Universe expansion is accelerating ?

[bwanaaa]

That's what I've read. But if that's the case why is the Doppler shift greater for objects further away? The further away you look, the further back in time is the light you see. That Doppler shift is greater the further back in time you look, that means they were traveling faster than what we see closer to us in time now. Doesn't that mean the universal expansion is decelerating, no?

 

[C1]

Hmm.....

Isnt it a relative thing?

Supposedly the galaxies near ours by their doppler shift are not moving as fast relative to ours and the further out (galactic objects) one views, the doppler shift increases. So those (farther) objects are moving in a direction away faster (based on doppler shift magnitude and direction) than the nearer ones. Hence, relative to us, they are moving away (or separating from us) at an increasing rate.

Anyways, does it matter as there really is no actual physical center reference point (ie, origin) for the universe anyways.

 

[Sunny129]

That's what I've read. But if that's the case why is the Doppler shift greater for objects further away?

i'm not sure i understand this question. the very fact that the Doppler shift increases as we look deeper into space suggests that objects farther from us are moving away from us at greater velocities than objects nearer to us, which in turn suggests that universal expansion is accelerating.

 

[Wizlem]

The fact that the farther away we look the faster objects are moving only implies that farther objects are moving faster. That's how they got farther away. You can't infer if they are accelerating or decelerating from those statements.

To tell if some object is changing velocity, you look at it at two different times and see if it is going faster or slower, you don't look at some other object.

IF gravity was the only force acting on these objects, you would predict that the expansion of the universe would be decelerating, but it is not.

 

[silverpig]

The fact that the farther away we look the faster objects are moving only implies that farther objects are moving faster. That's how they got farther away. You can't infer if they are accelerating or decelerating from those statements.

To tell if some object is changing velocity, you look at it at two different times and see if it is going faster or slower, you don't look at some other object.

IF gravity was the only force acting on these objects, you would predict that the expansion of the universe would be decelerating, but it is not.

This is mostly correct.

First off, doppler shift in this context doesn't necessarily mean the objects are moving faster, just that the space between us and the object is getting larger. That may sound like the same thing, but there is a subtle difference.

If an object is actually moving away from us, yes, the light will be doppler shifted towards the red end of the spectrum in a classical way. However, there are some objects we can see that have a redshift which implies the source is moving away from us faster than light. This can't occur because of relativity so something else must be happening.

The answer is that space is expanding. When a photon of wavelength L travels through expanding space, the photon's wavelength is lengthened as the photon stretches. This is what produces most of the redshift we see.

Now, how do we tell that the universe's expansion is accelerating? Well we look for what are known as standard candles - objects (actually events) that have a standard luminosity no matter where they occur in the universe.

There is a special type of supernova (type 1a) that always has the same luminosity - it's a standard type of explosion that gives off the same amount of energy always. By looking at a range of these events over a range of distances, we can deduce that the rate of the expansion of the universe is accelerating by just plotting distances and redshifts.

If the universe's expansion is constant, there will be a linear relation between distance and velocity. If the expansion is slowing, the graph should curve away from this line in one direction. if the expansion is accelerating, the graph should curve away from the line in the opposite direction. When we perform this experiment, we find the graph curves away from the line in the direction that indicates acceleration.

This was a really freaking amazing result and was not at all expected.

 

[SecurityTheatre]

This is mostly correct.

First off, doppler shift in this context doesn't necessarily mean the objects are moving faster, just that the space between us and the object is getting larger. That may sound like the same thing, but there is a subtle difference.

If an object is actually moving away from us, yes, the light will be doppler shifted towards the red end of the spectrum in a classical way. However, there are some objects we can see that have a redshift which implies the source is moving away from us faster than light. This can't occur because of relativity so something else must be happening.

The answer is that space is expanding. When a photon of wavelength L travels through expanding space, the photon's wavelength is lengthened as the photon stretches. This is what produces most of the redshift we see.

Now, how do we tell that the universe's expansion is accelerating? Well we look for what are known as standard candles - objects (actually events) that have a standard luminosity no matter where they occur in the universe.

There is a special type of supernova (type 1a) that always has the same luminosity - it's a standard type of explosion that gives off the same amount of energy always. By looking at a range of these events over a range of distances, we can deduce that the rate of the expansion of the universe is accelerating by just plotting distances and redshifts.

If the universe's expansion is constant, there will be a linear relation between distance and velocity. If the expansion is slowing, the graph should curve away from this line in one direction. if the expansion is accelerating, the graph should curve away from the line in the opposite direction. When we perform this experiment, we find the graph curves away from the line in the direction that indicates acceleration.

This was a really freaking amazing result and was not at all expected.

+1 great explanation. Very clear, concise and (as far as I am aware) accurate.

 

[bwanaaa]

tnx silverpig for a sincere and clear reply.

Yes, the whole idea that 'space is expanding' introduces an entirely new variable to calculating planetary distances. not only do relativistic effects have to accounted in regular velocity calculations, but now, relativistic calculations have to be applied to 'space expansion' as well.

That space expansion was even dreamed of, still bothers me. Your oblique reference to superluminal velocity data is intriguing. (a link?) How does one measure that? My understanding is that if any object *could* exceed velocity of light, it would be invisible when traveling towards us.

The analogy to sound comes back to me. A plane that is supersonic is inaudible on its approach. Only after it passes will its faint and rapidly fading roar be appreciated.

Is spatial expansion inferred from the absence of any stars being blue shifted? Why doesnt that simply mean that the actual stars are just moving away?

Your earnest effort at answering my question without a single obfuscating equation is remarkable. It truly satisfies my truth meter.
my understanding is thus:
constant spatial expansion (i.e. delta x as a function of x) can be written
δx/x = k where k is the hubble constant
but the further away you look, the further back in time you are looking.
do we need to figure out how to add the time variable to that differential equation?
How about:
δx/x = k(-t)
Integrating would give
log x= -kt

That equation in words is interpreted as:
As time goes on, space is shrinking.

Clearly I am wrong as many smarter people do this for a living, where I have the luxury of being an amateur-the bread on my table does not require I avoid controversy, naivete or honest ignorance.

 

[Sunny129]

That space expansion was even dreamed of, still bothers me. Your oblique reference to superluminal velocity data is intriguing. (a link?) How does one measure that? My understanding is that if any object *could* exceed velocity of light, it would be invisible when traveling towards us.

superluminal motion is an optical illusion. it is a byproduct of viewing relativistic jets traveling almost directly toward or away from us, but not *exactly* toward or away from us (it is the slight angle of incidence that produces the effect). wiki's explanation explains the phenomenon well enough for the layman...

 

[Biftheunderstudy]

I think a little bit of historical background might help here.

First, let's start with Einstein and his theory of general relativity. He firmly believed that the universe was static, that it is not changing with time. He quickly realized that there was nothing to oppose gravity and that his equations told him that the universe would be collapsing. Thus, he added a constant which had the effect of counteracting gravity. To be clear, he had no physical basis for doing so.

Later, Edwin Hubble measured redshifts of distant galaxies and showed that the universe is actually expanding, rather than collapsing or static. Embarrassed, Einstein removed the constant and called it his biggest blunder.

Fast forward to relatively recently (nobel prize in physics last year was for accelerating universe), using type 1a supernovae we can get distances and thus redshifts for incredibly distant objects and we see that the expansion of the universe is actually accelerating.

Queue Einstein again, his biggest mistake was actually right. This constant we call the "cosmological constant", you may have heard it called Dark Energy.

 

[maddie]

And then there's this. An infinite future. I find it more comforting than the old cosmologies of a big bang/big crunch, static state or cold death. In this' the multiverse of isolated universes is eternal even though an individual universe can experience one of the previous 3 fates.



Alan H Guth:

Eternal inflation and its implications


I summarize the arguments that strongly suggest that our universe is the product of inflation. The mechanisms that lead to eternal inflation in both new and chaotic models are described. Although the infinity of pocket universes produced by eternal inflation are unobservable, it is argued that eternal inflation has real consequences in terms of the way that predictions are extracted from theoretical models. The ambiguities in defining probabilities in eternally inflating spacetimes are reviewed, with emphasis on the youngness paradox that results from a synchronous gauge regularization technique. Although inflation is generically eternal into the future, it is not eternal into the past: it can be proven under reasonable assumptions that the inflating region must be incomplete in past directions, so some physics other than inflation is needed to describe the past boundary of the inflating region.
http://arxiv.org/pdf/hep-th/0702178v1

 

[PrincessFrosty]

This is mostly correct.

First off, doppler shift in this context doesn't necessarily mean the objects are moving faster, just that the space between us and the object is getting larger. That may sound like the same thing, but there is a subtle difference.

If an object is actually moving away from us, yes, the light will be doppler shifted towards the red end of the spectrum in a classical way. However, there are some objects we can see that have a redshift which implies the source is moving away from us faster than light. This can't occur because of relativity so something else must be happening.

The answer is that space is expanding. When a photon of wavelength L travels through expanding space, the photon's wavelength is lengthened as the photon stretches. This is what produces most of the redshift we see.

Now, how do we tell that the universe's expansion is accelerating? Well we look for what are known as standard candles - objects (actually events) that have a standard luminosity no matter where they occur in the universe.

There is a special type of supernova (type 1a) that always has the same luminosity - it's a standard type of explosion that gives off the same amount of energy always. By looking at a range of these events over a range of distances, we can deduce that the rate of the expansion of the universe is accelerating by just plotting distances and redshifts.

If the universe's expansion is constant, there will be a linear relation between distance and velocity. If the expansion is slowing, the graph should curve away from this line in one direction. if the expansion is accelerating, the graph should curve away from the line in the opposite direction. When we perform this experiment, we find the graph curves away from the line in the direction that indicates acceleration.

This was a really freaking amazing result and was not at all expected.

A great explanation.

From my armchair physicist understanding, light is red shifted for 2 reasons, one because things move away from us and that stretches the wavelength of emitted light. But that light is also further stretched due to the fact that the universe is expanding.

Standard candles are great for measuring distance because they have a known intrinsic brightness, but from what I understand that's not what allows us to tell what component of the red shift comes from velocity or expansion.

One trick that's used is in spiral galaxies you have rotation, if a spiral galaxy exists on a plane to us (we're looking at it side on) then one side the stars are moving away from us on average faster as they have real velocity of the galaxy + rotational velocity, and the other side has the real velocity of the galaxy - rotational velocity, so average wavelength on either side if the galaxy is different by a tiny amount.

I think that difference allows you to calculate the real velocity component of the galaxy away from us and the rest of the red shift is then due to expansion.

 

[disappoint]

However, there are some objects we can see that have a redshift which implies the source is moving away from us faster than light. This can't occur because of relativity so something else must be happening.

The answer is that space is expanding.

Right. Although the objects can't move through space FTL, space itself is not so constrained. Beyond the edge of visible space it is expanding faster than the speed of light away from us.

The evidence for this has been observed at the edge of visible space.

 

[bwanaaa]

A great explanation.

....
One trick that's used is in spiral galaxies you have rotation, if a spiral galaxy exists on a plane to us (we're looking at it side on) then one side the stars are moving away from us on average faster as they have real velocity of the galaxy + rotational velocity, and the other side has the real velocity of the galaxy - rotational velocity, so average wavelength on either side if the galaxy is different by a tiny amount.

I think that difference allows you to calculate the real velocity component of the galaxy away from us and the rest of the red shift is then due to expansion.

My interpretation of your statement is this:

redshift is the sum of real velocity and spatial expansion. redshift differences of opposite galactic arms, when they are perpendicular to our view, quantify real rotational velocity only.

but when the galactic arms have rotated so they are almost in line to our view, a star at the further tip is really much further away from us than a star at the closest tip. if there is no spatial expansion, then the stars (which are moving away from us like the cars of a train) should all have the same redshift.
Any difference in redshift (not due to experimental error) then is due only to spatial expansion.

Trouble is, doesnt it take a really long time for this kind of galactic rotation to be observed? Longer than humans have been staring at the stars?
But let's assume we could identify two stars at opposite ends of two galactic arms when they are in line with our view.
The largest galaxy is IC 1101
(http://en.wikipedia.org/wiki/IC_1101)
at 6 million light years in diameter (5.6 x 10^21 meters)
Hubble's constant is 68 km/s per Megaparsec or 68 km/s per 3.08 x 10^22 meters
That means we have to be able to measure a difference of 6.8 km/s when two stars (at the furthest tips of IC 1101) are in line with our view. Do we even have instruments that can resolve this?

 

[Sunny129]

My interpretation of your statement is this:

redshift is the sum of real velocity and spatial expansion. redshift differences of opposite galactic arms, when they are perpendicular to our view, quantify real rotational velocity only.

but when the galactic arms have rotated so they are almost in line to our view, a star at the further tip is really much further away from us than a star at the closest tip. if there is no spatial expansion, then the stars (which are moving away from us like the cars of a train) should all have the same redshift.
Any difference in redshift (not due to experimental error) then is due only to spatial expansion.

i would say that you interpretation is pretty spot on. when the major axes of two galactic arms opposite each other in another galaxy are perpendicular to our line of sight, the difference in the velocities of stars at opposite ends of these arms is (VE+VR)-(VE-VR), where VE is the recession/expansion velocity and VR is the rotation velocity at the instant the galactic arms are perpendicular to our line of sight. when the galactic arms are inline with our line of sight, and the motion of the stars at the end of these arms are more or less purely perpendicular to our line of sight (that is, their velocities along our line of sight is virtually zero), you will find that their redshifts are in fact equal.

Trouble is, doesnt it take a really long time for this kind of galactic rotation to be observed? Longer than humans have been staring at the stars?
But let's assume we could identify two stars at opposite ends of two galactic arms when they are in line with our view.

it would take millions of years to actually "see" any kind of rotation in a galaxy, but it only takes an instant to "know" how a galaxy rotates, just like it only takes an observational instant to "know" the motion of a star in our own galaxy relative to us. a simple snapshot of distant stars in such distant galaxies (or a several days/weeks/months long exposure, which is still an instant in time as far as we're concerned) and inspection of its EM spectrum tells us about its motion without us having to observe it for millions of years first.

The largest galaxy is IC 1101
(http://en.wikipedia.org/wiki/IC_1101)
at 6 million light years in diameter (5.6 x 10^21 meters)
Hubble's constant is 68 km/s per Megaparsec or 68 km/s per 3.08 x 10^22 meters
That means we have to be able to measure a difference of 6.8 km/s when two stars (at the furthest tips of IC 1101) are in line with our view. Do we even have instruments that can resolve this?

you do not have to take spatial expansion into consideration in "local" spacetime, i.e. when comparing velocities of two stars in the same galaxy. spatial expansion needs only be considered over cosmic distances. while a 6 million light year diameter is insanely large for a galaxy, it is by no means a cosmic distance. sure, one could argue that it is a cosmic distance because said galaxy spans almost two megaparsecs - enough to account for a ~150 km/s difference in velocities due to spatial expansion alone. but distances aren't typically considered "cosmic" until we're talking hundreds of millions, or perhaps billions of light years...besides, all stars in that galaxy are not moving apart because they are gravitationally bound. only non-gravitationally bound objects will move apart due to the expansion of spacetime (like whole galaxies for instance, but not the gravitationally bound stars within a galaxy).

 

[Biftheunderstudy]

Velocity Doppler shifts are relatively easy to measure. So we can get the radial velocity (the velocity along our line of sight).

For nearby objects, our spectrometry is good enough to measure 1 m/s radial velocities.

A characteristic profile for spectral lines can show rotation, one side is redshifted, the other is blue shifted. This provides a "rest frame" from which you can measure the cosmological redshift.

 

[bwanaaa]

Velocity Doppler shifts are relatively easy to measure. So we can get the radial velocity (the velocity along our line of sight).

For nearby objects, our spectrometry is good enough to measure 1 m/s radial velocities.

....

wow. tnx. but what is "nearby"? and why does error of spectrometry increase w distance?

The whole bit about a galaxy being gravitationally bound is kind of a squishy idea. After all, gravity goes a 1/r^2 so everything is gravitationally bound to some degree. haley's comet that visits us every 76 years is bound to our solar system. but voyager is not. yet both are subject to the gravitational force of our sun. How far away does voyager need to get before it can tell us about expansion of the universe? How many galactic diameters before we say that the milky way is not 'binding' it.?

If I knew how fast voyager was going, maybe I could figure out when that might be (Hopefully voyager is travelling perpendicular to the plane of the milky way)

 

[Biftheunderstudy]

There are many reasons why our accuracy will decrease as distance increases. Background and foreground objects "polluting" the spectra for one. I think the major cause is probably because you are taking a Fourier transform of whatever is in the "beam" of the telescope. For a resolved star, that's the whole star and so you can get the sort of average Doppler shift out. For a far away galaxy, your beam will include the whole galaxy and other "things" in it. We recently found a exoplanet around one of the centauri stars, it is a tiny planet but we can now measure wobbles on the order 1m/s -- pretty cool.

The whole concept of gravitationally bound is indeed kind of odd. We usually think of it in terms of a "Virial radius", inside which the gravitational potential energy overcomes the kinetic energy (U = -2KE).

What we mean when we say that cosmic expansion doesn't happen in this regime is that Dark Energy, the cosmological constant, the expansion force (whatever you want to call it) is incredibly weak. Any other force easily overwhelms it and so it is only important on the largest scales in the universe and most apparent where there is no "stuff".

 

[Sunny129]

The whole concept of gravitationally bound is indeed kind of odd. We usually think of it in terms of a "Virial radius", inside which the gravitational potential energy overcomes the kinetic energy (U = -2KE).

What I mean when I say that cosmic expansion doesn't happen in this regime is that Dark Energy, the cosmological constant, the expansion force (whatever you want to call it) is incredibly weak. Any other force easily overwhelms it and so it is only important on the largest scales in the universe and most apparent where there is no "stuff".

thanks for elaborating on my mention of "gravitationally bound vs non-gravitationally bound"...i'm at work right now and i wouldn't be able to provide an in depth explanation until later...:thumbsup:

 

[beginner99]

Is spatial expansion inferred from the absence of any stars being blue shifted? Why doesnt that simply mean that the actual stars are just moving away?

Note that a whole galaxy is blue shifted, namley the Andromeda galaxy. it's moving towards us due to gravity and in the very, very far future it will "collide" with the Milkyway.