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View Full Version : Couple of novice questions [early photons, Galaxy appearance]



Screwdriver
2009-Apr-05, 11:07 AM
Hi, i'm having trouble finding some answers to some thoughts of mine.. hopefully someone can help

Firstly, how can photons that were produced early after the big bang even be hitting our planet.. wouldnt they have passed us much much earlier?

Secondly, the milky way is 100,000 light years in diameter. Has anyone given a thought to the actual appearance to the galaxy (or ANY galaxy for that matter) if light was instantaneous? I often wonder how different they would really look with all the stars in their actual position.. or would the effect be minimal. Im sure this could be simulated on a computer

and just for good measure.. if space is curved.. and we could see duplicate galaxies.. could we see our own?

O_o

antoniseb
2009-Apr-05, 11:37 AM
Hi Screwdriver, welcome to the BAUT forum.

Concerning your questions, I have some short answers. Others may follow with more details:

1. The photons that are hitting the Earth from 380,000 years after the big bang did not start out near us, and they've been "swimming upstream" against universal expansion ever since.

2. If the speed of light was near infinite, we'd see some stars in the galaxy slightly more evolved, and in slightly different places. For almost any person looking the overall impression of the galaxy would be about the same as we have now.

3. Space would have to be *very* curved for us to see our galaxy as though it were distant from us.

antoniseb
2009-Apr-05, 11:45 AM
BTW, I moved this thread from Astronomy to Q&A.

dgruss23
2009-Apr-05, 12:44 PM
Secondly, the milky way is 100,000 light years in diameter. Has anyone given a thought to the actual appearance to the galaxy (or ANY galaxy for that matter) if light was instantaneous? I often wonder how different they would really look with all the stars in their actual position.. or would the effect be minimal. Im sure this could be simulated on a computer


The appearance of the Milky Way would be similar to NGC 3992 (M109). You can check out some links to images that explain the classification in this post (http://www.bautforum.com/1061059-post6.html).

One thing I did not mention in that post. NGC 3992 is also similar to the Milky Way in that it is a multi-armed spiral.

hhEb09'1
2009-Apr-05, 03:44 PM
2. If the speed of light was near infinite, we'd see some stars in the galaxy slightly more evolved, and in slightly different places. For almost any person looking the overall impression of the galaxy would be about the same as we have now.Yes, the diameter of the Milky Way (http://en.wikipedia.org/wiki/Milky_Way#Sun.27s_location_and_neighborhood)is about 100,000 light years, and we're about 25,000 ly from the center, so the farthest stars are no more than 75,000 ly. But the galaxy takes 200 million years to make a complete rotation. In 75,000 years, it would only move an eighth of a degree.

Screwdriver
2009-Apr-06, 01:47 AM
thanks for the answers all!

(the eighth of a degree info is very cool and was just what i was after)

01101001
2009-Apr-06, 01:59 AM
often wonder how different they would really look with all the stars in their actual position.. or would the effect be minimal.

This was recently done in more depth in topic Galactic Images (http://www.bautforum.com/space-astronomy-questions-answers/86062-galactic-images.html#post1456112)

cosmocrazy
2009-Apr-07, 08:15 PM
and just for good measure.. if space is curved.. and we could see duplicate galaxies.. could we see our own?

O_o

Hi welcome to BAUT,

this last question is an interesting one, there are many modern astronomers theorising that we could, if we look far enough actually see our own galaxy in the far distant universe, if the universe did indeed curve right back round on its self. Obviously this is yet to be observed and if it is curved in such a way then the actual size of the universe is extremely larger than we might think it to be. But as technology in astronomy improves this curvature might well be detected. Hold that thought. :)

Seeka
2009-Apr-07, 08:54 PM
Hi welcome to BAUT,

this last question is an interesting one, there are many modern astronomers theorising that we could, if we look far enough actually see our own galaxy in the far distant universe, if the universe did indeed curve right back round on its self. Obviously this is yet to be observed and if it is curved in such a way then the actual size of the universe is extremely larger than we might think it to be. But as technology in astronomy improves this curvature might well be detected. Hold that thought. :)

:confused: This has confused me hugely, how could we see our own galaxy in the far distant universe?!

cosmocrazy
2009-Apr-07, 09:05 PM
:confused: This has confused me hugely, how could we see our own galaxy in the far distant universe?!

If the universe (space/time) is curved then light paths will follow this curvature. Now if the universe is large and curvaceous enough to be bent right round on itself then any light paths will eventually come back to where they started. Theoretically therefore you could view all the way round back to where you started. :) bit like folding a piece of paper into a cylinder. :)

speedfreek
2009-Apr-07, 09:50 PM
:confused: This has confused me hugely, how could we see our own galaxy in the far distant universe?!

Perhaps you have heard of the balloon analogy, where we imagine our universe is the 2 dimensional surface of a balloon. We are reducing our universe to 2 dimensions and then wrapping it around a 3 dimensional sphere. The inhabitants of our little universe would also be 2 dimensional so they could not see that they lived on a 3 dimensional sphere - they are unaware of that 3rd dimension. But if they managed to circumnavigate the balloon, they might be able to work out its shape when they end up back where they started.

Now imagine we take our 3D universe and wrap it onto a 4 dimensional sphere! This is pretty much impossible to visualise without turning your brain inside out, but the principles we can already understand would remain, they would just have an extra added dimension. Just as when you draw a straight line across the surface of a balloon you find it circumnavigates the balloon and ends up back where it started, the same could apply in our universe if it had a closed shape, like the surface of a balloon.

If the universe were not expanding, all light would eventually circumnavigate the universe if it had a shape like this. This means you would be able to see what your galaxy used to look like, if the universe were old enough for light to have made the journey all the way round. You would see a dim image of your galaxy from eons ago, smeared across the whole sky! Depending on the shape of the universe, you might see multiple images in the sky and their pattern would give you clues to the possible 4 dimensional shape - the sphere, a torus, a dodecahedron etc.

But the universe is expanding and that expansion seems to be accelerating which limits the distance we will ever be able to see. We can see the whole of our observable universe but only as it was when the Cosmic Microwave Background Radiation was emitted, around 380,000 years after the Big-Bang. We can take measurements of the minor fluctuations in that background radiation and build up a picture of the temperature patterns across the early universe. Then we can look for areas in those patterns that look similar, as if we are seeing the same piece of space but from different directions.

There have been studies of the CMBR, looking for matching areas that might show us the overall shape, but so far we have come up with nothing. As far as we can determine, our observable universe is made up of unique space and we are not looking at multiple images of the same distant regions of space when we look in different directions.

So it seems as if we will not be able to use this method to determine the shape of the universe, but we are now pretty sure that the whole universe is indeed larger than our observable portion of it. There seems to be no measurable curvature to our observable universe, but there is always the chance that the universe is curved, but that curvature has a radius so large that it would seem to be flat across the parts we can see. If that is the case then a straight line might indeed actually be part of a stupendously massive circle that is much larger than our observable universe, but we will never be able to see our own galaxy.

For more information, see The Shape of Space (http://www.etsu.edu/physics/etsuobs/starprty/120598bg/startit.htm) (it is worth reading all the sections - they are short!)
and
Key et al, 2006 - Extending the WMAP Bound on the Size of the Universe (http://arxiv.org/abs/astro-ph/0604616)

alainprice
2009-Apr-07, 10:21 PM
I like the answer above because it also gives info about Olber's paradox, in case you run into it.

If the universe was curved in such a way that light from 10 billion years ago is getting back to us now, you would in fact see the milky way forming but with all the colors shifted towards the red from expansion.

That would imply that the universe gets brighter with age due to recycling of photons. This last idea seems silly to me; so I enjoy the common belief that the universe is much larger than what we can currently see.

Jeff Root
2009-Apr-09, 12:25 AM
I have read that the actual math shows that if the Universe were closed,
and the expansion not accelerating, light would just be able to go halfway
around the Universe in the time from the Big Bang to the Big Crunch.

-- Jeff, in Minneapolis

Noclevername
2009-Apr-17, 03:26 AM
...If light had infinite speed, wouldn't it be useless for carrying information? It would be, essentially, everywhere at once, so we couldn't actually see anything or tell what source light came from.

Not sure if this is correct or not. But something about the idea of infinite lightspeed just doesn't click.

Jeff Root
2009-Apr-17, 05:55 AM
I think that if light had infinite speed, you would just see whatever light
was being emitted at the time your eyes are open. Rather than seeing
the light that left your computer monitor two nanoseconds ago and the
light that left the Moon 1.2 seconds ago, you would see the light that
is leaving your computer monitor NOW and the light that is leaving the
Moon NOW. Ignoring all the other things that happen that depend on
the speed of light being what it is, things wouldn't look very different
unless you happen to be an astronomer, and what are the chances of
THAT?

-- Jeff, in Minneapolis

cjameshuff
2009-Apr-18, 05:04 AM
...If light had infinite speed, wouldn't it be useless for carrying information? It would be, essentially, everywhere at once, so we couldn't actually see anything or tell what source light came from.

It would violate causality horribly if you also kept relativity, but the time lag is of little importance in, say, household lighting. Light would still have sources, because light would still spread out with distance from them and get absorbed as it reflected off things. Rooms don't slowly "fill up" with light after you turn a light on or open a window (not on human timescales, anyway), and making light instantaneous won't change that.