Is the universe truly expanding?

[microtech]

the FLRW metic, the Friedmann equations, and the scale factor. You can look at Einstein's Field Equations

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?

 

[microtech]

What about it?

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?

 

[microtech]

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]

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]

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.

 

[microtech]

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 space — ρ ≈ 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.For more detailed discussion of these distances and misconceptions arising from their misuse read this article:
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)

 

[nitsuj]

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]

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.

 

[tressure]

This is clearly a difficult question to answer, i am starting to think that there is a whole lot happening in space that we don't 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

 

[rootone]

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]

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.

 

[tressure]

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?

 

[guywithdoubts]

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.

 

[tressure]

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

 

[MK5]

i think that that matters come from explosion of stars etc...thats why it get expands

 

[tressure]

you are correct mk5, explosion of starts give birth to asteroids, planets... and we all know that during that explosion gases are released, but eventually form chunks of matter, ultimately asteroids, and planets. but that does not explain the expansion of space, because expansion of space is entirely independent of exploding starts.
73% of everything that's there is space, planets and stars only make up about 3%, logically there is no way 3% can infulence 73%

 

[MK5]

if we imagine there are some layers outside our universe in which some amount of matter can travel from our universe BUT not all matter, as that matter remains in, that's why our universe gets expand

 

[tressure]

MK 5 can you be literal in you explanation, finding it difficult to understand you here!

 

[MK5]

This won't be the reason of universe expanding but if we imagine that that like layers of Earth there are different layers in our universe as the explosions in our universe the matter gets spread through and that matters remains there and gets expanded...".Also universe is getting cooled and amount of matter getting increased as amount of matter getting increased the matter is added into our universe and it gets expanded"

 

[MK5]

as another reason can be is that our universe is little round in shape it is getting flat and that's why we imagine that it its getting expanded

 

[tressure]

oh! thanks... you need to understand that there is no center of the universe, that means there isn't layers of the universe otherwise there would be center layer, which is clearly not the case. second thing is you need to also understand that matter (galaxies) are not getting bigger and are also not moving, only space between the is getting bigger.

 

[berkeman]

Thread is closed for Moderation...

 

[Drakkith]

This thread will remain locked. I remind all members that PF exists not to give personal opinions, but to learn mainstream science.