Do We Live in a Giant Cosmic Bubble?

By Clara Moskowitz

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 reined 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.

Original here

The Moon that went up a Hill but came down a planet

Edited to add: I have taken the topic of this post and logically continued it in a post about Pluto.

Every now and again I get an email from someone who tries to tell me that the moon orbits the Sun more than it orbits the Earth.

On the face of it, their claim appears to hold water. For example, they’ll say that if you compare the orbital velocity of the Moon around the Earth (about 1 km/sec) to the orbital velocity of the Moon around the Sun (the same as the Earth’s velocity around the Sun, about 30 km/sec), you’ll see that the Moon’s orbit is always convex to the Sun; in other words, it doesn’t make a loopy pattern around the Sun, as you might expect:

That diagram is exaggerated; on that scale the Moon’s combined path around the Earth and Sun would look pretty much like a circle (and to be clear, the link above doesn’t make the claim that the Moon orbits the Sun more than it does the Earth, it’s just pointing out the patterns).

Moreover, if you calculate the force of the Sun’s gravity on the Moon, you find it’s more than twice the force of Earth’s gravity on the Moon!

Whoa. So does the Moon orbit the Earth, or the Sun?

Turns out, it orbits the Earth, despite these claims. The above claims are true, but are not important in this argument. Instead, you have to look at something called the Hill sphere. Basically, it’s the volume of space around an object where the gravity of that object dominates over the gravity of a more massive but distant object around which the first object orbits.

OK, in English — and more pertinent to this issue — it’s the volume of space around the Earth where the Earth’s gravity is more important than the Sun’s. If something is orbiting the Earth inside Earth’s Hill’s sphere, it’ll be a satellite of the Earth and not the Sun.

The derivation of the math isn’t terribly important here (and it’s on the Wikipedia page if you’re curious), but when you plug in the numbers, you find the Earth’s Hill sphere has a radius of about 1.5 million kilometers. The Moon’s orbital radius of 400,000 km keeps it well within the Earth’s Hill sphere, so there you have it. The Moon orbits the Earth more than it orbits the Sun. In reality it does both, and saying it orbits one and not the other is silly anyway.

But what about those other claims? Well, the orbital shape is pretty meaningless for this argument. You’ll get that always-convex pattern for any moon that orbits a planet far enough out compared to the planet’s distance from the Sun. But as long as that moon is inside the planet’s Hill sphere, it orbits the planet more than the Sun.

And the fact that the Sun’s force effect of gravity on the Moon is stronger than the Earth’s is interesting, but not relevant. Why not? Because the Sun is pulling on both the Earth and the Moon! Since both the Earth and Moon are roughly the same distance from the Sun (the Sun is 400 times farther away from us than the Moon is), the force acceleration due to the Sun’s gravity on the Moon and the Earth is about the same. That means the Sun’s gravity is important in keeping the Earth and Moon together orbiting the Sun, but doesn’t affect the Earth and Moon as a system. The Moon is still gravitationally bound to the Earth.

Think of it this way: if the Sun were to disappear, the Moon would still orbit the Earth pretty much as it does now. That means the Sun’s gravity doesn’t affect the Earth/Moon system much. On the flip side, if the Earth disappeared, the Moon would continue to orbit the Sun as well. That means the Earth’s gravity doesn’t much affect the Moon’s orbit around the Sun, either.

Now, if we moved the Earth/Moon system closer to the Sun, then this all starts to matter. That’s because eventually you get to a point where the size of the Moon’s orbit gets to be significant compared to the size of the Earth’s orbit around the Sun. In that case, as the Moon orbits the Earth, when its closest to the Sun, the gravity of the Sun gets significant compared to the Sun’s gravity on the Earth (remember, right now that force effect is about the same). But that’s exactly how the Hill sphere idea works!

It turns out that if you moved the Earth and Moon to about 40 million km from the Sun — closer to the Sun than Mercury! — then the Hill sphere of the Earth would be 400,000 km, equal to the moon’s distance from the Earth now. That means the Moon could be stripped from the Earth by the Sun, and so then you could say the Moon orbits the Sun and not the Earth.

But out here, 150 million kilometers from the Sun, the Moon orbits the Earth quite nicely, thank you very much. You can keep your convex orbits and your forces and everything else, and examine them as matters of interest. But the Moon will keep pacing out its orbit just the same, and it’ll keep doing it that way for a long, long time.

The orbital diagrams in this post are from Helmer Aslaksen’s webpage at the Department of Mathematics, National University of Singapore. I modified them a bit to compare them. You should read that page, as it has an excellent description of the Moon’s orbital shape. Oh, and stay tuned, because I have another thing to say about all this in a later post…

Edited to add: in a couple of places I used the word "force" when I mean "acceleration". I fixed that, or used the word "effect". This may seem like a nitpick, but in fact they mean different things physically, and therefore their magnitudes, the sizes of their effects, changes. That’s what I get for writing this at 4 in the morning.

Original here

SPACESHIP DIES IN BLAZE OF GLORY

ESA The unmanned Jules Verne ATV cargo ship breaks up in a spectacular display during re-entry, as seen on Monday over the Pacific from an observation plane. The European Space Agency's first cargo mission to the international space station ended in a spectacular fireworks show today, with the fiery re-entry of the unmanned Jules Verne ATV spaceship over the South Pacific. "Jules Verne has now successfully completed its mission," ESA Director-General Jean-Jacques Dordain declared at the International Astronautical Congress in Glasgow, Scotland. The end came at around 9:30 a.m. ET, when controllers back at Europe's mission control in Toulouse, France, directed the 17-ton craft into its final plunge. Jules Verne, the first of Europe's Automated Transfer Vehicles, was launched from the ESA spaceport in French Guiana early March 9. It linked up with the space station almost a month later, delivering tons of food, water and equipment. During its stay, Jules Verne periodically boosted the space station's orbit, and in fact helped the station dodge a passing piece of Russian space junk last month. But all good things must come to an end: Unlike the Italian-built space cargo modules that are carried back and forth inside NASA's space shuttle, the Euroean-built ATVs are not designed for return or reuse. Instead, each spent craft has to be disposed of safely, by directing it remotely on a plunge through the atmosphere. The wide-open South Pacific is the favorite dumping ground for such space junk, as we saw back in 2001 when Russia's Mir space station fell to its doom. Jules Verne's re-entry was witnessed by an international team of scientists flying aboard a NASA DC-8 observation plane. Studying the spacecraft's controlled fall could lead to fresh insights about the chemical and radiation effects of falling meteors - as well as better computer models for predicting how objects fragment as the blast through the atmosphere. ESA The Jules Verne ATV cargo craft glows during its atmospheric re-entry, in a view captured Monday from a DC-8 observation plane flying over the Pacific. A lens diffraction flare can be seen in rainbow colors at lower right. Today's first pictures from the DC-8, posted to ESA's ATV blog, revealed a spray of fireworks similar to those seen during Mir's fall. More tragically, they also recalled what witnesses saw during the shuttle Columbia's breakup five years ago. Although experts still have to track all the bits of debris, it looks as if Jules Verne's plunge through the atmosphere provided a great light show, but no big impact. There were far more damaging plunges elsewhere on the planet today. Original here