Light, light years, and an expanding universe

BoJangles · Nov 12, 2008

Quote from live sciencehttp://www.livescience.com/space/081111-st-black-holes.html***The new conclusion comes from the discovery of two distant and interacting galaxies, both of which contain black holes at their hearts that will most likely merge. The pair is 12 billion light-years away, which means the light seen by astronomers left the scene 12 billion years ago. (A light-year is the distance light will travel in a year, or about 6 trillion miles, or 10 trillion km.)*** Q1. If a light source is 12 billion light years away, how could it possibly have taken 12 billion years to get here, after all, the universe is expanding, and we are moving away from everything, at a massive speed (including said light source).Q2. Additionally wouldn’t the light have travelled instantaneously, meaning it left the scene as soon as we see it? I.e. time dilation.I'm wondering if a bit of relativity is poking its head in here (in regards to my initial question).Further more, If the above is true (the articles wording), what would be the answer to the following scenario?Q3. Light leaves a source 12 billion years ago 12 billion light years away in an expanding universe, on that same day hour and second, I get in a space ship and travel 12 billion years at half the speed of light in the same direction (i.e. facing away from the light source); would that light which had left the original light source hit me the second I stop? I hope this makes sense and thanks in advance for your response. <div class="Discussion_UserSignature"> -------------- Let me start out with the standard disclaimer ... I am an idiot, I know almost nothing, I haven’t taken calculus, I don’t work for NASA, and I am one-quarter Bulgarian sheep dog. With that out of the way, I have several stupid questions... *** A few months blogging can save a few hours in research *** </div>

Saiph · Nov 12, 2008

You've got a good set of questions there, and they are all wrapped up in the fact that there are several different ways of looking at distance in astronomy.If we judge something to be 12 billion ly away because of light travel time, that isn't necessarily it's real distance from us now.  Just as you suspect, the expansion of the universe has carried the object away from us at the same time the light travels to us.  It's proper distance does not equal the distance the light traveled.Here's a good wiki page to help get you started:  http://en.wikipedia.org/wiki/Distance_measures_(cosmology) <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

a_lost_packet_ · Nov 12, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Quote from live sciencehttp://www.livescience.com/space/081111-st-black-holes.html***The new conclusion comes from the discovery of two distant and interacting galaxies, both of which contain black holes at their hearts that will most likely merge. The pair is 12 billion light-years away, which means the light seen by astronomers left the scene 12 billion years ago. (A light-year is the distance light will travel in a year, or about 6 trillion miles, or 10 trillion km.)*** Q1. If a light source is 12 billion light years away, how could it possibly have taken 12 billion years to get here, after all, the universe is expanding, and we are moving away from everything, at a massive speed (including said light source).Q2. Additionally wouldn’t the light have travelled instantaneously, meaning it left the scene as soon as we see it? I.e. time dilation.I'm wondering if a bit of relativity is poking its head in here (in regards to my initial question).Further more, If the above is true (the articles wording), what would be the answer to the following scenario?Q3. Light leaves a source 12 billion years ago 12 billion light years away in an expanding universe, on that same day hour and second, I get in a space ship and travel 12 billion years at half the speed of light in the same direction (i.e. facing away from the light source); would that light which had left the original light source hit me the second I stop? I hope this makes sense and thanks in advance for your response. Posted by Manwh0re</DIV> SpeedFreek wrote an excellent synopsis here regarding cosmological measurements and the speed of light that I think should be stickied in this forum after he gets a chance to fine-tune it.  It could help you in finding your answers. <div class="Discussion_UserSignature"> I put on my robe and wizard hat... </div>

weeman · Nov 12, 2008

Uh yeah....speedfreek's post pretty much explains your question, manwhore!   <div class="Discussion_UserSignature"> Techies: We do it in the dark. "Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity." -Albert Einstein </div>

BoJangles · Nov 12, 2008

Thanks guys for your help

<div class="Discussion_UserSignature"> -------------- Let me start out with the standard disclaimer ... I am an idiot, I know almost nothing, I haven’t taken calculus, I don’t work for NASA, and I am one-quarter Bulgarian sheep dog. With that out of the way, I have several stupid questions... *** A few months blogging can save a few hours in research *** </div>

SpeedFreek · Nov 12, 2008

Err.. you're welcome!  <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-wink.gif" border="0" alt="Wink" title="Wink" />Saiphs URL got truncated, so here it is againhttp://en.wikipedia.org/wiki/Distance_measures_(cosmology) along with another useful description:The Distance Scale of the Universe With reference to your question, the pair of galaxies have a light-travel time of 12 billion years so we are seeing those galaxies as they were when the universe was around 1.7 billion years old. The pair were around 4.8 billion light-years away when they emitted the light we are seeing and whatever they have become since, it is now thought to be something over 23 billion light-years away. <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

weeman · Nov 17, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Err.. you're welcome!  Saiphs URL got truncated, so here it is againhttp://en.wikipedia.org/wiki/Distance_measures_(cosmology) along with another useful description:The Distance Scale of the Universe With reference to your question, the pair of galaxies have a light-travel time of 12 billion years so we are seeing those galaxies as they were when the universe was around 1.7 billion years old. The pair were around 4.8 billion light-years away when they emitted the light we are seeing and whatever they have become since, it is now thought to be something over 23 billion light-years away. Posted by SpeedFreek</DIV> Speedfreek, there's nothing I'd love more than to sit down with you and pick your brain! <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-laughing.gif" border="0" alt="Laughing" title="Laughing" />Going back to your two posts in the thread, "Disprove this theory of everything", I can understand about 80% of what your explaining in your post about the Observable Universe. However, with your second post, about the apparent redshifts of galaxies, I'm only understanding about 30% of what your explaining. Can you elaborate more on the z scale? I think understanding how these scales are determined will give me a better understanding of cosmic expansion. Also, I think you somewhat answered a question I've pondered about distance. You said that 13.7 billion ly is a measurement of time, not a measurement of distance. So, when we look into space, isn't 13.7 billion ly the farthest that we can see? I also understand somewhat the concept of angular diameter (objects appearing larger in the sky), but what does this tell us about a galaxy's distance at the farthest reaches of our observable universe? Basically, all I'm trying to say is that, even though I feel I have a better understanding of cosmic expansion than the average American, I read your posts, and realize that I only know about 1%<img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-tongue-out.gif" border="0" alt="Tongue out" title="Tongue out" /> <div class="Discussion_UserSignature"> Techies: We do it in the dark. "Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity." -Albert Einstein </div>

marcel_leonard · Nov 17, 2008

My question would be if the powers that be that created the universe were a spantaneous Big Bang or a mischiefous Creator playing w/ matches; wouldn't said power be beyond the laws of physical Time/Space as we know it????? <div class="Discussion_UserSignature"> "A mind is a terrible thing to waste..." </div>

SpeedFreek · Nov 17, 2008

Wow, well I will do my best to help you, although I only actually have what I feel is the vaguest understanding of some of these concepts myself.In cosmology, they use the equations of general relativity to derive z from the shift in the spectrum of a distant object, and z comes in the form1 + z = the scale factor now / the scale factor then.Basically, this means for an object with a given cosmological redshift of z, the universe was 1 + z times smaller when the light was emitted, than the size of the universe today. Or, the universe is now 1 + z times larger than it was then.So if the most distant galaxy we have seen has a redshift of z=7, then the universe is now 8 times larger than it was when the light we are looking at was emitted. If the CMBR has a redshift of z=1089, the universe was 1090 times smaller when the CMBR was emitted than it is today. The universe is now twice the size it was when the light from a z=1 galaxy was emitted.It applies to distances at the cosmological scale. An object at z=1 is now twice as far away as when its light was emitted. All objects that are at z=1 relative to another object are now twice as distant from that other object. The universe is now twice the size it was at z=1. Now, for that 13.7 billion light-years thing.When you ask what is the farthest that we can see, you are asking for a distance. You should understand by now that distance can be measured in different ways, depending on what it is you want to know. Do you want to know for how long the light has been travelling, how far the light has moved, how far away the thing that emitted the light was when light was emitted, or how far that thing has moved since? The first is easy - 13.7 billion years, but the others more complicated.We have to consider different notions of distance which depend on the expansion of the universe. How far has the light moved? Well the light we are talking about when we talk of the size of the observable universe being 13.7 billion light-years is the CMBR. When they say they can see 13.7 billion light-years, they are referring only to the background radiation. When the CMBR was emitted and photons filled the universe they were moving in all directions. The CMBR photons that we receive today were the ones that were only around 42 million light-years away at that time. So have they travelled only 42 million light-years in 13.7 billion years? No, they have not. They have travelled 13.7 billion light-years. The universe was expanding as they travelled, constantly putting more distance in between those photons and their eventual target. As they reach us 13.7 billion years later, the place that they were emitted from has been "expanded away" to 46 billion light-years.But when you talk of the farthest thing we can see, perhaps you are meaning the most distant galaxy it would be theoretically possible to detect with the most perfect conditions and instruments. Well that is a little more complicated than it seems in an expanding universe too!The problem here is that there are two limits that put a damper on our observations. One is a limit in time - we can only see back as far as there were things to see. The other limit is the speed of light, or more accurately the objects that recede at the speed of light at the edge of our Hubble sphere.Let's look at the limit in time. The further back we look in time, the faster the universe was expanding. What this means to us in practical terms is that the longer the light has been travelling for, the closer the original emission point for that light (for distances larger than z=1.6). The expansion of the universe was so fast back then that any light emitted over a certain distance away was actually carried away from here by that expansion. The further back you look, the faster the things we can see were receding, and the closer those things were to us when they emitted the light we see.The photons that reach us at any given time were emitted throughout the universe at different times. At any given time we are looking at a "slice" of the history of the universe, looking backwards through time. That "slice" currently contains CMBR photons emitted 42 million light-years away, z=7 galaxies that were 3.5 billion light-years away, z=5 galaxies that were 4.3 billion light-years away, z=3 was 5.2 billion, z=1.4 was 5.7 billion, z=1 was 5.4 billion, z=0.5 was 4 billion light-years away and the lower redshifts get closer and closer. So the highest redshifted objects are only the most distant in terms of the time their light has been travelling, and where those objects, or whatever has become of them since, has receded to by now. What we actually see at high redshifts are relatively close galaxies, only a few billion light years away.So.. what we are looking for, further back still (higher redshifts), are early structures that were very close to us when their light was emitted! That's not really distant, is it?But lets look at something that had formed only 100 million years after the Big-Bang. This region would have a redshift of z=30 and whatever we are looking at would only be 1.2 billion light years away. Perhaps galaxies haven't formed yet and we are looking for individual stars or diffuse clouds of stuff... these things would be incredibly dim and hard to detect. Now how about the most distant, in terms of actual distance, objects we have seen? Well obviously we see things as they were when the light left them, and the most distant objects we have actually seen are galaxies that were receding at the speed of light when their light started its journey towards us. The light from anything actually more distant than that has not had time to reach us yet. Those galaxies have a redshift of only around z=1.4 were 5.7 billion light-years away when they emitted their light, and even if we had the ability to detect every photon ever emitted in this direction we could see no further. This is the edge of the Hubble sphere, as it was 9.1 billion years ago. Any light we see that has been travelling for more than 9.1 billion years was closer than 5.7 billion light-years when it was emitted. The most distant objects we have seen, in terms of proper distance, are galaxies that were 5.7 billion light-years away, 9.1 billion years ago. If our model is correct, we will eventually see CMBR photons that were emitted at distances further than 42 million light-years away and galaxies that were further away than 5.7 billion light-years. Hopefully we will also see the earliest galaxies that formed when they were close to us. But there is one final limit on what we can see.. the cosmological event horizon... <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

weeman · Nov 18, 2008

Thank you for the post, that helps clear things up...a little! <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-wink.gif" border="0" alt="Wink" title="Wink" /> <div class="Discussion_UserSignature"> Techies: We do it in the dark. "Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity." -Albert Einstein </div>

SpeedFreek · Nov 18, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Thank you for the post, that helps clear things up...a little! Posted by weeman</DIV>It is hard to know how much to cover in a post, and I can end up getting a little too enthusiastic about things.I would really recommend you read Expanding Confusion, and pay attention to the set of space-time diagrams on page 3. The whole picture of our expanding universe, as we understand it, is contained in those diagrams and once you understand how to interpret them, everything I have been saying should fall into place. If you have any specific questions I will try to address them as best I can. <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-smile.gif" border="0" alt="Smile" title="Smile" /> <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

Meric · Nov 18, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>It is hard to know how much to cover in a post, and I can end up getting a little too enthusiastic about things.I would really recommend you read Expanding Confusion, and pay attention to the set of space-time diagrams on page 3. The whole picture of our expanding universe, as we understand it, is contained in those diagrams and once you understand how to interpret them, everything I have been saying should fall into place. If you have any specific questions I will try to address them as best I can. Posted by SpeedFreek</DIV> As soon as my brain is reassembled I'll try and put together a few intelligent questions.  I may need to read in paragraphs instead of pages <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-laughing.gif" border="0" alt="Laughing" title="Laughing" /> <div class="Discussion_UserSignature"> Those who never make mistakes, are always led by those who do. </div>

weeman · Nov 19, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>It is hard to know how much to cover in a post, and I can end up getting a little too enthusiastic about things.I would really recommend you read Expanding Confusion, and pay attention to the set of space-time diagrams on page 3. The whole picture of our expanding universe, as we understand it, is contained in those diagrams and once you understand how to interpret them, everything I have been saying should fall into place. If you have any specific questions I will try to address them as best I can. Posted by SpeedFreek</DIV> Actually, the 'Distance Scale of the Universe' page helps shed a lot of light on the subject! That's certainly saved as one of my "Favorites".

<div class="Discussion_UserSignature"> Techies: We do it in the dark. "Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity." -Albert Einstein </div>

SpeedFreek · Nov 19, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Actually, the 'Distance Scale of the Universe' page helps shed a lot of light on the subject! That's certainly saved as one of my "Favorites".

Posted by weeman</DIV>Yes, that is a very good place to start - it is where I started from on my quest to understand the current mainstream cosmological model.Once you understand what the distance measures actually represent, you can plug any redshifts you are interested in into Ned Wright's Javascript Cosmological Calculator and plot your own graphs if you like. <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

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