Must (Ve) expansion of the universe be C?

[bobie]

Is there any experimental, practical reason or aspect of the theory/model that requires that the speed of expansion of the universe be ≥ C?

The Hubble constant, the only experimental datum, comes from a formula V_e/T₀*V_e

that allows any possibility. If we considered V_e = C/2 , R would be 7.2 Gly, would that be a problem, since distances are measured according to a model and are not absolute?

That would make inflation, if it really occurred, more plausible , as an increase of speed over a short period wouldn't change sensibly the size of the universe. That would also allow for an increase in the rate of expansion, now, without violating the founding law of the universe

Thanks for your attention.

 

[mfb]

What do you mean with "speed of expansion"? There is no unique, meaningful way to express the expansion of the universe as a speed (distance per time).

 

[marcus]

Hi Bobie, I'm always interested by your posts because you seem to be intelligent, but very confused about concepts. You ask questions that are intelligent too, but based on fundamental confusion.

Maybe you could kind of "reboot" your set of conceptions and then it would be easier for you to communicate and get into real discussion of stuff in cosmology. I can try to suggest some conceptual steps, hoping that you will understand it as well-intentioned and not pretentious of me or condescending.

1. We don't know the size of the universe, so we can't say how fast it is expanding, in the normal way you can if you know something's size. So the quantity "Ve" does not exist for us. It is not defined. So your whole post here, because it is about something that is meaningless, is itself meaningless.

2. The pattern of distance expansion IS NOT LIKE ORDINARY MOTION because in Hubble law distance expansion NOBODY GETS ANYWHERE. Nobody approaches any goal, the relative positions of everybody stay the same. Just that everybody gets farther apart.
That kind of distance expansion is allowed by 1915 General Relativity. It is not subject to the 1905 Special Relativity speed limit on ordinary motion.

3. When you say "increase in the rate of expansion, now, without violating the founding law of the universe" you probably are thinking that "the founding law of the universe" is the 1905 Special Relativity speed limit on ordinary motion of one object in another object's frame of reference. Basically the idea that nobody can catch up to, and pass, a flash of light, or if you like "nobody can pass a photon".
But that does not say that DISTANCES cannot increase faster than c, or for that matter HUNDREDS of times faster than c. Distances increasing affects everybody including photons. When distances scale up in all directions between all things, it does not cause anybody to catch up and pass a photon.

4. 1905 SR was about static flat non-expanding geometry. It is very useful because the geometry we live in is only very very slighty curved and only very very slightly expanding. So SR is an excellent approximation
But 1915 GR trumps SR. General Rel is about dynamic (ie. changing) geometry. The effects are almost imperceptible but they are real enough and they show up most clearly over extragalactic distances. We have no right to expect that distances will remain the same. Geometry interacts with matter. Geometry has a kind of "momentum". If it gets started expanding it will tend to continue although the rate may gradually change.

5. In cosmology we have a criterion of being AT REST relative to the background of ancient light. there is the so called CMB the cosmic microwave background that dates back to around year 370,000. It is the soup of ancient glow from the ancient hot gas that filled space at that time. If you move relative to the ancient soup of light, ie. relative to Background, you will detect a doppler hotspot ahead of you and a doppler cold spot behind you.

We know the speed and direction that the solar system is moving, relative to Background.

6. Strictly speaking, the Hubble law expansion of distances is not about any old distance. It is about distances between objects which are at rest with respect to Background---between objects "at CMB rest"---between objects for which the CMB is "isotropic" that is the same temperature in all directions. Normally the individual motions of objects are small---galaxies are pretty much at rest,when you look at the large-scale picture. But strictly speaking you'd have to take account of the fact that they have some small individual motions that aren't part of the Hubble law distance expansion pattern. The current percentage rate of distance growth (between stationary objects) is about 1/144 of one percent, per million years. There is a bit of uncertainty about it. Some people say it is around 1/140 of one percent per million years.

these are just basics (and I'm not an expert, some others here will hopefully correct or clarify as needed). But if you get past the basics there are a lot of fascinating things to learn in cosmology! I hope you stick around and get more into the subject!

 

[bobie]

1. We don't know the size of the universe, So the quantity "Ve" does not exist ..
The pattern of distance expansion IS NOT LIKE ORDINARY MOTION...

Hi marcus, it is true that sometime I use imprecise language, I rely on the intelligence of my readers to make little adjustments, but it does not seem to be the case now.

- I just called (for short) V_e what you call recession speed and I specified it means expansion of U. If I made a mistake because V_e implies MOTION, then that's what implies also recession speed and an expanding universe: we are both saying that the distance between us (at the center of the Hubble sphere) and a point/object at distance of a Hubble radius increases by≈C every second

The comoving radius of a Hubble sphere (known as the Hubble radius or the Hubble length) is c/H_0, where c is the speed of light and H_0 is the Hubble constant...
The Hubble length c/H₀ is 14 billion light years in the standard cosmological model, somewhat larger than c times the age of the universe, 13.8 billion years. This is because 1/H₀ gives the age of the universe by a backward extrapolation which assumes that the recession speed of each galaxy has been constant. However, modern observations indicate recession speeds are increasing slightly due to dark energy, so that 1/H₀ is only an approximation to the age of the universe.

You say we don't know the size of the universe,but you know the age of the universe (≈14 Gy)
and that the Hubble radius (U) is ≈C times the age of the universe (14 Gly), well, isn't that the size of the universe? I am using this term as wiki (and everybody) uses that.

- The fact that the size of the Hubble radius is ≈C times the age of U implies that ever since BB the universe has been expanding at ≈C, ergo the average V_e of U is ≈C according to the standard model. Where is the confusion? If the average V_e had been of C/2 the Hubble radius would be ≈7 Gly.
- Moreover, H₀ (the only experimental datum about the present), says that 1cm becomes 1cm+2.2*10^-17cm every second. Why you derive the size of the present universe dividing C by H₀ ? If you consider the present rate equal to the average equal to C/2 (or any other speed) the formula will work: .5 C/H₀*.5 C = T₀.
Why do you choose C and not C/2, C/4, I asked?Thanks for you kind words, btw, a friend can never be read as condescending

 

[Mordred]

Hi marcus, it is true that sometime I use imprecise language, I rely on the intelligence of my readers to make little adjustments, but it does not seem to be the case now.

- I just called (for short) V_e what you call recession speed and I specified it means expansion of U. If I made a mistake because V_e implies MOTION, then that's what implies also recession speed and an expanding universe: we are both saying that the distance between us (at the center of the Hubble sphere) and a point/object at distance of a Hubble radius increases by≈C every second

The comoving radius of a Hubble sphere (known as the Hubble radius or the Hubble length) is c/H_0, where c is the speed of light and H_0 is the Hubble constant...
The Hubble length c/H₀ is 14 billion light years in the standard cosmological model, somewhat larger than c times the age of the universe, 13.8 billion years. This is because 1/H₀ gives the age of the universe by a backward extrapolation which assumes that the recession speed of each galaxy has been constant. However, modern observations indicate recession speeds are increasing slightly due to dark energy, so that 1/H₀ is only an approximation to the age of the universe.

You say we don't know the size of the universe,but you know the age of the universe (≈14 Gy)
and that the Hubble radius (U) is ≈C times the age of the universe (14 Gly), well, isn't that the size of the universe? I am using this term as wiki (and everybody) uses that.

- The fact that the size of the Hubble radius is ≈C times the age of U implies that ever since BB the universe has been expanding at ≈C, ergo the average V_e of U is ≈C according to the standard model. Where is the confusion? If the average V_e had been of C/2 the Hubble radius would be ≈7 Gly.
- Moreover, if that is not enough, H₀ (the only experimental datum about the present), says that 1cm becomes 1cm+2.2*10^-17cm every second. When you derive the size of the present universe dividing C by H₀ you are stating that right now an object on the circumference of the H-sphere, of the universe, is expanding/receding (but not moving) at C.

I was asking if this is a necessity and what happens if we assume an average V_e/recession speed of C/2

Thanks for you kind words, btw, a friend can never be read as condescending

there is a point your missing, The Hubble sphere is less than the size of the observable universe, considerably less. Though the way your post reads that might be wrong in how I am interpreting your post.
The Hubble radius is more accurately the point at which we observe recessive velocities at c, however at the edge of the observable universe, we observe redshift as at roughly 3c.
so in answer to your question

"You say we don't know the size of the universe,but you know the age of the universe (≈14 Gy)
and that the Hubble radius (U) is ≈C times the age of the universe (14 Gly), well, isn't that the size of the universe? I am using this term as wiki (and everybody) uses that."

this is not the size of the Observable universe, Marcus also referred to the problem that we don't know the size of the entire universe, we only know the size of the observable portion(observable universe, often shortened to just universe many articles don't specify so its best to assume they are referring to the observable universe) Which is larger than the Hubble radius.

using the term moving in regards to expansion should be used carefully (though it is accurate in a sense). The distance is increasing as the volume of space expands, but this imparts no inertia

The term movement though implies momentum which is an inertia term. However as Hubble didn't know why galaxies were receding were stuck with the term recessive velocity (even though there is no velocity involved) in a sense its an apparent velocity rather than an actual velocity.

these two articles will provide some direction in the above
http://tangentspace.info/docs/horizon.pdf :Inflation and the Cosmological Horizon by Brian Powell
http://arxiv.org/abs/astro-ph/0310808 :"Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe" Lineweaver and Davies

 

[bobie]

Thanks for your explanation, Mordred. So universe is used both for Hubble sphere and observable universe.
Many think that it is there real size of U, if it is finite ( as you know I assume)
If it were so, then the average V_e is ≈3C.
But then my question remains with a different parameter: since all distances are conventional and model dependent what happens if we set the radius of the U_observable at 3/2C /H₀?

Is it a necessity, bound to some absolute, experimental datum or can be arbitrary?

 

[Mordred]

no your getting this wrong The Hubble radius is the radius at which the recessive velocity becomes greater than c.
it is not referred to as the universe, The observable universe is.
however you need to realize that recessive velocity is a distance dependent relation
Hubble's law states the greater the distance the greater the recessive velocity.

you can't set an average recessive velocity

[tex]v_{recessive}=H_oD[/tex]

 

[Mordred]

R (Gly) is the Hubble radius, D(par) is the observable universe. Z=000.0 bottom column is the universe today
Z=1089 is the universe when t=0, keep in mind these charts are done in terms of proper distance

the both the R(Gly) and D(par) are non linear you cannot use an average value and get a non linear growth rate

[tex]{\scriptsize\begin{array}{|c|c|c|c|c|c|}\hline R_{0} (Gly) & R_{\infty} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14.4&17.3&3400&67.9&0.693&0.307\\ \hline \end{array}}[/tex] [tex]{\scriptsize\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline z&T (Gy)&R (Gly)&D_{par}(Gly) \\ \hline 1089.000&0.0004&0.0006&0.001\\ \hline 767.343&0.0007&0.0011&0.002\\ \hline 540.606&0.0012&0.0019&0.003\\ \hline 380.779&0.0021&0.0033&0.005\\ \hline 268.117&0.0036&0.0057&0.009\\ \hline 188.701&0.0062&0.0097&0.016\\ \hline 132.721&0.0107&0.0165&0.028\\ \hline 93.260&0.0183&0.0280&0.048\\ \hline 65.444&0.0312&0.0475&0.084\\ \hline 45.837&0.0531&0.0805&0.145\\ \hline 32.015&0.0902&0.1363&0.249\\ \hline 22.272&0.1529&0.2307&0.427\\ \hline 15.405&0.2590&0.3901&0.731\\ \hline 10.564&0.4384&0.6592&1.248\\ \hline 7.151&0.7414&1.1130&2.128\\ \hline 4.746&1.2523&1.8740&3.621\\ \hline 3.050&2.1099&3.1334&6.148\\ \hline 1.855&3.5312&5.1412&10.399\\ \hline 1.013&5.8131&8.0532&17.449\\ \hline 0.419&9.2287&11.4928&28.801\\ \hline -0.000&13.7872&14.3999&46.279\\ \hline \end{array}}[/tex]

 

[Matterwave]

As to the points you list, I am just bewildered when I read your point 1, and:

The model most theorists currently use is the so-called Friedmann–Lemaître–Robertson–Walker (FLRW) model. According to cosmologists, on this model the observational data best fit with the conclusion that the shape of the universe is infinite and flat...
78 billion light-years
In 2003, Cornish et al. found this lower bound for the diameter of the whole universe (not just the observable part), if we postulate that the universe is finite in size ,
now , you can't have it both ways : must I conclude you know U is infinite, that you know it is finite (39 or 46? Gly), or that you don't know?

I just want to address this one point. The paper gave a lower limit for the size of the universe. There are no upper limits that we know of. So the Universe might be finite but very large, or it might be infinite. We don't know. If the universe is spatially infinite, we can not ever prove this since there is the existence of a Hubble horizon.

 

[bobie]

limit[/b] for the size of the universe. There are no upper limits that we know of.

Read the passage more carefully, Matterwave: it says it may be bigger, that is D> 78 Gly ( in fact many think it is 92) but finite

 

[bobie]

...Maybe you could kind of "reboot" your set of conceptions and then it would be easier for you to communicate...
1. We don't know the size of the universe,...

Thanks , marcus, for your resumé of cosmological conceptions. I am and was fully aware of those points, I surely must learn communicate. Let's see if I am able to ask some clear questions:
- which is the 'conception'?

The model most theorists currently use is the so-called Friedmann–Lemaître–Robertson–Walker (FLRW) model. According to cosmologists, on this model the observational data best fit with conclusion that the shape of the universe is infinite and flat..

...78 billion light-years :In 2003, Cornish et al. found this lower bound for the diameter of the whole universe (not just the observable part), if we postulate that the universe is finite in size... ,
do you know U is infinite, or not?, if it is finite, which size? Which way should I 'reboot'?
- 'observable' universe (U_o) is a misleading word, as most objects beyond Hubble radius are observed where/as they were in the past, but surely not observable where the stand now, correct?
- is T₀ ≈13.8 Gy generally agreed or not? if we change it to, say, 28.8 isit a problem?
- the fact that objects recessing at V_e=c are at 14.4 Gly, whil T₀ is 13.8 implies that the average expansion of the edge V_e-av from BB up to now is (14.4/13.8 =) 1.044 c, is this correct?
- H₀ is considered the V_e of 1 cm at present time, but, is it possible at all to determine it current value, aren't you determining it sudying distant galaxies? Suppose next minute the expansion stops suddenly (as it began) , when/how could you register that?

The question I asked in the OP is simple :
- if we suppose that V_e-av ,the edge of U has been expanding at ≈.5C, U is finite, its radius would be now(T₀*V_e*3.15=) 21.7 Gly (Hubble radius = 14.4 Gly) and the recession speed of U_o= 1.505 C ( below the threshold of relative motion of 2C), what happens? could we rescale the distances of galaxies without problems?, or what problems do arise?
- is the fact that a finite U has an edge a problem?

I hope you have the kindness and patience to address all points, marcus

 

[Matterwave]

Read the passage more carefully, Matterwave: it says it may bigger, that is D> 78 Gly ( in fact many think it is 92) but finite

How did...how did your original post get under my post?

Anyways, the "finite" part of that passage was a postulate. It's not proven. It is not known currently that the universe is finite.

 

[bobie]

Anyways, the "finite" part of that passage was a postulate. It's not proven. It is not known currently that the universe is finite.

Of course, generally speaking, matterwave, anything in this theory is a postulate, a conjecture: it is infinite, it is finite it is 39, 46, 78, 90..http://en.wikipedia.org/wiki/Observable_universe,

BTW can you address some points so that marcus can solve the more complex ones?
Thanks

I hope someone could restore the post in its proper place.

 

[Matterwave]

matterwave, anything in this theory is a postulate, a conjecture: it is infinite, it is finite it is 39, 46, 78, 90...http://en.wikipedia.org/wiki/Observable_universe,

BTW can you address some points so that marcus can solve the more complex ones?
Thanks

I hope someone could restore the post in its proper place.

The size of the observable universe is not a postulate. It is calculated based on the age of the universe and the theoretical history of the universe's past evolution. However, depending how how the universe expanded in the past, there might be different sizes for the current "observable universe". As we don't have a full and complete history of the universe (although we have some pretty good models) we can't pinpoint perfectly exactly what this size is. However, with better measurements, we should be able to pinpoint this better in the future.

The size of the ACTUAL universe is, by definition, not able to be pinpointed by experiment. This is because obviously if we have an "observable universe" then there is a possibility of an "unobservable universe" which is outside of our Hubble sphere. We can't take measurements of this un-seeable piece of the universe, so we can not say for sure whether it is finite or infinite.

The best we can do is use our current best model of the universe, the FLRW model based on general relativity, and extrapolate its mathematical consequences to the universe at large. The mathematical consequence of this theory is that if the universe is (spatially) FLAT or if the universe's (spatial) curvature is NEGATIVE, then the universe is infinite. If the universe's curvature is POSITIVE, then the universe can be finite. This is a result of the math. If you take the FLRW metric with a positive curvature and integrate over the whole 3-submanifold, you will get a finite answer, whereas if you take the FLRW metric with a flat or negative curvature and integrate over the whole 3-submanifold then the answer diverges.

 

[bobie]

Do you understand they mean this as a postulate or must I rather take it as a basic 'conception' of the theory?

The model most theorists currently use is the so-called Friedmann–Lemaître–Robertson–Walker (FLRW) model. According to cosmologists, on this model the observational data best fit with the conclusion that the shape of the universe is infinite and flat...

do you get the impression they are saying they do not know?

and what about this:

2. expansion IS NOT LIKE ORDINARY MOTION because in Hubble law distance expansion NOBODY GETS ANYWHERE.
3. ..But that does not say that DISTANCES cannot increase faster than c, or for that matter HUNDREDS of times faster than c.

 

[Matterwave]

I'm not sure what you're asking now.

The theory we use to describe the universe is general relativity. The model that we use is the FLRW model. In this model, a flat universe is infinite as I mentioned in my post above, and your quote says. Current observations is consistent with a flat universe. But flatness is basically a null result. You can't exactly rule out that the universe has SOME curvature but just one much larger scales than we can currently measure.

The above paragraph doesn't tell me the size of the OBSERVABLE universe. The size of the OBSERVABLE universe is something we can go out and make concrete measurements of. But the exact size, since it is dependent on the universe's expansion history, is still dependent on the model.

 

[bobie]

I'm not sure what you're asking now.

I am asking if a lay reader, like myself, should take both quotes as basic 'conception' of the theory and conclude they do know that the size of U is infinite and that the second quote is conflicting because if T₀ is finite (even if enormously greater than the current value) the rate of expansion must be infinite, which is impossible .But, are you hinting that it is known that U is infinite?

Anyways, the "finite" part of that passage was a postulate. It's not proven. It is not known currently that the universe is finite.

What are the parts of the theory that indeed are proven?

The size of the OBSERVABLE universe is something we can go out and make concrete measurements of..

it is not exactly like that:

A distance-redshift relationship is model-dependent.
We directly measure redshift and apparent magnitude .., we convert apparent magnitude to actual (aboslute) magnitude. We then find which values of the parameters Friedmann-Lemaitre-Robertson-Walker universe best fit the redshift-magnitude relationship.The values of these parameters give us a particlular model.

You don't go out there, you use your model to decide the values of the distances. If the model is wrong there is no way to know

 

[julcab12]

I am asking if a lay reader, like myself, should take both quotes as basic 'conception' of the theory and conclude they do know that the size of U is infinite and that the second quote is conflicting because if T₀ is finite (even if enormously greater than the current value) the rate of expansion must be infinite, which is impossible .But, are you hinting that it is known that U is infinite?

What are the parts of the theory that indeed are proven?

...Just a laymen's point of view. T₀ is a construction assuming we have a cosmic time/coordinate system that started as 0 (used in slicing spacetime into spacelike sllices). U=Infinite as a postulate is a prediction or limit of the FLRW when approaching time 0 (where density and temperature becomes infinite). We have evidence that our universe is 'statistically' homogeneous and isotropic and not exactly Homogeneous and isotropic due to very very small curve. But since we are dealing with approximation. The model/FLWR best explains this condition. The only natural drawback is when the metric leads to breaking point as it approached 0. It tells us that we need to have a extended mathematical model (QG- String or LQG).

The short answer and key point is that the theory works to a point.

... Finite part of the picture came from the very small curve (blackbody radiation spectrum). It's the same reason why cosmologist took the mental image of spherically symmetric geometry and use triangle to measure it like how we measure or calculate the circumference of Earth.

 

[phinds]

What are the parts of the theory that indeed are proven?

There is no such thing as "proven" in physics. The best you can do it "fits the data better than anything else and SO FAR has seen no counter-examples". Folks 100 years ago were pretty confident that Newton's Law of Gravity had been "proven" but physicists are more cautious these days

 

[Matterwave]

I am asking if a lay reader, like myself, should take both quotes as basic 'conception' of the theory and conclude they do know that the size of U is infinite and that the second quote is conflicting because if T₀ is finite (even if enormously greater than the current value) the rate of expansion must be infinite, which is impossible .But, are you hinting that it is known that U is infinite?

Nobody can "know" that the size of the universe is infinite. At most, we can use our current mathematical model and show that the model produces an infinite universe given the data. That is all we can say about that. That is all that I ever said.

What are the parts of the theory that indeed are proven?

There are no parts of the theory that are proven. There are postulates of the theory, and then there are parts of the theory that are in accord with data (currently this corresponds to the approximately flat, accelerated expanding, universe).

it is not exactly like that:
You don't go out there, you use your model to decide the values of the distances. If the model is wrong there is no way to know

I mentioned that the size of the observable universe is model dependent. But the size of the observable universe is fundamentally different than the size of the unobservable universe, because BY DEFINITION we can observe the observable universe. In other words, we can make measurements to fit our models. We can refine our models, we can use the observable universe to narrow down those models that give good predictions. We cannot use the unobservable universe to do this.

 

[bobie]

Thanks for your ample discussion of point 1.

Waiting for marcus to answer to the last point (the OP), can you discuss this point?

- H₀ is the V_e of 1 cm at present time, but, is it possible at all to determine its current value?, aren't we determining it observing distant galaxies? Suppose next minute the expansion stops suddenly (as it began) , when/how could we register that?

or give me a link where I can learn in detail how Plancks mission o WMAP determine H₀ and T₀:
I am not looking for technical details (wiki has plenty ) but for an explanation of the relation between the 2 values:
- which is determined empirically and how , I suppose it's H₀,
- if H₀ simply 1/ H₀, or it can be determined in other ways
- why 1/H₀ must be exactly the age of the U

Thanks

 

[marcus]

==quote bobie==
...
- H0 is the Ve of 1 cm at present time, but, is it possible at all to determine its current value?,
…
==endquote==

...give me a link where I can learn in detail how Plancks mission o WMAP determine H₀ and T₀:
I am not looking for technical details (wiki has plenty ) but for an explanation of the relation between the 2 values:
- which is determined empirically and how , I suppose it's H₀,
- if H₀ simply 1/ H₀, or it can be determined in other ways
- why 1/H₀ must be exactly the age of the U

Thanks

Hi Bobie, I've been busy lately! This past weekend, our chorus, that I sing in, has given three performances of the long oratorio by Mendelssohn called ELIJAH. Do you know of it? For some reason I assume you are a regular church-goer. If you are, you will quite likely have sung hymns with words set to music by Felix Mendelssohn. You might even be in the choir! I sing bass (either high or low bass depending on what the director needs.)

For the moment I'm at a loss as to how your needs can be met regarding cosmology.

I suspect you need first and foremost to learn to compartmentalize your brain and learn how to think like one of us with part of your mind. You can keep your own private thoughts in another mental "compartment". But whatever your ultimate purposes and interests, you would benefit from assimilating the basic ideas from the point of view of someone who accepts 1915 general relativity as a good approximation.
That is what the current cosmic model is derived from. We use the 1923 Friedman equation which is simplification of the GR equation. (GR is our "Law of Gravity" nowadays, having been repeatedly checked and cross-checked in numerous ways to exquisite degrees of precision.)

In a mathematical science like cosmology there are no final absolute truths. Cosmologists are constantly challenging the 1923 Friedman model and proposing alternatives, and they are constantly challenging the 1915 GR equation that it is derived from. So far these have stood up amazingly well. But I don't know of anyone who thinks they are FINAL. Everybody I know is skeptical in that sense, and expects some proposed alternative to eventually provide a better fit to the data and win out. There are literally MILLIONS of data points that a model must fit, more coming in all the time. Telescopes are automated. So far 1915 GR has stood up, as has its 1923 Friedman simplified version.

There are 3 or 4 basic numbers that need to be plugged in to Friedman equation and adjusted to make the curves fit the data. In that sense many statements (like about age since start of expansion, and about current rate of expansion) are MODEL-DEPENDENT. But keep in mind that these 3 or 4 basic parameters are now rather narrowly constrained. Within a few percent.

You can't make RADICAL changes in either the plug-in parameters or the equation itself without losing a snug fit with the data. So what I would propose is this: whatever your private thoughts may be, you learn the basic language and assumptions of the standard model first, before you try to challenge or find contradictions.

For example suppose your fundamental motivation were to challenge and find contradictions, then by understanding the standard cosmic model you would be able to do so more efficiently! ( Moreover you would be able to better understand current criticism and alternative proposals by professionals---there are professional cosmologists who've devoted their lives to trying to pick holes in the current model and think up alternatives.)

 

[marcus]

One thing to learn is not to assume U has an edge. There is no convincing evidence of an edge. There might be! But without evidence we must remain skeptical and not assume any kind of boundary. (The currently observable region, which is constantly growing as more light comes in from further matter, is of course finite but that is different. The model is of the whole cosmos because so far no indication of boundary.)

There is also no evidence that the U has finite extent. It might be finite and edgeless (analogous to the surface of the earth) or it might be infinite in volume. It is a media lie that expansion started out at a "point". If it is infinite volume now then it was infinite volume at the start of expansion. This cannot be ruled out based on what we know so far, so we have to include it as a possibility.

Go to Lightcone and put this number in for S_upper: 0.92956
You will get that with Lightcone's choice of parameters it will be over a billion years before T₀ = 1/H₀
that coincidence will not happen until year 14.856 billion according to Lightcone.

It is simply not true that T₀ = 1/H₀ at the present day. Lightcone's figure for the present day is year 13.787 billion. So that particular ephemeral coincidence will not occur for over a billion years. And one should not attribute excessive importance to a temporary coincidence. (The line have to cross some time or other and it just happens the current estimate is they'll cross some time a bit over a billion years from now.)

 

[bobie]

Hi Bobie, I've been busy lately!... I sing bass .. low bass ...
For the moment I'm at a loss as to how your needs can be met regarding cosmology...I suspect you need .. to learn... to think like one of us
It might be finite and edgeless (analogous to the surface of the earth) ...

Hi marcus, I am really, sincerely grateful for the time you find for my questions, if you sing low bass you must be quite different from Alfred Newman and more like Pavarotti, quite a great guy !
Please take yor time to reply, just , please, let me know when you've had enough of my whims, don't just drop out.

I hope I'll never think like one of you or luckily like anyone else. I know great scientists refuse to accept GR or other theories. I do not try to pick holes in cosmology: some elementary things puzzle me, and if you answer my question directly I can decide if I was sceptic because I lacked information or rightfully so. I do not hope, or even wish, you to say I was right, after my experience in other threads, but...
Very often physicists are not able to see the wood because of the leaves, so an ignorant schoolboy without microscope-nor-telescope may easily spot it. I pinpoint elementary issues like the fact that if you do not know the size of something, you are never allowed to say it is infinite, if you say something is finite and edgeless, you can refer only to the surface of a sphere, and can never say it is flat. If you forget basic coherence, I scream, if I trespass this boundary you rebuke me.

If you are a great projecting engineer at Ferrari and get in your car from the left, any schoolboy can tell you your car won't go, since you have 2 flat tires on the right side. That does not mean he doesn't appreciate your 'model'.

Now , whenever you have time, please let me know (or tell me where I can learn) a few concrete items not necessarily related to theory:
- if you determine H₀ examining galaxies as they were recessing billions of years ago, even if you do it today, that datum refers to U billions of years ago. Probably that galaxy does not exist and U stopped expanding ever since, I asked, " how/when would know if U stopped expanding next minute"
- how you determine T₀, and why it is 1/H₀, and if it is only determined by H₀
- why is it a problem if U has an edge

This is not, I suppose, picking holes, just curiosity.

 

[Matterwave]

Now , whenever you have time, please let me know (or tell me where I can learn) a few concrete items not necessarily related to theory:
- how do you concretely determine the present value of H₀, examining what, and how/when you would know if U stopped expanding next minute
- how do you determine T₀, and why it is 1/H₀, and if it is only determined by H₀

This is not, I suppose, picking holes, just curiosity.

You determine the present Hubble constant by looking at galaxies moving away from us. You plot their redshift vs the distance to those galaxies, and you will see a roughly linear pattern. The redshift gives you a recessional velocity, and you define H from the equation: ##v=H_0 D##. The Hubble constant is the slope of the best fit line.

When Hubble first discovered this constant, he could only see out to so far and the expansion was always linear. So H was constant as far as Hubble knew. However, the Hubble constant sadly is NOT actually a constant (go figure, what a weird way to name this). This is because if you plotted in your graph out far enough, the data points will no longer be linear. In that case there is no best fit line.

You determine the age of the universe depending on the model of the universe that is used. It is not always 1/H. In fact, the age of the universe is only T=1/H if the universe's expansion was and has always been constant. This is not the case. So the actual age of the universe depends on the model that best fits the expansion history of the universe. It so happens...by coincidence, that the actual age of the universe, using our best current model, IS actually quite close to 1/H. But this is, as far as I know, simply a coincidence of the current best fit model.

 

[mfb]

Very often physicists are not able to see the wood because of the leaves, so an ignorant schoolboy without microscope-nor-telescope may easily spot it.

This just does not happen. Advances in physical theories are made by professionals - so far, no one could show me even a single exception.

I know great scientists refuse to accept GR or other theories.

And still they accept that it is an extremely successful theory and an extremely good approximation. And they know GR very well. You can't compare this to your situation, where it is clear that you do not understand the theory you want to be sceptical about.See Matterwave's post for an answer how to measure the speed of expansion now and in the past.

 

[bobie]

You can't compare this to your situation, where it is clear that you do not understand the theory you want to be sceptical about.
.

You must have missed the Ferrari-bit in my post, mfb.
Can anyone in the world affirm that a body can be at the same time in 2 different places? (your alibi wouldn't save your life)
Can anyone affirm that any thing can exist and have no edge?
Does one need to be a scientist or know GR or cosmology to state that anyone who affirms that is wrong?
I am sceptical only about these simple issues, principles, not about the theory. When did I say that?

You determine the present Hubble constant by looking at galaxies moving away from us...

That is what I thought, matterwave, mfb, but,
- if you observe galaxies (maybe now exctinct like UDFJ) as the were receding billions of years ago how can you think it is the rate of expansion at present? If my thought is not clear: suppose an ambulance is speeding away from you, switches on the siren for one second, then stops. The redshift will reach you after t seconds. If, when you register the redshift, deduce ' the car is receding now', you are wrong: itwas receding t seconds ago
- if you observe objects on a smaller scale, you can never know the nature of the redshift.
so there ought be other concrete data to establish if U is expanding at all, now. What are they?

 

[bobie]

1)You determine the age of the universe depending on the model of the universe that is used. It is not always 1/H.
2) In fact, the age of the universe is only T=1/H if the universe's expansion was and has always been constant.

Can you expand on that, matterwave?
1) What do you mean by depending on the model? which models? how does it vary with models?
H₀ tells us that 1cm now is 1+2.2*10^-17cm next second, right?
In what way this expansion is related to the age of U , in any model?
2) Are you sure of that? uniform expansion k would affect the size of U that would be R= T*k, how can it affect the age?
The surface of a balloon is expanding at the rate 1cm+1/10mm/s, if you repeat observation you may establish if and how much/often the rate changes, you may conclude that a point at distance 10m is recessing at 1m/s,now,
- for how long has it been expanding?
can you tell me how do you answer that question?

 

[Mordred]

Hubble's law is only constant in space, it is not constant in time. As others have pointed out it is not truly a constant that always remains the same value.

Also Hubble's law is not the only reasoning that we know space is expanding.

thermodynamics also tells us that the volume of space has increased. You can tell that the volume of the universe has grown from the Ideal gas laws. An increase in volume in a thermodynamic system will lower the temperature, pressure and energy-density of that system.

So for example we measure the CMB, at the time of the CMB temperatures was 3000 K, today temperatures is 2.7 k roughly. Please note I just showed an expanding volume without referring to Hubble's law. If you perform the math involved in thermodynamics you can easily show how the temperature history can show an expanding volume of the universe

 

[mfb]

You must have missed the Ferrari-bit in my post, mfb.

To keep the analogy: physicists don't look at a car from just one side. They completely disassemble it, inspect every part with microscopes, x-rays, ultrasound, electrical tests, ... with multiple independent teams for every component. Then they assemble it in steps and perform the same tests again for every subsystem and then the final car. There is no way a flat tire would stay unnoted.

Can anyone in the world affirm that a body can be at the same time in 2 different places?

Depends on your favorite interpretation of quantum mechanics, but in general yes. How is that related to the thread?

Can anyone affirm that any thing can exist and have no edge?

Yes. The surface of Earth has been given as an example. Also note that the universe is not a thing in the usual meaning of that work.

- if you observe galaxies (maybe now exctinct like UDFJ) as the were receding billions of years ago how can you think it is the rate of expansion at present?

No one thinks that. At least no scientist. What gave you that impression? You always observe the accumulated expansion between emission of the light and now. This is always taken into account (together with tons of more technical details).

- if you observe objects on a smaller scale, you can never know the nature of the redshift.
so there ought be other concrete data to establish if U is expanding at all, now. What are they?

You can see the random local motion at galaxies nearby, but it becomes insignificant when you observe more distant objects with larger redshifts.

 

[bobie]

No one thinks that. At least no scientist. What gave you that impression? You always observe the accumulated expansion between emission of the light and now..with tons of more technical details

Thanks for your replies, mfb, I will not discuss the other points as they are not relevant.
The bolded passage is the key issue: does it mean the expansion of space? that explanation would be circular

A distance-redshift relationship is model-dependent.We directly measure redshift and apparent magnitude ... we convert apparent magnitude to actual (aboslute) magnitude. We then find which values of the parameters Friedmann-Lemaitre-Robertson-Walker universe best fit the redshift-magnitude relationship.The values of these parameters give us a particlular model. Once we have the model, we can calculate a distance-redshift relationship.

If mentor George is right please follow my interpretation, and correct my mistakes:
- you detect a redshift in a galaxy (z=11.9 in UDFJ) because the wavelength/ H-line is 12.9 times greater than the usual 21cm., and other lines confirm such a ratio,
- then you feed this datum into an 'arbitrary' model of your liking or creation and decide that (if UDFJ still exists) it is at a now-distance k of 32.644 Gly from here, the light was emitted 0.3719 Gy after BB, and then-distance was 2.531 Gly, just because it must be k/12.9. You interpret z as the factor of expansion of space.
If this is the actual procedure, then it is a circular argument : the conjecture becomes a proof of itself.
If UDFJ ceased to exist and U stopped suddenly to expand after one year, your conjecture would be exactly the same because your model would not detect it.
If I do not know the model well enough, which is probable , please explain how you would it detect such a change. The only concrete datum in your possession is (at least) 14 billion years old, and cannot tell you anything now or 1 billion years ago. A 1-Gy-old galaxy can tell you something about that period if the redshift is reliable, what can give you information about now?

 

[Matterwave]

Thanks for your replies, mfb, I will not discuss the other points as they are not relevant.
The bolded passage is the key issue: does it mean the expansion of space? that explanation would be circular


If mentor George is right please follow my interpretation, and correct my mistakes:
- you detect a redshift in a galaxy (z=11.9 in UDFJ) because the wavelength/ H-line is 12.9 times greater than the usual 21cm., and other lines confirm such a ratio,
- then you feed this datum into an 'arbitrary' model of your liking or creation and decide that (if UDFJ still exists) it is at a now-distance k of 32.644 Gly from here, the light was emitted 0.3719 Gy after BB, and then-distance was 2.531 Gly, just because it must be k/12.9. You interpret z as the factor of expansion of space.
If this is the actual procedure, then it is a circular argument : the conjecture becomes a proof of itself.
If UDFJ ceased to exist and U stopped suddenly to expand after one year, your conjecture would be exactly the same because your model would not detect it.
If I do not know the model well enough, which is probable , please explain how you would it detect such a change. The only concrete datum in your possession is (at least) 14 billion years old, and cannot tell you anything now or 1 billion years ago. A 1-Gy-old galaxy can tell you something about that period if the redshift is reliable, what can give you information about now?

IF the only way to tell cosmological distances were via redshifts, then indeed the arguments would be circular. Thankfully, there are other ways to tell distances. The standard candles that Hubble used were the Cepheid variable stars. These stars are very bright, and their luminosity varies in time in a predictable way (we know the relationship by looking at Cepheid stars in our own galaxy). Hubble could then take the luminosity and calculate a distance to this star. This is how Hubble first figured out his law. As it turns out, Hubble got his own constant wrong because he only knew about one type of Cepheid variable stars when in fact there are two kinds. But after adjusting for this, we now know a better estimate of Hubble's constant.

Cepheid variable stars are very bright, but they are not bright enough to allow us to see to the far reaches of the universe. To calculate farther distances, we need something brighter. And that something is provided by type Ia supernova. Because of the special mechanism by which type Ia supernovae occur (they are the result of an accreting white dwarf going over the Chandrashekar limit), they all have very similar luminosities. Furthermore, there is an empirical correlation between their luminosities and the width of their light curves (how long they are bright for). So, to see farther out into the galaxy we use type Ia supernovae (which are very very bright! Roughly 5 billion times brighter than our Sun).

 

[bobie]

IF the only way to tell cosmological distances were via redshifts, then indeed the arguments would be circular. Thankfully, there are other ways to tell distances. .

The point is not the current distance. It is circular even if you might confirm (indirectly) the absolute distance.
The key point is how you establish if and at what rate U is expanding now, how you interpret z, and how you derive the age of U.
Then, lastly, the OP : is the hypothesis that the recession speed is and was C/2 compatible with the standard model?

 

[Matterwave]

The point is not the current distance.
The key point is how you establish if and at what rate U is expanding now, how you interpret z, and how you derive the age of U.
Then, lastly, the OP : is the hypothesis that the recession speed is and was C/2 compatible with the standard model?

So your problem is that you think perhaps the redshifts we will see in the future might be different than the redshifts that we see today because somehow the far away galaxies have stopped in their expansion between the time that the light left them and the time that the light reached us now?

My argument for this point might not very well satisfy you. We are only given the data we have today (and some small amount of data from our History, which is non-existent compared to cosmological time scales). We use our current data to come up with the most coherent, and simple, model that explains the current data. We don't have access to data in the future, so we can't make models based on data we don't have. If, in the future, we saw that all our previous redshift values started to decrease, we might have to modify our model. Until then, our model fits our best data we have today.

There is no way to rule out experimentally that the far galaxies have suddenly stopped expanding (say, yesterday they stopped expanding all of a sudden) and we just haven't received the light yet. Our cosmological models rule out such a situation, but they are models which are not set in stone. We only get information as it comes to us. But until we get such information, we have no idea what such information might be, so why should we try to change our current best fit model?

Such a hypothesis would seem very contrived indeed.

 

[bobie]

So your problem is that you think perhaps the redshifts we will see in the future might be different ...? ...There is no way to rule out experimentally that the far galaxies have suddenly stopped expanding (say, yesterday they stopped expanding all of a sudden) ... Our cosmological models rule out such a situation,...We only get information as it comes to us. But until we get such information, we have no idea what such information might be,

I am glat you are acknowledging the information we receive is millions or billions of years old.

No, that is not my problem.
I want only to understand some obscure (for me) points of the model.
I ask simple questions in my threads and I get answers like " you are criticizing the theory""you should study more" " you do/can understand the theory" etc.
The question I asked, over and over, is so simple:

" Must the average recession speed be ≥C or is the hypothesis = C/2 compatible with the model?, if not, what are the problems?"This simple question may have only a few replies:

- I, personally, am not able to answer,
- I do not know, as nobody in the world knows,
- yes it is possible,
...a) but it contrasts with this evidence... that tells us it is ...3C or 67 C or...,
...b) but we chose another parameter because...
- no it is not possible, because the following problems make it not viable:...
- no, but I can't tell you why

Can you pick your option?
Can you describe how you derive the age of U from experimental data?,

Thanks for your efforts.