Expanding people in an expanding universe?

[Canute]

Someone elsewhere posted a theory that the space is shrinking. The conclusion of the discussion was that this would observationally equivalent to expansion. Each point of space would 'see' the universe expanding. I don't know if that makes sense or not.

Earlier someone said that the space between the particles within an atom do not expand. Why is that? Saying it is 'bound' by forces doesn't seem to work, the whole universe is bound by forces.

 

[davilla]

Originally posted by Sikz
The only think I can think of is that we could be contracting. If it was our galaxy that was contracting:

1) It would have to contract at HUGE speeds in order to produce the illusion of high universal expansion.
2) Galaxies in the same general direction as the center of our contraction would appear to be moving towards us (Earth), not away.

This addresses quite well the question as it relates to an expanding universe. I understand now that things are drifting apart from each other, and it makes sense with respect to the Big Bang theory. But what about the question with repect to a cosmological constant? Suppose it only appears that the rate of expansion of the Universe is accelerating. Then what assumptions are we making about our local reference frame? If we assume there isn't a cosmological constant, then what factor would explain this apparent contradiction in evidence?

Also, if I may be so bold, here are some unanswered questions:

Originally posted by davilla
Since the Universe is expanding then wouldn't all matter have been more concentrated billions of years ago, hence a denser gravitational field?

Also, out of curiousity, aren't celestial objects that are receding from us doing so not only in space but, to a lesser degree, in time (since more distant objects are seen as they were a longer time ago)? Does this affect redshift?

In a different line of though, I'm going to assume that there is a cosmological constant. This is normally explained by the expansion of space. Inspired by some of the topics of earlier threads, I'm wondering if, alternatively, this could be explained by a variable speed of light. Of course, to us the speed of light always appears to be constant. An understanding of relativity bears this out: the speed of light is the measuring stick of time by which we observe everything else. The cosmological constant might be the result of a universal variation in this absolute speed.

Someone made the keen observation that everything "falls" through spacetime at the same "rate". The faster something moves (relative to an intertial frame) the slower its clock. A photon moves at a maximum velocity spacially because is it still temporally. Now, a changing c could be understood by an acceleration in the "rate" at which we are "falling" through spacetime. Imagine the Universe as an "atomic object" in a macrosystem. The "faster" the Universe "falls", the "farther" a photon has to travel to connect the same two points in spacetime, and hence the slower its speed. A cosmological constant would mean that, universally, everything is slowing down. As it would appear to us, everything is ripping apart. Light connects everything in the universe, and we are losing our connection.

Certainly this has been proposed before. Where does dark energy fit into this account? Another difference between this and the standard explanation might be at the quantum level. I'm having trouble envisioning which would have a change in Plank length. A real change and an apparent change would be indistinguishable. Otherwise it might be possible to decide between the two experimentally.

Originally posted by Canute
Someone elsewhere posted a theory that the space is shrinking. The conclusion of the discussion was that this would observationally equivalent to expansion.

Did they mean that lengths are shrinking? This sounds like a question of nomenclature, how it is that you view the same mathematical phenomenon in R3.

 

[davilla]

Originally posted by Canute
Earlier someone said that the space between the particles within an atom do not expand. Why is that? Saying it is 'bound' by forces doesn't seem to work, the whole universe is bound by forces.

As I understand it, the space between atoms actually does expand, according to the theory. If there is a cosmological constant, then space is expanding everywhere (and uniformly, one would hope). But most of the stuff we humans have contact with is in an equilibrium because of atomic forces rather than gravity. The expansion in the fabric of space might slowly (weakly) tug at the atoms in your hand, but because the atoms are bound (by electromagnetic forces) in molecules, they will resist this, and rather than rip apart, the molecule will pull itself back together.

 

[Canute]

That's what I don't get. How can spacetime be bound within atoms? And if it is then why not between atoms? Hell, there seems to be some debate about whether spacetime even exists. It's very confusing.

 

[selfAdjoint]

What do you mean, "spacetime bound within atoms"?. In this model the atoms occupy places in spacetime, and forces act betrween and inside them to make them the size they are. Wheteher or not spacetime expands has nothing to do with it.

Now certainly in some notional background free theory of everything, the interaction of the forces with the spacetime geometry, and its expansion will have to be dealt with. But there is no such theory now and so nobody can say what that interaction will look like.

 

[Canute]

But I thought the current theory is that space is expanding. Are you saying that isn't the case?

 

[Nereid]

Echoes - sharp probes of dust

Nereid wrote (re dark matter and dust): Perhaps it's just dust? If it were warm, it'd glow in the far-IR/microwave bands (if hot, in near-IR and optical); we don't see any. It'd also absorb light (IR, etc); we don't see such absorption.

There's another, independent, technique: echoes. When a really, really bright point source suddenly shows up (and then fades), it's like a strobe light through all of space between the source and us. From our point of view (POV, does anyone use this shorthand?), we will see 'circular' echoes of the flash, which are reflections of the flash off things just outside the line of sight between us and the flasher (hope that's not porn to PF ). Study of these echoes can identify all kinds of interesting things, and confirm our understand of the amount (and distribution) of dust.

Historically light has been where such echoes have been seen (e.g. from the supernova in the LMC). Recently X-ray echoes have been observed from a particularly bright GRB:
http://t2wesa.r3h.net/export/esaSC/SEMUFH474OD_sensations_0.html