# Stars 13 billion light years away from 13 billion years ago

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Hi everyone. I'm not an expert, so forgive me for my ignorance.

My question is, if we can see a star that is 13 billion light years away, we see the light of this star as it was 13 billion years ago right?

So, if we see a star 13 billion light years away and 13 billion years ago, does it mean that this star was 13 billion light years away 13 billion years ago?

I don't know if I explained it right, and my english is not so good, but I hope somebody understands.

Thanks!!

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Hi everyone. I'm not an expert, so forgive me for my ignorance.

My question is, if we can see a star that is 13 billion light years away, we see the light of this star as it was 13 billion years ago right?

Correct.

So, if we see a star 13 billion light years away and 13 billion years ago, does it mean that this star was 13 billion light years away 13 billion years ago?

No, it was much closer because the universe is expanding. It would have been (I think) about 4 billion light years away. The light we see from those stars has taken 13 billion years to reach us because while it was travelling towards us, the universe was expanding so the light had an ever increasing distance to go. (A bit like walking the wrong way on one of those moving walkways - you will get to the ned but it will take you more time than if it were not moving.)

And that star is now about 45 billion light years away.

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• 2 weeks later...

Correct.

No, it was much closer because the universe is expanding. It would have been (I think) about 4 billion light years away. The light we see from those stars has taken 13 billion years to reach us because while it was travelling towards us, the universe was expanding so the light had an ever increasing distance to go. (A bit like walking the wrong way on one of those moving walkways - you will get to the ned but it will take you more time than if it were not moving.)

And that star is now about 45 billion light years away.

" ... the universe is expanding ..." Not possible to know that.

Is it not true that the only thing we can determine from the light from those distant stars we see today is what was happening when the light was emitted? We won't know how those stars were moving a million years earlier because their light has gone past us and we won't know how they were moving a million years later until a million years from now when the light reaches us.

We can not see what is, only what was. If those stars 13 billion years away stopped moving away from us 12 billion years ago we won't know about it for a billion years.

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" ... the universe is expanding ..." Not possible to know that.

It is known as well as any other scientific theory. It is the only model so far that is consistent with all the evidence.

We can not see what is, only what was. If those stars 13 billion years away stopped moving away from us 12 billion years ago we won't know about it for a billion years.

While it is true that in the period immediately before and after the time we see them, those stars might have danced a jig or turned into chocolate that sort of random speculation is not really helpful. We can only extrapolate from the evidence we have.

When that evidence is consistent with the predictions of theory, I'm not sure what the point is of inventing non-existent "what if" scenarios that have no basis in theory or evidence.

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It is known as well as any other scientific theory. It is the only model so far that is consistent with all the evidence.

While it is true that in the period immediately before and after the time we see them, those stars might have danced a jig or turned into chocolate that sort of random speculation is not really helpful. We can only extrapolate from the evidence we have.

When that evidence is consistent with the predictions of theory, I'm not sure what the point is of inventing non-existent "what if" scenarios that have no basis in theory or evidence.

The evidence we have is that the distance between us and those stars which we believe to have been the farthest away from us (13 billion years ago) was increasing and that the distance between us and those stars we believe to be closest to us (millions, not billions, ago) was decreasing.

So, which is the most recent evidence, light emitted by stars 13 billion years ago or light emitted from stars in the galaxies closest to us?

Are we to believe that the most current state of expansion or contraction of the universe is shown best by the most recent evidence or the oldest?

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Are we to believe that the most current state of expansion or contraction of the universe is shown best by the most recent evidence or the oldest?

Well, we can use all of that information. And that is how people came to the surprising conclusion that expansion appears to be accelerating. Apart from that, they all tell a consistent story.

(And, of course, the red-shift data is not the strongest evidence for the expanding universe. There was debate about alternative models for decades after that was first seen.)

Edited by Strange
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Well, we can use all of that information. And that is how people came to the surprising conclusion that expansion appears to be accelerating. Apart from that, they all tell a consistent story.

(And, of course, the red-shift data is not the strongest evidence for the expanding universe. There was debate about alternative models for decades after that was first seen.)

I'm more confused than ever. If expansion "is", not "was", accelerating it seems to me that what we should see is the stars closest to us moving away at a faster rate than those farthest away. How does what we see support this?

What observations do we have other than the red shift?

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I'm more confused than ever. If expansion "is", not "was", accelerating it seems to me that what we should see is the stars closest to us moving away at a faster rate than those farthest away. How does what we see support this?

Don't confuse the speed-distance relationship with acceleration. The acceleration appears when comparing the observed redshift-distance relationship with that predicted by constant expansion. (I am not intimately familiar with this research, so I will just point you to this as a starting point to read more: http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/press.html)

What observations do we have other than the red shift?

Well, it was the discovery of the CMB, with its precise match to theory, that really killed off other models. But there is also the observed proportions of hydrogen and helium, and the large scale structure of the universe.

http://www.universetoday.com/106498/what-is-the-evidence-for-the-big-bang/

http://www.astronomynotes.com/cosmolgy/s7.htm

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I guess my basic point is that I find it disingenuous for any observer to say that such-and-such "is" happening when the observations are based on light emitted millions and even billions of years ago. I we were to observe a man chopping wood on a planet a million light years away I would laugh at anyone who would say that the man is still chopping wood today. No, I can only accept that what we see way out there is what "was" happening.

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Correct, when we observe distant objects we observe them as they were in the past. This is built into the models and allows them to be tested at different times (which is how the surprising acceleration was found).

BTW There is another interpretation of your objection to "is" and that is one of definiteness. We use expressions like "the universe is expanding" or "there is a singularity at centre of a black hole" but this is (or should be) understood as meaning "according to current evidence and our best models it appears that ..." There is always uncvertainty and the possibility of being proven wrong in science.

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If I look out of a window and see a car crash in progress, I do not declare to a colleague. "Look, a car crash was in progress some nanoseconds ago, though I have no idea if it is still occurring, since I am basing my assessment on light that reached my eyes and was processed by my brain some time after the events that produced it." Instead I combine my general knowledge of the manner in which car crashes proceed and say "Look, a car is crashing."

Why don't you like that approach Fred?

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I guess my basic point is that I find it disingenuous for any observer to say that such-and-such "is" happening when the observations are based on light emitted millions and even billions of years ago. I we were to observe a man chopping wood on a planet a million light years away I would laugh at anyone who would say that the man is still chopping wood today. No, I can only accept that what we see way out there is what "was" happening.

You have half a point there, but in the motion of Galaxies there's going to be a fair bit of predictability.

What's more likely: all the Galaxies we see, since they emitted that light, were eaten by space Unicorns? Or, they continued "moving" in ways governed by things like Gravity and expansion that we've learned about?

We've seen the expansion and seen it accelerating, you can say "sure, but what's happening now" - but what's your alternative? Are you going to say everything has actually (just for example ...) stopped? Well - why? How? If you don't have an alternative, and just want to say "well, we don't know", that's fine. But also very pointless.

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I don't like quoting myself but I feel like I must say again: "The evidence we have is that the distance between us and those stars which we believe to have been the farthest away from us (13 billion years ago) was increasing and that the distance between us and those stars we believe to be closest to us (millions, not billions, ago) was decreasing." The most recent evidence we have seems to indicate that the expansion of the universe may have stopped and we may be in a contraction phase.

I am no expert in this area. All I know is what I've read. Can anyone explain why we should accept 13 billion year old data and ignore data that is only a few million years old?

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I don't like quoting myself but I feel like I must say again: "The evidence we have is that the distance between us and those stars which we believe to have been the farthest away from us (13 billion years ago) was increasing and that the distance between us and those stars we believe to be closest to us (millions, not billions, ago) was decreasing." The most recent evidence we have seems to indicate that the expansion of the universe may have stopped and we may be in a contraction phase.

Can you give a reference for this? It contradicts everything I have ever read.

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I don't like quoting myself but I feel like I must say again: "The evidence we have is that the distance between us and those stars which we believe to have been the farthest away from us (13 billion years ago) was increasing and that the distance between us and those stars we believe to be closest to us (millions, not billions, ago) was decreasing." The most recent evidence we have seems to indicate that the expansion of the universe may have stopped and we may be in a contraction phase.

I am no expert in this area. All I know is what I've read. Can anyone explain why we should accept 13 billion year old data and ignore data that is only a few million years old?

The closest stars are gravitational bound to us in the Virgo Supercluster (which includes us and Andromeda amongst others) - we are destined to stay together because gravity is strong enough between the components (because they are close enough) to overcome any background expansion. Background expansion and the accelerated nature of it that is more recently discovered happens on the Cluster/SuperCluster level - ie galaxies will never be ripped asunder as they are gravitationally bound; but on the largest cosmological scales the gaps between the great agglomerations of galaxies (and thus stars) are all increasing and increasing at an accelerating rate.

If you put 16 people in a rough circle and each of them walked off in the direction of a compass point (N NNE NE NEE E etc) the gaps between any two would increase - but the atoms making up the person would stay in the same relation to the body they comprise. all the stars we can see with the naked eye are in the Virgo Supercluster and will remain local - but all the stars outside our petty vicinity will eventually leave us forever.

Thus the physics theory predicts exactly what we see - until it stops doing that (or we come up with a viable alternative theory) we will work on the assumption that it is correct

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No, it was much closer because the universe is expanding. It would have been (I think) about 4 billion light years away. The light we see from those stars has taken 13 billion years to reach us because while it was travelling towards us,

Will time show new unknown stars for us always?

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The closest stars are gravitational bound to us in the Virgo Supercluster (which includes us and Andromeda amongst others) - we are destined to stay together because gravity is strong enough between the components (because they are close enough) to overcome any background expansion. ...

Ah, that. Of course. It amazes me that so many people insist a theory must be wrong when they have such a poor understanding of it.

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The light coming from star that are 13 billion light years away, was the light given off 13 billion years ago. That star is more than likely long gone by now. It may have gone nova and its parts made into another star, that we may not see for billions of years.

The observational question I have is, as we go further and further back into time, all the light from all the most distance objects should eventually be found in only one place, right, since at one time they were all close to each other by modern universe size standards; BB.

For example, in the solar system we can have planets all around the earth, If we were to fly out beyond the solar system, several light years away, all the planets will now appear to be touching due to the small angle on the horizon. Where is the oldest center; tiny angle.

Edited by puppypower
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The very earliest light we see is the cosmic microwave background. This is light that was release about 360,000 years after expansion started. But now it is stretched across the entire sky. So you aren't going to see everything squashed into a point. Because that point was the entire universe and is now stretched out across the whole universe.

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Bear in mind the BB isn't something that happened in one place in a pre-existing space, with everything then moving away from that one place.

The BB happened everywhere. At the end of your nose, and off the shoulder of Orion.

So while everything was once "closer", we're not going to see everything "nearing" one particular point distant from us.

(Edit: just to note, was typing this before the above reply, not meaning to disagree with (or even add to) Stranges' good reply.)

Edited by pzkpfw
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I will add another often overlooked piece of evidence for universe expansion that is often overlooked.

An expanding universe is one that is also cooling down. This follows the ideal gas laws. A contracting universe would heat up.

More often than not those new to cosmology are aware of the distance measurement aspects of expansion. Seldom are they aware that according to the ideal gas laws

pV=nRt An increase in volume results in a decrease in temperature when the number of particles remain roughly constant at $10^{90}$ particles.

We can see this via the temperature change in the measured blackbody change from the CMB to today. In point of detail the temperature varies inversely proportional to the scale factor "a" in the Friedman metric. Which correlates to the measured temperature.

Another method of measuring expansion rates which involves a specialized redshift measurement is the late time Sachs-Wolfe effect. There is two forms. The early non integrated Sache-Wolfe effect and the late time integrated (after CMB Sache-Wolfe effect.)

This helps us map changes in the average energy/mass density. (Which will effect temperature).

https://en.m.wikipedia.org/wiki/Sachs%E2%80%93Wolfe_effect

The Sachs-Wolfe effect is also a key aspect in mapping anistropies in energy/density. Ie overdense regions of the CMB.

Edited by Mordred
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...

An expanding universe is one that is also cooling down. This follows the ideal gas laws. A contracting universe would heat up.

...

Does this mean that space should be considered a gas? Gasses are particles; is space?

If space is/behaves like a gas, wouldn't we see objects moving faster with less "drag", and light moving slower, in less dense regions? The gas idea seems to give support for a wave model of light.

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Does this mean that space should be considered a gas? Gasses are particles; is space?

If space is/behaves like a gas, wouldn't we see objects moving faster with less "drag", and light moving slower, in less dense regions? The gas idea seems to give support for a wave model of light.

No, the cooling of the CMB refers to the photons in the universe, which behave like a gas. Sort of.

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Does this mean that space should be considered a gas? Gasses are particles; is space?

If space is/behaves like a gas, wouldn't we see objects moving faster with less "drag", and light moving slower, in less dense regions? The gas idea seems to give support for a wave model of light.

No not quite... you need to consider the volume of the entire universe to calculate the blackbody temperature of the universe, then calculate the number of fermions and leptons with their applicable degrees of freedom, chemical chain reactions and entropy.

Then using the correct formulas you can calculate the temperature of the universe per a given volume, or Even calculate the number of photons etc at a given temperature. However you must remember this is over the volume of the universe...

The average energy density of the universe will not be sufficient to slow light down as per a medium.

We can model the universe as an ideal gas, quite accurately but one must remember the scale of the volume involved.

These articles can provide greater detail

http://arxiv.org/pdf/hep-ph/0004188v1.pdf:"ASTROPHYSICS AND COSMOLOGY"- A compilation of cosmology by Juan Garcıa-Bellido

http://arxiv.org/abs/astro-ph/0409426An overview of Cosmology Julien Lesgourgues

http://arxiv.org/pdf/hep-th/0503203.pdf"Particle Physics and Inflationary Cosmology" by Andrei Linde

http://www.wiese.itp.unibe.ch/lectures/universe.pdf:"Particle Physics of the Early universe" by Uwe-Jens Wiese Thermodynamics, Big bang Nucleosynthesis

chapter 3 of the last article has a particularly good section on the subject.

Try this for thought, take everything in the universe today, now shrink it all down to a volume the size of a grapefruit... that grapefroot size universe is going to be extremely hot. Roughly 10^19 k hot...

Here is a chronology with temperature of the universe in case your math skills cannot handle the above articles..

https://en.m.wikipedia.org/wiki/Chronology_of_the_universe

Weinbergs "First three minutes" is a excellent read. (Though based on an older particle physics model) SO(5)

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