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View Full Version : flaw in epanding universe logic?

kenmogul
2007-Mar-05, 03:04 AM
Everyone says that the fact the universe is expanding at an accelerating rate means no big crunch. In any explosion, right at the beginning, the explosion proceeds at an accelerating rate--and slows down later. Maybe we are just right at the beginning, relatively speaking, of the big bang explosion. Putting the age of the universe at 14 billion years seems incredibly young to me. Our solar system has been around for 4 billion. Maybe the universe seems to be expanding at an expanding rate now--only becaiuse we are so close to the explosion, and the expansion will slow down much later, making a contraction possible, even likely.

Where is the flaw in my logic?

Amber Robot
2007-Mar-05, 03:15 AM
Well, you're sort of on the right track because the observations that showed the acceleration of the universe were originally intended to measure the deceleration of the expansion.

Ken G
2007-Mar-05, 03:33 AM
Where is the flaw in my logic?

Simple-- the Big Bang is not a model of an explosion, despite its horrendously misleading name. An explosion involves a high pressure region surrounded by a low pressure region, and all the physics you are talking about stems from that. The observations that spawned the Big Bang model are not those of a pressure gradient. The physics is that of general relativity, not a pressure-driven explosion. It's really very different, so your intuition simply doesn't apply, you have to learn the real theory.

Ufonaut99
2007-Mar-05, 03:36 AM
Hi Kenmogul, and welcome to the board.

You're right that we're right at the beginning. The universe will be around an awfully long time, and undoubtedly it has lots more surprises in store.

In any explosion, right at the beginning, the explosion proceeds at an accelerating rate--and slows down later.
Not quite. Say a bomb goes off, the matter goes from zero to high-speed in a fraction of a second. The matter then slows down due to friction with the air, etc.

That's basically what we expected to see - galaxies exanding away from eachother at a slower and slower rate, with mutual gravitational attraction doing the slowing. The big question used to be whether gravity was stong enough to stop the expansion.

What we found instead was accellerating expansion. This is equivalent to our bomb going off, but as we track debris flying past us (well after the initial impetus) we find it's getting faster instead of slower - despite the air resistance !

Maybe the universe seems to be expanding at an expanding rate now--only becaiuse we are so close to the explosion, and the expansion will slow down much later, making a contraction possible, even likely.

Who knows - You may be right :) However, we time the Big Bang from when the original "seed" started expanding. Something then gave an added impetus during Inflation. To the best of our knowledge, that was the end of the intial motion, so something else must be doing some pushing now.

At the end of the day, we don't know what Dark Matter nor Dark Energy are - together over 90% of the energy in the universe. That means we can't predict how they'll behave, nor what they'll do. Maybe Dark Energy is a further reverberation of Inflation that'll stop one day - or maybe not.

All we can say is, if DM and DE continue to behave as at present, then there won't be a big crunch.

Cougar
2007-Mar-05, 05:07 PM
In any explosion, right at the beginning, the explosion proceeds at an accelerating rate--and slows down later. Maybe we are just right at the beginning, relatively speaking, of the big bang explosion.... Maybe the universe seems to be expanding at an expanding rate now--only becaiuse we are so close to the explosion, and the expansion will slow down much later, making a contraction possible, even likely.

Where is the flaw in my logic?
In a couple places. First, as mentioned, the big bang is not really like an "explosion." As currently conceived, the big bang is theorized to have resulted in mass/energy AND space, all expanding together. And second, even if it was like an explosion, an explosion does not "proceed at an accelerating rate" after the first instant of detonation. Immediately the expanding debris starts slowing.

There is pretty good evidence that the accelerating expansion is due to some property of the vacuum energy of space itself.

jamini
2007-Mar-05, 05:14 PM
There is pretty good evidence that the accelerating expansion is due to some property of the vacuum energy of space itself.
And more specifically, negative pressure, the effects of which have been demonstrated by observations including gravitational lensing and Sn1 resdhift data and can be expressed mathematically as a cosmological constant.

Dubb
2007-Mar-05, 05:14 PM
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jamini
2007-Mar-05, 06:08 PM
Dubb - The more distant the galaxies, the faster they are accelerating away from us; this holds true from any point of observation within our observable universe. Expansion is occurring equally throughout the universe, less the local gravitational forces between galaxies and galaxy clusters.