Expanding people in an expanding universe?

[vera]

Help me please!
do'nt know if I'm right here, but any hint leading to a simple answer of the following question would be appreciated:

if the universe is expanding, are we expanding as well?
does my hand expand at the moment?
what about a center of expansion?
what about our point of view?

and how to explain it in a few simple but profound sentences to my physics professor on monday?

 

[wolram]

if the universe is expanding, are we expanding as well?
does my hand expand at the moment?
what about a center of expansion?
what about our point of view?
-------------------------------------------------------------------
the excepted view is that "space", is expanding not the
matter contained in it, so no your hand is not expanding.
and you won't be able to notice any "local", expansion.

 

[vera]

wolram, thanks, that's a start!
that means only "distances" between matter expands and not matter itself?
can you give me more details, referrences or a good link?

 

[HallsofIvy]

I know I'VE been expanding lately! Thanks for pointing out that that is simply a natural cosequence of the expansion of the universe. My wife keeps talking about "exercising"! (Not to mention givin up beer!)

 

[vera]

finally got it by the "raisins in an unbeaked dough" -metaphor.

 

[marcus]

Originally posted by vera
finally got it by the "raisins in an unbaked dough" -metaphor.

good! that metaphor is a favorite of mine too

you asked about a "center" of expansion

expansion does not require a center

In a rising loaf of raisin bread ANY raisin will do as a
center from which to see the others receding

If the dough is expanding uniformly then you can pick any raisin to take as your point of perspective and it will look the same
from any raisin's perspective. the other raisins (or galaxies) will
all be drifting farther and farther away

and the farther they already are the more rapidly they will be receding.

(one possibility cosmologists have to face is that the universe is infinite, which means picturing that the raisin dough extends, and has always extended, without limit in all directions. but even then it can still be expanding. and an infinite expanding thing has no center away from which it is expanding, no need to talk about this unless professor insists----we do not know for sure if U is finite or infinite so there is some danger of confusion and disagreement about it)


be careful if you try to use the balloon analogy, or if someone tries to use it with you. It LOOKS as if the balloon has a center from which expansion is occurring. But that is actually outside the "universe" which, in that analogy, is just the 2D rubber skin of the balloon. within the rubber skin there is no special point from which you can say the expansion emanates. All points are equally the center of the expansion.

Raisin dough is usually safer because it is 3D, like the space we see around us.

 

[davilla]

But don't the raisins also expand? I mean they're all shrivelled up out of the carton, but baked in bread they're so big and juicy. Speaking of which, I wonder what happens when grapes are substituted in the recipe. Grapes without seeds, for that matter. Why do they even grow seeded grapes any more? We have the technology, and it can't possibly be under patent still. So why put up with having to spit out the seeds? It's so annoying. I don't recall ever biting a seed when eating raisins, though. They must remove them or something, but you'd think the process wouldn't be one hundred percent perfect. Prunes, by contrast, have only one fairly large pit, but the machines don't always get it out. Strange. Anyways, in conclusion, would a contracting universe be akin to drying a plum, or would the pit also have to shrink?

 

[marcus]

Originally posted by davilla
But don't the raisins also expand? I mean they're all shrivelled up out of the carton, but baked in bread they're so big and juicy.

some people can't get their minds off food

yeah baked in bread those raisins do get big and juicy except
for the unlucky ones in the crust
which get burnt shriveled hard and bitter

but the dough is just a metaphor
and no metaphor is perfect
and in the differential equations model of the universe
objects like galaxies don't swell up as space expands
they just get farther apart from each other

the size of your hand, or of a rock, or a paperclip, is
determined ultimately by the distances between the atoms
in a molecule and the intermolecular distances.
Those are distances (selfAdjoint pointed out) that are arrived
at by the atoms---balancing their attractions and repulsions---to get the separation they feel most comfortable with
so space expanding does not affect those distances at all

it only affects collections of things which are not bound together in some fashion

 

[davilla]

So what binds planetary objects to the sun? I understand how atomic interactions could balance a big sac of molecules like myself with attraction and repulsion, but gravity doesn't repel! Shouldn't we find galaxies likewise drifting apart? Or are all orbits somehow slower than would otherwise be required, so that planets and the like are actually falling in at a rate that cancels the rate of dispursion of space? Like a very slow worm eating its way into an apple as it grows from the tree, never getting any closer to the core. Seems far-fetched to me.

 

[marcus]

may I frankly compliment you. i think you have performed a correct analysis thinking figuratively

I suspect that much of the time when we do not think mathematically we are led astray by words and get off track, but in fact two things in orbit around each other are always radiating off energy (by gravitational waves) and spiraling in

orbits naturally decay, even those of the stars around the center of our galaxy, indeed there are other losses (space not being a pure vacuum etc), but fortunately for all concerned this natural contraction is extremely slow

Expansion COULD be so fast that it would outrace the normal tendency of a galaxy to decay and contract. It could be so rapid that it would pull a galaxy apart. But is not, not by a long shot.

I sure like the way you thought about this, the worm and all

Originally posted by davilla
So what binds planetary objects to the sun? I understand how atomic interactions could balance a big sac of molecules like myself with attraction and repulsion, but gravity doesn't repel! Shouldn't we find galaxies likewise drifting apart? Or are all orbits somehow slower than would otherwise be required, so that planets and the like are actually falling in at a rate that cancels the rate of dispursion of space? Like a very slow worm eating its way into an apple as it grows from the tree, never getting any closer to the core. Seems far-fetched to me.

 

[Nereid]

Nemiroff and Bonnell, who put together Astronomy Picture of the Day, give some nice links (as they frequently do; goodness only knows how they find the time to do this, day after day after day):
http://antwrp.gsfc.nasa.gov/apod/ap030303.html

As marcus says, "the Big Rip" is little more than speculation at this stage. The next ten years or so should see a flood of data which will help a lot (projects such as Planck, SNAP, ESSENCE, ...)

 

[davilla]

Originally posted by marcus
I sure like the way you thought about this, the worm and all

That was really an afterthought, just trying to keep to the food theme!

I tried to post on this earlier, but i think I was being too specific trying to pin it on our supposedly unknown acceleration (maybe circular orbits are too much of an approximation?), when in reality I don't fully understand the equations involved. My main question with all of this expanding universe stuff is, how do we know that our perception of the Universe isn't somehow being distorted? For instance, what if the Great Attractor is a black hole that's slowly sucking us in, shrinking our space locally such that it appears the rest of the Universe is expanding? Maybe that is also an impossibility and I'm being too specific. Let me try again:

For a long time people thought the Earth was flat only because it appeared flat around them (save mountains and such). For even longer people thought the Sun orbited the Earth for no other reason than that's what it appeared to do. The fault with both was that our perceptions were in a local frame. Since we "knew" we weren't moving, eveything else had to be moving. Since we "knew" the sky was always in the same direction, that direction was universal. Maybe there is a similar mistake with this cosmological constant stuff. The rest of the Universe appears to be expanding... and that's what it must be doing, since we "know" our reference frame is infallible!

Instead of coming up with this far-fetched changing Univserse stuff, why not approach the problem from a fundamentalist viewpoint? The fabric of space isn't changing; there's something local that is influencing our perception of it. Now if I had an advanced degree in astrophysics then maybe I could come up with something credible, but as it stands I'll have to appeal to those of you who do. What assumptions are we making about our local reference frame? If we assume the Universe isn't expanding, what factor would account for the apparent contradiction in evidence? Doesn't that sound like a much more reasonable approach?

In keeping to the food theme, who ever said rat tastes like chicken? Maybe chicken tastes like rat.

 

[Sikz]

Well see one big problem is that if the universe isn't expanding, it's contracting. Since matter has mass and mass exerts gravity, all objects are exerting some level of gravity on all other objects- if the universe is expanding, if the matter has a velocity at which it is moving away from other matter, a high enough velocity and a low enough amount of matter will cause the galaxies to continually slow down slightly but never stop and turn around. If the universe is NOT expanding, however, there is no force to counteract gravity- so it must be contracting. Since evidence points to expansion, we assume that's what it is doing.

And that's some nice thinking, with the point of reference and all... The only think I can think of is that we could be contracting. If it was our galaxy that was contracting:

1) It would have to contract at HUGE speeds in order to produce the illusion of high universal expansion.
2) Galaxies in the same general direction as the center of our contraction would appear to be moving towards us (Earth), not away.

It would have to be the Earth that was contracting- but the Earth isn't big enough to contract at such speeds and still exist. In fact, neither is the galaxy; we would notice the stars getting a lot closer together over the years, I think :P

 

[Nereid]

davilla wrote: What assumptions are we making about our local reference frame? If we assume the Universe isn't expanding, what factor would account for the apparent contradiction in evidence? Doesn't that sound like a much more reasonable approach?

Let's take it step by step. Lots of simplification here, and many important qualifications that'll need to be made once we get more specific (experts with advanced degrees in astrophysics, please be gentle).

First, we observe that galaxies have 'lines' in their spectra which match the 'lines' we see in the spectra of stars. ('lines' is a historical legacy; it means certain wavelengths/frequencies are being absorbed, by various atomic species). We conclude that these lines arise because galaxies are made up of lots and lots of stars, like the ones we can see. (as telescopes have got more powerful, we've been able to 'resolve' ever more distant galaxies into stars).

Next, we observe that the further away a galaxy is, the more those lines are shifted into the red ('redshift').

Then we have a huge debate, lasting decades, on just how far away a galaxy is, and couldn't the redshifts be caused by something other than 'receding from us'? We conclude that, with some important exceptions, redshift does indicate recessional speed.

After that we see that a plot of distance vs redshift has some outliers on it - galaxies whose redshift indicates they should be further (or closer) than the distance we measure by other means. Another huge debate ensues. We conclude that we can account for the outliers, but only with something called 'dark matter' - something which has mass but does not emit (or absorb) light (or X-rays, radio, gammas, ...). We are comfortable with this conclusion because an awful lot of other, independent, observations point to the same conclusion, and are all consistent with each other.

(nearly there). Our redshift-distance plot is a straight line; the constant is called the Hubble constant, after the astronomer who first published a paper on the relationship. We finally - 80+ years later - agree on what value the Hubble constant has, to +/- 10% (or 5%?). Interestingly, if you look at the data Hubble used in his landmark paper, you'd conclude it didn't really make his case.

Last (actually almost first, historically), we apply the best theory of physics that we have, for 'big' things - Einstein's General Relativity - to the universe as a whole, and find that GR predicts an expanding universe!

Have we overlooked something? Are the data open to a completely different interpretation? Of course! All kinds of clever alternatives have been suggested, and some really wild ideas tossed about. Bottom line: nothing does as good a job of accounting for the data as an expanding universe consistent with GR.

Then along came a spider ... observations of quite distant supernovae appear to show that the rate of expansion of the universe is accelerating, and has been for the past x billion years. Wha? Enter 'dark energy', 'cosmological constant' (Einstein invented that too), 'quintessence', and much more. The debate rages; stay tuned, same station, same time, for the next exciting episode of "It's your universe, no need to eat, the Big Rip will bloat you anyway!"

 

[davilla]

Originally posted by Nereid
Then we have a huge debate, lasting decades, on just how far away a galaxy is, and couldn't the redshifts be caused by something other than 'receding from us'? We conclude that, with some important exceptions, redshift does indicate recessional speed.

That sounds along the lines of what Sikz was saying. I remember reading his arguments somewhere before--some debate about wether the Universe would expand forever, because gravity is too weak, or finally collapse back in on itself if too strong. Looking ahead, this is not cosmological constant-grade stuff.

We conclude that we can account for the outliers, but only with something called 'dark matter' - something which has mass but does not emit (or absorb) light

In this forgive-me-astronomy-gods simplified language, what distribution of dark matter explains outliners? And why is it necessary? Is it too much to presume that there are random variations in the expansion, the most notable of which are directed toward or away from us?

We are comfortable with this conclusion because an awful lot of other, independent, observations point to the same conclusion, and are all consistent with each other.

I would ask for additional reading but I'm sure I've never seen this much explained this way. Maybe now this additional knowledge will help clarify things, in particular how it is that "GR predicts an expanding universe," when I get back to it.

Then along came a spider ... observations of quite distant supernovae appear to show that the rate of expansion of the universe is accelerating, and has been for the past x billion years.

I guess this is what I really have trouble with. It's good to know that it isn't as set in stone as it had appeared when I read it earlier, because the Universe ripping apart is really rather troubling (unless it can somehow rip appart those monstrous blood-sucking black holes as well).

Since the Universe is expanding then wouldn't all matter have been more concentrated billions of years ago, hence a denser gravitational field?

Also, out of curiousity, aren't celestial objects that are receding from us doing so not only in space but, to a lesser degree, in time (since more distant objects are seen as they were a longer time ago)? Does this affect redshift?

 

[Nereid]

That sounds along the lines of what Sikz was saying. I remember reading his arguments somewhere before--some debate about whether the Universe would expand forever, because gravity is too weak, or finally collapse back in on itself if too strong. Looking ahead, this is not cosmological constant-grade stuff.

The decades-long debate was more fundamental than that - e.g. are quasars at the distances their redshift implies? What is the distance to {NGC xyz}? 55 Mpc? 110 Mpc?

In this forgive-me-astronomy-gods simplified language, what distribution of dark matter explains outliners? And why is it necessary? Is it too much to presume that there are random variations in the expansion, the most notable of which are directed toward or away from us?

I would ask for additional reading but I'm sure I've never seen this much explained this way. Maybe now this additional knowledge will help clarify things, in particular how it is that "GR
predicts an expanding universe," when I get back to it.

OK, more on dark matter next time. Let's leave 'dark energy' for a while - when you're in the middle of one of these debates the volume can sometimes be rather loud.

 

[davilla]

Dark matter: OK!

But what is the connection to outliers? And I'm reading that neutrinos are not distributed among galaxies the way dark matter is believed to. Hmmm...

 

[Sikz]

Dark matter- Galaxies don not have enough visible mass at their centers to keep the stars orbiting around them in orbit. Galaxies should, according to the amount of matter we see, fall apart, leaving only the more inner stars. Apparently there is more gravity than we see- and for various reasons the fault appears to be in our knowledge of what matter is out there, not in our equations for gravity. "Dark matter" is a way we explain this and similar things; it refers to gravitational forces that are emanating from something we cannot see (various theories on what it is: other-dimensional objects, huge dust clouds with low density, some new and exotic type of matter, etc).

Dark energy- the universe is expanding, and appears to be accelerating over time. Since gravity causes all things to attract all other things, the expansion should be slowing instead (since everything should be contracting together due to gravity). Dark energy is some sort of repelling force, like "antigravity". Various theories on how this works as well, the most notable being that vacuum fluctuations (no "vacuum" is a true vacuum- particles and their antiparticles are continually appearing and anihalating each other. This has to do with quantum physics) have negative energy while "normal" matter has positive. The positive is gravity, attracting, and the negative is the vacuum, repelling. Therefore when we have Object A receding from Object B, Object B is slowing Object A's recession- but when a certain amount of vacuum is between A and B (when they get a certain distance from each other), the dark energy of the vacuum will be greater in power than the gravity of Object B, and Object A will start to recede faster.

This is greatly simplified, but it is the basics. Hopefully it will be helpful for you...

 

[Nereid]

dark matter

To extend from Sikz's post; highlights only, and many details (some quite important) glossed over.

From Newtonian gravity, if an object is in a (circular) orbit, the time it takes to make one orbit and its speed are determined by the total amount of mass inside the orbit (lots of assumptions). Since the speed of stars and gas clouds in (spiral) galaxies can be measured, we can infer the radial mass distribution of those galaxies. We know how much light (and UV, IR, ...) is emitted; we can estimate the total mass in the stars (and gas clouds) which emit that light. As Sikz said, there's more mass than in the stars, gas clouds (and dust).

Looking at clusters of galaxies in the X-ray band, we see they are a pool of very hot, diffuse gas. The intensity of the X-ray emission tells us how much gas there is, and its radial distribution. We can also estimate its temperature, composition, and pressure. If we assume the hot gas to be in equilibrium, then it will have a distribution determined by the mass distribution within the cluster. We can add up the mass of the individual galaxies in the cluster. Again, there's a discrepancy - more mass from the X-ray observations than total mass in the galaxies (and hot gas).

The galaxies in a cluster are gravitationally bound to the cluster (mostly). Their speeds, relative to the centre of mass of the cluster, can be used to determine the mass of the cluster (virial theorem). Cluster masses estimated from the velocity dispersion of the galaxies are much higher than estimates of the total mass in the individual galaxies. BTW, cluster galaxies are some of the outliers I mentioned before; on a plot of apparent distance (determined by redshift) by position on the sky (the wedge-shaped plots in the 2dF and SDSS websites), they appear as a line of galaxies pointing towards us - 'fingers of god'. In fact, they don't really span such large distances for us, it's their 'peculiar (cluster) velocity' which makes it seem so.

One of the experiments which 'proved' GR was the apparent shift in position of some stars as the line of sight came close to the edge of the Sun - the famous solar eclipse of 1919 (never mind that the data were so poor they could've 'proven' almost anything!). This gravitationally-induced bending of light can be seen in cluster images too. Some is quite spectacular - 'gravitational lensing' - most needs careful analysis to detect. Basically, images of 'background' galaxies, seen through a cluster, can be used to map the distribution of mass in a cluster. Again, far more mass than in the galaxies alone.

So, three independent sets of data about the existence of dark matter - X-ray observations of the IGM, cluster velocity dispersions, and gravitational lensing. The good news: all three estimate approx the same amount of dark matter. Here's an example of some recent work: http://www.esa.int/export/esaCP/SEME3PXO4HD_FeatureWeek_0.html

More good news: when you build model universes, the ones with the 'right' amount of dark matter 'look like' the universe we see today, wrt the distribution of galaxies on the sky and in 3D. This may take some explaining; maybe later.

More good news: the tiny, tiny deviations from isotropy in the cosmic background radiation (CBR; aka cosmic microwave background, CMB) can be explained by the 'right' amount of dark matter.

More good news: the biggest 'local' pool of dark matter - the Great Attractor - can be modeled consistently. This giant object is primarily responsible for the CMB dipole, and it took quite a bit of work to characterise it, including finding galaxies on the other side of it (from us) that have a component of their redshift that is towards us.

More ...

 

[Nereid]

What is 'dark matter'?

Another decades long story ... (again, highlights only; much important detail skipped).

Perhaps 'dark matter' is just gas? Can't be, 'cause it would emit light (X-rays, IR) characteristic of its component atoms; quasars seen through such gas would have absorption lines from the gas (which aren't seen).

Perhaps it's just dust? If it were warm, it'd glow in the far-IR/microwave bands (if hot, in near-IR and optical); we don't see any. It'd also absorb light (IR, etc); we don't see such absorption.

Perhaps it's faint stars, lots of red dwarfs for example, or huge numbers of brown dwarfs? This is harder to rule out completely, and the evidence more indirect; however, it's pretty clear the maximum mass of such objects would still be insufficient to account for more than a small fraction of the dark matter (more than the 'max mass' estimate, and there'd be some observational evidence).

What about pebbles, rocks, comets, free-floating planets?? Also difficult to estimate; again, max mass estimates are too small to account for much of the dark matter.

So, plenty of evidence that 'dark matter' exists; some characterisation of its distribution; lots of evidence of what it's NOT; no hard data on what it actually IS!

 

[marcus]

Originally posted by Nereid
To extend from Sikz's post; highlights only, and many details (some quite important) glossed over.

...
...
So, three independent sets of data about the existence of dark matter - X-ray observations of the IGM, cluster velocity dispersions, and gravitational lensing. The good news: all three estimate approx the same amount of dark matter. Here's an example of some recent work: http://www.esa.int/export/esaCP/SEME3PXO4HD_FeatureWeek_0.html

More good news: when you build model universes, the ones with the 'right' amount of dark matter 'look like' the universe we see today, wrt the distribution of galaxies on the sky and in 3D. This may take some explaining; maybe later.

More good news: the tiny, tiny deviations from isotropy in the cosmic background radiation (CBR; aka cosmic microwave background, CMB) can be explained by the 'right' amount of dark matter.

More good news: the biggest 'local' pool of dark matter - the Great Attractor - can be modeled consistently. This giant object is primarily responsible for the CMB dipole, and it took quite a bit of work to characterise it, including finding galaxies on the other side of it (from us) that have a component of their redshift that is towards us.

More ...

Nereid, I liked this essay on dark matter, and the "recent work" link you gave on mapping the dark matter in a cluster.

Would especially like a link to an article about some of the "good news" you mention, especially

1. dealing w. what you say about the Great Attractor, estimates of its dark matter content and seeing "thru" it to galaxies falling towards it and thus towards us

2. computer models of, i guess, structure formation that show the outcome "looks like" our universe structure-wise if you put in the right amount of dark matter initially

 

[Nereid]

Great Attractor

marcus wrote: Would especially like a link to an article about some of the "good news" you mention, especially
1)dealing w. what you say about the Great Attractor, estimates of its dark matter content and seeing "thru" it to galaxies falling towards it and thus towards us

This page, brought to PF members by ranyart, is a good place to start:
http://www.solstation.com/x-objects/greatatt.htm

Here's the ranyart post/thread:
https://www.physicsforums.com/showthread.php?s=&threadid=12406

 

[Nereid]

marcus wrote: Would especially like a link to an article about some of the "good news" you mention, especially [...]
2)computer models of, i guess, structure formation that show the outcome "looks like" our universe structure-wise if you put in the right amount of dark matter initially

I don't have any good ones immediately to hand. However, this site has many excellent links:
http://msowww.anu.edu.au/2dFGRS/

In particular, this paper gives a flavour of how the work is done: "The 2dF Galaxy Redshift Survey: Cosmological Parameters and Galaxy Biasing", Ofer Lahev, in astro-ph/0205382

A couple more:
http://antwrp.gsfc.nasa.gov/apod/ap011219.html
and if you click on the 'computer simulation' link in this page, you will get:
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000MNRAS.311..793B

A pretty picture:
http://antwrp.gsfc.nasa.gov/apod/ap030611.html

 

[Nereid]

another 'outlier' - resolution

One set of observations seemed to contradict the redshift-distance hypothesis, images of apparently interacting galaxies with quite different redshifts. Are these just chance alignments? or are the galaxies truly interacting?

It took decades to answer this question (and some are still not convinced; there's a thread or two here in PF on this), but once Hubble got to work, 'chance alignment' matched the data better.

One good example is Stephan's Quintet:
http://hubblesite.org/newscenter/newsdesk/archive/releases/2001/22/
http://news.bbc.co.uk/1/hi/sci/tech/994116.stm

Here there really are some galaxies interacting. However, one of them is clearly a foreground galaxy (the stars are resolved, for one thing), and is not interacting with the others.

You can also see lots of much more distant galaxies in this image. It's through detailed analyses of such background galaxies that the weak gravitational lensing signal can be detected, leading to estimates of the distribution of dark matter (I don't know if such an analysis has been done for this group of galaxies).

(I really like this image; I have it as my desktop wallpaper, rotated 90 degrees).

 

[DW]

Originally posted by vera
if the universe is expanding, are we expanding as well?

No. The matter of you're made of is bound.

what about a center of expansion?

If it can be said that there is one it isn't within our universe. The expansion model has space itself expanding, not matter diverging from a point "within" space.

 

[marcus]

Originally posted by Nereid
...(I really like this image; I have it as my desktop wallpaper, rotated 90 degrees).

Nereid, many thanks for all the links!

 

[davilla]

Originally posted by Nereid
We know how much light ... is emitted; we can estimate the total mass in the stars (and gas clouds) which emit that light.

How can the mass of stars of a galaxy be estimated by its emitted light? Do we know of any galaxies for which the number of stars can be counted for comparison? I thought the fainter stars were exceedingly difficult to find even within the arm of our own galaxy. And what about the big mass of (one would presume) stars in the center of a galaxy; is there any way to resolve what that even is?

cluster galaxies are some of the outliers I mentioned before; on a plot of apparent distance (determined by redshift) by position on the sky (the wedge-shaped plots in the 2dF and SDSS websites), they appear as a line of galaxies pointing towards us - 'fingers of god'. In fact, they don't really span such large distances for us, it's their 'peculiar (cluster) velocity' which makes it seem so.

I was thinking that could be a drawback of using redshift to determine distance. If a rotational body seen edge-on is plotted this way, the galaxies spinning away from us (relative to the center of the cluster) would seem to be farther away, and on the opposite side those spinning toward us would seem to be closer. But I was thinking on the wrong scale; I was trying to Google outlier stars rather than galaxies.

This Great Attractor stuff is really cool. How close are we, technologically, to getting a better view of the cosmos on the other side of our galaxy?

 

[Nereid]

This Great Attractor stuff is really cool. How close are we, technologically, to getting a better view of the cosmos on the other side of our galaxy?

We can already see 'through' the Milky Way in gammas and X-rays (the high background in both bands around the bulge makes things tricky, but at least the absorption is low!). In radio and FIR, IIRC, the far side is also 'visible'. In all bands, the main difficulty is in being able to image with a similar resolution to the optical. However, Chandra does give arcsecond images, albeit somewhat small (and many radio surveys are also close).

Spizer (sp?), the new IR space telescope, can give excellent results, and the JWST (if Bush doesn't kill it) will do even better.

The other issue is that it'll take a decade or two more before we have the confidence to interpret images in other wavebands as we do for images in the optical.

 

[Nereid]

How can the mass of stars of a galaxy be estimated by its emitted light? Do we know of any galaxies for which the number of stars can be counted for comparison? I thought the fainter stars were exceedingly difficult to find even within the arm of our own galaxy. And what about the big mass of (one would presume) stars in the center of a galaxy; is there any way to resolve what that even is?

A good answer to this excellent question will take quite a few pages! So, in general terms, leaving out many important details ...

The key thing to find is a robust 'luminosity function' - the number (or mass) of stars with a particular absolute luminosity (optical brightness) vs luminosity; crudely, how many stars are there 2x, 5x, 10x, ... 100x as bright as the Sun? 0.5x, 0.2x, 0.1x, ... 0.01x? This may well vary, for example, it might be different in globular clusters than in the Sun's immediate neighbourhood; it's certainly different in newly formed open clusters (lots more very bright stars; they haven't had time to go supernova).

By looking carefully near the Sun, in nearby open clusters, in neighbouring galaxies (LMC, SMC, other nearby dwarfs, M33, Andromeda, ...), we can get a good idea of how this luminosity function varies. By looking at this function in reverse - if you see this much light, distributed from UV to NIR in the following way, with emission lines this strong, ... - you can get a pretty good idea of the mass (in stars!) that generates this amount of light.

The faint end of the luminosity function has been difficult to deal with; as you correctly point out, we don't even know for sure how many really faint stars there are near us!

However, you can look at things the other way - ask 'how many really faint stars must there be for our estimates of the mass, in stars, in a galaxy to be wrong by 2%, 5%, 10%? The answer has been, for several decades now, "an awful lot!" Several sets of observations have put constraints on the low mass end of the distribution, e.g. Hubble's search for really faint stars in globulars, in the galactic halo, and so on; MACHO, OGLE, etc (searches for gravitational lensing, towards the Milky Way centre, the LMC, ... the number of lenses found constrains the space density of low-mass objects, including Jupiters!).

Perhaps the most interesting discoveries were those of low surface brightness galaxies, and galaxies with very high gas content. It seems there are still large clouds of H (and He) from which small galaxies are forming even today.

The stars in the bulge of a galaxy aren't really a problem - if you can get a rotation curve, you will know how much mass is interior to your data points. In fact, the difficult part has been - until Hubble - to rule out explanations other than super-massive black holes as the nuclei of galaxies; we couldn't resolve the rotation curves sufficiently well (except for our own galaxy, and you can always argue it's special somehow).

 

[davilla]

Nereid, you've answered my questions volumetrically! I should thank you all for being so helpful. I haven't taken an astronomy class since high school or, perhaps worse, even considered it before now. But in truth there's a lot of things that I would like to study, if only I could have learned how to become a professional student!

 

[Canute]

Someone elsewhere posted a theory that the space is shrinking. The conclusion of the discussion was that this would observationally equivalent to expansion. Each point of space would 'see' the universe expanding. I don't know if that makes sense or not.

Earlier someone said that the space between the particles within an atom do not expand. Why is that? Saying it is 'bound' by forces doesn't seem to work, the whole universe is bound by forces.

 

[davilla]

Originally posted by Sikz
The only think I can think of is that we could be contracting. If it was our galaxy that was contracting:

1) It would have to contract at HUGE speeds in order to produce the illusion of high universal expansion.
2) Galaxies in the same general direction as the center of our contraction would appear to be moving towards us (Earth), not away.

This addresses quite well the question as it relates to an expanding universe. I understand now that things are drifting apart from each other, and it makes sense with respect to the Big Bang theory. But what about the question with repect to a cosmological constant? Suppose it only appears that the rate of expansion of the Universe is accelerating. Then what assumptions are we making about our local reference frame? If we assume there isn't a cosmological constant, then what factor would explain this apparent contradiction in evidence?

Also, if I may be so bold, here are some unanswered questions:

Originally posted by davilla
Since the Universe is expanding then wouldn't all matter have been more concentrated billions of years ago, hence a denser gravitational field?

Also, out of curiousity, aren't celestial objects that are receding from us doing so not only in space but, to a lesser degree, in time (since more distant objects are seen as they were a longer time ago)? Does this affect redshift?

In a different line of though, I'm going to assume that there is a cosmological constant. This is normally explained by the expansion of space. Inspired by some of the topics of earlier threads, I'm wondering if, alternatively, this could be explained by a variable speed of light. Of course, to us the speed of light always appears to be constant. An understanding of relativity bears this out: the speed of light is the measuring stick of time by which we observe everything else. The cosmological constant might be the result of a universal variation in this absolute speed.

Someone made the keen observation that everything "falls" through spacetime at the same "rate". The faster something moves (relative to an intertial frame) the slower its clock. A photon moves at a maximum velocity spacially because is it still temporally. Now, a changing c could be understood by an acceleration in the "rate" at which we are "falling" through spacetime. Imagine the Universe as an "atomic object" in a macrosystem. The "faster" the Universe "falls", the "farther" a photon has to travel to connect the same two points in spacetime, and hence the slower its speed. A cosmological constant would mean that, universally, everything is slowing down. As it would appear to us, everything is ripping apart. Light connects everything in the universe, and we are losing our connection.

Certainly this has been proposed before. Where does dark energy fit into this account? Another difference between this and the standard explanation might be at the quantum level. I'm having trouble envisioning which would have a change in Plank length. A real change and an apparent change would be indistinguishable. Otherwise it might be possible to decide between the two experimentally.

Originally posted by Canute
Someone elsewhere posted a theory that the space is shrinking. The conclusion of the discussion was that this would observationally equivalent to expansion.

Did they mean that lengths are shrinking? This sounds like a question of nomenclature, how it is that you view the same mathematical phenomenon in R3.

 

[davilla]

Originally posted by Canute
Earlier someone said that the space between the particles within an atom do not expand. Why is that? Saying it is 'bound' by forces doesn't seem to work, the whole universe is bound by forces.

As I understand it, the space between atoms actually does expand, according to the theory. If there is a cosmological constant, then space is expanding everywhere (and uniformly, one would hope). But most of the stuff we humans have contact with is in an equilibrium because of atomic forces rather than gravity. The expansion in the fabric of space might slowly (weakly) tug at the atoms in your hand, but because the atoms are bound (by electromagnetic forces) in molecules, they will resist this, and rather than rip apart, the molecule will pull itself back together.

 

[Canute]

That's what I don't get. How can spacetime be bound within atoms? And if it is then why not between atoms? Hell, there seems to be some debate about whether spacetime even exists. It's very confusing.

 

[selfAdjoint]

What do you mean, "spacetime bound within atoms"?. In this model the atoms occupy places in spacetime, and forces act betrween and inside them to make them the size they are. Wheteher or not spacetime expands has nothing to do with it.

Now certainly in some notional background free theory of everything, the interaction of the forces with the spacetime geometry, and its expansion will have to be dealt with. But there is no such theory now and so nobody can say what that interaction will look like.

 

[Canute]

But I thought the current theory is that space is expanding. Are you saying that isn't the case?

 

[Nereid]

Echoes - sharp probes of dust

Nereid wrote (re dark matter and dust): Perhaps it's just dust? If it were warm, it'd glow in the far-IR/microwave bands (if hot, in near-IR and optical); we don't see any. It'd also absorb light (IR, etc); we don't see such absorption.

There's another, independent, technique: echoes. When a really, really bright point source suddenly shows up (and then fades), it's like a strobe light through all of space between the source and us. From our point of view (POV, does anyone use this shorthand?), we will see 'circular' echoes of the flash, which are reflections of the flash off things just outside the line of sight between us and the flasher (hope that's not porn to PF ). Study of these echoes can identify all kinds of interesting things, and confirm our understand of the amount (and distribution) of dust.

Historically light has been where such echoes have been seen (e.g. from the supernova in the LMC). Recently X-ray echoes have been observed from a particularly bright GRB:
http://t2wesa.r3h.net/export/esaSC/SEMUFH474OD_sensations_0.html