Hubble's Law Predicts Constant Deceleration?

[patrickn]

Hubble's law can be interpreted to show that our Universe has undergone constant deceleration.
The value of deceleration comes out at a = -kc (where 'k' is Hubble's constant and 'c' is the speed of light.)
Following on from this are some very interesting predictions about the future behaviour of our Universe.
Has anyone got any compelling reasons why this interpretation might be invalid? It seems so obvious to me as an interpretation (as it seems to be the simplest) but I've never seen it proposed so I wonder why it has never been put forward?

 

[MeteorWayne]

Well, Hubble's law is more of an observation, than a law.
The problem is, that during the last few years new data has indicated that it's possible that the expansion of the Universe is speeding up. The place marker called "dark energy" is used to describe that effect.

The evidence for it, while fairly strong, is not outside of the error bars, so we cannot yet be sure that the expansion exists. Is it compelling? No not yet. Is it suggestive? yeah it is. Observations over the next few years will help refine that.

Just cause it's simplest doesn't mean it's right

 

[DrRocket]

Hubble's law can be interpreted to show that our Universe has undergone constant deceleration.
The value of deceleration comes out at a = -kc (where 'k' is Hubble's constant and 'c' is the speed of light.)
Following on from this are some very interesting predictions about the future behaviour of our Universe.
Has anyone got any compelling reasons why this interpretation might be invalid? It seems so obvious to me as an interpretation (as it seems to be the simplest) but I've never seen it proposed so I wonder why it has never been put forward?

Why and how do you interpret Hubble's Law to imply constant deceleration ? What Hubbles'sl Law (a statistical correlatin and not a "law" at all) says is that that red-shift is proportional to distance or equivalently that recession speed is proportional to distance. You would expect such a relatoinship from an explosion and subsequent constant speed of the dispersing particles -- neither acceleration nor deceleration.

In fact there is now data that indicates that the rate of expansion is accelerating, which would imply that the Hubble constant is not constant.


[Hubble's constant is in units of length^2/ time so Kc has units of length^3/time which is not the units that go with acceleration, so your equation simply doesn't hold water.] --- Edit, this is simply wrong. Acceleration is in units of length/(time ^2).

 

[drwayne]

I thought Ho was in units of 1 / time - but it has been a long time since my last astrophysics class,
so I could be, well, duhhhh....

Wayne

 

[DrRocket]

I thought Ho was in units of 1 / time - but it has been a long time since my last astrophysics class,
so I could be, well, duhhhh....

Wayne

No,you are correct. I screwed up. Hubbles constant is 77 (km/s)/Mpc which is Velocity/distance or 1/time.

http://en.wikipedia.org/wiki/Hubble's_law

That also makes Hc have dimensions of distance/(time ^2), which is consistent with acceleration and so my other contention is also incorrect. I blew it twice.

Thanks for pointing out the mistake (s).

But there is still no reason based on Hubble's data to deduce that the acceleration of the universe is slowing, although you would expect that to occur due to gravity. That expectation, which was widely believed up until the late 1990's, also seems to be wrong in light of the more recent data suggesting that the expansion is actually accelerating.

 

[drwayne]

OK, my fading memory is not AS awful as I think it is.

I have seen some "analysis" appear here in the past where it looks like the author has simply looked
at combinations of fundamental constants and their units, and postulated from that physical relationships.

Wayne

p.s. Ironically, another version of hc, where h is Planck's constant, has been a number that has stuck with
me through many decades - i.e. hc = 12.4 x 10^3 eV * Angstroms

 

[DrRocket]

OK, my fading memory is not AS awful as I think it is.

Yours seems to be OK. My problem is that my memoer is not as awful as I think it is either -- probably because my perception of my memory is also wrong.

I have seen some "analysis" appear here in the past where it looks like the author has simply looked
at combinations of fundamental constants and their units, and postulated from that physical relationships.

Yes. That can lead one badly astray. I have seen people think that torque is related to energy just because both can be measured in ft-lbs. There is a critical difference bewteen the cross product and the dot product, and between vectors and scalars.

p.s. Ironically, another version of hc, where h is Planck's constant, has been a number that has stuck with
me through many decades - i.e. hc = 12.4 x 10^3 eV * Angstroms

Where does this come up? It is the denominator in the fine structure constant (throw in a 2 pi) but I don't recognize the significance as a stand-alone number.

 

[drwayne]

Energy - wavelength conversion, i.e.

E = (hc) / lambda

I came along at the right time to where Angstroms were more commonly used. These days,
microns are more common.

Wayne

 

[DrRocket]

Energy - wavelength conversion, i.e.

E = (hc) / lambda

I came along at the right time to where Angstroms were more commonly used. These days,
microns are more common.

Wayne

I think chemists still use angstroms. Most other folks are more rigid in using the usuall metric terminology and 10 is not divisible by 3 (for folks in the peanut gallery who have forgotten an angstrom is 10^(-10) m ). But to me it is all just lambda, units to be determined later by convenience (I don't think I have ever seem the wavelength of light quoted in furlongs, but that might work out if c is given in furlongs per fortnight).

You must have done many such conversions to be able to recall the value for hc with such ease. I have done the calculation, on the few occasions that I needed to, as E=hf and I have to look up the value for h to do it. I tend to not remembe values for any but the most common constants and must look up the others. " hc "is not on my list of common quantities that are committed to memory (apparently neither is the date for my anniversary, and that gets me into a lot of trouble). I can usually recall the value for g, and I am OK with c to one significant figure (in meters or centimeters per second).

Individual numbers just don't fascinate me as they do some others (their opinion on the matter is as valid as mine). I subscribe more to a statement from an old topology teacher -- "If yoiu've seen one integer, you've seen them all". (BTW to a topologist an integer is just a point, and actually, as he and I well know, there are algebraic properties that do depend on the particular integer or class of integers chosen.)

 

[atlantisworp]

What ever it is now, all kind of motions will finally come to an end.

 

[MeteorWayne]

No they won't. Apparently you are unaware of the observations of the last few decades.

 

[DrRocket]

What ever it is now, all kind of motions will finally come to an end.

Apparently you are unaware of:
1) The observations due to Hubble that the universe is expandiing and the apparent lack of sufficient mass to halt the expansion in any reasonable time frame and likely forever (literally forever)
2) The more recent observational data that suggests that the rate of expansion is accelerating rather than decelerating
3) Conservation of energy which when applied to the universe as a whole makes sure that it does not all come to a screeching halt.
4) Conservation of momentum which puts an exclamation point on the previous observation.
5) Newton's first law -- An object in uniform motion will to stay in uniform motion unless affected by an outside force.
6) The fact that photons only have one speed, and it is pretty fast.
7) The exclusion principle that keeps things moving a little even at absolute zero.

 

[MeteorWayne]

Dr Rocket.... How dare you bring well established and documented physics into this discussion

 

[drwayne]

I have had occasion to do a number of band gap - cutoff wavelength type calculations over the
years (working with HgCdTe), but it it mostly a weird number that refuses to leave my brain.

When I was studying for part of my qualifiers - which involved taking the GRE Physics exam,
it was useful to remember a number of constants, and a few stuck for a number of years.
Of course, I got one question right because of my father telling me many times over the
years how far the moon was from the Earth.

Wayne

 

[flyingscot]

The observation that the universe can appear to be accelerating and the observation that the universe can appears to be decelerating can be equally correct if the universe is defined as a sphere whose surface is proportional to the square of its radius. Apply the inverse square law to time.

Acceleration can appear to be occurring because objects in the universe appear to be farther apart while physical laws imply that deceleration must occur. If time follows the same law (inverse square) that effects the dispersal/strength energy radiating from an initial source, then the assumption that acceleration is occurring because objects appear farther apart, over a given period of time, has to be reinterpreted by how time actually behaves.

 

[ramparts]

The observation that the universe can appear to be accelerating and the observation that the universe can appears to be decelerating can be equally correct if the universe is defined as a sphere whose surface is proportional to the square of its radius. Apply the inverse square law to time.

Acceleration can appear to be occurring because objects in the universe appear to be farther apart while physical laws imply that deceleration must occur. If time follows the same law (inverse square) that effects the dispersal/strength energy radiating from an initial source, then the assumption that acceleration is occurring because objects appear farther apart, over a given period of time, has to be reinterpreted by how time actually behaves.

Wow. Um... I'm not sure what you're saying here. By the inverse square law, do you mean Newton's inverse square law for gravity? (There are many inverse square "laws" in physics, so a simple "the" isn't very specific.) You say it's the inverse square law that "effects [sic] the dispersal/strength radiating from an initial source", but none of that actually means anything, so that's not very helpful. And if you do mean the gravitational law, how do we apply that to time? Incidentally, what do you mean by "how time actually behaves"? You seem to be using some very novel understanding of time without telling us what that is.

Also, the universe appears to be accelerating or decelerating based on how the redshift vs. distance data vary from Hubble's "law". Obviously the universe can't appear to be doing both at the same time, so you'll have to explain what you're getting at.

 

[flyingscot]

Assume that the effect of time is inversely proportional to the distance from its assumed beginnings-14 billion years minus 300 thousand years and imagine that its effect diminishes like gravity for this consideration.

If an object appears to be accelerating away from another object, the assumption has been that it is indeed accelerating if the "red shift" is determined. Another explanation might be that time is the variable. If it is that the effect of time diminishes like the effect of gravity, then a "red shift" could be observed without acceleration.

If after driving the old Corvette for an hour at one hundred miles an hour and we discover that a distance of 200 miles has been covered and the speedometer and odometer are in perfect working order, then we would have to look at another variable that has not been considered as variable-time.

Put time on the surface of an expanding sphere.

 

[ramparts]

Assume that the effect of time is inversely proportional to the distance from its assumed beginnings-14 billion years minus 300 thousand years and imagine that its effect diminishes like gravity for this consideration.

The effect of time? What? What is "the effect of time", and how can it be inversely proportional to "the distance from its assumed beginnings" (do you mean that it's inversely proportional to itself, or something else)? How do we quantify "the effect of time" - what are its units?

Put time on the surface of an expanding sphere.

Yeah, these things sound really easy until you have to start doing math and thinking about the meaning of what you're saying, don't they? :lol: Sure, put time on the surface of an expanding sphere, I do it every day!

 

[flyingscot]

I'll just share two of my favorite Einstein quotes:

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction"


"Imagination is more important than knowledge"

There's no need to be anxious about a new idea. There is no need to attack it or ridicule it. Just relax and think about time. This concept is very simple so try to see the idea now and save the analytical traps for later.

 

[SpeedFreek]

If an object appears to be accelerating away from another object, the assumption has been that it is indeed accelerating if the "red shift" is determined. Another explanation might be that time is the variable. If it is that the effect of time diminishes like the effect of gravity, then a "red shift" could be observed without acceleration.

Do you know why we think the expansion of the universe was decelerating to begin with but is now accelerating, rather than still decelerating as we had previously assumed?

 

[MeteorWayne]

The whole premise of this thread is ridiculous....nothing in Hubble's "Law" predicts constant deceleration. It is a measure of the rate of expansion based on the distince from us.

It seems destined for the Unexplained. Too many threads in Physics that ignore physics have been allowed to fester here lately. IMHO, of course. (but my "O" counts )

 

[ramparts]

I'll just share two of my favorite Einstein quotes:

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction"


"Imagination is more important than knowledge"

There's no need to be anxious about a new idea. There is no need to attack it or ridicule it. Just relax and think about time. This concept is very simple so try to see the idea now and save the analytical traps for later.

Einstein also understood the physics of his day really well before he was able to change it. Both of those things he said may be true, but they don't mean that anyone off the street can come up with some amazing new theory if they don't already have a very solid understanding of what we already know.

Put otherwise, "imagination is more important than knowledge" is not equivalent to "screw knowledge, just imagine something and it'll be awesome."

 

[ramparts]

To be more specific to this thread, I should add: you posited this new theory using your own terminology and absolutely no math. In other words, you weren't at all quantitative (which is KEY to physics), and you made no effort to put what you said in the context of what we already know. That's fine, I just expected that when I asked you questions to clarify those points, you would actually answer those questions.

That is nothing like what Einstein did (let me repeat that for emphasis: nothing). Einstein presented his ideas using math, made quantitative testable predictions, and responded to his critics with scientific arguments, not quotes about imagination.

That said, I'm still open to hearing your answers to my questions. If you think I'm ridiculing you, then silence that ridicule with the sheer logic of your argument.