Dark energy causing universe expansion?

[Kagrat]

What is the dark energy which apparently is causing the expansion of the universe to accelerate?

 

[mfb]

Find it out and get a Nobel Prize!

We observe the accelerated expansion, and a constant energy density everywhere is the easiest model to describe it, we can measure its density assume it is there, but that is all we know about it.

 

[rootone]

The model is that space is getting bigger and the bigger it gets the faster it expands.
Don't worry, nobody has any good cliue why it ls that way, but that is what is obsereved

 

[phinds]

What is the dark energy which apparently is causing the expansion of the universe to accelerate?

There is no "apparently" about it. SOMETHING is unquestionably causing the universal expansion to accelerate and we call that something "dark energy" as a placeholder name because we don't know what it is.

 

[phinds]

The model is that space is getting bigger and the bigger it gets the faster it expands.

That is NOT the model. Space does not expand, things just get farther apart. Google "metric expansion".

 

[Chronos]

According to science gravity is a fixed force with unlimited range, it merely decreases in strength as range increases. Assuming the force of gravity is an intrinsic propery of mass, the only way the universe can expand is to a] decrease the effective mass of individual bodies or change the rate at which the force of gravity dminshes with distance, or b] introduce another force that opposes the force of gravity. Given option a] requires mass or the force of gravity to eventually assume negative values at some point, scientists have widely preferred option b], - whose main appeal is it is an arbitrary factor Einstein was able to fudge into his field equations for GR. Scientist are as guilty as any sailor in seeking the nearest port in a storm. Since the nature of this mystery force is otherwise unknown, scientists christened it 'dark' energy in the same spirit they dubbed the unseen mass needed to hold galaxies together as 'dark' matter.

 

[mfb]

Assuming the force of gravity is an intrinsic propery of mass, the only way the universe can expand is to a] decrease the effective mass of individual bodies or change the rate at which the force of gravity dminshes with distance, or b] introduce another force that opposes the force of gravity.

The universe can expand well without both. Another force (or similar) is only necessary if you want the expansion to accelerate.

 

[Chronos]

Agreed. The old f=ma thing.

 

[nikkkom]

According to science gravity is a fixed force with unlimited range, it merely decreases in strength as range increases. Assuming the force of gravity is an intrinsic propery of mass, the only way the universe can expand is to a] decrease the effective mass of individual bodies or change the rate at which the force of gravity dminshes with distance, or b] introduce another force that opposes the force of gravity.

Er... no. Standard FLRW Universe can expand forever without any of the above. It only needs to have a low enough density of matter for that.

Given option a] requires mass or the force of gravity to eventually assume negative values at some point

Wrong. Not every decreasing real function is required to become negative. Example: -exp(x).

 

[Chronos]

Yes, as already addressed, linear expansion [coasting models] can be accommodated without any fudging, but, accelerated expansion - not so much. It [accelerated expansion] only leaves two obvious options: 1] kill the messenger [refute the data], 2] acquiesce to the evolution of nontraditional mass/gravity values.

 

[rede96]

That is NOT the model. Space does not expand, things just get farther apart. Google "metric expansion".

Although that is the way I’ve learned to think about expansion, from other posts I’ve made on the subject, not everyone sees it that way.

For me personally it’s statements like “light waves are stretched and red shifted by expansion” that really confuse me. If there is nothing expanding what causes light to become stretched for example?

 

[phinds]

Although that is the way I’ve learned to think about expansion, from other posts I’ve made on the subject, not everyone sees it that way.

For me personally it’s statements like “light waves are stretched and red shifted by expansion” that really confuse me. If there is nothing expanding what causes light to become stretched for example?

The problem is in the use of the English language word "expansion", which carries with it, intentionally or not, the implication of proper motion, but there IS NO proper motion in metric expansion. As things move apart, the source of light is moving away (but not with proper motion) and thus red shifted.

 

[Khashishi]

For me personally it’s statements like “light waves are stretched and red shifted by expansion” that really confuse me. If there is nothing expanding what causes light to become stretched for example?

The redshift can equally be explained as a Doppler shift or as a metric expansion. The difference is due to a choice of coordinates. The metric expansion explanation is simpler because the Doppler shift requires you to parallel transport the velocity of the source to yourself, but you get the same result. In relativity, all choices of coordinates are equally valid, but in cosmology, the CMB rest frame is special. By basing our coordinates around the CMB rest frame, we get metric expansion, but by using other coordinates, we don't necessarily have expansion. If it weren't for all the stuff in space (CMB, galaxies, etc.) comoving which gives us a preferred coordinate system, it would make no sense to say space is expanding. If space were totally empty, there would be no reference points, so we could as easily create coordinates such that space were contracting or expanding or stationary.

 

[rede96]

The problem is in the use of the English language word "expansion", which carries with it, intentionally or not, the implication of proper motion, but there IS NO proper motion in metric expansion. As things move apart, the source of light is moving away (but not with proper motion) and thus red shifted.

Proper motion is another term that always gets me confused. For example if we define proper motion as objects moving though space, then expansion isn’t due to objects moving though space.

But as they are not moving *though* space but distances are increasing, then either the space is expanding or new space is being created. So it leads to thinking of space as some physical thing, which it isn’t.

So for me I prefer to just think about metric expansion as describing how things move apart if no other forces, that can effect relative motion between objects, come into play.

 

[phinds]

So for me I prefer to just think about metric expansion as describing how things move apart if no other forces, that can effect relative motion between objects, come into play.

Yes, that is the right way to view it. See the link in my signature

 

[Martin0001]

What is the dark energy which apparently is causing the expansion of the universe to accelerate?

We don't know what it is.
There are scientists with very long beards who think that it might be a rather boring cosmological constant (eg time stable, constant energy per volume of space acting like antigravity) or a bit more exciting quintessence which is time dependent.
The most exciting form of quintessence would be so called phantom energy.
If the last version is correct then some unlucky beings in distant future may well face End of the World and they will be in position to see it up to a minute or two before final singularity.
Scientists living then and there will calculate the end of the world date with increasing accuracy and they may find out that it will happen on Monday 27 Nov. 22354788929 AD at 15.35'56.005" +/- 0.002". They may find it 1 year upfront (ableit less accurate estimations will be available long before that) so the Final Nobel Price in physics will be granted.
I bet, they will be arguing with each other and carrying on producing new and more accurate dark energy models up to 15.20 of that very day.
In final stages of affair these unlucky creatures will end up gravitationally unbound from their planet (so they will fly) and minutes later their bodies will be electromagnetically unbound, so they will cease to exist.
Minutes later everyting including supermassive BH will end up unbound too, rate of expansion of space will approach infinity, entropy will reach theoretical maximum and hence time will end.
All is known as a Big Rip.

 

[Khashishi]

Proper motion is another term that always gets me confused.

I don't think proper motion is the correct term to use in this context. Proper acceleration is a more accurate term. It is possible to measure proper acceleration with an accelerometer, without using any external reference points. It's not possible to measure your own velocity without using an external reference, since velocity is relative. There is no absolute rest frame to space.

But as they are not moving *though* space but distances are increasing, then either the space is expanding or new space is being created.

Seeing as there is no absolute rest frame to space, it is meaningless to say one is moving *through* space. On the other hand, you can say if one is moving with respect to the cosmic microwave background.

 

[rede96]

Proper acceleration is a more accurate term. It is possible to measure proper acceleration with an accelerometer, without using any external reference points.

Except objects that are accelerating due to metric expansion do not feel acceleration in a similar way that you don't feel acceleration when in free fall. So that's not really relevant in this case.

Seeing as there is no absolute rest frame to space, it is meaningless to say one is moving *through* space. On the other hand, you can say if one is moving with respect to the cosmic microwave background.

The point is expansion causes distances to increase between objects not gravitationally (or otherwise) bound. So there is relative motion between them. The issue I was highlighting is that some people view this relative motion being caused by something 'expanding' like space or something else. AFAIK, that isn't the case. The relative motion is simply a property of dark energy or what ever it is that is responsible for metric expansion.

 

[PeterDonis]

I don't think proper motion is the correct term to use in this context.

"Proper motion" in the context of cosmological models that cover the universe as a whole generally means motion relative to comoving observers. That is a well-defined concept even though "motion" in general is not.

Proper acceleration is a more accurate term.

Not for what is being discussed in this thread. As @rede96 pointed out, the objects that we use to measure the expansion of the universe--galaxies and galaxy clusters--feel no acceleration; that means their proper acceleration is zero.