Universe expansion

[mikeDnight]

Love to agree with you as red shift relies on ancient data which cannot be confirmed. However, to agree with you, one would have to deny General Relativity. By definition, photons travel at light speed and therefore if they had any mass at all it would have increased infinitely at light speed, making one photon the heaviest object in the universe.

Hmmm. If the minute mass of a photon causes it to become the heaviest object in the universe according to theory, then this would also become destructively apparent when subatomic particles are released in atom smashers such as the hadron collider. These particles and subatomic particles travelling at near light speed, have a significantly greater mass than a photon, yet they have no destructive effect on the collider, which they would have if their mass had increased to near infinity.

Even if we ignored the mass altogether and think purely in terms of energy levels, particles and subatomic particles when in collision lose energy and release quanta of lower energy. The same should therefore apply to a photon, which is a quanta of energy. The high energy photon would lose energy with each collision and move nearer to the red end of the spectrum and beyond.

This of course is just a personal hypothesis, which to me seems as good as anything else that's currently on the table. And as it is simplicity itself, I feel it is nearer to the truth than the ramblings of the lunatic minds of modern academia, many of whom follow blindly those who went before, for fear of ridicule and because their funding would be taken away.

Mike :-)

[kalasinman]

There is an infinite difference between  near the speed of light and at the speed of light. The objects in collider collisions are released at that near  speed, not accelerated to it.

[hjw]

Hmmm. If the minute mass of a photon causes it to become the heaviest object in the universe according to theory, then this would also become destructively apparent when subatomic particles are released in atom smashers such as the hadron collider. These particles and subatomic particles travelling at near light speed, have a significantly greater mass than a photon, yet they have no destructive effect on the collider, which they would have if their mass had increased to near infinity.

Even if we ignored the mass altogether and think purely in terms of energy levels, particles and subatomic particles when in collision lose energy and release quanta of lower energy. The same should therefore apply to a photon, which is a quanta of energy. The high energy photon would lose energy with each collision and move nearer to the red end of the spectrum and beyond.

This of course is just a personal hypothesis, which to me seems as good as anything else that's currently on the table. And as it is simplicity itself, I feel it is nearer to the truth than the ramblings of the lunatic minds of modern academia, many of whom follow blindly those who went before, for fear of ridicule and because their funding would be taken away.

Mike :-)

Particles in a cyclotron do travel virtually at the speed of light. They do so after they are shot from a linear accelerator into the ring. From then on energy is pumped into the system with each rotation to make the particles heavier according to Einstein's E=mc² or dE=dm*c². Once the particles collide with the target the stored energy is released and can form exotic matter/antimatter pairs. The more energy, the heavier the potential particle pair. ~2GeV give you a proton/antiproton for example. And yes, if you have a failure in the containment magnetic field during the run, the particles will do a lot of damage to the infrastructure.

Redshift, on the other had is a phenomenon of the Doppler effect. A wave which is moving away from the observer at a speed comparable to that of the propagation speed is significantly stretched while the wave is bunched up when moving towards the observer. In acoustics, we take it for granted and don't even notice it anymore. A car coming towards us has a higher pitch than after it passed us. So you get this "iiii-ooo-uuu" sound. Light does the same however the object has to move very much faster than the car to make us see the difference. Wouldn't it be cool to have a blue car approaching us and then turn red when it passes?

[George Jones]

What a fascinating discussion, most of which I struggle to understand

One serious question I have about the accelerating expansion theory is what is the force that is acting upon the objects in the universe to make them accelerate?

Gravity.

[DRT]

That's a concept I struggle with, George. It is not the first time I have heard that answer but I can't get my head around the idea that a universal force that pulls things together can seemingly also push everything apart. Very counter intuitive for the unscientific mind to comprehend.

[ronin]

Could you explain why space-time itself can go faster than  the speed of light? To understand this a bit better it would be also helpful if you could provide some example of this and what are the consequences of this. 

Thanks

The observable limit of the universe is 13.8 (or whatever billion years) that is because after that light from the bits of the universe beyond that have to travel agains space-time that is moving away from us at greater then the speed of light. So they are dragged away faster then the are heading our direction.

The speed of light is an apparent limit for anything in space-time, that does however not hold for space-time itself.

One consequence is that we cannot see anything beyond the 13.8 billion light year radius. This is the Hubble Radius, defined on the DD site nicely as:

The distance from the observer at which the cosmological recession velocity of a galaxy would equal the speed of light.

Beyond that radial value then the recession is greater the the speed of light. However it is not the objects that are moving at greater then "c" but space-time.

Consequece is simple, we can only see so far = the Hubble Radius.

I also suspect it means that as the acceleration increases then the Hubble Radius will decrease.

[George Jones]

Newton's theory of gravity uses one constant of nature (i.e., a number that must be determined empirically), "big" G, the gravitational constant.

Newton's theory, however, is not our best theory of gravity, Einstein's general theory of relativity is. Einstein's theory of gravity works wherever Newton's does, but Einstein's theory also works in situations where Newton's theory fails, like, for example, the orbit of Mercury. Einstein's theory has two constants of nature that must be determined empirically, G and the cosmological constant Lambda.

In an appropriate limit, the effect of Einstein's gravity is given by the sum of two terms, A + B, where term A depends on the constant G and term B depends on the constant Lambda. Term A is attractive, while term B is repulsive.Terms A and B also depend on distance, but in very different ways. Term A gets weaker as distance increases, while term B gets stronger (like the force of a spring) as distance increases. For distances that are not of large cosmological scale, term A dominates, and gravity is attractive. For distances that are of large cosmological scale, term B dominates, and gravity is repulsive.

In our everyday experiences, term B is absolutely completely negligible, so our intuition is for attractive gravity.

[beamer3.6m]

Newton's theory of gravity uses one constant of nature (i.e., a number that must be determined empirically), "big" G, the gravitational constant.

Newton's theory, however, is not our best theory of gravity, Einstein's general theory of relativity is. Einstein's theory of gravity works wherever Newton's does, but Einstein's theory also works in situations where Newton's theory fails, like, for example, the orbit of Mercury. Einstein's theory has two constants of nature that must be determined empirically, G and the cosmological constant Lambda.

In an appropriate limit, the effect of Einstein's gravity is given by the sum of two terms, A + B, where term A depends on the constant G and term B depends on the constant Lambda. Term A is attractive, while term B is repulsive.Terms A and B also depend on distance, but in very different ways. Term A gets weaker as distance increases, while term B gets stronger (like the force of a spring) as distance increases. For distances that are not of large cosmological scale, term A dominates, and gravity is attractive. For distances that are of large cosmological scale, term B dominates, and gravity is repulsive.

In our everyday experiences, term B is absolutely completely negligible, so our intuition is for attractive gravity.

I don't really understand everything said in this thread but I found this answer/information fascinating.

[beamer3.6m]

Newton's theory of gravity uses one constant of nature (i.e., a number that must be determined empirically), "big" G, the gravitational constant.

Newton's theory, however, is not our best theory of gravity, Einstein's general theory of relativity is. Einstein's theory of gravity works wherever Newton's does, but Einstein's theory also works in situations where Newton's theory fails, like, for example, the orbit of Mercury. Einstein's theory has two constants of nature that must be determined empirically, G and the cosmological constant Lambda.

In an appropriate limit, the effect of Einstein's gravity is given by the sum of two terms, A + B, where term A depends on the constant G and term B depends on the constant Lambda. Term A is attractive, while term B is repulsive.Terms A and B also depend on distance, but in very different ways. Term A gets weaker as distance increases, while term B gets stronger (like the force of a spring) as distance increases. For distances that are not of large cosmological scale, term A dominates, and gravity is attractive. For distances that are of large cosmological scale, term B dominates, and gravity is repulsive.

In our everyday experiences, term B is absolutely completely negligible, so our intuition is for attractive gravity.

I don't really understand everything said in this thread but I found this answer/information fascinating.

[ollypenrice]

Could you explain why space-time itself can go faster than  the speed of light? To understand this a bit better it would be also helpful if you could provide some example of this and what are the consequences of this. 

Thanks

This is my understanding but I hope the professionals will comment...

Spacetime is not 'going faster than the speed of light.' In a sense it isn't 'going' anywhere, it is simply expanding. It expands at something like 74 kilometers per second per megaparsec. This means that two objects separated by a megaparsec are moving apart at 74 km per second. Now at this point language starts to confuse us and impede our grasp of what's happening.

It's the word 'moving' which does the damage. In non-cosmological circumstances and scales you can only make an object 'move' by accelerating it. If you don't accelerate it it won't move. Anything which is already moving is moving because it has previously been accelerated. However, going back to our two objects separated by a megaparsec we find that they will 'move' apart due to the expansion of space between them and not because they they have been accelerated. From their point of view they have not been acclerated because the expansion of space has happened equally around them in all directions. Indeed they haven't, in a sense, moved at all.

So there are two kinds of movement. There's movement because you've been made to move (accelerated) and movement because new space has been created between you and your distant neighnbour. They are not at all the same and that's why it is perfectly sensible to envisage galaxies receding at far more than the speed of light. They are not receding because they are 'moving' in the normal sense of the term. 

Historical note; earlier in the thread the theory of the expanding universe was attributed to Edwin Hubble, which is incorrect. In fact Hubble, though happy to milk Einsein's approval for purposes of self-publicity, never fully accepted the idea. Hubble's Law simply describes the observation that the redshift of a galaxy is linearly proportional to its distance in any direction. The cause of that redshift is not specified. In fact the problems of a static universe under general realativity had been excercising several mathematical physicists for some time, starting with Friedman in 1922. The mathematics of the expanding universe go back to the (aptly-named ) Lemaitre in 1927. So, really, the non-static universe was discovered in parallel both mathematically and observationally.

Although I've yet to read it I'm told that Hawking's The Grand Design has things to say about how spacetime might have been in a compressed state at the beginning. On my list!

Olly

[Piero]

This is my understanding but I hope the professionals will comment...

Spacetime is not 'going faster than the speed of light.' In a sense it isn't 'going' anywhere, it is simply expanding. It expands at something like 74 kilometers per second per megaparsec. This means that two objects separated by a megaparsec are moving apart at 74 km per second. Now at this point language starts to confuse us and impede our grasp of what's happening.

It's the word 'moving' which does the damage. In non-cosmological circumstances and scales you can only make an object 'move' by accelerating it. If you don't accelerate it it won't move. Anything which is already moving is moving because it has previously been accelerated. However, going back to our two objects separated by a megaparsec we find that they will 'move' apart due to the expansion of space between them and not because they they have been accelerated. From their point of view they have not been acclerated because the expansion of space has happened equally around them in all directions. Indeed they haven't, in a sense, moved at all.

So there are two kinds of movement. There's movement because you've been made to move (accelerated) and movement because new space has been created between you and your distant neighnbour. They are not at all the same and that's why it is perfectly sensible to envisage galaxies receding at far more than the speed of light. They are not receding because they are 'moving' in the normal sense of the term. 

Historical note; earlier in the thread the theory of the expanding universe was attributed to Edwin Hubble, which is incorrect. In fact Hubble, though happy to milk Einsein's approval for purposes of self-publicity, never fully accepted the idea. Hubble's Law simply describes the observation that the redshift of a galaxy is linearly proportional to its distance in any direction. The cause of that redshift is not specified. In fact the problems of a static universe under general realativity had been excercising several mathematical physicists for some time, starting with Friedman in 1922. The mathematics of the expanding universe go back to the (aptly-named ) Lemaitre in 1927. So, really, the non-static universe was discovered in parallel both mathematically and observationally.

Although I've yet to read it I'm told that Hawking's The Grand Design has things to say about how spacetime might have been in a compressed state at the beginning. On my list!

Olly

Thanks for your reply Olly, you gave me a lot of material to think!

[beamer3.6m]

May I ask what is a megaparsec

[DRT]

May I ask what is a megaparsec

According to Google...

[George Jones]

A parsec is a distance of 3.26 light-years; a megaparsec is a million parsecs.

The name "parsec" is a combination of the terms "parallax" and "arc second". Parallax is a change in direction caused by change in position. For example, the direction in which you have to look to see a picture on the wall changes if you move from one side of a room to the other. An arc second is a very small angle, 1/3600 of a degree.

Imagine that the picture on the wall is a distant star, and you are the Earth. You change position because the Earth changes as it moves along in its orbit. In six months, the Earth moves from side of its orbit to the other. If the change in direction to star is 2 arc seconds, then the distance to the star is 1 parsec.

[ollypenrice]

Just crossed with George who's a professional but bung my post in anyway!

Parsec is a contraction parallax second. If you observe an imaginary star from point x and them move one astronomical unit (the Earth-Sun distance) away to point y for a second observation and measure the angle between these two lines of sight to be an arcsecond then the distance to the object is one parsec. It's easier with a diagram like the one on the right here. https://en.wikipedia.org/wiki/Parsec

A parsec is equivalent to 3.26 lightyears. A megaparsec is just a million of those.

When we say that the universe is expanding at 74 Km per second per megaparsec we have to be careful. 74 Km per second seems fast. However, it takes a megaparsec of space to generate that value. A megaparsec is 3.26 million lightyears - an utterly vast distance. If we look at this entirely in kilomtres we find that a megaparsec is about thirty million million million Km and it takes that much space to drive two objects apart at just 74 Kps.  Over small distances the rate of expansion seems a lot less astonishing.

Olly

[Tiki]

That's a concept I struggle with, George. It is not the first time I have heard that answer but I can't get my head around the idea that a universal force that pulls things together can seemingly also push everything apart. Very counter intuitive for the unscientific mind to comprehend.

Consider the following thought experiment:

We are in a rocket and hover above a test particle located half way between two streams of matter . We notice that the only forces that act upon the particle come from the gravitational influence of the two matter streams. We notice that the two matter streams are composed of identical particles which move with the same speed but in opposite directions. We are able to ascertain that the density of the two matter streams are identical and therefore influence the test particle in equal but opposite ways. It is no surprise that the particle remains at rest in such a symmetrical situation.

We now zoom off in the rocket in a big loop and pass by the test particle, however this time we are moving at the same velocity of one of the matter streams. By the symmetry of the situation and the fact that our idealized rocket doesn't interact with the test particle we are again not surprised to see the particle still at rest just as before. Why shouldn't it be?

When we whizzed by, we took a whole bunch of measurements. The upshot is that the densities of the matter streams no longer appear to be the same! On our flypast one of the matter streams appeared as a stationary line of particles whilst the other appeared to be composed of particles moving at twice the original speed. The moving particles would appear more dense for two reasons: a slight increase in mass of each particle and because we would measure the distance between particles to be less than in the static stream. (special relativity)

Why doesn't the test particle move towards the moving matter stream?

Ans: Because there is a pushing force which exactly counters the effects of the increase in density of the moving matter stream.

[hjw]

They way I understood the whole thing is that some time ago there was a huge explosion which started at a singularity. And because there was a lot of energy involved in an otherwise empty space, things flew apart at the speed of light (Olly, that's were your acceleration is). Because early on there was no matter only energy everything should have been evenly distributed but the universe isn't. So physicists came up with a nanosecond or so of "hyper-inflation" where things were ripped apart faster than light and because speed of light being the fastest thing on earth kicked in afterwards again the un-eveness could  not smooth out anymore and much later gave rise to galaxy superclusters and other fascinating stuff.

So since the "Big Bang" everything is flying apart at close to light speed. Two points in space which were initially close to each other, fly more or less in the same direction and their relative speed is low. The further two points were apart at the start, the more they fly in different directions and the faster they are moving apart from each other now. That would go on for ever if there wasn't another phenomenon called "gravity". Gravity does a lot of things on a "small" scale like forming stars and planets keeping solar systems in place, form galaxies and make them rotate. But on a large scale it also counteracts the expansion of the Big Bang. By how much, that's the billion dollar question.

The amount of gravity is dependent on the amount of mass in the universe. Above a certain mass value, gravity will eventually overpower the big bang and things will come together again and form whats called the "Big Crunch". If the mass is below a critical value the universe will expand forever to a point where no new stars can be formed anymore and it all ends in the Big Freeze. If the mass is exactly right, it will slow down expansion until it approaches zero velocity in the distant future. Its called a "Flat Universe". Fairly unlikely, you say. Latest measurements put us in a flat universe with a 0.4% error margin! How about that?

And just when you think you are getting somewhere, there are theories that the expansion rate of the universe is actually increasing. So despite gravity slowing everything down we seem to expand faster and faster. And as with many things in the universe there are parts which are unknown. Enter the stage for Dark Matter and Dark Energy. Both seem to be the predominant parts of the universe however, nobody has a clue what they are. Hence it is easy to blame dark energy for counteracting gravity. To be honest, here I am out of my depth - but I will believe it, when I see it .

Isn't it fun?!

[ollypenrice]

They way I understood the whole thing is that some time ago there was a huge explosion which started at a singularity. And because there was a lot of energy involved in an otherwise empty space, things flew apart at the speed of light (Olly, that's were your acceleration is). Because early on there was no matter only energy everything should have been evenly distributed but the universe isn't. So physicists came up with a nanosecond or so of "hyper-inflation" where things were ripped apart faster than light and because speed of light being the fastest thing on earth kicked in afterwards again the un-eveness could  not smooth out anymore and much later gave rise to galaxy superclusters and other fascinating stuff.

So since the "Big Bang" everything is flying apart at close to light speed. Two points in space which were initially close to each other, fly more or less in the same direction and their relative speed is low. The further two points were apart at the start, the more they fly in different directions and the faster they are moving apart from each other now. That would go on for ever if there wasn't another phenomenon called "gravity". Gravity does a lot of things on a "small" scale like forming stars and planets keeping solar systems in place, form galaxies and make them rotate. But on a large scale it also counteracts the expansion of the Big Bang. By how much, that's the billion dollar question.

The amount of gravity is dependent on the amount of mass in the universe. Above a certain mass value, gravity will eventually overpower the big bang and things will come together again and form whats called the "Big Crunch". If the mass is below a critical value the universe will expand forever to a point where no new stars can be formed anymore and it all ends in the Big Freeze. If the mass is exactly right, it will slow down expansion until it approaches zero velocity in the distant future. Its called a "Flat Universe". Fairly unlikely, you say. Latest measurements put us in a flat universe with a 0.4% error margin! How about that?

And just when you think you are getting somewhere, there are theories that the expansion rate of the universe is actually increasing. So despite gravity slowing everything down we seem to expand faster and faster. And as with many things in the universe there are parts which are unknown. Enter the stage for Dark Matter and Dark Energy. Both seem to be the predominant parts of the universe however, nobody has a clue what they are. Hence it is easy to blame dark energy for counteracting gravity. To be honest, here I am out of my depth - but I will believe it, when I see it .

Isn't it fun?!

The problem with the 'explosion' analogy is that an explosion propagates from a central point while cosmological expansion propogates from everywhere within the universe. That is why my acceleration cannot be provided by the BB. If a central explosion threw galaxies apart from this central point then they would have been accelerated. They would all be racing away from that point, so two that happened to be side by side would be moving in a similar direction and see a much smaller redhift in each other than they would see if they looked towards the centre and through to the other side where the blast would have thrown galaxies the opposite way. The most important thing about Hubble's discovery was that this is precisely what is not seen.

What is seen is that, in any direction, redshift is proportional to distance. This leads us to conclude that precisely the same observation could be made from any galaxy anywhere in the universe. This is why I don't see any acceleration of galaxies provided by the BB. Take our own galaxy. In which direction has it been accelerated by the expansion of the universe? If it had a direction of travel/acceleration this would give a reduced redshift in the direction of travel and in increased one in the direction behind us. But this is the opposite of what is observed. The expansion is going on equally in all directions around us so clearly we don't have a direction of travel engendered by the expansion. This means there has been no acceleration imparted to us by the BB.

It is vital to forget 'explosions' in thinking about the BB. Explosions are directional, outward from a centre. Cosmological expansion is universal. The universe has no centre, nor does it have an edge.

Olly

[kalasinman]

So, if no acceleration, and therefore no energy imparted to produce the apparent motion, does that not mean that "dark energy" is a misnomer?

[hjw]

Hi Olly,

no contradiction there. The best visualization I have seen is a  balloon. Draw some galaxies (dots) on it and blow it up. Each dot will move away from the others and the further away the dots are the faster they will separate. Yes, it doesn't matter which dot you observe. Hence red shift is universal. And I do disagree with the universe does not have a edge. The edge is located at (age of the universe) x c. If the big bang theory is correct, red shift is a direct consequence of the parts flying apart. And it also explains why we haven't seen anything older than ~13 billion years (or further away than 13 billion light years).

Cheers

HJ 

[ollypenrice]

So, if no acceleration, and therefore no energy imparted to produce the apparent motion, does that not mean that "dark energy" is a misnomer?

No. The expansion of the universe involves the expansion of space and it seems reasonable to suppose that this expansion requires energy to drive it. No problem there. The expansion can accelerate or decelerate. The expansion of space does not seem, to me, to be equivalent to the imparting of an acceleration to particular galaxies. Acceleration can be measured in metres per second per second. In what direction are these individual galaxies accelerating? (You can only accelerate in a particular direction.) What Hubble observed is that everything is accelerating away from us (on the face of it) but the linearity of that acceleration with distance suggests either an astonishing coincidence or the idea that something else is happening. The 'something else' is that every galaxy will observe exactly what he observed. The galaxies are at rest within an expanding universe. (They're not really at rest, of course, because they are pulling on each other and may be being pulled on by dark matter as well, but ignoring that, the expansion of the universe does not accelerate them.)

The problem with the universe is that it is unlike anything else in the universe because it is not in the universe, it IS the universe. For this reason it bahaves differently.

Hi Olly,

no contradiction there. The best visualization I have seen is a  balloon. Draw some galaxies (dots) on it and blow it up. Each dot will move away from the others and the further away the dots are the faster they will separate. Yes, it doesn't matter which dot you observe. Hence red shift is universal. And I do disagree with the universe does not have a edge. The edge is located at (age of the universe) x c. If the big bang theory is correct, red shift is a direct consequence of the parts flying apart. And it also explains why we haven't seen anything older than ~13 billion years (or further away than 13 billion light years).

Cheers

HJ 

Rather than draw galaxy dots on the balloon, stick coins onto it for galaxies. That way the expansion of the surface of the balloon will not cause the galaxies to expand. They don't expand because they are gravitationally bound.

The universe cannot have an edge and still be the universe. What is beyond the edge would also be part of the universe. Or, to challenge you another way, which galaxies are closest to the edge? Or where is the Milky Way relative to the edge? There most certainly is no edge in the BB theory. (This is absolute fact. The theory may be wrong, but it is absolute fact that within the theory there is no edge. No BB theory variant of which I know posits an edge. Certainly there is a limit called the 'observable universe' but that is a visual horizon created by the expansion. It isn't physically an edge.)

We do see things far more distant than 13 billion LY. Don't make the mistake of thinking that a light travel time of 13 billion LY means that the source of that light is 13 billion LY away. That would be true if the universe were not expanding, but it is. This means that the space through which the light has been travelling for 13 billion years has expanded during the light's flight so the object is vastly more than 13 billion LY away. I think that a 13 billion year light travel time equates to a current distance of at least 40 billion LY.

The BB universe (which may or may not be like the real one but it's our best guess!) is about 13 billion years old. That does not mean the most distant things we can see are 13 billion LY away. (see previous point.) However, it is true that in the BB interpretation we haven't seen anything older than about 13 billion LY because that is the age of the BB universe. There's a lot of compelling evidence to support this. For example, the white dwarf branch cuts off before there are lots of cooled and reddened white dwarfs because they have not yet had time to cool.

Olly