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The Pioneer Anomaly (wikipedia.org)
52 points by lisper on March 4, 2011 | hide | past | favorite | 10 comments

The first book on Astronomy that I read was very old. The author praised Einstein for his work, but he was skeptical about the expansion thing. His opinion was that even a slight unknown effect in how light travels between galaxies for millions of years could explain the red shift that's usually attributed to expansion.

I don't even understand the maths that justify it, so I won't dare to even have an opinion. But I find it strange how can "we" know so much about objects at monstruous distances. Most of this knowledge is based in a chain of facts that could be invalidated if only one of the links breaks.

If you are interested on the details of the measurements of galactic distances, Terry Tao has created an amazing pedagogical review: http://terrytao.files.wordpress.com/2010/10/cosmic-distance-...

PDF link for those unable to view Powerpoint:


It is good stuff, worth a read.

Thank you, I'll take a look next week at the office's Office :-)

Astronomy is one of those things where the current theory explains what you see it until it doesn't.

Why aren't all objects taking part in the expansion of the universe? Don't our ears move farther apart (ever so little) due to this effect?

IIRC, what's expanding is the distance between particles. Technically, the particles in your ears are moving farther apart, but the forces that keep your ears attached to your head then conspire to keep them in approximately the same spot. The expansion still happens, it's just that the effects get propagated to places where mass density is low, aka between stars.

Then again, I could be totally wrong about that.

I think you should not say that they are moving farther apart, but that they are getting farther apart.

The weird thing with this expanding universe is that things can get farther apart from each other without moving. Similarly, distances between objects can grow faster than the sum of the speeds of those objects.

Yes you're right, 'moving' isn't accurate there, whoops!

I have no idea how close my interpretation has been as IANAP but:

Take the balloon analogy for the expanding universe. Tape pennies all over an empty balloon. Now inflate it.

Every penny is moving away from every other penny.

Now imagine an ant walking along the balloon as you inflate it. He can get closer to a penny only if he is moving at a rate greater than the speed of expansion. Otherwise even if he's moving in the direction of a penny, it's still moving away from him.

As for your ears, maybe they're moving apart (I guess?) but the size of the universe is so immense that the inconsequential distance between your ears is not noticeable relative to larger distances, like between Earth and the next Galaxy.

OR Sure, your ears are moving apart, but so are the ends of the ruler you'd use to measure.

Or while expansion is happening on a grand scale, there really isn't anything preventing localized eddies of contraction or stability.

And that's how I sleep at night.

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