Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: HelpMe929 on 08/10/2019 15:36:31

Title: What is the cosmic microwave background?
Post by: HelpMe929 on 08/10/2019 15:36:31
Could somebody please explain this to me better than anybody else has so far?

My understanding of the CMB is it's some kind of leftover from the big bang. But just how is that possible when 'we' (all the atoms that make up you, me and our local universe) were part of that big bang?

Isn't it like seeing a lightning strike hit your garden shed before breakfast, but not actually hearing the thunderclap until after supper!

I just dont get how my atoms have managed to get where I find them today ahead of all those other atoms/photons/whatever that other sciences would suggest have travelled umpteen billion lightyears in distance a good deal faster than my own humble bits of matter could have done.

Just WHY are we still seeing this stuff that should have TOTALLY dissipated out of eternity a few umpteen billion years ago?!?!

Any answers, please?
Title: Re: What is the cosmic microwave background?
Post by: Janus on 08/10/2019 16:02:23
The problem is that you are thinking of the Big Bang as an explosion in space, rather than the expansion of space.
With an explosion in space, the Photons could have raced past the matter into the space beyond. But with the Big Bang, there is no "space beyond" for the photon to expand into. We are at the "forefront" of this expansion.
An analogy would be that the universe is like the skin of an expanding soap bubble. Everything in it, photons, matter, etc. is confined to the skin. When the bubble is small everything is much more crowded together and as it expands, Things on a whole move apart and it is less crowded.  With photons, when it is small their frequencies are high ( the universe is "hot"), but as the bubble expands, they are stretched out to longer wavelengths and lower frequencies ( the universe "cools"). The CMB are those early photons that have been stretched out, but still confined to the same "Bubble skin" as everything else.
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 08/10/2019 16:17:37
And here was I thinking there was no such thing as magic.

The fact that we're 'seeing' this radiation is proof that it's moving through space, right? It isn't just loitering around in our vicinity with nothing better to do. So the radiation we saw last week must now be gone forever. It sounds like this radiation must have been tavelling past us and 'disappearing forever' for the last umpteen billion years. why is it STIIL travelling past us in a way that we can register with instruments?

My apologies, but to me your answer does not really answer the question at all.
Title: Re: What is the cosmic microwave background?
Post by: Hayseed on 08/10/2019 16:40:02
Modern theory believes that mass and motion can change time.

They also believe that a change in clock tic rate, or a change in frequency is proof of a change in time.

This allows time to vary with motion.  They believe this is the only way to explain light velocity measurements.

Any theory or explanation of CMB must have these underpinnings.

So, if you capture a CMB wave, it would appear very old to us, but for the wave, it thinks it's only milliseconds old.

See what you can do with science?
Title: Re: What is the cosmic microwave background?
Post by: Kryptid on 08/10/2019 16:50:24
The fact that we're 'seeing' this radiation is proof that it's moving through space, right? It isn't just loitering around in our vicinity with nothing better to do. So the radiation we saw last week must now be gone forever. It sounds like this radiation must have been tavelling past us and 'disappearing forever' for the last umpteen billion years. why is it STIIL travelling past us in a way that we can register with instruments?

It's true that the radiation is constantly on the move. However, that radiation is travelling in all directions at very nearly the same rate. As many photons of this radiation are entering any given cubic meter of space as are leaving it. So there is always radiation present to measure.
Title: Re: What is the cosmic microwave background?
Post by: Janus on 08/10/2019 17:28:34
And just to add to What Kryptid already said. While the photons we detect today are different ones than the ones we detected yesterday or will detect tomorrow. The one we detected yesterday where a bit "younger" and and the one we will detect tomorrow will be a bit "older".   In other words, the CMB is still decreasing in temp as the universe continues to expand.  The decrease is very small, so it would take a long time before it became enough to be discernible by our measurement standards, but it exists.


 
So the radiation we saw last week must now be gone forever. It sounds like this radiation must have been tavelling past us and 'disappearing forever' for the last umpteen billion years. why is it STIIL travelling past us in a way that we can register with instruments?
This is like saying that because the average water molecule speed is something is in the 100's of meters per second, that a fish in a lake should not detect any water around him because the water molecules that he felt yesterday has traveled off to some other part of the Lake. New water molecules have replaced them.*

*Now it is true that with the lake being a confined body of fixed size, he can encounter the same water molecule more than once.  This is note completely inconceivable for the universe if it were of a finite fixed size.  a Photon could pass the Earth, "circumnavigate" the universe and pass it again. How long it takes between passes would depend on the size of the universe. ( in a really small "closed" universe you could conceivably look off into the distance with a telescope and see the back of your own head)
 But with an expanding universe, even if it is finite, the size and rate of expansion could be such that the "circumference" grows faster than light can traverse it, and light passing you at any given moment can never return. ( but this still doesn't mean that it won't be replaced by a different photon.)
Title: Re: What is the cosmic microwave background?
Post by: RobC on 08/10/2019 19:30:10
It is easy to understand that the sun has already gone down 8 minutes before one sees it set but I can understand the difficulty in visualising the formation of the CMB.
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 08/10/2019 21:00:13
I suppose my next obvious question has to be what has generated (or is generating) the CMB?

It must be more than from just the birth of everything...? For so much radiation to still be zipping passed us after so long, then whatever radiated the photons we're still seeing surely must have been a long way from where 'we' were at the time it was emitted.

Does that suggest the possibility of calculating the diameter of the universe at a particular moment in history? I mean, if it's possible to calculate how much total mass the entire universe holds by investigating the CMB, then why not be able to calculate how big it once was also...
Title: Re: What is the cosmic microwave background?
Post by: Kryptid on 08/10/2019 22:14:36
I suppose my next obvious question has to be what has generated (or is generating) the CMB?

It was formed when the first atoms formed. The bonding of an electron with a proton to form a hydrogen atom releases a photon. Those protons and electrons very uniformly filled all of the Universe, so the radiation was released from pretty much all locations in space, both near and far.

Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 08/10/2019 23:16:58
It was formed when the first atoms formed. The bonding of an electron with a proton to form a hydrogen atom releases a photon. Those protons and electrons very uniformly filled all of the Universe, so the radiation was released from pretty much all locations in space, both near and far.

But I didn't think there was a 'far'! Wasn't all matter/energy compresseed in a high pressure / high temperature 'soup' in which the first atoms were able to bond? How can they bond in a cold / empty envrinment?

Are we even talking aboout the same thing anymore?
Title: Re: What is the cosmic microwave background?
Post by: Kryptid on 08/10/2019 23:40:59
But I didn't think there was a 'far'! Wasn't all matter/energy compresseed in a high pressure / high temperature 'soup'

This happened when the Universe was about 380,000 years old. It was already pretty big by then.

How can they bond in a cold / empty envrinment?

It's actually easier for electrons to bind to protons when they are travelling more slowly (that is, the temperature is lower). If it's too hot, the electrons will be stripped away from the protons, forming a plasma. The environment wasn't exactly cold, though (it was about 3,000 kelvins at that time) and it was far from empty.
Title: Re: What is the cosmic microwave background?
Post by: evan_au on 09/10/2019 10:38:09
Quote from: ?
what has generated (or is generating) the CMB?
As Kryptid said, the CMBR photons last interacted with matter when the universe had a temperature around 3000K.
- A plasma at 3000K will continually absorb and reemit light with a characteristic "black body spectrum"

At that time, the universe was mostly hydrogen, some helium, and almost nothing else.
- When the temperature drops below 3000K, hydrogen and helium can capture and hold onto electrons, so emission of the black body radiation effectively stops, and emission becomes a "line spectrum".
- Hydrogen and Helium bind their electrons tightly, so it takes very energetic photons (Ultraviolet) to excite them and allow emission of a line spectrum.
- By the time the universe dropped below 3000K, there was very little ultraviolet around, so space became transparent to this 3000K black-body radiation.

The subsequent expansion of the universe by a factor of over 1000 has dropped the effective temperature of the black body radiation from 3000K to 2.7K.
- Since we are still within the "fireball", we are being continually bombarded by this black body radiation from all directions (but severely red-shifted).

See: https://en.wikipedia.org/wiki/Cosmic_microwave_background#Relationship_to_the_Big_Bang
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 09/10/2019 13:09:37
This is just going around in circles....

I'll just agree with the experts that they really do know how much mass is in the universe and that it just isn't enough, and that invisible matter really does exist (along with flying broomsticks and invisible cloaks).

Thanks everybody for their patience and attempts to explain something that only really exists in mathematics.
Title: Re: What is the cosmic microwave background?
Post by: Hayseed on 09/10/2019 15:33:19
I would not rely on any estimate of cosmos(or even galactic) mass.  All stars spew out isolated free charge continuously.  Think about that......think how long they have been doing that.   We can find no evidence of charge recombination from our sun.  The charge from our sun is still accelerating out past Neptune.  If that charge recombined, we would detect the emission.  Think about all the charge from the sun for eons.  And eons of stars.  There could be a good chance that most of the mass(isolated charge) in the universe, is not under gravitational influence.

And a good chance that there could be much, much more mass than thought.   Naked mass might be different than gravitational mass.

How about this, we believe that high velocity makes it difficult for recombination.  What if a simple pole flip on a particle made a difference?  What if there are certain velocity and density conditions where a high velocity recombination readily occurs with an unknown emission spectrum..........such as CMB.  CMB might be fresh.  A cold spectrum.

Astronomy is an unique science.  When we photograph motion on earth, all the objects are in present time. One time stamp. But a star field is not an image of objects........it's an image of time stamps, with unknown AND different times.  This is unique. We are foreign to this dynamic.  On earth, we have never had need of this way of thinking and understanding.  That does not include recognizing star field patterns with season.  That reasoning when to the gods.

We can not see the present universe or even our present galaxy. The present universe might be dark and scattered.

It's hard to fathom that our galaxy's center could have super nova-ed thousands of years ago.

Compare the number of assumptions that astronomy uses with the number of assumptions that chemistry uses.   Astronomy needs salt to swallow.

Not that there is anything wrong with astronomy.  Like I said, it's unique. A different kind of puzzle.
Title: Re: What is the cosmic microwave background?
Post by: Kryptid on 09/10/2019 17:13:59
This is just going around in circles....

What wasn't clear about what we said?
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 10/10/2019 00:06:53
This is just going around in circles....

What wasn't clear about what we said?

the statistics :p
Title: Re: What is the cosmic microwave background?
Post by: Colin2B on 11/10/2019 18:00:22
Thanks everybody for their patience and attempts to explain something that only really exists in mathematics.
Thatís not true, it doesnít exist in maths, it is as real as the radiation from a light bulb, we just canít see it with our eyes.

What wasn't clear about what we said?

the statistics :p
What statistics, what maths?. I donít see any of those here, just a genuine attempt by a number of people to help you understand what it is.
Title: Re: What is the cosmic microwave background?
Post by: Hayseed on 11/10/2019 18:05:43
At first, it was thought to come from pigeon poop.
Title: Re: What is the cosmic microwave background?
Post by: Bored chemist on 11/10/2019 18:33:52
- A plasma at 3000K will continually absorb and reemit light with a characteristic "black body spectrum"
Not reliably.
The flame of an oxyacetylene gas torch is a plasma at about that temperature, it emits a band spectrum.

https://rkdmb.home.xs4all.nl/rkd/flame/spectrum.html

To get a black body you either need lots of ionisation and or high pressure.
The aftermath of the BB had both.
Title: Re: What is the cosmic microwave background?
Post by: evan_au on 11/10/2019 22:06:57
Quote
oxyacetylene gas torch
This also has several things that didn't appear in the Big Bang, like the OH, CH, C2 and C3 species.
Title: Re: What is the cosmic microwave background?
Post by: Bored chemist on 12/10/2019 00:12:14
Quote
oxyacetylene gas torch
This also has several things that didn't appear in the Big Bang, like the OH, CH, C2 and C3 species.

Which makes things worse.
With no molecular species, only atomic transitions were available.
Neither hydrogen nor helium can give a black body spectrum at 3000K except under pressure.
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 12/10/2019 01:27:54
Now I'm interested again, with more advanced questions, if that's allowed.

The conversation seems to imply the CMB generation was quite a short phase (when the universe was a 3000k plasma). If its effects are still saturating the universe after 13.7bn years (due to its photons filling cubic meters of space as fast as they leave it), then the volume of this plasma must have been.... 'large'. But, why dont we see other radiation that must have been emitted before and after this 3000k period? Why isn't this still filling up cubic meters of space as fast as they leave it like the cmb? Or is this the dark matter that ppl are still looking for?
Title: Re: What is the cosmic microwave background?
Post by: Halc on 12/10/2019 01:45:24
The conversation seems to imply the CMB generation was quite a short phase (when the universe was a 3000k plasma). If its effects are still saturating the universe after 13.7bn years (due to its photons filling cubic meters of space as fast as they leave it), then the volume of this plasma must have been.... 'large'. But, why dont we see other radiation that must have been emitted before and after this 3000k period?
There was light before then, but the universe was opaque, so the light never got far before hitting something.
There is light since then.  That's all the stars and whatnot, most of them brighter than the CMB.

Quote
Why isn't this still filling up cubic meters of space as fast as they leave it like the cmb? Or is this the dark matter that ppl are still looking for?
It does fill space as fast as it leaves it, else the stars would not shine continuously.  There's nowhere you can be (except in places like clouds that obstruct light) where galaxies cannot be seen, so their light fills all space, same as the CMB.
Title: Re: What is the cosmic microwave background?
Post by: Colin2B on 12/10/2019 09:18:31
The conversation seems to imply the CMB generation was quite a short phase (when the universe was a 3000k plasma). If its effects are still saturating the universe after 13.7bn years (due to its photons filling cubic meters of space as fast as they leave it), then the volume of this plasma must have been.... 'large'. But, why dont we see other radiation that must have been emitted before and after this 3000k period?
To add to what @Halc said, Iím giving a very simplified analogy.
Think about a light bulb, lots of very bright light. Switch it off and you will see the light fade quickly and the filament still glow red for a while. Now touch the glass, still very hot, move the bulb 100m away and a thermal imaging camera will still pick up that heat (infrared) radiation.
Now imagine a universe filled with light bulbs (lets ignore for the moment the inflation that put them there) if they are all switched off together they begin to cool, but still radiating. When the ones next to us have cooled, all of them will have cooled, but the radiation reaching us from the ones billions of light years away was emitted when they were still quite hot - less hot than IR and was down to microwave frequencies.
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 12/10/2019 12:37:20
Thanks everybody for the explanations :)

They all make perfect sense and fit in with the logical laws of physics as I understand them. But there's still my original question that even now I cant get my head around.

I'll come back to it in a moment, because evan_au mentioned that the early universe was opaque due to the ultraviolet radiation. I must have misunderstood him because otherwise the universe would still be opaque due to ultraviolet radiation wouldn't it (or at least the more distant/older swathes of it would still be opaque?)

But forgetting that, I still dont understand WHY anything as old as 13bn years is still visible to us. The only way I can understand that is if the universe as early as one hour old (just to pick a sensible timeframe) was already as big as our visible universe now is, big enough so that anything travelling at lightspeed would still take 13bn years to travel between two given points in it.

If it wasn't already this big then common sense would sugest that  its expansion rate (in order to explain the 13bn year 'lag') would be too high for gaseous material to coalesce into stars and solar systems.

So was the Universe already universe sized very early in its life?

Have I just answered my own question...? (other than the ultraviolet bit).

Many thanks
Title: Re: What is the cosmic microwave background?
Post by: Colin2B on 12/10/2019 15:10:36
evan_au mentioned that the early universe was opaque due to the ultraviolet radiation. I must have misunderstood him
Yes, slightly.
Not opaque due to UV, but the temperature at that time was below the UV level and so there wasnít much around to excite the H & He.

If it wasn't already this big then common sense would sugest that  its expansion rate (in order to explain the 13bn year 'lag') would be too high for gaseous material to coalesce into stars and solar systems.
No it wasnít already size of current universe, but locally the force of gravity was still strong enough to pull the material together.
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 12/10/2019 17:44:58
Apologies Colin for the confusion.

I'm asking if there's a reason (yet) for this 13bn year delay in the cmb reaching our neck of the universe.
Title: Re: What is the cosmic microwave background?
Post by: Halc on 12/10/2019 18:26:06
But forgetting that, I still dont understand WHY anything as old as 13bn years is still visible to us.
Light doesn't decay.  If I emit a photon thataway, assuming it hits nothing, it will go forever at that speed and energy, so 13 BY is nothing to that process.
Problem is, when it is finally measured by something, that something is probably very far away and moving away from the source (here) of that photon, so that receding detector will measure a very red-shifted photon.  That's why the CMB currently appears so red-shifted from its original frequency: because we're moving just stupid fast away from the source atom of that photon.

Quote
The only way I can understand that is if the universe as early as one hour old (just to pick a sensible timeframe) was already as big as our visible universe now is, big enough so that anything travelling at lightspeed would still take 13bn years to travel between two given points in it.
It takes less time for light to travel between points closer together, else we'd not see the light of our sun.  So sure, points that far apart might take that long for light to travel between them.
All of the CMB today comes to us from some identical distance.  If the light was emitted closer by, it has already passed by us, and if emitted from further away, it hasn't reached us yet.  How far away that distance is depends very heavily on how one measures distance (what sort of coordinate system is used).

Quote
If it wasn't already this big then common sense would sugest that  its expansion rate (in order to explain the 13bn year 'lag') would be too high for gaseous material to coalesce into stars and solar systems.
The universe doesn't have an edge and thus a meaningful finite size.  Space has expanded some 40,000 times the distances that were between things back when the CMB was emitted.

Quote
So was the Universe already universe sized very early in its life?
That's the model, yes.  It's like dividing an unbounded size by 40,000, which isn't a meaningful thing to do since said size isn't a finite number.

I'm asking if there's a reason (yet) for this 13bn year delay in the cmb reaching our neck of the universe.
There's no delay.  It was always here, and will always be here, since the light is everywhere at all times.  We're just observing from a position where it has been 13 billion years since that light first flooded all space.
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 12/10/2019 20:06:20
I'm asking if there's a reason (yet) for this 13bn year delay in the cmb reaching our neck of the universe.
There's no delay.  It was always here, and will always be here, since the light is everywhere at all times.  We're just observing from a position where it has been 13 billion years since that light first flooded all space.

haha. OK. Enough questions I think. :)

Many thanks
Title: Re: What is the cosmic microwave background?
Post by: evan_au on 12/10/2019 23:38:20
Quote from: HelpMe929
why don't we see other radiation that must have been emitted before [and after] this 3000k period?
We don't see light emitted before this period because the plasma was opaque before this era.
- In a plasma (before this era), electrons are not attached to atoms, and so they can have every possible energy
- As electrons approach positive nuclei (or negative electrons), they are accelerated/decelerated by the electric field
- This produces "Braking Radiation" (with the German name "Bremsstrahlung") which can absorb and produce light of every possible energy level - but in thermal equilibrium, it will have a distinctive spectrum
https://en.wikipedia.org/wiki/Bremsstrahlung

In atoms (after this era), electrons can have specific energy levels, and thus produce a line spectrum
- Cosmology suggests that there was an even later phase (150 million to 1 billion years after the Big Bang), where hydrogen fusion started in early stars, which produced lots of UV light, causing hydrogen & helium to again become a plasma
- With larger telescopes, we are now able to see quasars that were active towards the end of this period in the early universe
- These have high red-shift (z=6 to 20), but not nearly as much as the big-bang radiation (z=1089)
- They do have many atoms beyond hydrogen and helium, since nuclear fusion produced them, and supernovas spread them into space
- Astronomers are hoping that the James Webb Space telescope will be able to see a lot more of these infra-red =high red-shift quasars which were active earlier in this reionization phase (if and when it is successfully launched & commissioned)
- But the universe was much less dense during the reionization phase than in the pre-3000K era, so we can still see the CMB at microwave frequencies through the infra-red haze of radiation from this later phase (and the visible-light of today's stars and galaxies).

See: https://en.wikipedia.org/wiki/Reionization
Title: Re: What is the cosmic microwave background?
Post by: yor_on on 13/10/2019 08:08:18
In a way it's a mystery HM. What we see is 13.8~ billion years away from us. That's the time of light existing. When it comes to the universe itself it may be infinite though. We have our 'bubble' of light and as we 'move' through the universe that 'bubble' still won't grow. One reason why there is no definite 'place' for the universe to start, although we still set a 'time' to it locally defined.
=

and yes, a good argument for the universe actually being 'infinite'.
=

Another thing is how we came to be, and how 'light' itself came to be. One idea that I like is the one of 'spontaneous pair productions' of particles. That one goes out from the vacuum itself having a ability to transform into particles. The vacuum has a time relation to what can be said to 'exist' as well as it can be under so called 'pressure'. If you imagine the original state of a universe as something under a tremendous 'pressure' as defined 'dimensionally' then this spontaneous pair production has a excellent chance to start. If you then also define 'c' as a 'universal constant' then that is the rate of 'information' connecting it to become our universe.  In such a case the universe didn't exist until the pair production started.
==

Sorry, forgot one most important part of that recipe :)

You need to add a 'inflation' at that origin, faster than the rate of information, which I hope we agree on being 'c'. Otherwise the pair production will just be a fluctuation recombining under a time limitation, unable to become 'real'. Then we just need those patches of 'space' to become united into a universe. And that is where this idea of 'information' helps us. If you think of quantum entanglements they are 'instantly connected' although outside 'c', but that is allowed due to their inability to transmit information, just as a spontaneous pair production is limited through a time relation (which I would define as Planck time). So we really need a inflation for it to work..

https://www.universetoday.com/79418/planck-time/
==

and yes, I think that would make the dimensions (universe) we define something constructed, non existing until information united it. The idea of 'pressure' creating 'pair productions' can be altered to 'energy densities' if you like, aka Hawking radiation.
Title: Re: What is the cosmic microwave background?
Post by: yor_on on 13/10/2019 15:01:48
https://en.wikipedia.org/wiki/Pair_production

" Pair production is invoked to predict the existence of hypothetical Hawking radiation. According to quantum mechanics, particle pairs are constantly appearing and disappearing as a quantum foam. In a region of strong gravitational tidal forces, the two particles in a pair may sometimes be wrenched apart before they have a chance to mutually annihilate. When this happens in the region around a black hole, one particle may escape while its antiparticle partner is captured by the black hole.

Pair production is also the mechanism behind the hypothesized pair-instability supernova type of stellar explosion, where pair production suddenly lowers the pressure inside a supergiant star, leading to a partial implosion, and then explosive thermonuclear burning. Supernova SN 2006gy is hypothesized to have been a pair production type supernova. "

One problem with those ideas is that we don't get a answer to what makes a 'photon'. Because 'photons' are what makes 'pair productions' exist. Then again, I don't know anything describing the idea of 'energy' better than just 'photons'. Then you also need to remember that all you see and experience is secondhand, as a result of outcomes. Photons are not footballs, you can't follow their propagation. I think Collin put it quite nicely in

So the question is this, is light an independent entity or is it somehow an observed effect of a larger mechanism.
It all depends what level you are talking about. Clearly, light is just a part of the overall electromagnetic spectrum, which is an effect of oscillating electric/magnetic fields. So you would say that light is an observed effect of the laws of electromagnetism. Remember, Einsteinís first paper on relativity dealt with a specific problem in electrodynamics (moving electromagnetic fields) that had been puzzling scientists; it was in that paper where he suggested light does not behave as if propagated in a medium so itís measured speed is not affected by the emitter or observer. This speed also comes from Maxwellís equations which are based on the findings of Amper, Faraday, Coulomb and Gauss - all leaders in electromagnetic theory.

==

Another difficulty with it is that if we imagine different patches uniting, then shouldn't there be a time difference?  Meaning that patches further away would have time to evolve? Then again, if we set a origin at null, would it matter? All patches would then be equivalent from some 'global perspective'?  But would they be it relative my local clock? Actually that difficulty is baked in into the theories we use normally too, if we define it relatively.
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 14/10/2019 05:46:19
In a way it's a mystery HM. What we see is 13.8~ billion years away from us. That's the time of light existing. When it comes to the universe itself it may be infinite though. We have our 'bubble' of light and as we 'move' through the universe that 'bubble' still won't grow. One reason why there is no definite 'place' for the universe to start, although we still set a 'time' to it locally defined.

If the universe (during the period of its CMB emition) was infinite, then wouldn't there be evidence of it in its red-shift? Wouldn't there be quite a large fluctuation in red-shift values for the CMB?

Is it possible to determine the size of the CMB universe from its red-shift fluctiaions? Assuming that it had a physically recognizable 3 dimensional shape...

and assuming that it has red-shift fluctuations.....................................
Title: Re: What is the cosmic microwave background?
Post by: evan_au on 14/10/2019 09:42:45
Quote from: HelpMe929
assuming that [CMBR] has red-shift fluctuations
There is one aspect of the CMBR which does represent a red shift; this "dipole anisotropy"  appears be due to the motion of our galaxy within the local cluster: 370 km/sec towards Leo.
See: https://en.wikipedia.org/wiki/Cosmic_microwave_background#Features

Apart from this, the CMBR does have variations, but they are very small - around 1 part in 100,000.
- This is a small variation on something which is pretty close to absolute zero ...
- A different red-shift would produce a black-body spectrum with an effective different temperature.
See map at: https://en.wikipedia.org/wiki/Cosmic_microwave_background#Microwave_background_observations

The variation in CMBR temperature is usually interpreted as differences in matter density in the early universe.
- By interpreting these spots and blotches, scientists are inferring many details about the early universe.
See: https://en.wikipedia.org/wiki/Cosmic_microwave_background#Primary_anisotropy

Roger Penrose has even controversially suggested that ring-like structures in the CMBR are relics of objects from before the Big Bang...
Listen:
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 14/10/2019 13:06:33
thanks for the video :)

It wasn't the easiest explanation to follow as he seems to keep drawing on a level of knowledge in his listeners that I don't have, and I seemed to miss a lot of patches in his theory. The death of one universe being the birth of another... with the implication of photons being the common denominator in the deaths and births of universes, and the evaporation of black-holes supplying the energy for the new births (eek).

But a really fascinating idea, and one which would seem to explain a high entropy universe evolving (or devolving) into a low "boring" one before its rebirth (low entropy because everything is so dispersed that there are no more configurations available).

But this is why I asked the question about red-shift fluctuations in the CMBR. Wouldn't a uniform red-shift indicate the CMBR was generated from just one very small area and time? There should be CMBR from the moment when space lost its opacity, and CMBR from the time when it stopped radiating and started 'forming'. Surely between these two periods the redshifts of the CMBR would have increased due to the intervening inflation? A uniform redshift suggests a sudden burst of energy from a flat surface!
Title: Re: What is the cosmic microwave background?
Post by: yor_on on 14/10/2019 14:17:13
I'm not sure. If you think of a origin as having no specific place even if we can set a time to it. Then it seems to have started 'everywhere' and if so a redshift/ blueshift should be due to different galaxies (stellar objects) relative motion versus each other, and inflation and its subsequent expansion. What strikes me in such a scenario is that there is a defined time zero for it all to happen, and that one we're pretty sure about as we constantly are in (relative) motion versus the rest of the universe, at least the way I think about it. If we use the skin of a expanding ballon then? I don't know, the ballon analogy is a halting one but if we used it we would need to think of it as a sphere as it seems to me, to keep a equivalent zero time? I might be wrong about this but that's how I think of a 'zero time being equivalent everywhere'. Wrinkles and a uneven form should be able to translate to a different time, ahem, sort of :)
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If we leave the balloon we could also imagine it as a sheet of a even tension as it seems to me. But it is pretty strange if we assume a infinite universe, isn't it? Then again, the size doesn't matter for it, it's still strange to me. The blue and redshift we  find have two reasons, one being between different objects (as suns) in relative motion versus each other, the other a (cosmic) redshift due to the inflationary period, and subsequent accelerating expansion. That one is a major effect and seems to be the same more or less everywhere on a large scale. There is one thing more that is strange and that is the idea of a vacuum expanding. How does it do it?
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You could also ask yourself how we would create CBR if we now could. It's as if the universe came to be in all 'spots' possible, and at a same time? And the same must go for the vacuums expansion, it comes also to be in all spots simultaneously? The problem with that , let's call it a question, is that simultaneity doesn't exist in relativity as far I know. But if I would be to assume that the vacuum expanded unevenly we should notice it from the CBR (cosmic background radiation).

And just for the fun of it. A expansion means that you're shrinking :) Relative the 'size' of the universe at least.
Title: Re: What is the cosmic microwave background?
Post by: Halc on 14/10/2019 14:37:27
But this is why I asked the question about red-shift fluctuations in the CMBR. Wouldn't a uniform red-shift indicate the CMBR was generated from just one very small area and time?
If it was generated from one small area in a small time, it would appear to us as a flashbulb on a camera going off at one random instant in time.  It would only be noticed if 1) There were humans around at the time, and 2) Somebody was looking in the exact correct place at the time.  So there would be no CMB.
No.  The CMB was emitted from everywhere, all at once, and as I said before, all of the CMB today comes to us from some identical distance.  If the light was emitted closer by, it has already passed by us, and if emitted from further away, it hasn't reached us yet. Being from an identical distance in all directions, it is uniform in all directions (or at least would be if we were comoving.  See the anisotropy that Evan discusses above), and forms the 'flat surface' you mention below.

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There should be CMBR from the moment when space lost its opacity, and CMBR from the time when it stopped radiating and started 'forming'. Surely between these two periods the redshifts of the CMBR would have increased due to the intervening inflation?
The two periods are quite close together, and the CMBR is not cleanly one frequency, but a narrow range, partially due to this brief bit of inflation that took place between these two periods.  It wasn't an instantaneous flash, but a brief one nonetheless.
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A uniform redshift suggests a sudden burst of energy from a flat surface!
That it does, and all points at some uniform distance from us is that flat surface, which actually appears as the inside of a sphere as viewed from the center.  It's curved, not flat, since light from a flat surface would get to us first from the point on that surface closest to us.
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 14/10/2019 15:32:02
If it was generated from one small area in a small time, it would appear to us as a flashbulb on a camera going off at one random instant in time.  It would only be noticed if 1) There were humans around at the time,

Quote from: Halc
The CMB was emitted from everywhere, all at once, and as I said before, all of the CMB today comes to us from some identical distance.  If the light was emitted closer by, it has already passed by us, and if emitted from further away, it hasn't reached us yet.

Quote from: wiki
According to the Big Bang model, the radiation from the sky we measure today comes from a spherical surface called the surface of last scattering. This represents the set of locations in space at which the decoupling event is estimated to have occurred[15] and at a point in time such that the photons from that distance have just reached observers

Sorry, but the above means I have to ask my question 'again'
Why the 13bn delay in the CMB reaching us?

It isn't like the big bang was an event that happened 'to somebody else'. We were there when it happened. We were in the middle of it. We've been part of the inflation, and it seems photons travelling at the speed of light from that place where we started from have only just caught us up... HOW?

I dont expect an answer because it seems everybody has a different 'explanation'.

Title: Re: What is the cosmic microwave background?
Post by: Halc on 14/10/2019 15:45:48
Sorry, but the above means I have to ask my question 'again'
Why the 13bn delay in the CMB reaching us?
There's no delay.  It is everywhere and has been here since it was formed and will be here for as long as you want.  It didn't just now 'turn on', so there's no delay to it.

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It isn't like the big bang was an event that happened 'to somebody else'.
No, but it happened everywhere, not in just some spot.  The CMB is not from the big bang.  It's from when the universe was a bit more than a third of a million years old.

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We were there when it happened. We were in the middle of it. We've been part of the inflation, and it seems photons travelling at the speed of light from that place where we started from have only just caught us up... HOW?
The CMB we see didn't come from 'here'. Similar light did, but the light that originated here is heading away from us, hitting perhaps some observer maybe 45 billion light years away (comoving distance).  So the light we see now is similarly coming from the birth of hydrogen atoms that are currently (comoving coordinates again) 45 BLY away.  It wasn't that far away when it was emitted.  That material is 'now' well outside our event horizon and any light emitted from there now will never ever reach us.
Title: Re: What is the cosmic microwave background?
Post by: yor_on on 14/10/2019 16:27:20
It's tricky. There are two ways to find out how old the universe might be. The most recommended is using the redshift we measure from the 'earliest' light reaching us, then extrapolate it in time, backing up its 'history'. Doing so the light that has been 'stretched' gains energy until it reach a point in where its energy is 'infinite', and that energy coincidence with the shrinking of our universe as we play the cosmic movie backwards. At that point we have a origin, at least for the most distant light we are able to find and measure. The other way is to use 'cosmic ladders' as astronomers have a good idea of what type of stars that existed at different stages of the universe's time evolution and then defining how far away they are from us. Those furthest away from us fits very well in age with the first method described. And defining how far those stars are from us is by using 'standard candles', an idea from https://www.famousscientists.org/henrietta-swan-leavitt/ at a time when only men was expected to invent new methods :)
=

Ok, 'infinite is a pretty strong word for it, before the light we can see it is supposed to have been a hot and dense plasma without radiation. There is one thing though, How do we guarantee that this redshift hasn't disappeared into oblivion? The only way you can be sure of that, as it seems to me, is by finding that there could be even more redshifted light that we would be able to measure, if it existed. That should be the answer to that one I think? (Not that the plasma couldn't radiate but as far as I got the idea it was more or less 'soaked up' before it could leave.)
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 14/10/2019 17:51:56
Halc, we're in agreement on this. So let me put it another way.

Imagine the post-opaque plasma universe from which the CMBR comes from.
For arguments sake it's close to the end of the plasma era.
Imagine you can locate the plasma which is destined to form our galaxy.
imagaine you can locate the same for the Andromeda galaxy.
For arguments sake lets say the two 'clumps' are ten light years apart.

There is another plasma clump 100 million light years away
A fourth clump 101 million LYs away

FREEZE TIME.
The andromeda plasma's photons will pass our plasma galaxy after 10 years (forget about inflation).
The 3rd and fourth clumps will go past us after 100 & 101 million years.

UNFREEZE TIME
add inflation
Andromeda's plasma photons go past ours galaxy's plasma after 10 * (inflation rate) years - or well before their stars even form.
group 3's photons take 13.4 BY to reach us (what we can detect right now)

logic says we wont see group 4's photons for another 1 million * (inflation rate) years (or apparently we can with just a bigger telescope  :o)
but if we were able to detect those photons they would have a longer red-shift than group 3's.

if we knew what the inflation rate was we'd be able to work backwards and know exactly how far group 3's plasma was away from our galaxy's plasma in the plasma usiverse, and have some idea of the size of the universe back then.

If we compared the red-shift difference between group3 & group4 photons we'd be able to say that group4 was 1 million lights more distant than group3 so we'd know the plasms universe was AT LEAST 1 million light years in diameter.

All I'm trying to find out is (haha) how big the early plasma universe must have been to make photons take 13bn years to reach us, modified of course by the inflation rate.

not a very good what-if perhaps, but I'm new to this.



Title: Re: What is the cosmic microwave background?
Post by: Halc on 14/10/2019 18:20:12
Halc, we're in agreement on this. So let me put it another way.

Imagine the post-opaque plasma universe from which the CMBR comes from.
For arguments sake it's close to the end of the plasma era.
Imagine you can locate the plasma which is destined to form our galaxy.
imagaine you can locate the same for the Andromeda galaxy.
For arguments sake lets say the two 'clumps' are ten light years apart.
Not a bad guess.  Given our current separation and inertial motion, it would be about 70 light years. In reality, it was probably further away than that since we're approaching each other, so in the past the separation was larger.  Our motion is not inertial, but accelerating.

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There is another plasma clump 100 million light years away
A fourth clump 101 million LYs away
And a 5th clump about 1.2 million LY away.

What is important here is how distance is measured.  We're probably using proper distance in a comoving coordinate system to be meaningful about these numbers.  That coordinate system has some nice properties like the entire universe going transparent pretty much at the same time, but speed of light is hardly constant in it.

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(forget about inflation).
We can simplify the model to a flat model where there is no expansion going on at all.  The universe is born infinite in size and that size doesn't change.  After a third of a million years, the universe goes transparent and all the light that exists suddenly has nothing to hit, and thus goes forever.  In that model, light speed is fixed at c, and light 13 billion years old and from 13 BLY away will get to us just now, and at full brightness to boot.  The entire night sky would by as bright as the sun due to CMB, and Earth would be burnt to a crisp.

It's actually a good model, except without expansion, there's no redshift so the light is its original wavelength and not microwaves.

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FREEZE TIME.
The andromeda plasma's photons will pass our plasma galaxy after 10 years
The 3rd and fourth clumps will go past us after 100 & 101 million years.
Exactly so.  Never mind my 5th clump, which is only important if there is expansion.

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UNFREEZE TIME
add inflation
Andromeda's plasma photons go past ours galaxy's plasma after 10 * (inflation rate) years - or well before their stars even form.
Not that simple since inflation rate is not a measure of time.  It's actually the inverse of time, or 1/sec.
But yes, it's close and the light goes by us at not much more than 10 years.

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group 3's photons take 13.4 BY to reach us (what we can detect right now)
No.  Group 3 was 100MLY distant from here after only 380,000 years, so it (and space there) is moving at 263x light speed.  After one year, that material is 263LY further away and light has only moved one LY away from it, so that light it is then 100,000,262 LY away from us.  It will never get here.  That material is beyond our event horizon and we will never see it.
That's what my 5th clump at 1.2MLY does.  That one moves at about 3c from here, and that's the light we're seeing just now.  The visible universe is just a bit beyond that material, but only visible via radiation other than light, which was blocked at first.  Gravitons for instance come from further away than the CMB.

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logic says we wont see group 4's photons for another 1 million * (inflation rate) years (or apparently we can with just a bigger telescope  :o)
A bigger telescope doesn't let you see light that isn't here yet.  You have to wait for it.

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but if we were able to detect those photons they would have a longer red-shift than group 3's.
When later photons are eventually observered, they will indeed have a longer wavelength than the current CMB.  It is always cooling in appearance, and the age of the universe can be computed from the current wavelength.

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if we knew what the inflation rate was we'd be able to work backwards and know exactly how far group 3's plasma was away from our galaxy's plasma in the plasma usiverse, and have some idea of the size of the universe back then.
Has nothing to do with the size back then since we are not looking at the edge of it.

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All I'm trying to find out is (haha) how big the early plasma universe must have been to make photons take 13bn years to reach us, modified of course by the inflation rate.
The universe was compressed by a factor of about 36000 back then compared to now.  Infinite / 36k is still infinite.
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 14/10/2019 19:34:18
oh...

so the universe was infinite even at only 380,000 years old? That's where I've been going wrong then   :-[
Title: Re: What is the cosmic microwave background?
Post by: Janus on 14/10/2019 21:41:03
oh...

so the universe was infinite even at only 380,000 years old? That's where I've been going wrong then   :-[
It could of been infinite.  We really don't know if the universe is infinite or finite. We are limited to the "observable universe", which, is roughly 93 million light years across at this point of time. This just means that the universe is at least this big.  How much further it extends beyond that, we just don't know. It could very well be infinite in extent.
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 15/10/2019 00:06:07
Well, I just don't understand it... If it is infinite then why all the to-do and palava about "We've calculated the mass of the entire universe and it's been found wanting, and in need of a shed-load more mass, which just happens to be invisible" (Dark Matter). It's this statement that inspired me to question what CMBR actually is and why somebody thought it suggested there wasn't enough of it (mass that is).

Is dark-matter another myth that's been blown out of proportion?
Title: Re: What is the cosmic microwave background?
Post by: Janus on 15/10/2019 01:38:04
Well, I just don't understand it... If it is infinite then why all the to-do and palava about "We've calculated the mass of the entire universe and it's been found wanting, and in need of a shed-load more mass, which just happens to be invisible" (Dark Matter). It's this statement that inspired me to question what CMBR actually is and why somebody thought it suggested there wasn't enough of it (mass that is).

Is dark-matter another myth that's been blown out of proportion?
We have looked at the observable universe how it behaves.  It behaves as if it contains a lot more mass than the mass we can see or detect via the electromagnetic spectrum.  So we have calculated the mass of the entire "observable universe", and found that visible mass only makes up a part of it.
On smaller scales, we look at galaxies. We can measure how fast the stars in them orbit.  But if we calculate how fast they they should orbit based on the matter we can see, they are orbiting too fast.  Not only that, but the way their orbital speeds change as you move out from the center of the galaxy isn't consistent with the way the matter we see is distributed. So for example with a typical spiral galaxy, instead of behaving like the matter is constrained to the visible disk shape, they behave as if the majority of the matter is spread out in a sphere which extends above and below the disk of the galaxy.
There are two possible explanations for this: Either there is something with mass there that we can't see, or our understanding of gravity is wrong.   Both possibilities have been examined and considered. But as more and more data comes in, the dark matter keeps gaining evidence and the other model loses it. For example, we've recently found some galaxies that behave as if they have little to no dark matter.  Now while is is perfectly reasonable for this to occur in the dark matter model, it is less reasonable that gravity itself behave differently in these galaxies then other.  If the rules governing gravity are different than those we think they are, they would still need to be consistent from galaxy to galaxy.
And It's not as if we don't already know of particles that behave similar to the way dark matter would. The neutrino is a massive particle, with gravity, which does not interact via the electromagnetic interaction. It is effectively invisible.  It is a WIMP (Weakly Interacting Massive Particle)*, which is one of the possible candidates for dark matter, In fact, one hypothesized type of neutrino( the Sterile Neutrino) has been suggested as a suspect.

*The other candidate is MACHOs or MAssive Compact Halo Objects
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 15/10/2019 10:31:03
That's an excellent explanation. Thanks very much Janus  :) 8)
Title: Re: What is the cosmic microwave background?
Post by: hamdani yusuf on 16/10/2019 07:20:03
The problem is that you are thinking of the Big Bang as an explosion in space, rather than the expansion of space.
With an explosion in space, the Photons could have raced past the matter into the space beyond. But with the Big Bang, there is no "space beyond" for the photon to expand into. We are at the "forefront" of this expansion.
An analogy would be that the universe is like the skin of an expanding soap bubble. Everything in it, photons, matter, etc. is confined to the skin. When the bubble is small everything is much more crowded together and as it expands, Things on a whole move apart and it is less crowded.  With photons, when it is small their frequencies are high ( the universe is "hot"), but as the bubble expands, they are stretched out to longer wavelengths and lower frequencies ( the universe "cools"). The CMB are those early photons that have been stretched out, but still confined to the same "Bubble skin" as everything else.
Is the expansion of space continuous or is it granular? Does the space between nucleus and electrons also expand proportionally? What about the space ocuppied by the nucleus and the electrons?
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 16/10/2019 09:58:44
How can something that's inifnite in size 'expand'?

I suppose it can expand because in an infinite void there's nothing to stop an infinite mass from expanding iniside it.

But wouldn't an infinite mass (or even 'effectively' inifinite) just crumple into itself from its own gravity? What stops it from doing this?

Gravity is a weak force, so expansion must be driven by a strong force. What strong force could that be?

Could any known strong force have an effect over an infinite (or effectively infinite) reach?

sorry - just musing out loud.

While on the topic of 'off the scale'.
Is 'infinite' a usefull measure of anything? Infinite means 'without limit'.
so if you have an inifinite universe expanding, then it implies an infinite void to expand into.
if you have in infinite void then there's nothing to stop you having an infinite number of infinite universes, all expanding, but not even over infinite time will any of them ever encounter each other.

however, could their overall gravity effects be pulling each other into expansion?
Title: Re: What is the cosmic microwave background?
Post by: Halc on 16/10/2019 12:47:57
How can something that's inifnite in size 'expand'?
The space between say galaxies will double in another 13.8 billion years.  That will effectively double the distance to any distant thing.  There is no number that represents the size of the universe, so there is no number there to double.

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I suppose it can expand because in an infinite void there's nothing to stop an infinite mass from expanding iniside it.
There's no void into which mass is expanding.  That would involve an edge to the universe with matter on one side and nothing on the other.  There would be a place with stars only in one hemisphere. The universe (the part with material) would have a finite size in such a model.

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But wouldn't an infinite mass (or even 'effectively' inifinite) just crumple into itself from its own gravity? What stops it from doing this?
Infinite mass would need to be within its own Schwarzschild radius to form a black hole.  Arguably, this is already true, and we're in a universal black hole.  The Schwarzschild radius of the visible universe is about the same as the radius of same.  Spooky.
On the other hand, the total mass of the universe has been posited to be zero (preventing violation of thermodynamic law at big-bang time).  The positive mass density of the particles is balanced by the negative energy density of gravitational potential.  Not sure of this myself because when I ran the numbers, the negative one was more than the positive one. I got a net negative total mass, but what do I know?  Maybe that negative mass explains the acceleration of expansion.

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Gravity is a weak force, so expansion must be driven by a strong force. What strong force could that be?
Expansion is inertial, so it requires zero force. Acceleration of the expansion on the other hand requires energy, hence the proposal of dark energy.

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While on the topic of 'off the scale'.
Is 'infinite' a usefull measure of anything? Infinite means 'without limit'.
That's right. It's quite a useful concept, but it isn't a number and it is a mistake to treat it as one.

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so if you have an inifinite universe expanding, then it implies an infinite void to expand into.
No it doesn't.  That's treating it as a number, a finite universe expanding into something larger.

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if you have in infinite void then there's nothing to stop you having an infinite number of infinite universes, all expanding, but not even over infinite time will any of them ever encounter each other.
They would be all finite universes in this infinite void, and yes, I suppose they'd be capable of collisions as their edges meet.  An infinite universe has no edge and cannot thus meet something 'coming the other way'.
Title: Re: What is the cosmic microwave background?
Post by: HelpMe929 on 16/10/2019 15:52:45
Thanks Halc :)

I love the idea that we're already inside a black hole. omg!  :'(

But inertia means something has set it in motion after one big push - like in a 'newton's cradle' toy (or one big bang), which doesn't fit well for an infinite universe (or does it?)  Also, going by your own reasoning 'it can't be infinite if it has an edge', aren't you then saying that the universe is finite because it is expanding? How can something that's infinite get 'more' infinite? (notice I didn't use any numbers there :) ) Even the inflating sphere analogy is a finite one because you could conceivably travel around it and get back to your starting point.

I think everything becomes simpler by using the phrase 'effectively infinite' rather than just 'infinite'. But are there any major implications for anything if the universe is one or the other?

(runs off to look up 'dark energy')

wow! Now I understand why the military rely on laser rangefinders rather than using Trig. But wwii Submarine commanders used it successfully to sink ships so trig can't be that inaccurate. On the other hand, I thought red-shift was originally calibrated by linking it to the body's observed distance anyway. I thought redshift only supplied a body's speed-of-retreat rather than its distance, and distance was achieved by calibrating it with known-distance bodies. Could it simply be that the early calibration of red-shifts were less accurate than today's methods?

You guys must have the patience of saints is all I can say :)
Title: Re: What is the cosmic microwave background?
Post by: Halc on 16/10/2019 20:53:01
Thanks Halc :)

I love the idea that we're already inside a black hole. omg!  :'(

But inertia means something has set it in motion after one big push - like in a 'newton's cradle' toy (or one big bang), which doesn't fit well for an infinite universe (or does it?)
A big push is only needed if there was mass that was in need of acceleration from some different velocity, but the BB model has expansion already there and symmentrical right from the start.  No force.
So the mathematics seems to be something like zero mass at first (and hence no force needed to 'accelerate' it), but positive energy formed positive mass as it was balanced by the negative energy of the stretching and negative gravitational potential energy.  Something like that.  I'm no expert.  Where the force (big push) and energy is needed is in the acceleration of the expansion, and dark energy is one attempt at explaining this.

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Also, going by your own reasoning 'it can't be infinite if it has an edge', aren't you then saying that the universe is finite because it is expanding?
No.  Not finite because it doesn't appear to have an edge.  Are you familiar with the balloon analogy?  That explains expanding space without it having an edge, but it is a local analogy only since few models have space curving back on itself like that.

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How can something that's infinite get 'more' infinite? (notice I didn't use any numbers there :) ) Even the inflating sphere analogy is a finite one because you could conceivably travel around it and get back to your starting point.
OK, you know about the analogy, but as I said, it is a local analogy, not a viable model of the universe.  The point is, the mathematics describes what we see, and the mathematics doesn't really describe the size of the universe.  By infinite, it just means that no size or edge is posited.  Almost all matter is comoving, which means it is pretty close to being stationary relative to all the material in the universe visible from its location.  Our solar system (more stationary than typical) is moving at about 0.0002c relative to its local mean.  A similar figure for almost anything else, which leads to a mathematical model where distant stuff is not moving away from us quickly, but just the space between us and it is increasing at some distance-proportional rate.

One could use an inertial coordinate system instead and then the universe becomes nicely finite in size, with an edge and all that.  Part of the misconceptions involved is the fact that the expansion of space is rarely described in inertial terms, but rather some form of comoving coordinates which are far more natural for very large descriptions, but leads to questions like you're asking.

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I think everything becomes simpler by using the phrase 'effectively infinite' rather than just 'infinite'. But are there any major implications for anything if the universe is one or the other?
I think the word 'effectively' means that there are no implications one way or another.  It means that if it is finite, the edge is far enough away to not have to work it in the model.  It is effectively infinite.
The inertial model has a very finite size (a frame dependent one) and an easily computable edge.  I've put out a post about that, but have thus far not managed to convey the event horizon or dark energy into the model, so it isn't very practical.  It is a model of expansion without dark energy, which is not entirely accurate.  Without dark energy, there is no event horizon and hence no star so distant that its light will not eventually get here, or at least what's left of 'here' at that time.

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(runs off to look up 'dark energy')

wow! Now I understand why the military rely on laser rangefinders rather than using Trig. But wwii Submarine commanders used it successfully to sink ships so trig can't be that inaccurate.
No idea why dark energy is related to military range finders.  Sinking a ship from a sub is an exercise in 2D (minus 1) space.  Sinking a ship with a shell from a gun is a 3D (minus 1) exercise and requires more precision.

On the other hand, I thought red-shift was originally calibrated by linking it to the body's observed distance anyway.[/quote]How was distance observed?  And red shift has to do with relative velocity and not so much distance, unless its really large distance.

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I thought redshift only supplied a body's speed-of-retreat rather than its distance
That's true, but speed of retreat is roughly proportional to distance if distance is really big.
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and distance was achieved by calibrating it with known-distance bodies.
Again correct. You need a 2nd way of measuring large distances, and this was done by observing the brightness of Type-A supernovas which have a precise brightness.  They're all identical, so they make excellent measures of distance.
Title: Re: What is the cosmic microwave background?
Post by: hamdani yusuf on 18/10/2019 04:36:11
The space between say galaxies will double in another 13.8 billion years.  That will effectively double the distance to any distant thing.  There is no number that represents the size of the universe, so there is no number there to double.
What about the space between center of a galaxy and its outerskirt? Is it also expanding?
Title: Re: What is the cosmic microwave background?
Post by: hamdani yusuf on 18/10/2019 04:40:16
That's true, but speed of retreat is roughly proportional to distance if distance is really big.
That's correct, if the assumption that space is expanding is correct.
Title: Re: What is the cosmic microwave background?
Post by: Halc on 18/10/2019 12:30:40
What about the space between center of a galaxy and its outerskirt? Is it also expanding?
Yes it is, but gravity pulls the outerskirt back into equilibrium.  This would not be the case if expansion accelerates to a rate larger than what gravity can pull back.

So for instance, the Earth orbits at pretty much the same radius all its life despite a ~30% expansion of space since it was born.  The moon is moving away, but that's not because of expansion.