intrastellar expansion

[Kevin Webb]

hi there,

if there is enough matter grouped together, the gravity can over come the expansion of space eg andromeda is fairly stable.

in the vast distances between galaxies the expansion of space can occur unhindered.

is this a fair (if not superficial) description ?

i am interested in how to determine at what point, lets say two 1solarmass stars in orbit would have to be separated to allow the expansion of space to overcome their gravitational attraction.

has there been measurements that show there is in fact intragalactic expansion in some galaxies, maybe near the outer edge ?

[StupendousMan]

hi there,

if there is enough matter grouped together, the gravity can over come the expansion of space eg andromeda is fairly stable.

in the vast distances between galaxies the expansion of space can occur unhindered.

is this a fair (if not superficial) description ?

Yes.

i am interested in how to determine at what point, lets say two 1solarmass stars in orbit would have to be separated to allow the expansion of space to overcome their gravitational attraction.

That's a good question. It depends on some cosmological parameters which we do not know precisely, so the answer will be rough .. but it should be possible to derive it. Alas, I don't have time to do that right now.

has there been measurements that show there is in fact intragalactic expansion in some galaxies, maybe near the outer edge ?

We can only detect (easily) galaxies which have a great deal of matter -- and such galaxies will have far more matter than required to prevent the expansion of space in their neighborhood. So, we can't make such measurements, almost by definition.

[antoniseb]

has there been measurements that show there is in fact intragalactic expansion in some galaxies, maybe near the outer edge ?

I don't think that any studies have *observed* this, but there have been papers that look at the scale of the expansion and quantify (within limits) the effect you are asking about.

Expansion is very small and weak. For a simplified example, just adding expansion into Newtonian mechanics, Our Sun orbits our galaxy in about 200 million years. In that time space has expanded about one and a half percent, so our orbit's semi-major axis (note that the orbit is very wavy, perturbed, and really not an ellipse), would have increased by about 300 light-years in that time. An analysis using General Relativity would give a result in the same ballpark.

[GOURDHEAD]

Are there sets of equations that relate rates of homgeneous (or non-) space stretching to mass density and/or gravitational field strength?

[John Mendenhall]

Are there sets of equations that relate rates of homgeneous (or non-) space stretching to mass density and/or gravitational field strength?

I happen to have just yesterday looked up the on-line references on this, for another thread. It was too dense to use on the other thread, the math is typical GR horror stuff, but essentially, for bound gravitational systems, which include the solar system, galaxies, and small groups of galaxies, the gravitational forces negate the expansion. The expansion in these systems is zero, as I read it. Not just overwhelmed by the gravitation, but mathematically zero. Expansion is a large scale phenomenon, not local.

edit: How's that for counter intuitive? Albert does it again.

Check Wikipedia, search for 'expansion of universe'

[Ken G]

IExpansion is very small and weak. For a simplified example, just adding expansion into Newtonian mechanics, Our Sun orbits our galaxy in about 200 million years. In that time space has expanded about one and a half percent, so our orbit's semi-major axis (note that the orbit is very wavy, perturbed, and really not an ellipse), would have increased by about 300 light-years in that time.

But to be clear, the orbit does not actually expand by that amount, antoniseb is just trying to show how weak the effect is and how easily it is set aside by the strength of gravity. If one is to go farther out to more distant orbits to look where gravity is similarly weak, then the problem is perturbations-- almost anything that happens to the system, like another galaxy swinging past, or even just a collection of stars, will cause a marked change in such a weak orbit. It would then be impossible to tease out the effect of the metric expansion in general relativity. Can you tell that the Earth is a sphere using careful measurements of the slope of your driveway?

[GOURDHEAD]

In the nitpicking department one should distinguish between intrastellar and intragalactic and whether the former is more restrictive than the latter. Does physics limit the domain of the expansion or the tendency to expand?

[antoniseb]

If the expanding can red-shift photons, it probably works at all scales.

[speedfreek]

I was just wondering if it were possible for the metric expansion of space to be happening everywhere in the universe, rather than just in the empty parts?

What I mean is, the theory basically says that where there is (almost?) no gravity around space expands, but there is no expansion where there are gravity bound systems.

Could it not be the case that space expands everywhere, but where there are gravity bound systems the gravity totally overrides the expansion and holds the systems together while the space expands around them, with gravity holding them in place relative to each other? Can space expand everywhere but distance between objects only increase when there are places with no gravity between those objects?

[Peter Wilson]

Our universe is dualistic: small-scale systems are by-and-large undergoing contraction, not expansion. Two stars in mutual orbit will generally be found to be spiraling towards each other, not away.

Even if stars in mutual orbit were expanding at the Hubble rate, this would be impossible to measure with today's instrumentation, being substantially less than 1 part per billion per year. Such expansion will be completely swamped by more prosaic effects, such as magnetic fields and tidal interactions.

To say the universe is expanding is to miss the forest for the trees. It is mostly contracting, with expansion at cosmic scales a "residual" effect

[Kevin Webb]

hello

StupendousMan Senior Member Join Date: Sep 2006 It depends on some cosmological parameters which we do not know precisely

what would those parameters be ? could a hypothetical system be used where these parameters could be ignored eg 2 stars far away from any other mass etc.

GOURDHEAD Are there sets of equations that relate rates of homgeneous (or non-) space stretching to mass density and/or gravitational field strength?

and are these GR field equations <insert shriek here> could some one please answer GOURDHEAD as my spider sense tells me these are the equations that will answer my initial question

hmm

lets take the milky way, at what distance away from the "centre" does gravity stop and expansion of space kick in ? there must be reliable measurements made for objects in the region just outside the milky way ?

[Peter Wilson]

...at what distance away from the (galaxy's) "centre" does gravity stop and expansion of space kick in ?

About 5 mega parsecs.

[Kevin Webb]

Peter Wilson Quote:About 5 mega parsecs.

wow that is nearer than i would of guessed ....

how did you determine that ?

[transreality]

Is it true, that here we mean that gravity is overcoming the increase in distance between mass due to expansion, however, it is not preventing the local regions contribution to the overall expansion of the universe?

[Cougar]

...at what distance away from the (galaxy's) "centre" does gravity stop and expansion of space kick in ?

About 5 mega parsecs.

wow that is nearer than i would of guessed ....

5 megaparsecs is over 16 million lightyears. That's 160 times the diameter of the Milky Way's disk, so it's not exactly close. I think Pete's just giving you a rough estimate. Sounds plausible, but I don't really know.

Andromeda galaxy is about 2.5 million lightyears away, and apparently the expansion over that distance isn't enough to overcome the gravity between our galaxy and Andromeda, since it's blueshifted and coming this way.

The huge Virgo Cluster is the central player in the supercluster we belong to. A large elliptical galaxy near its center (M87) has a redshift of z=.004. The cluster is about 50 million lightyears away (17 Mpc) [there's a little controversy over the exact distance]. Wikipedia claims, "The large mass of the cluster has the effect of slowing down the recession of the Local Group [that's us] from the cluster by approximately ten percent." (I wouldn't take this as gospel, but it could very well be approximately correct.)

Might give you a little better idea of expansion vs. gravitation.

[Kevin Webb]

using M87 (M1=5.174^42kg) and the Milky Way (M2=1.15^42kg) and a distance of 52Mly, I get F(grav)= 2^27N is this correct ?

[Peter Wilson]

Sounds right.

[Peter Wilson]

wow that is nearer than i would of guessed ....

how did you determine that ?

As Cougar suggests, that is actually an immense distance. I think the scale of our universe has been lost with the advent of really large telescopes. In our minds we label this distance, "5 Mpc," but it is vast beyond the human imagination. This distance is not "near" by any definition of the word.

As for how it is arrived at, if you look at a collection of matter out there, say a cluster of galaxies, they will all have local velocites with respect to each other. These relative velocities fall into the range of +/- 200 km/s. One could have a relative velocity of 200 km/s towards earth. In order for the Hubble expansion to off-set that local velocity towards us, the galaxy must be at a distance at which the average expansion velocity is at least 200 km/s. This occurs @ 200 km/s/(73 km/s/Mpc) = 2.7 Mpc. In order to "make sure" all local motions are swamped by the Hubble expansion, I "round up" the figure to 5 Mpc. Thus, it is safe to say that all visible matter beyond 5 Mpc will be redshifted.

Again, this distance is vast beyond any ability of the human mind to actually picture.

[Kevin Webb]

Originally Posted by Kevin Webb wow that is nearer than i would of guessed ....

i should clarify (since it has been mentioned a couple of times now) i was not saying that 5Mpc is close in terms of distance alone, but rather saying that i would of guessed that the gravitation affect of the milky way would have had a further reach (maybe i should not guess next time)

Today 05:54 AM Peter Wilson Sounds right.

i was trying to work out the gravitational force that must be exceeded to begin expansion between the two objects. i will keep going to see it i can nut it out. its an enormous amount of force to overcome, considering M87 has a recessional velocity of 1307km/s (NED). i wonder how, that from such a strong gravitational attraction between the milky way and M87, such a high recessional velocity come about ? is the "force" of expansion really that great in magnititude ? or is there a stonger attractor behind M87 ? alot to think about.

Today 06:13 AM Peter Wilson As for how it is arrived at

thankyou for your explanation. it is very much appreciated

[Peter Wilson]

i wonder how, that from such a strong gravitational attraction between the milky way and M87, such a high recessional velocity come about ? is the "force" of expansion really that great in magnititude ? or is there a stonger attractor behind M87 ? alot to think about.

Astronomical numbers have a habit of becomming...well...astronomical

Okay, power = force times velocity.

P = 2E27 N * 1.3E6 m/s = 2.6E33 watts, which does sound "astronomical."

This has to be put into perspective, however. Consider the radiant power output of M87. Visible matter in the universe radiates energy (that's how we see it!) at a rate of about 2E-5 w/kg. Ergo, radiant power output of M87 is about 2E-5 w/kg * 5E42 kg = 1E38 watts.

In other words, the radiant power output of M87 is more than 10,000 times the energy required to overcome the gravitational attraction.

Something to think about? Definitely

[Kevin Webb]

Peter thankyou for the help and info. i have only recently begun to supplement my interest in the universe with some basic number crunching. i think its my lack of experience with the magnitude of numbers that lead to mis-interpretations of how significant they are. i'll get more familiar as time goes by. i certainly hope i have enough time to understand the more complex math ...

[Kevin Webb]

been a while, but i found that the strength of repulsion can be expressed as

Fcosmic = lambda r m c^2,

where m is the mass of the repelled object, r is its distance from the repelling body, c is the speed of light and lambda is 10^-53m^-2 Davis The Accidental Universe

so all i need to do is find when Fcosmic is ever so slightly larger than Fgravity. Is it permissible to make Fcosmic = Fgravity ie

lambda r m c^2 = Gm1m2 / r^2

then rearrange the equation to isolate r and then add a few metres extra...

i get that two 1solarmass stars would have to be separated 2.6ly apart for the expansion of space to move them apart.

comments please

[winensky]

but essentially, for bound gravitational systems, which include the solar system, galaxies, and small groups of galaxies, the gravitational forces negate the expansion. The expansion in these systems is zero, as I read it. Not just overwhelmed by the gravitation, but mathematically zero. Expansion is a large scale phenomenon, not local.

edit: How's that for counter intuitive? Albert

Check Wikipedia, search for 'expansion of universe'

I have seen this expressed several times and not only is it counterintuitive but it attributes a seemingly overwhelming effect to cold matter at a local scale. I don't disagree with the observation or the maths but something is screaming, not right. The implication for the future morphology of the universe is that those of us safely tucked away in islands or strings of entropic cold matter will watch the rest of the universe expand as oceans of space and time. I have difficulties with two sets of rules especially given the established homogenaity of the background CMBR.

[grant hutchison]

i get that two 1solarmass stars would have to be separated 2.6ly apart for the expansion of space to move them apart.

I think you may have forgotten to convert solar masses to kilograms. It makes a big difference.

Edit: No that's not it. My mistake.

Grant Hutchison

[grant hutchison]

I have seen this expressed several times and not only is it counterintuitive but it attributes a seemingly overwhelming effect to cold matter at a local scale.

Yes, overwhelming.
The current accelerating expansion of the Universe in theory makes a tiny difference to your height, and the radius of the Earth, but it is so small as to be undetectable by any practical means.

Grant Hutchison

[grant hutchison]

... lambda is 10^-53m^-2 Davis The Accidental Universe

I'm wondering about the current relevance of that number, given that the book was published 27 years ago. Has there been a second edition?

Grant Hutchison

[grant hutchison]

Here is Lineweaver & Davis's Scientific American article about the expanding Universe, which has relevance to this topic. See page 5, Is Brooklyn Expanding?.

Grant Hutchison

[grant hutchison]

... lambda is 10^-53m^-2 Davis The Accidental Universe

I'm wondering about the current relevance of that number, given that the book was published 27 years ago. Has there been a second edition?

Ah, here we go.
I picked the book off the shelf when I got home and I find that Davies is giving the observed upper bound to lamda in 1982, writing at a time when (he states): "... there is no observational evidence that Λ differs from zero".

Grant Hutchison

[winensky]

Yes, overwhelming.
The current accelerating expansion of the Universe in theory makes a tiny difference to your height, and the radius of the Earth, but it is so small as to be undetectable by any practical means.

Grant Hutchison

Ah, Thanks. This was my understanding as well.

[Kevin Webb]

Here is Lineweaver & Davis's Scientific American article about the expanding Universe, which has relevance to this topic. See page 5, Is Brooklyn Expanding?.

Grant Hutchison

cheers

okay so the hint is At Earth's surface, the outward acceleration away from the planet's center equals a tiny fraction (10�30) of the normal inward gravitational acceleration.

can i take this to mean that if between the earth (mass 5.98E+24kg) and a 1kg mass (distance from centre 6.38E+06m) there is 10N of force, then this outward acceleration will equal the inward acceleration at a distance, r, where F equals 1/10E+30·10N ???

i get approx 2Mly.