# Is the universe still expanding?

#### Simetra7

When the HST looks out into the far off regions of space, the data it recieves (the light from distant galaxies etc.), has been travelling for millions of light years. When this data shows red shift, and therefore that the source of the light is moving away from us, does this mean that at the time that light was emitted (millions of light years ago), its source was travelling at the velocity indicated by the red shift, or does the red shift indicate the velocity of the source of the light at the present time? If we can only measure the velocity from the time the light was emitted, then how do we know whether the source is still moving away from us or not?

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#### Simetra7

I get the basic gist of the doppler effect, but what i want to know is, does the motion of the light source affect the wavelength of the beam of light along it's whole length, considering that we are talking about a beam of light that extends through millions of light years of space. As an example, if the source of light suddenly disappeared, it would be millions of years before we would know that it had disappeared. Would we still see the red shift and assume that the source of light was speeding away from us?

#### Simetra7

Ahh!! Now it makes more sense, Thanks. When you understand that it's the space between us and those distant objects that is expanding rather than just the objects themselves moving away from us it's easier to see why the whole length of the light beam is stretched. I think i will learn a lot from these forums.

#### Simetra7

I have been thinking about this subject some more, and referring back to my second post, if the source of light did disappear, (explode or collide with another star etc.) what would happen to the light that we observe from that source?

#### marcus

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Simetra7 said:
I have been thinking about this subject some more, and referring back to my second post, if the source of light did disappear, (explode or collide with another star etc.) what would happen to the light that we observe from that source?
what was already on its way would keep coming as if nothing had happened.
we would only learn of the disappearance, or explosion, or collision, later when the light from THAT event had a chance to reach us

#### Simetra7

marcus said:
what was already on its way would keep coming as if nothing had happened.
we would only learn of the disappearance, or explosion, or collision, later when the light from THAT event had a chance to reach us

So we would still assume that that object was there, and that it was speeding away from us at the velocity indicated by the redshift?
Is most of what is known about the universe based on similar assumptions.
What i mean is could much of what we have observed out there be long gone, or are there other ways of telling whether these objects are still there?

#### marcus

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Simetra7 said:
So we would still assume that that object was there, and that it was speeding away from us at the velocity indicated by the redshift?

... other ways of telling whether these objects are still there?
who is WE?

astronomer/cosmologists would not need to assume that
their models only require you to assume what is consistent with the laws of physics

(what is required by physical law is already a bit fuzzy, there is broad agreement on a lot but some disagreement and some gradual change in the consensus view)

If a star explodes, and a certain amount of energy escapes as light, neutrinos, etc. then (conservation of mass-energy law) whatever doesnt escape is going to still be there as material remnants, a cloud of crud, a neutron-star cinder etc.

the models cosmologists use DONT DEPEND ON THE MATTER BEING IN PARTICULAR FORMS , they are mathematical models that dont need to assume that a particular object remains that object----the atoms and particles are still there regardless of what arrangement they happen to be in.

==================

so I guess the answer to your question is NO we dont need to assume details that we dont actually know. The models are mathematical and not verbal. You just plug in what you have observed---from the light that already reached us----and you assume some math equations called physical laws----and you draw conclusions from that.

the conclusions are of a limited qualified nature. you cant deduce with certainty things you havent seen yet, only make predictions about what will be observed over there in the future "we saw it explode so that glowing expanding cloud of crud that we see now, if nothing has interfered with it, is probably still out there glowing and expanding according to the laws, and we can predict that we will still see it tomorrow"

then if ever anything happens that goes against prediction, everybody gets very excited because it is a chance to revise the laws!
Physical laws are used to make predictions until they are proven wrong (or of limited applicability) by making a wrong prediction and can be replaced or corrected by improved laws.

Fortunately or unfortunately, the laws only change very slowly. newton's laws are still more or less true (only slightly modified by einstein) and keep on being used, and keep on working. one just has to use a little common sense about not applying them to some extreme situations where they need relativistic correction.

that is good in the sense that a great deal of stuff is routinely predictable

but it is less fun for the theorists because they get their kicks when experiment and observation proves that some law is wrong and then the theorists get to look for how to tweak it so it will fit the data better, or replace it by some more clever law.

I am not all that knowledgeable about this -----basically your question goes into Philosophy of Science, or Foundations of Science, issues. So maybe somebody else here will answer in some more satisfactory way. this is the best I can do at the moment anyway.

========SUMMARY OF RESPONSE=====

NO, if "we" means scientists, then we do not claim to know about future observations, we know only what we have observed.

we make a lawful model that fits the past observations

and then we use that model to PREDICT about future observations

and that then serves to TEST the model and the laws that went into it

and if future observation goes against the predictions then everybody has fun because they get to refine the model and maybe even modify the laws.
but that doesnt happen very much because the laws have gotten pretty good and keep predicting right.

#### marcus

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Simetra7 said:
... speeding away from us at the velocity indicated by the redshift?
...
that "speeding away" is not such a good verbal image
did you read the Sci Am article by Chuck Lineweaver and Tamara Davis about expanding universe? it clears up a lot of common confusion
we have some links to that around here somewhere.

how do you imagine the recession velocity is "indicated" by the redshift?

if you think you can calculate it as a doppler effect, with the doppler shift formula, then you have misunderstood expansion

if you want to read that recent Sci Am article, ask and someone will get links----or you can probably google with Lineweaver Davis and get it

#### Simetra7

marcus said:
who is WE?

astronomer/cosmologists would not need to assume that
their models only require you to assume what is consistent with the laws of physics

Sorry! "We" was probably a poor choice of words. I meant the experts in the field. My undertsanding of cosmology is limited but i am fascinated by what i have read so far and am keen to learn more. I would be very interested in reading the article that you spoke of. Thanks.

#### marcus

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Simetra7 said:
marcus said:
who is WE?

astronomer/cosmologists would not need to assume that
their models only require you to assume what is consistent with the laws of physics

Sorry! "We" was probably a poor choice of words. I meant the experts in the field. My undertsanding of cosmology is limited but i am fascinated by what i have read so far and am keen to learn more. I would be very interested in reading the article that you spoke of. Thanks.
Great, also Yaaks recommended some Wikipedia articles which could be helpful too.
As it happens Charles Lineweaver and Tamara Davis are two of my favorite cosmologists, I am glad to dig up the links to their Sci Am article.

It is one main article, on maybe 6 pages plus some SIDEBARS. I will get the links to the sidebars because they address people's common misunderstandings and i think they are very good

#### marcus

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Simetra, see if these work!
MAIN Lineweaver and Davis article in March 2005 SciAm
http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147

SIDEBARS with pictorial diagrams and a question together with right and wrong answers explained.

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p39.gif
What kind of explosion was the big bang?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p40.gif
Can galaxies recede faster than light?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p42.gif
Can we see galaxies receding faster than light?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p43.gif
Why is there a cosmic redshift?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p44.gif
How large is the observable universe?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p45.gif
Do objects inside the universe expand, too?

If these links dont work because they have gotten old, please let me know.

#### johninf

Life becomes much simpler if you assume Hubble was wrong.
That the universe is not expanding!
That the red shift we see is partly due to the earths rotation on its axis,
round the sun, orbits the centre of the milky way.
We are always moving away from a lot of the sky.
But REd shift is also caused by light passing through gravity fields, light slows down.
Shapiro effect . Compton effect , Einstiens weak light.
The universe is infinite and not expanding!

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#### SpaceTiger

Staff Emeritus
Gold Member
johninf said:
Life becomes much simpler if you assume Hubble was wrong.
Indeed, ignorance is bliss.

#### johninf

The majority of people in the world now believe the big bang is a fact,
they are ignorant.and do not look at evidence to the contrary.
They fell safer in a finite universe with a beginning .
Infinity frightens them.
Nevertheless the universe is infinite.
nasa is coming to the same conclusion by lokking at the geometry of the observable Universe.

#### SpaceTiger

Staff Emeritus
Gold Member
johninf said:
The majority of people in the world now believe the big bang is a fact,
they are ignorant.and do not look at evidence to the contrary.
Please, share with us this evidence you're so knowledgable of.

nasa is coming to the same conclusion by lokking at the geometry of the observable Universe.
NASA has not found anything inconsistent with the Big Bang. The observable universe is flat, exactly as we've expected for over 20 years.

#### Simetra7

SpaceTiger said:
Please, share with us this evidence you're so knowledgable of.

Yes, please do....I'm all ears (eyes).

#### Chronos

Gold Member
Evidence favoring Big Bang theory: The cosmological microwave background radiation, primordial elemental abundance, WMAP power spectrum. large scale structure of the universe... Big Bang theory does not require a finite, or infinite, universe.

#### johninf

If scientists are so blissfully happy, why are they so paranoid
when the big bang THEORY is critisized?
In reality they are afraid of infinity because it cannot be described mathematically.
The present math is inaccurate and innappropriate to infinity.
They are also afraid of the idea of no beggining!

#### johninf

"The true size of the universe is probably much larger than the visible universe. The geometry of the universe suggests that it may have an infinite size and that it will expand forever. Even if the universe is not infinite, our visible universe must be a minute speck in a much larger totality."
NASA
If they concede that the univerese may be infinite NOW, then its a simple step in deduction to say that it always was.and always will be.

#### johninf

is not supporting evidence of the big bang.
Background radiation is conatantly caused by an infinite number of supernova.
Its the ambient temperature of the universe.

#### marcus

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Simetra7 said:
... "We" was probably a poor choice of words. I meant the experts in the field. My understanding of cosmology is limited but i am fascinated by what i have read so far and am keen to learn more. I would be very interested in reading the article that you spoke of. Thanks.
"We" is a fine choice---meaning mainstream working astronomers and cosmologists. Let's include ourselves, as long as we are trying to understand the mainstream consensus of experts. I regret having sounded persnickety at that point.

Did you have a look at the Sci Am article---Lineweaver and Davis? If it was not right for you let us know and I or someone can look for other stuff about the current picture in cosmology.

right now in this thread we are getting some trouble because of a very common misconception----a lot of people don't understand the big bang set of models and think that any big bang picture has to be spatially finite. that just isnt so. From the very start----going back I think to the NINETEEN TWENTIES----people were proposing and studying expanding universe models which were spatially infinite and which started off with a singular hypersurface of infinite spatial extent

the singularity in the model has always been considered possible to be either infinite in extent or finite.

Nick (Tiger) who is a gradstudent going on to be a professional can correct me if I am wrong but this is pretty standard.

the popular misconception is that expansion has to start from a point, which it doesnt, mathematically an infinite thing can start expanding----so this is a "FAQ" type of thing we encounter over and over again at messageboards like PF. I think it comes from several reasons

1. the famous physicist John Wheeler CALLED it onset of expansion by the NAME of "big bang". He loves colorful terminology and is very imaginative. I think he also invented the term "black hole".
After a good name crops up it will often take off on its own and develop its own signif. in the public imagination.

2. people find it hard to imagine a "bang" that happens in an infinite region, they picture some comicbook explosion all emanating from one point. It is a verbal thing where the original mathematical idea gets lost and a verbal preconception dominates.

3. another verbal trouble comes from the word "singularity". In mathematics a singularity can occur at an infinite set of points. It means a PLACE WHERE THE MODEL BREAKS DOWN and fails to compute, often because it starts giving infinite or otherwise unreasonable answers, or no answers.

the classical 1915 Einstein model fails to compute right at the beginning of expansion, so that is called a singularity. It also fails to compute right around the center of a black hole.

but singularity does not mean "single point". Nobody who knows what they are talking about ever said that universe expansion has to begin at a single point, or even at a finite set of points. IT COULD but that hasnt been settled yet. It could also very well begin at an infinite set.

even a black hole singularity does not have to be a single point, in some models it is a ring (I find this hard to picture but mention it only because it is just a common truism that singularties do not in general consist of single points. It is only the WORD that gives people that idea.)

instead of meaning "single point" a singularity is an "oddity" or "peculiarity" that happens when you push a model to the limit of where it applies and it breaks down.

in some proposed new cosmologies (e.g. Loop Quantum Cosmology) the big bang singularity is smoothed out and spacetime evolution extends back to an earlier contracting phase. Eventually something like this will probably become mainstream----some improved theory which does NOT fail at the beginning of expansion will check out and be accepted by cosmologists and will replace the classical 1915 Einstein based cosmology.
then the singularity (or failure) in the classical picture will be FIXED. this has happened in other situations---improved theories (like of the atom) eliminate singularities in older classical theories.

================
so don't be misled by the words "bang" and "singularity" into thinking that the mathematical model necessarily begins at a point or has to be spatially finite.

this is actually one of the most interesting questions in big bang cosmology!

there is a number Omega, which can be MEASURED, and for which the measured value is around 1.01 or 1.005 plus/minus something.

and the uncertainty is such that it could easily be exactly 1.000, in which case the universe might actually be of infinite spatial extent! and on a very large scale UNCURVED.

but if this number is slightly more than 1, say it is 1.005, then the universe can be very huge (much bigger than what has been observed so far) but still finite.

and we really dont know, this is one of the big questions in big bang cosmology, and why they sent up satellite observatories like COBE and WMAP----to measure Omega more precisely, to shrink down the errorbounds.

a lot of mainstream cosmologists seem to think Omega is exactly one (infinite bigbang, infinite universe) I guess just because the measurement gives an answer that is so close--maybe they think "it is so close to one, why shouldnt it be exactly one?" But in all honesty the issue is still undecided and there is still an errorbar and more accurate measurements need to be made.

Hope this is helpful, and not too long winded.

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#### Simetra7

marcus said:
Did you have a look at the Sci Am article---Lineweaver and Davis? If it was not right for you let us know and I or someone can look for other stuff about the current picture in cosmology.

Yes I read the article and looked at the other links too. It shed some light on what is definitely a hard concept to get your head around. Any other stuff you could dig up would be much appreciated. Thanks.

#### marcus

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Simetra7 said:
Yes I read the article and looked at the other links too. It shed some light on what is definitely a hard concept to get your head around. Any other stuff you could dig up would be much appreciated. Thanks.
this is normally SpaceTiger's beat. or Chronos. keep asking specific questions and see if you can get some more suggested reading. I am going to retire to my usual haunts and not take up Astronomy room for now.

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