S.o.L vs The Big Bang

  • #1

Main Question or Discussion Point

Hello,

My question is regarding the speed of the Big Bang explosion.

As I understand it, within the first 10 minutes of the Big Bang the universe had expanded hundreds of light years (this of course being faster than the Speed of Light). Bringing me to my question, if nothing can travel faster than the Speed of Light, then surely the Big Bang explosion would not have been able to exceed that speed at the suggested growth rate?
 

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  • #2
phinds
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The creation of distance is NOT the same as things moving in the same frame of reference. For example, at present, the objects that we are currently getting the light from at the edge of our observable universe are receding from us at approximately 3 times the speed of light. That's because the geometry is changing, NOT because anything is moving faster than c. Nothing moves faster than c, but distances CAN change faster than c. UN-intuitive, but true.
 
  • #3
Thank you for the reply phinds. I'm not an expert so please forgive my stupidity, but I am still finding it difficult to contemplate.

I can understand that the distance between Earth and a Star on the edge of the Galaxy are receding away at 3 times the speed of light and that is measured in distance, not speed, but that is the distance between two objects where as the Big Bang is a singular object (object may not be the best description here...).

The Big Bang came from one central point and expanded outwards. Which is what I am failing to comprehend?
 
  • #4
phinds
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The Big Bang came from one central point and expanded outwards. Which is what I am failing to comprehend?
No, there is no "point" from which the big bang expanded. If that were the case, then the universe would not exibit its two fundamental attributes of isotropy and homegeneity which are the foundations of modern cosmology.

The expansion of the very early universe ("inflation") is not well understood [actually, I'm not sure it is understood AT ALL] but it was surely a creation of distance, NOT a case of things exceeding the speed of light.
 
  • #5
Drakkith
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Thank you for the reply phinds. I'm not an expert so please forgive my stupidity, but I am still finding it difficult to contemplate.

I can understand that the distance between Earth and a Star on the edge of the Galaxy are receding away at 3 times the speed of light and that is measured in distance, not speed, but that is the distance between two objects where as the Big Bang is a singular object (object may not be the best description here...).

The Big Bang came from one central point and expanded outwards. Which is what I am failing to comprehend?
As Phinds stated the Big Bang is thought to have occured throughout the entire universe and is not an "explosion in space from a single point". The easiest way to understand inflation and expansion, although not technically correct, is that nothing can move THROUGH space at greater than the speed of light, however space itself has no such limit and can "carry" things with it as it expands.

Imagine you are in an above ground swimming pool and can only swim up to 5 mph through the water. If the swimming pool suddenly bursts and empties all over your yard while you were swimming away from your friend, you are still swimming 5 mph THROUGH the water, but now the water is carrying you with it at a velocity greater than 5mph away from your friend. Again that isn't technically what's happening but is just a way of visualizing it.
 
  • #6
Does this mean gravity can "not accelerate" me faster than the speed of light towards an object then? I have trouble understand force-less gravity motion so I think of it as the space being consumed by mass, so to speak, and dragging the objects with it
 
  • #7
Thank you for your patience. That swimming pool metaphor really helps!

However, Stephen hawking himself said that if you converge everything in the cosmos it returns to one central point in space. Which makes sense, for instance, if you take a nuclear bomb exploding, the explosion comes from one central point (specifically the point in space where the atom is split in two - which is in fact one central point) and expands outwards (or in the Big Bangs case, inflates).
 
  • #8
Does this mean gravity can "not accelerate" me faster than the speed of light towards an object then? I have trouble understand force-less gravity motion so I think of it as the space being consumed by mass, so to speak, and dragging the objects with it
As I understand it, gravity cannot accelerate you faster than the speed of light. You can use the force of gravity to accelerate, as shown with quite a few spacecraft throughout history, but whether it is possible to 'reach' the speed of light using this method, I'm not sure.
 
  • #9
DaveC426913
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As I understand it, gravity cannot accelerate you faster than the speed of light. You can use the force of gravity to accelerate, as shown with quite a few spacecraft throughout history, but whether it is possible to 'reach' the speed of light using this method, I'm not sure.
It is not.
 
  • #10
Drakkith
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Thank you for your patience. That swimming pool metaphor really helps!

However, Stephen hawking himself said that if you converge everything in the cosmos it returns to one central point in space. Which makes sense, for instance, if you take a nuclear bomb exploding, the explosion comes from one central point (specifically the point in space where the atom is split in two - which is in fact one central point) and expands outwards (or in the Big Bangs case, inflates).
I haven't read or seen whatever cosmos is, but I have to believe that he means the "observable universe", which is just the large sphere around us that we can see. Beyond a certain distance we cannot see anything however it is believed that there is still plenty more stuff out there and that the universe is either infinite or finite but without bounds. (Meaning that you go in one direction long enough and you eventually come back around to your starting point.)
 
  • #11
It is not.
Is what Drakkith said:
However space itself has no such limit and can "carry" things with it as it expands
correct? and if so, how does gravity works in relativity without being a force if not that space "carries" test particles nearer the object on a geodesic without changing their velocity?

and lastly, if those are both the case, how does the carrying differ in gravity and inflation?
 
  • #12
DaveC426913
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Is what Drakkith said:

correct?
Yes.

and if so, how does gravity works in relativity without being a force if not that space "carries" test particles nearer the object on a geodesic without changing their velocity?

and lastly, if those are both the case, how does the carrying differ in gravity and inflation?
If Milky Way and Andromeda were 2 million light years apart, and then an amount of space were inserted between them - say 2 million light years - they would now be 4 million light years apart. No force has acted on them and the galaxies themselves have not been accelerated at all, let alone faster than light.

Space doesn't literally get inserted this way, I'm simply trying to demonstrate how the expansion of space can result in distances increasing without acting as forces or acting to move objects.
 
  • #13
Right, but during inflation space did something different than normal, here it is being called creation of distance, surely this creation of distance carried the mass suspended within it along, as per the previous description. So if it is the case that mass being carried by "moving space" does not accelerate the mass, but causes it's distance to change, then is that also a description of what is going on in relativity's gravity, whereby distance changes without acceleration?
 
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  • #14
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Right, but during inflation space did something different than normal, here it is being called creation of distance, surely this creation of distance carried the mass suspended within it along, as per the previous description. So if it is the case that mass being carried by "moving space" does not accelerate the mass, but causes it's distance to change, then is that also a description of what is going on in relativity's gravity, whereby distance changes without acceleration?
EDIT: I misunderstood your question, but I'll leave the post anyway... :-)

Acceleration is defined with respect to some inertial frame. You must consider what happens to your particle as well as what happens to its inertial frame during the modification of spacetime through e.g. inflation. The particle is all the time at rest in its own inertial frame during this process, since the inertial frame itself is "carried along" with the inflation in the same way that the particle is "carried along".

Therefore, the particle is not accelerating. Newton's laws are still valid at low speeds in this inertial frame even during such an inflationary cosmological phase. And there is no law that limits how much the physical distance between two particles can increase due to modification of spacetime through the evolution of Einstein's equation.

At least that was my understanding of this.
 
  • #15
I haven't read or seen whatever cosmos is, but I have to believe that he means the "observable universe", which is just the large sphere around us that we can see. Beyond a certain distance we cannot see anything however it is believed that there is still plenty more stuff out there and that the universe is either infinite or finite but without bounds. (Meaning that you go in one direction long enough and you eventually come back around to your starting point.)
Yes but this still implies that there is a starting point for the Big Bang, which is my initial point. I think I understand the concept of what is being said here, though it is just the concept that the Big Bang did not have a central starting point that eludes me! Ohh my sleep will be disrupted this night!
 
  • #16
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Yes but this still implies that there is a starting point for the Big Bang, which is my initial point. I think I understand the concept of what is being said here, though it is just the concept that the Big Bang did not have a central starting point that eludes me! Ohh my sleep will be disrupted this night!
The inflating balloon analogy is pretty good. If you live on the surface of an inflating balloon. Also, you can only detect things that happen on the balloon surface, because e.g. light moves along the balloon surface in this analogy. There is no "ambient space" like we have when looking at a balloon, of course.

Then there is no starting point for the inflation process. At one instant there is just a point, i.e. the big bang, and at any time after that there is a 2-sphere. It just gets bigger and bigger. That's why some say that the big bang happened "everywhere" at once.
 
  • #17
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The concept of the Big Bang not having a starting point is not a problem if you redefine "Big Bang" to be in accordance with what the theory currently states (no starting point). The confusion arises because it's commonly taught as an explosion which we all know has a point of origin.

Don't think "explosion". Just think "spacetime that suddenly expanded a long time ago, and cooled, and now we live in it". Nobody ever said it magically acquired some extra dimensions and went from point to volume. It was volume, then it got bigger.

What happened before that? We can only guess. But putting any faith behind a guess based on everyday experience is probably not a good idea.
 
  • #18
Thank you for the replies everyone :)
 
  • #19
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Space doesn't literally get inserted this way, I'm simply trying to demonstrate how the expansion of space can result in distances increasing without acting as forces or acting to move objects.
I've heard of a lot of analogies to help explain spaceinflation, but these are always combined with the clause that space doesn't literally inflates in that way. What is what literally happens? How well understood is what exactly happens and what are the differences in observable results with inserting space like in all the analogies?
 
  • #20
phinds
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I've heard of a lot of analogies to help explain spaceinflation, but these are always combined with the clause that space doesn't literally inflates in that way. What is what literally happens? How well understood is what exactly happens and what are the differences in observable results with inserting space like in all the analogies?
All we really know is that distances increase. Everything else is just speculation, far as I know.
 
  • #21
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All we really know is that distances increase. Everything else is just speculation, far as I know.
I see, thank you. Even if not understood what exactly inflation is, are the hyperinflation at the big bang and the ever increasing inflation that is currently happening thought to be the same processes? Or is there such a difference between the two so that they couldn't be the same?
 
  • #22
Then there is no starting point for the inflation process. At one instant there is just a point, i.e. the big bang, and at any time after that there is a 2-sphere. It just gets bigger and bigger. That's why some say that the big bang happened "everywhere" at once.
So while you are saying "at one instant there is just a point", but during that instant if we were to measure this point, would we measure it as being the size of the universe due to the fact that our rules are also infintesimally small? If not, then how is that point not the starting point?
 
  • #23
phinds
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So while you are saying "at one instant there is just a point", but during that instant if we were to measure this point, would we measure it as being the size of the universe due to the fact that our rules are also infintesimally small? If not, then how is that point not the starting point?
That post used a poor choice of words. As it is currently understood, there was no "POINT" other than a point in time. The universe was an incredibly dense plasma of energy, of indeterminate size, at the Plank Time and expanded from there.
 
  • #24
phinds
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I see, thank you. Even if not understood what exactly inflation is, are the hyperinflation at the big bang and the ever increasing inflation that is currently happening thought to be the same processes? Or is there such a difference between the two so that they couldn't be the same?
There really seem to be 3 aspects of expansion, although I don't know but what they are just different attributes/states of the same thing.

The first is the inflation that occurred during the "inflationary period", some TINY fraction of a second after the Plank Time. This cause a staggerinly large increase in the size of the Universe. This was WAY different in magnitude than other expansion, but who knows if it was qualitatively different.

After that there was the "normal" expansion of the Universe that goes on to this day.

Then there is the ACCELERATION of that "normal" expansion starting about half-way back in the life of the universe when "dark energy" (whatever it is), which seems to have been around all along, finally was able to overcome gravity because "normal" expansion had moved everthing far enough apart that the effect of gravity COULD be overcome by this locally TINY "force" (or whatever it is).
 
  • #25
What do you mean by "at the Plank Time" I thought plank time was a unit of time?

Then there is the ACCELERATION of that "normal" expansion starting about half-way back in the life of the universe when "dark energy" (whatever it is), which seems to have been around all along, finally was able to overcome gravity because "normal" expansion had moved everthing far enough apart that the effect of gravity COULD be overcome by this locally TINY "force" (or whatever it is).
So you are saying that dark energy was there all along, contributing slightly to "normal expansion"? So "normal expansion" was able to expand objects beyond the range of gravity, to where dark energy's contribution to "normal expansion" could overcome gravity....this makes no sense to me. You start off saying normal expansion started half way back...then say...when normal expansion had moved everyhting...
 

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