[watsisname]

A random thought occurred to me, I have no clue what the answer is, but would this expansion happen at close to the speed of light? Would those rules even exist at that point of the Universe' life? I've read before that some constants may not be as 'constant' in deep time as we think.

Battuta is right: the expansion, especially during the Inflationary Epoch, is much, much faster.  But the speed of light, c, as far as we can tell, is a constant.

This sounds like a gross violation of special relativity, but it isn't.  The motion of material or signals through space is not the same as the motion due to the expansion of space, and this difference is very commonly misunderstood, even among cosmologists.  I like to use the classic balloon analogy here, with some further elaboration to help explain the difference.

Imagine a 2D analogue to the 3D universe as the surface of a balloon, with dots drawn on it to represent galaxy clusters.  Measure the distance between these dots by wrapping a tape measure across the surface.  Now inflate the balloon.  The distance between any two dots increases with time due to the expansion, and the more distant the dots are, the faster they are separated.  This is Hubble's Law: the rate at which two things (assuming they are not close enough to be bound by gravity or other forces) move apart is directly proportional to the distance between them.  And it doesn't matter which dot you measure from, you will always see the same behavior, as if you are always at the center of expansion.  This is a natural outcome of a uniformly expanding space, or what we call metric expansion.

I'm sure most people have heard this analogy already, but now let's look a little deeper:

Imagine that there are two ants on the balloon, at two different dots.  One ant tries to crawl over to the other while the balloon expands.  Does he ever make it over to his friend?

If the balloon is expanding too rapidly, or if they started out too widely separated, then he never makes it.  He gets dragged farther away by the expansion even as he crawls in the right direction.  The analogy is that the moving ant is a photon, crawling at the speed of light, while the stationary ant is an observer.  If the two began too widely separated, the photon never reaches the observer.  It is as if the galaxy it originated from is receding faster than the speed of light.

This doesn't violate special relativity because the galaxy isn't "really" moving -- it's just being dragged along with the expansion of space.  Or, equivalently, the space between them is expanding faster than a photon can cover the distance.  There is no limitation on how fast this can happen, because there is no limitation on how rapidly space can expand.  The expansion of space is described by general relativity, and depends on the amount of matter and energy it contains.
 
In the real universe there are galaxies that will never be observed because their light cannot reach us, just as if they are beyond an event horizon.  The comparison to black holes is apt -- relativity is not violated by things falling into black holes either.  Any superluminal behavior due to the movement of inertial reference frames toward the singularity is blocked from view by an event horizon, much as any superluminal behavior of galaxies due to the expansion of the universe is behind an event horizon.

[watsisname]

The Hubble radius isn't the same as that of the observable universe, though — the latter is about three times larger, because of expansion-related screwiness I can visualise but not explain.

No, they are the same, the difference is how the distance is defined.  13.7 billion years is the age of the universe, and thus is the distance to the edge of the observable universe according to light-travel-time.  Measuring by co-moving distance, which accounts for the expansion, makes it larger (46 billion LY if my memory serves), but it's the same region. 

The region is defined as the volume which is presently causally connected with Earth -- i.e. there has been the time and ability of photons to travel this distance since the Big Bang.  As the universe ages, this volume increases in size as photons from more distant regions reach us.  There is some maximal size that this can reach for the reasons described in the post above -- eventually there is some limit beyond which photons will never be able to make it here.  This boundary is determined by the expansion rate, which changes over time.

[Ace]

Inflation (which these gravity waves support) means that the observable universe is actually bigger then 13.7 billion light years. The light has been traveling for that time, but is highly stretched.

So, probably dumb-ish question since it's been a while since taking cosmology, depending on how you expand or play with space time you can cause events to do causality violations (e.g. Albucierre drives). So are there possibly hard limits on how negative energy (inflation, current expansion) get applied so avoid that? i.e. you can't do a localized field (i.e. anti-grav) it has to apply to all of space-time to avoid causality violations?

[watsisname]

There are solutions to general relativistic field equations that allow for closed timelike curves (CTCs), such as the Kerr metric describing rotating black holes, wormholes, and some other unusual ones.  It is generally thought [citation needed] that although CTCs seem to allow a particle to visit its own past, causality violations would never be observable for the same reasons singularities aren't observable -- an event horizon would shroud them.  I don't know if there has been a rigorous proof of this yet though; I'd need to brush up on the topic.  And whether CTC's can actually occur is entirely hypothetical -- they are special solutions to GR which might not hold valid in reality.

The current expansion is a consequence of both the impetus from the Big Bang and the accelerating expansion due to dark energy.  Dark energy is pretty weird stuff and we can't observe it directly.  It seems to be a property of spacetime itself.  It has an energy density which is constant regardless of the size of the universe (unlike radiation pressure or mass density), and is uniform everywhere (it isn't clumpy like dark matter is).  It has the really weird property of negative pressure, which is perhaps counter-intuitive because normally we think of pressure pushing out on things, like gas molecules inside a box.  But in cosmology, pressure does the opposite -- there is no boundary for it to push against, so it manifests as spatial curvature, braking the expansion rate.  Dark energy, with a negative pressure, enhances the expansion.  Eventually the universe will expand exponentially more rapidly due to dark energy.

[Ace]

I now have a mental image of a time traveler being a strange floating black hole distorting light around it screaming "IM NOT TOUCHING YOU" to the universe.

[Flipside]

Thanks for the explanation, it makes things a lot clearer to me now. I won't pretend 'understanding' but I do at least feel I have 'comprehension'

[redsniper]

But in cosmology, pressure does the opposite -- there is no boundary for it to push against, so it manifests as spatial curvature, braking the expansion rate.  Dark energy, with a negative pressure, enhances the expansion.  Eventually the universe will expand exponentially more rapidly due to dark energy.

So now let me get this straight. The radiation pressure from light and all the other EM radiation, is flying every which way from an infinite amount of matter in infinite space, and so that pressure pushes on... everything and slows down it's motion (that is, expansion). So it almost sounds like friction or viscosity to me, everything is expanding through some electromagnetic goop slowing it down... but then the dark energy pushes the other way even harder, so things keep expanding. Is that about right?

[watsisname]

Great question.   As counter-intuitive as it might seem, yes, pressure acts to slow the universe's expansion rate, but it is not so much through a frictional or viscosity effect.  Those don't really apply here.  And do not think of the expansion as due to stuff being 'pushed'.

To understand what's going on we'll have to delve a little bit into general relativity.  Let's start with the field equations -- here they are in their full glory:



Yikes, what the heck do all these crazy symbols mean?  Don't worry, it's actually not that complicated.  The left hand side describes spacetime curvature. G_μν is the curvature tensor, g_μν is the metric tensor.  Notice how Λ (cosmological constant, or dark energy) is associated with the metric tensor, which makes sense because it is a property of spacetime itself.   

The right hand side represents the matter and energy that the spacetime contains, with T_μν as the 'stress-energy' tensor.  Here is where our friend pressure comes into play.  Like matter, pressure also participates in the stress-energy tensor.

So in layman's terms, the equations say "curvature = matter and energy".  The curvature tensor depends on the stress-energy tensor, and thus pressure as well.

The curvature, in turn, affects the universe's expansion rate, or more precisely the rate of change of the expansion.  The field equations themselves don't describe this, as they do not have any time dependence.  But they can be used with the Friedmann equations to solve for the scale factor (size) of the universe as a function of time, or the first time derivative of this (the expansion rate), or the second time derivative (the acceleration).  Assuming a homogenous and isotropic universe (very good approximation at large scales), you'll end up with an expression relating the expansion to not just the density of matter, but the pressure as well.  Pressure acts to brake the expansion just like matter does, while dark energy acts to increase the expansion rate.

[Mongoose]

*is infinitely glad he never tried moving on to graduate-level physics*

[Dragon]

Note that the "cosmological constant" is the point where this whole theory starts smelling fishy for some physicists. It's possible that in reality, the matter is somewhat more complicated than that. When you get mysterious, unexplained energy that seems to compromise about 75% of the universe, some call it dark energy, some call it a model that needs revision. That said, the discovery of gravity waves seems to confirm the current model isn't so bad, and a pretty good candidate for dark matter has been discovered (though I'm not a fan of that part of the theory, either)... We can probably expect more news from that front in the following years.

[watsisname]

Well, dark matter and dark energy are very different things.  Dark matter is evident by the behavior of systems in which it seems like there is a heck of a lot of mass there that we cannot detect by EM.  This evidence is pretty robust and comes from many different sources, with gravitational lensing data from colliding galactic clusters being among the most persuasive.  The alternative is that our description of gravitation needs modification, and there are researchers working this angle, e.g. with MOND or MOND+GR, but so far these haven't been very successful in the ways of predictive power and validation.

Dark energy or cosmological constant on the other hand is a name we give for the cause of the observed accelerating expansion.  The simplest explanation follows from describing what 'substance' would have this effect within the framework of general relativity.  That would be a uniform fluid with a negative pressure, and crucially, whose density remains constant despite the universe's expansion.  Thus, it is easily explained as a property of the vacuum itself, and not so easily explained as a modification we need to make to the laws of gravitation due to matter.  Mass density does not remain constant as the scale of the universe changes.

And of course, we cannot yet detect either of these things directly (though particle accelerator experiments hope to detect dark matter, or at least demarcate the limits to the energies that dark matter particles [the WIMPs] have).  But we can quantify very precisely how much matter, dark matter, and dark energy there is by observations of the CMB.  A difference in these values will change the relative sizes of the fluctuations in this background, so a good test of cosmological parameters is to measure the CMB's angular power spectrum. 

Doing so shows that the total density due to matter is about 30%, of which only 5% is ordinary baryonic matter, and 25% is dark matter!  Dark energy makes up the remaining 70%.  (These are not exact values, but are within a percent or two).  Radiation pressure, via photons and neutrinos, make up far less than one percent... only about 8x10^-5!  Radiation pressure is very unimportant today, but it was very important in the early universe.  We use the terms 'matter dominated' or 'radiation dominated' to describe these two regimes.  As the universe ages and the mass density continues to decrease while the cosmological constant (presumably) remains constant, the universe will be said to be lambda-dominated, expanding exponentially faster.  Maybe (we can't yet rule it out) even ending in a Big Rip as all structures are torn apart.