# Light years

1. Feb 19, 2004

### Dagenais

A friend told me that if a galaxy is 13 billion light years away, it means the universise is at least 13 billion years old.

I don't get it? So, light years does measure time? Or is it because this new "galaxy" found is the oldest galaxy in existence?

He explained it to me that it took the image of that galaxy 13 billion years to get to our eyes, that's how we know our Universe is at least 13 billion years old.

?

Last edited: Feb 19, 2004
2. Feb 20, 2004

### Staff: Mentor

A light year is the distance light travels in a year.

If the universe were only 10 billion years old (for example) where would that galaxy have been 13 billion years ago to give off light for us to see today?

3. Feb 20, 2004

### Dagenais

So it took 13 Billion years for that light to get to our eyes?

Isn't it up to the power of the telescope used? Since it collects light better than our eyes.

So, if there is a galazy out there, 40 billion light years away, we have to wait 40 billion years to see it?

4. Feb 20, 2004

### Staff: Mentor

Yes. Distance = speed * time:
13 billion years * speed of light = 13 billion light years.
Yes, but thats because things that are further away look dimmer than similar objects near us. You only need to look at an approaching car's headlights to see that.
Now we're into hypothetical territory. IF a galaxy formed 13 billion years ago 40 billion light years away in a static universe, it would take another 27 billion light years for the light to reach us: 40-13=27.

I said a static universe because the expansion makes it a lot tougher for that question. Since the universe is expanding, a galaxy that we see as 13 billion light years away today wasn't that far away 13 billion years ago.

5. Feb 20, 2004

### Phobos

Staff Emeritus
Just to add to that thought...

Telescopes are passive receivers...they don't actively pull light toward us or reach out to capture light. (Not sure if that is what you were thinking, but I have seen that misconception before.) Telescopes capture more light than our eyes can, so fainter objects can be seen with the telescope.

Consider this...
The Big Bang created the boundless universe. Energy/matter was already distributed throughout the universe since the beginning. Stars and galaxies later formed (again, throughout the universe). Light travels thru the universe at finite speed. We see our surroundings "now". We see the moon as it was 2 seconds ago. We see the sun as it was 8 minutes ago, the planets hours ago, the stars years or hundreds/thousands of years ago, other galaxies millions/billions of years ago. Seeing a galaxy 13 billion light years away is seeing things near the beginning of time. It's a very old image and "now" for that galaxy is very different. But we can't see it yet. We can't see our past (that light is gone), but an astronomer in that distant galaxy would see now how our galaxy was 13 billion years ago.

From other data, we know the universe to be about 13.7 billion years (time) old. So, with the finite speed of light, we only have the potential to see about 13 billion light years (distance) away. We can say that our "visible universe" has a radius of 13.7 billion light years even though the entire universe is bigger than that (and may be infinite). So, even though that galaxy at 40 billion light years may exist, it's well outside our range of view (it's not part of the light shell currently bombarding the Earth). 27 billion years from now when that image reaches whatever is left of us, it will be an image from the beginning of the universe (even though the galaxy may be long gone "now").

6. Feb 22, 2004

### Dagenais

So is there a way for Astronomers to "see" stuff billions of light years away, without having to wait billions of years for the light to reach our view?

7. Feb 22, 2004

### marcus

There's no way to get around the delay
or shortcut the time it takes light to travel

If they want to study some distant objects
they have to rely on light which those objects emitted long ago
and which is just now arriving here

This is probably the same as what phobos and russ_watters were
saying

it is pretty neat how astronomers are able to figure
out things about the universe at present even though they only
have observations of light coming from the past

Last edited: Feb 22, 2004
8. Feb 22, 2004

### Nereid

Staff Emeritus
To add to what marcus wrote ... it's not only light, but neutrinos, cosmic rays, (whatever), nothing travels faster than light.

And that includes gravity. Although we don't have a 'theory of everything' yet, which would tell us all about gravitons, for example, we do know that gravity also travels at the speed of light, so the gravitational signal of a distant cataclysm will reach us at the same time as the light from that event.

9. Feb 22, 2004

### Staff: Mentor

I've never heard that one before. Yeah, it makes a question easier to answer if you understand the basis of the misconception.

10. Feb 23, 2004

### jjalexand

13 out + 13 back = 26 (not 13.7 billion years?)

"Big Bang" implies explosion.

Explosion implies exapnsion from a small central region to the current size of the universe.

Nothing travels faster than the speed of light.

If a star's light takes 13 billion years to reach earth, and the start took at least 13 billion years to separate from earth by that distance, doesn't this mean the age of the universe is at least 26 billion years?

(That is 13 billion years to travel to that distance + 13 biilion yearts for the light from it to travel back to us = 26 billion years)

However we seem now to have narrowed the age of the universe down to 13.7 billion years.

How do we reconcile these two results?

Sorry if this is a well known paradox, but it always seems to be not comprehended, glossed over, or ignored by science writers. (And physics forum participants!)

11. Feb 23, 2004

### jjalexand

Correction: "... and the STAR took..."

The last post should read "...and the star took...", not "...and the start took..."

12. Feb 23, 2004

### Dagenais

Isn't the big-bang a theory? Is it accepted by all astronomers?

13. Feb 23, 2004

### marcus

Re: 13 out + 13 back = 26 (not 13.7 billion years?)

https://www.physicsforums.com/showthread.php?s=&postid=147179#post147179

this is a link to a post in the "astronomy reference" thread that serves as a linkbasket

there is a fair amount of vague thinking and confusion about the expansion of the universe and Lineweaver and Davis (two people at the University of new south wales) have made a valiant effort to come to grips with it.

they have a pedagogical article called "Expanding Confusion: popular misconceptions about the ..." which is being published by the Australian Astronomical Society

I dont know that I can describe the situation in a nutshell
two things can start rather near to each other, back when the U was expanding very rapidly, and yet the light take a long time to get from one to the other because it has (in effect) to fight against the expansion

they draw a kind of picture of this called a space time diagram
(a picture with space on the horizontal (or "x") axis and time on the vertical ("y") axis)

on such a diagram one can draw the lines for two objects getting farther apart and a line for some light emitted from one and evenually getting to the other

a galaxy (now very distant) could have sent us some light back when we were within only a few million LY and yet that light could have taken several billions of LY to get here

dont know how to advise you except to look at pictures in
Lineweaver and Davis articles
since they take more trouble clarifying this than anyone else I know

here are some links (two of those posted in the "reference shelf")

------------------------
http://arxiv.org./abs/astro-ph/0310808

Davis and Lineweaver
"Expanding Confusion:common misconceptions of cosmological horizons and the superluminal expansion of the Universe"

----------------------

http://arxiv.org/astro-ph/0402278 [Broken]

Tamara Davis thesis (advisor Charles Lineweaver)
"Fundamental Aspects of the Expansion of the Universe and Cosmic Horizons"

the thesis is around 150 pages IIRC so it takes a while to download
--------------------------

Charles Lineweaver
http://arxiv.org/astro-ph/0305179 [Broken]
"Inflation and the Cosmic Microwave Background"

good general survey of cosmology covering may topics but
with some space-time diagrams
and some overlap with the other articles

Last edited by a moderator: May 1, 2017
14. Feb 23, 2004

### marcus

btw JJ the field of cosmology is fraught with apparent paradox

the Speed Limit we all know about
does not apply to the expansion of the universe

the distance between two widely separated galaxies can increase at
a rate which is greater than c

the rate that largescale distances increase is called
"recession velocity" and recession velocities can exceed c

the Speed Limit applies locally to things in a particular
neighborhood with a particur local frame of reference---
so nothing is allowed to whiz past the earth going over c
and nothing is ever allowed to catch up with and pass a photon
and information is not allowed to propagate faster than c

but there is a subtle distinction between something flying past you at a certain speed and the rate at which some large distance is increasing as part of the general expansion of space

and things that are not co-occupants of the same local system of coordinates are allowed to drift apart faster than c.

It is my hope that you will find some articles to browse around in that will clarify this distinction (between
the stretching out of space across great distances and
the relative motion of two things in the same locale)
to your satisfaction.

15. Feb 23, 2004

### Phobos

Staff Emeritus
Re: 13 out + 13 back = 26 (not 13.7 billion years?)

The event is poorly named. It was named by someone mocking the theory, but the name sounded catchy & stuck.

There was no "center" we know of. The points of the universe were closer together...now they're getting farther apart. The Big Bang kicked it off. The Big Bang "seed" (starting point/singularity/whatever) remains a mystery.

In short...the star and the Earth did not form in the same place. They formed billions of light years apart. The Big Bang did not explode matter outwards into empty space. It was the energetic expansion of all space in the universe.

The universe was boundless from the very beginning (after time = 0). It rapidly expanded thereafter. The huge energy from the Big Bang was everywhere. That energy included fundamental particles from the very beginning...throughout the universe. As the universe expanded, it cooled. Eventually it cooled enough for the fundamental particles to form atoms throughout the early universe. Gravity pulled matter into stars...throughout the early universe. Light started travelling from those stars. So, the light from a star that formed billions of light years from us is now just reaching us.

No problem...it's a weird & complicated concept. Hopefully other members can help the explanations you're reading here.

16. Feb 23, 2004

### Staff: Mentor

Re: 13 out + 13 back = 26 (not 13.7 billion years?)

Put simply, the term "big bang" is probably a poor choice of words. It was not an explosion from a single point, it happened everywhere at once.
Just in case you ahve a misconception there, the word "theory" is synonomous with "accurately fits the existing body of evidence." So yes, its a theory, and it is accepted by most astronomers in its general form. Don't fall into the "its just a theory" trap.

17. Feb 23, 2004

### Nereid

Staff Emeritus
You can make a couple of even stronger statements:
- there is no other theory of the origin of the universe which fits the observations and data as well as the Big Bang theory
- AFAIK, there are no other 'origin of the universe' theories being seriously investigated by astronomers (except as foils).

On the other hand, there are still many details to be worked out within the Big Bang theory, and data from new observations continues to flood in.

18. Mar 1, 2004

### gunblaze

quote:A friend told me that if a galaxy is 13 billion light years away, it means the universise is at least 13 billion years old.

It would rather be quite interesting to know the age of a star through the distance for its light to reach earth!(actually quite possible since no theory like this is proven wrong)

but since light yrs represent its distance for its light to reach us, do we presume that all star emits light???

Correct me if i'm wrong!
Then y doesn't the black hole emits light even though its a star before its death??? If it doesn't emits light, y would people noe of its existance(they guessed?)???

19. Mar 1, 2004

### jjalexand

Reply to Russ_Watters

While the big bang theory may have happened 'everywhere at once', maybe that 'everywhere' was a very small region in some enclosing (other dimensional) space?

Don't we talk about the expansion of the universe, the reason for the red-shift?

Don't we have a concept of the 'age' of the universe?

Doesn't that also imply a radius of the universe? (Or at the very least, surely the age in years is equivalent to some length in light years that might possibly have some physical significance in terms of it's size?)

Are we really saying the big bang theory is assoicated with an infinite universe, i.e. everything is rushing away from everything else without a center or a boundary? If it happened every_where_ at once, why not every_when_? If it had a beginning in time, then why not in space as well? If the big-bang happened 'everywhere at once', was that relative to some meta-universe containing ours, or only 'everywhere at once' relative to some (possibly expanding) balloon of local universe.

Russ, as a layman, I would personally be grateful for some further explanation, reference or logical argument clarifying your statement that 'The big-bang happened everywhere at once'.

20. Mar 1, 2004

### Phobos

Staff Emeritus
Re: Re: Light years

Yes, black holes that were created from the collapsed cores of large stars did emit light back when they were functioning stars. But if we're detecting a black hole, then its star-light is history...that light has passed by the Earth already.

So, how do we detect black holes? (1) By their gravitational influence on nearby matter. (2) By the light emitted from stuff falling into the black hole.

Black holes were predicted to exist based on math/physics. Now we see evidences...like (1) and (2) above...that are best explained by a black hole. Based on that, it is believed that black holes are very likely to exist.

Confusing? Consider the example of seeing a star rapidly orbiting what appears to be nothing. Not only that, but given it's orbit and mass, you can calculate the mass that should be in that place where you see nothing. With that calculated mass combined with the volume of empty space being orbited, math/physics tells you that it can only be one thing...a black hole.

21. Mar 1, 2004

### Phobos

Staff Emeritus
jjalexand - Since you asked Russ, I'll be glad to let him respond. But rest assured that the existence of a meta-universe is not ruled out. But at the moment, there's no conclusive evidence for it either. For now, Big Bang Theory doesn't rely on there being a meta-universe.

22. Mar 1, 2004

### jjalexand

Thanks everybody

Sorry, I have only just seen all these responses.

Thanks to Marcus, Phobos, Russ Watters, etc for all the excellent help on the 13+13 issue. It looks as if there was a whole area of 'knowledge' about the big-bang that I was not aware of.

I had always assumed the initial singularity was confined to a very small region, and that was why the cosmic background radiation had a very uniform temperature. However, of course it does get complicated that region is the whole universe, introducing an element of self-reference (which always complicates things, e.g. the types of mathematical statement that Godel showed may be unprovable seem to involve self-reference).

I suppose popular science writers may generally gloss over these difficult issues as they may be just too hard to explain in any short text.

I will have a look at the references, thanks.

23. Mar 1, 2004

### Nereid

Staff Emeritus
To amplify on this ...
http://www.eso.org/outreach/press-rel/pr-2002/pr-17-02.html [Broken] of stars orbiting the black hole at the centre of the Milky Way.

And this is a very recent observation of light (actually X-rays) from a star being ripped apart as it got near a supermassive black hole (and some of its mass fell into the black hole).

Last edited by a moderator: May 1, 2017
24. Mar 2, 2004

### gunblaze

Re: Re: Re: Light years

Thanks, that helps. So what u mean is that if some sort of a star is supposed to be orbiting in space but its light isn't visible means that the the presence of a black hole is near!

25. Mar 2, 2004

### jjalexand

Utterly cool after 3 Stella Artois

"Notice that in contrast to special relativity, the redshift does not indicate the velocity, it indicates the distance [12]. That is, the redshift tells us not the velocity of the emitter, but where the emitter sits (at rest locally) in the coordinates of the universe. "

This UNSW paper is so excellent, thank you Marcus, you have the magic touch. I have been puzzling about this for years, it is so well expressed too. (to the extent that I can follow it).

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