The discussion centers on the acceleration of the universe's expansion, specifically addressing the role of gravitational attraction and dark energy. Participants clarify that while gravitational attraction decreases with distance, it does not lead to acceleration; rather, it results in reduced deceleration. The cosmological constant, associated with dark energy, is identified as the primary driver of accelerated expansion. The Friedmann equation is highlighted as the correct analytical framework for understanding these dynamics.
PREREQUISITESAstronomers, cosmologists, physics students, and anyone interested in the dynamics of universe expansion and the interplay between gravity and dark energy.
The cosmological constant in modern cosmology is the vacuum energy (or "dark" energy) that drives an accelerated expansion. Without the vacuum energy there would still be expansion, but it would be slowing down.jEANNE RUPPERT said:How does/would this relate to the 'cosmological constant'?
That's like saying that a rocket fired from Earth will speed up as it gets further from the Earth, as the gravitational attraction reduces. Instead, of course, it slows down less quickly - which is equivalent to a decelerating expansion. And, depending on whether it has the required escape velocity, it either continues to move away at an ever decreasing rate, or eventually stops and falls back to Earth.Gannet said:It seems to me as the space between galaxies expands it reduces the gravitational attraction between them. Assuming that this is happening between the majority of the galaxies in our Universe it would cause the acceleration of expansion by a factor of 1/r2.
It's been explained clearly in this thread why that is very wrong.Gannet said:Prior to the discovery that the expansion of the Universe was accelerating in 1998, it was assumed that the expansion of the Universe would be decelerating because of gravitational attraction.
Since gravitational attraction is based on the Mass between the two galaxies of interest which is not changing and the inverse square of the distance between them which is changing because the space between the galaxies is expanding.
Then, what I am indicating the gravitational attraction between most of the galaxies are reducing at an inverse square rate and is contributing to the acceleration of the expansion of space. In other words, it is a positive feedback mechanism existing in the gravitational attraction formula
Yes, it would have speed up if the thrust remains constant and if it is outside the Earth's atmospherePeroK said:That's like saying that a rocket fired from Earth will speed up as it gets further from the Earth, as the gravitational attraction reduces. Instead, of course, it slows down less quickly - which is equivalent to a decelerating expansion. And, depending on whether it has the required escape velocity, it either continues to move away at an ever decreasing rate, or eventually stops and falls back to Earth.
Galaxies don't have engines, so the analogy is with a rocket cruising through space. If it has working engines, it can go where it likes.Gannet said:Yes, it would have speed up if the thrust remains constant and if it is outside the Earth's atmosphere
No, it hasn't been explained. The only onePeroK said:It's been explained clearly in this thread why that is very wrong.
The space between the galaxies is expanding, carrying the galaxies with it.PeroK said:Galaxies don't have engines, so the analogy is with a rocket cruising through space. If it has working engines, it can go where it likes.
And without dark energy that expansion must slow down. The correct analysis is to use the Friedmann equation, but heuristically it's like decelerating due to gravitational attraction.Gannet said:No, it hasn't been explained. The only one
The space between the galaxies is expanding, carrying the galaxies with it.
A post which you "liked".Bandersnatch said:No, it reduces the deceleration. Even as the force of gravity decreases with increasing distance, its direction of stays inward, and it's the only one acting (without counting dark energy).
The expansion of the universe is not like a force, but like inertial motion in a gravitational field with some initial impulse. In a universe without dark energy the expansion is always decelerating.
Same as with shooting a projectile or launching an (unpowered) space probe - the weakening gravitational field as you get higher/further doesn't cause acceleration, just reduced deceleration, and the objects rely on the initial velocity to carry them along.
Which came closest to understanding the questionPeroK said:And without dark energy that expansion must slow down. The correct analysis is to use the Friedmann equation, but heuristically it's like decelerating due to gravitational attraction.
This was explained in the second post:A post which you "liked".
We all understand the question. If you expect objects to speed up under gravity as they move further apart, then you're wrong. Unless, there is another "force" pushing them apart: the role of that force is played by dark energy. Without it, you simply have deceleration due to mutual gravity.Gannet said:Which came closest to understanding the question
What is being claimed is that Dark Energy is the cause of the acceleration of the expansion. And if the deceleration of the expansion is due to the gravitational attraction, then the opposite is true the acceleration is due to the reduction in gravitational attraction because of the positive feedback that continue to occur as the space expands even at a constant velocity.PeroK said:And without dark energy that expansion must slow down. The correct analysis is to use the Friedmann equation, but heuristically it's like decelerating due to gravitational attraction.
This was explained in the second post:A post which you "liked".
This is where the Newtonian model breaks down, as there is no such thing as "expansion of space" in Newtonian gravity. This question came up recently - how far can you push the heuristic Newtonian model?Gannet said:What is being claimed is that Dark Energy is the cause of the acceleration of the expansion. And if the deceleration of the expansion is due to the gravitational attraction, then the opposite is true the acceleration is due to the reduction in gravitational attraction because of the positive feedback that continue to occur as the space expands at a constant velocity.
OK Thanks I did not know that.PeroK said:This is where the Newtonian model breaks down, as there is no such thing as "expansion of space" in Newtonian gravity. This question came up recently - how far can you push the heuristic Newtonian model?
The model you're using maps the concept of expansion of space (GR and the Friedmann equation) to objects moving apart (Newtonian). It's one or the other. You're mixing the two.
In the Newtonian model, the expansion of space is modeled by objects moving apart. There's no additional expansion of space.
In the GR model, the objects are not moving, but space is expanding.
It's one or the other; but not both.
That's not strictly true, is it? Whether objects are moving apart or the space is expanding is due to the choice of coordinates. In comoving coordinates it's the latter, in proper coordinates it's the former. There's no real physical difference between the two.PeroK said:In the GR model, the objects are not moving, but space is expanding.
You're more than welcome to pick up from post #10:Bandersnatch said:That doesn't make it not true. And I don't see why you think comoving coordinates are more appropriate here. If anything, proper coordinates are more intuitive, as they map to Newtonian intuitions.
In any case, the issue here seems to be how the OP understands, or misunderstands, Newtonian mechanics. It doesn't look to me as having anything to do with using the wrong model.
I explained things using (implicitly) comoving coordinates. If that's not to your taste, then give an alternative explanation in proper coordinates or in a coordinate-free way.Gannet said:The space between the galaxies is expanding, carrying the galaxies with it.
Look. To ever get any acceleration, you need the nett force vector to change direction. The force of gravity, even as it's decreasing with the square of the distance, always points in the same direction. No matter how large r you choose, it is always decelerating. You need that additional effect of dark energy (or, for a rocket, it's engine being on, as opposed to unpowered flight after launch), acting as a force with opposite direction, to (eventually) cause acceleration. Only then can you talk about how the different scaling between the two forces (1/r^2 for gravity vs r for DE) leads to deceleration changing into acceleration.Gannet said:And if the deceleration of the expansion is due to the gravitational attraction, then the opposite is true the acceleration is due to the reduction in gravitational attraction
Thank you for helping me understandBandersnatch said:Look. To ever get any acceleration, you need the nett force vector to change direction. The force of gravity, even as it's decreasing with the square of the distance, always points in the same direction. No matter how large r you choose, it is always decelerating. You need that additional effect of dark energy (or, for a rocket, it's engine being on, as opposed to unpowered flight after launch), acting as a force with opposite direction, to (eventually) cause acceleration. Only then can you talk about how the different scaling between the two forces (1/r^2 for gravity vs r for DE) leads to deceleration changing into acceleration.
I assume you mean specifically the energy density associated with the cosmological constant (aka dark energy), correct?elcaro said:The feedback mechanism of the cosmological expansion is due to the simple fact that the energy density of space is constant and this energy also causes the expansion
Yes, you are right. Because (unlike any other form of energy or mass) it has negative pressure, and that is what causing the expansion. Normal forms of energy/matter and radiation only decelerate the expansion.PeterDonis said:I assume you mean specifically the energy density associated with the cosmological constant (aka dark energy), correct?