Universe expansion logic ?

  • Thread starter hitchiker
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by measuring redshift physicist conclude
1.most of the galaxies are moving away from our galaxy
2.universe is expanding

1.most of the galaxies are moving away from our galaxy

if we ride alongside a train on a motorbike with relatively same speed , do we don't hear the doppler effect . it is effective if one is static
like wise our galaxy must be moving alongside or in line with other galaxies so shouldn't there be blueshift on the direction of our galaxy heading and redshift on opposite side, but we see redshift every where :confused:

2.universe is expanding

how physicists concluded universe itself is expanding by judging the outward movement of galaxies :confused: universe is infinite ,isn't it? so how does universe expand .if space itself is expanding from a concentrated state stretching stuff within it, then one has to admit space is finite ,if space is infinite it must not be expanding.

if galaxies are moving away from each other ,well they are moving relative to each other but how does it prove space/universe is expanding
 
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  • #2
Drakkith
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if we ride alongside a train on a motorbike with relatively same speed , do we don't hear the doppler effect . it is effective if one is static
like wise our galaxy must be moving alongside or in line with other galaxies so shouldn't there be blueshift on the direction of our galaxy heading and redshift on opposite side, but we see redshift every where :confused:
There is, but only with galaxies close enough to us that the expansion of the universe is not the dominant source of redshift. For example, the Andromeda galaxy is highly blueshifted (for a galaxy) and is currently moving towards us. However, the overwhelming majority of galaxies are too far away to be bound to us and as a result the expansion is carrying them away and causing them to be redshifted. This expansion happens in all directions and affects everything. Everything in the universe that is not bound by gravity or another fundamental force is moving away from everything else.

2.universe is expanding

how physicists concluded universe itself is expanding by judging the outward movement of galaxies :confused: universe is infinite ,isn't it? so how does universe expand .if space itself is expanding from a concentrated state stretching stuff within it, then one has to admit space is finite ,if space is infinite it must not be expanding.

if galaxies are moving away from each other ,well they are moving relative to each other but how does it prove space/universe is expanding
A couple of things here. First and foremost, when we say that the universe is expanding, what we mean exactly is that objects within the universe are moving away from each other. That's it.

Second, even an infinite universe can expand. This may seem silly and counter-intuitive, but it is the truth. You are imagining the universe as a giant bubble or something related and that the universe itself is expanding into pre-existing space. This is not what we mean by expansion, nor is it required that something be outside the universe for it to expand into.

So, everything we see that is not bound to us is moving away from us due to expansion. Whether or not the universe is finite or infinite us currently unknown, yet either can be true and still be compatible with current models of cosmology.
 
  • #3
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[



.... ,if space is infinite it must not be expanding.
Why do you say that if space is infinite it must not be expanding?
 
  • #4
Chronos
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1. All of the galaxies in the local group are moving in the same general direction at a speed of about 600 km/s. There are additional peculiar motions of the individual galaxies, but, of lesser magnitude. Only the Andromeda galaxy is actually approaching the Milky Way - at the blistering speed of about 60 km/s. The are a few dwarf galaxies that are also blue shifted, but, are orbiting other larger galaxies so their blue shift is a temporary effect due to their orbital direction relative to the Milky Way.

The Hubble constant is about 70 km/s/Megaparsec. Assuming the velocity of the local group relative to the CMB [~600 km/s] is typical, it appears unlikely any galaxy more than about 9 Megaparsecs [~30 million light years] distant will ever be blueshifted.

2. The observable universe is definitely finite and has steadily increased in size for the last 13.7 billion years. Since, by definition, the observable universe is limited by the finite speed of light - it will forever be finite.
 
  • #5
jim hardy
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Is the pioneer anomaly in proportion to hubble contant dividec by c ?

methinks there's something yet to be figured out about this apparent expansion, redshift, and all that...
 
  • #6
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I see absolutely no reason why the unobservable universe cannot be finite. Unobservability certainly doesn't preclude finiteness. It merely means that it is beyond our observation. For all we know the unobservable universe could be an exact replica of the observable one including the observable one's finiteness albeit larger. The real difference is that the unobservable universe's finitness might very well be a real one instead of one based on our inability to see beyond a certain point. Actually, saying that the visible universe is finite based on that reason is tantamount reaching conclusions about a land area' size because we can't see beyond a certain mountain range.
 
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  • #7
Chronos
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Is the pioneer anomaly in proportion to hubble contant dividec by c ?

methinks there's something yet to be figured out about this apparent expansion, redshift, and all that...
Not even close. The hubble constant is only 70 m/s per Megaparsec. Our best guess for now is the pioneer anomaly is merely a thermal effect.
 
  • #8
Chronos
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Radrook, keep in mind the unobservable universe is causally disconnected from the observable universe.
 
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Radrook, keep in mind the unobservable universe is causally disconnected from the observable universe.

Interesting! Can you elaborate a little on that.
 
  • #10
Chronos
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You can't observe anything older than the observable universe.
 
  • #11
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Why do you say that if space is infinite it must not be expanding?
infinity is not an ongoing process, it is a fundamentality
 
  • #12
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You can't observe anything older than the observable universe.

Radrook, keep in mind the unobservable universe is causally disconnected from the observable universe.

Causality
Causality (also referred to as causation[1]) is the relationship between an event (the cause) and a second event (the effect), where the second event is understood as a consequence of the first.[2]

http://en.wikipedia.org/wiki/Causality
So how would observation be a causation?
 
  • #13
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infinity is not an ongoing process, it is a fundamentality
I agree 100%! If it can be increased then it wasn't infinite to begin with. The infinitely large cannot be made larger because there is nothing larger than the infinitely large. The same holds true for the infinitely small. If indeed it can be smaller larger then it had spacial or numerical limits. Limits and infinity are mutually exclusive.
 
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  • #14
Chronos
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Infinity is a logical construct and yields logical contradiction when treated as if it possesses any physicality. Is the set of all integers more 'infinite' than the set of all even numbered integers? There are any number [an infinite number, to be precise] of calculus problems that use infinity as a limit, yet, yield a finite solution.
 
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So how would observation be a causation?
If light cannot reach us from the unobservable universe, then nothing can. Therefore, nothing we ever do can have any effect on the unobservable universe, and vice versa.
 
  • #16
bapowell
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The infinitely large cannot be made larger because there is nothing larger than the infinitely large.
Are you familiar with Cantor's work on the cardinality of infinite sets? It is possible to construct infinite sets of different size.
 
  • #17
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If light cannot reach us from the unobservable universe, then nothing can. Therefore, nothing we ever do can have any effect on the unobservable universe, and vice versa.
It doesn’t have to for both universes to be causally connected. As the definition of causation shows, causal connection simply means that one universe would need to affect the other in a cause and effect manner. Does the visible universe do anything that affects the invisible one in a cause-effect manner? well, please note that it feeds matter to the other making it larger in the process. Also note that such matter can be assumed to interact causally with whatever it encounters on the other side of the boundary that separates one from the other from a human perspective.

BTW

I agree that humans are causally disconnected from that universe. But that doesn't justify a blanket statement that would imply that both universes are causally disconnected.
 
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  • #18
Drakkith
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Does the visible universe do anything that affects the invisible one in a cause-effect manner? well, please note that it feeds matter to the other making it larger in the process. Also note that such matter can be assumed to interact causally with whatever it encounters on the other side of the boundary that separates one from the other from a human perspective.
I don't believe matter is being fed from the observable universe to the unobservable universe.

BTW

I agree that humans are causally disconnected from that universe. But that doesn't justify a blanket statement that would imply that both universes are causally disconnected.
The border isn't a brick wall, its a barrier in time. Anything beyond that barrier CANNOT affect us yet. Objects just on the other side can and have been affecting objects on our side, we just haven't been able to observe this effect yet. Therefor we are not causally connected to it yet.
 
  • #19
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I don't believe matter is being fed from the observable universe to the unobservable universe.
Where is it all going from your standpoint then-tra la la land?


The border isn't a brick wall, its a barrier in time. Anything beyond that barrier CANNOT affect us yet. Objects just on the other side can and have been affecting objects on our side, we just haven't been able to observe this effect yet. Therefor we are not causally connected to it yet.


I never said that we are causally connected as human beings to anything that happens beyond that horizon. I never said that the horizon blocks all influences between these regions. Your misunderstanding is that you are focusing on people in relation to that horizon and I am talking about the two distinct regions themselves in relation to one another and people are totally irrelevant from the point of causality between these two areas.
 
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  • #20
bapowell
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1.Your last statement is self-contradictry and and places in serious doubt the reasoning ability of anyone who would express it. Are you aware of that or are you just jesting?

2. I never said that WE are causally connected as human beings to anything that happens beyond that horizon. Actually, and on second thought, after reviewing your blatantly self- contradictry statements , I decided that as far as I am concerned this discussion between us is over. Maybe when the arguments you present begin making at least a little bit of sense to me we might resume. Until then, no dice! This type of communication is extremely disagreeable. Yikes!
Your statements are confrontational and rude, Radrook. Drakkith is merely trying to engage in constructive discourse (the brick wall reference I'm sure was meant facetiously, as I know Drakkith to be a thoughtful commenter.) I'll advise you to read the PF rules on this kind of attitude -- this is a place to learn, not pick fights.
 
  • #21
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Radrook, keep in mind the unobservable universe is causally disconnected from the observable universe.
If we look back to z=6, we see galaxies as they were when the universe was about a billion years old. Those same galaxies can be seen from the other side at the same redshift by astronomers in galaxies far beyond our horizon. Thus can it not be said that parts of what we call the "observable universe" are causally connected to events we call "unobservable".
 
  • #22
phinds
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If we look back to z=6, we see galaxies as they were when the universe was about a billion years old. Those same galaxies can be seen from the other side at the same redshift by astronomers in galaxies far beyond our horizon. Thus can it not be said that parts of what we call the "observable universe" are causally connected to events we call "unobservable".
Yes, it can. And it IS. Correctly.

Amounts of the universe beyond our current horizon WERE causally connected in the early universe (read about the CMB uniformity) but they are NOT at present.
 
  • #23
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Those same galaxies can be seen from the other side at the same redshift by astronomers in galaxies far beyond our horizon
redshift varies with distance, if you mean astronomers in galaxies that lies in our 'forward horizon' redshift would be much greater

it is like driving a car on highway.. horizon in front of you will become connected and rear horizon shrinks

we may never see more past as we can see now ,as everything is moving away from its origin forward
 
  • #24
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redshift varies with distance, if you mean astronomers in galaxies that lies in our 'forward horizon' redshift would be much greater
I am talking of astronomers in a galaxy who observe the universe to be 13.7 billion years old as we do and at a co-moving distance of 57.5 billion light years from us. Call it "C".

The galaxy in the young universe, seen at a redshift of 7 by both the distant galaxy and us, is midway between C and us, call it "B". The causal connection from B to C is similar to that from B to us even though we consider B is observable but C is not.
 
  • #25
phinds
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I am talking of astronomers in a galaxy who observe the universe to be 13.7 billion years old as we do and at a co-moving distance of 57.5 billion light years from us. Call it "C".

The galaxy in the young universe, seen at a redshift of 7 by both the distant galaxy and us, is midway between C and us, call it "B". The causal connection from B to C is similar to that from B to us even though we consider B is observable but C is not.
Yes, that's correct. Are you asking if the statement quoted here makes sense, or what?

EDIT: This response is based on "half-way". I did not consider the red-shift and I assume that hitchiker's comment below on the red shift is correct.
 
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