Is the Universe Moving? Acceleration or Stationary?

[RunToFreeForFly]

It is moving, accelerating or stationary?

PS: hi, I am noob say hi to the new comers.

 

[Integral]

Motion must be relative to something so there is no way we can detect, or even speak of, the motion of the Universe.

 

[RunToFreeForFly]

relative to outside of universe?

 

[Integral]

There is nothing outside of the Universe

 

[kuenmao]

If the universe is defined as "all that there is", then what would be the meaning of saying "outside the universe"?
Instead, I'd like to ask a question...is the universe, as a whole, taking an average, an accelerating reference frame? Do we experience accelerations even though we do not produce any force? Is it possible that the universe is actually not a strictly valid inertial reference frame, but one that has very minor accelerations?

 

[Nereid]

If the universe is defined as "all that there is", then what would be the meaning of saying "outside the universe"?
Instead, I'd like to ask a question...is the universe, as a whole, taking an average, an accelerating reference frame? Do we experience accelerations even though we do not produce any force? Is it possible that the universe is actually not a strictly valid inertial reference frame, but one that has very minor accelerations?

And I've got a question for you ... how would you go about answering this question of yours?

 

[mijoon]

If the universe is defined as "all that there is", then what would be the meaning of saying "outside the universe"?
Instead, I'd like to ask a question...is the universe, as a whole, taking an average, an accelerating reference frame? Do we experience accelerations even though we do not produce any force? Is it possible that the universe is actually not a strictly valid inertial reference frame, but one that has very minor accelerations?

Very excellent questions. We admit that the universe may have intrinsic curvature, intrinsic expansion and, according to Godel, intrinsic rotation.
One may well ask what other intrinsic attributes it has.

 

[Gerinski]

To the physics as we undertstand them today, I would indeed say the question is meaningless.
We call the universe all what there is (at least all we may ever have knowledge about), so it can not have any movement related to anything else (any external frame). If it did, that external frame would by definiton become the universe itself ! . We may only wonder if we (the earth) have any movement related to the accepted universal frame (the cosmic background microwave radiation or CMR), and as far as I know we don't.

This is not just a semantical game, if we call the universe all what exists according to our best knowledge abilities, if we ever discover a larger frame in which our current knowledge can be integrated, we will consider that frame as the new universe description.
But as far as we don't have any objective reasons to believe the universe is broader than we now believe it is, our familiar universe is all what there is.

This is not saying we must not care about such new views of the universe. On the contrary, we know our current theories are incomplete, so we strive to find out more accurate descriptions to the universe we experience.

 

[kuenmao]

And I've got a question for you ... how would you go about answering this question of yours?

Actually...I was meaning to ask if anybody(or any organization) ever did such precise experiments in space to see if things not acted upon by an external force would suddenly start accelerating... ...
Or perhaps it would be better to say whether or not things would suddenly accelerate with no reason in this universe. But then again...that would be a kind of "energy creation", wouldn't it? Since there is nothing else beyond the universe that can supply the kinetic energy...

 

[Nereid]

Actually...I was meaning to ask if anybody(or any organization) ever did such precise experiments in space to see if things not acted upon by an external force would suddenly start accelerating... ...
Or perhaps it would be better to say whether or not things would suddenly accelerate with no reason in this universe. But then again...that would be a kind of "energy creation", wouldn't it? Since there is nothing else beyond the universe that can supply the kinetic energy...

So let's think about experiments involving spacecraft ... we've got Gravity Probe B, which is extraordinarily sensitive in many ways, to the kinds of very small effects predicted by Einstein's General Relativity ... in a year or so we'll know whether it's found anything odd (including, possibly, strange accelerations).

We've got GRACE, which has already told us lots of interesting things about the Earth's gravitational field (no strange accelerations).

There's the Pioneer anomaly, named after the US Pioneer 10 and 11 spacecraft . It remains just what its name says, an anomaly.

In the planning phase, there's LISA, which will certainly nail down the nature of the Pioneer anomaly (if it's real near LISA; if it's only apparent much further from the Sun, maybe not), and put pretty tight constraints on any strange accelerations.

Beyond spacecraft , there are observations of the motions of planets, stars, gas clouds, galaxies, quasars, ... AFAIK, the biggest surprise in the last 50 years or so was the CMBR dipole, which showed that the Local Group (our Milky Way, M31, M33, the SMC, the LMC, and ~50 other, small galaxies; plus possibly thousands of globular clusters, and gas clouds) is in the grip of the http://cow.physics.wisc.edu/~ogelman/guide/gr8a/ .

But, nothing to indicate any anomalous accelerations!

So, let me ask you again, how would you go about answering this question of yours? Can you describe, in principle, an experiment that would tell you whether the universe is 'an accelerating reference frame'?

 

[kuenmao]

So, let me ask you again, how would you go about answering this question of yours? Can you describe, in principle, an experiment that would tell you whether the universe is 'an accelerating reference frame'?

One suggestion is to make a box, and then within it put a mass fixed with weak springs, which would allow the piece of mass to move whenever the system was put in an accelerating reference frame. Obviously, the system is a few orders off in terms of precision and magnitude, but the same thing could be done with optical instruments such as lasers and optics. Another possibility would be to measure the deflection of light, but that obviously is impossible in terms of present day technology.
As GR already stated, one can't tell between the effect of a gravitational field and an accelerating reference frame, so I guess that you'll have to look for some place with a reasonably determinable and extremely small gravtiational effect...which again seems impossible.
Just a random thought, never thought it out clearly, so do cut me some slack...
And I'm only a high school student, so please don't go too technical in responding

 

[Nereid]

It's always interesting to try to think ideas through, at least somewhat. In actually doing science, the 'thinking through' of ideas usually takes far, far more effort than coming up with new ones

You might like to compare your ideas with some of those proposed at the recent Cosmic Visions conference, particularly those in the Fundamental Physics section (if you can, try to download some of the files; their contents are quite interesting).

One aspect of your idea that you might like to ponder further on: how to tell the difference between a purely 'local' cause of some otherwise unanticipated acceleration and a 'universal' cause? Bear in mind that 'local' could mean hundreds of millions of light years

 

[Garth]

Some years ago it was claimed that the whole universe was rotating. The polarisation of radio waves from quasar radio lobes was observed to depend on what part of the sky the quasars were situated.
As a Machian I asked the question, "Rotating with respect to what?" and never really got a satisfactory answer.
However it was subsequently discovered that the polarizations were due to a galactic electric field and the interesting question went away!
Garth

 

[A_I_]

There is nothing outside of the Universe

i just got an idea to my hand..
i don't know if it's philosphical..
but i think it's pure truth..
when you say "there's is nothing outside the universe"
you do mean (unconsciously maybe) that there's something..
because we can never describe what WE call "nothing"

 

[setAI]

If the universe is defined as "all that there is", then what would be the meaning of saying "outside the universe"?

the term "The Universe" used to have this all-encompassing meaning- but the language has evolved and now a universe is a region of a greater multiverse- the word "universe" has become a structure in the hierarchy above galactic superclusters- no longer the whole Cosmos [unless you use the formal capital U where it can retain it's older meaning]


THe Ekpyrotic scenario would suggest that the brane of our universe does move and drift through some higher dimensional space- so the idea of this [not THE] universe moving is not meaningless in all models-

 

[Gerinski]

This doesn't really answer your question (does our universe move "relative to anything else"?) but it's somehow related (do we move relative to our whole universe?)

There was a post about it not long ago, but I don't remember where / when. Anyway, this is picked from http://www.astro.ucla.edu/~wright/cosmolog.htm

The Hubble law defines a special frame of reference at any point in the Universe. An observer with a large motion with respect to the Hubble flow would measure blueshifts in front and large redshifts behind, instead of the same redshifts proportional to distance in all directions. Thus we can measure our motion relative to the Hubble flow, which is also our motion relative to the observable Universe. A comoving observer is at rest in this special frame of reference. Our Solar System is not quite comoving: we have a velocity of 370 km/sec relative to the observable Universe. The Local Group of galaxies, which includes the Milky Way, appears to be moving at 600 km/sec relative to the observable Universe.

 

[tony873004]

...As a Machian I asked the question, "Rotating with respect to what?" and never really got a satisfactory answer...

Rotating with respect to itself I would guess. For example, the Earth moves, but we have to say with respect to something. But the Earth also rotates, and we don't need an external reference point. The reference point would be the Earth's instantanious stationary position. The Earth's rotation can be proven through the Corialis (spelling?) force, where weather patterns tend to rotate. A rotating universe should produce some sort of corialis force too.

Also, as far as the Earth moving, it's possible to measure the Earth's motion relative to it's own instantaneous stationary position. In the Earth's journey around the Sun, it keeps changing direction, and that produces a force that can be felt (although too insignificant to feel).

For example, imagine I have a spider in a jar. And I attach the jar to a string and start swinging it in circles. Relative to a lizard sitting on the wall outside the jar, the jar is moving. But can the spider in the jar consider the jar to be stationary, and the lizard to be moving? No, because the jar's motion is not linear. The spider in the jar will feel an artificial gravity from the motion of the jar being swung in circles. Since declaring the jar to be the stationary center of the universe would not cause the artifical gravity to go away, it would not be correct to say that the jar is stationary. The jar is moving relative to the jar's instantenous stationary position. No outside reference necessary.

If we are to discover that the Universe itself is rotating, revolving or moving, we have to look for the subtle clues within the Universe that show the Universe is moving relative to its own instantaneous stationary position.

just my guess...

 

[Garth]

If we are to discover that the Universe itself is rotating, revolving or moving, we have to look for the subtle clues within the Universe that show the Universe is moving relative to its own instantaneous stationary position.

Well that is what I meant by saying I was a Machian.

Do you remember Newton's rotating bucket example, a little like your spider in a jar? Newton argued that spinning a bucket would cause the water within to spin too and rise up the sides of the bucket. The fact that the surface was flat would be proof that the bucket was not rotating according to an absolute frame of reference. Leibniz had argued that the rotation was with respect to material objects. Bishop George Berkeley and subsequently Ernst Mach asked what would happen if the walls of the bucket were "several leagues thick", would the rotation of that mass affect the 'direction compass'? They said that 'direction compass' would be orientated on the fixed stars, the mass distribution of the rest of the universe.

The Machian position is thus that without matter there is nothing to 'hang "its own instantaneous stationary position" on.

As a footnote Gravity Probe B is measuring at this moment the Lense-Thirring frame dragging effect, the dragging of the 'direction compass' by the spinning of the Earth - a thick enough rotating bucket!

Garth

 

[888eddy]

what about the speed of light?

you can't break the speed of light right. but we can get close enough to the speed of light to witness atoms getting more massive. so if we where moving through space firing an atom one way would bring it closer to the speed of light than firing one the other way. so we can easily see how we are moving surely?

 

[OSalcido]

One suggestion is to make a box, and then within it put a mass fixed with weak springs, which would allow the piece of mass to move whenever the system was put in an accelerating reference frame. Obviously, the system is a few orders off in terms of precision and magnitude, but the same thing could be done with optical instruments such as lasers and optics. Another possibility would be to measure the deflection of light, but that obviously is impossible in terms of present day technology.
As GR already stated, one can't tell between the effect of a gravitational field and an accelerating reference frame, so I guess that you'll have to look for some place with a reasonably determinable and extremely small gravtiational effect...which again seems impossible.
Just a random thought, never thought it out clearly, so do cut me some slack...
And I'm only a high school student, so please don't go too technical in responding

don't see how this would work... if there is a motion of the universe, everything will be moved, equally. there would be no way to discern it.

if you're in a car, how can you tell how fast the other passengers of the car are going relative to the outside of the car (without looking outside the car)? It's impossible.

Maybe there is some quantum mechanics effect that could probably be used

 

[BrainTeaser48]

Yep, got evicted, couldn't pay the rent, darn that trickle down economy.

The Universe must be moving if it follows the laws of thermodynamics, but as everyone else has stated that must be relative to something outside the universe such that you could mesure the distance and the rate of change. What about a change in physical laws outside our universe? not only might it be moving, but observed from another universe or point between it might appear to be squirting! If I were to put my foot down and state my belief, I would state that the universe is moving with respect to any internal point you choose, and we should call it expansion and we should say that some guy named Hubble discovered it, anybody want to second that one?

 

[888eddy]

theory of relativity

scrap my first post on how we could use particle accelorators and the speed of light to find our motion through space. apparently the speed of light is the same no matter how fast you are moving. if someone fired a beam of light and measured its speed relative to himself, and i was traveling at 99.9% the speed of light and measured the same beam relative to me we would get the same answer. very confusing and would love to know how they worked that out but completely blows my idea of measuring our speed through space out of the water. although i guess it means we are not moving through space, unless space is made of something and we are moving relative to that. without anything to measure our speed against velocity is meaningless, so i guess we just don't have a velocity. not 0 or light speed or infinity or anything at all. infact if we discovered space was made of something and we measured our speed relative to it how would we know how fast space was travelling? just found something else out. acording to the theory of general relativity the outer parts of the universe are expanding faster than the speed of light relative to us but since the mass of the universe is bending space, space is bigger there and therefore the outer parts of the universe arent going faster than the speed of light relative to space it just means we can't see them giving the illusion that the universe has an edge. my head hurts. infact if you where to travel an infinite distance from the universe a mile would be infinitly big so your velocity would be zero because you would never finish that mile. so iguess the universe isn't moving then because its the mass of the universe that creates the space for it to move through. i think. i'll probly write back in a while disproving this theory but its something for people to think about.

 

[888eddy]

disproving

writing back already to disprove myself. been all of 5 minutes. anyway; you would still be able to see the outer parts of the universe despite them traveling faster than the speed of light because they are going away from you. like a fighter jet breaking the sound barier, you can't hear it heading towards you but you can hear it traveling away. second mass just bends space there is no basis for saying it effectively creates it. infact it just pulls it in i.e. bends it, or squashes it depending on which way its facing relative to the mass. imagine i was right before and the universe was infinatly 'streched out' i.e. a mile was infinitly long. this would mean that this is how the universe is without any mass acting on it; in its base state. it would take an infinite mass to sqush an infintly stretched universe though and the universe doesn't weigh infinity. even if it did weigh infinity it would have to be infinitly big so even infinitly far away there would still be mass squashing the universe.

 

[Ruslan_Sharipov]

Killing vector fields

In GR the Universe is described in an internal way by the metric. The Universe is called sationary if this metric (averaged on large scales) admits a time-like http://en.wikipedia.org/wiki/Killing_vector" . It is known that our Universe is not stationary. It is expanding, i. e. its metric admits a time-like conformal Killing vector field. However, if its expansion goes along with some rotation, I don't know a formal definition of this state. Please, maybe someone can give such a definition?

 

[888eddy]

Gravitational ripples

i was just reading about gravitational ripples. gravity acts at the speed of light, not instantaneously as predicted by einstein. this means that if the sun where to just disappear it would be 8 minutes before our orbit changed. gravity is just a distartion in space and as this distortion rapidly changed (with the removal of the sun) a ripple of gravity would be emmited. this is much like a bubble on the surface of water. the bubble is distorting the surface but if it pops a ripple travels through the water. the same can be opserved with fast moving masses as they. they effectively leave a wake behind them like a boat. if we could measure features of this wake it would surely be possible to work out how fast the object was traveling relative to space and compare it to its motion through the universe then we could see how the universe was moving. I am probly wrong and i look forward to someone telling me/me realising why this won't work.

 

[Chris Hillman]

Revisiting this thread at a higher level of sophistication

In GR the Universe is described in an internal way by the metric. The Universe is called sationary if this metric (averaged on large scales) admits a time-like http://en.wikipedia.org/wiki/Killing_vector" . It is known that our Universe is not stationary. It is expanding, i. e. its metric admits a time-like conformal Killing vector field. However, if its expansion goes along with some rotation, I don't know a formal definition of this state. Please, maybe someone can give such a definition?

The keyphrases you want are the kinematic decomposition of a timelike congruence into acceleration vector, expansion tensor, and vorticity vector. See Hawking and Ellis, The Large Scale Structure of Spacetime or Poisson, A Relativist's Toolkit, or the Wikipedia article "Congruence (general relativity)" in the version listed here.

is the universe, as a whole, taking an average, an accelerating reference frame?

Very excellent questions. We admit that the universe may have intrinsic curvature, intrinsic expansion and, according to Godel, intrinsic rotation.
One may well ask what other intrinsic attributes it has.

Don't confuse "the universe", e.g. the FRW dust, an exact solution of the EFE which is the simplest grossly realistic cosmological model in gtr, with the preferred timelike congruence defined by the world lines of the dust particles in an FRW dust (or Goedel dust, or some other cosmological model).

Mathematically speaking, acceleration and vorticity (which indicates a kind of "rotation") and expansion are all properties which may or may not hold for a congruence of curves in a spacetime, whereas the Riemann curvature tensor (and tensor built from it such as the Einstein tensor) are properties of the spacetime itself. It is quite possible to have distinct congruences in a given spacetime which have very different properties. However, physically speaking, once we introduce the EFE and interpret congruences in terms of gtr, one congruence may be "preferred" or physically distinguished. In particular, in any fluid solution, the world lines of the fluid elements form a distinguished timelike congruence.

Thus, given any perfect fluid solution to the EFE, it makes sense to ask whether the timelike congruence corresponding to the world lines of the fluid elements is accelerating. In general, if there is a nonzero pressure gradient, the answer is "yes". However, most cosmological models are dust solutions, special cases of perfect fluids in which the pressure everywhere vanishes, which is equivalent to saying that the dust particles are in a state of inertial motion. Thus, they experience no accelerations.

Very excellent questions. We admit that the universe may have intrinsic curvature, intrinsic expansion and, according to Godel, intrinsic rotation.
One may well ask what other intrinsic attributes it has.

The FRW dusts are homogeneous and isotropic, but there are also anisotropic homogeneous solutions (e.g. the Kasner dust) and inhomogeneous solutions having no Killing vector fields at all (e.g. the Szekeres dust). In addition to these, there are solutions in which the dust has nonzero vorticity due to "swirling" about some axis of rotation (e.g. the Van Stockum dust). Last but not least, there some solutions such as the Goedel lambdadust in which the timelike congruence defined by the dust particles has vanishing expansion tensor ("rigid motion") but nonzero vorticity, yet the spacetime is homogeneous (but anisotropic). Indeed, in the Goedel lambdadust the vorticity vector is in fact constant, the "lightcones in the large" of the Goedel lambdadust have surprising properties, and this spacetime admits (nongeodesic) closed timelike curves (CTCs). This model is pedagogically important but very unlike the universe we live in!

The most interesting solutions are those which are in some sense "perturbations" of the FRW dust to include density variations or small "universal rotations" about a distintinguished locus. As Garth said, after correcting for other physical effects, observation thus far places tight upper bounds on any possible "universal rotation".

Some relevant Wikipedia articles (in versions I worked on) are listed here.
I can provide nearly arbitrary amounts of verbiage and math if called upon to do so

Gerinski mentioned an excellent tutorial by Ned Wright (Astronomy, UCLA), who writes

The Hubble law defines a special frame of reference at any point in the Universe. An observer with a large motion with respect to the Hubble flow would measure blueshifts in front and large redshifts behind, instead of the same redshifts proportional to distance in all directions. Thus we can measure our motion relative to the Hubble flow, which is also our motion relative to the observable Universe. A comoving observer is at rest in this special frame of reference. Our Solar System is not quite comoving: we have a velocity of 370 km/sec relative to the observable Universe. The Local Group of galaxies, which includes the Milky Way, appears to be moving at 600 km/sec relative to the observable Universe

He is talking (in nontechnical language) about comparing a nonspinning inertial frame field corresponding to the world lines of the dust particles in an FRW model with an inertial frame field which is moving with respect to these world lines (with, as you would expect given the Hubble expansion of the dust particles, nonconstant infinitesimal velocity, relating the two frame fields at each event by a variable boost, over the long run). In recent and old posts I have discussed this frame field and computed its kinematic decomposition.

Rotating with respect to itself I would guess. For example, the Earth moves, but we have to say with respect to something. But the Earth also rotates, and we don't need an external reference point. The reference point would be the Earth's instantanious stationary position. The Earth's rotation can be proven through the Corialis (spelling?) force, where weather patterns tend to rotate. A rotating universe should produce some sort of corialis force too.

It turns out that things are much trickier than you expect! There are in fact various distinct notions of "rotation in the large" which can be compared to the notions of "infinitesimal rotation" and "intrinstic angular momentum". One reason why the Goedel lambdadust is pedagogically important is that it and some other examples can be used to vividly illustrate some of these distinctions. (See the WP articles I cited, in the last versions I worked on.)

For example, imagine I have a spider in a jar. And I attach the jar to a string and start swinging it in circles. Relative to a lizard sitting on the wall outside the jar, the jar is moving. But can the spider in the jar consider the jar to be stationary, and the lizard to be moving? No, because the jar's motion is not linear. The spider in the jar will feel an artificial gravity from the motion of the jar being swung in circles. Since declaring the jar to be the stationary center of the universe would not cause the artifical gravity to go away, it would not be correct to say that the jar is stationary. The jar is moving relative to the jar's instantenous stationary position. No outside reference necessary.

It turns out that rotation is trickier than you expect, even in flat spacetime! Here too the multiplicity of operationally significant notions of "distance in the large" is key. Multiple confusions in the older literature caused the appearance of a vast and mostly bad literature on the so-called Ehrenfest paradox. As usual, Einstein's intution was basically correct but he lacked the requisite mathematical tools (the kinematic decomposition, quotient manifolds versus submanifolds, and so on). I don't want to awaken a crank who has turned up at PF and has been very active at WP but you can see the WP articles "Ehrenfest paradox" and "Bell's spaceship paradox" in the versions listed in the page cited above, and you can search for a long PF thread with contributions from myself, perfect, Greg Egan, and others. Best of all would be to study the review paper by Gron which I cited in the articles and in the thread. I stress that there is no genuine paradox here, just a lot of confusion caused by mostly bad physicists not thinking hard enough to see that they are confusing multiple distinct concepts.

Garth mentioned Newton's bucket and the various distinct so-called Mach principles. For a technical introduction to the latter, see http://www.arxiv.org/abs/gr-qc/9607009 and for a nontechnical introduction to both, see the prize-winning book by Lawrence Sklar, Space, Time, and Spacetime.

888eddy, slow down! You are not moving at relativistic velocity but too fast for safety, I think IOW, you appear to be trying to grok all of gtr before you've mastered str. I urge you to study Taylor and Wheeler, Spacetime Physics.

 

[zankaon]

Large scale peculiar motion?

Motion must be relative to something so there is no way we can detect, or even speak of, the motion of the Universe.

09-29-2004 06:31 PM
Gerinski This doesn't really answer your question (does our universe move "relative to anything else"?) but it's somehow related (do we move relative to our whole universe?)

There was a post about it not long ago, but I don't remember where / when. Anyway, this is picked from http://www.astro.ucla.edu/~wright/cosmolog.htm

The Hubble law defines a special frame of reference at any point in the Universe. An observer with a large motion with respect to Hubble flow would measure blueshifts in front and large redshifts behind, instead of the same redshifts proportional to distance in all directions. Thus we can measure our motion relative to the Hubble flow, which is also our motion relative to the observable Universe. A comoving observer is at rest in this special frame of reference. Our Solar System is not quite comoving: we have a velocity of 370 km/sec relative to the observable Universe. The Local Group of galaxies, which includes the Milky Way, appears to be moving at 600 km/sec relative to the observable Universe.

5-19-2007, 02:06 AM
zankaon

In regards to questions, one might be reminded of Hudson M. et al 2004 arXiv.org/astro-ph work with SMAC (streaming motion of Abell clusters) studies and the idea of any large scale peculiar velocity of such clusters, other than just infall into respective superclusters. This year more input from ACT (Atacama Cosmology Telescope), South Polar Telescope, and pending Planck Surveyor's better resolution of CMB, suitable for total Suneyev-Zeldovich effect, a redshift independent distance indicator, might enlighten matters. The latter effect is another (excellent?) standard candle, wherein one has CMB scattering off intra-cluster hot gas, leaving cool areas (clusters) in CMB band. So large scale peculiar velocity studies might address any speculation as to any sort of larger scale coherent component of motion of such clusters.