The expanding Universe

[Gian]

The Universe has been expanding since the Big Bang ye? so we can see the 'red shift' in galaxies moving away. Does this mean literally that galaxies are getting further away from us in miles? or does 'stretching space' mean something else? I'm not a scientist so I need some assistance on this one

[Strange]

 

 

Does this mean literally that galaxies are getting further away from us in miles?

 

Yes. (Not sure what else to say. )

[mathematic]

The point is that these galaxies are moving away from us because space is expanding and not because of their motion.

[Airbrush]

Only superclusters of galaxies are getting further away. All the galaxies in our Virgo supercluster are stuck together with us forever.

Edited September 8, 2016 by Airbrush

[rockeyracoon]

It's like we are inside a balloon and the balloon is being inflated.

BUT what if the so called echo (event horizon) of the big bang is not an echo at all?

What if the BIG BANG, was what happens when infinite number of strings (a singularity) and a Boson connect. When the boson is created (by the creation of a galaxy), it imparts vibrations to the strings in the singularity which in turn instantly became the new universe,

in a very excited state (HOT) VERY VERY VERY HOT.

It so simple yet so perfect from a this perspective a singularity could occupy no space and yet have the seeds of the galaxy just waiting for a boson because stings have 0 mass and oly a vibrational component.

This only leaves the question was there a 1st universe?

Edited September 9, 2016 by rockeyracoon

[Strange]

It's like we are inside a balloon and the balloon is being inflated.

 

 

It is more like we are on the surface of a balloon (a 2D analogy for the 3D universe).

 

Your description suggests that there is something "outside" the balloon. That does not match current cosmology.

 

 

The rest of your post seems to be baseless speculation. As there is no science in it, there isn't much to say.

[Daecon]

I have a question which may be slightly off-topic, but what might our measurements of the Universe look like if expansion was at a constant rate and not accelerating?

[Strange]

I have a question which may be slightly off-topic, but what might our measurements of the Universe look like if expansion was at a constant rate and not accelerating?

 

 

We would see Hubble's law showing a constant relationship between distance and speed (at all times and distances).

[fred2014]

The point is that these galaxies are moving away from us because space is expanding and not because of their motion.

 

One interpretation but I suspect will one day be shown to be incorrect (and somewhat embarrasing).

[Strange]

 

One interpretation but I suspect will one day be shown to be incorrect (and somewhat embarrasing).

 

Even if it did turn out to be incorrect (and you provide no reason why that should be the case) it certainly wouldn't be embarrassing. It is based on one of our most successful theories and is consistent with all the (current) evidence. Why would anyone be embarrassed by that?

[Daecon]

Sorry, I missed your previous reply, Strange.

 

Has the rate of acceleration been determined? For example, if the Universe expands by x% over a certain amount of time, then the expansion would naturally accelerate as time goes on, wouldn't it?

[Strange]

Sorry, I missed your previous reply, Strange.

 

Has the rate of acceleration been determined? For example, if the Universe expands by x% over a certain amount of time, then the expansion would naturally accelerate as time goes on, wouldn't it?

 

 

It has been measured (otherwise we wouldn't know about it ) but I don't think there is an simple way of characterising it (I certainly don't fully understand it).

 

Note that this acceleration has a very specific meaning: the deviation from a constant rate of expansion. So, although a constant rate of expansion means that things raced faster as they get further away, that is not referred to as acceleration (because it is not proper motion, just the effects of scaling the geometry of the universe).

[Daecon]

So if I suggested a scenario like the following:

 

"Say we had a hypothetical Universe that measured 500 units across with an expansion rate of 10% an hour. In the first hour, this Universe would expand from 500 to 550 units across - so the measured expansion would be 50 units per hour.

 

But in the following hour, the Universe would expand from 550 to 605 units across and the expansion would be measured at 55 units per hour.

 

This way, the expansion in our hypothetical Universe is measured to be accelerating, even though the percentage is constant."

 

But that's inaccurate to describe the accelerating expansion which is actually happening in our real Universe?

Edited October 2, 2016 by Daecon

[Strange]

Exactly. The expansion is defined by a scaling factor. And, as you say, a constant scaling factor leads to an increasing speed with distance (or time). This is the reason for Hubble's law.

 

But what was discovered ~20 years ago is that the scale factor is not constant but appears to have started increasing about 5 billion years ago. That is what is described as acceleration.

[Mordred]

When the universe was approximately 7.2 billion years old. According to dataset of Planck 2012. This is when the universe goes from the matter dominant era to the Lambda dominant. (cosmological constant aka dark energy).

 

The Seperation distance rate was gradually slowing down (but still expanding) till then.

 

You can fine tune the inflection point using the calc in my signature but you have to change the number of steps and start/end (stretch values) stretch is just inverted scale factor for ease of calcs.

 

(if I recall Marcus included this as an example in the calculator guide.) Provided Jorrie didn't change it after Marcus passed away.

 

(4 of us were involved in its development)

Edited October 2, 2016 by Mordred

[Ivanov2000]

Only superclusters of galaxies are getting further away. All the galaxies in our Virgo supercluster are stuck together with us forever.

 

Why is that? Matter does not like to move, without good reason, not though space anyway. It happily moves through time. Anyway, if you have two bits of matter x distance apart in expanding space, two things can happen 1. they move towards each other, to maintain distance x or 2. space distorts locally to maintain distance x. Maybe a bit of both, anyway, you can call either gravity. So is gravity just matter's reluctance to play ball with the expansion of space time? Problem is this would mean the effect is greater with larger values of x, which don't seem to be the way things work.

[Strange]

 

Why is that?

 

 

Because they are gravitationally bound: they are all in (complex) orbits around one another. The presence of this force between them stops them drifting apart (which is what things that are not gravitationally bound do).

 

 

 

Matter does not like to move, without good reason, not though space anyway.

 

If matter is moving then it will carry on moving unless something exerts a force on it. Matter may have a resistance to changing its state of motion (which is its mass) but once it is moving it is happy to carry on.

[Ivanov2000]

Yes, moving the same as it already is, is one of the 'good reasons' for moving.

 

"Problem is this would mean the effect is greater with larger values of x"

 

In fact, probably a constant acc, determined by rate of space time expansion somehow, but that still doesn't work.

 

 

I got here via mond. From what I understand, gravity no longer follows the inverse square below a certain acc, which does not make a lot of sense (since the inverse square comes from the surface area of expanding sphere - seems to me that a straight inverse relation would require 2d space). So I'm thinking leave gravity alone, there's something there in addition to gravity, something small that doesn't show up until the force of gravity is tiny.

 

But not dark matter, that doesn't really work neither.

[Strange]

Neither dark matter nor MOND have anything to do with acceleration expansion.

 

We don't know what causes accelerating expansion so the placeholder name for whatever the cause is, is "dark energy". Dark because we don't know what it is and "energy" because it can be most easily modelled by an extra energy term in the equations.

 

But I thought that it was expansion that was puzzling you. You should get to grips with that before worrying about the extra detail of its acceleration.

[Ivanov2000]

Not sure I ever mentioned acceleration expansion.

 

The point is, why would space time expand away from matter? Doesn't make any sense. So it does expand near matter but gravity holds matter close.Ok, but Newton's inverse square is based on fixed background. In expanding background gravity would have to be greater than that to hold everything in place, or... there is something else going on, which, if weak, but linear, will show up when gravity is weaker - which is where the reference to MOND and dark energy come in, and those stars doing odd things at the outer edge of some galaxies... where the acceleration due to gravity is less than a certain amount that has a curious relationship to c and H.

And the point of the point is that the other thing going on that mass just don't like moving.

[Strange]

Not sure I ever mentioned acceleration expansion.

I must have misunderstood this, then:

 

In fact, probably a constant acc ...

 

 

The point is, why would space time expand away from matter? Doesn't make any sense.

 

 

Space expands when there is a uniform (homogeneous) distribution of matter. This is a result derived from the same equations that give us gravity.

 

Where the distribution of matter is not homogeneous, then you don't get expansion. (Which is what is meant when we talk about galaxy clusters being held together by gravity).

 

So it does expand near matter but gravity holds matter close.Ok, but Newton's inverse square is based on fixed background.

 

Newtonian gravity isn't really relevant. This all has to be modelled using GR.

 

 

 

In expanding background gravity would have to be greater than that to hold everything in place

 

There is no force pushing things apart so this is not correct.

 

 

John Baez has written a good explanation of the mathematics which I think you should be able to get something out of, even if you just skip the math:

http://math.ucr.edu/home/baez/einstein/

Edited October 9, 2016 by Strange

[Ivanov2000]

 

 

Space expands when there is a uniform (homogeneous) distribution of matter. This is a result derived from the same equations that give us gravity.

 

Where the distribution of matter is not homogeneous, then you don't get expansion. (Which is what is meant when we talk about galaxy clusters being held together by gravity).

 

 

This does not agree with what I have read elsewhere.

[Mordred]

What Strange is detailing is the difference between a global metric ie universe, compared to local influence due to gravity GR. On the global metric, expansion is due to thermodynamic conditions. These thermodynamics are our standard model of particles interactions. We use temperature, pressure, density in the deceleration equations of the FLRW metric. Every particle has an equation of state, which correlates a particles pressure influence due to its kinetic energy. Now if you take a homogeneous fluid (no preferred location) then there is no gradient in pressure, this means you have no inherent outward flow. Even better as our expansion is isotropic ( no preferred direction). One can argue if expansion is due to pressure or temperature but the equations use density. (critical density formula)

 

Now gravity does influence expansion but not in an intuitive way. As more matter collapses into large scale structures (local), the average density on the global scale decreases. So the global gravity average decreases. Expansion occurs.

 

So why doesn't expansion affect the local structures? This is because locally gravity is too strong for the global expansion to affect. The average mass density is roughly 10^-27 kg/m^3. This equates to a mere 7.2 *10-10 joules/m^3. Pretty weak, far less than a Newton of force. When Local gravity exceeds the critical density, no expansion occurs.

[Strange]

 

This does not agree with what I have read elsewhere.

 

 

I suspect that is because most of what you have read comes from popular science articles and is, therefore, extremely inaccurate or even wrong.

 

 

This leads to an important point, namely that we should not expect the global behaviour of a perfectly homogeneous and isotropic model to be applicable when these conditions are not even approximately met. The expansion of space fails to have a ‘meaningful local counterpart’ not because there is some sleight of hand involved in considering the two regimes but because the physical conditions that manifest the effects de- scribed as the expansion of space are not met in the average suburban bedroom.

https://arxiv.org/abs/0707.0380

[Ivanov2000]

That does not say that space does not expand in your bedroom, it merely says that the effect is not obvious under those conditions. The implication is that space does, indeed, expand in your bedroom. In fact, as far as I can see, the 'homogeneous' and 'isotropic' conditions are only there to simplify the calculation.

 

So why doesn't expansion affect the local structures? This is because locally gravity is too strong for the global expansion to affect. The average mass density is roughly 10^-27 kg/m^3. This equates to a mere 7.2 *10-10 joules/m^3. Pretty weak, far less than a Newton of force. When Local gravity exceeds the critical density, no expansion occurs.

 

Are you saying gravity actually prevents space expanding, or gravity merely prevents matter moving apart as space expands? If the latter, I would agree, but that does not appear to be what Strange is saying:

 

"Space expands when there is a uniform (homogeneous) distribution of matter."

 

"Where the distribution of matter is not homogeneous, then you don't get expansion."

Edited October 10, 2016 by Ivanov2000