What happens to light when it reaches the edge of the universe?

shrumeo

i've always wondered about "expanding" space
are the objects (protons, photons, whatever) embedded? do they expand along with space? or do little distance markers move away from each other? Does matter expand along with space? isn't it part of spacetime or is it totally separate? Does space "expanding" simply mean that MORE space got put in between two objects? If 2 objects are attracted to each other gravitationally (sitting in little spacetime wells) then wouldn't the "expanding" space between them affect their gravitational pull on one another?

Chronos

marcus: 15 billion light years is the average of what is currently measured as the distance the hubble's constant approaches the speed of light [between 10 and 20 billion light years]. this approximation is supported by stellar evolution models that predict it would take no less than 10 billion years [by local inertial reference frame clocks] and no more than 20 billion years to result in globular clusters composed mainly of white dwarf stars. the fact these clusters are only observed at the extreme fringes of galaxies strongly suggests they are the most ancient gravitationally condensed collections of matter in any galaxy.

regarding recessional velocities. you are right. hubble's constant is not constant over time. the fact the red shift increase with distance, and therefore over time, insists the universe expanded more rapidly in the past than it does now. that is no surprise. the early universe had to expand with more force than the attractive force of gravitation. it would otherwise have collapsed upon itself before we had the opportunity to observe and ask the question 'what happened?'. we know gravity is attractive, hence, the existence of a repulsive [anti-gravity] force is virtually assured. the main question is which force will prevail? i would guess neither. the universe, as we perceive, will eventually reach a state of equilibrium. when the matter density exactly balances the energy density, the universe will acquire a state of perfect equilibrium. this implies other consequences, but, we dont have to deal with that for at least another 6 billion years.

Chronos

shrumeo: i think the real problem here [and with quantum gravity] is that gravity ceases to be a 'player' at the subatomic level. number one, the other 3 forces [strong, weak, and electromagnetic] are vastly more powerful than gravity at short distances. i think gravity is nonexistent at such short distances. quantum field theory requires forces to act at integer distances. from what i have seen, the wave length of gravity must be a recipocal of c^2. this is not consistent with the theoretical distance between an electron and proton in atomic hydrogen.

Hydr0matic

Doesn't a flat and infinite universe imply there's a center ?

Assume the universe is flat and infinite.
Is there an infinite amount of matter covering this infinite universe ? Not according to big bang, right ?
So is this limited amount of matter spread across the entire universe ? No, cause then there'd be infinite distance between these "pieces" of matter, right ?

The only alternative is that there is a limited amount of matter localised somewhere in the universe. So I guess it's a matter of definition, right ? Do we define "the center" as the center of all space (i.e. there is none), or do we define it as the gravity center of all matter/energy ? Or perhaps the point where the sum of all coordinates of matter equals zero ?
.. or what ?

If there is to be no center (of any kind) to the universe, isn't a closed borderless one the only option ?

jcsd

No a falt nfinite unievrse doe not imply a centre, the big bang means that if the unievrse is infinite in space it is infinite in energy.

Hydr0matic

hmm.. ok ?
So how did this energy get from being localised in a single point to being distributed in an infinite space with finite density ? Did this happen in an inifinitesimal timeframe ? Was there ever a moment where the energy was somewhere in between the singularity and the infinite distribution ?

Feels like I've got some reading to do ...

marcus

Hydr0 there is actually a linguistic (not physics) problem here
that confuses people again and again

In ordinary non-technical English, "singularity" means peculiarity, or oddness, or abnormality

(it has no connotation of happening at a single point!)

a theoretical model can experience a singularity at an infinite set of points

it simply means that there is a boundary or limit to its applicability

it means you cannot push the model past a certain limit, because (say) it blows up and fails to compute, or it computes infinities or meaningless numbers----then there is a singularity

this limit could be pictured as a 3D hypersurface bounding a 4D region where the model works well----a little bit like the 2D crust on a 3D loaf of bread is a boundary of the bread (no, I cannot think of a good image, all the images seem to make it more confusing)

But because "singularity" sounds like the word "single"
it suggests to many people that there is a single point
where the singularity happens!
so they imagine a single isolated point
this is a wrong image and leads to much confusion

the BB singularity may have been confined in a small point-like region, but this is not the prevailing view. It could also have occurred at every point of an infinite 3D hypersurface---this is nowadays a very common view

Hydr0matic

Ok, I see. So the difference between now and the beginning is simply a matter of energy density, where "the critical density" marks the point to which the theory is limited.

right ?

marcus

not right yet

the "critical density" is a very nice density of about 0.83 joules per cubic kilometer

which is either exactly (or else very near) the actual density
of the universe right at this moment!


at the bigbang time the energy density was probably "off the chart"
infinities are usually a sign that the model is breaking down
and in the usual model
things like density and curvature go off to infinity as one approaches
the instant that expansion began
and so one really must admit that if one wants to approach
that instant and continue computing them one needs a new model!


perhaps you are wondering what is the "critical density"
it is the density the U would need to have (in the simplified Friedmann picture) in order to be exactly spatially flat, at this moment

too much mass/energy and she will be positively curved
too little and she will be negatively curved
and critical is just right
and since the U is observed to be indistinguishable from flat, spatially, it is usually assumed that the actual density, smoothed out to a uniform average level, would be equal to or very near the critical

Hydr0matic

Ok .. "critical" sounded more dramatic so I assumed it was the point when the model brakes down.

thnx for clearing things up

shrumeo

hmm, maybe i'm missing something...(total non-physicist here trying to understand these things in plain english)

i didn't really mean to imply anything about gravity among subatomic particles. it was just a string of nonsequential questions. But still, lets say we are at the subatomic level and we are "looking" (forgive me Heisenberg) at an electron surrounding a proton. Now, space is expanding even at this scale, right? Now, are the electron and proton something that is not part of the fabric of spacetime? In other words is there more "vacuum" being added to the existing "vacuum" that spearates them? is the electromagnetic force constantly overcoming the expansion OR are they wrinkles in spacetime that also expand along with the vacuum?

On the other scale, say a planet and a moon. Is the space between them expanding to where gravity is constantly overcoming all the new space between them?

Also, there is amount of space between the atoms and molecules that make up the planet and moon (forgot tht scale). Is the electromagnetic force constantly overcoming the expansion of space between them?

If objects and matter remain intact (which they appear to do), do any equations that, say, deal with their, say, group velocity, for example, automatically have built in them, from experiment or derivation, a term that overcomes the expansion of space???

mouseonmoon

http://www.space.com/scienceastronom...ay_040524.html

Universe Measured: We're 156 Billion Light-years Wide!
By Robert Roy Britt
Senior Science Writer

......

Stretching reality

The universe is about 13.7 billion years old. Light reaching us from the earliest known galaxies has been travelling, therefore, for more than 13 billion years. So one might assume that the radius of the universe is 13.7 billion light-years and that the whole shebang is double that, or 27.4 billion light-years wide.

But the universe has been expanding ever since the beginning of time, when theorists believe it all sprang forth from an infinitely dense point in a Big Bang.

"All the distance covered by the light in the early universe gets increased by the expansion of the universe,"

The scientists studied the cosmic microwave background (CMB), radiation unleashed about 380,000 years after the Big Bang, when the universe had first expanded enough to cool and allow atoms to form....

.....findings have shown "no sign that the universe is finite, but that doesn't prove that it is infinite."

The results do render impossible a "soccer ball" shape for the universe.....



"If the universe was finite, and had a size of about 4 billion to 5 billion light-years, then light would be able to wrap around the universe, and with a big enough telescope we could view the Earth just after it solidified and when the first life formed," Cornish said. "Unfortunately, our results rule out this tantalizing possibility."


"The problem is that funny things happen in general relativity which appear to violate special relativity (nothing traveling faster than the speed of light and all that).

=====
might help-might not

Chronos

how did that guy earn a doctorate? i agree GR has potential flaws, but, not on the basis of such patently flawed arguments. you cannot look into a telescope and see the back of your head. that violates causality. i can play along with any kind of universe anyone predicts, but, i absolutely reject any model that violates causality. even GR forbids that. god may play dice, but a universe without causality is impossible to observe.

shrumeo

i think you are agreeing with the article(?)

GodAdoresU

According to the most accurate measurements of the position of "planet earth" in relation to the rest of the universe, we are in the Dead center. And so one could assume 2 things about the universe: 1) we are extremely lucky. or 2) the universe is closed. The reason being, if we do indeed live in a closed universe, then it would appear that we we're in the center. For example, Imagine yourself standing on planet earth (pretty hard huh? XD) But without all of the mountains, valleys, trees, grass etc. Just you and a nice two dimensional surface. If you assumed that the earth was flat (and if it was), then you would observe that you were in the very center (just as we observe). However, it is very unlikely that we are at the very center of the universe. Therefore, there is only one other possibility:We live in a closed universe. However, this is only hypothetical. And I don't think that we will know for sure until we are able to venture out into the universe and make more observations. So the answer to your question in my opinion is that Light propagating from stars and galaxies eventually comes back around to the point of origin.

EMFsmith

I though the speed of light was the speed at which "Stuff" (Pardon my ignorance here) of 0 mass can travel, ie photons. So how can there be ANYTHING that travels faster than this?

EMFsmith

This is what lead me to post this, something Marcus wrote on a post somewhere, i thought it would take infinite energy to move objects of mass at c ?


(For example, galaxies are routinely observed at redshifts greater than 3.

In fact, one was recently detected to have z = 10 (by Roser Pello's group).

A galaxy observed at z = 3 must have been receding from us, at the time it emitted the light we are now receiving from it, at a speed greater than light).