New issue of Scientific American....

[heyo]

An interesting article in the new SciAm about the expansion of the universe, and it clued me in to some things that I thought I understood but was off on.<br /><br />One thing is, I thought that the redshift of distant galaxies was due to an effect similar to the doppler shift of sound here on Earth, and that is the analogy that is commonly used.<br /><br />Not so however, because the galaxy's local motion relative to it's local spacetime is relatively low compared to the speed of the expansion of the universe. Most of the redshift is actually caused by space strrrreeetching out while the light waves are en-route over all those billions of years, thereby making them longer.<br /><br />Draw a picture of a light wave on a rubber band, then stretch the rubber band. I think that's a good analogy.<br /><br />Also, we know that astronomers can look about about 14 billion light years or so, and are therefore looking back about 14 billion years, but what the article said (which makes sense) is that we see those galaxies where they were at that time, and that the actual current radius of the observable universe is about 46 billion light years, extrapolating out to where those galaxies would be today.<br /><br />Also, we know that objects can't move at speeds greater than C, but the expansion can proceed at />C. But, I did not know that galaxies with a redshift of only 1.5 (which IIRC means the light is stretched to 2.5 times it's original wavelength) are already receding from us at >C.<br /><br />It is a neat article. I reccomend checking it out.<br /><br />Heyo

 

[rogers_buck]

Yes, that was an excellent article. I think you are summarizing some of its best points.

 

[centsworth_II]

<font color="yellow">"...objects can't move at speeds greater than C, but the expansion can proceed at >C."</font><br /><br />I wonder what speed of expansion was achieved during inflation. <div class="Discussion_UserSignature"> </div>

 

[heyo]

I don't know what speed was reached during the inflationary period.<br /><br />However, I read an interesting tidbit that I will have to try and remember the source of. But it explains that if a certain theory of inflation is correct, the following statement would be true: If the entire universe was represented by a sphere the size of the Earth, the radius of the sphere that we have been able to see so far would be represented by a grain of sand. Pretty mind boggling stuff.<br /><br />I am going to have to try and dig that up, I can't even remember now if it was in a science magazine, a book, or online. I will start by thumbing through my books.<br /><br />Heyo

 

[aetherius]

Is the term "stretching" of spacetime just used to give the uninformed, like myself, a mental image? Or does spacetime actually have this property?<br /><br />If it does stretch then does it become less elastic the longer it is stretched? This would seem to imply an effect on gravity.<br /><br />If spacetime gets more rigid as it stretches then that implies gravity should decrease for a given mass as time passes. (If you think of it in terms of the planet on the sheet analogy). If the sheet of spacetime is stretched and becomes more rigid then a given object will not make as big of a "dent" in spacetime as it did in the past.<br /><br />Or, could it be just the opposite? Does spacetime apply pressure when it is displaced? If you think in terms of a a planet displacing spacetime the same way a bowling ball displaces water in a bucket, then if spacetime is less flexible it will apply more pressure to the object.<br /><br />By either intepretation it seems like the stretching of spacetime should affect gravity. Is this possible?

 

[bobw]

There is one analogy that works pretty good for that question; raisin bread. As it bakes the dough expands from the gas and carries the raisins apart. Since all the bread is expanding at the same rate the farther apart the raisins are the more dough there is between them expanding so they travel apart faster than two raisins close together. I didn't make that up but I can't remember who did. <br /><br />Edit: It's the space-time that expands and the light that stretches as it travels through the expanding universe. I can see it in my mind but I don't know, yet, if I can write it out <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> </div>

 

[alokmohan]

Raisin bread.Very good analogy.als feel like eating it and understanding physics together.Ha ,Ha ,LOL.

 

[robnissen]

"we know that astronomers can look about about 14 billion light years or so, and are therefore looking back about 14 billion years, but what the article said (which makes sense) is that we see those galaxies where they were at that time, and that the actual current radius of the observable universe is about 46 billion light ."<br /><br />That statement seems to violate relativity. Two items can never have a relative speed to each other greater than c. Under relativity, if Item A is moving at .9c to the east, and item B is moving at .9c to the west, their relative speed is not 1.8c but rather c, because two items can never move faster than c relative to each other. Therefore, a galaxy that we are now viewing that is 14 billion light years away, can now be no farther than 28 billion light years away. The only exception would be if inflation had continued to take place during the last 14 billion years. So apparently the article is arguing that there is some sort of inflation i.e. expansion currently going on. But is there any evidence for current expansion other than the mystery of Dark energy?