Speed Of light Question

4Xcut

Please excuse my ignorence if this seems like a stupid question.
If the universe is 28 billion light years from edge to edge and it is only 14 billion years old would that not suggest that there is something faster than the speed of light.

(Size and age of universe taken from newscientist.com)

Chalk

well if the universe started in the centre and expanded 14bill light years in all directions then would that make sense?

Son Goku

Well, the Universe doesn't really have a center.

Basically the Universe can be as big as it wants without anything travelling faster than light, because it is the space that is moving, not the matter.

Capt'n Midnight

4Xcut wrote:

Please excuse my ignorence if this seems like a stupid question.
If the universe is 28 billion light years from edge to edge

that's the visible universe. The universe could be a lot bigger and we wouldn't know, and it it was then all bets are off re where the centre is.

One way of working out where the centre is would be to work back from the observed doppler shifts n' stuff to see where everything came from, but the expansion is happening in such a way that it's impossible. to my mind it's a bit like standing on a rubber sheet that is being stretched , and you know that at one point the sheet was all at one point. Take a step back and imagine the sheet is part of a balloon, there is no centre on the surface of the baloon and yet everything can expand.

Gurgle

Capt'n Midnight wrote:

One way of working out where the centre is would be to work back from the observed doppler shifts n' stuff to see where everything came from

But that would tell you the gravitational shape of the universe, not the physical shape.

CiDeRmAn

Perhaps one should consider the fact that while the speed of energy/matter through space time is limited to the speed of light this limit may purely be a point of perspective, an actual faster than light particle would appear to be travelling backwards in time.
Also it could turn out that within a given universe every point may consider itself to be at the centre, as everything is expanding away from it, relatively speaking (no pun intended).
Centre of an expanding universe may well be a point that may not exist within our reference frame anymore, to reduce the dimensions available for a moment, an object on a balloon skin will only be aware of two dimensions yet we can see the centre of the balloon is within, no longer part of the expanding structures frame of reference, or something like that anyway.
Thats what has occured to me anyway.
There should be tons of info on the early inflation of the universe out there that will answer yuor questions.
Also check out open and closed topographies and the relative eveness of the distribution of energy and matter and what this says about the early life and behaviour of an expanding universe.
Also check out the possibility of other non-parallel universes with different basic rules, such as an altered pi, different strengths of fundamental forces of nature,
Interesting stuff.

Son Goku

Capt'n Midnight wrote:

that's the visible universe. The universe could be a lot bigger and we wouldn't know, and it it was then all bets are off re where the centre is.

There isn't really a center at all. In certain analogies you can consider the Big Bang the Center, but there isn't truly a center at all. Considering all points trace back to the same origin.
All points are equals. (In a casual sense)

If the Universe is bigger than the speed of light allows that is simply because space doesn't have a conventional speed limit.
No matter would be truly moving beyond light speed.

Spear

4Xcut wrote:

Please excuse my ignorence if this seems like a stupid question.
If the universe is 28 billion light years from edge to edge and it is only 14 billion years old would that not suggest that there is something faster than the speed of light.

(Size and age of universe taken from newscientist.com)

It's called the horizon problem normally. Since all the universe is at the same termperature of ~2.7K, as shown by the background microwave radiation, the universe must have been close enough to be in thermal equilibrium. But separate regions are too separated now, for that to be possible. This and other issues in cosmology can all seem to be explained by a period of early inflation when space expanded faster than light can travel through it.

Capt'n Midnight

Son Goku wrote:

No matter would be truly moving beyond light speed.

Anything further than 16Bn light years away would be invisible, think of it like a schwartzchild radius without the one way effect, we could hypothetically see a civilisation 15Bn light years away broadcasting what they could see. But they would back then only see a universe with a radius of 1Bn LY including where we were then and stuff we could never see on the far side of them.

Son Goku

Capt'n Midnight wrote:

that's what the second half of my post said

I know, I was just emphasising what you said.

Although I should emphasis that the expansion comes from the Universe being dynamic rather than "new space" flowing in from anywhere.

Capt'n Midnight

Son Goku wrote:

I know, I was just emphasising what you said.

Oh thanks

Although I should emphasis that the expansion comes from the Universe being dynamic rather than "new space" flowing in from anywhere.

Reminds me of that guy working in the infinite motel that's full and has to make an infinite number of spaces available for a bus with an infinite number of passengers. He simply asked each guest in room number N to move to room 2N

WhiteWashMan

Chalk wrote:

well if the universe started in the centre and expanded 14bill light years in all directions then would that make sense?

hahaha.

the best answer so far

it doesnt matter what the gravitational shape of the universe is.

classic.

Capt'n Midnight

Spear wrote:

This and other issues in cosmology can all seem to be explained by a period of early inflation when space expanded faster than light can travel through it.

Interesting 'cos early on the universe would have a higher density than now, would it have been enough to slow light down ?
Could we detect the breaking radiation ?

Spear

Capt'n Midnight wrote:

Interesting 'cos early on the universe would have a higher density than now, would it have been enough to slow light down ?
Could we detect the breaking radiation ?

Oh damn, an original question. Er, I don't know. At a vague guess, the expansion of space doesn't constitute an acceleration so that in itself wouldn't produce braking radiation. Then again the inflation could have occured during the period of force unification, so there'd be no electromagnetically charged particles to give rise to braking radiation. The inflation would separate whatever matter there was anyway, also stopping or greatly reducing the capacity for it. So assuming, there were charged particles, in great enough densitys before the inflation, but after force unification eneded then I gues there could have been some produced. But, braking radiation is a continuous spectrum, all of which would have since been greatly redshifted to very low energies, and probably absorbed by interstellar matter, even if it wasn't simply outshone by other more usual radiation sources, as well as the more recent braking radiation emissions. So I guess my answer is maybe, under certain circumstances, and even if there was some, it'd be damned hard to detect.

Professor_Fink

Capt'n Midnight wrote:

Interesting 'cos early on the universe would have a higher density than now, would it have been enough to slow light down ?
Could we detect the breaking radiation ?

The Cosmic Microwave Background radiation is a remenant from the time at which matter and light decoupled. An Bremsstrahlung radiation before that point would have come into thermodynamic equilibrium with the rest of the universe, and would no longer be observable. Since atoms are generally neutral there would be little Bremsstrahlung radiation after that point since it is caused only by charged particles.

Capt'n Midnight

Professor_Fink wrote:

Since atoms are generally neutral there would be little Bremsstrahlung radiation after that point since it is caused only by charged particles.

IIRC (not from actual experiance mind you) when the energy density dropped enough for matter to condense out it would have almost certainly been superheated plasma which if I'm not mistaken would be ionised. (or was it electrons that appeared first?, same difference)

Professor_Fink

Capt'n Midnight wrote:

IIRC (not from actual experiance mind you) when the energy density dropped enough for matter to condense out it would have almost certainly been superheated plasma which if I'm not mistaken would be ionised. (or was it electrons that appeared first?, same difference)

The universe became transparent to radiation because atoms formed. This happened at about 3000K.

Also I'm not quite sure what you mean by the "energy density" dropping. Energy is incredibly dense in matter (E^2=M^2C^4+P^2C^2).

Capt'n Midnight

Professor_Fink wrote:

The universe became transparent to radiation because atoms formed. This happened at about 3000K.

Also I'm not quite sure what you mean by the "energy density" dropping. Energy is incredibly dense in matter (E^2=M^2C^4+P^2C^2).

At 3000K you don't have any neutral atoms just ions.
Energy density as in X amount of energy per volume of space - eg: a black hole has a higher density than deep space. AFAIK there were 11? dimensions very early on (7 were small loops or something) but as the universe expanded it dropped down to 4.

Professor_Fink

Capt'n Midnight wrote:

At 3000K you don't have any neutral atoms just ions.
Energy density as in X amount of energy per volume of space - eg: a black hole has a higher density than deep space. AFAIK there were 11? dimensions very early on (7 were small loops or something) but as the universe expanded it dropped down to 4.

Do you have any idea what you're talking about?

Carbon melts at ~3773K, and boils at ~5100K
Lots of transition metals have high melting points too.
They're not even gases let along ionized, at 3000K.

Neutron mass: 1,6749 * 10^(-27) kg
Neutron radius: ~1.3 fm

Volume of a sphere of this radius: ~ 9.2*10^45
Energy enclosed (E=mc^2): 1.5 *10^-10

Total energy density: 1.6*10^34 J m^-3

Thats pretty high! So please don't make up stuff about the energy density dropping.

Equally this all happened quite a while after the big bang. There were not 11 extended dimensions at this point regardless of whether there were in the first fraction of a second.

Capt'n Midnight

Professor_Fink wrote:

The universe became transparent to radiation because atoms formed. This happened at about 3000K.

That's where I got the 3000K from.
Course if had the old thinking cap on I would have said , hang on , radiation peak depends on temperature and suggested a higher temp like well above 6500K (you can get a shadow from a candle's flame using a brighter light) and an opaque universe just means photons don't move far before bouncing off something , later on you'd be into atomic absorption spectra of H and He

The 11 dimensions was just an example of how different the universe was when there was more energy in a smaller space - don't forget at the BB it wasn't even as thick as two Planck's .

http://teresi.us/html/writing/big_bang.html
before 10-43 second, physicists’ formulas for space, time, and matter break down.
Near the age of 10-10 seconds evidence of the unification of some of the forces can be seen. Before this point, the weak and electromagnetic forces were combined. Let there be light !
At three minutes old, the universe began to cool enough to allow the strong nuclear force to take control and pull together atomic nuclei. Plasma
when the universe was about 300,000 years old, the primordial soup of elementary particles cooled enough to allow nuclei to reign in electrons and form atoms. finally some un-ionised matter

http://www.thekeyboard.org.uk/The%20Big%20Bang%20Theory.htm

By just over 30 minutes after time zero, all of the positrons had annihilated with almost all of the electrons - with again one in a billion left over - to produce the background radiation proper, and the temperature had dropped to 300 million K, and the density was only 10% of that of water. At this temperature stable atoms were still not able to form.

http://www.madsci.org/posts/archives/feb98/888690999.Ph.r.html

speed of light in air) = c * (1 - .00029*(d / d0))

where d0 = 1.29 kg/m^3 is the density of air at
room temperature and pressure. The equation
is approximately true even for densities 100 times
greater than d0.

This equation works for visible light only.
If you're considering other parts of the spectrum
(infrared, ultraviolet, X-ray, etc.) the
form of the equation will be the same, but the constant
will be different from .00029.

seems to suggest that at 30 minutes light speed would have been low enough for a bit of Cerenkov radiation since we can probably assume that a lot of matter was moving pretty fast back then.

Professor_Fink

Capt'n Midnight wrote:

That's where I got the 3000K from.
Course if had the old thinking cap on I would have said , hang on , radiation peak depends on temperature and suggested a higher temp like well above 6500K (you can get a shadow from a candle's flame using a brighter light) and an opaque universe just means photons don't move far before bouncing off something , later on you'd be into atomic absorption spectra of H and He.

Thats not accurate at all. CMB is from a time when the universe was, to a very good approximation, in thermal equilibrium. Plasmas can absorb a very wide range of wavelengths. Google for "debye length" is you feel the need.

Capt'n Midnight wrote:

The 11 dimensions was just an example of how different the universe was when there was more energy in a smaller space - don't forget at the BB it wasn't even as thick as two Planck's .

http://teresi.us/html/writing/big_bang.html

Do you even check out who wrote these websites? That one seems to be written by a computer programmer with no background in physics. Not exactly "A Brief History of Time". And by the way, physics works great back as far as about 10^-30 of a second.

Capt'n Midnight wrote:

http://www.madsci.org/posts/archives/feb98/888690999.Ph.r.html
seems to suggest that at 30 minutes light speed would have been low enough for a bit of Cerenkov radiation since we can probably assume that a lot of matter was moving pretty fast back then.

Again I say "thermal equilibrium". The cerenkov radiation, if there was any, would have been well before the universe became transparent to radiation. i.e. it would all have interacted with the matter in the universe, coming into thermal equilibrium, and would just be CMB like everything else.

my pants are on

4Xcut wrote:

Please excuse my ignorence if this seems like a stupid question.
If the universe is 28 billion light years from edge to edge and it is only 14 billion years old would that not suggest that there is something faster than the speed of light.

(Size and age of universe taken from newscientist.com)

Poo