Something I don't understand...

JohnR7 · Dec 7, 2005

Trillian said:
... about the Big Bang theory.

1- Is the basic understanding that there was nothing at all and then all the sudden there was a sort of explosion and then an entire universe? That can't possibly be right... I must have missed something so please explain it to me.

The Big Bang is basicly quantum physics. They do not say there was nothing at all. What they say is that energy and matter followed a different set of laws than they follow after the "big bang". It was a beginning, but it was only a beginning in as much as what was here before changed.

The two main factors with the Big Bang theory was compression and intense heat. This was actually a theory from Jewish mysticism that began at least 1000 ago. It just climbed out of the religious books into the science book in recent years.

dad · Dec 7, 2005

Jet Black said:
careful:

http://www.phys.lsu.edu/mog/mog17/node9.html

or if that is too heady:

http://www.cnn.com/TECH/space/9802/27/accelerating.universe/[/QUOTE]

"Changes in expansion rate are estimated by comparing the redshifts of distant galaxies with the apparent brightness of Type 1a supernovae found in them. These measurements suggest that the expansion of the universe is accelerating. "
Distant supernova are relied on heavily here, it seems. The assumption that closer supernova are the same, basically. If they were, say much much smaller, or etc, the assumption would be quite wrong then?

A way to account for this perceived expansion, then is to come up something that explains it. That is where dark matter and energy 'came from' (Correct is wrong here, but this seems to be what they claim). So, now we have a majority of the physical universe made up of this 'stuff', they claim!
"The universe is made mostly of dark matter and dark energy," says Saul Perlmutter, leader of the Supernova Cosmology Project headquartered at Berkeley Lab, "and we don't know what either of them is."
http://www.lbl.gov/Science-Articles/Archive/dark-energy.html
!!!!!!
Comedy at it's finest, I'd say.
But, yes, the latest opinion is that there is an acceleration of the (perceived) expansion!

Mathematician · Dec 7, 2005

MartinM,

MartinM said:
No, GR is perfectly consistent with a Universe with a centre. It just happens to be the case that the simplest model within GR that is consistent with observation has no centre.

I have no idea what you're referring to here. The CMB certainly has a rest frame, but that's got nothing to do with a centre.

GR allows a center? Please be kind enough to show me how?

The QM rest frame defines a center of a somewhat abstract yet very real sort. We are moving wrt universal rest. We can determine the direction and speed. Hubble's Law allows us to convert velocity to distance. There is conceptually, a point in space that represents universal rest and everything is moving away from that point. I claim that point is the center of the universe. Quibble away if you must.

Mathematician · Dec 7, 2005

Loudmouth,

Loudmouth said:
I thought it stated that there is no such thing as a true frame of reference, or an ideal frame of reference. Is this wrong?

A true or ideal frame of reference would define a center. We've said the same thing.

MartinM · Dec 7, 2005

Mathematician said:
GR allows a center? Please be kind enough to show me how?

Try a spherically symmetric mass distribution with finite radius. Fairly obvious centre.

The QM rest frame defines a center of a somewhat abstract yet very real sort. We are moving wrt universal rest.

The CMB rest frame is not a universal rest frame, and GR needs no modification to deal with it. The laws of physics are the same in every reference frame, which is all that GR requires. The CMB frame is convenient for the purposes of calculation, but there's nothing fundamental about it. Any other reference frame will yield identical results. There is no such thing as 'absolute rest'.

We can determine the direction and speed.

Relative to the CMB rest frame. Calling it absolute doesn't make it so.

Hubble's Law allows us to convert velocity to distance. There is conceptually, a point in space that represents universal rest and everything is moving away from that point.

Even if the CMB rest frame were to define absolute rest, which it doesn't, it's not one point. It's every point in the Universe, and Hubble's Law applies equally well about any point. Every point is moving away from every other.

Alex1210 · Dec 7, 2005

Lignoba said:
No longer accepted? By whom? You? Because its still the theory we are learning while I am getting my B.S. in Physics, and IIRC it is the theory still held at the University of Nevada where I am getting my PhD in Physics after I graduate. I have never heard of this "Dark Energy" theory, so I cant really verify, but all matter has gravity, therefore the only way for our universe to not have the same amoutn of gravity it had at the Big Bang would be for it to lose matter... but how does that work? Where did it go? Did it just transpose time and space and go into another universe?

Although someone seems to have already explained this, evidence has been showing that the universe cannot collapse. I'm not surprised the idea is still taught. Many disproved theories are taught, because they help to show many key ideas. I suggest you read up on Dark Energy. If you're a physics major, you should find it interesting. Most universities have regular colloquiums and this topic bound to show up.

Dr.GH · Dec 7, 2005

I have the rough recollection of the notion that if the universe were to expand infinitely but had finite mass that there are two ways for a "new" universe to occurr.

First was the argument that you "reignite" the vacumn. (This seems to me to be just a variation of Hoyle's "steady state" idea).

Second was the notion that when light/communication/information is physically unable to be transmited from one region of an expanding universe to some other, they are disassociated from one another at the quantum level and then exist as new universes.

This is not my area of interest, so perhaps someone who actually knows something about it could chime in now.

Lucretius · Dec 8, 2005

I think most cosmologists rely on Inflation now, GH, because it so nicely gets rid of problems like:

Horizon
Monopole
Flatness

Having just finished Alan Guth's book on Inflation, he explained several ways in which Inflation could lead to new universes. Creating a false vacuum is one though currently it seems impractical you would have to cram an insanely large amount of mass into an area about 10^-22 cm wide.

MartinM · Dec 8, 2005

Lucretius said:
I think most cosmologists rely on Inflation now, GH, because it so nicely gets rid of problems like:

Horizon
Monopole
Flatness

Having just finished Alan Guth's book on Inflation, he explained several ways in which Inflation could lead to new universes. Creating a false vacuum is one though currently it seems impractical you would have to cram an insanely large amount of mass into an area about 10^-22 cm wide.

Fortunately, we don't have to create new Universes ourselves; several inflationary models do it quite naturally without any help from us - good job, since I'd imagine getting funding for something like that would be pretty tricky

Loudmouth · Dec 8, 2005

Mathematician said:
Loudmouth,
A true or ideal frame of reference would define a center. We've said the same thing.

I admit that my physics is pretty shaky, so bear with me.

GR, from what I understand, says that an ideal frame of reference is not required to relate movement. However, I don't think that it ruled it out. From my understanding, GR made the ideal FoR inconsequential, not non-existent.

MartinM · Dec 8, 2005

Dr.GH said:
First was the argument that you "reignite" the vacumn. (This seems to me to be just a variation of Hoyle's "steady state" idea).

Sounds somewhat reminiscent of a Poincare recurrence. Basic idea is that for any suitable system, if you wait long enough you'll eventually see it return arbitrarily close to any initial position in phase space. So given enough time, the Universe will eventually return to a state capable of reproducing the Big Bang, albeit with slightly different initial conditions. Whether or not this is actually possible is an open question, I believe.

Second was the notion that when light/communication/information is physically unable to be transmited from one region of an expanding universe to some other, they are disassociated from one another at the quantum level and then exist as new universes.

I really think that depends on what we mean by 'Universe.' A spatially infinite Universe is already covered by an infinite number of causally isolated patches; one could consider each of those patches a Universe in and of itself.

MartinM · Dec 8, 2005

Loudmouth said:
GR, from what I understand, says that an ideal frame of reference is not required to relate movement. However, I don't think that it ruled it out. From my understanding, GR made the ideal FoR inconsequential, not non-existent.

As far as GR is concerned, questions about velocity and acceleration make no sense except in relation to a particular reference frame, and it's simply meaningless to ask which frame is 'correct' - they're all just as good as each other. Some may be more convenient for the purposes of calculation, but that's a different matter.

Mathematician · Dec 8, 2005

Martin,

MartinM said:
Try a spherically symmetric mass distribution with finite radius. Fairly obvious centre.

Thanks. Yes it's so obvious, I'm banging my head on the wall for being so dense as to not think of it.

As for the rest, it's gone beyond my understanding and it will take some study on my part to catch up. Thanks.

Loudmouth · Dec 8, 2005

MartinM said:
As far as GR is concerned, questions about velocity and acceleration make no sense except in relation to a particular reference frame, and it's simply meaningless to ask which frame is 'correct' - they're all just as good as each other. Some may be more convenient for the purposes of calculation, but that's a different matter.

Thanks. That was my understanding as well, just not as well written as yours.

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