# Expansion Greater than Light Speed?

1. Apr 26, 2004

### Antonio Lao

Does anyone know whether the expansion of the universe ever exceeded the speed of light taking the early accelerated phase into consideration?

If not then the early photons must have bounded back and forth within the boundary of the early universe and a few or maybe many of all photons we are seeing now are rebounded photons.

Last edited: Apr 26, 2004
2. Apr 26, 2004

### Staff: Mentor

Yes, some galaxies appear to be recedinig at >C.
Even a finite universe need not have boundaries. There wouldn't be any bouncing - no edge to bounce off of.

3. Apr 26, 2004

### marcus

confirming what russ says,
most of the galaxies in the observable universe have redshifts z
greater than 2
(indeed one has been observed with z = 10, much more distant!)

and when we look at a galaxy with z = 2 or bigger

we are looking at an object which was already receding from us at a speed greater
than c
at the very moment when it emitted the light which we are now receiving
so we witness it in the act
of receding faster than c

that might also be true for z = 1.9, or z = 1.8
I would have to use a calculator to be sure
but certainly it is true for z = 2 and bigger.

At first sight it seems paradoxical and some essays have been written
to help people assimilate this, but it is a standard fact, just mindboggling when you first encounter it

Last edited: Apr 26, 2004
4. Apr 26, 2004

### marcus

I just used Morgan's calculator
with Hubble parameter = 71
and darkenergy 0.73
and matter 0.27
and
it said z = 1.7 implies the object was
receding at speed 1.02 c
that is just a little faster than light
so to a good approximation the cutoff is z = 1.7

using the present best estimates for the cosmological parameters
anything we see with redshift greater than 1.7
was already receding from us faster than light
when it emitted the very light which we are now receiving

Morgan's calculator
http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html [Broken]
is in the links in the reference sticky at the General Astronomy forum.

This thread could as well be in General Astronomy
instead of Theory Development. When I responded I didnt realize it was
in Theory Development. We've had similar questions there

Last edited by a moderator: May 1, 2017
5. Apr 26, 2004

### DrChinese

Useful reference:

http://arxiv.org/PS_cache/astro-ph/pdf/0310/0310808.pdf [Broken]

And thanks to some of the mentors above for being patient with me while I was trying to absorb this.

Last edited by a moderator: May 1, 2017
6. Apr 26, 2004

### LURCH

The second paragraph does not necessarily follow from the first. If the universe never expanded at a speed exceeding light-speed, it could still expand at light speed. Keep in mind that the "expansion of the universe" simply means, "how far stuff has gotten". So, for as long as there has been light, and that light has been traveling outward, the universe has been expanding at least at the speed of light.

7. Apr 26, 2004

### kurious

Inflation theory was thought up to explain, among other things, how regions separated
by distances greater than light can have possibly travelled, seem so alike- the idea being that if light couldn't bounce around and smooth things out , the faster than light stretching of space-time itself could.But what if there are particles out in space moving faster than light-most of the mass of the universe is still unaccounted for.

8. Apr 26, 2004

### Antonio Lao

Is this a stupid question? But if some of these galaxies are really receding beyond light would their relativistic mass be more than infinite?

9. Apr 26, 2004

### DrChinese

Not a stupid question at all. But read the Lineweaver reference first and save yourself a lot of grief.

10. Apr 26, 2004

### Staff: Mentor

...short answer: no, because expansion isn't motion. The galaxies aren't necessarily moving, but rather the space between them is expanding.

11. Apr 27, 2004

### kurious

...short answer: no, because expansion isn't motion. The galaxies aren't necessarily moving, but rather the space between them is expanding

But if that space is made of mass there is a relativistic mass problem!

12. Apr 27, 2004

### Antonio Lao

Thanks. It also dawn on me after I posted the "stupid" (not thinking) question that the galaxies were in a rest frame of their own and that it is the space that is expanding. But as kurious pointed out, if mass is attributed to space, then again the relativistic mass increase would be a problem.

Are modern theories trying to explain exotic matter, dark matter and dark energy to the mass property of space?

13. Apr 27, 2004

### kurious

Theorists are saying that dark energy 's density stays the same as space expands.
Normal energy like the cosmic microwave background gets less dense according to
1/ r^4. Dark matter is not thought to be a property of space but rather some exotic particles like axions or maybe even large accumulations of neutrinos.I have a problem with dark matter hypotheses: why doesn't it block our view of some stars and why don't we suddenly see new stars if dark matter moves around a bit, as it must surely do?

14. Apr 27, 2004

### Antonio Lao

What I am trying to do in my own research is to show that the mass we commonly used to know about can be splitted into two kinds of mass. The potential and the kinetic. The potential is the same as inertial and gravitational mass. And the kinetic mass has more to do with the true meaning of energy and momentum. There are logical distinctions between them. Their combination gave us the experimentally detectable mass.

When the potential mass and kinetic mass of a particle are equal then the experimental mass is zero (e.g. the mass of photon). With this idea in mind, we can say that the potential and kinetic mass of space are also equal so that the experimental (detectable) mass of space is also zero.

The W's and Z's bosons in the electroweak theory are all kinetic mass. And the mass responsible for gravity is the potential mass. The mass inside black holes and singularities are dominated by potential mass.

The universal expansion of space is the constant process of separation between potential and kinetic mass. All the potential mass are trying to get together, forming black holes, galaxies, stars, and planets while the kinetic mass spreadout uniformly causing the phenomenon of expansion in order to keep the detectable mass of space at the constant value of zero.

15. Apr 27, 2004

### Staff: Mentor

"space" itself is not a substance with mass.

16. Apr 27, 2004

### kurious

potential and kinetic energy

space" itself is not a substance with mass

Then what is it? What if the 10^120 Joules /m^3 of vacuum energy is Antonio's two cancelling mass types?
I thought of something similar a few weeks ago-the idea being that vacuum particles appear when positive energy moves away from a cloud of negative energy and then the particles disappear again by re-entering the cloud.

17. Apr 27, 2004

### Antonio Lao

Space is the seat where the potential and kinetic mass are exactly equal. I think, it is the same as saying the potential and kinetic energy are also equal. When the potential and kinetic energy are exactly equal, the Lagrangian at the point of spacetime is zero.

When the Lagrangian is exactly zero, the Hamiltonian is just the sum of the products of generalized space dimension (position coordinates) and the generalized momenta.

18. Apr 27, 2004

### Antonio Lao

The time independent Hamiltonian (Lagrangian=0) is equivalent to the quantum uncertainty condition of $\Delta \psi \Delta \phi \geq \frac{h}{2 \pi}$.

19. Apr 27, 2004

### AWolf

The zero mass of a photon is at rest mass. Since a photon cannot be at rest, it cannot be compared to the zero mass of space.

20. Apr 27, 2004

### Antonio Lao

if we take the point of view of the photon in vacuum, one photon can never overtake any other photons from the same source and along the same straight line (geodesic). So all photons from the same source appear motionless with respect to each other if the are along the same geodesic line. But can photons from a different source, or photon from same source but different direction, always appear to be traveling at light speed or less?

21. Apr 27, 2004

### Antonio Lao

But if the angle between two geodesic line is very small then the adjacent photons are practically collinear and they will also appear to be motionless with respect to each other. Integrating the whole 3D space, all photons from same source with varied directions can all appear to be motionless at a local region but when the small angle diverges from collinearity at far distances then and only then where th photons appear to be traveling at some speed. But space should always be filled with photons since we can see the same source (e.g. a star) at any angle and at any location, therefore all photons are motionless with respect to each other at all time and place.

22. Apr 27, 2004

### Staff: Mentor

Analogy: fish swim in a lake. A lake is not a fish.

Vacuum energy occurs in space, but it is not, itself, space.

23. Apr 27, 2004

### AWolf

Photons from the same source would have a relative velocity between zero and twice the speed of light (opposite directions).

Two objects travelling in the same direction at 163,000 miles/sec relative to a stationary third point would have doubled their mass and halved their time.
Relative to each other their masses would appear the same as if they stationary and their time would appear normal.
Has the relativistic mass suddenly disappeared ? Has time dilation ceased ?
No. (Assuming Lorentz's position that mass really does increase and that it's not just a thought experiment)

Because the two objects are experiencing the same effects, neither can detect them in the other.

All photons travel through space at the speed of light. The relative velocity, unless the photons are interacting, is of little or no consequence.

24. Apr 28, 2004

### kurious

Vacuum energy is not space.
What if negative energy is space and positive energy pops out of it to yield w bosons etc.
Just as relativity would say space is not a passive background I am agreeing but
trying to find a more physical reason why it is not passive.I understand the mathematics of SR and some of GR but neither theory leaves you with a gut feeling that its right!

25. Apr 28, 2004

### Antonio Lao

In general relativity (GR), the concept of force for gravity was replaced by the concept of spacetime curvature. The greater the curvature, the greater is the gravity force. This implies an equivalence between force and spacetime. Furthermore, GR's field equations are more equivalent relations between spacetime and the energy-momentum tensor. Tensors are higher dimensional transformed vectors. If the 4-dimensionality of spacetime can be treated as 1-dimensionality embedded in higher fifth-dimensionality contracted to 2-dim then spacetime's worldlines are analogous to vectors embedded in 2 dimensional space. When these worldlines are closed loops, the spacetime can again be splitted into closed timelike loops and closed spacelike loops. The interactions of these timelike loops and spacelike loops can be the existence of the electric and magnetic forces of the vacuum. These can be renamed into the spacelike force and the timelike force respectively.