# Thread: How can we see galaxies from 13 billion years ago?

1. From time to time in the news I see articles about galaxies 10.2 or 13.1 billion light years away. How is this possible? If the universe is only about 14 billion light years away how can we possibly see a galaxy from 12 billion years ago? Shouldn't the light be billions of light years past us by now?

Some popular articles state that a given galaxy is 10 or more billion light years away. How is this possible? Wouldn't this require galaxies to be moving at about the speed of light or at least half the speed of light?

Thanks

2.

3. Originally Posted by Fishguy2727
From time to time in the news I see articles about galaxies 10.2 or 13.1 billion light years away. How is this possible? If the universe is only about 14 billion light years away how can we possibly see a galaxy from 12 billion years ago? Shouldn't the light be billions of light years past us by now?

Some popular articles state that a given galaxy is 10 or more billion light years away. How is this possible? Wouldn't this require galaxies to be moving at about the speed of light or at least half the speed of light?

Thanks
There is no problem here. The expansion of the universe can lead to arbitrarily high relative velocities between two distant bodies. This does not mean that the galaxies are actually flying with that velocity, but the expanding space is dragging everything along. Therefore, we can see objects that are about 13.1 billion lightyears away. They were closer at early epochs of the universe, but expansion has put them at distances that far from us. The cosmic microwave background is even farther away, because it was emitted about 380 000 years after the "big bang".

4. So now let's take one of those distant galaxies, say 10Bly away. That's where it was 10B years ago, not today; it's a time-delayed image ::: with me so far? In order to figure out where it is today / right now, we'd need to know it's direction, velocity, everything along its 10By path that would influence it along the way, include a slight tweek for expanding space, and voilą ::: we know where it should be today. Or is this not possible?

Also, has there even been any attempts to determine the real-time picture of the known universe with the light-year lapse removed?

5. I see what you are saying.

But in order for it to be 10bly away today, even if that and our galaxy are traveling in exact opposite directions each would have to travel 5bly in ~14by and would have to both be traveling at over 1/3 the speed of light.

What about the articles saying that the galaxy in this picture is 12 byo and we are looking back in time. That means that back then it was around and gave off light that we are seeing now, but how can we possibly see that light? Shouldn't it be well past us by now?

Space itself is expanding, got it. That contributes to the distance apart in the end, got it. How does this alter light's travel and the speed of light?

6. The speed of light is absolute. Correctly stated, this principle says that no cause and effect relationship can be propogated at greater than the speed of light.

However, where there is no possibility of any cause and effect relationship - not even one photon's worth - then things can happen at speeds that are greater. In this case, the universe can expand at a rate greater than light speed, if that expansion prevents such a relationship happening.

Thus, a galaxy near to us can be retreating from a galaxy far away at a relative speed greater than light if no cause and effect relationship between those two galaxies are possible. The two galaxies are not necessarily actually moving relative to each other. Just space itself expanding.

Of course, this means that we cannot ever detect such a galaxy. It will remain forever unknown.

7. Quoted: "Just space itself expanding".

Why do you say that space itself is expanding.

jsaldea12

11.16.10

8. The more advanced our astronomical telescopes, like Hubble telescope or the more giant earth-based telescopes , that reach the recesses, the deeper recesses of the visible universe, always there is that beyond, the unfathomable deeper, wider invisible outer space with indwelling galaxies. I dont think any good astronomer, worth his grain, who has peered through his astronomical telescope, will deny this. Space is there beyond, unfathomable, always there beyond the reach of the strongest astronomical telescope as visible universe is being pushed, is being made larger. One good implication is that space is just there, with or without the theory of expanding universe which actually pertains to matter, re-galaxies, moving outward? Big Bang, if there is, pertains to matterfrom the size smaller than a proton.

Einstein original concept of cosmological constant appears correct.

Jsaldea12
11.15.10

9. Hello,

An average star burns for a few billion years. For example, let us take a star that burns up for about 6 billion years. After it burns up, the old light is spreading, and until it reaches the Earth, it needs that much time (6 billion years, for example). So, for us, the star seems to be burning for 6 billion years after the star has burned out. Now, I think I have put it in a very straight forward manner, and I hope it helps you understand. So, it is not as hard as it seems to be, now is it?

10. Originally Posted by GioNikola
Hello,

An average star burns for a few billion years.
Er, no. The life of a star depends on its mass, the smaller the mass, the slower it burns its fuel, and the longer its lifetime. A Red Dwarf (the most numerous type of star), can burn for 100's of billions of years. A blue giant could detonate in a few million.

Our Sun has a total estimated lifetime of 12 billion years.

An average star masses about 1/2 the mass of our Sun and would burn for an estimated 50 billion yrs.

11. Originally Posted by Janus
Originally Posted by GioNikola
Hello,

An average star burns for a few billion years.
Er, no. The life of a star depends on its mass, the smaller the mass, the slower it burns its fuel, and the longer its lifetime. A Red Dwarf (the most numerous type of star), can burn for 100's of billions of years. A blue giant could detonate in a few million.

Our Sun has a total estimated lifetime of 12 billion years.

An average star masses about 1/2 the mass of our Sun and would burn for an estimated 50 billion yrs.
I slight misinterpretation from my side when I said "few". Yes, the average star burns for about 50 billion years. Thank you for the correction.

12. Originally Posted by 6nqpnw
So now let's take one of those distant galaxies, say 10Bly away. That's where it was 10B years ago, not today; it's a time-delayed image ::: with me so far? In order to figure out where it is today / right now, we'd need to know it's direction, velocity, everything along its 10By path that would influence it along the way, include a slight tweek for expanding space, and voilą ::: we know where it should be today. Or is this not possible?

Also, has there even been any attempts to determine the real-time picture of the known universe with the light-year lapse removed?
Actually, a galaxy whose light took 10 billion years to reach us was a lot less than 10 billion light-years away when that light was emitted, and is a lot further than 10 billion light-years away as that light reaches us.

Light that has been travelling for 10 billion years comes from galaxies with redshifts of around z=1.9, which were only 5.7 billion light-years away when that light was emitted, and are now thought to be around 16.6 billion light-years away as we detect that light.

13. Where it "actually' is is not valid. A galaxy is for all intents and purposes, where we see it. All information is constrained to move at c, no faster.

14. There are a few distance measures used in cosmology, and they are all as valid as each other.

Light-travel time or look-back time:
The time between the emission and our detection of the light from a distant object, which is often expressed in light-years, but is not a measure of distance through space, but rather is a measure of elapsed time.

Angular diameter distance or proper cosmological distance:
The distance between our worldline and the worldline of an event, at the (cosmological) time the event occurred. We see galaxies at the distance they were when they emitted the light we see.

The distance "now" between our worldline and the worldline the event occurred on, if those worldlines are co-moving with the expansion of the universe. By the time we see the light from a distant galaxy, the expansion of the universe has put that galaxy a lot further away than its proper distance.

15. Originally Posted by MigL
Where it "actually' is is not valid. A galaxy is for all intents and purposes, where we see it. All information is constrained to move at c, no faster.
If, by "where we see it", you mean how large it looks, how much space it takes up in the sky - i.e. angular diameter, then, as we look at objects with increasing redshift, they look smaller and smaller (or I should say further and further away!) with increasing "distance" as would be expected.

But this relationship only holds true up to a certain distance - the Hubble distance (a redshift of around z~1.4). For galaxies whose light was emitted when they were beyond the Hubble distance (galaxies that have always had apparent recession speeds faster than c), their apparent angular diameter increases with increasing redshift - they start looking closer again!

This is because we are looking back across the history of the universe, and are seeing the most distant objects as they were when the universe was a lot smaller!

The highest redshift galaxies, whose light has been travelling for 13 billion years to reach us, look to be only a little over 3 billion light-years away, in terms of proper distance. These galaxies look very dim, but relatively close to us.

Whereas a lower redshift galaxy, whose light was emitted a little over 9 billion years ago (a galaxy at the Hubble distance), looks to be nearly 6 billion-light years away. These galaxies look brighter than the higher redshift galaxies, but look to be further away.

For even lower redshift galaxies, whose light was emitted less than 9 billion years ago, the relationship between redshift and angular diameter returns to something more intuitive!

http://www.astro.virginia.edu/class/...ght_cones.html

Check out the diagram at the top of this page. The red teardrop shaped line - the light cone, represents what we can see, the slice of spacetime we view across the history of the universe.

The vertical axis in the centre represents our path or "worldline", across time, and the horizontal axis represents the proper distance across space to the events we are seeing.

If you follow the light cone downwards from the top - "now", on the right hand side of the diagram you see the redshifts we are seeing as we look backwards through time and space. Notice how the light cone reaches its maximum proper distance between a redshift of 1 and 2, where it crosses the Hubble distance, and the proper distance starts to decrease again as we look further backwards towards the Big-Bang.

If it were possible to see all the way back to the Big-Bang, we would be seeing what happened right here! :wink:

So, when we ask how far away a galaxy is, there is more than one way to answer the question.

16. How do we know that there arent galaxies 600 billion light years away? If the universe was over 600 billion years old and there were super distant clusters of galaxies, as distant proportionally from all the ones we see as the galaxies are distant from one another with very little in between, would our instruments be able to detect them??

17. Originally Posted by icewendigo
How do we know that there arent galaxies 600 billion light years away? If the universe was over 600 billion years old and there were super distant clusters of galaxies, as distant proportionally from all the ones we see as the galaxies are distant from one another with very little in between, would our instruments be able to detect them??
We don't know that there aren't galaxies 600 billion light-years away. There may well be galaxies 600 billion-light years away, it all depends on how much of the universe was put forever out of our view during the inflationary epoch.

Our instruments should be able to detect any galaxy whose light has "beaten" the rate of the expansion of the universe and managed to reach us.

18. "Our instruments should be able to detect any galaxy whose light has "beaten" the rate of the expansion of the universe and managed to reach us."
THanks, I would not have thought it possible for our instruments to be sensitive enough to measure sufficient amounts of photons to create an image from a 600 billions light year distant galaxy. I imagined the signal to be so weak or sporadic as to be indistinguishable from the background.

I wonder if there is a distance far enough in trillions of light years where a star only manages to get one photon across (to a specific point on a sensor at the other end) every 10 years or something.

19. Originally Posted by icewendigo
"Our instruments should be able to detect any galaxy whose light has "beaten" the rate of the expansion of the universe and managed to reach us."
THanks, I would not have thought it possible for our instruments to be sensitive enough to measure sufficient amounts of photons to create an image from a 600 billions light year distant galaxy. I imagined the signal to be so weak or sporadic as to be indistinguishable from the background.

I wonder if there is a distance far enough in trillions of light years where a star only manages to get one photon across (to a specific point on a sensor at the other end) every 10 years or something.
There is a distance so far away that, because space is accelerating its expansion, no photon from galaxies beyond that distance will ever reach us.

http://en.wikipedia.org/wiki/Hubble_volume

20. Hi All
My name is Terry

When astro physcistist say they can look back in time with the Hubble etc and say they are seeing/detecting the moments after or the remnants of,the big bang.This has to be
impossible.Everything created when the big bang took place (if it was just one big bang) i.e. matter all forms of radiation and subatomic particles and so on went out in all directions away from the big bang and is in our time or has past us by billions of years i.e. for the case of light etc .What they are detecting is the big bang itself it has not stopped it is still an on going process. the big bang is still taking place. the fabric of our universe is expanding because of the continual injection of space time fabric,dark matter into our universe this is why the expansion of our universe is speeding up. it has as it were an engine driving it. When and if the engine ever stops for whatever reason that's when are universe may start to contract due to gravity.When our universe collapses back to the singularity maybe all the fabric of our universe becomes the engine of a new
universe.

Best Wishes
Terry Wareing

21. Light, and all information associated with it, travels at a finite speed. It travels one light year in one year. Therefore light which started a journey 13.7 billion years ago, started from a point at least 13.7 billion light years away ( it is modified by universal expansion ).
You cannot look 13.7 billion light years away and see current conditions; it takes time for information to travel.
If you are familiar with relativity you should also know there is no such thing as simultaneity.

22. Originally Posted by Terry2
Hi All
My name is Terry

When astro physcistist say they can look back in time with the Hubble etc and say they are seeing/detecting the moments after or the remnants of,the big bang.This has to be
impossible.Everything created when the big bang took place (if it was just one big bang) i.e. matter all forms of radiation and subatomic particles and so on went out in all directions away from the big bang and is in our time or has past us by billions of years i.e. for the case of light etc...
Terry Wareing
I think you're misunderstanding the concept of the big bang. It was not a big explosion that sent everythig flying in all directions the way we normally think of such things.

Try to think of it this way: The very early universe was like a "small" white hot kiln like they use for firing pottery. Everyting in it was white hot, includung the photons (electromagnetic radiation - that is, the radiant heat).

Over the last 13.7 billion years the kiln (the universe) has gotten a lot bigger. Because it's a lot bigger everything in it has cooled down a lot (including the photons - the radiant heat). It is this "radiant heat" that we detect as the Cosmic Background Radiation. It's all around us and it fills the universe (the big kiln) to this day.

Chris

23. Thus i conclude that "The Universe is not weirder than we think but it is weirder than we can ever,ever think." as my Grandpa says. though i promote all to add more posts to this topic.

24. It's all light's fault if not for it traveling and reaching us we will never see anything and just be blind earthlings

25. Originally Posted by MiguelSR1
It's all light's fault if not for it traveling and reaching us we will never see anything and just be blind earthlings
Actually, I would blame time rather than light. It's not light's fault that there hasn't been time for it to reach us!

26. I appreciate the clear explanation of what we see as we look deeper into the universe as this was plaguing me for a while. I still have some questions to clear up; Namely when we are looking at the deepest level, as was stated earlier these early galaxies appear closer to us but by their redshift and other quantitative measurements on their apparent evolution we can tell they are very old and representative of early galaxies, but even these galaxies have evolved to a certain point, correct? In other words is there an event horizon that continues to recede into the past that we cease to have access to and so can't ever look at the big bang itself or even a few billion years past it? Does the event horizon correspond to the point at which space is expanding faster than light? Along the same lines can someone explain how fast the inflationary period was and how much space was actually created during that event? I take it that space expanded faster than the speed of light and much of the matter is and always was beyond reach of observation? I suppose that is enough for now, although I am sure I have more questions heh.
Thanks so much
Justin

27. Originally Posted by Fishguy2727
From time to time in the news I see articles about galaxies 10.2 or 13.1 billion light years away. How is this possible? If the universe is only about 14 billion light years away how can we possibly see a galaxy from 12 billion years ago? Shouldn't the light be billions of light years past us by now?

Some popular articles state that a given galaxy is 10 or more billion light years away. How is this possible? Wouldn't this require galaxies to be moving at about the speed of light or at least half the speed of light?

Thanks

I mean this is the problem the general population has with scientists.................who cares,.............<can you prove it like it matters........>,,,,,,,,........

28. Originally Posted by theQuestIsNotOver
I mean this is the problem the general population has with scientists.................who cares,.............<can you prove it like it matters........>,,,,,,,,........
If you don't care, then there is no need to argue, is there?

29. A previous poster mentioned that the BB is an ongoing process. I believe this to be true. Since part of this process results in the space between galaxies increasing then I wonder if the actual expansion of space is the reason we even have galaxies. I'm trying to come up with an analogy but all I can think of is filling a painted balloon with air, eventually on the surface, gaps appear between fractured sections of paint. I hope that's reasonable. So the expansion of space contributed to the breaking up of the early universe into separate sections and then gravity took over for each clump.....Is that about right?

I think what the OP is intimating is this: If I were to try to shine light on an object moving away from me at a speed greater than light then will that object ever be illuminated? However we do see distant light which probably suggests that the expansion of space was less than c in the beginning but has been or is increasing. Correct so far? Does that mean that once the now distant light reached us it will always remain, stretched out, but it can't be outrun?

30. Originally Posted by zinjanthropos
I think what the OP is intimating is this: If I were to try to shine light on an object moving away from me at a speed greater than light then will that object ever be illuminated? However we do see distant light which probably suggests that the expansion of space was less than c in the beginning but has been or is increasing. Correct so far?
Far from correct, unfortunately. Take the expansion of space (an analogy) you mentioned before - here, nothing is moving through space, but space is expanding between things such that, at a certain distance, an object would have had to have travelled through space at the speed of light to get there, if it was moving through space.

As soon as the light leaves an object that has a cosmological recession speed of c, that light is heading towards us at c, through regions that are receding at less than c. Thus, that light will; eventually reach us.

We see the light from galaxies that have always been receding faster than c.

Have a read of this pdf article from Scientific American - it is a good starting point:

SciAm Lineweaver Article

31. That PDF really helped and totally cleared up many misconceptions I had in my visualization of the Big Bang and the relationship between space and things traveling through space. Really appreciate it.
J

32. Fishguy2727 - Not sure if this answers you question, but a galaxy is not a flashbulb. Once it is formed, it will emit light light until its end. That is if the galaxy was in existance 10 byo then it formed more than 10 byo. The light from its constituent stars will travel throughout the universe from that point in time until the galaxy ends (for whatever reason). Since stars can have lives of 100's of billions of years (or only several million years), the light from this galaxy will be a continuous stream for all of that time.

Stating that all the light ever emitted from this galaxy will have passed us by is not understanding that this galaxy will emit light, and be observable, for a time after it ceases to exist which is related to its distance from our viewpoint.

That is we will not know that the sun has ceased to exist until 8 minutes after the event has occurred.

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