How Galaxies Are Disappearing From Our Universe

StartsWithABang writes: You've heard of dark energy before, and you know that it causes the expansion of our Universe to be accelerating. Instead of slowing down, distant galaxies are speeding up in their recession from us, rendering them unreachable from our point of view. But even though we can't see the light emitted from them today, we can still see the galaxies themselves! This article explains how this works, how no information gets lost, and what it means for the Big Bang.

Comments

Balloons

[Intrepid imaginaut]

What always bothered me about the balloon analogy was the implication that this expansion of space is mostly taking place where there's little actual matter, ie the space between galaxies. If it really was expanding like dots on a balloon, we'd see equivalent expansion within galaxies and as far as I'm aware we don't, at least not to any significant degree.

Actually the whole thing is bothersome, if a galaxy was x light years away at some point in the past and it's now 2x light years away due to space expanding, doesn't that mean space has been created between the galaxies, and doesn't that violate some fairly fundamental laws of physics?

Re:Balloons

[wonkey_monkey]

As the AC's have said, gravity and other forces between particles completely swamp the "force" of expansion - for now. Imagine putting sheets of paper on a rubber sheet, and stretching the sheet. To an approximation, the centers of the pieces of paper would remain on the same spot on the sheet, while the rest of the paper would have the rubber sheet sliding out from under it, but the piece of paper would resist this "pull" on its edges easily.

If the expansion of space is expanding, though, there will come at a time when this "force" (if it is a force, I'm the wrong person to ask) becomes more significant. First, gravity won't be able to hold galactic clusters together, then galaxies, then solar systems, then the forces between and within atoms won't be enough to keep them together.

Sorry, bit of a downer to end on.

Re:Balloons

[Anonymous Coward]

If it really was expanding like dots on a balloon, we'd see equivalent expansion within galaxies and as far as I'm aware we don't, at least not to any significant degree.

The dots are not permanently fixed to a given location on the balloon. Imagine the dots are some mark on the balloon, but some slightly sticky raisin or such. If you attach a small spring between two near by raisins, they will stay close together, if the spring is strong enough, when the balloon expands.

created between the galaxies, and doesn't that violate some fairly fundamental laws of physics?

No? Exactly which fundamental law do you think it violates?

Re:Balloons

[ITRambo]

Yes. Space has been "growing:" since the beginning of time. A galaxies mass warps space/time so it is not affected by the ongoing expansion of space itself. Like adding water to soup already on the pot, the solids in the soup do not expand from within when the water is added, Instead of being pushed apart, galaxies are attracted to one another. Our local group is an example of that. http://en.wikipedia.org/wiki/L...

Re: Balloons

[tysonedwards]

The counterpoint is that all that mass, ergo gravity, existed 91 billion light years closer together just 13 billion years ago. As such, if such immense gravity was ultimately incapable of overcoming the "expansion problem" that you describe, how could a galaxy hope to with its much, much, much smaller mass by comparison?

At the same point, whose to say that over the time scales we're talking about that galaxies, solar systems and everything *haven't* increased in size proportionally relative to the expansion of the universe, minus the drag coefficient of gravitational forces?

There certainly is precedent for that, albeit ones with other quite plausible preeminent explanations, such as the gradual increase of orbital distance of our moon relative to the earth, the earth moving ever so slightly away from our sun year over year, and similar trends observed elsewhere within other orbital bodies within our solar system.

Re: Balloons

[Anonymous Coward]

The counterpoint is that all that mass, ergo gravity, existed 91 billion light years closer together just 13 billion years ago. As such, if such immense gravity was ultimately incapable of overcoming the "expansion problem" that you describe, how could a galaxy hope to with its much, much, much smaller mass by comparison?

As you may recall, gravity is a 1/r^2 force. Things in a galaxy are very close together (by Universal standards), so their gravity is very strong relative to the expansion of the Universe as a whole. In some scenarios with bizarre kinds of dark energy (that are ruled out by experiment, by the way), the expansion can become so rapid that galaxies themselves get torn apart, followed by planetary systems, stars and individual atoms... culminating in a disruption of all matter called the Big Rip. Again, ruled out by experiment, but it's the conceptual possibility you noted; just doesn't happen in our Universe.

Re: Balloons

[gestalt_n_pepper]

But if space is full of virtual particles which represent energy and mass, then you are saying that energy and mass are created from nothing. No?

Re:Balloons

[nine-times]

What I've wondered, and I don't expect to get a satisfying response here, but could the balloon analogy be used to explain why space is expanding between galaxies. Like imagine you had a balloon that wasn't expanding, but was just filled up with a fixed amount of air. Now everywhere that you've drawn a galaxy, you're start pinching the balloon. Like somehow you find a way to squeeze and compress those places where a galaxy is drawn, and you don't let it rebound. You just keep pulling and pinching and squeezing the balloon in and in, which is sort of what gravity does.

If you did that, you'd see the areas of the balloon between the galaxies becoming more and more stretched out. Maybe that's just a weird thought with no scientific basis, but we have space constantly being drawn in and condensing around any gravitational center, and meanwhile expanding between all the gravitational centers. I think if I were a scientist studying these things, I'd be looking for a link.

Re: Balloons

[Immerman]

No. At least I don't think so. I'll admit my understanding is fuzzy, but a few points to think on:

As I understand it (poorly), virtual particles appear to violate mass/energy conservation by their very nature - but they don't actually possess mass or energy themselves. Instead they sort of run on an "energy debt" for a few moments before either annihilating or capturing some "normal" energy so that they can persist. I think the energy is believed to be borrowed from the vacuum energy field - so that essentially you've temporarily lowered the energy of space itself in order to create some virtual particles, and there is no net energy change within the volume.

Vacuum energy is *not* conserved - like dark energy it is created along with new space-time. Again this simply appears to be one of the fundamental laws of the universe - mass/energy conservation apparently doesn't apply to space-time itself, only to things *within* space-time.

And finally there's a real possibility that matter can be created from whole cloth without violating conservation: In creating mass you also create a gravitational field, which in turn reduces the gravitational potential energy of everything else in the universe (gravitational energy is always negative). That reduction in the potential energy of the entire universe *may* perfectly balance the increase in mass-energy of the new matter. This is actually one of the theories about the big bang - it may be that the entire universe, taken as a whole, contains exactly zero net mass-energy: in essence everything really was created from nothing, and if you squeezed it all back together again you'd find there was still nothing there. It's just that the nothingness has been separated from itself in such a manner that things can be built out of the pieces.

Re:Balloons

[Immerman]

Not quite.

The rate of dark-energy expansion is not actually changing - it's simply that as things drift apart you get more and more space between them, and since the space itself is expanding, the further apart you are, the more total expansion is occurring between you. Like stretching a rubber band with a couple marks near the middle - the ends will be getting further apart at a much faster rate than the marks near the middle, even though the entire rubber band is stretching at a constant rate.

That means anything that is *already* drifting apart will accelerate apart indefinitely, but if the forces between two objects are currently holding them at a fixed distance from each other, then the amount of space (and dark energy) between them is not changing, and thus the repulsive "force" of expansion between them will likewise remain unchanged.

Now for galaxies and even star systems - they're constantly shedding mass and energy, so *eventually* the gravitational bonds that hold them together may weaken to the point that they fly apart. And once that happens, and the space between objects begins to increase, only then can expansion begin to accelerate them apart even faster. For individual planets and stellar-remnants though, gravity is more than sufficient to keep the individual pieces in place.

As for atoms... the distances between subatomic particles are fundamental - you can't raise an electron further from the nucleus without adding an extremely precise amount of energy as a photon, and it will rapidly shed that photon to return to its stable base state. It's simply not possible for things to "drift apart" - they can only exist at very precise distances determined by their quantum wavefunctions. And since things can't get drift apart, expansion can't get it's foot in the door. Atoms have nothing to fear from expansion except loneliness.

Re:Balloons

[visualight]

Gravity slows down the expansion, and, it slows down the passage of time. Time and space are not merely relative they are one and the same. The expansion of space and the passage of time -are the same thing- .

Perhaps at some point space-time expands so fast that it actually rips and a huge expanse of the universe is flooded with quantum energy made matter. Uniformly, throughout the expanse. This matter then slows down the expansion -temporarily though, as matter collects together due to gravity and creates new "pockets" where the speed of expansion can pick up again.

Re:Balloons

[Dutchmaan]

That all depends on a gravity relative to where you are in it. IANAS. Say you are decending into a black hole. As you approach the speed of light space time becomes compressed. If you are in this compression your world around you would still seem normal subjectively speaking, but the space relative to you outside the black hole would not only be expanding, but it would be accelerating. That's my best laymans stab at it anyway.

Re: Balloons

[Anonymous Coward]

It's just that the nothingness has been separated from itself in such a manner that things can be built out of the pieces.

Ah, brilliant, the Emo-Legos theory.

Re:Balloons

[boristhespider]

"doesn't that violate some fairly fundamental laws of physics?"

Do you not think that one of the many thousands of theoretical and observational physicists who've worked on this model for decades would perhaps have spotted this flaw at some point in the last eighty years...? Of course it doesn't violate fundamental laws of physics. The whole thing is based tightly on general relativity, so regardless of whether you believe that relativity is being applied accurately to cosmology or not (I don't, not entirely) there is no suggestion of it violating any fairly fundamental laws. Conservation of mass/energy is absolutely guaranteed in relativity. (In two tightly-coupled ways - directly, and via the Bianchi identities which are nothing more than geometric identities along the lines of, but more complicated than, the Pythagoras theorem. Which one you take as more fundamental depends on your philosophy but in relativity the one implies the other.)

The balloon analogy is basically flawed. It's also flawed because it relies on one imagining (to the extent that one can, and no-one can actually do so since our brains didn't evolve to imagine 4d let alone 5d) a 3+1d balloon embedded in a 4+1d spacetime, through the analogy with a 2d balloon embedded in 3d space. This inevitably leads to people understandably querying where the centre is and wondering if it's in the middle of this 4+1d space. It also leads people to understandably ask why the galaxies aren't expanding.

Basically, they're not expanding because the theory doesn't apply in them. There are two ways of viewing this - the simple (but inaccurate) and the headfuck. The simple way of looking at it is that the cosmological expansion is extremely weak and is very easily overpowered by other, more local, forces. So galaxies are easily held together because the gravitational pull between stars in a galaxy is overwhelmingly stronger than the pull of the cosmological expansion. This, unfortunately, does suggest there's some kind of balancing of forces and some kind of spatial expansion, which isn't strictly speaking true.

The headfuck is something that's actually almost impossible to model but straightforward to understand in relativity. The theory that the balloon analogy is based on is Friedman-Lemaitre-Robertson-Walker (FLRW - we're probably missing a name or two in there, as well) cosmology, based on what's known as the FLRW metric, which does nothing more than give the Pythagoras theorem in a 3+1d universe made up of an inverted pyramid of flat 3d spatial surfaces stacked one on top of the other along some time direction. (They could also be a load of nested spheres, or more bewilderingly a pile of saddles, but the data supports the flat model and there's currently no real reason to favour the so called closed or open models.) The FLRW metric applies on scales at which the universe seems to look the same in every direction and wherever you move to. In the jargon, it's "homoegenous and isotropic". Things like the SDSS surveys demonstrate how this can happen quite well -- take a look at http://www.a.phys.nagoya-u.ac.... which is the collection of data from the first SDSS survey (which ended about a decade back, I think; we're on SDSSIII or thereabouts now but I like this figure). On small scales this is obviously really knotty and far from homogeneous, but if you zoom out and squint slightly (to give a form of smoothing) then everything looks the same. Doing this a bit more rigorously, which is notoriously model-dependent, gives the "homogeneity scale" at somewhere in the order of 100Mpc, or about a hundred times larger than a typical galaxy cluster. That's the scale at which the FLRW model applies -- and that's the scale at which every single consequence can be said to hold. Below that, nothing that it says should be taken without a massive pinch of salt. This is particularly true in clusters, which are what is known as 'virialised' and detached from the cosmological expansion -- t

Re:Balloons

[boristhespider]

He thinks it violates conservation of mass/energy. It doesn't, of course, but it's not entirely unreasonable to ask whether it does if you don't know where the theory actually comes from.

Re:Balloons

[Trax3001BBS]

if a galaxy was x light years away at some point in the past and it's now 2x light years away due to space expanding, doesn't that mean space has been created between the galaxies, and doesn't that violate some fairly fundamental laws of physics?

I read a book that mentioned our on expansion, this was before dark energy was attributed to it.

It stated that E=MC^2 also means Energy can be converted into Mass, and in the universe particles are popping up all over the place, and mass is gravity (not that this mass couldn't be dark matter).

The cite -I'd use besides the books name (I've read so many on Quantum Physics I'm at a loss to name to name it) - or hidden within this link http://en.wikipedia.org/wiki/M...–energy_equivalence

I notice a submission just posted "Experiments create matter from nothing using nutrinos".
http://slashdot.org/submission...
A second way?

So any space between them is being filled up or in the books case the cause of the expansion.

Re:Balloons

[tyme]

AC wrote

I think you understand. Yes, the hypothesis is that space itself is being created, and that this is a fundamental law of physics. There's no fundamental law for it to violate, there's conservation mass and energy, no conservation of space.

But there is conservation of angular momentum. If you insert extra space between two objects that are rotating about a common center of mass, then either their rotation must slow down, or you are getting fee energy from the expansion of space. I'm not personally opposed to either outcome, but I'm curious to know which is the observed fact.

Re: Balloons

[rtb61]

Much like the very small where we can still only make guesses, the very big also is something we can only guess about. What is really happening between galaxies, the place where all the solar winds from billions of suns within each galaxies goes to die and what impact does it have on light as it travels those distances. Likely things are happening between galaxies that we for quite some time will only ever be able to guess at. It is not like they are empty, after all the suns from each galaxy are pouring high energy matter into them all off the time.

Re: Balloons

[KiloByte]

If space is expanding everywhere, including the distance between atomic sub particles, wouldn't they collapse again due to gravity/electromagnetic forces back to the normal energy well positions?

That would generate energy out of nowhere, making every atom, nucleus and baryon unstable.

Re: Balloons

[Immerman]

Well, it's pretty close to completely empty, estimates for the mass-energy density of the intergalactic medium put it at roughly equivalent to one proton per cubic meter - compared to the one proton per cubic centimeter of interstellar space, or the . And of course the vast bulk of the universe's volume is actually inter-cluster space (or probably more accurately and inter-super-cluster space), and you'll probably get a similar density drop off as you move into each progressively more isolated region.

For another reference point - it's estimated that the average mass-energy density of the universe - including dark-matter and dark-energy as well as normal matter and energy, is roughly 6 protons per cubic meter - with virtually all of that being concentrated into galaxies (except dark energy, which is distributed uniformly) .

Re: Balloons

[Immerman]

Oh, though I suppose it's also worth mentioning the possibility that our universe exists as a false-vacuum, in which case there may be immense energies existing in the fabric of space-time just waiting for a single point to decay into a lower-energy state, releasing (some?) of those energies as new matter and energy, and catalyze a vacuuum decay cascade that would sweep through the universe at almost the speed of light, destroying everything in it's path in a new "big bang". It's theorized that that may have been the nature of the original big bang as well: completely empty false vacuum decayed into all the matter and energy we see today.

Re:Balloons

[wonkey_monkey]

The rate of dark-energy expansion is not actually changing

Isn't it? I thought we still didn't know.

Re:Balloons

[sjames]

There is no conservation of spacetime, so no violation of anything.

The space within a galaxy expands as well, but it doesn't amount to much Compared to intergalactic space, there's not much space in a galaxy. The expansion is counterbalanced by gravity.

Re: Balloons

[Intrepid imaginaut]

Perhaps those mysterious and very distant massive gamma ray bursts without the China syndrome chaser might be related?

Re:Balloons

[HiThere]

How would you go about observing this? For that matter, how could you tell how fast the universe was spinning (and in which dimensions)?

Perhaps the universe is spinning in n dimensions around some axis in the n+1th dimension. That would seem to mean that there would be a force seeking to expand the universe acting perpendicular to the axis in every dimension less than n. Unfortunately, it would also seem to mean that the force acting along different dimensions could well be uncorrelated in magnitude, so this is probably a bad model.

Re:Balloons

[rubycodez]

the expansion rate is believed to be about 75 kilometers per second per 3 megaparsecs (about three million light years). That is very small

Re:Balloons

[Trax3001BBS]

It stated that E=MC^2 also means Energy can be converted into Mass, and in the universe particles are popping up all over the place, and mass is gravity (not that this mass couldn't be dark matter)

Energy also can cause gravity, so the conversion process involved in e=mc^2, e.g. photons converting to a positron-electron pair, would not change total gravity involved (short of the fact you've now spread things out to different places).

No it might not change the total gravity involved but it would localize it.

It takes something to interact for this to happen, the interacting forces being mobile prior to this.

I'm not short on fact, just not as good a memory as I once had; it's replies like yours that help. and for that I thank you.

Re: Balloons

[Immerman]

Probably not - if the cascade propagates at just under the speed of light, then our first warning would probably be an insanely rapidly expanding disc of "fire" consuming the night sky as the nearest edge of the interface between universes rushed towards us, creating a seething expanse of mass-energy from the void itself - assuming photons are able to cross the boundary at least. Otherwise we'd probably see a blotting out of the stars instead, possibly accompanied by scintillating reflections of stars still on our side. Either way the sky would go mad, and relatively swiftly thereafter we'd experience total annihilation as the conversion front crosses us and makes it impossible for mass-energy as we know it to exist.

Re:Balloons

[Immerman]

True, I spoke from a point of ignorance. From what I can tell though it seems like most of the people arguing for a changing rate of expansion are arguing that it's slowing down rather than speeding up. In any case though, I've yet to find a reputable source on the subject.

Re: Balloons

[Waccoon]

Space is a giant roll of bubble wrap?

Re: Balloons

[hackwrench]

Over what seem to us as long periods of time all atoms are unstable.

Re: Balloons

[hackwrench]

According to the article, the universe closest to us is or maybe was decelerating but the universe further away is observed to be accelerating. The article doesn't seem to be clear that the light we are seeing from both originated at different times so everything might have been accellerating at the time the further away light originated but everything was decelerating at the time the nearest light originated.

Re:Balloons

[vandamme]

>> doesn't that violate some fairly fundamental laws of physics?

Yeah, if there was something in the space, instead of just plain nothing.

Re:Balloons

[cavebison]

> What always bothered me about the balloon analogy

That's because it's an analogy - they're not meant to be perfect representations of reality.

Re:Balloons

[boristhespider]

A bit of both an objection to FLRW itself, and to dust solutions. On a fundamental level, the FLRW solution can never be more than an approximation to the real universe until we find a way of mapping the small scale physics up to a universal scale - since no well-defined averaging procedure exists (regardless of whether we're doing 3d averaging, 4d averaging, or some kind of statistical averaging) that does this with any rigour and generality, we can't actually ever state that the universe is FLRW "on average". On a more practical level, I don't think anyone would seriously question the applicability of FLRW+perturbations in the radiation-dominated universe, or indeed in the matter-dominated universe up to a pretty late redshift (arbitrarily, somewhere between z=5 and z=1). I certainly wouldn't, myself, and have done quite a bit of work on perturbation theory in the early (ie pre-CMB) universe without feeling the slightest twinge. Where it gets a bit more dubious is when those perturbations actually begin to grow, and in particular when the linear perturbations grow through 1 (ie when delta = delta rho/rho >=1) at which point the expansion is very definitely long since dead, and also when the *second-order* perturbations grow through the first order, at which point the expansion is also basically dead. This happens in the relatively recent, and therefore dust-dominated, universe. It's one reason averaging became so in vogue is that if we could find an effective dark energy, we'd solve both the cosmological constant problem *and* the coincidence problem in one go -- the dark energy is dominant now exactly because we live when there are massive inhomogeneities in the universe.

Of course, it hasn't worked, but that doesn't mean it isn't still an issue - in particular that we simply do not know how to write this theory down, so all we're working with is (extremely successful) phenomenology.

FLRW is unarguably a subcase of LTB; I'm not sure anyone would disagree with that. (LTB is also a subcase of Szekeres, while Minkowski space is a subcase of FLRW with constant scale factor. Minkowski is also a subcase of Schwarzschild with a vanishing mass, etc. There are reams of inhomogeneous solutions that are related in some pretty convoluted ways; "Inhomogeneous Cosmological Models" by Krasinski attempts to at least catalogue them. It's not the most readable of books, but he did try and make it comprehensive.)

Re:Balloons

[boristhespider]

It's impossible to say for certain at the minute since we can't prove anything, but my guess would be both -- we don't understand what can generate a metric on cosmological scales (in the sense of how it's composed of billions of billions of metrics that are best modelled by Schwarschild or Kerr-Newman), but if we did understand how to do so it would most likely by necessity still be a phenomenological description. The ideal would be that we'd end up with a situation similar to that of thermodynamics, which is an emergent theory and a phenomenological description, based ultimately via statistical mechanics on small-scale physics. Without that, we'll be as thermodynamics was in the 19th century, with a phenomenological description but no convincing way of demonstrating its validity.

Balloons

[Anonymous Coward]

What always bothered me about the balloon analogy was the implication that this expansion of space is mostly taking place where there's little actual matter, ie the space between galaxies. If it really was expanding like dots on a balloon, we'd see equivalent expansion within galaxies and as far as I'm aware we don't, at least not to any significant degree.

At that scale, gravity massively dwarfs expansion. For any system which is gravitationally bound, you can assume the "force" of expansion is trivial.

Actually the whole thing is bothersome, if a galaxy was x light years away at some point in the past and it's now 2x light years away due to space expanding, doesn't that mean space has been created between the galaxies, and doesn't that violate some fairly fundamental laws of physics?

I think you understand. Yes, the hypothesis is that space itself is being created, and that this is a fundamental law of physics. There's no fundamental law for it to violate, there's conservation mass and energy, no conservation of space.

Save the Galaxies!

[vortex2.71]

I am personally doing my part to conserve galaxies and I hope that all of you are too. Please, please, please help do your part to conserve this valuable resource before it is too late. Not just for today because it's Universe Day, but for life.

Re:Save the Galaxies!

[ArcadeMan]

I just donated 25 Dogecoin to the Save the friggin' Galaxies project to show how much I care!

Re:Save the Galaxies!

[Livius]

100 000 000 000 should be enough galaxies for anyone.

Re:Save the Galaxies!

[Tablizer]

Okay, but we'll have to kill whales to do it.

What?

[MichaelMacDonald]

Ok... What a long and convoluted way of saying galaxies are getting so far away that we can't see them anymore. He doesn't even explain why photons can't reach us from those distances. Not to mention, light can still reach us from a billion light years away, but travellling there at the speed of light is still instantaneous for the traveller. What mechanism changes this with greater distance? It makes no sense.

Re:What?

[pauljlucas]

The photons can't reach us from those distances because the amount of space between the far-off galaxies and us is increasing faster than light can travel.

Where's Sarah MacLachlan when you need her?

[jpellino]

Off merely saving a single breed of Doberhuahua. Piker.

Re:Pretty simple, really

[BarbaraHudson]

It's more likely they can't stand the insufferable whiners on the internet who can't stop talking about how horrible the world is.

The ping time would be horrendous,
The energy required tremendous,
But they don't want your stash of p0rn
To them naked walrus are stupendous.
Burma Shave

What would happen if the expansion of our knowledge outpaced the expansion of the universe? Is there a cross-over point, so that we (or our robotic descendants) will be able to literally control the universe? And if so, should we?

Or not

[wonkey_monkey]

You've heard of dark energy before, and you know that it causes the expansion of our Universe to be accelerating.

How do you know? Maybe I haven't, and maybe I don't.

This article explains [...] how no information gets lost

I'll admit I only skimmed the article - it is medium.com after all, and contains a ridiculous seven exclamation marks (plus an interrobang and a double exclamation mark in an image note, for pity's sake) - but I didn't see where this got explained.

Curious

[koan]

I read quite often that galaxies are moving away from each other at increasing speed.

In fact faster than light.

While special relativity constrains objects in the universe from moving faster than light with respect to each other when they are in a local, dynamical relationship, it places no theoretical constraint on the relative motion between two objects that are globally separated and out of causal contact. It is thus possible for two objects to become separated in space by more than the distance light could have travelled, which means that, if the expansion remains constant, the two objects will never come into causal contact. For example, galaxies that are more than approximately 4.5 gigaparsecs away from us are expanding away from us faster than light. We can still see such objects because the universe in the past was expanding more slowly than it is today, so the ancient light being received from these objects is still able to reach us, though if the expansion continues unabated, there will never come a time that we will see the light from such objects being produced ‘'today (on a so-called "space-like slice of spacetime") and vice-versa because space itself is expanding between Earth and the source faster than any light can be exchanged.

So that's confusing to me, wouldn't their mass increase as well and possibly lead to a massive attraction then collapse of the Universe back to the point prior to the Big Bang?
Or is it just the distance not the velocity relative to each other.

Re:Curious

[koan]

Ooops forgot this bit:
https://en.wikipedia.org/wiki/...

Re:Curious

[thrich81]

I going to take shot at this and press my luck, as I've already posted a comment about cosmology which is beyond my real understanding -- but ... The apparent mass increase you refer to is a Special Relativistic effect and Special Relativity only holds strictly in a flat, non-expanding space-time. Once you bring in General Relativity and curved or expanding space-times then your notions from Special Relativity don't hold, especially at cosmological distances. And the relation of velocity to red shift is not the same as in Special Relativity, since the cosmological redshift is caused by an actual expansion of the space between objects which is somewhat different from the notion of just relative velocities of the two observers' (flat) reference frames in Special Relativity. So the short answer is: General Relativity, especially at cosmological distances, is really strange and any intuition you (and I) have from a reasonable understanding of Newtonian physics and Special Relativity don't hold. Sorry, I can't do any better.

How galaxies are disappearing from our universe.

[BarbaraHudson]

Granted we can no longer see them, but that's a pretty arbitrary assumption to say that they have disappeared from our universe, any more than it's okay to say that it's us who disappeared. Even if we can't see them, it's a safe bet that they're still governed by the same laws of physics we are. It would be really strange if their (or our) laws of physics suddenly changed just because we can't see each other any more. Not being able to see each other is just one consequence of those laws in the universe we continue to share.

The long and short of it.

[Chas]

The universe is expanding.
As it expands, attractant forces (like gravity) hold less and less sway over things.
Without that "drag", more distant objects are speeding up.
We're starting to get to the point that certain objects are far enough away that, unless we find a BIG loophole in physics someplace, we'll never be able to reach them. And unless we find it SOON, we'll lose track of these objects, thus pretty much negating our ability to plot a course to them at all.

Re:The long and short of it.

[Immerman]

Actually we're not "starting to" get to that point, I believe the article stated that 97% of observable galaxies have *already* crossed that threshold - we're just still seeing the light that was emitted (mostly) long before they did so. And, like watching something fall into a black hole, we will continue to see their fading, increasingly red-shifted image for as long as we can construct instruments sensitive enough to detect the photons emitted in the last instants before it crossed the "event horizon".

Re:The long and short of it.

[vinlud]

The speed of light limit applies to matter flying in space, it doesn't apply to the expansion of space itself. The inflationary epoch shortly before the Big Bang is an example of this

Re:The long and short of it.

[vinlud]

No the increase of the expansion velocity is mainly because of the creation of space in between galaxies, not because of a decreased effect of gravity.

This is one of the weird properties of what is being observed, because it implies that our universe will be increasingly dominated by dark energy. Current dark energy accounts for 70% of the energy balance of the universe. With the space being created also containing 'new' dark energy this number will increase in the future, quite possibly exponentially based on the existing observations.

Re:Pretty simple, really

[binarylarry]

Or perhaps GP is a really sly Samsung slashvertisment.

"World so bad, I just need to feel my Galaxy S5!"

Re:How galaxies are disappearing from our universe

[pauljlucas]

It should really be stated as, "... galaxies are disappearing from our observable universe ..." which is little more than simply disappearing from view and not the actual universe.

Re:How galaxies are disappearing from our universe

[Concerned Onlooker]

Yep. From the article:

"And while no galaxy has literally disappeared to the point where it's invisible, 97% of them have disappeared in the sense that they're unreachable to us, and that the light they're emitting today will never reach us. The galaxies are still visible, but only due to their old light."

They're not disappearing from THE universe, they're disappearing from OUR universe.

'Big Rip' better than Heat Death

[Roger W Moore]

Sorry, bit of a downer to end on.

Not really. Before we had Dark Energy the ultimate fate of the universe was to expand up to a finite size and sit there for ever until all the stars died and the Black Holes evaporated leaving and empty, dead universe going on forever.

Now we have an unknown fate since we have no idea what will happen when the Dark Energy density causally disconnects points at the Planck-length, the so-called "Big Rip". I'll take the unknown over permanent, eternal heat death any day.

Re:'Big Rip' better than Heat Death

[Immerman]

Well, either that, or the "Big Crunch", or infinite expansion at an ever-decelerating rate. The "Critical universe" you describe was always vaguely improbable - one atom more or less and you end up with one of the other two. You almost need a deity fine-tuning things for it to be a viable option. We discovered dark energy accidentally when trying to determine which of the three possible outcomes mandated by existing physics was most likely.

It had been assumed that the universe was in a steady state primarily for historical, religious reasons: God created the Heavens and the Earth to last forever and ever amen, and that myth wound its way into our assumptions about the world. Had this latest burst of scientific advances originated within Hindu cultures rather than Christian ones, the idea of a steady-state universe would likely have been considered ridiculous - probably the "Big Crunch" (and subsequent Bang) would have been the default assumption. In fact that would probably have been the assumption of most religions - cyclicity is a rather fundamental theme in most religions that don't completely disregard the realities of the world.

Re:'Big Rip' better than Heat Death

[Roger W Moore]

Yes the critical universe was always rather improbable but the early supernova data pushed us into eternal expansion (before it was realized that it was actually accelerating) which ultimately is the same thing: heath death.

I don't buy the religious input at all though. The reason for assuming a steady state universe was simply because the local universe appears relatively constant and unchanging i.e. in a steady state. It is only when you look at the largest possible scales that you realize that things have changed very significantly and that has only been possible in the past century. Indeed one of the strongest proponents for the 'steady state' universe was Fred Hoyle (he actually coined the term Big Bang to deride that theory) who was a lifelong atheist.

Re:'Big Rip' better than Heat Death

[Anonymous Coward]

Before we had Dark Energy the ultimate fate of the universe was to expand up to a finite size and sit there for ever until all the stars died and the Black Holes evaporated leaving and empty, dead universe going on forever.

Without dark energy, what you describe would be impossible, and the idea of it reaching some static point was not a serious idea since Hubble et al demonstrated expansion. If the universe expanded up to some finite size, then gravity would eventually overcome that and recollapse. Heat death was achieved by having things expanding at an ever decreasing rate but not reaching zero expansion, because there was more momentum in a sense originally than could be overcome by gravity.

Re:'Big Rip' better than Heat Death

[RelaxedTension]

Heat death gives us an awful long time to work on figuring out extracting vacuum energy, and leaves a viable universe for us to maintain a bubble of livable space in. Given the trillions of years we would have, it should be long enough to figure out how to big-bang our own new universe as well, assuming we still need to inhabit that kind of physical space after that long.

Of course, the big rip would give us a reason not to procrastinate for billions of years, so I guess there's something to be said for having a literal drop-dead timeline to motivate you.

Re:'Big Rip' better than Heat Death

[Immerman]

Being an atheist in no way insulates you from religious-based preconceptions - at least not when those preconceptions are interwoven into fundamental, unstated assumptions upon which the culture you grew up in is based.

Take the idea of a clockwork universe as another example - which pervaded western thought for many centuries, and still does among many armchair physicists and a few professional grade theoreticians. Such a concept is directly in line with the idea that the universe was made perfect by God, but conflicts with the the fundamental assumptions of many/most other major religious creation myths.

Re:Balloons speckled with paint...

[taiwanjohn]

A spot of paint on your balloon would locally restrict expansion as it inflates, as galaxies seem to do in our expanding universe. My understanding of current hypotheses is that dark matter plays the role of "paint" in this analogy. However, there's an intriguing alternative explanation, which only becomes apparent when you think of space as a fluid.

Ironically, I stumbled upon this notion after musing on the strong interaction. (And I confess I was a bit high at the time.) Something that repels at a distance but attracts in proximity... that reminds me of bubbles interacting in the surface tension of fluids. So I googled "space as a fluid" and found that there's a whole branch of inquiry in this direction. It doesn't get as much attention as String Theory, but it's not dismissed out of hand either. (Correct me if I'm wrong... IANA physicist.)

Anyway, I'm curious to hear others' thoughts on this.

What happens to the photons?

[kheldan]

If energy can neither be created or destroyed, only change forms, then what happens to the photons emitted by starts when those photons reach the edge of the Universe? I'm just looking for theories, here.

Re:What happens to the photons?

[thrich81]

I'll probably get busted by some cosmologist for this, but -- there is no "edge" to the Universe, at least not one a photon or anything else can can travel to. For one thing, if you try to look far enough in any direction, from anywhere, you will eventually see space expanding away from you faster than light so the photons at the speed of light can go forever without catching up. More philosophically, because the Universe is "practically" (this is where I will get busted) defined by the space-time manifold which provides the coordinate systems to measure any events in it, there is no definition of an "edge" where there is space-time on one side and something else on the other, at least which anything in this Universe can get to or observe. Any notions of Galilean motion where you go far enough for long enough in one direction and you will eventually get to the end of any bounded region is defeated by the structure of space-time.

Re:What happens to the photons?

[boristhespider]

As a cosmologist, I can comment that the entire theory being talked about is based on a particular solution of GR, which involves defining a particular metric living on a manifold and endowing it with dynamics via the Einstein field equations - meaning I don't have much argument with what you said.

Re:Pretty simple, really

[Anonymous Coward]

This should be modded funny not interesting, seriously people (who have mod points)

Appearences can be decieving.

[Z00L00K]

What we see is that the expansion of the universe appears to be accelerating, but what if there's another explanation?

Re:Appearences can be decieving.

[itzly]

but what if there's another explanation

Then we'll consider it.

Re:How galaxies are disappearing from our universe

[Wraithlyn]

disappear [verb]

1) to cease to be seen; vanish from sight.

First it was planets ... now galaxies

[Hognoxious]

They're probably just being reclassified as "dwarf star conglomerations" or somesuch.

Re:How galaxies are disappearing from our universe

[Immerman]

Well, it's a lot more than simply disappearing from *view* - passing behind a dense dust cloud would do that. They are disappearing from all causal contact with us - unless special relativity is wrong, and it *is* possible to travel faster than light (with all the causality-breaking problems that would entail) those galaxies no longer exist from our perspective: it is theoretically impossible for there to be any further interaction between us, ever. For all intents and purposes they have completely ceased to exist when they cross that threshold. At least from our perspective - from their perspective of course it would be *us* who ceased to exist. Which makes me wonder where the whole "information can't be destroyed" aspect of the discussion came from: just because *I* no longer have access to a book doesn't mean the book has been destroyed.

Re:Listen!

[Immerman]

That sounds like an intro to a "Look Around You" style pseudo-science humor piece. If so, please point me to it, I could go for a good laugh.

Re:How galaxies are disappearing from our universe

[BarbaraHudson]

Can there still be interaction between the galaxy that just disappeared, and a galaxy mid-way between us? Yes.
Can there still be interaction between the middling galaxy and us? Yes.
Just because we can't interact directly with it doesn't mean that all influence and interaction is gone.

Re:How galaxies are disappearing from our universe

[BarbaraHudson]

But can those far-away galaxies still interact with their closest neighbors? Yes.
And they interact with others that are closer to us.
Wash, lather, rinse, repeat enough times, and they still indirectly interact. They have not exceeded C in relation to all their neighbors.

The Hubble non-Constant?

[Immerman]

I'm no expert, but I'm pretty sure he's wrong towards the end when describing the Hubble Sphere "catching up with" light emitted from objects already outside it.

Wikipedia's article on the Hubble Volume states:

However, the Hubble parameter is not constant in various cosmological models[4] so that the Hubble limit does not, in general, coincide with a cosmological event horizon. For example in a decelerating Friedmann universe the Hubble sphere expands faster than the Universe and its boundary overtakes light emitted by receding galaxies so that light emitted at earlier times by objects outside the Hubble sphere still may eventually arrive inside the sphere and be seen by us.[4] Conversely, in an accelerating universe, the Hubble sphere expands more slowly than the Universe, and bodies move out of the Hubble sphere.[1]

Observations indicate that the universe is accelerating,[6] so that some objects that we can currently exchange signals with will one day cross our Hubble limit.

So it sounds like he's describing events that could happen in a decelerating universe, but evidence suggests that we're not in such a universe, so it's irrelevant. As I understand it the reason we can see beyond the Hubble limit is that, when the light we're seeing was emitted, it was actually still within the Hubble limit. But thanks to the expansion of the space it has traveled through, it has actually traveled a lot further than that by the time it reaches us. As an extreme example, consider the light being emitted by a galaxy just as it crosses the limit - it's traveling towards us at lightspeed, but the space it's traveling through is itself expanding away from us at lightspeed - the result being that it will travel an infinite distance towards us without ever getting any closer. Light emitted a fraction of an inch closer though will gradually gain on the expansion of space, and eventually reach us, though it will still have traveled far further than the distance between us when emitted.

Hmm, okay... a bit more reading and it sounds like maybe that wikipedia article is itself flawed. Basically the Hubble "Constant" (the rate at which the universe is expanding, currently ~70km/s per megaparsec), which determines the radius of the Hubble Sphere, is presumed constant in space, not necessarily in time, and is currently believed by at least some to be diminishing over time: i.e. the universe is expanding more slowly today (per unit size) than in the distant past - in which case if the trend continues the expansion will eventually slow down enough for our infinitely traveling light to start making some headway and the video would be correct.

I can't find any information though on what evidence we have that the constant is shrinking, or even any reputable sources that it's commonly accepted that it is - just forum discussions and popular science magazines - notoriously bad places to get reliable scientific information from. All the Hubble Constant-determining graphs appear to show a linear change in expansion with distance - which would seem to suggest that the Hubble constant is in fact constant over time - otherwise you'd see non-linearities emerge as a result of light emitted many billions of years ago passing through space expanding at a much faster rate. Wouldn't you? Or am I misunderstanding what the graphs are actually showing?

Re:The Hubble non-Constant?

[jonfr]

As he says (Veritasium) in a comment to his video.

"Some people have been sending wikipedia references saying parts of this video are wrong, but I think it's wikipedia with the misconceptions. For further reference check out this paper: http://journals.cambridge.org/..."

I don't know where this misunderstanding has come from.

Re:How galaxies are disappearing from our universe

[stevelinton]

Can there still be interaction between the galaxy that just disappeared, and a galaxy mid-way between us? Yes.

Can there still be interaction between the middling galaxy and us? Yes..

Both true, but these interactions don't combine. Suppose you have three galaxies in a line A--- B---C and A and C are just leaving causal contact.
Suppose a light-speed message is sent from A towards B and C. B will indeed receive it, and be able to reply to it (maybe) but that will happen just as B and C leave causal contact (the universe having carried on expanding), so that if that message is forwarded towards C it will still not arrive. The photons in the forwarded message cannot overtake those in the original message that are still flying from B towards C.

If B and C are close enough to be gravitationally bound then A will lose contact with both of them at the same time.

Re:How galaxies are disappearing from our universe

[stevelinton]

Doesn't work. If you try and relay light (or any other message) along the line from the distant galaxy to us, what happens is that it reaches each relay station just as the relay station loses contact with us. It never arrives.

Re:How galaxies are disappearing from our universe

[Thiez]

No, because it would take some time for the far galaxy to interact with the mid-way galaxy, and it would take some time for the mid-way galaxy to interact with our galaxy, and if we add those times we find that while we were waiting the mid-way galaxy has also moved out of our range.

Re:How galaxies are disappearing from our universe

[BarbaraHudson]

If B and C are close enough to be gravitationally bound then A will lose contact with both of them at the same time.

Objects don't have to be gravitationally bound to influence each other. A rogue plantoid passing through our system isn't gravitationally bound to it, but our gravity still can modify its path.

Re:How galaxies are disappearing from our universe

[BarbaraHudson]

Prove it, say, with gravity.

Re:How galaxies are disappearing from our universe

[stevelinton]

If B and C are close enough to be gravitationally bound then A will lose contact with both of them at the same time.

Objects don't have to be gravitationally bound to influence each other. A rogue plantoid passing through our system isn't gravitationally bound to it, but our gravity still can modify its path.

You're right, but you've misunderstood my point. If A, B and C are all "far" apart then all the distances are increasing at an accelerating velocity and the situation is as I described it. The last paragraph deals with the special case where B and C are close enough that they are not accelerating apart. In this case B and C will remain in contact forever, and so A will lose touch with both of them at the same time.

Re:How galaxies are disappearing from our universe

[stevelinton]

What sort of proof do you want, and how does gravity come into it? Happy to try.

QM

[StripedCow]

Perhaps somebody can explain this to me.

If in Quantum Mechanics anything can happen with a certain (perhaps very small) probability, then in an infinite amount of time anything will in fact happen.
This proves that the universe will in fact never collapse.

Or does it?

Re:Where is the center?

[boristhespider]

Yes you are, and because you're not educated in the field.

"The article assumes that planet earth is the center of the universe"

No it doesn't. Cosmology does not assume that the Earth is in the centre of the universe. It assumes the exact opposite. It's even known as the "cosmological principle" -- and it's a fundamental axiom in cosmology. Without it we wouldn't have the model that we're talking about. Instead we'd have Lemaitre-Tolman-Bondi models, which are isotropic around the Earth but definitely not homogeneous.

Basically, building the cosmological model goes like this:

1) Observe the CMB. This is all around us, at 2.7K, and is absolutely the same in every direction. It is, in the jargon, isotropic around the Earth.
2) Assume that gravity on large scales is accurately modelled by a geometric theory of gravity (such as, but not restricted to, general relativity). We now know that on average the universe should be described by a metric that is at least isotropic about a point near to Earth.
3) Since this is obviously absurd, as you've picked up on, apply the cosmological principle. If the Earth is not in a special position in the universe, which it would be an astonishing act of hubris to assume it is, but the universe looks isotropic around the Earth, then there are only two choices. We can either dump the cosmological principle and assume the universe is centred on Earth -- which is... untenable, given the vast scale of the observations -- or we can assume that the universe looks isotropic around every point. This implies that it is homogeneous and isotropic: every point is the same in every way.
4) We can now tighten our previous assumption and assume that the universe is modelled by a metric that is isotropic around every point. That means that it is composed of what are known in the jargon as "maximally-symmetric" 3d surfaces. This leads us naturally and inevitably to the Friedman-Lemaitre-Robertson-Walker metrics, which give rise to the "big bang" theory you dislike so strongly.

There are obviously problems here. The phrase "on average" is used frequently and without rigour. That rigour cannot, as yet, be provided. We have assumed twice the nature of gravity - first that it is geometric in origin, and second that it is described by general relativity, which is basically the simplest geometric theory of gravity. Fitting to observation also leads us, naturally and inevitably -- unpleasantly so, if we're being honest -- to dark energy and dark matter. But there is a need to "create these terms", in that the theory demands them, and the theory is *astonishingly successful*. One of the main successes of FLRW cosmology is that it first predicted a characteristic wavelength of ripples on the cosmic microwave background, which was then observed (and which can be used to determine how much dark matter there is relative to normal matter), and that that same wavelength should also be imprinted on the large-scale distribution of galaxies. This was *also* observed, and is exactly where it was predicted by combining CMB and supernovae observations. This is amazing not least because the theory predicts the CMB forming when the universe is around 300,000 years old, while the large-scale distribution of galaxies is observed when the universe is pushing on a bit, probably around 10bn-12bn years old. The wavelength on the galaxy distribution is therefore extremely stretched compared to that seen on the CMB. And, as one might expect, the level to which it is stretched is extraordinarily sensitive to the cosmology - it doesn't take much of a change in the levels of matter, dark matter and dark energy to put it slap bang in the wrong place entirely.

Doing this unfortunately means we need to put dark energy in the model. Unsurprisingly, this isn't as ad-hoc as it seems, since there are multiple candidates for a dark energy, but it's still a bit unfortunate since not many of them are profoundly appealing. (Perhaps the most appealing is also the original, proposed by Wetterich in 1987, s

Re:How galaxies are disappearing from our universe

[BarbaraHudson]

But they were already interacting before this all started.

Re:How galaxies are disappearing from our universe

[BarbaraHudson]

To get very far away from us they started receding from us at a higher speed than objects that are closer. However, nobody can point to where an object "disappeared" - it's all conjecture unsupported by experiment or direct observation. Who knows, maybe when the fabric of the universe gets too thin, the repulsive force becomes an attractive force. We simply don't know enough yet.

Re:How galaxies are disappearing from our universe

[Immerman]

But they weren't interacting with each other's *present*. When we interact with a galaxy a billion light years away, we're experiencing the influence of what they did a billion-plus years ago, while they experience the influence of what we did a billion-plus years ago.

They may still be under the influence of "Galaxy 3" that has already left our Hubble Sphere, but it will be the influence of that galaxy's distant past, while it was still within our Hubble Sphere. By the time they get influenced by Galaxy 3's present, they will have themselves moved outside our Hubble Sphere.

(please ignore the fact that, strictly speaking, concepts like "present" are poorly defined over relativistic distances - we need some sort of absolute reference frame to keep this conversation from getting really complicated)

Re:How galaxies are disappearing from our universe

[stevelinton]

To get very far away from us they started receding from us at a higher speed than objects that are closer. However, nobody can point to where an object "disappeared" - it's all conjecture unsupported by experiment or direct observation. Who knows, maybe when the fabric of the universe gets too thin, the repulsive force becomes an attractive force. We simply don't know enough yet.

Of course. Anything could happen, but there is a remarkably consistent, and mathematically simple, if somewhat unintuitive picture emerging of how the universe has evolved on the largest scales. The picture in general (dark matter, dark energy, etc,) is consistent with a number of independent sets of data, for example supernova surveys and detailed analysis of the cosmic microwave background. The article is trying to explain the consequences of this picture.

What we can see are galaxies at very high redshifts and evidence for accelerating expansion. If the dark energy explanation for the expansion is right, then lighjt emitted from those galaxies (which we can see) a few billion years after the light we see them by now, will never reach us. Of course some unknown thing could intervene to prevent this happening, but we see no sign of such a thing yet.

Re:Pretty simple, really

[Lumpy]

Nope, the heavy accelerations away from ours started the day microsoft windows was first announced.

What tfa fails to mention

[tpjunkie]

And helps make sense of the situation is that when a galaxy becomes causally disconnected from us, it's not that the distance between us has expanded so far that light no longer has "time" to reach us, it's that the photons carrying that information have become so redshifted that they have a wavelength equal to or larger than the observable universe and are thus undetectable, although in practice this happens well before reaching a wavelength that large

Re:How galaxies are disappearing from our universe

[BarbaraHudson]

Obviously at one point these objects were closer together. Therefore, they had some (maybe infinitesimal, but still not zero) effect on each other, either through bending of space/time, or through gravitons (take your pick which). If two galaxies are receding from each other in exactly opposite directions, and from a frame of reference between the two each is apparently receding at 2/3 c, then neither object appears in the other's frame of reference. However both can influence, and be influenced by, the object between the two, so even though we can't see them, they' still continue to (indirectly) affect our visible universe, and us. Anything that affects us, even indirectly, is still by definition "in our universe." We just can't detect it directly.

Or, simpler, shine two flashlights at each other. The photons from both are traveling at c relative to you, the observer. It should not be possible for them to interact with each other because they're traveling towards each other, relative to each other, at 2c. But observation tells us they interact. To an observer on either photon, it appears as if the other photon doesn't exist, since it would "disappear" at the same time that it "appeared" (or worse, disappear before it appeared). Time of its' existence in the other's frame of reference is zero, but they still interact, even with relative velocities greater than c, and information IS exchanged.

The only apparent way out of this is to say that time and space are both quantized at our scale. Time doesn't flow smoothly, but rather is a series of "ticks". So, even if the theoretical frame of reference of each photon viz the other is well over c, for a minimum of 1 tick (since you can't have half a tick) they can interact. Of course, this brings with it another set of problems, but that's the fun of it.

Re:How galaxies are disappearing from our universe

[BarbaraHudson]

One simple mechanism for reversing the expansion that I came up with while walking the dogs: As "space" gets thinned out, the length of time that particles can exist before returning to the quantum foam gets longer, until it passes a threshold and is no longer "part of the foam." Given enough of this, the added mass causes the universe to reverse its expansion.

Re:How galaxies are disappearing from our universe

[stevelinton]

Could do, although there is no evidence of such an effect up to now. The laws of physics could also just change tomorrow for no particular reason, in thi sarea, or in some much more down-to-earth one, like whether the proton is stable. We can never know.

The article is essentially in the business of explaining the consequences of the laws as we currently conjecture them to be (which fit what we can observe pretty well). It can't make any stronger claim to be "correct" than that, but, apart from refining "pretty well" to "very well" nor can any physical theory.

Re:How galaxies are disappearing from our universe

[stevelinton]

If you're dealing with constant velocities, you are in the territory of special relativity. In this world there are no event horizons and every object can interact with every other. If two galaxies are each receding in opposite directions from a third central one at 2/3 c they will each see the other receding at 12/13 c according to https://en.wikipedia.org/wiki/... (section 2). Velocities do not add up the way you think they do and when they get to a decent proportion of light speed it starts to matter. This has been experimentally checked using moving atomic clocks. Thus they can keep on exchanging messages, although the messages will be quite redshifted when they arrive and take longer and longer to make the journey.

However, the original article deals with accelerating motions, since that is what the universe seems to be doing. This is crucial.

One way of seeing what happens is to imagine two galaxies accelerating away from one another. Assume there are clocks freely falling in both galaxies.
Define a function f so that a signal sent from one galaxy at lightspeed (could be photons, gravitons, neutrinos, doesn't matter) at time t on the local clock arrives at the other galaxy at time f(t) on its local clock. It's not hard (for anyone with a degree in astrophysics) to work out exactly what function f is. It turns out that there is critical time T such that as t approaches t from below, f(t) approaches positive infinity. In other words the last few signals emitted by one galaxy as it's clock ticks towards T are spread out across the whole of the rest of time when they finally catch the other galaxy and no signal emitted at or after time T can ever arrive. The critical time T depends on the current separation, velocity and acceleration of the galaxies in a fairly straightforward way. After local time T nothing you do can affect the other galaxy. After its time T you can never find out what happened to it.

Re:How galaxies are disappearing from our universe

[BarbaraHudson]

If we posit a standard distribution for the length of time that a particle from the quantum foam can exist, then it's inevitable that some will stay in existence for a very long time - long enough to start clumping together as larger chunks of matter. At that point, it's no longer a part of the quantum foam, but "real" particles. Maybe that's how this universe started - the particles in the vacuum came together in the Big Bang, and will do so in the future. All that's needed is some sort of distribution of the length of time that a quantum foam particle can exist, and lots of time and space.

Re:How galaxies are disappearing from our universe

[BarbaraHudson]

I noticed that you ignored the case of objects approaching each other, each with a substantial fraction of the velocity of c. I used that thought experiment to pose the question of how they interact, because we know from observation that they do, even though each one is approaching the other faster than c. From the frame of reference of one, the other never exits, unless time and space are not continuous at the smallest scales.

Let's make it two planets approaching each other, as seen from an observer slightly off to the side at the center point where they should collide. Does the observer see two objects collide? Yes. Do the objects interact? Yes. They don't just zoom through each other instantaneously for their own frames of reference and not for others, same as photons moving toward each other.

People are too focused on the "receding" behavior, and not the "approaching" behavior, and what it implies.

Re:How galaxies are disappearing from our universe

[Anonymous Coward]

I used that thought experiment to pose the question of how they interact, because we know from observation that they do, even though each one is approaching the other faster than c.

In flat space-time (i.e. under conditions suitable for special relativity, like things just moving around on a non-cosmological scale and not next to a black hole), you can never produce a situation where things travel slower than the speed of light in one frame look like they are traveling faster than the speed of light in another frame. And since in every frame the speed of light in vacuum will be the same, you never get a frame that has something disappear like that, due to moving faster than light.

If two planets approach each other at 99% of the speed of light as seen by someone in the center of them, then in the frame of each planet they will see the other planet approaching them at 99.995% of the speed of light. There is no approaching faster than c involved, in those or any legit frames of references.

Situations under general relativity can be different, where a photon traveling at c can no longer reach something. It doesn't matter what frame you are in, because all will agree that such a photon, or anything else at or slower than c (e.g. including gravity that propagates at c) is unable to reach certain places at certain times.

People are too focused on the "receding" behavior, and not the "approaching" behavior, and what it implies.

No, people are trying to focus on your lack of understanding of relativity, special or general. I don't mean this as anything personal, but you seem to fundamentally be missing some key pieces of how relativity work, and your point is moot because it depends directly on that faulty understanding.

Re:How galaxies are disappearing from our universe

[BarbaraHudson]

The quantum zeno effect is one example.

Galaxies Moving Away? Really?

[OYAHHH]

Alright all you experts out there riddle me this one. If the universe is expanding then exactly why are the Milky Way and Andromeda Galaxies supposed to collide in the future????

Re:Where is the center?

[m1h41]

What if the universe is actually falling in a black hole? Can that explain the accelerating expansion of space (and dark energy with it)? It would be consistent with what I read about falling in black holes: stuff gets streched in all dimensiona.

Re:How galaxies are disappearing from our universe

[Tablizer]

Only if cats occupy those galaxies

Re:Nonsense.

[Tablizer]

I like your sense of scrutiny, Sir.

Re:Where is the center?

[hyperfine transition]

I just read a SF story that used an idea like this - that analogous to the time dilation experienced as you fell into a 'normal' black hole, you would see spatial dilation as you fell into a black hole 'in time'. the idea that the universe is inside a black hole has been bandied about but I can't find a reference for the 'temporal black hole' idea ( I feel sure that the story I read was based on a scientific paper).

Re:How galaxies are disappearing from our universe

[stevelinton]

Constant approaching velocity is special relativity again, and again the velocities don't add the way you expect.If the planets in your example are approaching at 2/3 c they each see the other approaching at 12/13 c and they will very definitely and messily interact. Each exists for the other.

In this case acceleration makes no essential difference though. In either planets frame of reference there is an event horizon behind it (in GR acceleration and gravity are equivalent) but none in front of it, so they can see each other and interact freely.

Re:Galaxies Moving Away? Really?

[pedestrian crossing]

Because they are really close together (on a cosmological scale). Gravity rules at this scale.

Re:Where is the center?

[boristhespider]

I can't actually imagine a setup that would lead to that in vanilla relativity, but even if we assume it could exist it would introduce strong anisotropies into the universe. The very nature of falling into a black hole introduces a directionality, which would immediately produce anisotropies that we don't observe. If the hole is on a rough order of magnitude in scale with the universe, this would be even more obvious since rather than just having a general directionality constant throughout the universe, we'd now have a directionality depending on space (and time), focused on the centre of the hole. This would leave some characteristic signatures on the CMB that aren't observed.

If you wanted instead to embed this in higher-dimensional theories - so the universe is, say, falling into a 7+1d hole - then frankly no-one can give a full answer since it's not a setup amenable to full analysis (but then, neither is the one I've been discussing in the previous paragraph). I'd imagine it's possible to get a setup that doesn't introduce such anisotropies in the dimensions we're observing. I'm thinking of a setup where for instance we're on a 3+1d brane and falling along a 5th dimension into a hole of some higher dimensionality, which extends infinitely (or as near as is sufficient to kill any anisotroies) parallel to the 3+1d brane. You might even be able to get a toy setup along these lines using something like the Randall-Sundrum models that were all the rage 15 years back -- these are composed of two 3+1d branes suspended in a 4+1d spacetime, parallel to one-another. If you make one brane entirely "black" then you'd have a setup with one brane on which a universe can live, separated along a 5th dimension from a black brane. I have genuinely no idea if anyone's looked at such a system, nor whether it can be realised in an RS model, but I wouldn't be surprised if someone's actually examined it. If not it would certainly be interesting to look at.

Re:How galaxies are disappearing from our universe

[BarbaraHudson]

That doesn't have to do with things changing because you could see something or not, but because you keep forcing it into a specific state through interaction.

Seeing IS interacting :-)

Re:How galaxies are disappearing from our universe

[Concerned Onlooker]

And why would you ignore the semantics of my sentence except to be obtuse? Quite likely my meaning was understood by everyone but you.

You and your hair-splitting ilk are what take the fun out of online interaction.

Re:Where is the center?

[m1h41]

*Head Explodes*
Also thanks for the answer.

You wouldn't notice any space anisotropies if in relation to the black hole, the universe were positioned in a 'smooth' area which you could get if either the universe in sufficiently far away or the black hole is orders of magnitude larger than the universe. This scenario only makes directinality more subtle, to confirm I suppose you would look for the rate of expansion in different areas of the sky and see if there's a gradient, can that be done with sufficient precision?

Re:Where is the center?

[boristhespider]

Surprisingly, no. There are suggestions of an anisotropy in the Hubble rate in different halves of the sky, but the errors are too big for this to be significant. That's the main problem with doing anything of the sort -- the error bars on the observations are just too big until we get far enough away (as in, taking velocities from galaxies far enough away that there are loads of them) to beat them down by sheer power of numbers.

But what that could let us do is put a constraint on how far away a black hole of one form or another would have to be, since the level of anisotropy that would be introduced would be related to the distance, in one way or another -- for instance, in Randall-Sundrum braneworld models it would depend on what is known as the "dilaton" which is basically the distance between the branes but manifests on the branes themselves as another scalar field -- although such would obviously be model-dependent, meaning that the results for, say, a universe moving towards a hole in an RS-type model, may or may not be very different to those in a more sophisticated model in some other approximation to M theory. About the only thing I'd say in general is that *any* directionality is going to induce an anisotropy. If the directionality is subtle enough that it's drowned in noise of very local observations then the influence of the whole is itself going to be correspondingly minor. How minor I obviously can't say since I've not looked at it in anything more than speculation entirely unbacked by analysis, but I'd be stunned if it was going to introduce more than relatively small errors.

That doesn't say that it wouldn't be an interesting scenario (if of course it hasn't already been examined and as I said I'd be surprised if someone hasn't already looked at it, at least in the context of RS models), and it's also not to say whether or not the corresponding impacts may or may not be significant, but it would be a careful balancing act to keep something like a black brane far enough off to avoid anisotropies that aren't covered by the existing error bars and yet produce significant impacts. Not to say it can't be done, just it would take a bit of care.

Shape of the universe

[minyard]

What is the shape of the universe? Maybe that explains it.

http://www.youtube.com/watch?v=mFTMiVs4VhY

Re:Listen!

[DeputySpade]

It's a reference to a Dr. Who episode in the most recent series wherein The Doctor theorizes that a creature may have a perfectly evolved ability to hide from all physical observation yet your subconcious knows it's there. Like the feeling one sometimes gets of being watched when in what appears to be an obviously empty room.

Re:Listen!

[Immerman]

Ah. I've got to get back into watching those - I'm a long-time fan but it's just not worth paying for cable/Netflix/etc for a single show.

Re:Where is the center?

[boristhespider]

I may not have explained it very clearly. The point is that while near to Earth it's obviously in a "special place" -- no other planet in the entire universe has exactly the same conditions around it, in a relatively sparse arm relatively distant from the centre of a relatively large spiral galaxy in a relatively small galaxy cluster that's on the outer edge of a supercluster -- but that if you zoom out a bit and look at things on average, on scales roughly around a megaparsec in scale and above (which is the approximate size of a galaxy cluster, something like 100 times larger than our galaxy), it all begins to look eerily similar. On larger scales (let's say around 50Mpc and upwards) it all turns into a similar-looking mush of little bubbles where everything is basically indistinguishable from everything else. Attempting to pin it down properly, this "homogeneity scale" appears to be at somewhere between 75 and 250Mpc or so.

That's the point -- that the Earth isn't in a special place in the universe in that where we are isn't marked out as anything special. In an average sense, picking a random spot in the universe will lead to a view indistinguishable from that we have from the Earth -- if you ignore local eccentricities such as stars, voids and mighty blasts of raw radiation from supermassive black holes in galactic centres. That is, the assumption is that from anywhere in the universe, if you ignore everything within perhaps a kiloparsec or so, it's all going to look very much of a muchness. In particular, the CMB is going to look basically the same, a featureless wash of radiation at a constant temperature, with little ripples of about one part in 10,000.