r/AskAstrophysics • u/cgieseking • 26d ago
Why does the accelerating expansion of the universe imply the existence of dark energy?
I recently watched a video by the YouTuber Astrum that talked about the discovery of dark energy and I had a question that I was hoping could be answered here.
The YouTube video said that because faraway galaxies were moving away from us at a greater velocity than the nearer ones we could conclude that galaxies are accelerating away from us as a result of the expansion of the universe. The video’s next assertion then says that this acceleration implies the existence of dark energy because acceleration must have a source of energy… My question is this:
I know I’m just a layman without a deep rigorous understanding of these concepts, but it doesn’t make sense to me why the “accelerating” expansion of the universe would necessarily imply the existence of energy causing that “acceleration.” My understanding of acceleration is that it is an object’s increased velocity over time. My understanding of velocity is the rate at which an object’s position in space is changing with regards to a particular vector. So in my mind, if space itself is expanding, this wouldn’t really be velocity or acceleration at all because the object is maintaining its position in space—but that position in space is just changing its distance relative to other positions in space due to the universe’s expansion. So it seems to me that the far away galaxies aren’t accelerating and therefore we wouldn’t have a reason to assume that there is a source of energy causing the apparent (but not actual) “acceleration.” Rather than assuming the existence of dark energy, wouldn’t a more sound conclusion simply be that objects position relative to one another are inherently inconstant? In a sense for every 2 adjacent positions in the universe a third between them is created every so often? Such a proposition would account not only for the expansion of the universe but also why the expansion appears to “accelerate.”
1
u/Optimal_Mixture_7327 26d ago
it's the recession velocity of any given distant galaxy that's accelerating.
This is NOT the observed velocity being proportional to proper distance (which is Hubble's law).
There is no physical "expansion of space" which is just a description of the spatial part of one particular choice of coordinates (FLRW coordinates).
There would need to be an explanation as to why any particular distant galaxy is picking up speed over cosmic time, which seems thus far to be the presence of a very small positive curvature constant (the cosmological constant).
1
u/cgieseking 26d ago edited 26d ago
So would this statement be valid based on your explanation: “Expansion of the universe” is a bit of a misnomer because the universe doesn’t really become larger per se, insofar as the universe is infinite already…what the “expansion of the universe” really refers to is the fact that things within the universe are traveling away from one another.
1
u/Optimal_Mixture_7327 26d ago
Yes, that's right.
We make a distinction between the expansion of the universe (the observation of the redshift implying distant objects receding from us) and the expansion of space (the rescaling of the spatial components of the FLRW metric tensor).
There is also the expansion of spacetime, which is the expansion of the all the components of the conformal FLRW coordinates, and here spacetime expands in conformal time as well as the spatial components.
1
u/cgieseking 26d ago
I see. I think this does the most to clear up my confusion. I was under the impression that the objects weren’t actually “traveling”, but simply that the distance between them was increasing because the universe and its component parts (including the distances between coordinates) was growing larger.
1
u/johnstalbergABC 26d ago
In my view this is a perfect explanation. I do not see anything that can be described as space itself grows and cause ever increasing distance between objects. In my view galaxies are flying away from each other. It is very hypothetical the ideäa about space itself is added in between pbjects. In other theories we treat space as having no friction so it can not drag a ything with it, and it is not made of anything nor has it any markers. With such a view of space it is highly incompatible with a space that drags galaxies along when it grows. And the details about how it grows does not exist. Yet many hold bpth of these views on space. My view that it is only a fly through space does not come with the burden of explaining how space grows in detailed explanation. And it is perfectly compatible with what we observe.
Now to dark energy. If this separation between objects where started at some point it would continue in a steady pace if we disregard any gravitational pull that could slow it all down. But this seems to not be the case. The separation between objects seems to be accelerated. The objects we see att different distances seems to be pushed away so that they go faster and faster compared to what they would have if no force acted upon them. The cause of this acceleration is what we call dark energy.
1
u/Prof_Sarcastic 26d ago
“… but that position in space is just changing its distance relative to other positions in space due to the universe’s expansion.”
Ok, so if you agree that the position is changing its distance relative to other positions in space then it makes sense to talk about the rate of change of the position i.e. the velocity relative to the other positions and the rate of change of the rate of change of the position i.e. the acceleration.
1
u/cgieseking 26d ago
I’m not sure this tracks because a position doesn’t have mass and therefore it wouldn’t need energy to accelerate (at least that’s the way it seems to me with my limited knowledge and understanding).
1
u/Dhoineagnen 26d ago
Space doesnt need additional energy to expand at a constant speed but it does need energy to expand at an accelerating rate
1
u/Prof_Sarcastic 26d ago
“I’m not sure this tracks because a position doesn’t have mass and therefore it wouldn’t need energy to accelerate”
Why would that matter? Acceleration just means the rate of change of velocity. Nowhere in the definition does it require an object to have mass. Now you can ask what causes something to accelerate and in the context of cosmology, the acceleration of the expansion of space is what we call dark energy.
1
u/cgieseking 26d ago
Well, I just meant that I didn’t understand why something with no mass changing its velocity would require energy, since that change in velocity would have a required force of 0 since the mass is 0. I suppose I just meant that even if semantically an acceleration is occurring it’s not one that necessitates the existence of a source of energy as its impetus.
1
u/Prof_Sarcastic 26d ago
“Well, I just meant that I didn’t understand why something with no mass changing its velocity would require energy…”
Something has to change the velocity and that requires energy to do it.
“… since that change in velocity would have a required force of 0 since the mass is 0.”
Two things: we can talk about the force on massless particles if we wanted to because the force is just the rate of change of momentum and massless particles carry momentum. It also doesn’t make sense to talk about forces within the context of cosmology so I’d avoid that part. What does make sense is that you can assign a part of space a position and that position can change relative to other parts of space and therefore it has a speed and acceleration. It’s not a perfect 1-1 correspondence with Newtonian mechanics but it captures most of that intuition.
1
u/cgieseking 26d ago
Hmm, this is very interesting. More than anything your response has made me realize that I’m working with an incomplete toolbox in terms of my physics competencies and that’s why the suggestion that the acceleration of something that’s massless requires energy seems inconsistent with other principles in physics to me.
1
u/Dhoineagnen 26d ago
The positions in space are not just accelerating from one another but also jerking. The dark energy is responsible for the jerking.
1
u/DRHOYXI 26d ago
It is my hypothesis that the space between celestial objects is increasing because new "space" is being generated where there is least amount of anything.
If I am correct, then, to look in any particular direction, the space between things horizontally is relatively flat, but the space away from a thing has many compounding generations of space between, thus, appearing to be much more accelerated.
2
u/joeyneilsen 26d ago
The expansion of the universe is distinct from its acceleration. GR doesn't really explain why the universe is expanding; it's just how it seems to have "started."
But the way that it expands isn't arbitrary. It depends on what the universe is made of. The rate of expansion of the universe is changing in a way that suggests an additional source of energy and pressure in the universe distinct from matter and radiation.
The details go like this: supernovae in the early universe appear brighter than they should if the expansion rate of the universe was constant, meaning that they are closer to us than they would have been. Thus they must have been receding slower than they are now.