Stuff I always wondered about astrophysics

[FayeKane]

I've numbered them so you can just include it in your reply instead of screwing around with

tags.STELLAR IGNITION AND LIFE

1 -- How long it takes for a protostar to ignite, once hydrogen ignition begins? For a million-mile wide star, it can't be less than two seconds (distance to center at c), but is it on that scale, or hours, or what?

2 -- If you're outside the thing, will it take decades for the photons to work their way to the surface as happens in fully switched-on stars? Does the event look like a sudden flash, or do more and more photons leak out and it gets brighter slowly?

3 -- Probably a dumb question and I'm too lazy to renumber the rest.

4 -- Do neutron stars support fusion? If not, why can we see them? Is it just blackbody radiation? Is it synchrotron radiation from the rotating magnetic field? Is the emission spectrum of neutron stars radically different than normal stars?DEATH

5 -- Is a supernova just an ordinary-but-large nova which finally destroys the star, or is there a different mechanism going on in supernovae?

6 -- When they finally burn out and nova, I doubt that stars become completely dark all of a sudden because the photons have to find their way through all that hydrogen. So what does the light curve look like between when the star dies, and when it collapses enough to retrigger fusion? Is the temporary decline in brightness even detectable?

7 -- For a million-mile-wide star, when it finally goes out, how long does it take for the star to fall into the center before it rebounds as a nova?

8 -- What fraction of its original diameter does a star collapse to before it rebounds as a nova? Is it the same for supernovae?

9 -- Is the fractional diameter before exploding the same for a supernova, or do supernovae crush much much smaller before they're totally destroyed?

10 -- If a star cannot support fusion before going supernova, what causes the bright flash? Does fusion restart, but too late to stop the collapse?

When a supernova "goes black hole", the whole star does NOT collapse into it. That surprises me because most of the star is under huge inward pressure and there's something in the middle pulling it in even harder than the weight from above is pushing. SO:

11 -- For how many Swartzchild radii does material get sucked into the hole, as opposed to being blasted into space? How far in miles, and what fraction of the supernova is that?

12 -- Does any of the former star remain orbiting the black hole, or is it all blasted away?

13 -- What is the typical rotation rate for a black hole? It must be, uh... astronomical. Does this affect the Swartzchild radius in the same way the the Earth is oblate at the equator? What's the radius of a typical black hole, anyway?

14 -- Does stuff being pulled into the hole contribute significantly to the brightness of a supernova?

15 -- Are heavy supernovae a lot brighter than ones originating in stars which do not form black holes due to this?

16 -- Do atomic nuclei exist in a neutron star, or is it just a mass of pure neutrons? (That one may be a stupid question).

17 -- Is there a name for the process of electrons and protons being squeezed so tightly that they form neutrons?

18 -- If neutrons stars are pure neutrons, where do the photons in its blackbody radiation come from, and what does that do to the neutrons?

19 -- How much wider is a neutron star at the equator than the poles, due to rotation?

20 -- I think I remember that pulsars slow down due to the drag of their magnetic field on the interstellar medium. Is that correct, and if so, why doesn't the rotational axis eventually become the magnetic axis due to this force? Do we see this happening? Could we even see it? I guess the evidence would be a pulsar dimming, but that's surely happening anyway.ELEMENT FRACTION AND DISTRIBUTION

21 -- Do heavy elements fall to the center of stars like in planets, or is element weight irrelevant in the middle of a nuclear explosion? Does layering occur outside the fusion core?

22 -- What percent metals (i.e., > H) can a star can be and still support continuous fusion?

23 -- What percent metals finally disallows fusion restart, and a supernova occurs?

24-- What percent of a supernova remnant becomes metals? How many star populations can there be before the hydrogen density is too low to support ignition of another star? When that happens, what will condense out in stellar nurseries, huge planets?

25 -- How wide can a non-fusionable mass get from pure accretion before it collapses into a neutron star just due to gravitational pressure at the center (with no supernova)? Will that EVER happen, or does the rate of increase of the radius due to new material eventually exceed the rate at of core pressure increase, since the new stuff is so far away from the center? Will a continually-accreting neutron star ever collapse into a black hole for this reason?GALAXIES AND GALACTIC NUCLEI

Since college, I haven't kept up like I should have.

26 -- Did "active galactic nuclei" and quasars turn out to both be the same thing?

27 -- Is there any difference between galaxies with an extremely bright center object and normal galaxies? That is, is there a continuum of galactic nucleus activity, or are there discontinuous classes of galactic centers?

28 -- Why are there no nearby quasars (or are there?)

29 -- Does the black hole at the center fa a galaxy run out of stuff to fall into it and (presumably slowly) become dark?

30 -- What is the eventual fate of very old galaxies? Do they all either become torn apart by close encounters, merge, or wrap themselves into an elliptical? Do very old ellipticals become spherical and just sit there forever?

31 -- Are there any galaxies we know of which have entirely burned out, or does the material of dead stars ALWAYS form new ones?

32 -- How can the leftovers of supernovae condense into a viable star when there wasn't enough hydrogen fraction there to support fusion when it was a star the last time? Is there some process that isolates hydrogen in supernova remnants? Does free interstellar hydrogen dilute the metals in supernova remnants enough to "replenishes" the fusion capability?

33 -- Eventually, will the metals fraction in old galaxies become too high to support any stars at all, and if so, how long will that be? How many star populations? Or is there some process that breaks heavy atoms apart into hydrogen again? (probably thermodynamically impossible at a large scale)

34 -- Where does a galaxy's angular momentum come from? That is, why did the gas and dust it condensed from have an angular momentum? I would think the atoms and molecules are bouncing around randomly. Is it due to angular momentum in the quantum fluctuations during the big bang? Is there net angular momentum in a pressurized cylinder of air? Do galaxy clusters have net angular momentum, or just galaxies?TOTALLY IRRELEVANT

35 -- In Clarke's 2010, the crappy sequel to 2001, Jupiter ignited because more monoliths were placed on it until its mass was sufficient for fusion. Is that impossible since the monoliths were not made of hydrogen and would sink to the center, with the hydrogen rising to the top where it's not dense enough to ignite no matter how many monoliths you add?THANK YOU,

-- Faye Kane, padwan among Jedi

PS:
You know you're old when the "25 and 50 years ago in Sky and Telescope" column is about articles you've already read. It happened to me with the "25" ones. There ought to be a small celebration among his peers when this happens to an astronomer--although it's scary, like when you're little and you see a dead mouse in the fall and you come back in the spring and the mouse is gone

 

[bombadil]

FayeKane,

I'm overwhelmed! Come back with your top 1-3 questions, and maybe we can explore them a few at a time.

-bombadil

 

[mgb_phys]

STELLAR IGNITION AND LIFE

Fusion doesn't occur throughout a star, it happens only in the core or in a thin shell around the core. The rest of the star heats up to a few 1000 deg.

It takes a lot longer than a few decades for photons to make their way out of star - for the sun it's around 30-40,000 years

Neutron stars don't fuse anything. We detect them by the radio jets from their magnetic field or occasionally from X rays produced by material in their solar system crashing into the surface.
They are blackbodies and are created at around 1M degrees, cooling by radiation fairly slowly.

 

[turbo]

To follow on with mgb_phys, when stars are born in dust-clouds like we see in the Orion complex (M42) it not only takes a very long time for the proto-star to develop, but the star continues to accrete from the dust and grow, until the star is so potent that the radiation pressure from the star becomes strong enough to push away (evacuate) the dust and gases from its environs and become visible to us.

Stars forming in such environs can be more easily detected in the longer wavelengths (deep infra-red), but their visibility in (human) visible light is limited by local systematics. SDSS's survey has been studied in great detail and there are probably a lot of proto-stars, mixed in with the brown dwarfs that can mimic quasars.

Observational astronomy, astrophysics, and stellar evolution (all wonderful fields) often have to give up observing hours on huge telescopes with wonderful new sensors as researchers strive for "the faintest", "the most distant" etc. Pushing the envelope is admirable, but there are local galaxies that have insufficient spectroscopy and that situation could be resolved in short order. Spectroscopy of very faint objects requires long, long hours of photon-collection to get anything resembling usable data.

Are we looking too deeply, squandering our resources on too many large instruments pursuing too many similar goals? I think we may be. I would love to see at least one large instrument each in the northern and southern hemispheres dedicated to mapping our local universe, in the hopes of illuminating some of the mysteries that confront us regarding redshift anomalies in interacting galaxies, redshift distribution in tight and loose clusters, etc. We don't know everything about our universe, and it is presumptuous to pretend that we do.

Please remember that a prime directive of the Hubble Space Telescope was to make observations that would help determine how Hubble's distance/redshift relation could be refined. There have been lots of papers written about those observations, with seemingly warring camps led by Sandage and Freedman. Are there other possibilities outside of this conventional (and artificial) dichotomy? We ought to consider this.

Edit: BTW, there are some pretty wide variances in the constants proposed by both groups, and if you wish to consider secondary distance indicators, things still don't get that much tighter.

 

[mgb_phys]

DEATH

5, Yes a novae is generally just material falling onto a star (typicaly a white dwarf)
A SN is an entire star blowing itself apart. Other than they appeared to observers as a new star they aren't linked.

6/7 - The SN light curve is pretty fast, material collapses into the core at relativistic speeds. The rise certainly less than hours observationally

8, Novae aren't collapses. On a supernovae only the inner core of the star collapses. The core reaches almost neutron star densities before rebounding - ie. 10s of km

9 -- Don't know, there are different types of SN depending on the source star.

10 For small SN (typeI) energy the energy comes form the decay of isotopes formed by fusion in the collapse. For large (typeII) stars a lot of the energy comes from just the gravitational collapse of 100solar masses crashing into a point at 25% c !

11 -- Most of the mass of the core probably ens up in the S radius, but most f the star is thrown out at 0.1C so never gets a chance.

12 Depends on the mass of the star, supernova remnents are found around most stars but a lot of the mass is too far out to really be considered as orbiting the black hole.

13 The black hole will rotate at high speed just from conservation of angular momentum. The neutron stars formed by a smaller collapses start their life spinning at 10,000 Hz. Non-spherical rotating black holes are a bit out of my field!

14 The envelope is pretty opaque, most of the light is from the envolope not the core.
Hardly anything falls into a black hole because of angular momentum it tends to form a disk around it.

15 Type I/II SN are different just because of the mechanisms of energy formation. Decay vs gravitational

16 Nope it's pretty much neutrons. Some of the neutrons on the surface might be in crystals rather than purely degenerate - there is a whole field of NS chemistry,

17 Degeneracy?

18 Same place that photons come from when an electron moves in a magnetic field.

19 In theory quite a lot. Models suggest about 2:1 don't know if anyone has ever measured this in the wild

20 The magnetic axis is almost the rotation axis. The jets come out very near the poles, we only see their rotation because of precesion.

 

[Chronos]

1. Faye, what the heck are you talking about? Proto stars ignite at their leisure [millions of years].
2. See stellar fusion 101.
4. What process do you have in mind that accounts for neutron star emissions?

 

[FayeKane]

FayeKane,

I'm overwhelmed! Come back with your top 1-3 questions, and maybe we can explore them a few at a time.

Never mind then.

(Geez, it's a good thing that I only asked the astrophysics questions, and not all my questions about astronomy.)

No offense, but I want to know ALL of those things, and I've wanted to know them since college, when I learned about all about all these interesting things.

But between all the Hyashi tracks and nuclear chemistry and stuff, I don't think they told us any of those particular things when I was an astronomy major.

I did ask many, many questions then, but I couldn't ask anything in class if the professor was telling us about something else, and I didn't want to ask for free individual lessons at his office. It would be gluttonous, like eating all the burgers at mcdonalds, and when it closes, following the guy home and asking him to grill more in his backyard.

I'm like a homeless person begging for food. I don't mean to intrude or imply that you OWE me anything, it's just that no matter how much I eat, it just makes me more hungry.

--flk

 

[FayeKane]

Fusion doesn't occur throughout a star, it happens only in the core or in a thin shell around the core.

Wow, I knew about the core of course, but I didn't know that it happened in a shell around the core.

Why doesn't the fusion happen deeper too, where there's even more pressure?

--flk

PS:
this is an example of how eating makes me more hungry.

 

[FayeKane]

To follow on with mgb_phys, when stars are born in dust-clouds like we see in the Orion complex (M42) it not only takes a very long time for the proto-star to develop, but the star continues to accrete from the dust and grow, until the star is so potent that the radiation pressure from the star becomes strong enough to push away (evacuate) the dust and gases from its environs and become visible to us.

Isn't there a moment when fusion begins? And when it does, doesn't that make a huge immediate difference in the appearance of the star?

Pushing the envelope is admirable,

Well I don't think they do it for the Guiness Book of World Records. Amateurs do, like http://blogs.myspace.com/index.cfm?fuseaction=blog.viewcustom&friendId=150103974&blogId=511606888&swapped=true" specifically so she would be the youngest.

but there are local galaxies that have insufficient spectroscopy

I'm sure your're correct, but I'm curious why spectroscopy of local galaxies is important. Is it just for survey reasons, for completeness? Surely you're not looking for something new in them. Being local, their supernovae don't tell us anything about the Hubble constant.

I can see galaxies' positions being important to cosmology, and I can also understand if what you're really studying is the absorption spectra of nearby gas in the light path. But why is the compositions of a billion stars added together not just like the spectra of all the stars in other galaxies?

Maybe they're trying to map stellar populations.

Are we looking too deeply, squandering our resources on too many large instruments pursuing too many similar goals? I think we may be.

Not working in the field, my opinion is like being a backseat driver. Nevertheless, my opinion is that we are not.

When I was a little kid, the largest scope was the 200", and I was actually sad because (I was told) you can't make telescopes larger than that (MMT and adaptive optics were still in the future), and that meant that there would never be any really new discoveries, like galielo and the moons.

But you need new, more sensitive instruments to discover the new stuff. And dedicating them to filling the blank spaces in the Next General Calalogue doesn't seem, to me, to be the best use of new, unique capability.

I would love to see at least one large instrument each in the northern and southern hemispheres dedicated to mapping our local universe, in the hopes of illuminating some of the mysteries that confront us regarding redshift anomalies in interacting galaxies, redshift distribution in tight and loose clusters, etc.

Hmm, so what we're missing is the dynamic stuff; it's not just about not pictures of new phenomena. That makes sense.

There have been lots of papers written about [Hubble redshift data], with seemingly warring camps led by Sandage and Freedman.

Woh, I should read more than just sky and telescope. What could they possibly disagree on, much less vehemently? Is the value of the Hubble constant still in question? Is the positive cosmological constant not accepted by everyone (I hope)?

Alan Sandage is still alive! Cool! At least SOMEBODY'S still left.

Are there other possibilities outside of this conventional (and artificial) dichotomy? We ought to consider this.

I'd like to know more about the dichotomy, please. I don't even know what the issue in contention involves. Cosmology? Galactic evolution? Calibrating the Hubble constant?

there are some pretty wide variances in the constants proposed by both groups,

"Constants", as in more than one? What?

and if you wish to consider secondary distance indicators, things still don't get that much tighter.

??
Why not? If you've got redshift, supernovae, cepheids, and probably new stuff I've never even heard of, don't they all agree? Which one is different than the others?

--faye

PS
I always hate it when people apologize to me for asking so many questions, but now I know how they feel.

 

[FayeKane]

1. Faye, what the heck are you talking about?

I'm not "talking about" anything. I'm asking how long it takes for stars to switch on.

Proto stars ignite at their leisure [millions of years].

Well that's the answer to my question then. Thanks

2. See stellar fusion 101.

I was drunk that day

4. What process do you have in mind that accounts for neutron star emissions?

If I knew that, I wouldn't have asked the question, as that's just a rephrasing of it (but someone else answered it: pure blackbody)

thanx,

-flk

 

[mgb_phys]

I'm sure your're correct, but I'm curious why spectroscopy of local galaxies is important. Is it just for survey reasons, for completeness? Surely you're not looking for something new in them. Being local, their supernovae don't tell us anything about the Hubble constant.

Metalicity distribtuion of our own and local galaxies might tell us how galaxies form and why there are galaxies.
This might be more interesting than reducing the uncertainty in H from 5% to 4%

What could they possibly disagree on, much less vehemently? Is the value of the Hubble constant still in question? Is the positive cosmological constant not accepted by everyone

It's pretty much nailed down now at around 75. For years there was a row between Sandage (H=50) and some others ,including Wendy Freeman, for H=100.
Freeman's research with Hubble has pretty much nailed it at 72 - which ironically is close to what Sandage originally picked.
That top scientists are not generally very conciliatory by nature and the fact they worked in the same building didn't help matters.

 

[turbo]

??
Why not? If you've got redshift, supernovae, cepheids, and probably new stuff I've never even heard of, don't they all agree? Which one is different than the others?

--faye

There are other distance indicators. One of my collaborators has written extensively about some of them, including some interesting trends that seem to correlate with galaxy morphology.

http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0503432
http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0310284
http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0503440
http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0402001
http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0408348

 

[FayeKane]

There are other distance indicators. One of my collaborators has written extensively about some of them, including some interesting trends that seem to correlate with galaxy morphology.

http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0503432
http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0310284
http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0503440
http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0402001
http://www.citebase.org/abstract?id=oai:arXiv.org:astro-ph/0408348

Woh, THANKS! Too bad it's just abstracts, but they do tell me all kinds of new ways to measure distance.

Now I got to look up "Tully-Fisher relationship". That will lead to something else, which will lead to...

It's going to be another Wikinight.

 

[turbo]

It's not just abstracts, Faye. You can link to the full PDF documents from CiteBase. Just look at the abstract pages to see.

Dave's papers are very well-researched, and like all papers that get published in well-respected journals, they are refereed and vetted. Our paper on M51-type galaxy associations was published in Astrophysics and Space Science - a subscription-only Springer journal. The editor urged us to post it on Arxiv as soon as the final version was accepted for publication, so there is a free version available on-line.

http://arxiv.org/abs/0805.1492

We have accomplished the data-collection and most of the analysis for our second paper, and are in the process of writing that one. Like Dave's papers, it won't get many cites because it will address a very unpopular subject - the possibility that some astronomical bodies posses intrinsic redshifts in addition to the redshift-distance relation discovered by Hubble. There are still puzzles in the Universe, and observational astronomy holds the key. The cosmology tail is wagging the astronomy dog these days, IMO, and has been since Gamow and the ascendance of the Big Bang Theory.

Some observational astronomers are concerned about this situation. Among them is the outspoken Michael Disney. BTW, I agree with him.
http://arxiv.org/abs/astro-ph/0009020

 

[PhantomOort]

35. If the monoliths have mass then they have gravity. Thus, with enough of them the added gravitational force will be enough to compress the hydrogen and initiate fusion, just as happens in the real universe.

 

[FayeKane]

35. If the monoliths have mass then they have gravity. Thus, with enough of them the added gravitational force will be enough to compress the hydrogen and initiate fusion, just as happens in the real universe.

I'm sorry Frank, I think you missed it.

They won't do fusion unless they're made of hydrogen. You may be thinking of them forming a black hole.

--flk

 

[FayeKane]

Woh! I don't know if you're Ari or Skip, but your paper was fascinating! I've wondered how the little stub on the Whirlpool Galaxy got there ever since I saw it at the end of Outer Limits each week as the credits rolled over it. But I didn't realize there were others like that.

I think that if there is a mechanism for "discordant redshifts", it ought to be in all the newspapers. That means that our primary distance metric is unreliable.

Note that the difference between BC and BNC is very very much an opinion, though the way you showed how to weed out HII regions was cool!

Also, you don't suggest a mechanism for discordant redshifts (yes I know that wasn't the purpose). But what is the mechanism? The only thing I can think of is if the galaxy is nearly edge-on and the "addendum" galaxy is approaching us along with the spiral arm it's attached to. But that effect would be seen in every edge-on spiral (and as I remember, it is).

Is one approaching and one receding because it's a collision? Or what? And why is the position angle distribution so skewed? One would think it would be completely random.

--faye

 

[Chronos]

There is no discordant redshift, that is a myth according to the vast majority of astrophysicists. There are plenty of up and coming youngsters who would have jumped on that bandwagon if it had wheels. Physicists are not dogmatic fools.

 

[turbo]

Woh! I don't know if you're Ari or Skip, but your paper was fascinating! I've wondered how the little stub on the Whirlpool Galaxy got there ever since I saw it at the end of Outer Limits each week as the credits rolled over it. But I didn't realize there were others like that.

Interacting galaxies are actually quite common. We decided to concentrate on pairs that fit the M51 paradigm (large spiral with smaller companion at the end of one arm) because bridged systems are generally considered to be interacting and not chance projections of background galaxies with foreground spirals.

I think that if there is a mechanism for "discordant redshifts", it ought to be in all the newspapers. That means that our primary distance metric is unreliable.

We can leave that to the theorists. Our concentration is on observation, data collection, and analysis. Our second paper will address the statistical probability of chance projection vs interaction in such pairs. We will approach this cleanly, without regard to secondary evidence for interaction, such as arm asymmetry, tidal distortions, or enhanced star formation.

Note that the difference between BC and BNC is very very much an opinion, though the way you showed how to weed out HII regions was cool!

If you will take the time to learn how to navigate the SDSS data, you'll see that many HII regions were targeted as possible QSO's and that spectroscopy was required to sort them out. We managed to do this by studying differences in blue, red, and infrared survey plates, but it sure would be nice to get new spectroscopy on relatively nearby galaxies to firm up the results.

Also, you don't suggest a mechanism for discordant redshifts (yes I know that wasn't the purpose). But what is the mechanism? The only thing I can think of is if the galaxy is nearly edge-on and the "addendum" galaxy is approaching us along with the spiral arm it's attached to. But that effect would be seen in every edge-on spiral (and as I remember, it is).

Is one approaching and one receding because it's a collision? Or what? And why is the position angle distribution so skewed? One would think it would be completely random.

--faye

As for a mechanism for a redshift differential, that is a matter for theorists. We observe and constrain, and that is the role of astronomers, since astronomy is a purely observational science. Trends in the data show that about 1/3 of the small companion galaxies in bridged systems are blueshifted WRT to the large host galaxy, and those differentials are small - well within the range commonly accepted for the peculiar motions of gravitationally-bound companions. The remaining companions are redshifted WRT their hosts, and the differentials can be quite large, despite secondary evidence for interaction.

This result is robust, and not really unexpected, given the trends in other observations. The difference with our survey is that we attempt to be as inclusive and complete as possible. There have been other studies of M51-type galaxy associations, and we reference the most prominent of those in our paper. We believe that our study is the most comprehensive and the most tightly-constrained treatment of this class of galaxy associations. For the thoughts of a professional observational astronomer on redshift asymmetry:

Non-velocity redshifts in galaxies

There are certain peculiarities, claimed or accepted, that suggest either strange behavior of redshifts or that we don't know how to measure them as well as we think. These take the forms of an inescapabale asymmetry in redshifts of binary galaxies, and claims that such redshift differences are quantized and completely disallow a dynamical interpretation.

Redshift asymmetries

Redshift asymmetries are found in almost all samples of paired galaxies with precise redshifts, especially where spirals are involved. The tendency is for the fainter galaxy to have a slightly larger redshift, with a peak in the distribution at 50-80 km/s. The form of the distribution suggests that this is independent of background contamination. Conventional explanations have focussed on problem in measuring redshifts of dusty rotatig disks (for example, if dust is stronger on the inside or outside of arms, the nuclear velocity may be distorted) or, for small groups, on expansion and perspective effects in unbound groupings (Byrd and Valtonen 1985 APJ 289, 535; 1986 ApJ 303, 523). This problem is not directly related to AGN, but letting one camel's nose into the Hubble tent might weaken its defenses for other applications.

(emphasis mine) You may be wondering how such trends can be possible. After all, if we think redshift is entirely due to cosmological expansion or peculiar motion (with some smaller component due to gravitation) the only way such a trend could appear is if the smaller companion galaxies are preferentially rushing away from us. We have no reason to believe that the Earth is at any special or privileged place in the universe, so that's out.

Observational astronomers are well-aware of such trends in the spectroscopic data. The trends cannot be accommodated by current cosmological models, so they are left unaddressed at a minimum, and sometimes derided - often by people with no appreciation for the rigors of observational astronomy. It's easy to engage in nay-saying - it's a lot tougher to actually do the work and come up with a rational defensible argument against the trends. For an example of how precise spectroscopy can be, just visit NASA's NED website and start looking up objects. You'll see their redshifts expressed in many reference frames and often to an impressive number of significant digits. Spectroscopy is precision science. Trends in such data cannot be ignored without grave danger to the integrity of the fields that rely upon it.

 

[matt.o]

Why did you omit the last sentence from the paragraph on redshift asymmetries, turbo-1? If anyone wants to read the complete entry at Bill Keel's website, http://www.astr.ua.edu/keel/galaxies/arp.html" I note that that site was last updated in 2003 and thus only contains references prior to then. Has any work on redshift asymmetries, or indeed galaxy pairs, been done using the large SDSS or 2dFGRS databases?

 

[turbo]

Why did you omit the last sentence from the paragraph on redshift asymmetries, turbo-1? If anyone wants to read the complete entry at Bill Keel's website, http://www.astr.ua.edu/keel/galaxies/arp.html" I note that that site was last updated in 2003 and thus only contains references prior to then. Has any work on redshift asymmetries, or indeed galaxy pairs, been done using the large SDSS or 2dFGRS databases?

The last line in that particular paragraph got into how spectra are analyzed, and did not address trends in the data. I pulled that quote from a text entry, and not his AU page with images.

You can quote the whole of Bill Keel's thoughts on this situation, if you wish. It won't change the fact that there are redshift anomalies in binary pairs and galaxy clusters. If extra entities are required to "explain" such anomalies, then we have a situation that needs to be explored. If you believe that Dr. Keel is wrong, along with other observational astronomers, it is incumbent on you to make your case. My collaborators and I have invested years of work to explore redshifts in interacting galaxy associations. You are welcome to our data if you'd like to construct a refutation.

It is available on-line, and I would be happy to link you up with the raw data so you can construct your own statistical model.

 

[matt.o]

The last line in that particular paragraph got into how spectra are analyzed, and did not address trends in the data.

Not true. The last sentence is, and I quote:

"Detailed study of how one measures redshifts from optical spectra in complicated velocity fields shows that some of this effect comes from the different weightings of continuum and emission-line radiation (Keel 1996 ApJS 106, 27)."

Which seems to run counter to your claim (emboldened by me):

Observational astronomers are well-aware of such trends in the spectroscopic data. The trends cannot be accommodated by current cosmological models, so they are left unaddressed at a minimum, and sometimes derided - often by people with no appreciation for the rigors of observational astronomy. It's easy to engage in nay-saying - it's a lot tougher to actually do the work and come up with a rational defensible argument against the trends. For an example of how precise spectroscopy can be, just visit NASA's NED website and start looking up objects. You'll see their redshifts expressed in many reference frames and often to an impressive number of significant digits. Spectroscopy is precision science. Trends in such data cannot be ignored without grave danger to the integrity of the fields that rely upon it.

Which led me to believe you were being a little dishonest by removing that last sentence, which points to at least one detailed study where such a trend is addressed and is partially accounted for, in order to strengthen your argument that people are burying their heads in the sand with respect to these trends.

 

[twofish-quant]

turbo-l: Like Dave's papers, it won't get many cites because it will address a very unpopular subject - the possibility that some astronomical bodies posses intrinsic redshifts in addition to the redshift-distance relation discovered by Hubble.

I don't think that subject is unpopular. I'd be very, very surprised if astronomical bodies *didn't* possesses non-cosmological redshifts. However, right now I don't think that whatever intrinsic redshifts there are are large enough to require a paradigm shift.

Also cosmology is having it big right now because there is massive amounts of cosmological data coming down from the skies.

One final thing is that I had one conversation with a cosmologist in which he was of the opinion that Hubble just got lucky in the same way that Clyde Tombaugh did. He said that if you take Hubble's original data and then use the most modern measurements, you get a scatter diagram with no obvious redshift correlation.

The other thing is that it's not an observationist versus theoretician split. Pretty much all of the observationalists that I know have been rather scathing in their opinions of non-cosmological redshift papers.

 

[twofish-quant]

You may be wondering how such trends can be possible. After all, if we think redshift is entirely due to cosmological expansion or peculiar motion (with some smaller component due to gravitation) the only way such a trend could appear is if the smaller companion galaxies are preferentially rushing away from us. We have no reason to believe that the Earth is at any special or privileged place in the universe, so that's out.

There are two big and easy explanations

1) closer galaxies tend to be brighter galaxies which means that you have massive statistical biases
2) closer galaxies are much younger galaxies and it's known that galaxies evolve radically

Right now, if you see any sort of redshift bias, most people I know would start with the easy explanations. The trouble with attributing them to non-standard cosmology is that you have a lot of independent evidence for the standard cosmology.

Observational astronomers are well-aware of such trends in the spectroscopic data. The trends cannot be accommodated by current cosmological models, so they are left unaddressed at a minimum, and sometimes derided - often by people with no appreciation for the rigors of observational astronomy.

Observational astronomers are well-aware of these trends, but the ones that I know see no reason to question the standard cosmological models because of them.

Spectroscopy is precision science. Trends in such data cannot be ignored without grave danger to the integrity of the fields that rely upon it.

No one is ignoring any data trends. The trouble is that at least among the observationalists I know, any intrinsic redshifts can be explained in terms of

1) statistical biases
2) galactic evolution

The fact that observation is hard and we don't know how galaxies evolve means that right now there is no trend that anyone can think of that seriously challenges the standard cosmology, and that's the consensus among the observationalists that I know of, many of whom have rather scathing views of people that argue that there are such challenges.

In fact there is hardly an effort to cover-up any observations of galaxies since piecing together 2) is a very active area of research.

 

[twofish-quant]

Also no theorist that I know of thinks that redshifts are *entirely* cosmological. There are some pretty clear evidence of non-cosmological redshift in the form of "fingers of god". The belief is that any sort of intrinsic redshift is not large enough to invalidate cosmological models and that any trends can be explained by

1) statistical biases, or
2) galactic evolution

 

[twofish-quant]

Off the top of my head, what might be happening is that the faint galaxy isn't red shifted, but rather than the main galaxy is massively blue shifted. The interaction with the faint galaxy causes massive disruption in the main galaxy, and you are much more likely to see disrupted gas and dust moving toward you than away from you (because the red shifted stuff is on the other side of the galaxy). There are similar effects in accretion disks (i.e. on the average you are more likely see the blue shifted stuff because it is heading to you.) The speeds of the discordant redshift (i.e. 1000 km/s) isn't out of line with the speeds of known objects that have been gravitiational disrupted.

Anyway there are enough weird and complicated things that happen with interacting galaxies, that you have to rule out a lot of effects before you end up with something that you need cosmology to explain. So the statement "the trends cannot be accommodated by current cosmological models" seems wildly premature.

 

[turbo]

Anyway there are enough weird and complicated things that happen with interacting galaxies, that you have to rule out a lot of effects before you end up with something that you need cosmology to explain. So the statement "the trends cannot be accommodated by current cosmological models" seems wildly premature.

If you take the interacting systems one-by-one you might be able to make such a claim. If you take them as a whole and apply statistitics, the statement is consistent and not even "wildly premature", as in "not even wrong".

All 11 of M81's companions are redshifted with respect to the larger, gravitationally-dominant host. Want to venture an explanation of how that can be the case? Is it possible that all 11 of M81's companions are rushing away from the Earth? If so, why?

BTW, if you think that the main galaxy can be "massively blue-shifted" then you ought to be prepared to explain why that might be the case. Grasping at blue-shifts for host galaxies while denying the reality of red-shifts for companion galaxies is a bit ridiculous.

 

[matt.o]

All 11 of M81's companions are redshifted with respect to the larger, gravitationally-dominant host. Want to venture an explanation of how that can be the case? Is it possible that all 11 of M81's companions are rushing away from the Earth? If so, why?

Reference?

 

[turbo]

Reference?

Do you have access to NED? Everybody with the Internet does.

I'll make it easy for you, since you can't be bothered.

http://seds.org/MESSIER/more/m081gr.html

 

[matt.o]

How does one determine if a galaxy is part of the M81 group from NED?

 

[turbo]

How does one determine if a galaxy is part of the M81 group from NED?

NED provides the spectroscopy - familiarity with the literature is essential if you are going to make sense of it.

 

[matt.o]

NED provides the spectroscopy - familiarity with the literature is essential if you are going to make sense of it.

Indeed, and perhaps you should familiarise yourself the literature, too. Try http://adsabs.harvard.edu/cgi-bin/bib_query?2002A%26A...383..125K" second. Then read the papers citing those two papers, and also the references therein. After you've done this, you can answer this: Are all of the members of the M81 group redshifted with respect to M81?

 

[turbo]

Indeed, and perhaps you should familiarise yourself the literature, too. Try http://adsabs.harvard.edu/cgi-bin/bib_query?2002A%26A...383..125K" second. Then read the papers citing those two papers, and also the references therein. After you've done this, you can answer this: Are all of the members of the M81 group redshifted with respect to M81?

Perhaps you can refer to my link from seds and prove from your links how the 11 companions are NOT redshifted.

Also, you you claimed that discrepancies can be explained by some galaxies being "massively blue-shifted". That puts you firmly in the camp of Arp, the Burbidges, etc who claim that galaxies can have intrinsic redshift. Jumping onto the blue side of spectral discrepancy does not make your ideas "conventional" in the least. Any divergence from the Hubble relationship (either red or blue) needs to be investigated.

 

[matt.o]

Perhaps you can refer to my link from seds and prove from your links how the 11 companions are NOT redshifted.

I said all of the members of the M81 group, not the 11 you cherry-picked.

Also, you you claimed that discrepancies can be explained by some galaxies being "massively blue-shifted". That puts you firmly in the camp of Arp, the Burbidges, etc who claim that galaxies can have intrinsic redshift. Jumping onto the blue side of spectral discrepancy does not make your ideas "conventional" in the least. Any divergence from the Hubble relationship (either red or blue) needs to be investigated.

Can you point to where I've made such a claim? Perhaps you are confused?

 

[twofish-quant]

All 11 of M81's companions are redshifted with respect to the larger, gravitationally-dominant host. Want to venture an explanation of how that can be the case?

Sure thing.

M81's companions = Hubble flow + peculiar motion
M81 = Hubble flow + peculiar motion + weird blue shift

As far as what causes the weird blue shift, it's not hard to imagine.
Suppose M81 has an exploding shell of gas or massive jets spewing out.
You'll see the blue shifted part coming toward earth, but not the red
shifted part going way, since that is hidden by the rest of the
galaxy. Take a picture of M81 and imagine two jets coming from the
core. You'll see the one coming toward you, but not the one on the
other side of the galaxy.

I'm not an exgal-guy, my background is in supernova with a smattering
of accretion disk knowledge, but this sort of thing happens all the
time.

Is it possible that all 11 of M81's companions are rushing away
from the Earth?

If I'm right about this then you are using the wrong baseline
here. M81 companions are the baseline, and M81 is weird.

BTW, if you think that the main galaxy can be "massively blue-shifted" then you ought to be prepared to explain why that might be the case.

We are looking at what sort of velocities? 1000 km/h? Accretion disk
jets can pump things to close to the speed of light. Getting 1000
km/h because you can see the stuff coming toward you but not the stuff
going away is not a problem.

Grasping at blue-shifts for host galaxies while denying the reality of red-shifts for companion galaxies is a bit ridiculous.

See above. It might wrong, but I'm not grasping at straws because you
see this in accretion disks all the time. If you have a central
object then the gas around the central object will appear redshifted
with respect to the central object, but you have enough resolution to
figure out what is going on.

 

[twofish-quant]

That puts you firmly in the camp of Arp, the Burbidges, etc who claim that galaxies can have intrinsic redshift.

Except that it's pretty well established that galaxies do have intrinsic redshift. If you plot galaxies on a Hubble diagram, you get quite a bit of scatter. Are you familar with "fingers of God"? If you plot galaxies on a distance diagram, you get elongated fingers pointing at earth, and those are due to galactic redshift scatter.

Any divergence from the Hubble relationship (either red or blue) needs to be investigated.

We are talking about what? 3000 km/h, which is about z=0.01? That's not going to make much of a difference in your Hubble diagrams.

 

[twofish-quant]

Can you point to where I've made such a claim? Perhaps you are confused?

He is. I proposed a model which I think nicely accounts for the observational results he claims. It might not be right, but by the principle of minimum surprise, you need to explain what's wrong with it first before claiming that what is going on *can't* be explained simply.

 

[twofish-quant]

One other thing. The observationalist that I know tends to be rather suspicous of any statistics that you get from a catalog. The trouble with using statistics from a catalog is that you need to read the catalog very, very carefully in order to make sure what the selection effects are (and in galaxy statistics there are always selection effects).

 

[twofish-quant]

Also keep in mind that I'm a theorist. If an observationalist comes up to me and tells me that it seems to them that the moon is made of green cheese, I'll start calculating the size of the cow that produced it. So if someone tells me that M81 has 11 components all with red shift, I start coming up models for showing how this can happen. If someone says, no they all are blue shifted, I toss that model and come up with a new one.

But the short answer is that even *if* M81 has companions that all are red-shifted, it's not hard to come up with a model that explains that without having to resort to non-standard cosmology.

 

[matt.o]

But the short answer is that even *if* M81 has companions that all are red-shifted, it's not hard to come up with a model that explains that without having to resort to non-standard cosmology.

Yes, the simplest being that M81 does not lie at the dynamical centre of the group.

 

[matt.o]

As for a mechanism for a redshift differential, that is a matter for theorists. We observe and constrain, and that is the role of astronomers, since astronomy is a purely observational science. Trends in the data show that about 1/3 of the small companion galaxies in bridged systems are blueshifted WRT to the large host galaxy, and those differentials are small - well within the range commonly accepted for the peculiar motions of gravitationally-bound companions. The remaining companions are redshifted WRT their hosts, and the differentials can be quite large, despite secondary evidence for interaction.

Let's examine this veracity of this claim. Looking at the data from Table 1 in Jokimaki, Orr & Russell (2008) (downloaded http://www.jorcat.com/tables.htm" ). Of these, 7/21 are blueshifted and 14/21 redshifted.

If those galaxies classified as "bnc" (bridge not connected) are discounted, since the evidence that these are interacting is weak when compared to the "bridge connected" sample, there are 6/16 blueshifted and 10/16 redshifted companions. Is this really a significant difference? Given the small numbers, and the chance that some of these systems could still be chance projections, I would be skeptical.

As expected, a larger number of bnc companions are redshifted (4/14 cf. 1/7 in the blueshifted). This is because it is more likely that a background chance-projection will occur compared to a foreground one, since a larger region is being probed in the background at these low redshifts. This claim is verified by considering those galaxies classified as having "discordant" redshifts in the Jokimaki, Orr & Russell paper (i.e. those with \Delta V > 1000\, km\, s^{-1}). There are 17 of these systems and 10 of these are classified bnc. The percentage across the whole M51-like sample which were classified as bnc was 16%, compared to ~60% of the discordant redshift objects. So are these really discordant redshifts or just plain old projections?

 

[turbo]

matt, you are right to question statistics on small samples, but if you understand our methodology, you know that our sample size is limited by restricting the set to galaxy associations that fit the M-51 template. Ours is not the first such project, as you can see from our reference list. The sample size (of systems with published redshifts) is also restricted by the dearth of spectroscopy in apparently-interacting galaxy pairs, so that hampers our work. In addition, we consider uncorroborated redshift data from an individual source as tentative, at best.

As for chance projections, there is also a chance that a nearby dwarf galaxy will appear projected over the arm of a more distant spiral and be discordantly blue-shifted WRT it. We found no such examples - and not for lack of trying. Over the course of 2 years, we studied images of many thousands of galaxies looking for interaction.

We address chance projection in our paper, and have done additional work on the statistics of chance projection, placement of companions, etc, in the interim. We will publish that in the future.

 

[twofish-quant]

Also from a theorist point of view, all this is "interesting" but not "we have to totally rethink cosmology interesting." Even if you were to establish conclusively that the interaction is not a statistical artifact (which is *incredibly* difficult), explaining that within the framework of standard cosmology is not terribly difficult. The velocity of these differences are in the hundred to thousand km/s range and there is no shortage of effects that could cause these sorts of effects. The other issue is that even if you were to establish that galaxies have redshift effects that cause things to be off by less than a few thousand km/s, this would cause corrections to distance/velocity plots, but it would not invalidate standard cosmologies.

If you want to work on something that *would* validate standard cosmology, then I think that statistics is just the wrong approach. The problem with statistics is that there are so many possible sources of statistical bias, and that's really hard to convince someone that you have a smoking gun as opposed to statistical bias, and it's messy for theorists because trying to disentangle what's going on when you might have very, very different things going on in each sample, is an impossible task.

If you want theorists to be interested, then rather than look for statistics, it's better if you do high resolution studies of one or two particularly weird objects. If you have an example where there is an interaction between a large main galaxy and a companion with a difference in redshift of z=0.3, then *that* would be interesting since it's hard to come up with an easy explanation. The other thing is publication bias.

Suppose you spend six months studying something and then it turns out that its just statistical bias, you have no consolation prize and nothing publishable. If you have an interaction between a single galaxy and a companion with redshift differences of z=0.3, and you have some very strong reason to think that it's not a coincidence, then after spending six months on it, if you've established that it's some bizarre gas jet effect, then you still have something publishable.

One trick in science is to set up experiments so that even null results are earth-shattering. I remember back in the mid-1990's, when the first results from COBE came in and we were talking to each other saying, "well if we don't see CMB anisotropies soon, then we have a lot of explaining to do." Same, with LIGO and LHC, if those come online and we don't see any gravity waves or the Higgs boson, then this will be something of an earthquake.

The fact is that there are lots and lots of things in the sky that we just don't understand, and if you theorists to be interested in your mystery, you have to provide some reason why your mystery is more likely to completely change people's understanding of the universe more so than the other mysteries out there.

 

[twofish-quant]

Something that would turn heads would be a single object with a huge redshift difference which is obviously not a projection. You can establish non-projection by having a gas bridge with an intermediate red shift. Or high resolution redshifts of the companion object in which you can show that the core has a massive redshift but the edges do not.

Also from a theoretical point of view, all these is much less important than it was in the 1970's because...

1) until the late-1970's, there was to mechanism to account for cosmological quasars. The gas-accretion model basically removed that objection

2) the bar for something Earth'shaking has increased enormously. If all you see is low-redshift objects than rather small intrinsic-redshifts can change greatly impact your data interpretation. Today, we have far, far more data on high redshift objects, which means that if you did establish high intrinsic redshifts in galaxies, it would have less of an impact. Suppose after we *did* establish that faint companions of interacting galaxies had intrinsic redshifts of say 2000-3000 km/s. It would affect calibration, but it wouldn't immediately change the standard cosmology much, because you have a mass of data that is unaffected by this change. If you have 3000 km/s redshifts, this wouldn't affect interpretation of objects with z=0.7 very much.

From a theorist point of view having an interacting galaxy with an apparent intrinsic redshift difference of z=0.1, would be very interesting, not withstanding any impact on cosmology, so as an observational strategy, you get more theorist interest by focusing on particular weird objects than you do by statistical arguments.

 

[turbo]

There is already an impressive grouping that should have been visited in more detail, instead of continually moving the goalposts - NGC 7603.

http://arxiv.org/abs/astro-ph/0203466

In this association, the main galaxy is a Seyfert and it is connected to its companion with 2x the redshift by a luminous bridge. The companion is roughly comparable in brightness to the host, so if it was a chance projection of an object twice as distant it would have to be 4x as bright in absolute magnitude. Embedded in the arm are two emission-line galaxies. One with over 8x the redshift of the Seyfert, and the other with almost 14x the redshift of the Seyfert. This system has been waved off as "chance projection" by the mainstream because there is no way to explain its appearance without invoking intrinsic redshift.

There is no other object visible around NGC 7603 that might have been responsible for the bridge, so the large companion is the most likely culprit despite its redshift. The two emission-line galaxies in the arm are of approximately the same size and brightness, despite the vast differences in their redshifts. If this association had been photographed in detail, with no spectroscopy, it would have been held out as a wonderful grouping of interacting bodies, with plenty of evidence for interaction. When redshift values are published, the entire association becomes "chance projection". To suggest otherwise is impolitic and dangerous for your career in astrophysics and astronomy.

N.A. Sharp made this quite clear in the follow-up paper of 1986.

However conventional the conclusions, there is considerable resistance to work on anomalous systems, and I am grateful to those who supported me, including the time allocation committees of the AAO, the MSSSO, and the KPNO, and to those who encouraged me who may prefer not to be named.

http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1986ApJ...302..245S

Sharp's paper was published 16 years before the nature of the bright blobs in the arm was known, well before the mesh of coincidences required for the appearance of the system (with conventional explanations) got so far-fetched.

 

[matt.o]

As for chance projections, there is also a chance that a nearby dwarf galaxy will appear projected over the arm of a more distant spiral and be discordantly blue-shifted WRT it. We found no such examples - and not for lack of trying. Over the course of 2 years, we studied images of many thousands of galaxies looking for interaction.

Again, that doesn't surprise me. This is a low redshift sample (|z| < 0.02) which is further limited by the M51-like criteria which limits the areal region which can be probed. Both of these limitations make the likelihood of finding one chance projection extremely small so I would not expect to find many at all. A further limitation of the sample is the requirement that the companion galaxy must be smaller than the main one, which again increases the likelihood that any chance projection will be a background one, rather than foreground.

In any case, looking at Table 1 again, I see NGC 0646 has a companion blueshifted by ~900km/s, but this difference could be less (~700km/s) depending on which redshift is used from NED. Looking at the Supercosmos sky survey images, it does look like there may be some interaction, but I would say this is a good candidate for being a blueshifted projection. Verification would require better imaging.

 

[matt.o]

There is already an impressive grouping that should have been visited in more detail, instead of continually moving the goalposts - NGC 7603.

http://arxiv.org/abs/astro-ph/0203466

In this association, the main galaxy is a Seyfert and it is connected to its companion with 2x the redshift by a luminous bridge. The companion is roughly comparable in brightness to the host, so if it was a chance projection of an object twice as distant it would have to be 4x as bright in absolute magnitude. Embedded in the arm are two emission-line galaxies. One with over 8x the redshift of the Seyfert, and the other with almost 14x the redshift of the Seyfert. This system has been waved off as "chance projection" by the mainstream because there is no way to explain its appearance without invoking intrinsic redshift.

Do you realize that the magnitude system is on a log scale, so, at a given distance, a magnitude 13 object is much more luminous than a magnitude 15.5 object? Also, there is a galaxy just off to the west with a similar redshift which could be responsible for the tidal features observed.

Edit to add: The absolute magnitude of NGC 7603 at z=0.029 is -22.5, while its NGC 7603b, at z=0.057, has absolute magnitude -21.5. The ratio of the luminosities is 0.36, i.e., NGC 7603 is ~3 times more luminous than its "companion" if the redshifts are cosmological. This is using the r-band magnitudes available from the SDSS http://cas.sdss.org/dr7/en/tools/chart/navi.asp?ra=349.73606&dec=0.24387&opt="

 

[twofish-quant]

turbo-l: There is already an impressive grouping that should have been visited in more detail, instead of continually moving the goalposts - NGC 7603.

True. If those things are interacting then there isn't any easy of explaining *that* particular galaxy without invoking weird physics. What I'd be interested in is looking at the high redshift galaxies to see if there is anything peculiar about them that makes them look different from standard quasars. One thing that would be interesting to see is see if the interacting bridges causes a shadow.

From a theory point of view, one reason for doubting in the 1970's that quasars were cosmological was that there was no good model for getting the required energies. There is now, and the model of quasars as being at cosmological fits so many things that it's going to take more than interacting galaxies to unseat that. The problem is that even if you to show that galaxies do have intrinsic redshifts, you have to also show that they are enough to invalidate distance measurements.

So, yes the goal posts do move over time.

turbo-l: To suggest otherwise is impolitic and dangerous for your career in astrophysics and astronomy.

I don't see why. If you talk to any theorist, you'll find that everyone has some nutty ideas, and you are not a decent theorist unless you have a few totally nutty ideas. Mine is "quantum immortality". To suggest that some redshifts are non-cosmological is hardly going to kill your career as a theorist. Most professionals realize that the evidence for things is nowhere is solid as intro astronomy textbooks suggest, and there are lots of anomalies out there.

But there is a difference from *suggesting* that redshifts are non-cosmological and *insisting* that redshifts are non-cosmological.

 

[twofish-quant]

One other thing (and I'm putting on my explain "how big a cow you need to make a moon of green cheese" hat).

It occurs to me that if galaxies form themselves in cosmological filaments which appears to be the case, then when you see a nearly galaxy, then there is a good chance that you are seeing the near end of a filament in which there are also other galaxies. If you look down an end of a filament of galaxies, you are likely to see a lot of galaxies lining up on a row, and I suspect that for large numbers of coincidences that a inhomogenous universe will give you a lot more galaxies lining up than you'd expect by chance. If the Earth is lined up with a wall of galaxies then you could easily see a dozen galaxies at difference distances right next to each other, whereas the changes of a random coincidence in a homogenous universe is likely to be nil.

In the unlikely event I have some free time, I might try calculating the odds of a chance alignment in a homogenous universe versus that you'd expect in an inhomogenous one, and I suspect that for large numbers of coincidences, that you'd expect a lot more alignments. In a homogenous universe, you'd never have large numbers galaxies at different distances next to each other, but I'm guessing that if the standard cosmology is correct, then this sort of thing ought to be quite common.

(And this has something to curiously this is very much related to my day job on Wall Street. It turns out the improbable events happen much more than by chance, because if two stock go down by 10%, then it's likely that you are in a situation that all of them will go down at the same time. The same thing might be operative here, in that if you see two galaxies at different distances next to each other, you are looking edgewise in a filament, in which you are likely to see twelve galaxies at different distances.)

 

[twofish-quant]

One other thing is that you can rule in and out geometry effects through correlations. Imagine a cone, with the foreground galaxy being a certain fraction of the distance down the cone, as you increase the limit at which the base of the cone registers a galaxy then you ought to see the red/blue asymmetry massively increase as you increase the number of detected objects.

So it occurs to me that one way of seeing whether this is a geometry effect is to look at the magnitudes of the galaxies. If this is chance coincidence then you ought to not only see a red/blue asymmetry but also an asymmetry in the relative brightness of the blueshifted galaxies/core galaxy and the redshifted ones/core galaxy. Also if this is a geometry effect, then as you ought to see the red/blue asymmetry start disappearing as you start limiting your samples to galaxies with a given luminousity ratio.

One other test you can use involves projection angle. I need to think about this some more but if you have one situation in which you have mostly interacting galaxies, then you'd expect to see the interacting galaxies randomly distributed in angle around the main galaxy (maybe). If the galaxies aren't interacting then I think you'd be able to tell the difference be doing statistics of the projection angles.