Expansion of the Universe question

[Drakkith]

Hrmm. I'm not sure on all the math to calculate this, but I have to ask if you took Inflation into account.

 

[WannabeNewton]

If you want the simplest way to see the rate of expansion of the universe and things such as the change in matter density or temperature just start with the scale factor in the FRW metric. You can find a lot by solving the friedman equation for open, closed,or flat universes and then observing the divergence of the energy momentum tensor.

 

[narrator]

Unfortunently MANY people misunderstand the big bang theory...

Well... it is only a tv show after all

I got to get me one of http://www.sheldonshirts.com/" !

 

[phinds]

As a slight aside on the discussion of expansion amount, I saw a VERY amusing article some time back called something like "the entire history of the universe in 200 points" and one of the things this guy pointed out was that although the universe is expanding, it still isn't getting any easier to find a parking spot.

His actual point was that the local amount of expansion over a modest amount of human time is trivial so even if your parking lot was out where the expansion is going on, it isn't going to get appreciably bigger during your lifetime. It was a helpful point to contemplate, but what really drew me in and helped me rember it was his irreverent way of putting it.

 

[cepheid]

We should probably start a new thread. IMO, this one has outlived its usefulness.

@marcus & Cepheid,

What I can't seem to find is an accurate number on the measured rate of expansion of the universe.

I attempted to derive the rate of expansion of the observable universe in my previous post (https://www.physicsforums.com/showpost.php?p=3274439&postcount=34"). I'm talking about the quantity that I called \dot{r}_H, which is the first derivative of the radius of the observable universe (so, basically, its the velocity of the boundary -- I've switched to the common notation of using an overdot to represent differentation with respect to time). I *think* that this expression is correct. I'll have more on how you can actually apply it to compute a numerical answer below.

There's 70km per second per megaparsec (which I take to be close to the Hubble Constant?)

Yes, this is the Hubble Constant, H₀. I think the current best-determined value of it is ~72 km/s/Mpc.

which is (oddly) given in one-dimensional terms rather than volume.

Well, there's a spherical symmetry there. It doesn't matter in what direction you're looking, as long as an object is x Mpc away, then it will appear to be receding from you with a velocity of v = H₀x km/s.

But this number doesn't seem to include the accelerating rate of expansion detected in the late 90's by Perlmutter et al.

The parameter that tells you the ratio of distance to recessional velocity is the Hubble parameter, H. This parameter changes with time. The Hubble Constant, H₀, is just the value of H today.

In some sense, H expresses the history of the rate of the expansion of the universe, since H = \dot{a}/a. In this expression, a is the scale factor, and \dot{a} is its derivative (the rate at which it changes with time). The value of the scale factor at time t is basically the ratio of the separation of any two objects at time t to their separation now. So a is 1 today, and a < 1 in the past.

That number (70km/s/megaparsec) seems equivalent to a 2.27 x 10^-16% increase in volume per second.

I'm not sure how you computed this, and I would ask, which volume are you considering? H₀ is not a velocity, it's the ratio of velocity to distance. So, the farther out you look, the faster things are moving away (the essence of Hubble's law). That leads me nicely back to my specific example where the volume being considered is the volume of the observable universe. The expression I derived for the velocity was:

\dot{r}_H = \dot{a}x_H + c

How do you actually compute this? Well, plugging in the values that apply today, remembering that H = \dot{a}/a, and that a = 1 today, and that H₀ is the value of H today, we just end up with \dot{a} = H_0 (today). The quantity x_H is the co-moving horizon radius, which is just equal to the physical radius of the observable universe today (about 46 billion light years).

EDIT: I get about a billion m/s, which does mean a fractional increase in radius in 1 second of only about 3e-18. So maybe you weren't too far off.

 

[AC130Nav]

I don't see all veiwpoints represented yet. I see an entrenched viewpoint that the Universe is expanding. Let's call that the "flat-earther" viewpoint in that it believes what we can see is all there is.

I see creationists--I won't even comment on them except to note that the Big Bangers are their favorites.

I see people who want to know what the formulae are for what is observed. OK.

Given, however, that black mass and black energy are in many cases accepted by cosmologists (the real ones, not the ones the magazines quote) as simply quotients applied because the formulae don't work, perhaps a more serious discussion is, indeed, necessary. Starting a new thread will only allow the old trash to be more easily be reenterd.

The real problem throught this whole discussion has been that people seem to not understand that space is being re-defined; there is no expansion in the usual sense. If it is the universe, it is not expanding into new space (absurd) it is redefining the space within it such that motion and gravity are different. Postulate two inconceivable masses approaching each other (one has been detected), how does that affect the structure of the space between?

Einstein didn't catch this but this is still Einsteinian and what has followed him is not.

 

[phinds]

Given, however, that black mass and black energy are in many cases accepted by cosmologists (the real ones, not the ones the magazines quote) as simply quotients applied because the formulae don't work...

So they, whoever they are, believe that the gravitational lensing exhibited by dark matter is just a quotient in an equation? I don't see how that works.

 

[Misericorde]

I don't see all veiwpoints represented yet. I see an entrenched viewpoint that the Universe is expanding. Let's call that the "flat-earther" viewpoint in that it believes what we can see is all there is.

I see creationists--I won't even comment on them except to note that the Big Bangers are their favorites.

I see people who want to know what the formulae are for what is observed. OK.

Given, however, that black mass and black energy are in many cases accepted by cosmologists (the real ones, not the ones the magazines quote) as simply quotients applied because the formulae don't work, perhaps a more serious discussion is, indeed, necessary. Starting a new thread will only allow the old trash to be more easily be reenterd.

The real problem throught this whole discussion has been that people seem to not understand that space is being re-defined; there is no expansion in the usual sense. If it is the universe, it is not expanding into new space (absurd) it is redefining the space within it such that motion and gravity are different. Postulate two inconceivable masses approaching each other (one has been detected), how does that affect the structure of the space between?

Einstein didn't catch this but this is still Einsteinian and what has followed him is not.

Your view of Dark Matter strikes me as already largely falsified unless GR is wrong about things it APPEARS to be correct about on a huge scale. Do you have anything to support the other views you mentioned, which I have to say, strike me as far more exotic/flat-earth than simply drawing conclusions from the best current observations?