Expansion of the Universe question

[narrator]

If it wasn't your quote that I used in the post, and I did remain on-topic, why are you offended? Do you disagree? Can you take science beyond the end of the universe's relevance? Can science describe that which doesn't exist?

From what I've read and seen on doco's, in the dying phase of the universe, it will just go dark and have a lot of lumps of cold rock floating at great distance from each other (far greater than they are now). Thus it endeth. What else is there to say? (Except that there's still the recycle theory.. lol)

If there is a deity responsible for it all, then we have as much chance of attributing anything to it as an ant would of understanding cosmology. Until that deity changes its policy of non-disclosure, science will never discover it. Thus, this forum is about what science HAS uncovered, and not about speculations outside its charter. (Have I got that right, mods?)

I do contemplate such questions, but not while I'm posting here.

 

[CutterMcCool]

@Marcus,

Thanks for your thorough answers. If space is expanding exponentially, shouldn't it be possible to follow the "leading edge" of space (the outer edge of the observable universe) and describe its acceleration in m/s^2? Much as the gravitational acceleration of a falling body on Earth (about 9.8 m/s^2) is expressed in that form? Which says its velocity would be increasing while its acceleration rate is constant.

Also in the Friedmann equations you mention, what is p and a'?

 

[marcus]

I hope someone else will help answer, McCool. I got distracted elsewhere and didn't see your question. Did you look up the Friedmann eqns. on wikipedia? Let us know what online thing you are looking at, give a link, so we can see what you see.

Typically in those equations, a(t) is a handle on distances called scalefactor, and a-dot or a-prime is the time derivative. a'(t) = da/dt.
Do you know some elementary calculus so that the notation da/dt means something. It is the increase per unit time.

The scalefactor a(t) is normalized so that it equals 1 in the present. So if you go back in time to some time when distances were just half as big then a(t) would = 0.5.
And if you go to some time in future when distances are three times as big then a(t) would = 3.

the Friedmann eqn usually has a greek letter rho in it which stands for the density. The grams per unit volume---in whatever units.

RHO LOOKS LIKE A LETTER P
which is confusing, sorry about that. And also one of the equations you often see together with the main one has an actual letter p, standing for pressure.

If there is something in the U that exerts a pressure that could also have an effect.

Most often though when you see something vaguely like a lowercase letter p it is actually rho, the density.

If you've had even a bit of calculus it's a lot easier to understand, so I hope you will let us know that you have. And then others may help explain the friedmann equations. they are pretty basic actually.

Did you ever visit Ned Wright's cosmology tutorial website?
if not google "ned wright"
and "wright balloon model"
and "wright cosmo calculator"
He has an FAQ
He has lots of diagrams showing the observable U, and the past lightcone, etc. How expansion affects the shape of things.

Let us know if you find anything especially good online stuff on your own. Share the goodies

 

[cepheid]

@Marcus,

Thanks for your thorough answers. If space is expanding exponentially, shouldn't it be possible to follow the "leading edge" of space (the outer edge of the observable universe) and describe its acceleration in m/s^2? Much as the gravitational acceleration of a falling body on Earth (about 9.8 m/s^2) is expressed in that form? Which says its velocity would be increasing while its acceleration rate is constant.

I hope someone else will help answer, McCool. I got distracted elsewhere and didn't see your question. Did you look up the Friedmann eqns. on wikipedia? Let us know what online thing you are looking at, give a link, so we can see what you see.

Hey marcus,

It occurred to me a few days ago that it should be possible to do what CutterMcCool is proposing by simply twice-differentiating the expression for the horizon radius as a function of time (the second equation here: http://en.wikipedia.org/wiki/Observable_universe#Particle_horizon)

Differentiating it once (assuming I did it right) leads to a neat result, in the sense that I think it has a ready interpretation. I use x for co-moving distance and r for physical distance, and I use the subscript H to mean, "horizon." The co-moving horizon x_H is the same as eta in that link, except that I explicitly include a factor of c in front of the integral. Then we get (using a ' symbol to mean differentiation w.r.t. proper time):

r'_H(t) = [a(t)x_H(t)]' = a'x_H + ax'_H

But ax'_H just equals c (since if you differentiate that integral, you just end up with the function in the integrand, 1/a, and ac/a = c). That leaves us with:

r'_H = a'x_H + c

So it seems that the radius of our observable universe (namely r_H) is getting bigger for two reasons. The first reason, associated with the first term above, is that everything in the current
horizon volume is getting bigger due to expansion, and this just occurs at a rate a'. The second reason is that the horizon volume itself is expanding to encompass more stuff that was not contained within it before. This just occurs at rate c, as more photons reach us! This second reason can be thought of as an expansion of the co-moving horizon volume, which is why it is associated with the second (x') term.

What do you think of my interpretation? If it's correct then, it is pretty neat. So, if you wanted to, you could think of this quantity a'x_H + c as the velocity with which the horizon boundary recedes from us in m/s.

To get the acceleration, we just differentiate again, giving us:

r''_H = a''x_H + a'x'_H

I have no ready interpretation for this!

 

[jbwright]

by The Angry Citizen:

I hope you all (4/8/11 to 4/30/11) won't get too angry when you discover that Creation (of something to go "bang", necessary for a Big Bang) requires an act of magic and that magic is not allowed when one studies the Universe. Instead, you must recognize objective reality as your tool in this epistemological arena.

The Cosmological Redshift? How else could it be developed? Try studying the Galactic Clusters that thoroughly dominate our Universe, and observe that redshifts (a side show) are like light light from a forest fire. And see where else a Galactic Cluster will lead you.

 

[Misericorde]

by The Angry Citizen:

I hope you all (4/8/11 to 4/30/11) won't get too angry when you discover that Creation (of something to go "bang", necessary for a Big Bang) requires an act of magic and that magic is not allowed when one studies the Universe. Instead, you must recognize objective reality as your tool in this epistemological arena.

The Cosmological Redshift? How else could it be developed? Try studying the Galactic Clusters that thoroughly dominate our Universe, and observe that redshifts (a side show) are like light light from a forest fire. And see where else a Galactic Cluster will lead you.

Am I reading you correctly, and you're saying that you believe in magic, as in divine or other magic? That's nothing to do with science; nothing at all.

 

[phinds]

I hope you all (4/8/11 to 4/30/11) won't get too angry when you discover that Creation (of something to go "bang", necessary for a Big Bang) requires an act of magic and that magic is not allowed when one studies the Universe. Instead, you must recognize objective reality as your tool in this epistemological arena.

The Cosmological Redshift? How else could it be developed? Try studying the Galactic Clusters that thoroughly dominate our Universe, and observe that redshifts (a side show) are like light light from a forest fire. And see where else a Galactic Cluster will lead you.

What in the world are you talking about? I don't get any of what you're saying.

• do you not believe in the big bang?
• how is redshift like a forest fire?
• how is redshift a "side show"

I'm willing to believe I am just not able to interpret you, but I think you need to explain yourself better instead of just making statements that don't seem to make sense.

 

[narrator]

jb seems to be asking possibly valid questions, but in the wrong forum.. perhaps it comes from an emotional investment.. but as this forum is less about theoretical, I'll stop there ;)

 

[Misericorde]

Theoretical? The issue with magic is the same as with a bad theory; lack of falsifiability and support. The need for physics which contradicts existing theories which work, and more. The notion that "before" the big bang or other event is no longer the work of science is a valid statement right now, but it doesn't lead to magic, and it isn't a license to go off the rails.

 

[TrickyDicky]

What's the use of substituting a theory because it is too "magical" with another even more fantastic?
BTW, personal theories are not allowed in this site.

 

[jbwright]

Some 80 years ago, astronomers had discovered a downward shift in the frequency of the light from distant galaxies (from the blue end of the spectrum towards the red, or a "Redshift"), and their even more recent discovery that this redshift became greater the farther away was the light-source galaxy. In an attempt to account for this redshift they decided that it must be a Doppler effect caused by the movement of the galaxies away from us, and ended up with an Expanding Universe. This resulted in the invented the Big Bang as the explosion that would start the Universe on its journey.

So, aside from having had to have been a monstrous explosion to have driven 200 billion galaxies (of 100 billion stars each) out into space, even more disturbing is the question of where all of this mass would have come from? Where , but perhaps through creation, an act of creating something out of nothing? I.e., and act of Magic? So, as we are determined not to use magic we are left with a Universe of some hundreds of billions of galaxies, with the redshifts in their light, and with little else. And this, of course, meant that we needed to look further to find what could take its place.

It is here that I suggest Galactic Clusters, a gravitational grouping of galaxies throughout the Universe, from only a few galaxies to over two thousand. These build over billions of years, collecting stellar winds as well as the galaxies, compressing all into an ever more dense aggregations of galaxies and gases, and eventually building a dominant spiral galaxy, a Seyfert. In time the mass and density in the core of the Seyfert Galaxy reaches a critical point at which a nuclear convulsion occurs and two Quasars are ejected, one on each side of the Seyfert.

From here it is necessary to go to the studies of the sky's by Halton Arp, where he recognizes that these Seyferts and Quasars are connected in a well defined way, occurring about once every 7 billion years, and that the Quasars themselves blossom into galaxies over a period of time, only to become swallowed by the galactic clusters. Hence, a galactic recycling happening at each of (say) 10 million Seyfert galaxies throughout our Universe, and perhaps on into an infinity of Existence.

Now, the major problem herein is the nature of the explosion within the Seyfert galaxy that takes the ashes and other debris from the Galactic Clusters and turns them into the well organized masses that are to become the Quasars from which the new galaxies are to be formed.

The Cosmological Redshifts can surely become real when one considers the constantly compressing gases all through the Galactic Clusters, and the other wealth of resources.

 

[Drakkith]

The Cosmological Redshifts can surely become real when one considers the constantly compressing gases all through the Galactic Clusters, and the other wealth of resources.

All the other nonsense in your post aside, no. This does not explain redshift in any way whatsoever. You are asking to get banned with your post.

Now, the major problem herein is the nature of the explosion within the Seyfert galaxy that takes the ashes and other debris from the Galactic Clusters and turns them into the well organized masses that are to become the Quasars from which the new galaxies are to be formed.

This isn't anything like a real quasar, sorry.

So, aside from having had to have been a monstrous explosion to have driven 200 billion galaxies (of 100 billion stars each) out into space, even more disturbing is the question of where all of this mass would have come from? Where , but perhaps through creation, an act of creating something out of nothing?

Prove it came from nothing. There are multiple hypothesis about what was before the big bang. Not all of them assume that NOTHING was there.

 

[phinds]

Drakkith,

I was happy to see your response to this post. I'm not knowledgeable enough to be sure but this all sounded like nonsense to me. I did check out Halton Arp and apparently he's a guy who's done some valuable work but is now considered to be on the fringe, refusing to give up his belief in intrinsic redshift hypothesis even though it has been discredited. I THINK some of jbwright's argument requires intrinsic redshift, but it's all so incoherent to me that I can't be sure.

 

[Drakkith]

Drakkith,

I was happy to see your response to this post. I'm not knowledgeable enough to be sure but this all sounded like nonsense to me. I did check out Halton Arp and apparently he's a guy who's done some valuable work but is now considered to be on the fringe, refusing to give up his belief in intrinsic redshift hypothesis even though it has been discredited. I THINK some of jbwright's argument requires intrinsic redshift, but it's all so incoherent to me that I can't be sure.

Unfortunently MANY people misunderstand the big bang theory and incorrectly assume that science says everything came from nothing. Not that this is purely their fault. I've seen many a TV show or similar where supposedly credible scientists claim that all this came from nothing. Or they at least pose the question "Where did all this come from?" in the context that it seems to come from nothing. This, obviously, leads to MANY arguments and forays into philosophical areas and leads many people away from science because they simply can't believe that everything came from nothing and science must be wrong. My own grandfather, a devout christian and pastor, has brought this argument up to me several times.

 

[mayflow]

It appears that dark energy is speeding the expansion and works like antigravity and is a repellent force rather than attracting force. Einstein once proposed this but discarded it as his greatest blunder. It may not have been. In his theory of special relativity, he uses light as a universal constant. I personally think that is a greater blunder.

I think he called it a cosmological constant or something like that, and he was maybe on to something, but he seemed so hung up on things being constants. I don't personally believe in constants, but in variables and flows and infinite probabilities.

 

[Misericorde]

Unfortunently MANY people misunderstand the big bang theory and incorrectly assume that science says everything came from nothing. Not that this is purely their fault. I've seen many a TV show or similar where supposedly credible scientists claim that all this came from nothing. Or they at least pose the question "Where did all this come from?" in the context that it seems to come from nothing. This, obviously, leads to MANY arguments and forays into philosophical areas and leads many people away from science because they simply can't believe that everything came from nothing and science must be wrong. My own grandfather, a devout christian and pastor, has brought this argument up to me several times.

Too true; how often do you hear it described as an "explosion"? No wonder people tend to think of it as something within space-time, instead of the beginning of both, the expansion of both from a pinprick.

 

[CutterMcCool]

rate of expansion?

@Marcus & Cepheid,

What I can't seem to find is an accurate number on the measured rate of expansion of the universe. There's 70km per second per megaparsec (which I take to be close to the Hubble Constant?) which is (oddly) given in one-dimensional terms rather than volume. But this number doesn't seem to include the accelerating rate of expansion detected in the late 90's by Perlmutter et al.

That number (70km/s/megaparsec) seems equivalent to a 2.27 x 10^-16% increase in volume per second. This is fantastically low!--were it an interest rate on a bank account, your $1000 deposit would still be only $1000 after 14.7 billion years. Which doesn't seem right financially or cosmologically. To calculate the changing rate of acceleration of space, a large enough number is needed to show significant change in the volume of space since inflation ended about 14.7 billion years ago.

Anybody have a more accurate number?

 

[Drakkith]

Is 70,000 km/s for a billion parsecs a small amount? (Thats 3.26 billion light years)

 

[CutterMcCool]

@Drakkith,

If the number I found is off by orders of magnitude that would explain the discrepancy--in that 10^-16% doesn't generate any noticeable expansion.

What I'm trying to do is calculate the rate of expansion by using a simpler formula, that for calculating compound interest, than those given above. In that analogy space expands its bank account at some interest rate per second. That "interest rate" can then be used to figure out its rate of acceleration.

That formula is:

I=V(1+r)^t

where I is the increased volume, V is the initial volume, r is the rate of expansion per second, and t is the time passed in seconds.

 

[CutterMcCool]

See above for formula.

 

[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?