Distance calculation and growing space

Hi,

Can someone explain how distances to stars are calculated, and how does 'new space' that has been 'created' from the time of emission of light and the time it reaches us figure into that calculation? Thanks.

 

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The distances to the nearset stars are determined by parallax, which is the difference in angle observed on different sides of the Earth's orbit.
The nearest stars jiggle back and forth when seen from different sides of the orbit, so an accurate measurement of the distance can be made.
The satellite Hipparchos has made accurate measurements of this movement (measured in parsecs, or parallax seconds)
out to 100 light years, and good estimates of distance can be made out to 400 ly.
In that 400 ly radius are examples of nearly every class of star, and by observing more distant stars of the same classes in clusters around the galaxy the distance to each cluster can be measured. giving a distibution of clusters for the entire galaxy.
Also a particular class of star called the Delta Cepheid has a regular fluctuation in intensity which is directly related to it's brightness;
the closest Delta Cepheids have had their distances determined by the parallax/cluster method;
Delta Cepheids are bright enough to be seen in nearby galaxies, and so can be used to determine the distances of these objects.
To find the distance of more distant objects Delta Cepheids have allowed the distance to galaxies which have had supernovae in them; all supernovae of a particular type (1a) are the same brightness, so you can find the distance to very distant galaxies this way. The red shift of the light of distant galaxies is also used as a mesure of distance, and is consistent with the other methods, so is used most frequently.

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As far as the expansion of the universe goes, the local gravity of the galaxy is strong enough to counter that, so the stars of our galaxy are not receding from each other; the galaxies of our local group (Andromeda, Triangulum and a few others) are also not expanding away from each other (at this moment in time).

But the more distant galaxies are, and this means that any galaxies more than 13 billion light years away will never be seen, and eventually all the galaxies outside the local group will expand out of our sight forever.
The distances astronomers are interested in are usually the apparent distances (as determined by standard candles and red shift observations); it is not useful to say that the galaxy in question has moved on sice emitting the light, because of simultanety problems.
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burek i jogurt

ok, so the measurements of supernovae that suggest the universe is expanding at an accelarating rate are made in our galaxy or in some far away ones?

 

I thought the 13 billion year light cone was due to a big bang...how else can it be explained? If it is due to a big bang why would we not be able to see galaxies beyond 13 billion years in the future?

 

That's not how it works. In a few billion years, what we now see as the Cosmic Microwave Background radiation will appear to form into galaxies, and we will be seeing a more distant, more redshifted CMBR. No galaxy will ever expand out of our sight.

Sure it's useful, you just have to be careful about specifing your reference frames.

 

if far-away galaxies are accelerating away from each other then they will reach the speed of light in a finite amount of time (assuming no big crunch or static universe) and we will never be able to see or contact these galaxies again.

of course, by the time we can observe this effect, the galaxies would have been long gone billions of years ago.

It seems ligical to me that the CMBR could be a culmination of light from the rest of the universe just at the limit of our current lightcone.

 

In a few billion years, what we now see as the Cosmic Microwave Background radiation will appear to form into galaxies, and we will be seeing a more distant, more redshifted CMBR. No galaxy will ever expand out of our sight.
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Hmm, should have qualified that a bit- I should have said 'if the Big Rip theory is right' I personally hope it is wrong, but we don't get to choose the cosmology of the universe.

http://cooltech.iafrica.com/science/215595.htm
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<font color="gold">if far-away galaxies are accelerating away from each other then they will reach the speed of light in a finite amount of time</font>

Not so. You are assuming linear acceleration.

 

Don't forget, the space in between is the thing which is expanding;
it has expanded at a superluminal rate before, and if thr Big Rip theory is right, it will do again (although I hope not).

 

linear acceleration

why would the inflation of the universe not be seen as a linear acceleration between galaxies?

 

Why would it be neccessarily linear?
Why assume anything without reason?

 

If Guths inflation theory is correct. It might not be.

 

well

I was not assuming this without reason. here is my reason....

I am considering vector components of motion. (one of the first things you learn in physics)

even if the galaxies aren't moving directly away from each other, they would have a corresponding vector component of motion directed away us.

EX) a galaxy is moving 10 miles/hour in a diagonal direction neither directly away from us or parallel to us. Then in physics, we see the galaxy as moving 5 miles/hour directly away from us (PLUS) 5 miles per hour perpendicular to us.
(Actually, the motion is described by 3 vectors since we live in 3 spatial dimensions)


SO... no matter which direction the galaxy is moving it would allways have a component of motion either directly away or toward the earth.

AND because of the universe's expansion, this direction of motion will allways be accelerating in the direction away from the observer. (or DEcelerating for those few galaxies moving towards earth)

 

You are assuming that the expansion of the universe in accelerating linearly, and using that assumption to deduce linear apparent acceleration of galaxies.

Why do you assume that the expansion of the universe is accelerating linearly?

 

it doesn't matter which direction they are moving, as far as we are concerned, the expansion does not have a central point, every part of space is expanding, in every direction, and as long as this is happening at an accelerating rate and does not stop, then galaxies will reach the speed of light relative to us, not that they will actually have any local velocity at all.

dont' take my word for this man, i just read it out of a stephen hawking book, and i'm confident that he has some grounds to believe this and is just not making it up

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later,
Jonathan

 

Yes, there is no centre to the expansion. That is irrelevant.

You might like to get your information from more recent sources, and perhaps from something more reliable than a pop-science work.

 

he he he, hawking, pop-sci, he he

 

right now hawking holds the lucasian professor of mathmatics, a professorship at cambridge once held by isaac newton.

he's a slightly more accepted authority than pop-sci sources in the theoretical astrophyysics field.

 

the point has allready been stated by me more than once and won't continue this