Do We Live in a Giant Cosmic Bubble?

[fgalkin2]

linky

If the notion of dark energy sounds improbable, get ready for an even more outlandish suggestion.

Earth may be trapped in an abnormal bubble of space-time that is particularly void of matter. Scientists say this condition could account for the apparent acceleration of the universe's expansion, for which dark energy currently is the leading explanation.

Dark energy is the name given to the hypothetical force that could be drawing all the stuff in the universe outward at an ever-increasing rate. Current thinking is that 74 percent of the universe could be made up of this exotic dark energy, with another 21 percent being dark matter, and normal matter comprising the remaining 5 percent.

Until now, there has been no good way to choose between dark energy or the void explanation, but a new study outlines a potential test of the bubble scenario.

If we were in an unusually sparse area of the universe, then things could look farther away than they really are and there would be no need to rely on dark energy as an explanation for certain astronomical observations.

"If we lived in a very large under-density, then the space-time itself wouldn't be accelerating," said researcher Timothy Clifton of Oxford University in England. "It would just be that the observations, if interpreted in the usual way, would look like they were."

Scientists first detected the acceleration by noting that distant supernovae seemed to be moving away from us faster than they should be. One type of supernova (called Type Ia) is a useful distance indicator, because the explosions always have the same intrinsic brightness. Since light gets dimmer the farther it travels, that means that when the supernovae appear faint to us, they are far away, and when they appear bright, they are closer in.

But if we happened to be in a portion of the universe with less matter in it than normal, then the space-time around us would be different than it is outside, because matter warps space-time. Light travelling from supernovae outside our bubble would appear dimmer, because the light would diverge more than we would expect once it got inside our void.

One problem with the void idea, though, is that it negates a principle that has reigned in astronomy for more than 450 years: namely, that our place in the universe isn't special. When Nicholas Copernicus argued that it made much more sense for the Earth to be revolving around the sun than vice versa, it revolutionized science. Since then, most theories have to pass the Copernican test. If they require our planet to be unique, or our position to be exalted, the ideas often seem unlikely.

"This idea that we live in a void would really be a statement that we live in a special place," Clifton told SPACE.com. "The regular cosmological model is based on the idea that where we live is a typical place in the universe. This would be a contradiction to the Copernican principle."

Clifton, along with Oxford researchers Pedro G. Ferreira and Kate Land, say that in coming years we may be able to distinguish between dark energy and the void. They point to the upcoming Joint Dark Energy Mission, planned by NASA and the U.S. Department of Energy to launch in 2014 or 2015. The satellite aims to measure the expansion of the universe precisely by observing about 2,300 supernovae.

The scientists suggest that by looking at a large number of supernovae in a certain region of the universe, they should be able to tell whether the objects are really accelerating away, or if their light is merely being distorted in a void.

The new study will be detailed in an upcoming issue of the journal Physical Review Letters.

Pretty interesting implications, if true.

Have a very nice day.
-fgalkin

[cosmicalstorm]

Stuid question perhaps but why would it have to be so special for Earth to be in this bubble, couldnt it be that these bubbles are fairly common? (Or that there are a few really big ones spread out in the universe)

[Patrick Degan]

The only "bubble" I'm aware of our star system residing in is the Cygnus Loop, which does have a lower free hydrogen density than much of the rest of the surrounding galaxy. Don't know if that would make such a big difference in bollixing-up the observations, though.

[Ariphaos]

Stuid question perhaps but why would it have to be so special for Earth to be in this bubble, couldnt it be that these bubbles are fairly common? (Or that there are a few really big ones spread out in the universe)

Well, we're between two voids for certain. I'm not sure if it's 'special', it may well be that close proximity to a boatload of active galaxies is inimical to life.

The voids themselves are fairly common, but the thing is that, by their very nature, there is less matter in the voids, so being in one is 'more special' than not being in a void.

[Covenant]

Argh, no. Why all this when there's a perfectly good explination for the low density of material around Sol?

http://www.daviddarling.info/encycloped ... minga.html

Head to Atomic Rockets and scroll down a bit in the Bussard Ramjet section to learn about the local bubble and the other bubbles in the area, places where the gas density has been carved out by supernovae.

http://www.projectrho.com/rocket/rocket3aj.html#bussard

[Surlethe]

I don't think they're talking about a region of low ISM density. The way I read it, it sounded instead like they were talking about a much larger region of low galactic density (like the "Great Attractor", IIRC). The reason why hypotheses that violate the Copernican Principle are rejected is Occam's Razor, but that presumes that the hypotheses are otherwise indistinguishable. If it is possible to experimentally determine between two hypotheses, then Occam's Razor is mute. In this particular case, we don't want to unjustifiedly presume against the Copernican principle, but if we can actually gather experimental evidence for or against it -- as the authors suggest -- then there's really no philosophical problem following the evidence wherever it leads.

[Ariphaos]

Yar, this has absolutely nothing to do with the local Bubble/Fluff. We are basically right next to the northern local supervoid, and are basically a part of the filament that divides the northern and southern local supervoids. This article might imply that they are larger and closer, by some definitions, than we previously suspected, though it doesn't give specifics and that's just a guess on my part. Since we can't see certain major features of the Universe (such as the Great Attractor), it may be that this is indeed the case.

[Kuroneko]

As Surlethe said, what they're considering is actually a gigantic bubble on the scale of at least as large as the observable universe, i.e., the hypothesis that the overall density of the universe is much higher than what we observe around us--on the supergalactic scale, rather than around Sol. In effect, they're testing the hypothesis that the universe is isotropic but not homogeneous--which would mean we are in a very special place indeed. This has actual observational consequences among supernovae with mid-range redshifts--or at least, so they say in their paper--so fortunately it's a bit more than chucking out Ockham's razor.

[Covenant]

I'm a little confused. Am I not getting some aspect of the theory--how big would this bubble be? As big as the solar system? Or a few light years... or as big as the universe? Do they propose that the universe is the low-density bubble (which seems nonsensical) or that the solar system/local area to Sol is where the bubble is and thusly we percieve the outside world odder?

It just doesn't make sense to me, so I assume I'm not catching something. No matter where you place the bubble's boundaries, doesn't that change the frame of reference we have here so greatly that it would not match what we're actually seeing? For this theory to be true, wouldn't the bubble need to be in our region of the galaxy--or just around the Milky Way? How would that make any sense? Would it be around just the local group? But then how do we explain our relationship to the other galaxies in our supercluster?

I just can't find a good spot to put the bubble barrier in my head without it causing more problems than it solves, the issue of inflation and supernovae being far from the only thing we can use to tell where we are located in the Universe. Am I crazy, or is the theory?

[Kuroneko]

The bubble would have to be at least on the order of a Gpc or more in radius--much bigger than our supercluster. And you're right; we would have to be very close to the center of the bubble for us to see anything like what we actually do. Hence the reference to the Copernican principle: our galaxy would be in a very special place--the point around which the universe is isotropic, even though it is not homogeneous.

[Covenant]

The bubble would have to be at least on the order of a Gpc or more in radius--much bigger than our supercluster. And you're right; we would have to be very close to the center of the bubble for us to see anything like what we actually do. Hence the reference to the Copernican principle: our galaxy would be in a very special place--the point around which the universe is isotropic, even though it is not homogeneous.

Okay, so it is at least a pretty specific circumstance. This just seems like a backdoor method of putting Human Exceptionalism back out into play, a sort of anti-Pale Blue Dot. He mentioned research--how would one test for this. Is it a testable hypothesis at all? If they're going to be looking at many, many supernovae outside of our entire supercluster, how are they going to determine the difference between inflation and warped perspective?

They would need to choose a sample of both very distant supernovae and near supernovae, and compare them, right? But if their 'near supernovae' are too near then they wouldn't be distorted by inflation anywhere, since gravitational binding will still hold many of the universe's metastructures together. What's the smallest level where inflation starts to drag things apart from each other... I assume, less than a supercluster?

This just seems all kinds of bullshit with the additional problem of being hard to test for.

[Kuroneko]

It is indeed a very specific circumstance, and I'm sure that no competent scientist would take this as a representation of reality without some very hard data, so let's take a step back from the news article, which, as those things tend to go, is a tad over-sensationalized, and give the authors just a bit of the benefit of the doubt. In the original paper [1], they did actually fit both the near and far supernovae to this very peculiar model, but they also went on to describe a qualitative difference in the mid-range supernovae. In other words, the two models--this very special, inhomogeneous model and the standard homogeneous ΛCDM--can be made to agree on both ends of the SNe data, but not the middle, and the point of the paper was really to find this difference. There is actually a dearth of observations in that range because astronomers tend to concentrate their efforts on the very near or the very far, but it does mean that the model is falsifiable.

Without significant evidence, replacing dark energy with this sort of model is simply absurd. Expansion due to dark energy--especially the Λ-driven one in the standard model--is a very natural fit to GTR, whereas this treats our place in the universe as something extraordinarily special for no apparent reason. But there's nothing wrong with exploring alternatives and trying to falsify them; that's just normal science, and if at the same time we have an opportunity to test a very basic assumption about the Universe, the Copernican principle, all the better.

[NoXion]

How would this bubble relate to the recently discovered Dark Flow, if at all?

[Kuroneko]

Probably little relation; it addresses a different issue. However, it is probable that evidence for one constrains the other--but I don't know enough about that.

[TheKwas]

The bubble would have to be at least on the order of a Gpc or more in radius--much bigger than our supercluster. And you're right; we would have to be very close to the center of the bubble for us to see anything like what we actually do. Hence the reference to the Copernican principle: our galaxy would be in a very special place--the point around which the universe is isotropic, even though it is not homogeneous.

Sorry, I'm still confused. In the article they say:

Scientists first detected the acceleration by noting that distant supernovae seemed to be moving away from us faster than they should be. One type of supernova (called Type Ia) is a useful distance indicator, because the explosions always have the same intrinsic brightness. Since light gets dimmer the farther it travels, that means that when the supernovae appear faint to us, they are far away, and when they appear bright, they are closer in.

But if we happened to be in a portion of the universe with less matter in it than normal, then the space-time around us would be different than it is outside, because matter warps space-time. Light travelling from supernovae outside our bubble would appear dimmer, because the light would diverge more than we would expect once it got inside our void.

Don't we observe numerous supernovae within 1 gigaparsec of earth (the entire observable universe is about 14 GPC in radius), and wouldn't these supernovae indicate whether or not we live is such a bubble?

[Kuroneko]

It must be large enough for us to not directly observe the jump in density that the model postulates. In their paper, they reference another work that used WMAP data to constrain our location to within 15Mpc of the center, meaning 15Mpc is 'small' compared to the bubble. Also there, they subdivide the SNes into three qualitative classes based on redshift factor: 0<z<0.1, 0.1<z<0.4, and 0.4<z, and fit their model to the first and last of these. With the WMAP Hubble constant H = 70.1km/(s·Mpc), z = 0.10 corresponds to a distance of about 410Mpc, and z = 0.40 to 1.4Gpc. This numbers may be different in the void model, but they do show the order of magnitude.

Initially, I didn't bother reading the paper closely, just skimming their introduction and conclusion instead, and the above was the basis of my earlier statement (although I simply estimated rather than computing the distances, leading to a slight overestimate)--that they had the near z<0.1 SNes inside, the far z>0.4 SNes outside, and the middle region as a transition. Now, since you asked, I went back to check, and they explicitly state that the 'best fit' void models have a scale of 850±240 Mpc. This seems to be the radius; at least, earlier void models (by the authors of the 15Mpc reference) had a transition radius of 1.35Gpc and a transition width of 40% more.

[Surlethe]

Even if the hypothesis is confirmed or rejected, I don't think that this would kill the Copernican Principle, which is a consequence of Occam's Razor as applied to cosmology. This is an empirical question, not one of model elegance.

By the way, could possibly impact this creationist argument? I have the book Starlight and Time (thanks to my in-laws), and the author rejects the assumption that the universe is homogeneous, and builds his cosmology from there. If the experimental evidence points to the inhomogeneity of the universe, he may have some media attention, at the very least.

[Covenant]

Even if the hypothesis is confirmed or rejected, I don't think that this would kill the Copernican Principle, which is a consequence of Occam's Razor as applied to cosmology. This is an empirical question, not one of model elegance.

By the way, could possibly impact this creationist argument? I have the book Starlight and Time (thanks to my in-laws), and the author rejects the assumption that the universe is homogeneous, and builds his cosmology from there. If the experimental evidence points to the inhomogeneity of the universe, he may have some media attention, at the very least.

I would think so, and was one of the reasons I was instantly a bit iffy about this on general principle. It seems like a bridge too far in terms of intellectual hoop-hopping just to explain something we basically already explain well enough, with the only upside being that it would allow for us to not only be in a special, almost unique portion of creation, but open up the idea for lot of other creationist garbage like re-interperting the Microwave Background as only being 6000 years old, or who knows what. It would certainly give them another tool in their shed. I don't think this theory holds water, but if they can test it and find it is true, well, so be it.

[Kuroneko]

Even if the hypothesis is confirmed or rejected, I don't think that this would kill the Copernican Principle, which is a consequence of Occam's Razor as applied to cosmology. This is an empirical question, not one of model elegance.

Well, not quite. Ockham's razor in this context tells models which do not treat our location as special are preferable, lacking strong evidence otherwise. The Copernican principle, however, is the claim that this is in fact the case--our place is not special--so it is an empirical claim that is in principle falsifiable.

As to YECs, there is nothing for them here. The near SNes, not affected by these concerns, are still far enough away to establish the age of the universe many orders of magnitude higher than they claim, and none of this would overturn geological evidence of Earth's age. Although I've no doubt that they would jump on it anyway.