Questions about the Universe: Big Bang, Dark Matter & Expansion

[BenClark]

Let me start by stating a few points indicating my current understanding.

1. The known universe started with a big bang. The Big Bang created all matter which exists in the known universe.
2. Space (not empty space but the fabric of space - for lack of a better description) is expanding in all directions.
3. Light travels at 186,000 miles per second. (Vacuum of space, and all that).
4. The age of our universe is approximately 15 billion years old.
5. The universe is expanding. Everyone thought that it should be slowing or even collapsing.

Now for my questions. Not necessarily in any order.
1. If I can look through a telescope and see an object 15 billion light years away,
then that means that the light has been traveling for 15 billion years. That indicates to me that the source of the light was 15 billion light years away when the light started to travel to earth.
So how did the source get to be 15 billion light years away? Even if it could travel at the speed of light, it would have taken 15 billion years to get to that point. Judging by this, the universe would be at least 30 billion years old.
I know it is not as simple as just seeing how long the light took to travel the distance, etc..
However, maybe it should. Light travels at 186,000 miles per second, reqardless of the velocity of the source or the observer.
In other words, even if the source and the observer are moving away from each other at rapid speeds. If the light travels 15 billion light years, then it traveled 15 billion light years.
So, how can the universe be 15 billion years old.

2. What is all this Dark Matter and energy stuff? The argument I hear is that if you observe a spiral galaxy, you will see that the outer bands are moving around the center too fast for the amount of matter in the galaxy, so there must be some kind of Dark Magic stuff making this happen. It seems to me that since everyone pretty much agrees that space is expanding, since the outer edges of the galaxy cover a much larger section of space, then they would be effected more than the center
of the galaxy. Now it seems to me, that if space itself is expanding, then the actual distance between two objects is not really getting farther apart, only the space is increasing. We see this as an increase in distance looking from afar. So in other words, if you have two points in space, and you measure them as being 1 light year apart, then when the space between them expands, the
measured distance between them would still be 1 light year apart. So, if you have a moving object, which is moving through expanding space, then we would see the object as if it were accelerating even though its speed is constant. That being said, we would then see the outer bands of the galaxy moving faster then they should because they have to traverse the expanding space.
So, is there really some dark matter or dark energy at work?

3. How do we know that the universe is not collapsing? If space is expanding at a rate faster than the objects in space are collapsing, our observations would show that the universe is expanding, even though its just expanding space.
Continuing from number 2 above, expanding space on such a grand scale as the known universe, we would see objects that appear to be not only speeding away but also accelerating. However, if you negate the rate of expansion, I don't think that the objects would be moving away at all.

4. The Big Bang. I've heard it over and over again. But if there was a big bang, should we see all objects in the universe moving away from some point in space. I've heard that this is not the case, that objects seem to be moving is all different directions.
Some of this could be explained away by expanding space. But what if the location of the big bang was outside our known universe.
From our point of view, there may not be an origin. If our known universe is a bubble, the point in space where the big bang took place, could be far outside our bubble. If we look hard enough, we might see all the objects moving in a V shape pattern, with the point of the V being the origin of the big bang.

I am not very good at explaining this stuff so I hope I made more since than not.
I would really like to get some replies on these and even some discussions. I may be completely wrong about all or some of it but I can at least learn something from people telling me I'm crazy, as long they explain why. :-)

Thanks

 

[mgb_phys]

1. The known universe started with a big bang. The Big Bang created all matter which exists in the known universe.
2. Space (not empty space but the fabric of space - for lack of a better description) is expanding in all directions.
3. Light travels at 186,000 miles per second. (Vacuum of space, and all that).
4. The age of our universe is approximately 15 billion years old.

Correct

5. The universe is expanding. Everyone thought that it should be slowing or even collapsing.

It's currently still expanding. It's not clear wether the total amount of matter is such that it will expand for ever, stop or collapse.

1. If I can look through a telescope and see an object 15 billion light years away,
then that means that the light has been traveling for 15 billion years. That indicates to me that the source of the light was 15 billion light years away when the light started to travel to earth.

No - space can expand faster than light. Although the speed of light is the limit for information, two points in the universe can move apart faster than light.

2. What is all this Dark Matter and energy stuff? The argument I hear is that if you observe a spiral galaxy, you will see that the outer bands are moving around the center too fast for the amount of matter in the galaxy, so there must be some kind of Dark Magic stuff making this happen. It seems to me that since everyone pretty much agrees that space is expanding, since the outer edges of the galaxy cover a much larger section of space, then they would be effected more than the center

A galaxy is too small for the effects of expansion to have an effect.
If you look at the rotation speed of a galaxy and assume that all the stars are moving because they are in orbit then you can work out the mass of the galaxy.
If you do this you get a mass which is 10x as much as if you add all the stars in the galaxy together. So either our law of gravity is wrong or there is 10x as much mass that we can't see.

3. How do we know that the universe is not collapsing? If space is expanding at a rate faster than the objects in space are collapsing, our observations would show that the universe is expanding, even though its just expanding space.

We measure the speed of distant galaxies, they are rushing away from us with a speed proportional to their distance.

4. The Big Bang. I've heard it over and over again. But if there was a big bang, should we see all objects in the universe moving away from some point in space. I've heard that this is not the case, that objects seem to be moving is all different directions.

It wasn't an explosion in space - it was an explosion OF space.
Look at the sticky thread about expanding balloon model.

 

[grahammtb]

Nah, you're not crazy - its all interesting stuff!

One of my lecturers told me an interesting way to think of the Big Bang. It could well have come from a point in space, in fact it had to if everything was condensed to a 'point'. But at the same time, all the space everything was inside was also compressed into that point too. It is false to think of viewing the big bang from the 'outside' because there was no 'outside'. The big bang happened everywhere, there was no space to expand into - the space is expanding from the big bang.

I don't know if this is right, but that's what I can remember from our lecture! It was a pretty interesting one :tongue:
It doesn't really answer your questions either but oh well...

 

[BenClark]

Grahammtb - Thanks. That makes perfect sense.

mgb_phys Thanks.
I asked:
1. If I can look through a telescope and see an object 15 billion light years away,
then that means that the light has been traveling for 15 billion years. That indicates to me that the source of the light was 15 billion light years away when the light started to travel to earth.

You answered:
No - space can expand faster than light. Although the speed of light is the limit for information, two points in the universe can move apart faster than light.

That doesn't sound right. Two points in the universe cannot move apart faster than light.
(This is not a statement of fact. Could very easily be wrong)
If space is expanding, then I don't think the objects are moving. Only space is expanding.
If I placed two sticks in space that were 1 light year apart, then the space begin to expand,
it might appear to me that the two sticks are moving apart. If I go measure the distance
between them, they would still be 1 light year apart. One light year is one light year. Expansion of space doesn't translate into greater distance. Just more space.
So if the space I occupy were to expand (double its size), I would not even know it. A ruler
is one foot long. If space expands 10 times, the ruler will still be 1 foot long. If someone were
looking at the ruler from a distance, then they might see it as being longer, until they move closer.

 

[mgb_phys]

1. If I can look through a telescope and see an object 15 billion light years away,
then that means that the light has been traveling for 15 billion years. That indicates to me that the source of the light was 15 billion light years away when the light started to travel to earth.

No it doesn't. Even ignoring relativity two objects could have been a foot apart and started moving apart at very nearly the speed of light such that it took 15B years for the light form one to catch up to the other. It's no different from an (American) football player throwing a ball while running to another player running.

That doesn't sound right. Two points in the universe cannot move apart faster than light.
(This is not a statement of fact. Could very easily be wrong)

Says who?
The radius of the observable universe - ie the distance of the furthest visible objects is around 45Bn lyr but they were only a few 100M lyr away from us when the light was emitted 13Bn years ago!

If space is expanding, then I don't think the objects are moving. Only space is expanding.

No space is expanding and objects are moving. You can measure the movement of objects relative to other objects such as distant galaxies

If I placed two sticks in space that were 1 light year apart, then the space begin to expand, it might appear to me that the two sticks are moving apart. If I go measure the distance between them, they would still be 1 light year apart.

No expanding space is real - if you took a tape measure the distance would increase.

So if the space I occupy were to expand (double its size), I would not even know it. A ruler is one foot long. If space expands 10 times, the ruler will still be 1 foot long.

The rule would sty the same length because of the force between the atoms. Even clusters of galaxies are bound gravitationally and so stay together as space expands.

 

[cam875]

is there any theory that it could just be the matter within space itself which could be just a pure vacuum that is moving. So the matter which was condensed to a point at one time was basically exploded which is the big bang and the forces that acted on that matter caused it to go flying and this matter is still expanding. So basically what I am saying is there any theory stating that it is just the matter moving outwards from that original central point and not space itself that is expanding.

 

[BenClark]

mgb_phys - Before proceeding, I just want to make it clear that I don't in any way persieve this to be an argument of any kind. If I write something that could be interpreted as a negative comment, I do apologize in advance. I freely admit that I don't know very much about this stuff. I want to learn more, however, I am the type of person that wants not only to know how and why, I want to know HOW the how and why came to be accepted.
mgb_phys, thanks for your conversation and insite. I hope you will stick with me while I try to grasp all this and I hope you don't think I am trying to convince you that you are wrong and I am right, as I know I will most certainly try your patience.
:tongue2:
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BenClark
1. If I can look through a telescope and see an object 15 billion light years away,
then that means that the light has been traveling for 15 billion years. That indicates to me that the source of the light was 15 billion light years away when the light started to travel to earth.

mgb_phys
No it doesn't. Even ignoring relativity two objects could have been a foot apart and started moving apart at very nearly the speed of light such that it took 15B years for the light form one to catch up to the other. It's no different from an (American) football player throwing a ball while running to another player running.

BenClark
I believe that is incorrect. I just recently watched a show called 'The Universe' in which several world class scientists stated that 'Regardless of the objects speed and direction, the speed of light is constant' (kaku ? was one of them, didin't catch the names of the others)
In fact, they gave an example similar to your football player. There example was a guy throws a tennis ball while standing still versus throwing the ball while riding a bike. The ball went farther when being thrown from the moving bike. However, when he shines a light, standing still versus riding a bike, regardless of speed, light always moved the same distance at the same speed.
Do you not agree that the faster a body moves, the slower time passes reletive to that body?
If so, two objects moving away from each other at high speeds would each be experiencing the slow passage of time. So if we are standing on one of those bodies, how do we know that our perception of 15 Billion years is really only 100 years in normal time?
I know they measure speed and distance based on red shift of light. However, if the source of the light were stationary, and the light traveled through expanding space, the light would still show a red shift, which would indicate movement of the source object, even though the object is stationary.
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BenClark
That doesn't sound right. Two points in the universe cannot move apart faster than light.
(This is not a statement of fact. Could very easily be wrong)

mgb_phys
Says who?
The radius of the observable universe - ie the distance of the furthest visible objects is around 45Bn lyr but they were only a few 100M lyr away from us when the light was emitted 13Bn years ago!

BenClark
From the outside looking in, it appears that the two objects are moving faster than light. What I am saying is there is a differece between actual objects moving and the appearance of objects moving do to expanding space. When space expands, it carries the objects with it and we see this as if the objects are moving. So my question is, how do we know that the universe is expanding. (Not the expansion of space, but objects moving outward from each other) If you could instantly stop the expansion of space, would we see a bunch of objects moving away from each other, standing still or moving towards each other?
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BenClark
If space is expanding, then I don't think the objects are moving. Only space is expanding.

mgb_phys
No space is expanding and objects are moving. You can measure the movement of objects relative to other objects such as distant galaxies

BenClark
You say that you can measure the movement of objects relative to other objects. How do you know that you are not measuring the expansion of space?
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BenClark
If I placed two sticks in space that were 1 light year apart, then the space begin to expand, it might appear to me that the two sticks are moving apart. If I go measure the distance between them, they would still be 1 light year apart.

mgb_phys
No expanding space is real - if you took a tape measure the distance would increase.

BenClark
So if the space I occupy were to expand (double its size), I would not even know it. A ruler is one foot long. If space expands 10 times, the ruler will still be 1 foot long.

mgb_phys
The rule would sty the same length because of the force between the atoms. Even clusters of galaxies are bound gravitationally and so stay together as space expands.

BenClark
How do you know this to be a fact? If the space I occupy expands, and I expand with it, how could I possibly know? We see the universe as this extremely large (infinit) thing. How do you know that our universe is not just a quark (or some other sub-atomic particle) in someone elses universe? (Obviosly I don't believe this but, how do we know that its not true, or at least something similar)

 

[mgb_phys]

'Regardless of the objects speed and direction, the speed of light is constant'

That's correct but not quite the same thing. I was responding to your statement that for light to have taken 15Gyr to reach us the object must have been 15Gyr away when the light was emitted - that is only true for two fixed objects.
The speed of light is (say) 300,000,000 m/s. Suppose you have two objects that are 1 m apart when a light beam is sent from one to the other, but they are moving apart at 299,999,999.999 m/s.
After 1 sec they will be 1+299,999,999.999m apart but the light will only have gone 300,000,000 m, but eventually the light will catch up.

Similarly with the edge of the observable universe (or the CMB), it was 100M lyr away when the light was emitted around 14Gyr ago and we have moved apart as space as expanded so that we are now 45G lyr apart.

The expanding space/expanding ruler is a classic question.
The expansion of space moves distant objects apart, we see this in Hubble's law - distant galaxies are moving away from us in all directions at a speed proportional to their distance. Note this doesn't mean we are at the center, any galaxy would see all the others expanding away form it - that's because it is space that is expanding.
But on smaller scales objects are held together by gravity (such as the solar system) or atomic forces in the ruler. so if we look at the closest galaxies around us they aren't moving away due to the expansion of the universe - they are swirling around because of gravity , the expansion is only a bigger effect on large scales.
A good picture might be dropping some rubber ducks in the ocean - in general they will all drift apart but the individual ducks don't expand.

 

[BenClark]

OK. I get what your saying, but it still doesn't make since.

When I talk about expanding space, I don't mean space that can be measured in distince.
I mean, for lack of a better description, the fabric of space. The place where we exists.
It seems to me, that is what is expanding.
Just like a large body warps space around it, we call it gravity.
We exist in this space (fabric) but are not aware of it. We can't see it or touch it, just as we can't see the warping of space (fabric) caused by large bodies.
However, we can feel the effects of the large body, Gravity.

On the other hand, there is space, as in a measurable distince.
What I am saying is that space (fabric) is expanding but the space that we see as distance is not expanding. 1 foot is 1 foot. It doesn't change.
So, if the space (fabric) that the 1 foot occupies, expands, the 1 foot is still 1 foot because the distince didn't increase, only the space (fabric) increased.
In other words, everything that exists in this space (fabric) is expanding with the space (fabric) but they are not growing in size.
So as the space (fabric) expands, objects appear to excellerate as they move in their own space because they are having to move across the greater streched out space (fabric)
If an object is not moving, sitting absolutely motionless, it would appear to us to be moving because the space (fabric) is expanding. The effect becomes greater with distince, so object which are billions of light years away seam to be moving faster. The farther away they are, the faster they seem to be moving.
As far as the red shift, that still holds. As light passes through the streched space, the leading edge of the wave passes into the expanded space first, which causes it to expand. So the wave becomes streched out, giving the red shift.

So, I'm not saying you are wrong, but this makes more since to me than your explanation.
What am I missing?

My explanation even explains the rotational speeds of a galaxy. If you take a small circle at the center of a galaxy, say with a circumference of 1 light year, and compare it to the outer edges of the galaxy with a circumference of tens of thousands of light years or even millions, the expanding space would have a much greater effect on the outer edges. If a given point in space is expanding a 100% a year (just made this number up), then the center would expand to 2 light years (not really accurate but you get the idea) and the outer ring would double as well. So the inner 1 ly becomes 2 ly and the outer changes from 500,000 ly to 1,000,000 lr. A much greater expansion. What we would see when we look at a galaxy is a galaxy that the outer area is spinning too fast for its total mass.
Does this not make since?
If everyone was saying that the entire galaxy was spinning to fast, then that would be different. But everything I have seen and heard says that the spiral arms are spinning too fast.

 

[DrChinese]

OK. I get what your saying, but it still doesn't make since.

You ask what you are missing... and the answer is that there is a tremendous amount of experimental evidence which has brought us to this point. These are not random guesses or wishful thinking.

For example: numerous objects - ones that act as "standard candles" to astronomers - have been discovered with high z (i.e. indicating high redshift). These high z objects are very old, and we are seeing them as they looked in the first billion years of the universe. Importantly, they are receding from us at over twice the speed of light. This is very convincing evidence that spacetime itself is expanding.

In addition, the older objects are receding faster than the newer objects, indicating that the expansion is continuing. Current simulations indicated that the expansion is accelerating.

So the point is: you may find it more useful to read more rounded works to gain a better understanding of the situation. The TV shows tend to put a lot of emphasis on the "WOW" and "GEE WHIZ" factors, because it takes a lot of time to develop the actual reasoning that leads to our current understanding. The show is too short to present this, and besides it requires the integration of a lot of different elements of physics knowledge.

One thing: this is a VERY exciting time for Cosmology, as the last 10 years or so has been filled with many exciting new and unexpected discoveries (dark matter, ongoing expansion of space-time, etc.)

 

[CosmologyTPA]

I have some of these same questions, but more related to gravitational lensing and distance measurements... but I won't hijack this thread with that topic.

Since the space is being created as the matter expands, then the matter is "riding a wave of warped space" and traveling faster than the speed of light, but with no frame of reference because there is nothing outside of that space to measure it's speed. We cannot determine how fast space is expanding because we do not have a reference point for measurement.

Which leads me to a very interesting question, recently it has been theorized that light matter actually "floats" on a flat, dark matter brane. It was the intersection of two dark matter branes that caused the "big bang" (which would be better described as the "big impact") and created light matter. IF that is the case, then as we look back in time to this "big impact", are we looking across a flat, plane of the dark matter brane?