Why is Dark Energy required ?

[The Code]

Milne's model is a particularly simple instance of a class of models known as Robertson-Walker metrics. When Chris made his claim, I assumed that he was envisioning an argument for it that would work in all Robertson-Walker metrics. Such an argument would necessarily be flawed, as I have shown by exhibiting some instance of a Robertson-Walker metric in which the conclusion is false.

Oh you mean this?
http://en.wikipedia.org/wiki/Friedmann% ... ker_metric
Can You show me where its flawed? Please, And Why.

There is nothing wrong about Robertson-Walker metrics.

There is (unless I'm mistaken, and then I'd appreciate having pointed out where) something wrong with the the proposition that
IF (a) the universe is described by a Robertson-Walker metric,
THEN (b) two galaxies that recede from each other at speed >c are causally decoupled until their mutual speed drops below c.

That,s easy no information can travel faster than light at c. Distant parts of the Universe are expanding greater than c. Which is why we can't see it.

tc

[Henning Makholm]

There is (unless I'm mistaken, and then I'd appreciate having pointed out where) something wrong with the the proposition that
IF (a) the universe is described by a Robertson-Walker metric,
THEN (b) two galaxies that recede from each other at speed >c are causally decoupled until their mutual speed drops below c.

That,s easy no information can travel faster than light at c. Distant parts of the Universe are expanding greater than c. Which is why we can't see it.

The information in my example is in the form of a photon that always travels at speed c relative to the galaxies it passes on its way towards us. Nevertheless, given enough time it eventually reaches us even though it started at a galaxy whose distance to us increases by more than one lightyear a year.

As a concrete example with invented numbers, start the experiment 10 gigayears after the Big Bang. At that time Galaxy A is 20 gigalightyears from us and receding at speed 2c. It emits a photon. Initially the photon recedes from us at speed c.

Now let's predict its trajectory roughly using 10-gigayear integration steps.

At T=20 Gy, galaxy A is 40 Gly from us. The photon, not receding quite as fast as galaxy A, is only 30 Gly away. At that point it passes galaxy B, which recedes from us at 2c*(30/40) = 1.5c. So the photon is receding from us at speed 0.5c.

At T=30 Gy, galaxy A is 60 Gly from us. The photon is now 35 Gly away. At that point it passes galaxy C which recedes from us at 2c*(35/60) = 1.166 c. So the photon is receding from us at speed 0.166c.

At T=40 Gy, galaxy A is 80 Gly from us. The photon is now 36.66 Gly away. At that point it passes a galaxy which recedes from us at 2c*(36.66/80) = 0.9166c. So now the photon is actually getting 0.0833 light years closer to us for each passing year.

At T=50 Gy, galaxy A is 100 Gly from us. The photon is 35.833 Gly away, passing a galaxy that recedes from us at 2c*(35.833/100) = 0.7166c. It is approaching us at 0.2833 lightyears per year.

At T=60 Gy, galaxy A is 120 Gly away, the photon is 32 Gly away, passing a galaxy that recedes at 2c*(32/120) and itself closing in at 0.466c.

At T=70 Gy, galaxy A is 140 Gly away, photon is 27.33 Gly away, closing in on us at 1c - 2c*(27.33/140) = 0.61c.

T=80 Gy, galaxy A 160 Gly away, photon 21.1 Gly away, closing at 1c - 2c*(21.1/160) = 0.73c.

T=90 Gy, galaxy A 180 Gly away, photon 13.8 Gly away, closing at 1c - 2c*(13.8/180) = 0.85c.

T=100 Gy, galaxy A, 200 Gly away, photon 5.3 Gly away, closing at 0.95c.

About 105 gigayears after the Big Bang, the photon finally reaches us.

[The Code]

The information in my example is in the form of a photon that always travels at speed c relative to the galaxies it passes on its way towards us. Nevertheless, given enough time it eventually reaches us even though it started at a galaxy whose distance to us increases by more than one lightyear a year.

Unless your Photon, is 13.3 billion light years away, And expansion is accelerating and your Photon will never get here.

tc

[Henning Makholm]

Unless your Photon, is 13.3 billion light years away, And expansion is accelerating and your Photon will never get here.

It won't necessarily get here in an accelerating universe, no. Whether it does, depends on how fast the acceleration is. My calculation above assumes an acceleration of zero. Clearly, by way of continuity, some small amount of acceleration can be tolerated without preventing the photon from reaching us.

In any case, all I claim is that Chris' assertion is not true in all Robertson-Walker universes. It is enough for me to show it to be false in one of them (namely the non-accelerating case).

[The Code]

Unless your Photon, is 13.3 billion light years away, And expansion is accelerating and your Photon will never get here.

It won't necessarily get here in an accelerating universe, no. Whether it does, depends on how fast the acceleration is. My calculation above assumes an acceleration of zero. Clearly, by way of continuity, some small amount of acceleration can be tolerated without preventing the photon from reaching us.

In any case, all I claim is that Chris' assertion is not true in all Robertson-Walker universes. It is enough for me to show it to be false in one of them (namely the non-accelerating case).

And the point is: What has been observed, (It has been seen) Parts of the Universe can not be seen because of the Above.
And that is what Chris was telling you.

tc

[Henning Makholm]

And the point is: What has been observed, (It has been seen) Parts of the Universe can not be seen because of the Above. And that is what Chris was telling you.

Could you try to reexpress that? Your text does not make much sense to me here?

If you are talking about something you claim to be true just for our universe, and not for all possible Robertson-Walker universes, then I'd like to know more about the particular observed features of our universe that makes your claim true for us, but not generally.

[The Code]

then I'd like to know more about the particular observed features of our universe that makes your claim true for us, but not generally.

There ya go.

http://en.wikipedia.org/wiki/Observable_universe

tc

ps. I Don't do universes if its more than ours. We need to understand this one first Huh?

[Henning Makholm]

then I'd like to know more about the particular observed features of our universe that makes your claim true for us, but not generally.

There ya go.
http://en.wikipedia.org/wiki/Observable_universe

That article appears to contain no claim even remotely like the one I'm objecting to here.

On the contrary, it states:
The comoving distance from Earth to the edge of the visible universe (also called the particle horizon) is about 14 billion parsecs (46.5 billion light-years) in any direction.
Objects that are 46.5 Gly away from us now will be receding from us at about 3 times c. Due to the acceleration of the universe they will have receded slightly slower than that in the past, but never (checking with a graph in arXiv:astro-ph/0205166v2) by a factor as large as 3. So most of the observable universe actually consists of galaxies that recede from us faster than c and always have done so. Nevertheless, they are observable in our current era.

ps. I Don't do universes if its more than ours. We need to understand this one first Huh?

If we don't endeavor to understand the structure of our theories (and, in particular, make sure we understand how our theories lead to which conclusions and which assumptions in the theories each of those conclusions rests on), then we have no reasonable hope of ever using those theories to understand the universe.

[swainy]

then I'd like to know more about the particular observed features of our universe that makes your claim true for us, but not generally.

There ya go.
http://en.wikipedia.org/wiki/Observable_universe

That article appears to contain no claim even remotely like the one I'm objecting to here.

On the contrary, it states:
The comoving distance from Earth to the edge of the visible universe (also called the particle horizon) is about 14 billion parsecs (46.5 billion light-years) in any direction.
Objects that are 46.5 Gly away from us now will be receding from us at about 3 times c. Due to the acceleration of the universe they will have receded slightly slower than that in the past, but never (checking with a graph in arXiv:astro-ph/0205166v2) by a factor as large as 3. So most of the observable universe actually consists of galaxies that recede from us faster than c and always have done so. Nevertheless, they are observable in our current era.

ps. I Don't do universes if its more than ours. We need to understand this one first Huh?

If we don't endeavor to understand the structure of our theories (and, in particular, make sure we understand how our theories lead to which conclusions and which assumptions in the theories each of those conclusions rests on), then we have no reasonable hope of ever using those theories to understand the universe.

The Universe is a lot bigger than 46.5 billion light years. That is all we can see. It is probably twice that size. Or more.

TC

[swainy]

Why has nobody mentioned this?

The Flatness Problem?

http://en.wikipedia.org/wiki/Flatness_problem

tc

PS. I May have some idea's.

[Chris Peterson]

Why has nobody mentioned this? The Flatness Problem?

What's to mention? There are a number of "problems" associated with the core Big Bang theory, and each has resulted in refinement of the theory. The flatness "problem" is no different- it isn't like this problem poses some contradiction to the standard theory- there are several possible solutions, the main one being inflation.

[swainy]

These questions are too easy

What's to mention?

Ummm, Something from beyond nothing, to something with to many zero's, in less than a second, Slow down for a while, then start accelerating again. You could almost think, this was an ongoing unknown mechanism.

There are a number of "problems" associated with the core Big Bang theory, and each has resulted in refinement of the theory.

Is there a few missing items? making the theory debatable? One like, what caused inflation? And many other problems that we have not got to the questions let alone the answers?

The flatness "problem" is no different- it isn't like this problem poses some contradiction to the standard theory- there are several possible solutions, the main one being inflation.

Don't you need to understand inflation properly before you can make the assumption it takes away other problems?

Why no runaway expansion universe? Or contraction? Looks like Goldilocks porridge was just right in more ways than we could ever imagine.

tc

[Chris Peterson]

Ummm, Something from beyond nothing, to something with to many zero's, in less than a second, Slow down for a while, then start accelerating again. You could almost think, this was an ongoing unknown mechanism.

No, I think it is probably explained by inflation. There are other, less likely explanations as well. I don't know what "beyond nothing" means; the transition was from one finite value to another, and no matter how large the difference, that's still very different from your characterization.

Is there a few missing items? making the theory debatable? One like, what caused inflation? And many other problems that we have not got to the questions let alone the answers?

Any theory is potentially debatable. Nobody considers any of the BB theories to be complete; that doesn't mean they are unlikely to be substantially correct, however. An inability to explain the cause of inflation does not mean it didn't occur, nor that it can't be a valid component of a BB theory.

Don't you need to understand inflation properly before you can make the assumption it takes away other problems?

No. If there is observational evidence that it occurred, and its existence doesn't otherwise break any important theories, it can be used effectively and reasonably.

[swainy]

What is the name of the only natural satellite which orbits our planet, Earth?: Ummm hang on this is tricky. Is it a Dog?

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You answered all my post with this reply Chris:

Any theory is potentially debatable. Nobody considers any of the BB theories to be complete; that doesn't mean they are unlikely to be substantially correct, however. An inability to explain the cause of inflation does not mean it didn't occur, nor that it can't be a valid component of a BB theory.

No. If there is observational evidence that it occurred, and its existence doesn't otherwise break any important theories, it can be used effectively and reasonably.

What is the observational evidence that it occurred? I noticed you used the word "IF" Can you Show me, observational evidence that it occurred?

tc

[Chris Peterson]

What is the observational evidence that it occurred? I noticed you used the word "IF" Can you Show me, observational evidence that it occurred?

Inflation predicts a flat Universe, which is observed. Inflation explains why there are no magnetic monopoles, which is observed. Inflation explains how quantum fluctuations evolved into the current large scale structure of the Universe, which is observed. Inflation predicts a very specific structure in the CMB, which is observed. Of all the theories that attempt to describe how the Universe got from its earliest form to the present form, inflation is the most parsimonious, requiring the least number of free parameters.

It is precisely because inflation is so well supported by observational evidence that most cosmologists accept it as a very likely component of the early Universe, even in the absence of a well defined "cause".

[swainy]

swainy wrote:What is the observational evidence that it occurred? I noticed you used the word "IF" Can you Show me, observational evidence that it occurred?


Inflation predicts a flat Universe, which is observed. Inflation explains why there are no magnetic monopoles, which is observed. Inflation explains how quantum fluctuations evolved into the current large scale structure of the Universe, which is observed. Inflation predicts a very specific structure in the CMB, which is observed. Of all the theories that attempt to describe how the Universe got from its earliest form to the present form, inflation is the most parsimonious, requiring the least number of free parameters.

That is me taking your word for it, and not been shown anything!

Tomorrow

tc

[Ann]

Swainy, I don't know how long you have been interested in the fate of the universe, whether it is going to expand forever or if it's going to collapse. When I first heard this question explained to me, in the 1970s, I was absolutely horrified at the prospect that the universe might collapse. After that I read everything I could find about the fate of the universe and how astronomers could learn about it. I read about all kinds of attempts to measure the expansion rate of the universe. For the longest time astronomers couldn't make any good measurements, because the expansion is almost constant in the moderately local universe. (And anyway, astronomers back then were always looking for signs that the universe was slowing down, never that it was speeding up.) So they found nothing.

But in the late 1990s two teams began searching for very distant supernovae of type Ia, which are good "standard candles" - that is, their absolute brightness is known - and whose lightcurves can be recognized. The astronomers measured the redshifts of the supernovae, and they compared the redshifts with the measured brightnesses versus the known absolute brightness of the supernovae. What the redshift of a supernova measures is how much the universe has expanded since the light of the supernova was emitted. Redshift says nothing about how far away the supernova was when it emitted its light. Astronomers can easily calculate how bright or faint the supernova should appear to us if the universe has expanded at a constant rate. If the universe has slowed down, then the supernovae with really high redshifts ought to be quite bright for their redshifts. That is because they would have been comparatively close to us when they emitted their light in a universe that is gradually losing speed. In a universe which is forever expanding at a constant rate, the supernovae with high redshifts would have been farther away when they emitted their light, so they would look fainter to us.

The two supernovae teams in the late 1990s were trying to find out if the universe was slowing down (in which case the really distant supernovae would be bright for their redshifts) or if the universe was expanding at a constant rate (in which case the really distant supernovae would be as faint as their redshifts predicted). To their amazement, the two teams of astronomers found that the really distant supernovae were fainter than their redshifts predicted they would be. This could only mean that the supernovae were farther away when they exploded than their redshifts predicted they would be. But if these distant supernovae were farther away than their redshifts predicted they would be, that could only mean the the universe was speeding up.

And that's why we need an accelerator for the universe. That accelerator is known as "dark energy".

Ann

[swainy]

Thanks for your time Ann.

I understand standard candles and that they prove accelerated expansion. What i don,t understand is: The expansion either way, with out tipping the scales. To a big crunch, or a big Rip. Its like inflation stopped and something else took its place. Or are they both the same thing? In a short space of time, to the gargantuan size we see the universe today.

tc

[Chris Peterson]

What i don,t understand is: The expansion either way, with out tipping the scales. To a big crunch, or a big Rip. Its like inflation stopped and something else took its place. Or are they both the same thing?

That is an important, and open, question. Most people consider the two to have fundamental similarities, based on vacuum energy. But whether dark energy as we now observe it is the same thing that caused inflation, or is just similar, is not known.

[The Code]

What i don,t understand is: The expansion either way, with out tipping the scales. To a big crunch, or a big Rip. Its like inflation stopped and something else took its place. Or are they both the same thing?

That is an important, and open, question. Most people consider the two to have fundamental similarities, based on vacuum energy. But whether dark energy as we now observe it is the same thing that caused inflation, or is just similar, is not known.

That is interesting and something I was unaware scientists considered. Although I had thought about the possibility of expansion been the little brother of inflation myself. So just for the interest, and to further this debate, could we take it to the next possible step? On the pretext (Dark Energy, Expansion, Inflation) are the same thing.

I know its all conjecture, but It may be interesting to rewind the clock and try to understand how in rewind Dark Energy=Expansion and Possibly = Inflation Could have came about.

A propagator that lost its power to increase the size of the universe at the end of inflation? What Could that be? What Propagator? Inwards? Outwards?

tc

[Chris Peterson]

I know its all conjecture, but It may be interesting to rewind the clock and try to understand how in rewind Dark Energy=Expansion and Possibly = Inflation Could have came about.

Well, that's precisely what a good many researchers are working on. But this work is generally very theoretical, very mathematical, and doesn't translate well into ideas easily discussed in a forum like this.

[swainy]

A propagator that lost its power to increase the size of the universe at the end of inflation? What Could that be? What Propagator? Inwards? Outwards?

I know its all conjecture, but It may be interesting to rewind the clock and try to understand how in rewind Dark Energy=Expansion and Possibly = Inflation Could have came about.

Well, that's precisely what a good many researchers are working on. But this work is generally very theoretical, very mathematical, and doesn't translate well into ideas easily discussed in a forum like this.

Why, I,m going to explain here in very simple terms, an idea I have that everyone should understand. Here goes:
Everything has a finite speed. Light, Gravity, Electricity, Inflation, oops I made a mistake there. Or did I ?

How would that work? If mass Or more over, the amount of Mass in the universe effects time, space time expansion. Would not the amount of Mass in the inflated universe put a stop to inflation via a finite expansion rate? That was easy enough to understand was it not?

tc