Why is the expanding velocity increasing?

[xersan]

Why is the expanding velocity increasing?

 

The astronomers say that "The expanding velocity of universe is increasing because of dark matter or energy".

 

But how do they determine this increasing? How do they know the value of velocity at past?

[sanctus]

Short answer: the further away you look the more you look into the past so you can know velocities in the past. Also, what you miss in the quoted sentence is the expanding velocity is increasing with increasing distance (...). This is Hubble's law.

 

Dark matter/energy, Cosmological constant, negtive pressure are things used (trying) to explain it...

[Tormod]

Are you quoting from somewhere or is it a typo?

 

There is no "expanding velocity of the universe" (the universe on the whole does not appear to have a motion in any direction that we can know of).

 

I assume you mean the increasing cosmic expansion?

 

The long of it:

Hubble's diagram and cosmic expansion ? PNAS

 

The short of it:

Astronomers measure the redshift of distant objects. This redshift is apparent in the spectrum of the light we see. A shift in the light spectrum towards the red means that the lightwave has been stretched, which in turn implies that the space through which the light has traveled has expanded.

[xersan]

The blue line appears Hubble's rule in figure. But the graphic of observational data is the red graphic. This graphic approaches to the axle y. If it would be the graphic like blue line, we don't say "the velocity is increasing".

 

Did I understand correctly?

[sanctus]

Yes in both cases you can say the velocity is increasing, because in both cases the velocity gets bigger for increasing distance.

The difference is how they increase, the blue shows a linear increase (usually called "expansion of the universe"), it shows that the (visible) universe is expanding. The red curve shows an accelerated increase (i.e. the further you are the more the velocities increase for a fixed distance-difference), this is called "accelerated expansion". The red curve shows what is happening at really huge distances, at very small distances (the ones Hubble could use-or did not know yet how toget them) there is nowadays a non-measurable difference between the two types of expansion.

Read the article Tormod linked to, it is explained well there.

[xersan]

Yes in both cases you can say the velocity is increasing, because in both cases the velocity gets bigger for increasing distance.

The difference is how they increase, the blue shows a linear increase (usually called "expansion of the universe"), it shows that the (visible) universe is expanding. The red curve shows an accelerated increase (i.e. the further you are the more the velocities increase for a fixed distance-difference), this is called "accelerated expansion". The red curve shows what is happening at really huge distances, at very small distances (the ones Hubble could use-or did not know yet how toget them) there is nowadays a non-measurable difference between the two types of expansion.

Read the article Tormod linked to, it is explained well there.

 

But, I think the top points of red curve shows the values at past. We must compare the values of expanding velocity about present and past; and if the value of past is higher than the value of present, does it need to discrease? Is it natural that the velocity of past is higher for discreasing?

[xersan]

Human has linear thinking. And it can be confused his mind easily.

 

The master arguement of increasing for expanding velocity: The value of redshift or escaping speed don't appear the expanding velocity directly.

 

If we want to compare, we must use the Hubble constant. Because Hubble constant is similar characteristic with expanding velocity.

 

Hubble constant is determined as the value ~80 km/sec/mpc by near field (for example Virgo) and ~40 km/sec/mpc by distant units. We know that data of distant units presents for past.

 

So, we can say "the expanding velocity is increasing".

[Natural]

The problem with the current view is that scientists refuse to give a shape to the universe but they have no problem with saying that the universe is expanding. But by doing so they are negating their own postulates. First of all if everything is expanding away from "us" (us being the Milky Way) then we would either be in the exact center of the universe or one of the directions is "expanding" directly towards it's opposite direction. In other words if we are not in the center of the universe then we would have to be "expanding" away from something. (the big bang) Which would mean that we are on some kind of vector. But if some other galaxy's vector is in the opposite direction to our vector then it would be heading back towards the point of expansion. Which would put it directly on a vector towards another galaxy which would be expanding away from something else and so on. So what I'm saying is that if everything is expanding away from everything else you will soon run out of directions to expand in. And just as many things will be expanding towards each other as expanding away from each other. Don't forget that we live in a 3D universe... not 2D.

And I don't want to hear the tired old line of "space is expanding" because space has nothing to expand. Expansion is only relevant to physical objects or the distance between them. (point A to point :) But since "space" has no points and nothing to put a point on it doesn't really exist, it is a theoretical drawing board. (even theoretical sub quantum particles do no make up "space"... they are particles in "a space")

I digress... but concerning increasing expansion... what about the idea of the momentum of the mass of the universe and the square of the distance rule allowing increased expansion rate due to decreased gravity with the square of the distance? Especially since we can not say exactly what our velocity is since we have no origin point of reference and therefore can not say what exactly our momentum is.

WHEW... that was a mouthful...:)

:hihi:

[sanctus]

Natural, you have for sure already hear of the thought experiment of the cake with raisins put in the oven where it grows. Every raisin moves away from each other but none is in the center, neither one has to be opposite to another. I do not understand your argument about opposite vectors...

[xersan]

 

But if some other galaxy's vector is in the opposite direction to our vector then it would be heading back towards the point of expansion.

 

Cosmic units have redshift except Andromeda* (*because of peculiar speed in local cluster). Therefore all of them have opposite vector for absolute form of universe. But we can not say same things for visible universe.

[Natural]

Natural, you have for sure already hear of the thought experiment of the cake with raisins put in the oven where it grows. Every raisin moves away from each other but none is in the center, neither one has to be opposite to another. I do not understand your argument about opposite vectors...

 

Actually I've often heard that example used, but to me it is not a very good example, since there are always raisins in the center of raisin bread. (that's what makes it so good)

What made me refer to this situation is that I am always hearing learned scientists saying how the universe is expanding "away from us in all directions". Implying (and misleading the general public) that every other object is flying directly away from the earth. (as if it is the center of the universe) And yet they refuse to even acknowledge that the universe has any general shape. So what I am talking about is the physics of an expansion. All expansions (explosions) have several required factors like, a point from which the expansion begins, an outside limit at a specific time and vectors of all particles. These factors may be extremely hard if not impossible to define, but never the less they are required for the physical event. And while I know that gravitational interactions will be constantly changing the parameters of the equations, that is no reason to deny the fact that general physics will always apply to the event as a whole.

So what I am saying is the following rules will apply to the universe:

That expansion begins at a single point. (the big bang)

And that all particles will basically (at first) be on a linear trajectory away from the point of expansion. (not from each other)

And that the general shape of every expansion (explosion) will be spherical. (meaning that the universe must by definition of expansion be roughly spherical)

However, I know that it must also be said that all particles will be expanding away from each other on varying degrees of trajectory. For example if you have 3 particles, (A, B and C) If A is on a linear trajectory and B is 1 degree off of a parallel trajectory from A, and C is 1 degree off of a parallel trajectory from A in the opposite direction from B. Then C will be expanding away from B faster than it expands away from A. This line of thinking will mean that all other particles will be expanding away from particle A in an array of different angles off of a parallel trajectory with A. But what this also means is that many particles will be on an almost parallel trajectory with A and many will be on an opposite trajectory with A.

But what this also means is that what all particles are actually doing is expanding away from the initial point of expansion.

And as I said before I realize that gravitational interactions will completely change the whole picture. But I just wish that scientists wouldn't be so afraid to say that the universe MUST be roughly spherical and that the expansion is mainly away from the point of the big bang. And to stop using the CMBR data to try to define the "shape" of the universe, since they are only defining what is visible from an outer point in the expansion and acting as if that perspective is the center of the universe.

And I also realize that somewhere in here the idea of measuring by red shift will come into play but the problem of spacial medium wavelength absorption of photons should probably be brought up in another post. (even though it relates heavily to the measuring of the "expansion")

:cup::):Alien:

[sanctus]

The thought-experiment of the raisin bread just shows that earth must not be at the center in order to have every raisin acelerating away...

 

Anyway I agree with you there has probably been a center of the explosion, but what you say here:

And that all particles will basically (at first) be on a linear trajectory away from the point of expansion. (not from each other)

does not seem right to me if you have everything moving out radially from a center of explosion, then everything moves also away from everything else, since there are no parallel trajectory.

 

Also I think one has to consider 3+1 dimensions, so it should be a kind of 3-sphere no?

 

But what this also means is that what all particles are actually doing is expanding away from the initial point of expansion.

two things about this:

1) due to gravity it may have changed so much that it is no longer true.

2) since we can see only the raisins "close" to us, as said above, we more likely see how the galxies recede from us due to 2 different initial radial directions (then additionally changed by gravity)

 

What I think we have to keep in mind here that the things you are saying refers to the universe as a whole, not the visible one (i.e. the one in causal contact at some time with us), which is very often meant when talking about the universe, so all this has forcefully a speculative part in it.

[Pluto]

G'day from the land of ozzz

 

In theory its the spacetime expansion and not metric.

 

The observable universe is gravity bound to gravity sinks creating a clustering effect.

 

Go to the hubble site and observe as many images as you like.

The European Homepage For The NASA/ESA Hubble Space Telescope - Image Archive

[freeztar]

The thought-experiment of the raisin bread just shows that earth must not be at the center in order to have every raisin acelerating away...

 

Anyway I agree with you there has probably been a center of the explosion

 

A "center" to the "explosion"?

 

As useful as the raisin bread analogy can be, I prefer the balloon analogy. Picture a white balloon with a grid painted in black. As you inflate the balloon, the nodes separate from each other. It's similar to the raisin analogy, but uses the inflation to manipulate the 2d/3d balloon surface. No node is at the center. :hihi:

[xersan]

According to big bang theory The form of universe may be spherical surface. But we can perceive it by the form of asymmetric elipsoidal surface because of the limited speed of light.

Edited March 31, 2013 by xersan

[sanctus]

A "center" to the "explosion"? :)

 

As useful as the raisin bread analogy can be, I prefer the balloon analogy. Picture a white balloon with a grid painted in black. As you inflate the balloon, the nodes separate from each other. It's similar to the raisin analogy, but uses the inflation to manipulate the 2d/3d balloon surface. No node is at the center. :)

 

But your analogy seems to me imply that we have a closed universe...

[freeztar]

Re-reading this thread just now, I have to agree with Tormod. The title of this thread is misleading as "velocity" is a vector quantity. It suggests that the universe is expanding in a certain direction, which is incorrect.

 

But your analogy seems to me imply that we have a closed universe...

 

How so?

 

Here's a link with a better explanation of the analogy and a good animation:

Balloon Analogy in Cosmology