13.8 lightyears?

[saac]

1 hour ago, Malpi12 said:

Would it confuse things even more if this simple soul said :-

Although the "observable" universe is getting bigger as time passes, there is going to be less  & less to observe, as more "things" will be expanded beyond c (or 3c ?)
Only the Local Group and the Milky Way will remain, gravitation-bound, for awhile. Until the increasing rate of expansion trumps even that.
So make the most of what Webb can still see.

But I could be wrong ? I have a feeling that I am missing something ! Cos that bit about Webb dont feel right

No you are essentially correct, at some point in the future the bottom will fall out of the second hand astrophotography market as we all try to off load our kit.  Pending on what value the critical density of the universe turns out to hold, our islands of light will move further and further apart until we eventually become isolated - the last star goes forever dim.  Either that our we all get cosy or fly apart. Now who wants to off load their Takahashi quick?  

Jim 

[ollypenrice]

3 hours ago, AstroKeith said:

 

A useful way to think of the expansion, is not that things are moving through space to be further away, its that the space itself between us that is expanding.

I'm just a superannuated English teacher but I have tried to find a way of distinguishing clearly between the two forms of 'moving' which we seem to need when discussing the universe. In the normal sense of moving, a thing needs to be accelerated in order to move. It then moves in a direction relative to things around it which have not been accelerated. It will get closer to some and further from others. This is not at all what happens in the expanding universe. None of the galaxies moves towards some of the others and further from some of the others. Every galaxy finds itself to be at rest and sees all the others as moving away from it. None experiences acceleration.

It's tempting to say that there is a difference between moving through space and space expanding, yet I thought we were not supposed to think in terms of any kind of fixed grid of space. That's why I focus on acceleration as a way of distinguishing between the two kinds of movement. Feel free to put me out of my misery!

Olly

[vlaiv]

4 minutes ago, ollypenrice said:

It's tempting to say that there is a difference between moving through space and space expanding, yet I thought we were not supposed to think in terms of any kind of fixed grid of space. That's why I focus on acceleration as a way of distinguishing between the two kinds of movement. Feel free to put me out of my misery!

Actually - it helps if you think in terms of fixed grid in space.

Imagine that you have 3 galaxies in a row.

There is 7 grid spaces between first and second and 3 grid spaces between second and third.

Such setup represents expanding space - as long as you don't equate grid space with constant physical distance. If one grid space is variable in terms of "kilometers" and depends on the moment we are talking about - this setup represents expanding universe.

Conversion factor between "grid space" and actual physical length is called scale factor and is main variable of the Hubble law that can be expressed like this:

Or in another words - Hubble parameter (rather than constant) is equal to how scale factor is changing at some time divided with value of scale factor at that time.

 

[mergatroid]

18 hours ago, michael.h.f.wilkinson said:

If the current, most accurate estimate of the age of the universe of 13.8 billion years (i.e. obtained by scientific method, nothing to do with theology) is correct, we can only see photons arriving from 13.8 billion light years away at this point in time.

I see no reason to believe that figure because I don't believe it's real science. It also makes no logical sense. It assumes we're at the center of the universe, which is a theological argument, i.e. humans putting themselves at the center of everything because some god put them there.

If the universe has a finite size (it doesn't) and there is no reason at all to think we're at the center of it, then we are closer to the edge of it. In that case, we'd see more galaxies in one direction than on the opposite. But we don't.

[mergatroid]

18 hours ago, ollypenrice said:

Light from very distance objects can never reach us because its source is receding from us a more than the speed of light

That isn't a proveable hypothesis however. It's observation leading to conjecture. It's illogical though. If it were true, then you would expect to see a few half-galaxies i.e. ones that are traveling at exactly the speed of light, where the part moving toward us is visible, and the part moving away is not.

I understand that tired light theory isn't popular but there may be some additional factor that causes light to shift besides motion. Photons after all carry energy. If they interact with something in space that causes a general loss of energy that would also explain some of the red shift.

[mergatroid]

11 hours ago, AstroKeith said:

The radiation is weaker and even more 'red shifted'. Actually 'red shifted' is now not quite right as we have shifted beyond red.

I still don't understand why astronomers insist that the wavelength shift has to have just one cause. That seems overly simplistic and leads to absurd-sounding ideas like the Big Bang, which is pseudo-religious, and stars necessarily moving away at light speed. Sure, there is a Doppler effect for light. But to say that is the only cause of wavelength shift is just silly and unfounded. There is a rule of thumb that the more absurd an explanation is, the more is needs to be revised.

[Gfamily]

2 hours ago, mergatroid said:

I still don't understand why astronomers insist that the wavelength shift has to have just one cause. That seems overly simplistic and leads to absurd-sounding ideas like the Big Bang, which is pseudo-religious, and stars necessarily moving away at light speed. Sure, there is a Doppler effect for light. But to say that is the only cause of wavelength shift is just silly and unfounded. There is a rule of thumb that the more absurd an explanation is, the more is needs to be revised.

The point is that there is good evidence for all the conclusions that you reject. You don't seem to have given any serious consideration of the evidence before you reject it, except for your feeling that it doesn't make sense to you. 

That's fine, we can't make it so that you actually consider the strands of evidence that give us the general understanding of how the universe it, but please don't assume that your rejection of the current model is based on a stronger evidence base than the explanations given here.   

Oh, and where has anyone said that the wavelength shift only has one cause?  We know that there are multiple causes for wavelength shift, because we understand the science.   

Edited January 31, 2023 by Gfamily

[michael.h.f.wilkinson]

3 hours ago, mergatroid said:

I see no reason to believe that figure because I don't believe it's real science. It also makes no logical sense. It assumes we're at the center of the universe, which is a theological argument, i.e. humans putting themselves at the center of everything because some god put them there.

If the universe has a finite size (it doesn't) and there is no reason at all to think we're at the center of it, then we are closer to the edge of it. In that case, we'd see more galaxies in one direction than on the opposite. But we don't.

No it doesn't mean we are the centre, it just says we are at the centre of what we can observe, because the universe has a finite age (which is really the only logical conclusion to draw from Olber's Paradox). This argument has been given in this thread before, but you apparently didn't understand it, or otherwise missed it. You give no argument as to why this is not real science, so I do not feel the need to reply that non-argument.

[ollypenrice]

3 hours ago, mergatroid said:

That isn't a proveable hypothesis however. It's observation leading to conjecture. It's illogical though. If it were true, then you would expect to see a few half-galaxies i.e. ones that are traveling at exactly the speed of light, where the part moving toward us is visible, and the part moving away is not.

 

Could you describe in detail what an observation of such half galaxies would reveal, bearing in mind that galaxies are gravitationally bound systems? 

Edit: And bear in mind, also, that we can detect remarkably little of the far side of our own galaxy, let alone of those at the limit of what is observable.

Olly

Edited January 31, 2023 by ollypenrice

[apaulo]

the only definate in astronomy is, we definately  know very little. lol

[andrew s]

4 hours ago, mergatroid said:

I still don't understand why astronomers insist that the wavelength shift has to have just one cause. That seems overly simplistic and leads to absurd-sounding ideas like the Big Bang, which is pseudo-religious, and stars necessarily moving away at light speed. Sure, there is a Doppler effect for light. But to say that is the only cause of wavelength shift is just silly and unfounded. There is a rule of thumb that the more absurd an explanation is, the more is needs to be revised.

They don't.  They make assumptions for example the "cosmological principle" and build theoretical models based on them. These are shaped by observation but put to the test but seeing how well there predictions match experiments.

This is difficult for astronomy as we can't directly experiment but it can still be done.

As an example, it is often said that space becomes discrete at the Planck scale but time of flight observations of  light of different wavelengths from very distant astronomical sources does not support it.

Similarly tired light is not supported by observation. 

On the cosmological red shift there are strong observations that support it. Tests of general and special relativity,  the Layman forest the CMB and more.

Alternative, theories do get published, say to avoid dark matter, but none as yet as been accepted as they don't explain other observational facts.

In the end a scientific theory stands or falls on how it fits the data and its predictive power.

Regards Andrew 

Edited January 31, 2023 by andrew s

[andrew s]

22 minutes ago, apaulo said:

the only definate in astronomy is, we definately  know very little. lol

Given it was once said we would never know what stars are made of I think we know an amazing amount. 

I guess it's a matter of perspective. 

Regards Andrew 

Edited January 31, 2023 by andrew s

[Mr Spock]

Remember this: Due to the vast distances in space and the speed of light, everything we look at is as it was not as it is. We have no concept of how the universe looks now, only how it looked up to 13.8 billion years ago...

We can only witness from our location objects going back in time in all directions equally. We have no idea where the hell we are in the universe as a whole.

[apaulo]

do we know what stars are made of. who has been on 1 or even near 1 to know for definate.

[andrew s]

3 minutes ago, apaulo said:

do we know what stars are made of. who has been on 1 or even near 1 to know for definate.

We know in just the same way we know two lumps of stuff in our hands are the same.

We make measurements and compare the results.  In the case of stars it's their spectra. 

Regards Andrew 

[michael.h.f.wilkinson]

9 minutes ago, apaulo said:

do we know what stars are made of. who has been on 1 or even near 1 to know for definate.

Largely hydrogen and helium, plus varying amounts of what astronomers call metals. It's all determined from spectra. Of course you might want to invent some exotic models in which esoteric substances are involved which just happen to have the same spectral lines (down to Zeeman-splitting properties), but Occam's Razor will cut these models down quite easily

[Mr Spock]

Spectroscopic analysis is a known science. It's results are predictable and can be repeated. 

[apaulo]

personally i wouldnt say you guys are right or wrong, but i wouldnt put my life savings on you being definately right, but i would have a bet you are not 100% right

[andrew s]

3 minutes ago, apaulo said:

personally i wouldnt say you guys are right or wrong, but i wouldnt put my life savings on you being definately right, but i would have a bet you are not 100% right

Science is never about 100% certainty.  I am personally very happy to accept we have very good current theories.  I also look forward to better ones. They get harder to find as they need to encompass an ever growing set of data.

I hope JWST opens up new insights  as most new instruments have in the past.

Regards Andrew 

[ollypenrice]

7 hours ago, mergatroid said:

I see no reason to believe that figure because I don't believe it's real science. It also makes no logical sense. It assumes we're at the center of the universe, which is a theological argument, i.e. humans putting themselves at the center of everything because some god put them there.

If the universe has a finite size (it doesn't) and there is no reason at all to think we're at the center of it, then we are closer to the edge of it. In that case, we'd see more galaxies in one direction than on the opposite. But we don't.

It does precisely the opposite, which is the whole point. If galaxy redshifts are the result of the proper motion of galaxies then they are all moving away from us and we are at the centre of a kind of explosion. This offends the Copernican Principle which urges us to doubt any observation which requires our position to be special. If, instead, the distance-redshift relation is taken as an indication of the expansion of space then all galaxies observe precisely what we observe and our place is not at the centre of anything.

Olly

[ollypenrice]

2 hours ago, apaulo said:

do we know what stars are made of. who has been on 1 or even near 1 to know for definate.

Could you tell us what parts you found hard to believe in the books you've read about the astrophysics of stars?  If you haven't read any, perhaps you should. This forum is not Facebook.

Olly

[apaulo]

forums for discussion, not insult.

[AstroMuni]

2 hours ago, apaulo said:

personally i wouldnt say you guys are right or wrong, but i wouldnt put my life savings on you being definately right, but i would have a bet you are not 100% right

Our impressions of 'what is right' is at a point in time. As more evidence becomes available, our understanding will evolve and may change. Thats life - whether its science or morality and ethics.

EDIT: And even history for that matter. Its said that the victors define the history, so do we really know our past?

Edited January 31, 2023 by AstroMuni

[andrew s]

19 minutes ago, apaulo said:

forums for discussion, not insult.

Very happy to have a discussion on astronomy and cosmology but your position seems to be that you don't belive any of our existing "knowledge and theories " and we know very little about the subject.

This makes a discussion one about personal beliefs rather than astronomy the core of this site.

I think that was what @ollypenrice was getting at. We need specifics to have a debate about.

For example,  one can discuss "tired light" v LCDM as we can look at the studies done on it. Current evidence from supernova supports LCDM rather than tired light.

" Two new papers provide the best direct evidence yet. The first, slated to appear in Astrophysical Journal, measures the brightening and dimming of a certain type of supernova. Thanks to Einstein's theory of relativity, if distant supernovae are speeding away from us, they will appear to flare and fade at a more leisurely pace than close-by ones. A team of scientists led by Gerson Goldhaber of the Lawrence Berkeley National Laboratory in Berkeley, California, has shown that this is, indeed, the case with 42 recently analyzed supernovae."

From here https://www.science.org/content/article/tired-light-hypothesis-gets-re-tired

Regards Andrew 

[Mr Spock]

There's no point in asking a question if you don't want to hear the answer