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Edd
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 Posted: Fri Mar 09, 2007 11:21 am Post subject: |
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| kmisho wrote: | | Edd, my understanding of inflation was that the restriction of the speed of light simply would not apply to space itself. |
Yeah that's it. Just that I find a good way to spot such situations is that expanding space doesn't let objects pass one another. No photon overtaking allowed, no getting from A to B faster than light, and everything behaves properly.
| Quote: | | As far as I know, many still accept some version of Guth's early inflationary period, where the universe (space itself) would have expanded at many times the speed of light for a short period following the big bang. |
Yup, some period of inflation is generally accepted as the most likely explanation for a number of issues still, although of course everyone's ready to change their mind if some evidence comes in otherwise  |
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kmisho
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| Posted: Fri Mar 09, 2007 1:22 pm Post subject: |
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| Edd wrote: | | kmisho wrote: | | Edd, my understanding of inflation was that the restriction of the speed of light simply would not apply to space itself. |
Yeah that's it. Just that I find a good way to spot such situations is that expanding space doesn't let objects pass one another. No photon overtaking allowed, no getting from A to B faster than light, and everything behaves properly.
| Quote: | | As far as I know, many still accept some version of Guth's early inflationary period, where the universe (space itself) would have expanded at many times the speed of light for a short period following the big bang. |
Yup, some period of inflation is generally accepted as the most likely explanation for a number of issues still, although of course everyone's ready to change their mind if some evidence comes in otherwise |
Of course. I have no religious convictions regarding current astrology or cosmetology.
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tonyman1989
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| Posted: Fri Mar 09, 2007 5:13 pm Post subject: |
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thank you edd.
one question how fast does the present universe expanding and how do you measure or how do they now that.
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Edd
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| Posted: Fri Mar 09, 2007 6:35 pm Post subject: |
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| That's Hubble's Constant. It's about 72 km /s /Megaparsec. So a Mpc away (3.3 million light years) the universe is moving away from us at 72 km /s - 160,000 mph. |
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tonyman1989
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| Posted: Fri Mar 09, 2007 6:44 pm Post subject: |
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so it is no longer moving at faster then light speed?
I heard it was speeding up.
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"If people are good only because they fear punishment, and hope for a reward, then we are a sorry lot indeed." Albert Einstein
"For then we will know the mind of God." Stephen Hawking
"We admit that we are like apes, but we seldom realise that we are apes." Richard Dawkins |
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Edd
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| Posted: Fri Mar 09, 2007 7:12 pm Post subject: |
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| Well the further away you get the faster its expanding. But on the other hand the further away you get the further you look back in time, which kind of confuses things. But no, its not expanding at anything like the rate during inflation. It's actually looking like it's pretty close to the slowest expansion the universe is going to see. |
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tonyman1989
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| Posted: Fri Mar 09, 2007 9:19 pm Post subject: |
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| Edd wrote: | | Well the further away you get the faster its expanding. But on the other hand the further away you get the further you look back in time, which kind of confuses things. But no, its not expanding at anything like the rate during inflation. It's actually looking like it's pretty close to the slowest expansion the universe is going to see. |
then when you look further back and then universe is expanding faster the further you go out and that in is further back in time dosn't that prove that the universe is exanpading slower with time.
and is there anyway that you can measure this expantion ratio to the current size of the universe (ex. say the universe is 40 billion light years across its 160,mph, but if it doulbes to 80 billion light years it expands at 40 ,000 mph.)
I looked up hubbles law on wikipedia and couldnot understand the math and the saigns can someone explain it? thank you.
ps: I'm big in Cosmology so talking about the universe and trying to understand it is like a day at candyland. 
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"If people are good only because they fear punishment, and hope for a reward, then we are a sorry lot indeed." Albert Einstein
"For then we will know the mind of God." Stephen Hawking
"We admit that we are like apes, but we seldom realise that we are apes." Richard Dawkins |
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Edd
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| Posted: Sat Mar 10, 2007 5:12 am Post subject: |
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| Quote: | | then when you look further back and then universe is expanding faster the further you go out and that in is further back in time dosn't that prove that the universe is exanpading slower with time. |
Errr, yes and no. To start off you've got Hubble's Constant which isn't constant. So it's probably better called the Hubble Parameter (H), but I've yet to get myself properly into the habit of that. What that tells you is over relatively small scales how fast something is moving away from us at a given distance. Something twice as far away will be carried by the expansion twice as fast, because there's twice as much space between us and it to do the expanding.
As you rightly point out, the further back you go the further back you look in time though, and you can work out much how the universe has expanded and what that means the Hubble Parameter was back then.
How H changes as the universe expands is described by the Friedmann Equations - http://en.wikipedia.org/wiki/Friedmann_equations. If you just look at the first equation on that page, we'll start by ignoring that -Kc^2/a^2 term, as the universe looks like it is flat and that then is zero. That 8 pi G rho term is the gravitational attraction of the matter in the universe. It's trying to slow the universe down, but because it's related to the density (rho) it means that as the universe gets bigger it gets weaker, since the density drops. Early on in the universe however, the universe was much smaller than it is now, so this was a big term as the density was high. So for much of the early life of the universe the expansion was slowing down.
The second term, Lambda, is the dark energy term, which is looking like its a cosmological constant. It likes to make the expansion accelerate, and it remains constant with the size of the universe. Relatively recently the matter density dropped below the level where it overpowers Lambda and so now the universe is starting to accelerate again (which is why I said earlier that we're pretty close to the slowest expansion). So the further you go out the faster things expand away from you, and the further you go out the further back in time you go, and the expansion rate is faster at high redshifts once you get past that point where dark energy has taken over.
| Quote: | | is there anyway that you can measure this expantion ratio to the current size of the universe (ex. say the universe is 40 billion light years across its 160,mph, but if it doulbes to 80 billion light years it expands at 40 ,000 mph.) |
It's the kind of thing I prefer not to do! It can be a bit of a complicated problem as do you want to know the apparent recession velocity of an object you look at in the sky, or do you want to know how fast the expansion is carrying it away if you ignored the light travel times? There's other questions about how to define it too, I'm sure. I'm also rather fearful that the answer's divergent as you approach the Big Bang, so if you wanted to look right back you'd find an infinite apparent recession velocity as the redshift of the Big Bang itself is going to be enormous - classically infinite in fact. It can probably be worked out for the CMB but I'd not trust myself to get it right without my textbooks as backup! |
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tonyman1989
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| Posted: Sat Mar 10, 2007 6:15 am Post subject: |
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too edd
thank you your info has been verying helpful and I will go to wikipeida and try to figure out the probelm forwards and backwards it very interesting. thats what I like to do.
ps: If you remember anything or have anything else to say please post it. thank you.
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"If people are good only because they fear punishment, and hope for a reward, then we are a sorry lot indeed." Albert Einstein
"For then we will know the mind of God." Stephen Hawking
"We admit that we are like apes, but we seldom realise that we are apes." Richard Dawkins |
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tonyman1989
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| Posted: Sat Mar 10, 2007 8:35 am Post subject: |
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to Edd
I still have two more qeustions after looking at wikipeida and other sites I still can't want the p in the Friedmann Equation values is and I can't find the value of the Cosmological constant.
Thank you very much for your help.
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"If people are good only because they fear punishment, and hope for a reward, then we are a sorry lot indeed." Albert Einstein
"For then we will know the mind of God." Stephen Hawking
"We admit that we are like apes, but we seldom realise that we are apes." Richard Dawkins |
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Edd
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| Posted: Sat Mar 10, 2007 8:56 am Post subject: |
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The cosmological constant has an OmegaLambda value (Lambda/3H02) of 0.7. The p is the pressure of the material - for matter this is essentially zero. It's still included in the acceleration equation because quite early on the radiation is the most important component and this has significant pressure, and the cosmological constant has negative pressure, although that seems to be accounted for in the Wikipedia versions of the equations.
(oh for a way to include greek letters here!) |
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tonyman1989
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| Posted: Sat Mar 10, 2007 10:26 am Post subject: |
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thank you
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"If people are good only because they fear punishment, and hope for a reward, then we are a sorry lot indeed." Albert Einstein
"For then we will know the mind of God." Stephen Hawking
"We admit that we are like apes, but we seldom realise that we are apes." Richard Dawkins |
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tonyman1989
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| Posted: Sat Mar 10, 2007 10:33 am Post subject: |
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but isn't p the mass (kg) of the universe divide by the area (sp. meters)?
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"If people are good only because they fear punishment, and hope for a reward, then we are a sorry lot indeed." Albert Einstein
"For then we will know the mind of God." Stephen Hawking
"We admit that we are like apes, but we seldom realise that we are apes." Richard Dawkins |
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Edd
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| Posted: Sat Mar 10, 2007 10:36 am Post subject: |
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| Got to distinguish p from rho. rho is the density - the mass in a volume divided by that volume, and p is the pressure. They look a bit too similar annoyingly! |
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