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Kagrat
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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.Kagrat said:What is the dark energy which apparently is causing the expansion of the universe to accelerate?
That is NOT the model. Space does not expand, things just get farther apart. Google "metric expansion".rootone said:The model is that space is getting bigger and the bigger it gets the faster it expands.
The universe can expand well without both. Another force (or similar) is only necessary if you want the expansion to accelerate.Chronos said: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.
Chronos said: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
phinds said:That is NOT the model. Space does not expand, things just get farther apart. Google "metric expansion".
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.rede96 said: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 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 said: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 said: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.
Yes, that is the right way to view it. See the link in my signaturerede96 said: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.
We don't know what it is.Kagrat said:What is the dark energy which apparently is causing the expansion of the universe to accelerate?
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.rede96 said:Proper motion is another term that always gets me confused.
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 said:But as they are not moving *though* space but distances are increasing, then either the space is expanding or new space is being created.
Khashishi said:Proper acceleration is a more accurate term. It is possible to measure proper acceleration with an accelerometer, without using any external reference points.
Khashishi said: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.
Khashishi said:I don't think proper motion is the correct term to use in this context.
Khashishi said:Proper acceleration is a more accurate term.
Dark energy is a hypothetical form of energy that is thought to be responsible for the expansion of the universe. It is believed to make up around 70% of the total energy in the universe, but its exact nature and properties are still not fully understood.
Dark energy has a repulsive effect on matter and causes the expansion of the universe to accelerate. This means that the space between galaxies is constantly increasing, causing the universe to expand at an increasingly faster rate.
No, dark energy is still a theoretical concept and has not been directly observed. However, its existence is supported by numerous observations and measurements of the expansion of the universe.
It is highly unlikely that dark energy will cause the universe to collapse. In fact, its repulsive effect is expected to continue accelerating the expansion of the universe indefinitely.
Dark energy and dark matter are two separate and distinct concepts. While dark energy is responsible for the expansion of the universe, dark matter is a type of matter that does not interact with light and has a gravitational effect on visible matter. Dark matter is thought to make up around 27% of the total energy in the universe, while dark energy makes up around 70%.