Misconceptions about the Big Bang and Expanding Universe

[Pangloss]

The March '05 issue of Scientific American has an interesting article about misconceptions regarding the Big Bang and the expanding universe. The full article is now available on their web site at:

 

http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147

 

Here are some of the more interesting bits:

 

Question: What kind of explosion was the Big Bang?

Wrong Answer: The Big Bang was like a bomb going off at a certain location in previously empty space.

Right Answer: It was an explosion in space itself.

 

The space we inhabit is itself expanding. There was no center to the explosion; it happened everywhere. The density and pressure were the same everywhere, so there was no pressure difference to drive a conventional explosion.

 

 

Question: Do objects inside the universe expand, too?

Wrong Answer: Yes. Expansion causes the universe and everything in it to grow.

Right Answer: No. The universe grows, but coherent objects inside it do not.

 

Neighboring galaxies initially get pulled apart, but eventually their mutual gravity overpowers expansion. A cluster forms. It settles down into an equilibrium size.

 

 

Question: How large is the observable universe?

Wrong Answer: The universe is 14 billion years old, so the radius of the observable part of is 14 billion light-years.

Right Answer: Because space is expanding, the observable part of our universe has a radius of more than 14 billion light years.

 

As a photon travels, the space it traverses expands. By the time it reaches us, the total distance to the originating galaxy is larger than a simple calculation based on the travel time might imply -- about three times as large.

 

 

Question: Can galaxies recede faster than light?

Wrong Answer: Of course not. Einstein's special theory of relativity forbids that.

Right Answer: Sure they can. Special relativity does not apply to recession velocity.

 

In expanding space, recession velocity keeps increasing with distance. Beyond a certain distance, known as the Hubble distance, it exceeds the speed of light. This is not a violation of relativity, because recession velocity is caused not by motion through space, but by the expansion of space.

 

 

Question: Can we see galaxies receding faster than light?

Wrong Answer: Of course not. Light from those galaxies never reaches us.

Right Answer: Sure we can, because the expansion rate changes over time.

 

The photon initially is unable to approach us. But the Hubble distancs is not constant; it is increasing and can grow to encompass the photon. Once that happens, the photon approaches us and eventually reaches us.

 

 

Fascinating stuff. The full article is well worth a read.

[star dust]

that actually answered a lot of questions that i had been pondering on. Thanks!

[Ophiolite]

SciAm is good at synthesis articles. With the dollar so weak it may be an opportune time for Europeans to take out five year subscriptions!

[1veedo]

I read the article, learned one thing I didnt used to know, and that gave me another question (In the same Missconception). Pertaining to like, something in the end.

 

I was also going to post my question but I forgot what it was

 

I'll go back and read the last two pages to see if I get the question again

[KholdStunner]

you said the big bang exploded in 'space itself'. please explain

[Ophiolite]

The phrase 'an explosion of space' would have been a better choice. Space and time did not exist prior to the big bang. They were created at that 'instant'. (Alledgedly.)

[J.C.MacSwell]

The phrase 'an explosion of [/b']space' would have been a better choice. Space and time did not exist prior to the big bang. They were created at that 'instant'. (Alledgedly.)

 

Why does the expansion OF space have MOMENTUM? Is it assumed to hold to the same equations as an explosion IN space?

[Ophiolite]

Does it? In the classical sense, surely, it cannot. Space does not have mass.

[J.C.MacSwell]

Does it? In the classical sense, surely, it cannot. Space does not have mass.

 

I'm thinking in terms of the masses themselves. Their receding velocities (regardless of why) would require the same change in momentum to slow down or speed up??

[Deified]

I don't think they do follow the same formulas. First of all, every point in space is expanding in all directions at once. In an explosion it only expands in one direction. What I just said means that the farther things are from us, the faster they recede. Not too hard to grasp. I'm sure you know about the hubble distance and such, so from what I just said I don't really see how the two explosions could be related in anything but name.