45 replies to this topic

[Flummoxed]

 

What are you confused about? It's actually very simple.

If galaxies were able to recede from each other at or over the speed of light then all the paradoxes that arise from that would still apply. Time dilation, length contraction and the mass of the other object would all become infinite at the speed of light because they all measure the same speed of light relative to themselves despite their velocities relative to each other.

 

According to the model the distance between the galaxies is not increasing at a constant rate, the distance between the galaxies is increasing at an ever increasing rate.

 

This is not what's observed though. If galaxies were moving away from us faster the greater the distance between us and them then that's just another way of saying that the rate of the supposed expansion is increasing over time but if that were true then we'd have to see galaxies moving away from us slower the further we look back in time.

Yes they'd actually be moving away from us faster the further away they are but we're seeing them as they were, not as they are now. The further away in space we look, the further back in time we see.

In an expanding universe in which the rate of the expansion is increasing over time we'd be seeing the closer galaxies moving away from us faster than the more distant galaxies because we'd be seeing the closer galaxies at a time closer to the current rate of expansion and further galaxies at a time when the expansion rate was slower.

 

This might help explain where I think you are wrong http://curious.astro...ht-intermediate Is there any point in this link you disagree with.

 

The outer edge of the universe is receding away from us at approaching the speed of light. Observed galaxies are eventually red shifted out of view due to the rate of expansion and the fact they are moving away above c. This in no violates SR or GR. The space between us and the outer galaxies is expanding, at approx 70km/s for every parsec away they are. This means the distance from us to galaxies on the outer edge of the visible universe is increasing faster than the distance between us and galaxies that are closer to us. Yes the light arriving from the outer universe left those galaxies a few billion years ago, and light they transmit today will probably never reach this planet, as those galaxies will be receding at above c, and will be red shifted out of existence.

[OceanBreeze]

This might help explain where I think you are wrong http://curious.astro...ht-intermediate Is there any point in this link you disagree with.

 

The outer edge of the universe is receding away from us at approaching the speed of light. Observed galaxies are eventually red shifted out of view due to the rate of expansion and the fact they are moving away above c. This in no violates SR or GR. The space between us and the outer galaxies is expanding, at approx 70km/s for every parsec away they are. This means the distance from us to galaxies on the outer edge of the visible universe is increasing faster than the distance between us and galaxies that are closer to us. Yes the light arriving from the outer universe left those galaxies a few billion years ago, and light they transmit today will probably never reach this planet, as those galaxies will be receding at above c, and will be red shifted out of existence.

 

I don't think Awal is arguing against the fact that the universe is expanding and galaxies at the edge are moving away faster from our reference point than galaxies closer to us.

 

What he is arguing is that the expansion is accelerating. Sorry for speaking for him, but he is saying the fact that galaxies that are farther away are moving faster and the fact they are further back in time, shows that the expansion is slowing down, not speeding up.

 

I'm not sure how to answer that except this: even though the farther away a galaxy is, the faster it is moving away, it is NOT moving away as fast as predicted by the Hubble constant! That means the galaxies were moving apart slower than they are today, so the expansion is speeding up!

[A-wal]

This might help explain where I think you are wrong http://curious.astro...ht-intermediate Is there any point in this link you disagree with.

Cheers, I'll check out the link later.

 

The outer edge of the universe is receding away from us at approaching the speed of light. Observed galaxies are eventually red shifted out of view due to the rate of expansion and the fact they are moving away above c. This in no violates SR or GR.

Still no! The velocity addition formula would still apply, it doesn't go away with increased scale, and the cause of the increase of the relative velocity in no prevents the paradoxes and mass increase that would arise with a relative velocity > c.

 

Try going through the 'expansion of space' method of acceleration on a smaller scale for the examples of SR type thought experiments and then explain how that method of acceleration makes any difference to the SR physics of relative velocity.

 

The space between us and the outer galaxies is expanding, at approx 70km/s for every parsec away they are. This means the distance from us to galaxies on the outer edge of the visible universe is increasing faster than the distance between us and galaxies that are closer to us.

But if that were true then we'd have to see galaxies moving away from slower the further we look back in time and that's not what we see.

 

Yes the light arriving from the outer universe left those galaxies a few billion years ago...

At a time when the the supposed expansion rate was slower, meaning we'd see them moving away slower than closer galaxies.

[Flummoxed]

I don't think Awal is arguing against the fact that the universe is expanding and galaxies at the edge are moving away faster from our reference point than galaxies closer to us.

 

What he is arguing is that the expansion is accelerating. Sorry for speaking for him, but he is saying the fact that galaxies that are farther away are moving faster and the fact they are further back in time, shows that the expansion is slowing down, not speeding up.

 

I'm not sure how to answer that except this: even though the farther away a galaxy is, the faster it is moving away, it is NOT moving away as fast as predicted by the Hubble constant! That means the galaxies were moving apart slower than they are today, so the expansion is speeding up!

 

If that is what he is trying to say he was not making it clear. 

 

Galaxies which we see light from at the outer edge of the universe today takes a long time to get here, and was transmitted some time in the past. Those galaxies are moving away faster today than when the original light was transmitted. 

 

Yes galaxies were moving apart slower in the past than they are today, the rate of increase of distance has increased to the outer galaxies.

 

BREAK

 

I did not realize the hubble constant is not constant, I need to check this https://www.forbes.c...e/#7224ab8c9c9a variations in hubble constant over time. 

 

If the hubble constant is not constant does that explain what is confusing me 

 

The wiki link on the hubble constant is better https://en.wikipedia...i/Hubble's_law 

Edited by Flummoxed, 04 January 2019 - 10:03 AM.

[Flummoxed]

 

 

Still no! The velocity addition formula would still apply, it doesn't go away with increased scale, and the cause of the increase of the relative velocity in no prevents the paradoxes and mass increase that would arise with a relative velocity > c.

 

Try going through the 'expansion of space' method of acceleration on a smaller scale for the examples of SR type thought experiments and then explain how that method of acceleration makes any difference to the SR physics of relative velocity.

 

Observers on a Galaxy on the edge of the visible universe looking in our direction will see us on the outer edge of their visible universe. We know that beyond us there are other galaxies moving away from us in the opposite direction to the distant observer galaxy, further more it is likely more galaxies lie over our visible horizon as the distant observer galaxy would confirm. The distant observer galaxy would also be able to confirm the outer edges of their visible universe is moving away from them at approaching c . Andromeda and about 100 other galaxies are moving towards us, Andromeda is closing the distance at approx 0.3c. 

 

The galaxies themselves are stationary in an expanding space, "think surface of expanding ballon", the galaxies are not being accelerated the distance is just increasing, due to dark energy/cosmological constant. 

 

When the light left distant galaxies x billion years ago the galaxies were moving away slower, now time has passed and the distance between us and them has increased, any new light transmitted today will take longer to get here and be more red shifted.

 

A little thought experiment back at you, Observers from a galaxy at the edge of the universe, will calculate we are moving away from them at lets say 0.7c would they be able to observe Andromeda moving towards us and blue shifted at 0.3c 

[Dubbelosix]

This might help explain where I think you are wrong http://curious.astro...ht-intermediate Is there any point in this link you disagree with.

 

The outer edge of the universe is receding away from us at approaching the speed of light. Observed galaxies are eventually red shifted out of view due to the rate of expansion and the fact they are moving away above c. This in no violates SR or GR. The space between us and the outer galaxies is expanding, at approx 70km/s for every parsec away they are. This means the distance from us to galaxies on the outer edge of the visible universe is increasing faster than the distance between us and galaxies that are closer to us. Yes the light arriving from the outer universe left those galaxies a few billion years ago, and light they transmit today will probably never reach this planet, as those galaxies will be receding at above c, and will be red shifted out of existence.

 

 

I don't think you quite understand what either me or awall were talking about. The receding of spacetime on the outer-rim [if that is indeed what is happening] is still a phenomenon that would lie in the past history of the universe [simply because] it has taken so long for that light to reach us. Basically what this means, is that the further you look back at signals from more and more distant objects, you are looking further into the past, not the present. 

[A-wal]

I don't think Awal is arguing against the fact that the universe is expanding and galaxies at the edge are moving away faster from our reference point than galaxies closer to us.

 

What he is arguing is that the expansion is accelerating. Sorry for speaking for him, but he is saying the fact that galaxies that are farther away are moving faster and the fact they are further back in time, shows that the expansion is slowing down, not speeding up.

Yes! Someone finally understood the question.

I do have other issues with the expansion interpretation, like it being a violation of SR because it does not mean that the velocity addition formula doesn't apply to galaxies moving away from each other. If they agree on the speed of light then the velocity addition formula has to apply for the same reasons it usually does. But let's just stick the supposed increasing expansion rate for the moment.

 

I'm not sure how to answer that except this: even though the farther away a galaxy is, the faster it is moving away, it is NOT moving away as fast as predicted by the Hubble constant! That means the galaxies were moving apart slower than they are today, so the expansion is speeding up!

That's the first real answer I've had. I didn't actually know that more distant galaxies move away slower than the Hubble constant predicts, I thought it was pretty much a direct distance to redshift correlation.

My issue is that if galaxies were following the Hubble constant then that would mean an expansion that's slowing over time. They take the Hubble constant as a starting point in which the universe expands at a constant rate.

 

 

Edit:

I've done some digging and as far as I can tell they do consider a direct distance to recessional velocity correlation as an increasing rate of expansion rather than as a constant rate of expansion.

 

I can find no acknowledgment at all that the further away the galaxy, the further back in the supposed expansion we're seeing them.

Edited by A-wal, 04 January 2019 - 06:32 PM.

[OceanBreeze]

This is the best explanation that I have found of the expanding universe theory.

 

OK, it is a Scientific American article but it cannot be dismissed as pop sci because of who the authors are:

 

 ADAM G. RIESS and MICHAEL S. TURNER have led the way in exploring the history of the universe’s expansion. Riess is an associate astronomer at the Space Telescope Science Institute (the science headquarters for the Hubble Space Telescope) and adjunct associate professor of physics and astronomy at Johns Hopkins University. In 1998 he was lead author of the study published by the High-z Supernova Team announcing the discovery of an accelerating universe. Turner, the Rauner Distinguished Service Professor at the University of Chicago, is now serving as the assistant director for mathematical and physical sciences at the National Science Foundation. His 1995 paper with Lawrence M. Krauss predicted cosmic acceleration, and he coined the term “dark energy.”

 

 

The entire article is worth reading but here are the relevant paragraphs for what we are discussing:

 

“Cosmologists have other reasons to expect that the expansion of the universe has not always been speeding up. If it had been, scientists would be at a loss to explain the existence of the cosmic structures observed in the universe today. According to cosmological theory, galaxies, galaxy clusters and larger structures evolved from small inhomogeneities in the matter density of the early universe, which are revealed by variations in the temperature of the cosmic microwave background (CMB). The stronger attractive gravity of the overdense regions of matter stopped their expansion, allowing them to form gravitationally bound objects—from galaxies such as our own to great clusters of galaxies. But if the expansion of the universe had always been accelerating, it would have pulled apart the structures before they could be assembled. Furthermore, if the expansion had been accelerating, two key aspects of the early universe—the pattern of CMB variations and the abundances of light elements produced seconds after the big bang—would not agree with current observations”

 

“Nevertheless, it is important to look for direct evidence of an earlier, slowing phase of expansion. Such evidence would help confirm the standard cosmological model and give scientists a clue to the underlying cause of the present period of cosmic acceleration. Because telescopes look back in time as they gather light from far-off stars and galaxies, astronomers can explore the expansion history of the universe by focusing on distant objects. That history is encoded in the relation between the distances and recession velocities of galaxies. If the expansion is slowing down, the velocity of a distant galaxy would be relatively greater than the velocity predicted by Hubble’s law. If the expansion is speeding up, the distant galaxy’s velocity would fall below the predicted value. Or, to put it another way, a galaxy with a given recession velocity will be farther away than expected— and hence fainter—if the universe is accelerating”

 

               “If the dimming is the result of a recent cosmic speedup that followed an earlier era of deceleration, supernovae from the slowdown period would appear relatively brighter”

 

               “The Advanced Camera for Surveys, a new imaging instrument installed on the space telescope in 2002, enabled scientists to turn Hubble into a supernova-hunting machine. Riess led an effort to discover the needed sample of very distant type Ia supernovae by piggybacking on the Great Observatories Origins Deep Survey. The team found six supernovae that exploded when the universe was less than half its present size (more than seven billion years ago); together with SN 1997ff, these are the most distant type Ia supernovae ever discovered. The observations confirmed the existence of an early slowdown period and placed the transitional “coasting point” between slowdown and speedup at about five billion years ago”

 

 

I bolded the most relevant sentences to try and make the correlation clearer as the article does jump around a bit. In a nutshell, they found very distant (ancient) type Ia supernova that appeared brighter (closer) than they should be based on the predictions of Hubble’s law, and concluded that the ancient expansion was slower than it is today, so the expansion is speeding up according to their findings.

 

 

 

 

[A-wal]

Ah cheers. I've just glanced at it so far and can already tell it's a much better explanation then anything I found. Most articles about expansion and dark energy are watered down to the point of just being flat out wrong.

[Dubbelosix]

I must admit it is one of the better explanations; I keep up with the news as you will know in the scientific world and the model is suspect. There are arguably discrepancies in the way we think about expansion. I certainly do not believe the universe is accelerating, but at best, it is still expanding and possibly slowing down.

 

I think the issue has to be drawn up logically - the findings can be interpreted in different ways.

 

1. The further we look at distant objects, the further we look back in time.

 

Fact.

 

2. The further an object is according to Hubbles law, the faster it appears to recede in conjunction with 1.

 

Fact.

 

3. The further a system is, the light encodes a situation about that system in the past, not the present we asymptotically agree with.

 

Fact.

 

This leaves open the interpretation that the universe is in fact slowing down, because it would mean that the oldest radiation is telling us something about how the universe once accelerated. How do you get an accurate reading of the expansion of a universe? We can roughly make out an approximation to its age, but locally systems are not moving away faster than light (vacuum drag). Recently the so-called acceleration of the universe has recently be called into question - for instance, Forbes has entered an article justifying that we actually don't know how fast our universe is expanding

 

https://www.forbes.c...g/#2cb408da36cf

 

If you take all three statements above, my conclusion is that expansion is in fact slowing down, not accelerating.

[Dubbelosix]

Because it is not as simple as saying there was a slow down rate, that first of all is an assumption, let alone one for a distant past. The fact signals from the past tell us about the past only, we cannot use it as a true gauge to state what type of expansion the universe is experiencing. You didn't understand this implication before and you probably don't see it still.

[Dubbelosix]

Well if you consider it non-standard, maybe you could explain what the Forbes article tells us, then try and refute the list of [fact] claims I made about acceleration?