i believe that i heard that the universe was currently expanding, however, someone just told me that it was contracting… any insight on this one?
~jonas
um, maybe someone lied to you, or was misinformed, or you misheard…
As far as I know, under accepted theory the universe is currently expanding. This is because we see a Doppler shift in the spectrum of stars. Analogy - as a car moves closer, the pitch of its noise changes - and then, as it moves farther away, the pitch of its noise changes again. The stars don’t give off any noise, but the Doppler effect works for all kinds of frequencies, including the electromagnetic spectrum, and thus light.
What was SUPPOSED to happen was, eventually the universe would reach a point where it would no longer expand, as the energy from the Big Bang had been expended. It would then begin to contract as gravity drew all the celestial objects closer to each other, meaning, eventually our universe would collapse upon itself again. (Time frame incredibly huge for this to happen, of course.)
Recently, however, scientists have verified one of Einstein’s weirder and lesser-known discoveries. There is a “dark energy”/“dark force” that appears to counteract the effects of gravity in our universe. I don’t remember exactly how this one was discovered, but the gist of it was, a super nova (exploding star) didn’t behave like it should have under current theory, and these “dark forces” seemed to kind of explain why it didn’t work. So - some scientists now think that our universe will continue to expand forever, and never be destroyed.
Hopefully, this “working” explanation should do it.
Scientists have not verified one of Einstein’s discoveries with the accelerating-apart measurements. Back when astronomers were figuring out the really hard questions about gravity and relativity, they thought we lived in a stable universe, where stars were not moving relative to each other. But Eintein’s own equations said that gravity should be drawing them together if they started out sitting still, so he proposed some unknown “cosmological constant” as a fudge factor to make up the difference.
Soon thereafter, Hubble discovered that the universe is expanding, so the cosmological constant wasn’t needed after all. The whole thing was somewhat of an embarrassment to Einstein.
Then recently, astrophysicists have been trying to detect whether we’re decelerating as they think we should, and their initial measurements seem to indicate that our speed apart is accelerating (the measurements were based on looking at the rate that supernovas happen in distant galaxies). No one knows for sure that the measurements are correct, or what might cause the acceleration. I’ve heard speculation about virtual particle creation somehow having something to do with it, but it sounds to me like they’re desparate for explanations. The fact that the acceleration of the universe would correspond to Einstein’s embarrassing cosmological constant has been played up by news reports.
The evidence for the cosmological constant (or dark energy, or quintessence, or whatever the heck it is) is getting stronger with every experiment. The best current evidence is that it makes up about 70% of the stuff in the Universe, so we’re pretty sure now that the Universe is, indeed, accelerating. For the expansion itself, though, we’re more than just pretty sure: We’re rock-solid certain that it’s definitely expanding. The only thing we’re more certain about, with regards to the Universe, is the fact that it exists in the first place.
The universe is expanding. Lots of “redshift” evidence for that. (question - - what other evidence supports it?)
Less definite, but likely, is that (1) the expansion of the universe is accelerating (as others mentioned here) and (2) that it will expand forever (it will not collapse someday).
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What I never understood was how “Eintein’s own equations said that gravity should be drawing [stars] together if they started out sitting still…” Wouldn’t Newton’s laws have implied that in a static universe the stars should be attracting each other? How come no one thought this was a problem before Einstein?
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-b
Under Newtonian physics, gravity is shown to be a weak force (inverse square law)…so if stars are far enough apart and moving fast enough, then gravity would not pull them together. For example, the Earth does not fall into the sun because it is moving fast enough in the right direction at it’s distance from the sun to stay in a stable orbit.
But maybe a real physicist can answer this better.
Newton’s explanation was that the Universe is infinite, so there’s always stars on all sides of you, and no center to collapse to. This explanation just barely cuts it, if you’re doing classical gravity, but it doesn’t work for GR.