What i have a hard time visualizing is the thought of symmetry in an infinitely expanding universe. If space between galaxies is infinitely expanding in all directions wouldn't that scatter the photons as they travel through space time? Not necessarily as in refract/reflect but create sort of a density effect where the density of incoming photons is so low because of the expanded (expanding) universe between them that we wouldn't see galaxies as we do?

just imagine the power of the light sources..thats very powerful..altho technically i cant give you a answer that makes sense....and maybe there's more to it than just density effects

just an opinion

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it shows you how much i know....not much i guess----"sohhfly"

For reasons to become apparent, let me interrupt here to say that many people ask how the Hubble expansion can fail to have a "central point" from which the expansion occurs.

Even in Newtonian dynamics, one can use transparencies to show that a dilation of form z -> exp(t) z is translation invariant (all points are equivalent).

But there's still a problem: as Olber noticed, if you use Newtonian theory to compute the intensity of starlight, assuming a homogeneous universe, the entire night sky should be as bright as the surface of the Sun. Obviously, it isn't. In Olber's time this was a profound mystery. With the advent of the Hot Big Bang theory, formulated gtr, it became apparent that repeating Olber's computation using a cosmological model such as an FRW dust solution of the EFE (the field equation of gtr), the expansion of the universe explains why the intensity of the night sky is much smaller than expected.

You are going to the opposite extreme and asking, I think, why the intensity is not below the threshold of measurement. One answer would be to say that when you crunch the numbers, it turns out not to be (in agreement with nightime experience for rural citizens). Another would be to say that in cosmological models featuring a sufficiently rapidly expanding universe, the intensity does indeed vanish in the limit as t -> infty.

Suggested reading:

• Olber's Paradox from Scott Chase, part of the UsetNet Physics FAQ, from John Baez (Mathematics, U.C. Riverside) and many others,
• Olber's Paradox, part of the Hyperphysics, website from Dept. of Physics & Astronomy, Georgia State University,
• Cosmology Tutorial from Ned Wright (Astronomy, UCLA),
• Cosmology FAQ from Ned Wright,
• 1995 Briefing from the National Academy of Science (not entirely out of date)
• Steven Weinberg, The First Three Minutes (same comment).

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Chris Hillman

Read these PF posts. Avoid Wikipedia--- except for these versions. Read this and this suggested sticky. When asked for advice, I always say: never take advice!

Space is not infinitely expanding. It is just expanding and suggests that the expansion might not stop. Regardless, the inverse square law does not change with spatial expansion as much as the speeds of distant galaxies" recessions from us are not measuring the displacement of those galaxies through space but are measuring the accumulation of all the small differential spatial expansions taking place between us and those distant galaxies we are observing. If newer dimensions were being created by spatial expansion then one would be tempted to expect a violation to the inverse square law and the density of photons would decrease in such a manner to reflect that.

Sounds like you are wandering into the domain of ATM here

Bryonm: very roughly speaking, the inverse square law (the central idea in Newton's non-relativistic theory of gravitation) and curved spacetime (the central idea in Einstein's relativistic theory of gravitation) don't mix, and blueshift's other suggestion appears to be based upon a misunderstanding about current speculations involving superstring theory.

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Chris Hillman

Read these PF posts. Avoid Wikipedia--- except for these versions. Read this and this suggested sticky. When asked for advice, I always say: never take advice!

Maybe I don’t understand your question correctly, but it seems to me that the density of incoming photons is quite low. That’s why the night sky is mostly dark.

Suppose the galaxies were not moving apart, then we should be able to see a whole lot of large bright nearby galaxies in a very bright sky filled with stars and nearby galaxies.

But if the galaxies are moving apart, which they seem to be doing, and if they've been doing so for 14 billion years, then what we now see are a lot of very tiny-looking distant galaxies that are a long way away from us, and our local “photon density” at night is quite low.

If you think of a bunch of light bulbs near you, and then move some half a mile away, and others a mile away, and others 2 miles away, etc. They are still just as big and bright if someone is near each bulb, but if someone is a long distance away from each one, then they appear to be tiny and dim, and the local photon density at that viewer is quite low to a person who is a long way from the light bulbs.

The radiation and shear power of galaxies is astounding, i don't doubt the ability to receive the light over great distances. Its amazing, mind boggling, scary and beautiful at the same time when you think of the sheer forces.

I guess in a way its fascinating that there is so much harmony that survives the chaos of an expanding universe and that the expansive energy of the universe may not just be the antonym of gravity in a sense but the antonym of relativity itself. Allowing the paradigm of harmony to survive chaos of infinite expansion hehe

You see the issue i have with this scenario is that space is finite and your moving the lights away from you - its a change of your relative position and perspective and not an increase of space between you and the source. The amount of photons released occupy the same volume and only the perspective of that volume changes.

If space is increasing then the photons should be relative to the increasing space and diffuse according to the lower density per volume.


What you describe is a sprawling universe but as i understand it the current concept is an inflating universe where space is being created between everything and not that everything is moving. So its not only a change in perspective but a change in volume thus altering the perspective even more as the volume increases and the density per volume decreases.

Yes, it's amazing.

I'm thinking of them in a visual sense. The antonym of hot is cold so the antonym of gravity is anti-gravity. Since i can't prove anti-gravity i'm using English to describe how i would visualize something.

Uhh, doh, someone else will have to answer those questions. A more full understanding of the complexities you are talking about might require a full 8-years of university courses in astro-physics.

Honestly it was that Cosmology Faq that got me thinking about this. The author seemed so biased to spend so much effort into debunking other theories yet not admit a single flaw of his own beliefs.

I'm not saying his beliefs are flawed either

sorry if this is "ATM"..


I still just wonder how you would visualize an expanding/inflating universe where not only the perspective changed but the volume/density of space changed as well and not because of the force of gravity but because of the forces of a yet to be determined energy that appears to have no relativity or bearing on the light we see.

Is that better?

hehehe.. i'm trying to do that in 8 weeks i guess

really, its a question. i'm not here to debunk anything but if someone knows why the volume of space can increase but the density of photons in that space doesn't appear to be relative i'm all ears! And i guess when i say density I'm thinking as photons as being particles that are emitted out to a mass volume of empty space and we only receive a finite amount of those particles and if you increase the volume by which those particles must travel then wouldn't that "Density" be lower or more diffused in an expanding universe?

i should put in my sig that i'm a computer nerd by day and just cherry picking my limited concepts of cosmology to try and understand them.

Bryonm.
The density of the first photons from the initial time when the universe became transparent has decreased as the volume has increased. That is called the CMB.

The density of the photons that were emitted shortly after that has also decreased as the universe increased...just not by so much as the CMB ones.

The density of the photons that were emitted shortly after that has also decreased as the universe increased...just a little bit less the the previous ones.

so what does this say about the amount of photons we can seen today?
Not much.

What you are running up against is our inability to imagine an expanding four-dimensional space instead of an expanding 3-dimensional ball of matter. That is why the best way to understand it is with maths, since we can always understand that one number is bigger or smaller than another number.

ETA: Its 5:30am and I've been working all night so don't expect any math from me!

Math definitely is a universal language, but I'm a visual person

I'm going to push myself to understand the math though, i'm thrilled to learn everything i can.

Well I'm not sure that space is "infinitely" expanding, and I'm not sure that the space between galaxies is expanding in all directions. If galaxies that were, say, one billion light years apart are now two billion light years apart, the space has expanded between those galaxies.

If you are considering two galaxies and imagine the space in between them to have expanded, how can you say it expanded in "all directions"? You can only say it expanded between those two galaxies, along an imaginary line that joins those two galaxies. Now I know what you mean by expanding in all directions, but this might be misleading, as that expansion is happening over time and time seems to only move in one direction!

In fact, the whole idea that "space expands" might be a misnomer. All we really think we know is that distances increase but we don't understand the mechanism behind this increase in distance. It is often described in the following terms:

The "background" metric that defines distance has changed over time.

This paper - Expanding Space: the Root of all Evil? explains the dangers of attributing space with physical properties that are not consistent with General Relativity.

Excellent. Very good. Thanks for the link to that paper.

Here is a 2001 Davis-Lineweaver paper you might also want to read:

http://arxiv.org/PS_cache/astro-ph/p.../0011070v2.pdf

I understand that the Genius that is Einstein theories is the concept of it being a thought process and not necessarily a visual or testable experiment in a lab, so its asking a lot to visualize GR itself. However when you use GR to explain the visible universe in theory you should be able to translate that to a visualization on a controlled environment such as a computer simulation.

Interesting read for sure.

"A recent example of the dangers of thinking
of expanding space as a real physical theory is contained
in Table 2 of Lieu (2007) in which the expansion
of space is lumped together with the Big Bang,
Dark Energy, Dark Matter and Inflation as a physical
theory demanding verification."

What does that mean?

I'm guessing this is comparing one idea of inflation vs the expansion that happens in GR if you remove the cosmological constant. Are we mixing multiple ideas of expansion or inflation here? (or am i mixing these??)

more reading to do

Theories are not hypotheses or thought processes that are going on. In order for something to graduate to theory it must have validity in the lab. When we speak of the theory of flying in physics we are describing how flight occurs and we can come up with predictions from our theory as to what will hinder flying and what will make it work better when we design a new jet. All the new jets fly. Whether or not they perform as predicted (better mileage, higher speeds, etc) can be a hypotheses but the theory and act of flying is not.

GR has been verified by gravitational lensing, the delays of pulses from binary neutron stars precisely matching Einstein's predictions and the accuracy of GPS systems along with the precession of the planets' orbits. You can visualize GR by just watching water droplets fall from the top of apartment buildings in your neighborhood and measuring the rate in which any two raindrops separate on the fall...the rate is 9.8 m/sec^2. By measuring the rate at which objects separate from one another in any free fall and measuring how much that rate changes gives an indication of the size of the mass the two objects are falling toward.

There are many computer simulations involving the collisions of galaxies and those simulations do take GR into account. As I recollect, the Grape 6 supercomputer makes 64 trillion calculations per second and takes the equations of GR into account.

Lieu’s paper is here, see Table 2:
http://arxiv.org/PS_cache/arxiv/pdf/...705.2462v1.pdf

It seems to me that Lieu is saying that each “Phenomenon” has a suggested “Explanation”, but none of the explanations are currently provable. He says that cosmologists only know how to use “unknowns” to explain “unknowns”. So it seems that some of the stuff we are often told are true “facts” of cosmology, might not be true facts at all but might just be guesses and hypotheses, which in themselves have no logical explanations. We are supposed to accept them while nobody can explain how they are supposed to work.

This is a complaint some people have had for years about the "expanding space" term. It is basically meaningless. What's happening is that the galaxies appear to be moving radially away from each other. Francis, Luke, Barnes, Berian, James, and Lewis don’t like to say the galaxies are “moving” so they call it “an increase in distance over time”. Well, most people call that “moving”.

The galaxies were said to be “moving” for 60 years. Slipher in 1912-1915 discovered their redshifts and he said the galaxies were “moving”. He gave some of their radial velocities in this paper in 1915 (scroll down to the bottom of this page):
http://articles.adsabs.harvard.edu/f...A.....23...21S

In the 1930s and ‘40s Hubble said they were moving. Eddington said they were moving. Here’s a page from Eddington’s 1933 book in which he gives some of their speeds based on their Doppler redshifts:

http://i28.tinypic.com/4i0w2a.jpg

Einstein in 1932 gave their approximate radial speeds, based on a “Hubble constant” that related their speeds to their distances away from the earth (see the top of the right column in this link):

http://i29.tinypic.com/ih8lee.jpg

In a 1934 paper, Wilhelm de Sitter expressed the galaxy redshift/motion situation this way:

”One still often meets with an expression of doubt by careful and scrupulous authors whether it is permissible to interpret the observed redshift as a ‘real velocity’. The observed fact is that z in (equation 12) is different from zero and positive. It follows that y is not a constant, and therefore according to (equation 9) dr’/dt is positive, and this velocity of recession has the same claim to reality as any other based on Doppler’s principle.”

http://articles.adsabs.harvard.edu/c...IF&classic=YES

Lets not confuse things here more than they are. Theory does in fact have multiple distinct meanings. You can have a theory such as the one you describe where you can test things through experiment and observation and then you can have theory where you hypothesize that something happens for one reason or another. An hypothesis can even predict things that may come true so that's where they're more alike than not, sort of an unproven but educated guess (at least how i feel about it.. may not be the correct scientific distinction..)

For example the big bang requires the expansion of space which in a way is distinct from the concept of expanding space per the paper quoted above because the expansion of space is critical to cooling down the CMBR and thus used to justify the theory of the big bang as "provable". So is big bang a hypothetical theory or do people consider it a "Theory" theory because they can observe testable components of such theory such as the CMBR and the redshift in galaxies?

That is exactly why i LOVE GR and i love Einstein - he conceptualized the visual world with math that somehow was able properly predict the motions of mass in our cosmos.

Right - all of these simulations relate to our observable universe. None of which were taking on a scale of applying GR to these galaxies and then GR to the universe that contains these galaxies to see if indeed there is some "expansion". Perhaps its still beyond our realm of computational abilities but somehow i doubt that, we're getting good at pulling off computational miracles these days

Found some cool notes on these simulations here: http://www.ifa.hawaii.edu/research/galaxies.htm

Question regarding expansion, don't we use the redshift/blueshift to also predict collisions of galaxies? Would there not be a great "Zig-zag" expansion in the universe vs the nice & smooth liner expansion that is currently shown? Is our local group just that unique in the universe that it has overcome what everything else is succumbing to? Or is the linear progression of expanding universe based entirely upon galaxy clusters and super clusters themselves?

Most of what you are saying here sounds good but times have changed and the quantum universe and our particle accelerators have changed that and our outlook at mathematics. The number of mathematical calculations that predicted that particles of a given mass were going to reveal themselves directly or indirectly has been enormous over the last century. Mathematicians never see theory as a hypothesis or postulate or axiom. They always distinguish between them. In fact, it was mathematicians that have accused physicists of mathematical ad hoc practices. While a mathematician adheres to rigid procedures like two column proofs and verifies new theories only being proven if its components consist of previously proven theories, physicists like Feynman don't always abide. He comes along and takes ancient Greek math, the physical features of old hand clocks and allows probability to grow or shrink by the size of those vector-like hands and then makes rules concerning adding angles to add time,etc, ( students even need an entire semester to do one problem!) Yet, in the end, that butchered up procedure can calculate the magnetic moment of an electron accurately to match lab results right to 12 decimal places. Trouble is that such a procedure doesn't fit anything else.Here you might be venturing a touch off of topic. (There has been other evidence that suggests expansion and a PM can clear that up later if you like. Just ask.) The thread was focusing on photon density in an expanding universe and really no one is arguing with you over that. In optics the angular area of a DSO does shrink with distance but it never disappears. The reason we do not see it all is because out equipment cannot resolve it all. As equipment advances, we see more. The waves associated with photons spread more with expansion. Wave density does not change per given area.Gravity is winning locally. You and I do fall to the ground and Andromeda is heading our way. It is between the local groups that the expansion is seen. It exists locally but is too weak compared to local gravity. A similar comparison can be made as to why so many objects like stars and planets seem round in our telescopes while asteroids and people and plants look all twisted into various configurations. Gravitational forces do exist between our bodies and all the parts within them and they do within asteroids and plants. But the electrical forces are greater locally and so we have shapes dictated mostly by electrical forces that are only slightly influenced by the much weaker gravitational forces. If you ate the whole planet gravitation would exceed the electrical forces within your body. Your shape would be round.

The same line of reasoning applies to local spatial expansion. It is too weak locally to notice. The rate per cm is likely on some order of 10 raised to the minus 640 trillion power (my exaggeration). You cannot measure any effect on such a small rate until you take into acccount enough space and that adds up between the local clusters in addition to what is in them.

Thanks for the link. I have most of the later Davis-Lineweaver papers on my hard drive already and they revisit these concepts, but it is nice to see the original.

I think their most useful paper is Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe.


Ahh but that is the key issue. The galaxies (or more correctly the clusters of galaxies) are considered to be at rest relative to the "Hubble Flow". The Hubble Flow represents the increase in distance between those clusters. This type of "moving" is not thought to be the same type of movement as is observed between galaxies that are gravitationally bound to each other - i.e. it is not inertial movement and as such is not subject to the same constraints as inertial movement which is how distant galaxies can have apparently superluminal recession velocities.


Gravity binds galaxies into clusters and within those clusters the galaxies swirl around, sometimes hitting each other. These clusters are receding from their neighbouring clusters due to the expansion of the universe and the more distant a cluster is from another, the faster its apparent recession velocity.

Outside of our cluster, all galaxies are redshifted as far as I know.

So to answer up, CMBR infers a physical expansion to describe the relatively cool and less dense radiation, otherwise inferring the time of the big bang would have a hotter CMBR. Correct?

Knowing that CMBR is just a different wavelength of electromagnetic radiation why would a microwave background be the only measurement? Would there not be similar or linear (depending on the energies released) backgrounds of gamma/x-rays and visible light spectrums as another CXBR or CGBR or CVBR?

Otherwise if you don't infer a physical expansion of the universe then you could conclude that the CMBR is rather what - 30-40+ billion years old?

Well, it's not just an inference. The CMBR was also hotter in a region of space 6 billion lightyears away (younger), for example. This has been observed by measuring the temperature (atomic transitions) of such distant gas clouds. After accounting for local effects, such clouds are still undergoing more transitions than similar local clouds, and that's because the CMBR in that region is hotter and more energetic.

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Everyone is entitled to his own opinion, but not his own facts.

I'm no expert but that sounds like the lyman-alpha forest density/temperature measurement and all the redshift craziness that comes out from that. Now you're really messing with my head

No, those measurements come from the study of molecular clouds. Cyanogen gas has many quantum states that it can leap up to and down from due to temperature surroundings. When its nuclei tumble in one of those states, it can give off a family of radio frequencies that radio spectrographs can read. In a lab cyanogen only does that when it is surrounded by a specific temperature. Cyanogen has other states where the nuclei spring apart from each other slightly and another where the electrons spring apart from the nuclei. Still another has those electronic transitions acting like a pogo stick, leaping from one particular state to another at a specific frequency. Each of those states reveals the surrounding temperature. In the far ultra-violet region Lennox Cowie made observations with FUSE (Far Ultra-violet Spectograph Explorer) looking at distant cyanogen gases in a state that revealed the CMB to be at 5.4 kelvins at a redshift of 2.2, in agreement with what Cougar was posting.

Oddly enough the only thing i can find out Cyanogen is a thread where you talk about it previously and mention studies referenced by Alex. (and how difficult it is post 911 to find this research).

Still, i guess i'm curious at how you would visualize a physically expanding universe.

After further research there appears to actually be x-ray, gamma ray and other variants of cosmic background radiations.. still begs the question of why these would be considered "background" but have absolutely no visible energies in this background.

But i guess that just leads me further down my quest of research

Excellent clarification, blueshift.

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Everyone is entitled to his own opinion, but not his own facts.