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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.
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?
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.
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.
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?
"Transparent" is a shit show that trades on every stereotype going. A man in drag is NOT a transsexual.
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.
Chas - The one, the only.
THANK GOD!!!
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.
If you reply, do so only to what I explicitly wrote. If I didn't write it, don't assume or infer it.
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.
http://www.rootstrikers.org/
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.
XML is like violence. If it doesn't solve your problem, you're not using enough of it. --AC
This should be modded funny not interesting, seriously people (who have mod points)
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.
--- Most topics have many sides worth arguing, allow me to take one opposite you.
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
Because they are really close together (on a cosmological scale). Gravity rules at this scale.
A house divided against itself cannot stand.