Expanding Universe Slowing Down and Then Speeding Up

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The discussion centers on the expansion of the universe, highlighting that it began with a rapid expansion rather than an explosion, known as "inflation." Initially, the universe's expansion was decelerating due to gravitational forces, but approximately 5 billion years ago, dark energy began to dominate, causing the acceleration of expansion. Participants clarify that dark matter contributes to gravitational effects, while dark energy has properties that counteract gravity. There is also a distinction made regarding Hubble's observations, emphasizing that he identified the relationship between distance and speed of galaxies but did not directly measure acceleration. Overall, the conversation seeks to clarify misconceptions about the nature of cosmic expansion and the roles of dark matter and dark energy.
robdelory
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Hubble observed that not only is the universe expanding but that it is accelerating. With the model of the Big Bang as I understand it, the universe began as a big explosion. That would mean that it expanded at a rate exceeding the speed of light or at least close to it. If that is correct, that means the universe over the course of 15+ Billion years had expanded at a high rate of speed, slowed down and is currently speeding up it's expansion again. Is this a correct way to describe it or has this been explained? If so, how can that be?
 
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robdelory said:
Hubble observed that not only is the universe expanding but that it is accelerating. With the model of the Big Bang as I understand it, the universe began as a big explosion. That would mean that it expanded at a rate exceeding the speed of light or at least close to it. If that is correct, that means the universe over the course of 15+ Billion years had expanded at a high rate of speed, slowed down and is currently speeding up it's expansion again. Is this a correct way to describe it or has this been explained? If so, how can that be?

Well, sort of.

First, it most emphatically did NOT start as a big explosion, it started as a big expansion, not from a single point as an explosion does but from everywhere at once (a bit hard to grasp at first, I know). Yes, there was (not 100% conclusive, but strongly believed to be true) a massive expansion in a tiny amount of time, called "inflation", followed for some 8 or 9 billion years by a more sedate expansion, and then at some point dark energy (whatever that is) which had been around all along but was overcome by gravity started, due to the spreading out of and thus the decreased density of matter, to overcome gravity and the expansion began to accelerate.
 
http://en.wikipedia.org/wiki/Metric_expansion_of_space

According to measurements, the Universe's expansion rate was decelerating until about 5 billion years ago due to the gravitational attraction of the matter content of the Universe, after which time the expansion began accelerating.

Edit: Is it a coincidence that the transition from decelerating to accelerating occurred about the time the Earth formed (4.5b years ago)?
 
phinds said:
Well, sort of.

First, it most emphatically did NOT start as a big explosion, it started as a big expansion, not from a single point as an explosion does but from everywhere at once (a bit hard to grasp at first, I know). Yes, there was (not 100% conclusive, but strongly believed to be true) a massive expansion in a tiny amount of time, called "inflation", followed for some 8 or 9 billion years by a more sedate expansion, and then at some point dark energy (whatever that is) which had been around all along but was overcome by gravity started, due to the spreading out of and thus the decreased density of matter, to overcome gravity and the expansion began to accelerate.
I think that was dark matter slowed down the expansion at first. then dark energy accelerates the expansion? I didn't study inflammation theory much, but I know the dark matter is gravity, dark energy is anti gravity.
 
sunmaggot said:
I think that was dark matter slowed down the expansion at first. then dark energy accelerates the expansion? I didn't study inflammation theory much, but I know the dark matter is gravity, dark energy is anti gravity.
ALL of the mass of the universe was slowing down the expansion, until the spreading out of all matter allowed DE to overtake it as the main consideration for the expansion.

Dark matter is not "gravity", it's just matter. All matter creates the spacetime curvature that we call gravity.

Dark energy is not exactly what the term "anti-gravity" normally means, although it does have an effect opposite that of gravity so from an English language point of view, that's not a terrible way to describe it.
 
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robdelory said:
Hubble observed that not only is the universe expanding but that it is accelerating. With the model of the Big Bang as I understand it, the universe began as a big explosion. That would mean that it expanded at a rate exceeding the speed of light or at least close to it. If that is correct, that means the universe over the course of 15+ Billion years had expanded at a high rate of speed, slowed down and is currently speeding up it's expansion again. Is this a correct way to describe it or has this been explained? If so, how can that be?
Hubble didn't observe that. The way we find out expansion speed is accelerated is by calculating mass density of the whole universe. And I have also asked about the question of expansion speed exceeding light speed in a talk. The professor (not from my university) said you don't need energy to accelerate something, therefore exceeding light speed is no problem, bla bla bla. I don't understand... dark energy is a kind of energy. What was he even talking about lol.
 
sunmaggot said:
The professor (not from my university) said you don't need energy to accelerate something, therefore exceeding light speed is no problem, bla bla bla. I don't understand... dark energy is a kind of energy. What was he even talking about lol.
The expansion of the universe is not acceleration in a local sense, which DOES require energy, and the expansion is not a local "speed" so is not limited to the speed of light.
 
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phinds said:
The expansion of the universe is not acceleration in a local sense, which DOES require energy, and the expansion is not a local "speed" so is not limited to the speed of light.
wow thanks, this explanation makes more sense!
 
sunmaggot said:
Hubble didn't observe that. The way we find out expansion speed is accelerated is by calculating mass density of the whole universe. And I have also asked about the question of expansion speed exceeding light speed in a talk. The professor (not from my university) said you don't need energy to accelerate something, therefore exceeding light speed is no problem, bla bla bla. I don't understand... dark energy is a kind of energy. What was he even talking about lol.
What do you mean Hubble didn't observe that. He found the red shift in distant galaxies. The farther away, the higher the shift. Again, I'm a novice here, but still...
 
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robdelory said:
What do you mean Hubble didn't observe that. He found the red shift in distant galaxies. The farther away, the higher the shift. Again, I'm a novice here, but still...
he only found out stars farther away from us move away with higher speed, which makes hubble's law: v = Hd, he didn't find out the acceleration.
 
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sunmaggot said:
he only found out stars farther away from us move away with higher speed, which makes hubble's law: v = Hd, he didn't find out the acceleration.
Cool, thanks for the clarification.
 
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