Universe accelerating?

[bmaynard]

I have a brief question, I read an article at space.com today:

http://www.space.com/scienceastronom...euniverse.html

It referenced an older article:

http://www.space.com/scienceastronom...nd_011212.html

Both of these reference dark matter accelerating galaxies to beyond the speed of light....how is this possible? I was under the impression that according to Einstein nothing to move faster than light, and that regardless projecting matter to that speed to would require more energy that exists in the universe.

Any insights?

[Cougar]

Both of these reference dark matter accelerating galaxies to beyond the speed of light....

Well, it's dark energy that is accelerating the expansion, not dark matter. But one doesn't even have to talk about accelerating expansion to encounter the issue of very distant objects "moving" away at superluminal velocities. Plain old expansion alone will have this effect. The main point is, the objects are not moving through space at faster than light speed. It is the billions of lightyears of space in between that is expanding, and this all adds up to an apparent recessional velocity faster than light. Einstein says nothing can move faster than light, but his theories do NOT say that an accumulation of expanding space can add up to an apparent recessional velocity faster than light.

[jamini]

Einstein says nothing can move faster than light, but his theories do NOT say that an accumulation of expanding space can add up to an apparent recessional velocity faster than light.

You explained that really well Cougar. But I think you made a typo. Shouldn't it read: his theories do NOT [preclude or prevent] an accumulation of expanding space adding up to an apparent recessional velocity faster than light.

[bmaynard]

Okay Cougar thank you that does clear it up for me a little, but the statements in those articles are very misleading since they specify moving beyond the speed of light:

--
"Any given source accelerates away from us and eventually reaches a speed larger than the speed of light so that photons emitted from it cannot catch up with the cosmic expansion, relative to us," he said.
--

Your explaination does make a lot of sense, thank you.

[speedfreek]

Yes, the use of the term accelerate is misleading in this context, as there is no relevant acceleration of the object itself.

The object is apparently accelerating away, due to the metric expansion of space. Understanding the concept of the metric expansion of space is one of the cornerstones of cosmology, and unfortunately it is one of the most misunderstood concepts. The popular media (and indeed some science websites) often misuse terms and confuse a lot of people.

Once you understand how the metric expansion of space works in theory, a lot of other problems you might have with cosmology will make a lot more sense, I have found.

The metric expansion of space is a process where the metric that defines distance changes over time. That statement is pretty hard for someone who hasn't studied the concept to understand in itself, but luckily the concept is quite simple, but as I said earlier is often misunderstood.

If the metric that defines distance changes over time, this means that any unit of measurement will change by the same factor as any other unit of measurement, over the same time period.

As an example (using arbitrary figures for simplicity), if it takes 5 billion years for distances to double, then all distances will double in that time. So in 5 billion years, 1 meter expands to become what 2 meters used to be, 1 billion light years expands to become what 2 billion light years used to be, 5 billion light years expands to become what 10 billion light years used to be, and the universe doubles in size. All in the same period of time.

So what would this look like? Well if it took 5 billion years for 1 meter to become 2 meters, then the expansion is very very small at the scale of a meter. But if you look at something that has expanded from 5 billion light years away to 10 billion light years away, then it has moved away by 5 billion light years in 5 billion years and is thus apparently receding at the speed of light!

So, although our cluster of galaxies and our nearest neighbouring cluster are moving very slowly relatively to each other, we are receding from the most distant objects in the universe at a speed that is faster than light and this is due to the space in between us and the distant objects expanding metrically.

You can visualise this process if you imagine the empty space between us and distant objects is filled with ping-pong balls which are pressed together and touching the balls around them, and are all expanding at the same rate. As the balls expand, their centres are pushed apart from each other. All balls are expanding at the same rate and the centres of the closest balls to us would be moving away very slowly, but the most distant balls would be receding at the speed of light or more!

It is as if every point in empty space is moving apart, creating new space in between those points. The effect is cumulative, for the more expanding space you look though, the faster the objects you see are moving away.

Another issue that often gets confused is the difference between (relativistic) doppler effect and cosmological redshift. It is cosmological redshift which indicates the expansion of space, where the wavelengths of light from distant objects are actually stretched by the metric expansion of space, causing an absolute shift in the lines of an objects spectrum.

Relativistic doppler effect on the other hand, is caused by the relative velocities between us and the object we observe, which imparts an apparent shift in the lines of that objects spectrum. In this case, the spectral lines would look different depending on the relative speeds of the source and the observer, whereas with redshift caused by the expansion of space the change to the light is absolute.

[Cougar]

You explained that really well Cougar. But I think you made a typo. Shouldn't it read: his theories do NOT [preclude or prevent] an accumulation of expanding space adding up to an apparent recessional velocity faster than light.

Quite right. Thanks for the catch.

I also wanted to add to what I said before:

Plain old expansion alone will have this effect... if you're viewing something that's far enough away.

But now that I think more about it, more questions arise. The farthest we can see (currently) is the last scattering surface. And it's all around us. Where in the Universe are the objects that are superluminal due to the expansion?

[bmaynard]

Thanks all for the explaination, it does make a lot more sense to me now. I especially like the ping pong ball example, I guess it all does come down to relativity the object from our perspective is accelerating at a much higher rate than it actually is due to the space in between.

Thanks again all.

[Cougar]

Another issue that often gets confused is the difference between (relativistic) doppler effect and cosmological redshift. It is cosmological redshift which indicates the expansion of space, where the wavelengths of light from distant objects are actually stretched by the metric expansion of space, causing an absolute shift in the lines of an objects spectrum.

Relativistic doppler effect on the other hand, is caused by the relative velocities between us and the object we observe, which imparts an apparent shift in the lines of that objects spectrum. In this case, the spectral lines would look different depending on the relative speeds of the source and the observer, whereas with redshift caused by the expansion of space the change to the light is absolute.

Just a nitpick here. You're quite right that these two mechanisms of redshift are often confused, especially when books refer to cosmological expansion as a doppler effect. As you say, it's different. The mechanism is different. The math is different. However, in the spectral lines that we measure, there is no distinction. There is simply a shift in the lines due to cosmological redshift, doppler redshift (or blueshift), or both combined. But at larger and larger distances, doppler shifts play a smaller and smaller part, becoming overwhelmed by the large cosmological redshift.

[speedfreek]

However, in the spectral lines that we measure, there is no distinction. There is simply a shift in the lines due to cosmological redshift, doppler redshift (or blueshift), or both combined. But at larger and larger distances, doppler shifts play a smaller and smaller part, becoming overwhelmed by the large cosmological redshift.

Indeed. We cannot tell from looking at an objects spectrum alone what form of redshift it is. There is no definite distinction in the redshift between relativistic or cosmological redshift (although we can sometimes get clues from things like line broadening). We usually have to decide using other means which mathematical translation we should be applying to the redshift data. (If the object is close enough we use the transformation for relativistic redshift, if it is distant we use the transformation for cosmological redshift). It is the distances from around 1-5 billion light years where it is more difficult to separate the cause of the redshift. Is it close to us and moving fast due to gravity, or further away and moving fast due to expansion?

The cosmological redshift of relatively close objects is so small to be unmeasurable, and is dominated by the relativistic redshift, caused by relative inertial movement. As you said, the opposite is true for distant objects over 5 billion light years away - their redshift is considered to be primarily cosmological, caused by the expansion of space.

To put this into context, redshift is described by the value z. Objects in our local cluster of galaxies all tend have a value of less than z=0.1, which is attributed to relativistic doppler effect caused by the relative inertial movements between us and the object we measure. A redshift of over z=0.1 indicates cosmic expansion.

Any object with a redshift of over z=1.5 is considered to be receding faster than light. We have measured redshifts of galaxies and quasars with values of z=7.0 or more. The CMB has a redshift of z=1089!

But now that I think more about it, more questions arise. The farthest we can see (currently) is the last scattering surface. And it's all around us. Where in the Universe are the objects that are superluminal due to the expansion?

It seems counter-intuitive doesn't it? But the limit on what we can observe is imposed upon us by the hubble parameter, and this parameter has changed during the history of the universe, due to the change in the rate of metric expansion. The expansion was slowing down in the past, and this allowed light to be seen that, if the expansion had been constant we might never have seen, but due to the past slowing of the expansion and thus the change in the hubble parameter, the light from these objects has 'slipped' inside our observable universe.

It is worth considering the nature of the metric expansion of space (again!). Locally, space is expanding very very slowly. It was a lot faster in the past, slowed down and then sped up again but the overall amount of expansion is always relatively small locally (except maybe at the very beginning!). So the light from a distant object is always travelling through space that is expanding, but expanding by a very small amount which changes over time. This means the light will make make progress towards us as we expand away from the light. To put this into perspective, it is theorised that the radiation from the most distant objects was only around 40 million light years away from this point in space when the radiation was emitted, over 13 billion years ago. It took that long for it to reach us, through space that is expanding.

As the light is travelling, the object it came from is expanding away from 'behind' where the light is, and we are expanding away along the direction of travel. In the past, as the expansion decelerated, the space 'behind' the light was expanding faster than the space in 'front' would be doing when it got there! It is the rate of expansion over distance that defines whether the light will ever reach us or not. So far we can see back to before any stars or galaxies formed. If the expansion continues to accelerate, there will come a point where we won't be able to detect those distant objects any more (or perhaps well will still detect them, but their signature will be 'frozen' in time, with no new information contained in it, and then they will gradually fade out!).

We have observed thousands of objects with a redshift of z=1.5 or over, meaning they are apparently receding at superluminal speeds. It seems like all the distant objects are receding superluminally.

[Cougar]

Indeed. We cannot tell from looking at an objects spectrum alone what form of redshift it is...
* * *
It seems like all the distant objects are receding superluminally.

I believe all of that was very well put.

One question... or clarification: All the distant objects over z=1.5 are receding superluminally now, in our time frame? Apparently they weren't when they emitted the light that is currently reaching us.

[speedfreek]

One question... or clarification: All the distant objects over z=1.5 are receding superluminally now, in our time frame? Apparently they weren't when they emitted the light that is currently reaching us.

It is assumed that the most distant galaxies formed something up to 750 million years after the big bang, and at that time they were around 1-2 billion light years away from the area in space where our galaxy formed, and already receding from this area of space superluminally due to the rapid metric expansion at the time.

But the expansion was decelerating, and the hubble parameter is always increasing, so there came a time when their light found itself inside the hubble distance - inside our observable universe. But that light still took around 13 billion years to cross what was originally only 1-2 billion light years of space.

It is that extremely fast rate of expansion early on that causes space-time to be highly curved over the largest distances. The CMB was emitted around 300,000 years after the big bang (when the space where the distant galaxies would eventually form was only 40 million light years from this point in space) and has a redshift of z=1089 (super-superluminal!) and by the time we reach 750 million years after the big bang, by which time the oldest galaxies are theorised to have formed (1-2 billion light years away), their redshift is something over z=7.0.

And of course we must always remember that however fast galaxies are receding, in their local space they aren't moving very much at all. But the most distant objects would see us (or this point in space at it was 13 billion years ago) receding superluminally. Which brings us neatly to the comoving distance - the distance we think those objects are from us right now. This distance is estimated to be around 46 billion light years.

[astrocat]

Anybody ever think that the Cosmos is Speeding Up because it's falling? It is pretty heavy, you know, and there really is nothing holding it up. I'm deadly serious.