The Expanding Universe

[RBerman]

At a recent trip to a planetarium in Manhattan, I saw a nifty show about dark matter. During the show, it was mentioned that we can only see a small piece of the whole universe, because light from the rest of the universe hasn't reached us yet. That made me wonder about the Big Bang. Is the universe supposed to be expanding faster than the speed of light? If not, we should be able to see the whole universe. Even when two objects move apart from each other at the speed of light in opposite directions, each appears to the other to only be moving at the speed of light, right? I'm sure there's a relativistic explanation. I'm not sure I could understand it, but fire away!

[Leonhard]

At a recent trip to a planetarium in Manhattan, I saw a nifty show about dark matter. During the show, it was mentioned that we can only see a small piece of the whole universe, because light from the rest of the universe hasn't reached us yet. That made me wonder about the Big Bang. Is the universe supposed to be expanding faster than the speed of light? If not, we should be able to see the whole universe. Even when two objects move apart from each other at the speed of light in opposite directions, each appears to the other to only be moving at the speed of light, right? I'm sure there's a relativistic explanation. I'm not sure I could understand it, but fire away!

Basically there's a difference between something that exhibits relative velocity, and the distance itself changing between two objects. The distance to an object can be scaled as quickly as you want. This can be couched in terms of "This galaxy is moving away from us faster than the speed of light" what is actually meant though is that distance out to that object is lengthened more than 3e8km every second.

Its mindboggling but there is a distinction between these two, because basically every point in space on a large enough scale are moving away from other points. We're not expanding away from a central point. No matter were you are in the universe you'd see galaxies "moving" away from you, and their "velocity" would be proportional to the distance to them.

Its a nasty ambiguity between motion and space-time itself stretching.

We don't know how small our piece of the universe is, I've seen estimates, but its all based on physics we don't have but are only guestimating about. If the theory of inflation is true, then the visible universe could comprise as small a part of the universe as one part out of 10e30 (that's ten to the power of thirty).

[Cerebrum123]

If space is expanding, then shouldn't the space between our molecules, and atoms, etc. be expanding too? Shouldn't certain things be falling apart in that scenario?

[Leonhard]

If space is expanding, then shouldn't the space between our molecules, and atoms, etc. be expanding too?

Yup, but because the distance between atoms is so tiny this becomes just a very weakly repulsive force which is vastly overcome by the bonds. All it means is that atomic orbits are infinitesimally larger than they would be in a static universe. Its only on the scale of the distances between galaxies... actually even further... between local groups that nothing can overcome it.

[Cerebrum123]

Yup, but because the distance between atoms is so tiny this becomes just a very weakly repulsive force which is vastly overcome by the bonds. All it means is that atomic orbits are infinitesimally larger than they would be in a static universe. Its only on the scale of the distances between galaxies... actually even further... between local groups that nothing can overcome it.

Okay.

[RBerman]

Basically there's a difference between something that exhibits relative velocity, and the distance itself changing between two objects. The distance to an object can be scaled as quickly as you want. This can be couched in terms of "This galaxy is moving away from us faster than the speed of light" what is actually meant though is that distance out to that object is lengthened more than 3e8km every second.

Its mindboggling but there is a distinction between these two, because basically every point in space on a large enough scale are moving away from other points. We're not expanding away from a central point. No matter were you are in the universe you'd see galaxies "moving" away from you, and their "velocity" would be proportional to the distance to them.

Its a nasty ambiguity between motion and space-time itself stretching.

We don't know how small our piece of the universe is, I've seen estimates, but its all based on physics we don't have but are only guestimating about. If the theory of inflation is true, then the visible universe could comprise as small a part of the universe as one part out of 10e30 (that's ten to the power of thirty).

I had a feeling I wouldn't be able to understand the explanation. Are you saying that an observer in one galaxy only observes the other galaxy moving away at speed X, but An observer watching the two galaxies separate would see the space between them grow at speed 2X, or perhaps more?

The planetarium show did mention that the universe is not expanding away from a central point, but I still don't understand that part. I mean, if stuff is moving farther away, and if the space in between them is independently expanding, didn't it all start at a single point? (A point in what?)

Is this expansion of space/time itself something that can be measured directly, or is it more of an inference from the idea that we can't see the whole universe?

[shunyadragon]

Yup, but because the distance between atoms is so tiny this becomes just a very weakly repulsive force which is vastly overcome by the bonds. All it means is that atomic orbits are infinitesimally larger than they would be in a static universe. Its only on the scale of the distances between galaxies... actually even further... between local groups that nothing can overcome it.

Actually no the objects, ie galaxies and black holes are moving outward through the medium of the Quantum Vacuum. The Quantum Vacuum itself is not expanding.

[klaus54]

Actually no the objects, ie galaxies and black holes are moving outward through the medium of the Quantum Vacuum. The Quantum Vacuum itself is not expanding.

"Outward", "Medium", "Quantum Vacuum" -- I don't understand these terms in this context.

K54

[shunyadragon]

"Outward", "Medium", "Quantum Vacuum" -- I don't understand these terms in this context.

K54

'Outward' means expansion from the point of origin of the Big Bang, The 'Medium' we expand through is the Quantum Vacuum at zero point energy.

Source: http://en.wikipedia.org/wiki/Zero-point_energy

Zero-point energy, also called quantum vacuum zero-point energy, is the lowest possible energy that a quantum mechanical physical system may have; it is the energy of its ground state. All quantum mechanical systems undergo fluctuations even in their ground state and have an associated zero-point energy, a consequence of their wave-like nature. The uncertainty principle requires every physical system to have a zero-point energy greater than the minimum of its classical potential well. This results in motion even at absolute zero. For example, liquid helium does not freeze under atmospheric pressure at any temperature because of its zero-point energy.

The concept of zero-point energy was developed in Germany by Albert Einstein and Otto Stern in 1913, as a corrective term added to a zero-grounded formula developed by Max Planck in 1900.[1][2] The term zero-point energy originates from the German Nullpunktsenergie.[1][2] An alternative form of the German term is Nullpunktenergie (without the "s").

Vacuum energy is the zero-point energy of all the fields in space, which in the Standard Model includes the electromagnetic field, other gauge fields, fermionic fields, and the Higgs field. It is the energy of the vacuum, which in quantum field theory is defined not as empty space but as the ground state of the fields. In cosmology, the vacuum energy is one possible explanation for the cosmological constant.[3] A related term is zero-point field, which is the lowest energy state of a particular field.

© Copyright Original Source

[RBerman]

Actually no the objects, ie galaxies and black holes are moving outward through the medium of the Quantum Vacuum. The Quantum Vacuum itself is not expanding.

So in your understanding, is the distance between the galaxies increasing more rapidly than the speed of light? They'd have to be, I would have thought, in order for some parts of the universe have gotten so far from us that light from them has never reached us. Or perhaps in this theory, they got very far from us and then later started emitting light?

[oxmixmudd]

So in your understanding, is the distance between the galaxies increasing more rapidly than the speed of light? They'd have to be, I would have thought, in order for some parts of the universe have gotten so far from us that light from them has never reached us. Or perhaps in this theory, they got very far from us and then later started emitting light?

In fact there are galaxies we can observe whose redshift implies a motion away from us greater than the speed of light. How to we still see them?

This is how I understand it:

A simple way to sort of get a feel for what is happening is to imagine the surface of a balloon that is being inflated, with dots on its surface. As it expands, all the dots move apart. But if you were an ant traveling on the surface, your proper motion relative to a dots would be the sum of your own speed and the 'speed of stretching'. So that, if you were to start a journey to another dot, even if its speed away from you starting at the source dot was greater than you could walk, you would still (eventually) get there because as you move toward the dot, the speed of stretching relative to your target dot would always be decreasing and eventually falls below your walking speed*. So, even though some galaxies are so far away their red shift implies they are moving away faster than light, their light still reaches us because light is traveling along the medium that is streatching. But the light itself gets stretched too in the process, hence the 'red shift' we observe.

*nasty caveat: We now know the universe expansion rate is increasing. So, it is theoretically possible then that the increase in expansion rate can create a scenario where light can never cross the distance. Indeed, it is postulated that in many trillions of years all observable galaxies will be invisible due to this phenomenon.


Jim

[oxmixmudd]

'Outward' means expansion from the point of origin of the Big Bang, The 'Medium' we expand through is the Quantum Vacuum at zero point energy.

Source: http://en.wikipedia.org/wiki/Zero-point_energy

Zero-point energy, also called quantum vacuum zero-point energy, is the lowest possible energy that a quantum mechanical physical system may have; it is the energy of its ground state. All quantum mechanical systems undergo fluctuations even in their ground state and have an associated zero-point energy, a consequence of their wave-like nature. The uncertainty principle requires every physical system to have a zero-point energy greater than the minimum of its classical potential well. This results in motion even at absolute zero. For example, liquid helium does not freeze under atmospheric pressure at any temperature because of its zero-point energy.

The concept of zero-point energy was developed in Germany by Albert Einstein and Otto Stern in 1913, as a corrective term added to a zero-grounded formula developed by Max Planck in 1900.[1][2] The term zero-point energy originates from the German Nullpunktsenergie.[1][2] An alternative form of the German term is Nullpunktenergie (without the "s").

Vacuum energy is the zero-point energy of all the fields in space, which in the Standard Model includes the electromagnetic field, other gauge fields, fermionic fields, and the Higgs field. It is the energy of the vacuum, which in quantum field theory is defined not as empty space but as the ground state of the fields. In cosmology, the vacuum energy is one possible explanation for the cosmological constant.[3] A related term is zero-point field, which is the lowest energy state of a particular field.

© Copyright Original Source

I don't think it is correct to consider the expansion as 'movement' through the vacuum as that is what the speed of light limit is all about. Nothing can propagate through space-time at >c. However, space-time can change/expand. and that is what expansion is, and the galaxies themselves are simply carried along by it because there is not a force sufficiently strong to overcome it as there is within the galaxies themselves, or in planetary systems etc.

However, theoretically this expansion will become the dominant force in the universe such that nothing, not even the atomic forces can overcome it and the universe at that point becomes effectively nothing like anything we can observe today.

Big_Rip

But perhaps I misunderstood your point?



Jim

[shunyadragon]

So in your understanding, is the distance between the galaxies increasing more rapidly than the speed of light? They'd have to be, I would have thought, in order for some parts of the universe have gotten so far from us that light from them has never reached us. Or perhaps in this theory, they got very far from us and then later started emitting light?

Yes, in simple terms objects my move through the Quantum Vacuum accelerating at speeds to more then the speed of light relative to each other, but that is the limit of the speed of light, which limits our ability to see distant objects to ~14 billion years ago. I may add sources that give a better explanation.

[Carrikature]

*nasty caveat: We now know the universe expansion rate is increasing. So, it is theoretically possible then that the increase in expansion rate can create a scenario where light can never cross the distance. Indeed, it is postulated that in many trillions of years all observable galaxies will be invisible due to this phenomenon.

With the even nastier caveat that some other unknown phenomenon might prevent that from happening.

[RBerman]

In fact there are galaxies we can observe whose redshift implies a motion away from us greater than the speed of light. How to we still see them? This is how I understand it:

A simple way to sort of get a feel for what is happening is to imagine the surface of a balloon that is being inflated, with dots on its surface. As it expands, all the dots move apart. But if you were an ant traveling on the surface, your proper motion relative to a dots would be the sum of your own speed and the 'speed of stretching'. So that, if you were to start a journey to another dot, even if its speed away from you starting at the source dot was greater than you could walk, you would still (eventually) get there because as you move toward the dot, the speed of stretching relative to your target dot would always be decreasing and eventually falls below your walking speed*. So, even though some galaxies are so far away their red shift implies they are moving away faster than light, their light still reaches us because light is traveling along the medium that is streatching. But the light itself gets stretched too in the process, hence the 'red shift' we observe.

*nasty caveat: We now know the universe expansion rate is increasing. So, it is theoretically possible then that the increase in expansion rate can create a scenario where light can never cross the distance. Indeed, it is postulated that in many trillions of years all observable galaxies will be invisible due to this phenomenon.

I understand the balloon part. If light traveling through a stretching medium is like a man walking on a treadmill, then it can only make headway if it's traveling faster than "new space" is stretching in front of it (but behind the object which, having emitted the light, is now moving away), which is I think what your "nasty caveat" is about. But I bet there's some relativistic qualifier to that observation.

Makes we wonder how we know that the apparently red-shifted light isn't actually just redder in the first place. I know that we extrapolate the original (non-moving) color of the light based on spectrographic analysis of what we think the light-emitting objects are, but there still seem to be a lot of assumptions floating around. But I'm far below even an amateur cosmologist.