[Pr. snoerkel]

As far as I understand, calculation the age of the universe is based on the present expansion rate of the Universe (when was all matter located at the same point?). However, that seems to simplistic. We know that the expansion rate is speeding up (and was slower in the past?), and according to the theory of relativity also time is variable. So I see no reason to believe that the big bang happened exactly 13.8 billion years ago

[dlorde]

As far as I understand, calculation the age of the universe is based on the present expansion rate of the Universe (when was all matter located at the same point?). However, that seems to simplistic. We know that the expansion rate is speeding up (and was slower in the past?), and according to the theory of relativity also time is variable. So I see no reason to believe that the big bang happened exactly 13.8 billion years ago

As I understand it, they feed their observational data into their mathematical models, which are based on general relativity, and so the accelerating expansion is included, and any relativistic effects of time dilation are also accounted for. Whether the precision of the resulting figure is really meaningful to us is not really the issue; as long as the same mathematical models are used for related calculations, the results should be commensurate. If there are contradictions, either the observations are faulty or the models need tweaking.

[evan_au]

The old arcade game 'Asteroids' worked like that. Whenever you left the screen on one edge, you'd reappear at a corresponding point on the opposite edge.

This technique has been used in a 3D simulation of the universe. They simulated the largest cubic space that would fit into their supercomputer, and then made the matter near the edge of the cube "feel" the effects of matter outside the cube by making it "adjacent" to the far side of the cube - effectively a "periodic" universe in the X, Y and Z dimensions. This is good for simulating structures which are smaller than the cube.

1 hour Podcast @ normal speed: http://omegataupodcast.net/2010/04/31-the-millennium-simulation/

[evan_au]

So dark energy is 'holding the universe together' ?  by gravity or by some other means?

My understanding of current theories is that dark energy is pushing the universe apart.

Earlier in the history of the universe, gravity was stronger than dark energy, but as the universe became less dense, the effect of gravity became less and the expansive effect of dark energy started to dominate.

[yor_on]

I don't know dlorde, but I'm not sure it is absurd? To me it's a question of who defines it, and from 'where'. In this universe it is us that does it, and we expect our universe to be isotropic and homogeneous, as well as 'infinite'. Possibly a 'God' would disagree with that, although we find it impossible to experimentally define a 'border' for it, even when divinely informed .

[dlorde]

My understanding of current theories is that dark energy is pushing the universe apart.

This is my understanding too. Which is why I was surprised when you said:

As noted in other posts, energy levels tend to decrease to the lowest possible level, over time.
This suggests that the level of dark energy is decreasing over time, and showing itself in the increased acceleration we have seen in the expansion of the universe.

Which implies that dark energy slows the acceleration, which increases as dark energy decreases...

[dlorde]

I don't know dlorde, but I'm not sure it is absurd? To me it's a question of who defines it, and from 'where'.

You can define infinity to be whatever you like, but if your definition is so different from the standard definition that means something quite different (e.g. allowing finite metrics to be considered infinite), you shouldn't be surprised if people find it absurd. What use is it to consider closed topologies to be infinite?

[evan_au]

Which implies that dark energy slows the acceleration, which increases as dark energy decreases...

One of the theories I have heard is that Dark Energy is a weak field that has a small non-zero value across the universe (or in other formulations, it is the difference between two very strong fields which cancel out, but not quite...).

If this field accelerates visible matter, then the Dark Energy is being converted into Kinetic Energy (or the stretching of space-time, etc), and the level of Dark Energy would be decreasing over time.

If the universe is being held tightly together by gravitation (such as in the early universe, or in our galaxy), or being held tightly together by electric fields (such as in our planet or our bodies), then the Dark Energy cannot push it apart - at least, at its present weak strength.

So rather than saying "dark energy slows the acceleration" I would say that "Strong Gravity & Electric fields inhibit dark energy from accelerating matter".

[Of, course, there is also the "Big Rip" hypothesis that Dark Energy gets stronger and stronger over time, until it rips apart galaxies, solar systems, planets and us.... But this doesn't explain where the Dark Energy gets this increased energy.]

PS: At its most basic, the term "Dark" means "We don't know what it is, so give us lots of money to find out...".

[yor_on]

Well, then we're absurd if now the topology of whatever universe we define as infinite, is finite? As some ideas suggest.

[Pr. snoerkel]

As far as I understand, calculation the age of the universe is based on the present expansion rate of the Universe (when was all matter located at the same point?). However, that seems to simplistic. We know that the expansion rate is speeding up (and was slower in the past?), and according to the theory of relativity also time is variable. So I see no reason to believe that the big bang happened exactly 13.8 billion years ago

As I understand it, they feed their observational data into their mathematical models, which are based on general relativity, and so the accelerating expansion is included, and any relativistic effects of time dilation are also accounted for. Whether the precision of the resulting figure is really meaningful to us is not really the issue; as long as the same mathematical models are used for related calculations, the results should be commensurate. If there are contradictions, either the observations are faulty or the models need tweaking.

Did anybody else hear that?

[dlorde]

... If this field accelerates visible matter, then the Dark Energy is being converted into Kinetic Energy (or the stretching of space-time, etc), and the level of Dark Energy would be decreasing over time.

Ah, OK. This suggests that, over time, the acceleration should slow and eventually stop, as Dark Energy reduces to zero. That potentially introduces the prospect of the expansion slowing and even reversing, as gravity takes effect. I guess the timescale depends on how much DE is out there...

[Of, course, there is also the "Big Rip" hypothesis that Dark Energy gets stronger and stronger over time, until it rips apart galaxies, solar systems, planets and us.... But this doesn't explain where the Dark Energy gets this increased energy.]

Yes, it seems odd.

[Zavenoa]

When was matter created?

13.8 billion years ago in a single instant, also known as the singularity where the general laws of relativity break down.  Cosmic inflation theory states after the Big Bang the Universe expanded exponentially in 10^-37 seconds.  Once the period of inflation stopped, the Universe was essentially a plasma of elementary particles moving at relativistic speeds slamming into each other creating and destroying particle-antiparticle pairs.  A few minutes after the big bang, while temperatures were still in excess of a billion kelvin, primordial nucleosynthesis took effect and deuterium (H-2), the helium isotopes He-3 and He-4, and the lithium isotopes Li-6 and Li-7 were created among some less stable radioactive isotopes like tritium (H-3) were formed but either decayed or fused to become more stable nuclei.  The rest pretty much settled into hydrogen.

Is the amount of matter still increasing?

Matter is increasing and decreasing, but Matter is only the stuff we can actually see, I believe your question is whether or not Mass is increasing.  The two must be differentiated.  As far as Mass goes, you have to consider energy as well.  Einsteins formula E=mc² (Energy = Mass * Speed of Light Squared) is called the mass-energy equivalence which states mass is a property of all energy and vice-verse.  There are different forms of the equation for different instances, but essentially you can't create or destroy mass, only convert it into energy.  Similarly, energy can be converted into mass, it just takes quite a lot of it (I believe this has been done at the Large Hadron Collider, and detected by the TRIUMF team).

Is the amount of dark energy increasing?

No, above would apply to dark energy as well.  On the dark energy note, evan_au is correct in stating that dark energy is responsible for the acceleration of the expansion of the Universe.  That's the only reason we know it's there, right now both dark energy and dark matter are the same thing the neutrino was before direct detection in 1956, a variable that made a formula balance.  Without dark energy, there is no way to explain why our Universe's expansion is accelerating, the expansion should be decelerating due to gravity.  Similarly, without dark matter, the solar systems at the tips of the spirals in the Milky Way Galaxy would get tossed out into intergalactic space by the rotation of the galaxy.

I believe we are closer to understanding dark matter after the direct detection of the Higgs boson particle in 2012.  After all, if there was one sub-atomic particle out there we couldn't find, may there not be more?  Dark energy may be more difficult, it may not.  One theory is the Chameleon model which states there is another particle that can act differently in different situations.  Where there is matter, the chameleon would be heavier and would be more sluggish like the weak or strong forces, while in the vacuum of space where there is little matter the chameleon would be far lighter and more reactive like the electromagnetic force.

How can something be created from nothing?

The only way we know this can happen is in quantum mechanics, and that's where it just gets weird.  In complete honesty, quantum mechanics baffles me.  I have no idea how anything can pop in and out of existence, or how two particles can be in different places but linked, known as Quantum Entanglement.  Einstein called it "spooky action at a distance," and I think that about sums it up.

As far as how the Universe was created from nothing, there are many theories on the subject and most deal with parallel universes, like M-Theory.  The theory I like the best deals with our Universe starting as a white hole when a black hole was created in a parallel universe connected by an Einstein-Rosen bridge (aka a wormhole).  Essentially, mass and energy falling into the black hole in the parallel universe would have traveled through the wormhole and out the white hole on the other side.  Maybe the black hole stopped feeding and went dormant after that, which is why we don't see the mass of the Universe constantly increasing, but then what happens if the black hole in the parallel universe starts feeding again?  Would mass and energy start shooting out of the middle of our Universe?  Where is the center of our Universe?  How come quantum particles are allowed to break the rules and I'm not?

Disclaimer, I am not a physicist and none of these ideas are my own (and any or all may be wrong), please ask Michio if you have any further questions.

[Pr. snoerkel]

... If this field accelerates visible matter, then the Dark Energy is being converted into Kinetic Energy (or the stretching of space-time, etc), and the level of Dark Energy would be decreasing over time.

Ah, OK. This suggests that, over time, the acceleration should slow and eventually stop, as Dark Energy reduces to zero. That potentially introduces the prospect of the expansion slowing and even reversing, as gravity takes effect. I guess the timescale depends on how much DE is out there...

[Of, course, there is also the "Big Rip" hypothesis that Dark Energy gets stronger and stronger over time, until it rips apart galaxies, solar systems, planets and us.... But this doesn't explain where the Dark Energy gets this increased energy.]

Yes, it seems odd.

I would believe that the accelleration will continue, with or without DE and I base this on the following observations:
In a few billion years, at the latest, the rim of the Universe will move away at near light velocity (at present max observed redshift is only 8.6, but that is just a lower limit). As the velocity of an object near the rim approaches light speed, the mass will also increase according to relativity. The increased mass will of course attract the rest of the Unirve with ever increased force, and consequently the expansion rate will increase.
This may already be the case in the present Universe and be the source of the dark energy

[dlorde]

As the velocity of an object near the rim approaches light speed, the mass will also increase according to relativity. The increased mass will of course attract the rest of the Unirve with ever increased force, and consequently the expansion rate will increase.

Wait, you're saying that in the absence of DE, gravity will increase the expansion rate?

[Pr. snoerkel]

That what I'm saying, but not clearly enough, it seems. Using the Stephen Hawking trick and use a 2-D model it could be described as:
Imagine that you have a thin flexible disc with steel ball evenly distributed over it. Now add more steel ball near the rim, and you will see the whole disc bend down, but especially near the rim. The balls will then roll towards the rim at higher speed.
The relativistic equivalent is of course that any object approaching the speed of light will gain mass.

[Zavenoa]

Imagine that you have a thin flexible disc with steel ball evenly distributed over it. Now add more steel ball near the rim, and you will see the whole disc bend down, but especially near the rim. The balls will then roll towards the rim at higher speed.

Makes sense in a 2D model, the further an object moves from the fulcrum the greater the force exerted, but I guess you would have to think of space-time as the thing that is warping instead of a disc.

[Pr. snoerkel]

The disc was only introduced as a symbol for space-time warping, so we agree. And of course local mass concentrations have local influence. Andromeda will in the future collide with the Milkyway and the expansion of space does not prevent that, since that expansion takes place on a scale that dwarfs even superclusters. Compared to its size, the expansion of space is incredibly SLOW. If a man grew on the same scale it would take him about five million years to grow another millimeter

[Zavenoa]

Pr. snoerkel I misread your post the first time I responded, axed that response.  I was still trying to think of the universe as a 2D disc covered in steel balls without actually getting to the point of adding more (read, think, then post?).  To explain a little more about what I was thinking with the fulcrum, I started thinking about the basic concept of a lever, and keeping all else the same the further you move the same amount of mass from the fulcrum the more force it will exert, physics 101.  If you continue to increase that distance enough, the force will be great enough to stretch the beam thus further increasing the distance.  Assuming the beam is infinitely malleable and will not break (space-time) the more it stretches, the further it will stretch in a given period of time, thus accelerating the expansion of the beam.  There are a lot of assumptions here, like the fulcrum & beam being permanently attached at one end with the mass permanently attached at the other, as well as nothing to prevent the beam from, say, hitting the ground, but I think you get the idea.

This idea of gravity accelerating expansion has my mind reeling.  I've never really liked the whole Dark Matter & Energy explanations.  I understand why we think they exist, maybe not the equations behind them, but the general principles, and something doesn't fit.  I'm not saying I agree with Hawking on this one either, but I like that he's at least challenging the assumption of an unknown utilizing existing principles.  Both of Einstein's theories, the geodetic effect (gravity warps space-time) and frame-dragging (spinning objects can pull space time with them), were confirmed by Gravity Probe-B's gyroscopic readings.  Both of these could be used explain Hawking's theory here, the geodetic effect would drag the disc down and frame-dragging would cause one steel ball to drag others with it.

If you apply the same idea of the stretching beam to the disc, couldn't this tie into the idea of metric expansion of space?  They seem to contradict each other in some ways, as Hawking's idea seems to suggest the mass is moving with space being fixed, while metric expansion says the mass is fixed while it's space itself that's expanding.  However, if space is more akin to a thick liquid as frame-dragging suggests, why couldn't the movement of mass cause the expansion of space, thereby causing the mass to appear to be moving faster than it actually is?

I might be coming to the wrong conclusion here, but if both mass and space can move, wouldn't that make faster than light travel possible?  For example, if I'm moving at 99.99% the speed of light and I warp the space behind me by dragging and stretching it, that would increase the distance I've traveled meaning I actually traveled faster than light.  However, it doesn't seem you would be able to use this to move to a destination faster than light, simply further away from one.

[dlorde]

The relativistic equivalent is of course that any object approaching the speed of light will gain mass.

Yes, but in the universe, that velocity is relative to an observer. All observers will have their own 'relativistic rim' at their observable limit. The 2D steel disc is analogous only to a single observer's view.

[Pr. snoerkel]

slorde's last remark is indiputable, but I do not see that the overall picture being the same for all observers will change anything. This is also the case in the present model of Cosmos. Unfortuneately my idea does not explain dark matter, just dark energy. But the idea uses only general relativity with no additions and seems to answer two of the original questions.
That idea with the faster than light travel needs a little more detailing, at least I did not grasp it fully, but that might be because I am focused on formulating an alternative to the traditonal big bang theory. The present one seems a little odd. First the Universe was created out of nothing or by collision of branes or something. Then it expanded faster than light. Finally it did not collapse into a black hole when matter was created, as it should have done according to the laws of gravity. Official science says that the laws of nature were different then - so who changed them - and when?
In the universities they teach that when the Universe was 10E-32 sec. old, it had a size of 0.1 m. I have been unable to find out how they are able to substantiate that. Had anybody been there how could they know the time? In a universe consisting entirely of photons there is in principle no way to construct a clock or a yardstick. Or is there?