Expanding universe?

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  • #1
aib
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This has been puzzling my mind for a long time. How do we really know the universe is expanding? Based on observation? Redshifts?

Observation based conclusions cannot be made IMO. We have observed the universe for a few hundred years and that is way too little to use to predict 13+ billion years.

Redshifts are based on certain assumptions, IIRC even Hubble himself has pretty serious doubts. Furthermore, gravitationally bound galaxies *thus in proximity to each other* have demonstrated totally different redshift values, way out of what the values for galaxies that are supposed to be almost the same distance away should be.

Everything in the universe seem to work in cycle manners, then why would the universe expand instead of for example, pulse in and out, and it just happens so that we live in a moment it is pulsing out?

Also, if it is expanding, shouldn't we observe distant galaxies popping OUT of existence?
 

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  • #2
sylas
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This has been puzzling my mind for a long time. How do we really know the universe is expanding? Based on observation? Redshifts?
Redshifts, background radiation, angular size and luminosity relations, supernovae light curves, nucleosynthesis predictions, and the theoretical predictions from the extremely well tested theory of general relativity.

Observation based conclusions cannot be made IMO. We have observed the universe for a few hundred years and that is way too little to use to predict 13+ billion years.
Would you apply this curious argument to the age of the Earth, at 4.56 billion years? Or even identifying the ages of dinosaurs, between 220 and 65 million years?

A well tested and repeated set of observations over a space of a few years can overturn conclusions from a millenia of less accurate observations, and reveal new information which lets us discover things never known before to anyone. I find that exciting.

Redshifts are based on certain assumptions, IIRC even Hubble himself has pretty serious doubts. Furthermore, gravitationally bound galaxies *thus in proximity to each other* have demonstrated totally different redshift values, way out of what the values for galaxies that are supposed to be almost the same distance away should be.
Redshifts are not in doubt. Various idea for how they arise have been proposed over the years, and Hubble himself was dubious of the leading and most obvious explanation. He had good reason, in fact. His original observations inferred a relation between redshift and distance which was incorrect by a factor of about 8, because he used an incorrect "distance ladder" based on the Cephid variable stars. The relation he obtained, of about 500 km/s/Mparsec, would have made the universe only about 2 billion years old.

Already by then, scientists knew with good confidence that the Earth was older than this, and that presented a problem. It was resolved with the discovery of two classes of Cephid variable stars, and that altered the distance ladder substantially, to give expansion rates more in line with what we have measured now.

In the meantime, continued study continued to refute all the various alternative explanations for how the redshift arises, leaving the obvious (if initially disconcerting) conclusion... the universe is expanding. That's now confirmed well beyond any reasonable doubt as scientists continue to observe and measure and test more and more thoroughly.

Everything in the universe seem to work in cycle manners, then why would the universe expand instead of for example, pulse in and out, and it just happens so that we live in a moment it is pulsing out?
Not everything in the universe works in cycles. There are all kinds of things that are not cyclic at all. The dinosaurs, for example, are extinct... they were here a good long time, and now they are gone.

Better to go where the evidence indicates, thant base conclusions on philosophical preferences for cycles.

Also, if it is expanding, shouldn't we observe distant galaxies popping OUT of existence?
No.

Cheers -- sylas
 
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  • #3
Wallace
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This has been puzzling my mind for a long time. How do we really know the universe is expanding? Based on observation? Redshifts?
There is a lot more to it than simply redshifts. This was the original and most simple peice of evidence, but everything else points to it as well. We clear see evidence that the Universe has evolved over its history by for instance seeing how the structures in the early Universe (at high redshift) differ in nature, abundance and distribution compared to those at lower redshifts.

Observation based conclusions cannot be made IMO. We have observed the universe for a few hundred years and that is way too little to use to predict 13+ billion years.
In fact the time of over which we observe is not relevant, because we can observe the Universe from today back to around 13 Billion years ago, due to the finite speed of light. It sounds like you are making the assumption that we 'directly' observe the expansion, like you wuld observe a car moving away by getting progressively smaller or dimmer in your view. This is not the case (we'd like to make such observations, but they are not technically feasible at this point). In fact the redshift gives you an instantaneous measure of the 'speed' (speaking very loosely) at which some galaxy is receeding from us, we don't need to make repeated measurements over time. It is by looking at galaxies at different redshifts that we can learn how the Universe evolved through time.


Redshifts are based on certain assumptions, IIRC even Hubble himself has pretty serious doubts. Furthermore, gravitationally bound galaxies *thus in proximity to each other* have demonstrated totally different redshift values, way out of what the values for galaxies that are supposed to be almost the same distance away should be.
This is not true. There have been claims that interacting galaxies have differing redshifts, but in every case under further examination (better observations etc) they have been found to be simply chance alignments of galaxies that are at different distances. The frequencies of such chance alignments is consistent with expectations from a random distribution of galaxies in the forground and background.

Everything in the universe seem to work in cycle manners, then why would the universe expand instead of for example, pulse in and out, and it just happens so that we live in a moment it is pulsing out?
I'm not sure what you mean by everything working in cycles? It's possible that the Universe does have a cyclic nature in terms of expansions and contractions, but at present the only thing we can say based on observations is that it is expanding. Certainly the possibility of cyclic models is often discussed, and some proposals are promosing in terms of explaining some things we don't understand well about the early universe, but we can't say with any confidence at this point that the Universe must be cyclic; we simply don't know for sure.

Also, if it is expanding, shouldn't we observe distant galaxies popping OUT of existence?
In fact in an expanding universe in general you would not see galaxies pop out of existance, however in an accelerating universe (such as we think we are in) you would see this. However it occurs over very very long times scales compared to Human lives. Again, we don't actually see individual objects actually moving and changing, we simply live too short to have a reasonable hope of doing so. The exception is the motions of stars in our own galaxy, but that doesn't have a bearing on broader cosmology.
 
  • #4
Chronos
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Nothing ever 'pops' out of view in our universe. Receeding galaxies will merely redshift into obscurity no matter how fast they flee us.
 
  • #5
Chalnoth
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A well tested and repeated set of observations over a space of a few years can overturn conclusions from a millenia of less accurate observations, and reveal new information which lets us discover things never known before to anyone. I find that exciting.
Just to add a little bit to this, the current rate of increase of gathering astronomical data is about a doubling of the data every year. That means that each year, we obtain as much new data about our universe as was collected in the entire previous span of human history.
 
  • #6
Redshifts are not in doubt. Various idea for how they arise have been proposed over the years, and Hubble himself was dubious of the leading and most obvious explanation. He had good reason, in fact. His original observations inferred a relation between redshift and distance which was incorrect by a factor of about 8, because he used an incorrect "distance ladder" based on the Cephid variable stars. The relation he obtained, of about 500 km/s/Mparsec, would have made the universe only about 2 billion years old.

Already by then, scientists knew with good confidence that the Earth was older than this, and that presented a problem. It was resolved with the discovery of two classes of Cephid variable stars, and that altered the distance ladder substantially, to give expansion rates more in line with what we have measured now.

In the meantime, continued study continued to refute all the various alternative explanations for how the redshift arises, leaving the obvious (if initially disconcerting) conclusion... the universe is expanding. That's now confirmed well beyond any reasonable doubt as scientists continue to observe and measure and test more and more thoroughly.
Cheers -- sylas
Has gravitational redshift been suggested and how does the equivalence principle relate to the acceleration of the universe?
 
  • #7
Chronos
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Gravitational redshift is an inadequate explanation. By the time gravitational redshift becomes a factor, a huge amount of photonic energy has already been expended by infalling matter. It is there, but the light already emitted renders it unobservable.
 
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