# A daft question…..

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2 minutes ago, Stu said:

Thanks Jim.

This shows accelerated expansion in the present, which implies we see higher expansion locally that in the most distant regions, or am I misinterpreting the diagram/what you are saying?

Do we know anything about what is happening to those galaxies now ie 12 or 13 billion odd years from when we are seeing them now….?

I get what you are asking, and one of the ways it can be done is to look at densities.

Since we look in past when we look into distance - we can do following - given speed of recession of each galaxy and time elapsed - we can calculate how much galaxies there should be per cubic volume of space.

We then start counting galaxies in every direction - at certain distances and this should give us cumulative speed up to that point which resulted in such density of galaxies per volume.

Maybe simplest analogy would be with water pouring out of a container. Let's say that we don't really know the speed at which water is flowing out, but we have snapshots of how much water there was at the beginning, and then after 1 minute and then after 2 minutes and so on.

Knowing that - we can reconstruct "curve" of water flow out of the container.

In reality - that is not what happens, or at least that is not primary method of how things are calculated.

In reality, mathematical model is developed by taking certain assumptions - like how amount of radiation in space impacts geometry of space time (how that energy curves the space), how amount of stuff / matter impacts geometry of space time (again energy bends space time) - how both of these change as you change volume of space.

You then add cosmological constant, for a good measure and because it seems reasonable (and maybe later say that it is your blunder, but people find that data best fits what we observe when it is there ) and get set of differential equations of how things should behave.

These equations depend on some values - like amount of stuff in universe and amount of radiation in universe and all of that.

Then you take measurements and let computer try to best fit equation parameters to that what you observe. In the process you find out that energy content of universe is roughly 71.4% of that dark energy stuff that has constant density with respect to space, that regular matter that you can see is only 4.6% that energy in form of EM radiation is very small percentage at the moment and that there seems to be huge chunk of stuff missing - that behaves like ordinary matter but can't be seen anywhere.

You call that dark matter and scratch your head where you have gone wrong

In the mean time - you start hearing that many others report findings that would be best explained if there was this dark matter that can't be seen but it interacts via gravity with stuff that can be seen.

In the end you conclude that most evidence points to your model being correct.

Then you take calculations from your model and it tells you things like - what is the age of universe, how much stuff of different kind there is in universe - how fast it was expanding depending on time and how that relates to content of the universe at particular time - and everything seems to fit rather nicely.

You are able to plot nice graphs of expansion curving this way or that way

If you want to get a bit better understanding of how these equations are derived - there is very good lecture by Susskind on cosmology. In one of his first lectures on the topic - he explains and derives these equations without use of General Relativity - with Newtonian gravity only - easy to follow if you have very basic understanding of differential equations.

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1 minute ago, mikeDnight said:

Spot on!  Photons that leave a distant galaxy or star are not the same photons that reach us. Each interaction between high energy photons with other particles or subatomic particles, causes a loss in the energy level of the photons, causing a move towards the red end of the spectrum, so the more distant the object the stronger the move towards the red. Therefore red is an indicator of distance and not of recession. All these fancy hypotheses remind me of the Copernican model of the solar system, clever but overly complicated and ultimately wrong.

What you are talking here about is interstellar reddening.

It can fairly easily be calculated how much it would impact light from a distant galaxy depending on density of matter in intergalactic space.

I won't bother with that - I'm just going to say that it fails completely as wavelengths get longer as micro waves and radio waves don't really scatter all that well of atoms due to difference in size.

CMB at that temperature could not possibly form due to scattering.

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9 hours ago, Ags said:

Is an alternate explanation possible that there is no expansion at all but "light gets tired" and over the course of billions of years simply shifts to a lower frequency, just like neutrinos change between different types as they travel?

There is still a theory ( amongst others ) that as light travels through the "Aether" it is slowed down by about 20km/s every million lightyears it travels through.

This may give the same appearance as the universe expanding.

( Aether = spacetime & all associated gravitational fields etc......basically the bits between the stars )

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Posted (edited)
1 hour ago, Stu said:

Thanks Jim.

This shows accelerated expansion in the present, which implies we see higher expansion locally that in the most distant regions, or am I misinterpreting the diagram/what you are saying?

Do we know anything about what is happening to those galaxies now ie 12 or 13 billion odd years from when we are seeing them now….?

Locally we would not see any evidence of expansion as our local place in space is dominated by the effect of gravity - evident that we can actually see and predict local mergers of galaxies such as Andromeda and our own Milky Way.  I'm not 100 % sure but I think we would need to be careful with words like "present"  as of course we are observing data from past events albeit we are seeing it in our present.  To be honest I'm not sure I can get my head properly round that !  As far as the change in the rate of expansion is concerned it is thought that this happened around 9  billion years post big bang - this is the timeline shown on the graphics as the "Dark Energy Dominated Era". You may heard/read of people talk about the Lambda Cold Dark Matter model  (Lambda CDM) or standard model - this is our best understanding to date of the evolution of the universe. The lambda element is in effect a derivative of Einstein's hated cosmological constant  - it points to a positive value which infers a positive pressure fuelling the acceleration.  At a popular level this is what is called dark energy.  The galaxies from 12 - 13 billion years past will be moving away from each other, rather the spacetime they occupy will be  expanding but the galaxies themselves would remain I think gravitationally bound.  If the expansion continues to accelerate then it is postulated that eventually the galaxies, including our own, would truly be islands of light. We would be so far distant from each other that all other galaxies would be lost from our sight !  No more DSO challenges I guess.   So when we observe those very first galaxies we see them set in a time when the universe was expanding but at a lower rate than we experience today.

Jim

Edited by saac
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Just wanted to thank everyone for the very informative responses to my daft question. I am very glad I asked, because it has prompted some excellent discussion and input, and has helped me understand a little more. I still need to go back and re-read the posts and links but will slowly get there I’m sure. I also haven’t tackled the entanglement posts yet, will try those tomorrow

Hopefully we are not done yet!

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1 hour ago, Stu said:

I still need to go back and re-read the posts and links

Me too.

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4 hours ago, Kev M said:

There is still a theory ( amongst others ) that as light travels through the "Aether" it is slowed down by about 20km/s every million lightyears it travels through.

Sounds like an easy theory to test, if we can find an asteroid occulting a distant quasar.

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10 hours ago, mikeDnight said:

Spot on!  Photons that leave a distant galaxy or star are not the same photons that reach us. Each interaction between high energy photons with other particles or subatomic particles, causes a loss in the energy level of the photons, causing a move towards the red end of the spectrum, so the more distant the object the stronger the move towards the red. Therefore red is an indicator of distance and not of recession. All these fancy hypotheses remind me of the Copernican model of the solar system, clever but overly complicated and ultimately wrong.

Well, except that experimentally photons have never  been found afaik to suffer from a 'loss in energy level'. It's all or nothing - planck's constant. If what you say is true it would be very easily proven by interacting photons with said 'particles or subatomic particles'.

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52 minutes ago, powerlord said:

Well, except that experimentally photons have never  been found afaik to suffer from a 'loss in energy level'. It's all or nothing - planck's constant. If what you say is true it would be very easily proven by interacting photons with said 'particles or subatomic particles'.

Well the CMB light started of as peaking in the  UV and is now down to microwave frequencies. So the have lost energy. The accepted explanation is due to the expansion of the Universe.

Other theories, such as tired light, don't match all the observations.  As I posted before take a look at this

An unrelated effect, as  @vlaiv mentioned,  is interstellar reddening where objects appear redder due to scattering much as why the sky is blue and the sun  redish (especially noticeable when its low on the horizon)  and not white.

Regards Andrew

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There is a clear difference between true Redshift and reddening caused by scattering in that the Fraunhofer  lines will be shifted in the former but not the latter.

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Posted (edited)
41 minutes ago, andrew s said:

Well the CMB light started of as peaking in the  UV and is now down to microwave frequencies. So the have lost energy. The accepted explanation is due to the expansion of the Universe.

Fair point, I'd not thought of that. It does appear to be an area which is still somewhat of a puzzle though. interesting area.

Edited by powerlord
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3 hours ago, powerlord said:

Well, except that experimentally photons have never  been found afaik to suffer from a 'loss in energy level'. It's all or nothing - planck's constant. If what you say is true it would be very easily proven by interacting photons with said 'particles or subatomic particles'.

Photons do regularly suffer from loss in energy levels and I think it has been measured (at least indirectly with clocks).

Whenever they start of in strong gravity well and leave it - they loose some energy and experience gravitational red shift.

They also experience gravitational blue shift when "falling" into our galaxy cluster, our galaxy, our solar system and finally down to earth.

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Has anyone mentioned Halton Arp and discordant red shifts yet? 3c273 anyone….

Marv

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Posted (edited)

Do I recall correctly that there is a theory that photons can interact with strong magnetic fields (I vaguely recall a dark matter experiment is based on this principle).

Edited by Ags
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On 31/03/2022 at 21:48, vlaiv said:

What you are talking here about is interstellar reddening.

Correct. the effect of the interstellar medium on the spectrum of distant objects is well defined and different to that seen from cosmological expansion. See Lyman forest for example

The animation there is particularly nice

Here are some measurements made by me of high redshift distant quasar spectra clearly showing the Lyman Alpha line shifted into the red from the UV and the Lyman forest from absorption by intervening material. The photons forming the rdshifted Lyman alpha emission line are the same ones that left the quasar that were not absorbed/scattered by intervening interactions

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