PDA

View Full Version : how long ago is 13 b ly?

damian1727
2009-May-11, 05:58 PM
does anyone know how long ago light would have left a galaxy 13 billion ly away?

(taking into account the expansion of space)

thanks

damian
:)

01101001
2009-May-11, 08:16 PM
does anyone know how long ago light would have left a galaxy 13 billion ly away?

What kind of distance measure (Wikipedia (http://en.wikipedia.org/wiki/Distance_measures_(cosmology))) are you employing?

NHR+
2009-May-13, 02:18 PM
does anyone know how long ago light would have left a galaxy 13 billion ly away?

I would say 13 billion years ago, but I might be wrong. I don't think the expansion concerns TIME at all, just SPACE. So 13 billion light years means that the light has been on its way for 13 billion years, right? :)

DISTANCE however is much more complicated, as mentioned above...

01101001
2009-May-13, 06:03 PM
Besides needing a decision on which distance measure is used in the question, it is missing something else: a description of which light you are speaking.

A galaxy, formed, begins constantly emitting light and does so until it's no longer able. So, an unuseful answer to how long ago light would have left, might be: pretty much any time since it started lighting up.

Do you have some particular emission in mind, like one whose photons are reaching Earth today? (Note that your selection of a particular light emission puts some severe constraints on the distance measure you select to match the 13 billion light year distance figure.)

geonuc
2009-May-14, 09:26 AM
Besides needing a decision on which distance measure is used in the question, it is missing something else: a description of which light you are speaking.
I think it's safe to assume the OP means the light we are seeing now.

01101001
2009-May-14, 02:05 PM
I think it's safe to assume the OP means the light we are seeing now.

robross
2009-May-14, 07:53 PM
does anyone know how long ago light would have left a galaxy 13 billion ly away?

(taking into account the expansion of space)

thanks

damian
:)

I don't have the exact answer to your question, because it involves complicated math using integrals and/or derivatives to calculate the rate of expansion over the lifetime of the universe. This has not been constant. If inflation occurred, that would also affect the answer. (Someone with the proper education *could* calculate this for you though).

You have to take into account the non-linear acceleration of the expansion of the universe from t=0. At some point it is believed the rate of expansion was slowing. Then recently that rate started increasing.

But as just an example, a star could have been 9 B-ly away from Earth's current position relative to that star when it released a particular photon, and because of expansion it took that photon 13 B years to reach us "now". And in the meantime, that star could have moved because of expansion and is now 30 B-ly away from us, in Earth's "now".

Again, those numbers are totally made up, just to give an example of how expansion would have affected these measurements.

Rob

damian1727
2009-May-16, 04:55 AM

damian1727
2009-May-16, 05:01 AM
i was thinking of that recent photo of a far away galaxy... everyone kept saying that as it was 13 billion light years away it was 13 billion years old ... hence having formed when the universe was 700 000 years old ..... but its not that old as far as i can see or how old would something we saw as 20 billion ly away be?

i think rob hit the nail on the head really ... the expansion of space has not been constant...

mmmm

kewl

damian1727
2009-May-16, 05:08 AM
.....nhr said

I don't think the expansion concerns TIME at all, just SPACE

?

i have often wondered about this as in many ways they are different aspects of the same thing (?!)

trust the universe to do your nut in ! lol

damian1727
2009-May-16, 06:18 PM
very interesting wiki pagel 0101010 :)

quote

The edge of the observable universe is now located about 46.5 billion light-years away.

end quote

dirty_g
2009-May-17, 09:42 AM
does anyone know how long ago light would have left a galaxy 13 billion ly away?

(taking into account the expansion of space)

thanks

damian
:)

13 billion years??

robross
2009-May-17, 01:29 PM
13 billion years??

That would only be true in a static, non-expanding universe.

Rob

Sam5
2009-May-17, 11:50 PM
does anyone know how long ago light would have left a galaxy 13 billion ly away?

(taking into account the expansion of space)

thanks

damian
:)

I think when popular science articles say a galaxy is “13 billion light years away”, they mean that the light that we now see left the galaxy 13 billion years ago, so the galaxy “appears” to us to be 13 bly away from us now. I think this has been the traditional meaning of the light-year distance to the galaxies since the late 1920s, and, in the case of distant galaxies, it is based on simple classical Doppler calculations. In other words the redshift of the galaxy suggests we are seeing it as being 13 bly away from us now.

However, this presents us with a problem that is sometimes explained this way: The galaxy was much closer to us when it first emitted the light we now see, but because of the expansion of the universe the light was delayed during its travel to us (because the space between us and the galaxy was expanding), and, of course, the galaxy has moved further away from its present “visual” position during all that time, and it is much further away from us now (further away than 13 bly).

Anyone feel free to correct me if I’m wrong about any of this. :)

Rift
2009-May-18, 03:45 AM
very interesting wiki pagel 0101010 :)

quote

The edge of the observable universe is now located about 46.5 billion light-years away.

end quote

I don't see that quote anywhere on that wiki page...

01101001
2009-May-18, 04:49 AM
I don't see that quote anywhere on that wiki page...

It's in Wikipedia: Observable universe (http://en.wikipedia.org/wiki/Observable_universe), perhaps arrived at by a link to a link.

damian1727, did you ever decide what distance measure (http://en.wikipedia.org/wiki/Distance_measures_(cosmology)) you want for the source being 13 billion light-years away?

nokton
2009-May-18, 03:21 PM
I think when popular science articles say a galaxy is “13 billion light years away”, they mean that the light that we now see left the galaxy 13 billion years ago, so the galaxy “appears” to us to be 13 bly away from us now. I think this has been the traditional meaning of the light-year distance to the galaxies since the late 1920s, and, in the case of distant galaxies, it is based on simple classical Doppler calculations. In other words the redshift of the galaxy suggests we are seeing it as being 13 bly away from us now.

However, this presents us with a problem that is sometimes explained this way: The galaxy was much closer to us when it first emitted the light we now see, but because of the expansion of the universe the light was delayed during its travel to us (because the space between us and the galaxy was expanding), and, of course, the galaxy has moved further away from its present “visual” position during all that time, and it is much further away from us now (further away than 13 bly).

Anyone feel free to correct me if I’m wrong about any of this. :)

If I may, we view, and evaluate time and distance within the time frame we are in.
Allow me to explain, if we, on earth, were in a much higher gravity field, and moving
through space at a much greater speed, our measurements would differ from what
we have now. We evaluate the universe within our time frame. To another intelligence
in a different time frame, our evaluations non sequitur within our present concept of time.
Nokton

damian1727
2009-May-18, 10:43 PM

so the universe was 700 000 years old at the time the light left the galaxy?

damian1727
2009-May-18, 10:52 PM
im going to read all your posts some more and think it thro

thanks

AndrewJ
2009-May-19, 02:01 AM

so the universe was 700 000 years old at the time the light left the galaxy?

Photons could only freely propagate once electrically neutral atoms predominated. This was a few hundred thousand years after BB (700 000 years would be later than most datings).

damian1727
2009-May-19, 08:23 AM
this is the object in question

http://www.dvhardware.net/article25228.html

01101001
2009-May-19, 12:13 PM
this is the object in question
http://www.dvhardware.net/article25228.html

The Hubble Space Telescope spotted one of its youngest and brightest galaxies ever. NASA and ESA researchers say the galaxy seen by Hubble has a redshift significantly above 7 and is more than 13 billion years old.

OK, then. They specify what distance measure they were using. Its redshift is over 7 and was seen about 13 billion years in the past (Lookback Time). (That should answer your basic "how long ago" question right there.)

But I sense you were really after a "how far away", using an alternate measure.

You can look on the comparative distance scales chart at Atlas of the Universe (http://www.atlasoftheuniverse.com/redshift.html) and see that would put its Light Travel Time distance at about 13 billion light-years and also a Comoving Distance of about 29 billion light-years.

nokton
2009-May-19, 04:10 PM
im going to read all your posts some more and think it thro

thanks

Hi Damian,
Albert was so concerned about the position of the observer, and how that
would effect the conclusion of the observer.
Consider this, you are an observer above a Pulsar rotating 30 times per sec.
what would you see? A beam of light as a lighthouse beacon, as determined in
so many astro mags. Or a spiral of light to you the observer. No, light is not bending,
it is following the known laws of physics, the spiral is a straight line for light, but it is
discrete. So the light we see from a pulsar is a wave, not a beam Thankyou for your contact
Nokton

nokton
2009-May-19, 04:29 PM
OK, then. They specify what distance measure they were using. Its redshift is over 7 and was seen about 13 billion years in the past (Lookback Time). (That should answer your basic "how long ago" question right there.)

But I sense you were really after a "how far away", using an alternate measure.

You can look on the comparative distance scales chart at Atlas of the Universe (http://www.atlasoftheuniverse.com/redshift.html) and see that would put its Light Travel Time distance at about 13 billion light-years and also a Comoving Distance of about 29 billion light-years.
Red shift is a spacetime dialation effect that has to do with speed and distance.
How we measure it as truth depends where we measure it from.
We really think we know it all, that is how we will fail as a species, our arrogence
is what will kill us.
Nokton

damian1727
2009-May-19, 07:56 PM
:eek:

damian1727
2009-May-19, 07:59 PM
1010100 are you saying that the light has been traveling 13 billion years but

in actual distance it would be around 29 billion ly away?

makes sense

:)

slang
2009-May-20, 08:06 AM
We really think we know it all, that is how we will fail as a species, our arrogence is what will kill us.

Can't have that! Cancel all science! Forget stuff, quick!

1010100 are you saying that the light has been traveling 13 billion years but

in actual distance it would be around 29 billion ly away?

Careful with the word 'actual' :) Yes, it is that actual distance, using that particular method of describing the distance.

robross
2009-May-20, 09:35 AM
does anyone know how long ago light would have left a galaxy 13 billion ly away?

(taking into account the expansion of space)

thanks

damian
:)

This is an old article but it looks interesting and discusses part of your question:

http://www.space.com/scienceastronomy/mystery_monday_040524.html

This is not exactly what you originally asked, but from the article

"the starting point of a photon reaching us today after travelling [sic] for 13.7 billion years is now 78 billion light-years away"

EDIT:

That 78 B-ly figure is actually the diameter, not radius, of the observable universe. So it's not 156 B-ly wide, it's 78 B-ly wide.

Rob

nokton
2009-May-20, 03:59 PM
This is an old article but it looks interesting and discusses part of your question:

http://www.space.com/scienceastronomy/mystery_monday_040524.html

This is not exactly what you originally asked, but from the article

"the starting point of a photon reaching us today after travelling [sic] for 13.7 billion years is now 78 billion light-years away"

Rob
Hi Robross, grasp you the concept. Time and gravity is the clue and the answer, can you not see it?
Nokton

damian1727
2009-May-22, 12:19 AM
:confused:

PraedSt
2009-May-22, 12:27 AM
damian1727, see if this helps. It's a reference (http://www.bautforum.com/astronomical-observing-equipment-accessories/88557-redshift-calculator.html) to something in WolframAlpha.

Which is here: wolframalpha.com

cjl
2009-May-22, 07:49 AM

so the universe was 700 000 years old at the time the light left the galaxy?
Seven hundred million, actually. You're off by 3 orders of magnitude :)

(A good thing too, because as far as I know, no current theories allow for the existence of galaxies just 700 thousand years after the big bang)

damian1727
2009-May-22, 08:00 AM
:p:pooops

AndrewJ
2009-May-23, 08:40 PM
Hi Robross, grasp you the concept. Time and gravity is the clue and the answer, can you not see it?
Nokton

:confused:

I agree.

winensky
2009-May-26, 10:35 AM
1010100 are you saying that the light has been traveling 13 billion years but

in actual distance it would be around 29 billion ly away?

makes sense

:)

There is a good physical discription of this.http://en.wikipedia.org/wiki/Expanding_universe

Go to "Understanding the Expansion of Space" and click on the isometric space-time embedding diagram, (the 'cone' shaped diagram). This demonstrates how light from an object 28BLY away can have reached us in 13 billion years. It is based on the currently predominant Lambda-CDM cosmological model.

nokton
2009-May-26, 04:18 PM
:confused:
Was trying to demonstrate that the position of the observer is crucial
to the observers measurment. One can only measure something from
within an established frame of reference.
Our established frame of reference is determined by our speed and local
gravity.
An intelligent lifeform on a planet with higher local gravity and a much greater
speed would have a frame of reference different from ours. To them, the age
of the universe would not coincide with ours because the passage of time to them
slower than we experience.
Nokton

damian1727
2009-May-26, 09:43 PM
thats another can of worms right there !

:)

damian1727
2009-May-26, 09:45 PM

nokton
2009-May-27, 05:25 PM
thats another can of worms right there !

:)
Sorry you see it that way damian, Ok tell me this, the present depiction
of a black hole is in two dimensions, like water draining down a plug hole.
Nothing further from the truth, understand it as it is, in 3 dimensions. If you
can, I can.
Nokton

speedfreek
2009-May-27, 07:24 PM
For a redshift of z=7.5

# The age at redshift z was 0.710 Gyr. (The universe was 710 million years old)

# The angular size distance DA is 3.4486 Gly. (That galaxy was 3.4 billion light-years away when it emitted the light we are now seeing)

# The light travel time was 12.955 Gyr. (The light took nearly 13 billion years to reach us)

# The comoving radial distance, which goes into Hubble's law, is 29.313 Gly. (That galaxy is now 29.3 billion light-years away)

source 1 (http://www.wolframalpha.com/input/?i=redshift+z%3D7.5) source 2 (http://www.astro.ucla.edu/~wright/CosmoCalc.html)

damian1727
2009-May-27, 09:19 PM
nokton you are a a complete knob :(

very condesending

not talking about the subject and misunderstanding peoples posts....is english your first language? i hope not for your sake

im sorry but i let it pass the 1st time

now bog off

Josh
2009-May-27, 11:05 PM
Hi damian,

I'm sure you're aware that calling people names and being generally rude doesn't cut the mustard here. If you don't like what someone says you can do one of a number of things: choose to ignore it, argue your point with them without calling them names, or report the post (using the red triangle on the top right of the post) that offends you and a moderator will decide whether any action is then appropriate.

These are your only options. If you take matters into your own hands, you will be suspended.

We now return you to normal programming ...

damian1727
2009-May-28, 12:02 PM
:whistle:

:shifty:

that better?

Swift
2009-May-28, 01:34 PM
:whistle:

:shifty:

that better?
No! The smileys indicate to me you are not taking this warning seriously. We also frown upon editing your posts well after the fact. Just admit you made a mistake and move on.

This is a final warning; the next step is a suspension.

damian1727
2009-May-28, 02:25 PM
sorry i thought u wanted me to edit it i'll but it back

im very sorry i called a nokton a nasty name

im also very sorry i tried to lighten the issue with smilies

im very very sorry i misunderstood you and removed the offensive word in question

sorry

?

nokton
2009-May-30, 03:28 PM
Hi Damian,
I do hope the moderaters here accept your contrition, and not bar
you from this forum. We all step over the line sometimes......
You have caused me no offence, but please, if you take issue with my concepts,
please indulge me by using reason and knowledge to argue your point with me.
Look forward to further dialogue with you,
Nokton

dirty_g
2009-May-30, 08:24 PM
That would only be true in a static, non-expanding universe.

Rob
Can I ask quite honestly why that is? Every school boy is taught that a star 20 light years away would therefore take 20 years travelling at the speed of light to reach. Why does this appear not to be true in a Galaxy 13 billion light years away (is it due to the fact that at that distance it would be moving away due to expansion)? Does it mean measurements of light years distance is unreliable at an intergalactic level and only reliable within the same galaxy? Please let me know Im quite interested now.

robross
2009-May-31, 01:33 AM
Can I ask quite honestly why that is? Every school boy is taught that a star 20 light years away would therefore take 20 years travelling at the speed of light to reach. Why does this appear not to be true in a Galaxy 13 billion light years away (is it due to the fact that at that distance it would be moving away due to expansion)? Does it mean measurements of light years distance is unreliable at an intergalactic level and only reliable within the same galaxy? Please let me know Im quite interested now.

I think if you read the entire thread again it should be clear.

In a static universe, a photon released from a star in a galaxy 2 billion light years away does indeed take 2 billion years to reach Earth, traveling at the speed of light.

In an expanding universe, as that photon travels through space, the space it is passing through is expanding, growing. Thus it actually must travel much farther than 2 billion light years before it reaches Earth.

Think about walking up a slope that's 100 feet long , and you travel at 10 feet per second. It would take you 10 seconds to walk up the slope, correct?

Now say the slope is really an escalator (or people mover, moving walkway, etc.) and you are going the wrong way on it, i.e., against the flow. And say the moving walkway is moving at 5 feet per second. Well, since you are moving 10 feet per second you overcome the opposing motion of the walkway, but now you find it takes you 20 seconds before you reach the end of the walkway, even though you've only traveled 100 feet at 10 feet/second.

If the walkway is moving very close to 10 feet per second, it could take you years before you reach the other end. And if it is moving at 10 feet per second exactly (or faster) you will *never* reach the end of the slope, because it's moving faster than you can travel through it.

That's sort of what is happening to the photon as it travels the vast distances between galaxies - the space it travels grows, though its speed is constant, so it takes longer to cover the original distance. I don't have the exact calculations handy but a photon released from a galaxy that was originally 3 billion light years away from Earth when the photon started traveling would take like 9-13 billion years to reach us now at earth, because of the expansion of space.

Rob

robross
2009-May-31, 01:35 AM
Can I ask quite honestly why that is? Every school boy is taught that a star 20 light years away would therefore take 20 years travelling at the speed of light to reach. Why does this appear not to be true in a Galaxy 13 billion light years away (is it due to the fact that at that distance it would be moving away due to expansion)? Does it mean measurements of light years distance is unreliable at an intergalactic level and only reliable within the same galaxy? Please let me know Im quite interested now.

Your original example was also valid, the light from a star 20 light years away does take 20 years to reach us. These distances are tiny in the cosmic sense, and a star 20 ly away is gravitationally bound to the same clump of matter as we are, so we are not expanding apart. Expansion becomes more dominant at very large distances between galactic super clusters.

Rob

AndrewJ
2009-May-31, 02:46 AM
Every school boy is taught that a star 20 light years away would therefore take 20 years travelling at the speed of light to reach.

Light from a receding galaxy that appears to us to have been 2 billion lya when the photon was emitted must have taken 2 billion years to get to us. Because of the expansion of space that galaxy must actually have been less than 2 billion lya from us 2 billion years ago and must currently be further than 2 billion lya from us.

robross
2009-May-31, 03:07 AM
Light from a receding galaxy that appears to us to have been 20 lya when the photon was emitted must have taken 20 years to get to us. Because of the expansion of space that galaxy must actually have been less than 20 lya from us 20 years ago and must currently be further than 20 lya from us.

Well, 20 light years is right next door in galactic terms, since our galaxy is 100,000 light years across itself. But your point is valid if we start talking about distances in the 2 Billion + light year range.

Rob

AndrewJ
2009-May-31, 03:20 AM
Rob - I missed out the billions, I'll edit my post to stick 'em in.

nokton
2009-May-31, 04:25 PM
Can I ask quite honestly why that is? Every school boy is taught that a star 20 light years away would therefore take 20 years travelling at the speed of light to reach. Why does this appear not to be true in a Galaxy 13 billion light years away (is it due to the fact that at that distance it would be moving away due to expansion)? Does it mean measurements of light years distance is unreliable at an intergalactic level and only reliable within the same galaxy? Please let me know Im quite interested now.
Dirty G, the light from a distant Galaxy has a red shift. That is how we measure its
distance away from us. Point is, ok if all things are equal, they are not. Space is
expanding, and with it, time, whether a star 20 light years distant, or a remote Galaxy.
Measurement of distance in such circumstances could be explained by math. The more
distant it is, the more the expansion of spacetime would distort the measurement.
The problem is, the cosmological constant, how fast is spacetime expanding?
Pleased you brought this concept up.
Nokton.

nokton
2009-Jun-01, 05:32 PM
Your original example was also valid, the light from a star 20 light years away does take 20 years to reach us. These distances are tiny in the cosmic sense, and a star 20 ly away is gravitationally bound to the same clump of matter as we are, so we are not expanding apart. Expansion becomes more dominant at very large distances between galactic super clusters.

Rob,
That the expansion of spacetime is greater in a much lower gravity field, as
between galaxies, than within a high mass environment such as a galaxy itself.
This would indicate that spacetime dialation is a variable depending on local gravity
conditions, therefore a cosmological constant cannot be a constant, and the expansion
of spacetime is not universally uniform, but determined more by the absence of mass
within space itself.
Nokton.

eburacum45
2009-Jun-02, 06:15 PM
I hope no-one missed speedfreek's excellent answer.
http://www.bautforum.com/1496112-post40.html
the distance to this galaxy is given in three very important and different ways.

Empyre
2009-Jun-03, 06:03 AM
,
That the expansion of spacetime is greater in a much lower gravity field, as
between galaxies, than within a high mass environment such as a galaxy itself.
This would indicate that spacetime dialation is a variable depending on local gravity
conditions, therefore a cosmological constant cannot be a constant, and the expansion
of spacetime is not universally uniform, but determined more by the absence of mass
within space itself.
Nokton.Unless I misunderstand it myself, a definite possibility, I think you are misunderstanding what robross is saying. Even where gravity is keeping matter bound together, like within a galaxy or even a galaxy super-cluster, space is still expanding at the same rate. The gravity is merely keeping the matter from being carried away by the expanding space.

Chip
2009-Jun-04, 02:52 AM
Keeping in mind that a framework of past to future is meaningless in a context where there is no time, if you could ride on the light from a galaxy 13 billion light-years away, the trip from there to here would be instantaneous.

Outside of that context, the effect of the gravity from other galaxies and dark matter and the expansion of the universe are complex factors as others have pointed out.