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Author Topic: How do we "know" that the universe is expanding?  (Read 744 times)

Offline PmbPhy

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How do we "know" that the universe is expanding?
« on: 28/05/2016 04:51:07 »
This question keeps coming up in this forum and it never gets the treatment that it deserves and which is required to understand the answer. Therefore I took Chapter 14 Expansion of the Universe from of Cosmology: The Science of the Universe - 2nd Ed. by Edward Harrison, Cambridge University Press, (2000) and placed it on my website for all to download. It's at http://www.newenglandphysics.org/other/expansion_of_universe.pdf

For those who really have a desire to learn why cosmologists believe that the universe is expanding then please read that chapter. Otherwise you won't be able to properly understand it.

When reading that chapter please keep in mind that theory is in general necessary in order to carry out meaningful experiments and observations. When cosmologists say that cosmological redshift is evidence of the expansion of the universe they mean just that, evidence. It does not mean proof. The geometry of the universe is provided by the Friedman metric, derived using general relativity, and the evolution of the universe is determined by Einstein's field equations. From those results the nature of the cosmological redshift is predicted. The observations of the cosmological redshift are in agreement with those predictions. That's precisely what it means to be called evidence.


 

Offline Thebox

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Re: How do we "know" that the universe is expanding?
« Reply #1 on: 28/05/2016 08:48:59 »
This question keeps coming up in this forum and it never gets the treatment that it deserves and which is required to understand the answer. Therefore I took Chapter 14 Expansion of the Universe from of Cosmology: The Science of the Universe - 2nd Ed. by Edward Harrison, Cambridge University Press, (2000) and placed it on my website for all to download. It's at http://www.newenglandphysics.org/other/expansion_of_universe.pdf

For those who really have a desire to learn why cosmologists believe that the universe is expanding then please read that chapter. Otherwise you won't be able to properly understand it.

When reading that chapter please keep in mind that theory is in general necessary in order to carry out meaningful experiments and observations. When cosmologists say that cosmological redshift is evidence of the expansion of the universe they mean just that, evidence. It does not mean proof. The geometry of the universe is provided by the Friedman metric, derived using general relativity, and the evolution of the universe is determined by Einstein's field equations. From those results the nature of the cosmological redshift is predicted. The observations of the cosmological redshift are in agreement with those predictions. That's precisely what it means to be called evidence.

I have you a proper question Pete if you are willing to answer or anybody else is willing to answer. 

You say the understanding is that the Universe is expanding, firstly can you please describe exactly what you are referring to as the Universe?

Are you making and saying an assumption that even if space was an unbounded quantity and infinite , this is expanding?


Are you saying we are in a finite ''bubble'' and the ''bubble'' is expanding?


Or do you really mean and to be precise in what we are discussing, mean the visual Universe is expanding by the observation of point sources gaining a greater radius?

added(just to add more pressure on your answers)

Are you saying the Hubble ''observation'' is that we can see this redshift or are you saying the redshift is recorded by device and not an actual real observation in the form of sight?

If it is the case that it is detected by device, then quite obviously light from light years away will be recorded to be ''weak'' at this end, the same reason the ambient light from distant stars at night does not allow us to see very good at night. Light at night from distant stars is ''red-shifted''.

Is a rainbow or any other ''colour'' a shift of light?




« Last Edit: 28/05/2016 08:59:58 by Thebox »
 

Offline Alan McDougall

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Re: How do we "know" that the universe is expanding?
« Reply #2 on: 28/05/2016 09:30:24 »
The dobpler effect of red-shift of the light spectrum of galaxies receding from us!
 

Offline PmbPhy

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Re: How do we "know" that the universe is expanding?
« Reply #3 on: 28/05/2016 16:35:36 »
The dobpler effect of red-shift of the light spectrum of galaxies receding from us!
Doppler shift alone cannot account for the redshift. The space itself between emitter and receiver increases with time and using the Doppler relation for large values of z gives the wrong answers.
 

Offline Alan McDougall

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Re: How do we "know" that the universe is expanding?
« Reply #4 on: 28/05/2016 21:21:33 »
The dobpler effect of red-shift of the light spectrum of galaxies receding from us!


You are correct!, it is also my opinion is that the Doppler shift alone cannot account for the red-shift of receding galaxies. The space itself between emitter and receiver increases with time and using the Doppler relation for large values of z gives the wrong answers.

The Hubble Law itself, though derivable from General Relativity, is not sufficient grounds to conclude that redshifts are a reliable proxy for distance in the universe.

One counter-example where an astronomical object with a very large redshift,  is seen ejected towards us out of the nucleus of a relatively low redshift spiral galaxy is sufficient to prove that the Hubble Law as a method of determining distance is not robust. High redshift should always mean great distance if the Hubble Law is true, hence this counter-example calls into some doubt into the present scientific understanding of cosmological expansion.

« Last Edit: 29/05/2016 01:52:58 by Alan McDougall »
 

Offline agyejy

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Re: How do we "know" that the universe is expanding?
« Reply #5 on: 28/05/2016 22:04:46 »
The Hubble Law itself, though derivable from General Relativity, is not sufficient grounds to conclude that redshifts are a reliable proxy for distance in the universe.

For starters redshifts are not really considered a reliable proxy for distance. Redshift is used to judge distances only as a very last resort when a standard candle can't be found.  We only roughly know the correspondence between distance and redshift through the use of standard candle and something called the cosmic distance ladder (which is a good thing to know about before you try to discuss cosmology). Further, General Relativity helps you interpret Hubble's Law (which itself is just an observed relation between distance and redshift not really a law) by telling you that the relationship you observe is probably due to expansion but can't really be used to derive it.

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One counter-example where an astronomical object with a very large redshift,  is seen ejected towards us out of the nucleus of a relatively low redshift spiral galaxy is sufficient to prove that the Hubble Law as a method of determining distance is not robust. High redshift should always mean great distance if the Hubble Law is true, hence this counter-example calls into some doubt into the present scientific understanding of cosmological expansion.

For starters if the object was ejected towards us that would produce a blueshift not a redshift. Secondly as pointed out above redshift is only used as a proxy for distance when there isn't any other option. Also, astronomers and astrophysicists do not analyze data out of context. They would not use an object that is clearly unusual (say having been ejected for a galactic core) for cosmological redshift determination as they would be well aware of the potential of skewed results. Finally, at cosmological distances (really any further than our own local cluster of galaxies) the redshift due to the expansion of the universe dwarfs any possible Doppler or gravitational shift. It would take velocities or amounts of mass that are outrageously inconsistent with our observations of stars and galaxies to produce shifts comparable in size to cosmological redshifts. So far the fastest stellar or larger objects we've found (we use stellar objects to measure cosmological redshifts) have a z value (a measure of this shift) of about 0.035. Cosmological redshifts at relevant distances start at somewhere around z=1 and go up considerably from there. So for there to be a star (or group of stars) moving fast enough to seriously impact the measurement of cosmological redshift at a distance that actually matters (where other methods of measuring distance are harder to come by) it would need to me moving at least 28 times faster than the fastest stars we've observed (which by the way are stars orbiting very close to supermassive black holes so there really isn't anyway you could get a star moving faster than that).   
 

Offline PmbPhy

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Re: How do we "know" that the universe is expanding?
« Reply #6 on: 29/05/2016 01:30:34 »
Quote from: agyejy
For starters redshifts are not really considered a reliable proxy for distance.
Hi agyejy: Please don't change the subject of this thread. The subject of this thread is all about how physicists (astrophysicists, astronomers, cosmologists, etc) "know" that the universe is expanding. I place quotes around "know" because in science we can never say with 100% certainty that we know something. However there are instances such as this when the evidence is so overwhelming that almost all physicists believe that it's true.

I'll address your response here because it's incorrect and I don't like to leave things like that alone. If you'd like to discuss it further, which I'm sure you will, then please start another thread. Thanks.

Re - ...redshifts are not really considered a reliable proxy for distance.

That statement needs to be addressed. In the first place the method used to measure the distance to astronomical objects depends on the distance to the object. For more on this please see: https://en.wikipedia.org/wiki/Cosmic_distance_ladder

When it comes to measuring the distance to galaxies which are very far away then using Hubble's law is the best way to determine its distance. If the galaxy is so far away that individual stars can't be resolved then Hubble's law is used. Of course there are other methods too. It all depends on the particular galaxy.

What I said above comes from a text I have on astronomy: The Cosmic Perspective - 2nd Ed. by Bennett, et al, page 578. My other texts on cosmology say the same thing. The authors write
Quote
Dividing both sides of Hubble's law by H0 puts it into a form in which we can use a galaxy's velocity to determine its distance

d = v/H0

In principle, applying this law is one of the best ways to determine distances to galaxies.
This assertion is uniform throughout the literature.


Quote from: agyejy
Redshift is used to judge distances only as a very last resort when a standard candle can't be found.
It's hardly a "last resort" since most galaxies are so far away that the method which uses standard candles can't be used. The further away a galaxy is the more accurate the distance to the galaxy can be determined.

In any case, this is not the appropriate thread to discuss this subject.
 

Offline agyejy

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Re: How do we "know" that the universe is expanding?
« Reply #7 on: 29/05/2016 03:32:24 »
Hi agyejy: Please don't change the subject of this thread. The subject of this thread is all about how physicists (astrophysicists, astronomers, cosmologists, etc) "know" that the universe is expanding. I place quotes around "know" because in science we can never say with 100% certainty that we know something. However there are instances such as this when the evidence is so overwhelming that almost all physicists believe that it's true.

I didn't.

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I'll address your response here because it's incorrect and I don't like to leave things like that alone. If you'd like to discuss it further, which I'm sure you will, then please start another thread. Thanks.

There is no reason to do any of that.

Quote
Re - ...redshifts are not really considered a reliable proxy for distance.

That statement needs to be addressed. In the first place the method used to measure the distance to astronomical objects depends on the distance to the object. For more on this please see: https://en.wikipedia.org/wiki/Cosmic_distance_ladder

Oh you mean like I mentioned here?

We only roughly know the correspondence between distance and redshift through the use of standard candle and something called the cosmic distance ladder (which is a good thing to know about before you try to discuss cosmology).

In the future please bother to read my responses fully. I do not like to be ignored and I really don't like to be lectured about not knowing things I've clearly demonstrated that I know. You might call it a pet peeve of mine especially when it happens right after I get finished demonstrating that I know.

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When it comes to measuring the distance to galaxies which are very far away then using Hubble's law is the best way to determine its distance. If the galaxy is so far away that individual stars can't be resolved then Hubble's law is used. Of course there are other methods too. It all depends on the particular galaxy.

You will note that I did not say it wasn't used I said it wasn't considered reliable and it isn't. I guess I could have been more specific in saying that it is the least reliable method. Also, you don't actually have to resolve individual stars to use Type 1a supernova. You just need to have a good number of consecutive measurements of the galaxy containing the supernova through a good deal of the actual event. For starters generally speaking the supernova outshines the galaxy while it is occurring and so tends to dominate the spectrum recorded from the galaxy. Further proper normalization using observations of the galaxy before and/or after the event can be used to remove the contribution of the galaxy from the observed spectra thereby isolating the light curve of the Type 1a supernova and allowing for a fairly accurate distance estimate. There are some complications in terms of estimating the amount of dust between you and the supernova but the systematic error due to that is far far less than the systematic errors inherent in using Hubble's constant if for no other reason than because distance measures (I believe mostly Type 1a supernova) were used to calculate Hubble's constant (especially at great distances where it's value differs) and simple propagation of errors means that any distance measure using the constant must be at the very least as uncertain as the measurements of Type 1a supernova with the addition of other uncertainties. Ignoring these facts and pretending like these measurement difficulties don't exist is exactly why laymen misunderstand cosmology.

Quote
What I said above comes from a text I have on astronomy: The Cosmic Perspective - 2nd Ed. by Bennett, et al, page 578. My other texts on cosmology say the same thing. The authors write
Quote
Dividing both sides of Hubble's law by H0 puts it into a form in which we can use a galaxy's velocity to determine its distance

d = v/H0

In principle, applying this law is one of the best ways to determine distances to galaxies.
This assertion is uniform throughout the literature.

Again I never said it couldn't be used or wasn't used. I said it wasn't considered reliable and other methods are used if at all possible. In fact the preferred method is to use as many independent methods as possible and average them to reduce systematic uncertainty. That's kind of how experimental science is done. First you use the method with the lowest systematic uncertainty that you can manage. If you there are several independent measurement methods (meaning their systematic errors are not correlated) and it isn't prohibitive you do as many as possible and use the results to reduce you systematic error.

Quote
Quote from: agyejy
Redshift is used to judge distances only as a very last resort when a standard candle can't be found.
It's hardly a "last resort" since most galaxies are so far away that the method which uses standard candles can't be used. The further away a galaxy is the more accurate the distance to the galaxy can be determined.

In any case, this is not the appropriate thread to discuss this subject.

Type 1a supernova work to on the order of 10 billion light years and methods like the Tully–Fisher relation can work out to similar distances. This covers basically all of the visible universe back to not long after the formation of the first galaxies. Furthermore, it is thought that Hubble's constant has actually changed with time which means accurate estimates of distance relying solely on Hubble's constant have considerable uncertainty. All contributing to why astronomers and astrophysicists will use any other method (usually standard candles because it generally is the easiest and most abundant way) to measure distance if at all possible.

There are objects for which no other method yet exists (the CMB for example) and the distances to those objects are much more approximate than other distances and if you could come up with any other way to measure that distance even slightly more accurately (that didn't involve Hubble's constant) they would start using it immediately (and you get super famous at least in the field).

As for being offtopic it should be abundantly clear that the exact methods by which we measure distances and the errors in inherent in our measurements are of extreme importance to any discussion about the size of the universe and any changes thereof. Thus discussions of the Cosmic Distance Ladder and the systematic uncertainties are in fact central to the topic of this thread.
 

Offline Alan McDougall

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Re: How do we "know" that the universe is expanding?
« Reply #8 on: 29/05/2016 04:04:59 »

The Hubble Law itself, though derivable from General Relativity, is not sufficient grounds to conclude that redshifts are a reliable proxy for distance in the universe.

One counter-example where an astronomical object with a very large redshift,  is seen ejected towards us out of the nucleus of a relatively low redshift spiral galaxy is sufficient to prove that the Hubble Law as a method of determining distance is not robust. High redshift should always mean great distance if the Hubble Law is true, hence this counter-example calls into some doubt into the present scientific understanding of cosmological expansion.

Alan
 

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Re: How do we "know" that the universe is expanding?
« Reply #8 on: 29/05/2016 04:04:59 »

 

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