Why was the big bang not an explosion?

[astroscott]

The concept of time before matter is meaningless - no matter, no clocks. Eternal time has no persistent reference frame.

Sorry but who was talking about time before matter.

 

[marcus]

Sorry to ressurect this thread but I've started a blog on this and was hoping that some of you could visit it and tell me what you think.
The address is http://phil-astroscott.blogspot.com/2010/06/hi-im-phil-and-ive-decided-to-take-my.html"
...

Your blog is really nice visually and the tone is modest and friendly. New Zealand seems like a nice place. The NZ scenery carries over and suggests that the universe is also a nice place.

I think you should broaden the scope of your blog and allow for both some LEARNING and some recent NEWS about the universe. Focus less on your alternative ideas about the bb and inflation.

Are you familiar with those blue and red oval maps of the microwave sky? The CMB temperature maps? They show the temperature as it would be measured by an observer who is at rest relative to the expansion process, or if you like relative to the ancient matter.
The actual measured temp map, because the solar system is moving at about 370 km/s ina certain direction relative to the CMB, would have a hot spot in that direction and a cold spot in the reverse direction. Simple doppler. That doppler dipole, that hotspot coldspot artifact of our absolute motion, is taken out of the data before the map that you see is made.

Absolute time, absolute rest, absolute motion, and the "proper distance" measure that goes with them are everyday familiar appliances to cosmologists. They even come with the standard model of the universe that virtually everyone uses. Technically called the LambdaCDM version of the Friedmann Robertson Walker Lemaître model, but the technical terminology is a nuisance.
Proper distance is what you would measure if you had enough time and used radar, or timed a flash of light, having first STOPPED THE EXPANSION at some given instant in time.
You stop the expansion process at a given instant and then see how long it is for a flash of light to reach the other galaxy. That tells its proper distance at that given instant.

Some people call that the "freeze-frame" distance. Because the distance doesn't change while you are measuring it. The distance one would measure like that at this present moment, today, has a special name: "comoving". Again the jargon words are a nuisance but that's life.

Ned Wright's calculator converts redshift to the today proper distance. Try it out.

THE DISTANCE TO MOST of the GALAXIES we can see using a telescope are INCREASING FASTER THAN THE SPEED OF LIGHT not because of inflation but just because that is how it is. General Relativity allows this (although the earlier "special" theory did not) and in fact the standard model with the jargon name REQUIRES it.

This doesn't have anything to do with time dilation (your reasoning in your blog). It is true using our plain old earthbased clocks. It is true using the "universe time" built into the standard cosmology model. It is true simply because Gen Rel gives a more accurate picture of dynamic geometry than one gets from Euclid or from Special Rel---and in the more accurate form of geometry distances are allowed to increase. Distances can increase between objects which are stationary relative to the ancient CMB light, or the ancient matter, or the expansion process, or however you like to think of it.

The Hubble law v = Hd is formulated in terms of proper distance. Whenever you use the Hubble law you are using proper distance. d is the distance and v is the rate it is increasing (Since we are only going 370 km/s absolute motion, our time is approximately the same as universe standard time, so the idea of rate of distance increase is not ambiguous.)

If you look at v = Hd you will see that if d is big enough the rate of increase v must exceed lightspeed c. In fact MOST galaxies which we are now looking at have distances more than enough to make the recession rate exceed c.

There are other distance measures, and Ned Wright's calculator gives some of them as well. But for starters, I would suggest getting familiar with the type of distance that goes into Hubble's law.

Google "Wright calculator" and put in a few sample redshifts.

For other types of distance (like how long the light took to get here etc etc.) there is a survey paper by Hogg that I recall reading some years back. Probably googling "Hogg astronomy distances" would get it. But if you want a link, just ask.

There is another simpler calculator, by Morgan, that I like. Google "cosmos calculator".
The only drawback is that at the start of a session you have to enter your three parameters (.27, .73, 71) for matter fraction, cosmo constant, and Hubble constant. You get the proper distance out, and the rate it is increasing, for different redshifts.

There is a nice Scientific American article by Lineweaver and Davis (Australians). I have a link in my signature. It is the "anu.edu.au/~charley" link.

You may know all this stuff and want more advanced sources and comment. If this is too rudimentary for you please say! I wanted to start with very basic stuff. Especially if you want Hogg's explanation of astronomy's alternative distance measures, ask for the link!

 

[Fredrik]

There is a nice Scientific American article by Lineweaver and Davis (Australians). I have a link in my signature. It is the "princeton.edu" link.

The link doesn't work anymore. I don't know a direct link that works. The only way I know to read it for free is to allow your browser to accept cookies from scientificamerican.com and then click the link in the Wikipedia article.

https://www.physicsforums.com/showthread.php?p=2724014

 

[Ich]

THE DISTANCE TO MOST of the GALAXIES we can see using a telescope are INCREASING FASTER THAN THE SPEED OF LIGHT not because of inflation but just because that is how it is. General Relativity allows this (although the earlier "special" theory did not) and in fact the standard model with the jargon name REQUIRES it.

This doesn't have anything to do with time dilation (your reasoning in your blog).

It has to do with time dilation.
I didn't understand what you're talking about in your blog, astroscott, but speeds in cosmology always are coordinate artifacts. You're simply free to choose the coordinates you like, and FRW coordinates are quite different from SR-like (or "static") coordinates. So no use demanding "velocities" being less than c.

That said, inflation is described by http://en.wikipedia.org/wiki/De_Sitter_space" .
In that article, you'll find 5 sets of coordinates for the exactly same condition. If you use static coordinates, like in SR, all velocities are less than c. There is time dilation. There is a horizon, like in a black hole. Which means that the coordinates don't cover the whole universe, but only the observable universe. And within the observable universe, all those "superluminal" galaxies conform to the SR speed limit.
If you use FRW coordinates (ok, too difficult to explain, but "flat slicing" corresponds to FRW coordinates after introducing "proper distance", which is necessary to get superluminal motion as standard FRW coordinates are constant for every comoving thing), there is exponential expansion and superluminal motion. And no time dilation. But it is the same universe.

So, most of the confusion exists because cosmologists use different coordinates that you'd think, but fail to communicate that situation.
What is left is the following unsatisfactory - but true - statement: "relative velocity" at a distance is quite arbitrary if you use v = "distance change" per time. Because distance is relative, and time is also.

 

[marcus]

The link doesn't work anymore. I don't know a direct link that works. The only way I know to read it for free is to allow your browser to accept cookies from scientificamerican.com and then click the link in the Wikipedia article.

https://www.physicsforums.com/showthread.php?p=2724014

Fredrik, thanks for alerting me to that! There is an alternate copy at Charles Lineweaver's site at his university. The quality is just as good, and no magazine advert nuisance or cookies. So I changed the link in my signature.

 

[robheus]

Hi, Just a quick question.

Does anyone know the original reason for discarding the idea of the big bang as an explosion?

Because there were no matches available at that time to ignite the explosion.

hmmm.

 

[russ_watters]

Because there were no matches available at that time to ignite the explosion.

I think part of what motivates this question is the fact that not only do people not understand what the Big Bang was, but most people don't really know what explosions are either. Just seeing a fireball in a movie doesn't tell you anything about what is going on inside it or how it works.

For example, it was mentioned earlier, but not picked-up by the OP that the primary component/effect of an explosion in air (or water) is an extremely powerful shock wave in an existing medium. Behind that expanding shock wave and outside the fireball (if there even is one) there may be very little else happening and all of the material in the shock wave (and much of what makes up the fireball in many explosions) is pre-existing material that wasn't part of the object that exploded. In other words, what kills most people and destroys most property in a bomb blast isn't the pieces of the bomb flying at you and hitting you or the fireball burning you, but rather the shock wave crushing you. Most movie "explosions" are heavy on the fireball part, but barely qualify as explosions. These days, with many "explosions" being CGI, you'll notice the building isn't even badly damaged by the blast, just by the fire. That's not an explosion, it's just a big fire.

Even if there is a fireball in a real explosion, the fireball itself isn't uniformly expanding (and there are many different types of explosions). Expansion of gases happens with a pressure gradient, so the outder edges are going to initially expand much faster than the inner parts and both the temperature and pressure will have a substantial gradient.

None of this except for the fireball bears even superficial resemblance to what happened at the Big Bang. And the fireball's resemblance is very superficial - it isn't any better than calling the Sun a big fireball.