faster than light," how is the needed "delay" in providing the ZPF energy dealt with?

Indeed, they are actually accelerating over time. But there is no force being applied to accelerate these objects.
Indeed? What would Newton say about acceleration without force? We know that a = F/m. So, unless some new physics is involved, we need F to act upon m in order to get a. You claim this is not necessary. A rather hefty claim, IMO. Could you elaborate on how this would work?

Drakkith
Staff Emeritus
Thanks, I am somewhat familiar with those. What I'm asking for (because I cannot find it anywhere) is: where do these equations predict lightspeed expansion?
I believe that's all part of the scale factor, but I'm not familiar with the actual math.

Well, if space is expanding, energy must be coming from somewhere, in order to keep a supply of "vacuum energy". The question is, from where is this energy coming? Further, if "space expands faster than light," how is the needed "delay" in providing the ZPF energy dealt with?
It's not a problem if you think of the expansion of space not as a literal expansion of space, but as a description of how objects behave within space. In any case, GR and QED/QFT are not fully compatible with each other, so it's no wonder that their predictions don't agree. (Such as the prediction of a vacuum energy density over 100 orders of magnitude too large)

On top of that, remember that these are still active questions within cosmology, and there may not be a good answer at this time.

Drakkith
Staff Emeritus
Indeed? What would Newton say about acceleration without force? We know that a = F/m. So, unless some new physics is involved, we need F to act upon m in order to get a. You claim this is not necessary. A rather hefty claim, IMO. Could you elaborate on how this would work?
Understanding this requires an understanding of General Relativity and explaining all the concepts of GR is beyond both myself and this thread. I recommend hitting up the Relativity forum for more information on GR.

Cosmological space is mostly a hard vacuum with 10^-6 molecules per cubic centimeter, or, inverting, 10^6 cubic centimeters per molecule.
According to NASA/WMAP (wmap.gsfc.nasa.gov/universe/uni_matter.html), the average density — that is, all known and hypothesizied matter evenly spread (“averaged”) over all known spaceρ ≈ 9.9×10–30 g cm–3 ≈ 5.9 protium atoms/m3 (with only 4.6% of this being “normal baryonic matter” (!), that is ≈ 0.3 baryons/m3), this tells us that the vacuum of “empty space” (as in the actual intergalactic voids) is truly empty.

Of interest is the “fact” that the critical density —said to determine whether the Universe is open, closed, or flat — calculated as ρc = 3H2/8πG (where H is the “Hubble variable” ≈ 70 km∙s–1∙Mpc–1 (arxiv.org/abs/1406.1718) — you are all familiar with π and G?) ≈ 9.2×10–30 g∙cm–3, which makes Ω = ρ/ρc ≈ 1.1 a rather good match (good enough for NASA to declare The Universe is flat (as of January 2013))!

The reason I find this of interest is that the H value is said to determine the Universe “radius” r = c/H ≈ 1.3×1026 m ≈ 13.9 billion lightyears. Which should mean, according to NASA/WMAP, that anyone trying to tell you that “the radius of the observable universe is 46.5 billion lightyears” is full of gas... (Example: Wikipedia’s article (en.wikipedia.org/wiki/Observable_universe), where the “average Universe density” is cited as “9.9×10−30 g/cm3 (equivalent to 6 protons in a cubic meter of space)” — the exact same value reported by NASA/WMAP, in a Universe vastly larger than what H (currently) indicates! Wikipedia’s “new math” at work?

So Tressure's very good question remains: What is this "space" thing that is supposed to be "expanding"?

Bandersnatch
The reason I find this of interest is that the H value is said to determine the Universe “radius” r = c/H ≈ 1.3×1026 m ≈ 13.9 billion lightyears. Which should mean, according to NASA/WMAP, that anyone trying to tell you that “the radius of the observable universe is 46.5 billion lightyears” is full of gas... (Example: Wikipedia’s article (en.wikipedia.org/wiki/Observable_universe), where the “average Universe density” is cited as “9.9×10−30 g/cm3 (equivalent to 6 protons in a cubic meter of space)” — the exact same value reported by NASA/WMAP, in a Universe vastly larger than what H (currently) indicates! Wikipedia’s “new math” at work?
No, microtech, they're not full of gas. You just misunderstood.
The issue here is that when describing the expanding universe there is more than one useful definition of radius of the universe, and these get often confused, sometimes by people who should know better. It's not as simple as 'how far the light can travel in the age of the universe', as both the expansion and its changing rate conspire to throw off our everyday conception of distance.

There are: the Hubble radius, proper distance to the source at the time of emission, proper distance to the source at the time of reception, particle horizon and cosmic event horizon.

Hubble radius ##c/H_0##, equal to about 14 billion light years (using ~70 km/s/Mpc for Hubble constant), is the distance at which the recession exceeds the speed of light (for the specific time). This value being close to the age of the universe times the speed of light is a coincidence - it wasn't always so in the past, and will further diverge in the far future.

Proper distance to the source at the time of emission (let's call it ##D_{then}##) is how far the source was when the now-observed light was emitted in terms of 'proper distance' - i.e., distance you would measure if you could stop the expansion and walk the distance with a measuring stick.
Taking the farthest observable thing in the universe - the CMBR - as an example, its ##D_{then}## was about 42 million light years.

Proper distance at the time of reception, ##D_{now}## is where the object you see is now in terms of proper distance - again, meaning the distance you'd get if you could stop the expansion and measure it disregarding any further expansion.
The ##D_{now}## of the CMBR, or proper radius of the observable universe, is about 46 billion light years. This is the value used for the radius of the universe in the wikipedia article, as it most closely conforms to our everyday idea of how big something is.

Particle horizon is the proper radius of the observable universe at the time of emission of the observed signal. It's not the same as ##D_{then}## due to the changing rate of expansion - what we see now at distance A, was back then at distance B, but it didn't lie within the particle horizon C back then. It wasn't part of the observable universe.
For example, the CMBR is now 46 billion ly away, at the time of emission it was 42 million ly away, while the particle horizon back then was about 1 million ly. That we can see it now, is only due to the fact that in the past the rate of expansion of the universe was decelerating, which allowed objects initially beyond the particle horizon to be eventually observed.

Cosmic event horizon is the largest proper distance that light emitted at a given time can ever bridge. It is now about 16 billion ly and will asymptotically approach about 16.5 billion ly in the future. It is lower than the current radius of the observable universe due to the accelerating expansion.

http://arxiv.org/abs/astro-ph/0310808

This calculator might help you familiarise yourself with the interplay of the aforementioned values, as well as redshift, age and recession velocities:
http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html
(made as an effort of PF member Jorrie with some help from others; there's a related sticky thread in the cosmology section)

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Thanks, I am somewhat familiar with those. What I'm asking for (because I cannot find it anywhere) is: where do these equations predict lightspeed expansion?
It takes vastly long distance to sum to the speed of light + for the expansion of space. This is obvious as locally we don't see space expanding at the speed of light :/

Did you find an equation to determine at what distances the expansion is about the speed of light?

mfb
Mentor
Thanks, I am somewhat familiar with those. What I'm asking for (because I cannot find it anywhere) is: where do these equations predict lightspeed expansion?
What do you mean with "lightspeed expansion"?

Further, if "space expands faster than light," how is the needed "delay" in providing the ZPF energy dealt with?
Distances for objects very far away increase faster than the speed of light. Nothing is moving faster than the speed of light.

There is no need for some energy to appear in any way.
What would Newton say about acceleration without force?
It is not an acceleration in the way Newton studied it, and Newton did not know general relativity.

In post 66, you found two values for the matter density that are in agreement with each other. Where is the point? The density value is independent of the size you consider.

This is clearly a difficult question to answer, i am starting to think that there is a whole lot happening in space that we dont know of.

Is it okay to say:
If space is expanding, then there is no multiverese... only one univ erse exists. BEcause all universes would expand, bond and ultimately be one thing.

To support the idea that there is multiverse, you would have to think that Albert Einstein was right with his cosmological
Constant theory. In that case, our universe would have an edge that separate it from other universes

One thing that is very important to remember is the difference between the observable universe and the whole universe.
We can make convincing arguments about what is observable, but beyond what is observable always will be speculative - even if we do have some neat ideas.

Drakkith
Staff Emeritus
Is it okay to say:
If space is expanding, then there is no multiverese... only one univ erse exists. BEcause all universes would expand, bond and ultimately be one thing.
No, because you're thinking of a multiverse as multiple universes in the same spacetime, such that as they expand they end up colliding with each other. This is not the case.

No, because you're thinking of a multiverse as multiple universes in the same spacetime, such that as they expand they end up colliding with each other. This is not the case.
okay, what is the case then?

okay, what is the case then?
Your example is about regions of space that belong to the same universe. What a "multiverse" is depends on which model you use, but they are all speculation.

let me ask this then, was space there in the beginning of time or was it also born during the big bang?