Dark Energy

Kevster

Hey,


If the Big Bang theory is correct then there is a center to the Universe. So, my first question is: Why can't they infer the position in space-time of this center (or have they done this already)?


My second question is: Just after the Big Bang, could many enormous Black Holes have been formed and could these then make-up the 'missing' amount of matter/energy (Dark Energy) that is apparent in the Universe.


Take care,
Kevin

Spear

Kevster wrote:

Hey,


If the Big Bang theory is correct then there is a center to the Universe. So, my first question is: Why can't they infer the position in space-time of this center (or have they done this already)?


Take care,
Kevin

Big bang theory does not predict a centre of the universe.

http://math.ucr.edu/home/baez/physics/Relativity/GR/centre.html

See also

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

Kevster

The first link provides a very good explanation - Thank you.

SonOfPerdition

Kevster wrote:

Hey,

My second question is: Just after the Big Bang, could many enormous Black Holes have been formed and could these then make-up the 'missing' amount of matter/energy (Dark Energy) that is apparent in the Universe.


Take care,
Kevin

Black holes form when certain types of stars die. So the earliest would require the timespan of star formation plus the life span of the star that was it's parent (so to speak).

As far as i know, black holes have no direct relation to dark matter.

SOP


edit: this might be of use to you http://www.bbc.co.uk/science/space/deepspace/darkmatter/index.shtml

Son Goku

Kevster wrote:

My second question is: Just after the Big Bang, could many enormous Black Holes have been formed and could these then make-up the 'missing' amount of matter/energy (Dark Energy) that is apparent in the Universe.

That's a good question and was investigated during the 1990s to the early 2000s. Shortly after the Big Bang (about 500,000 - 1,000,000 years after) enormous black holes did form from the primordial hydrogen. Most of the black holes formed at this time would have been supermassive black holes.

However, the interesting thing (at least I think it's interesting) is that these early massive black holes gathered all nearby dust and gas into large spirals. These spirals were eventually drawn around the hole until they became dense enough for stars to begin forming in them. Eventually these spirals became the galaxies we see today and the holes are the supermassive black holes at the centre of galaxies.
So galaxies are "just" the accretion disks of ancient supermassive black holes.

SonOfPerdition

Hi son Guru,

Is that the accepted theory of how super massive black holes formed? I thought it was only one of a number of theories? Coalescing of smaller black holes, or gravitational collapse off dense cluster of stars being some.

SOP

Son Goku

SonOfPerdition wrote:

Hi son Guru,

Is that the accepted theory of how super massive black holes formed? I thought it was only one of a number of theories? Coalescing of smaller black holes, or gravitational collapse off dense cluster of stars being some.

SOP

Supermassive blackholes form in a number of ways and there are a few instances of galaxies forming without supermassive black holes, but currently the evidence strongly suggests that:
(a)Most (practically all) galaxies form as accretion disks around supermassive black holes.
(b)Most supermassive black holes are found in the centre of galaxies.

In other words, the co-evolution scenerio, the vast majority of galaxies and supermassive black holes formed together. Although there are exceptions.

SonOfPerdition

Son Goku wrote:

Supermassive blackholes form in a number of ways and there are a few instances of galaxies forming without supermassive black holes, but currently the evidence strongly suggests that:
(a)Most (practically all) galaxies form as accretion disks around supermassive black holes.
(b)Most supermassive black holes are found in the centre of galaxies.

In other words, the co-evolution scenerio, the vast majority of galaxies and supermassive black holes formed together. Although there are exceptions.

hmmmm, i way out of touch on this stuff. I knew of the existence of super massive black holes in the centre of most galaxies, but i never realised the black holes were there first. I always assumed the black holes only formed once stars of appropriate type started dying and that stellar sized holes jopined together over time to form the bigger ones. It is really interesting to consider that the galaxies are the result of accretion disks.

and er, apologies for mis spelling your name in my last post! bad form on my behalf!

SOP

Son Goku

SonOfPerdition wrote:

It is really interesting to consider that the galaxies are the result of accretion disks.

Yeah, the deadly thing is that since black holes are a prediction of General Relativity and galaxies are the result of black holes, it kind of means galaxies are a consequence of the rules of general relativity.

SonOfPerdition wrote:

and er, apologies for mis spelling your name in my last post! bad form on my behalf!

No bother man.

Kevster

hehe, I was waiting for you to reply Son Goku. You always appear to know what you're talking about.


The explanation you gave is good. I actually recall hearing about that now in some documentary on Discovery Science many months ago. These [primordial] Supermassive black-holes obviously don't make-up the lost Dark Energy then? That's what I was trying to get at in the first post.

The Hill Billy

Kevster:

There's an interesting article here regarding how they hope to locate dark matter using pulsars.

Kevster

Thanks for that. I'm printing it off and will give it a good read later.

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Edit: Just read it - Very interesting article but it makes me wonder thus: What are the chances that all of this searching for Dark Matter will be fruitless? ...and then what are the chances that Dark Matter and Dark Energy 'exist' solely because the maths we use to describe the Universe is flawed?

My only basis for this assumption is the fact that scientists are capable of using maths to describe the formation of Earth, Venus, and the other rocky planets... BUT they cannot use the same maths models to describe how the gas giants are created. Therefore, what works on a small scale fails to work on a large scale.

Son Goku

These [primordial] Supermassive black-holes obviously don't make-up the lost Dark Energy then? That's what I was trying to get at in the first post.

Essentially Black Holes can't be Dark Matter, because they don't get to be "Dark". There usually surrounded by galaxies or accretion disks e.t.c., that make them really obvious in the night sky. Plus they aren't distributed in the right way, most holes are found near galactic cores, while dark matter effects galactic rotation out near the edges of the spirals.

They can't be Dark Energy because they come from the wrong part of the field equations.

Kevster wrote:

Edit: Just read it - Very interesting article but it makes me wonder thus: What are the chances that all of this searching for Dark Matter will be fruitless? ...and then what are the chances that Dark Matter and Dark Energy 'exist' solely because the maths we use to describe the Universe is flawed?

My only basis for this assumption is the fact that scientists are capable of using maths to describe the formation of Earth, Venus, and the other rocky planets... BUT they cannot use the same maths models to describe how the gas giants are created. Therefore, what works on a small scale fails to work on a large scale.

I appreciate that Dark Energy and Dark Matter seem a little bit like they were pulled out of a hat.
I'll give decent enough (I hope) explanation of them both tomorrow, along with the motivation behind them.

Kevster

Son Goku wrote:

I appreciate that Dark Energy and Dark Matter seem a little bit like they were pulled out of a hat.
I'll give decent enough (I hope) explanation of them both tomorrow, along with the motivation behind them.

Thanks - I appreciate your input.

Son Goku

Okay, first of all I'll deal with Dark Energy because it's easier and actually less "out of a hat" than Dark Matter.

If you attempt to derive Einstein's Field Equations for Gravity you get the following equation:


The symbol Λ is called the cosmological constant. If you set it equal to zero you get:


For most of the last sixty years people set Λ(Cosmological Constant) to equal zero, because that's what worked.
Then in recent years observations of supernova showed that the universe's expansion was accelerating. When people went back to Einstein's Field Equations, they noticed that the Expanding universe they were observing corresponded to solutions of the first, more general, field equations.
Hence Dark Energy was already part of General Relativity, but usually set to zero until we recently say its effects.

The one complication is that people noticed that the expanding universe could be equally well modelled by using the second equation, where Λ = 0, if you made the assumption that the universe is filled with an unusual substance called quintessence.

Hence the Dark Energy debate is really if our universe is described by the first equation, with Λ>0 or by the second equation, with Λ=0 and quintessence.
i.e. There is a mysterious (Dark) energy density in our universe, is it quintessence or the Cosmological constant?

In both cases General Relativity is right, it's just unfortunate that two different solutions to General Relativity can model the current universe equally well.

Some people think General Relativity itself might be wrong. However anybody who has ever tried to create another theory has either ended up accidentally making one equivalent to General Relativity (i.e. GR expressed in another mathematical language) or have made a theory which doesn't match observations.

Now even though the two solutions give the same results with regard to the structure of the cosmos, quintessence and cosmological constant cause slightly different accelerations. As well as this they act very differently from each other in the early universe.

Hence by making early universe observations and measuring the acceleration very precisely, we might be able to rule out one in favour of the other.
(At this point in time the Cosmological Constant option is doing a bit better than quintessence, but I wouldn't make any definite statements yet.)

Anyway, that's Dark Energy. I'll make a post on Dark Matter later.

Kevster

That's excellent. You explained it so well. Thank you.

mr_angry

Personally, I think that flawed mathematical thinking is a more likely actuality then the theory of dark matter. I guess it really stems from the idea that the speed of rotation of galaxies doesn't correspond to the predictions of classical Newtonian mechanics. One solution to this problem is to say that "dark matter" exists and makes up the majority of the mass in the Universe. Another solution is to suggest that traditional Newtonian mechanics is unsuitable for modelling the rotation of galaxies. The problem with the latter axiom is that nobody can yet explain why Newtonian mechanics should be unsuitable. Nevertheless, to me, it seems more likely than suggesting that 95% of the matter in the universe is totally invisible and intangeble.

Dark energy is much more interesting in my opinion. As spectroscopy became an accepted scientific technique, it was expected that a slowdown in the expansion of the universe would be observed, fitting the big bang theory nicely. However, instead, it was observed that the expansion of the universe is speeding up. Why? The only reasonable theory seems to be that a mystery form of energy (dubbed "dark energy") is responsible for the acceleration. Thus far, we have yet to learn anything meaningful about this, but it certainly is fascinating.

Son Goku

Okay, now Dark Matter.

Around the 1950s, when most physicists were working on Quantum Field Theory, a small group started a resurge of interest in Einstein's Theory of General Relativity. At the time these people were called Relativists and for a while being a theoretical physicist meant you were either an expert in Quantum Field Theory or General Relativity, rarely both.

Most Relativists were interested in finding out more about the mathematical structure of General Relativity. The big people in this area would have been Hawking and Penrose.
(e.g. Are singularities common in the solutions?, How does Causality work in the theory?)

However, a few began to apply General Relativity to Cosmology. One of the first problems this group faced was that General Relativity is far too difficult to work with, and find solutions to, in practical situations. The needed a simpler theory if they were to have a hope of tackling Cosmological problems.
Newton's theory wouldn't do, because it was inaccurate. So they created a halfway house theory called Linear-GR, that had most of the accuracy of GR without the complexity.
They used it to model how galaxies moved and rotated.

This formalism worked well until in the early 1970s when a young astronomer called Vera Rubin made observations that galaxies did not rotate in the fashion predicted by Linear-GR.
This was a big problem for the community at the time and the first explanation given was that maybe full-on, proper General Relativity would match the observations, even though Linear-GR did not. This was eventually shown to be incorrect during the 70s, it was proved that General Relativity and Linear-GR gave the same predictions in such scenarios.

Eventually it was suggested that if you assumed there was a Halo of unseen (Dark) Matter surrounding every galaxy, then General Relativity would give the correct galactic rotations.
This was a poor suggestion to say the least, however there was reasoning behind it. First of all observations should that General Relativity worked at the Solar System level and the Universe level. It would be odd that General Relativity would work within Solar Systems, break down at the Galactic scale and then start working again at the Universe scale. People reasoned it was more likely that General Relativity was correct and rather some unknown mass factor operated at the galactic scale.
Secondly any attempts to build a theory which matched the galactic predictions didn't work at the level of the whole universe.

Debates went on for a long time and nothing much happened. As the 1970s rolled into the 2000s, the evidence for Dark Matter became stronger than opposing ideas, but still pretty weak.

However in August 2006 observations of the Bullet Cluster, a pair of colliding clusters of galaxies, gave direct evidence of Dark Matter. While the visible matter could be seen colliding, a vast quantity of unseen material (matching the predicted amounts Dark Matter expected in such situations) caused gravitational lensing each side of the collision.

In this picture the pink is visible matter and the blue is the region of gravitational lensing, which can only be caused by large amounts of mass. Further more it's shape is the same as predicted Dark Matter flows.


This gave a wealth of support to the Dark Matter hypothesis, as other theories have a lot of trouble matching this observation and matching observations of the universe's expansion.

However we still don't know what Dark Matter is made of, i.e. What it is.

We've learnt a few things:
(a) It doesn't feel the electromagnetic force. Which really narrows it down.
(b) It only weakly feels the weak force, if it does at all. Experiments still haven't been able to prove it doesn't.
(c) It doesn't feel itself that strongly. Something only learnt in the last few months. Dark Matter mostly passes straight through other dark matter. This really narrows it down.
(d) It doesn't move very fast, i.e. it isn't composed of fast moving particles like neutrinos, because otherwise it would have messed up galaxy formation.
(e) Research indicates that dark matter only comes in clumps larger than about 1,000 light-years across, with an average particle speed of 9 km/s and temperature of 10,000 kelvins.

These combined results indicate that Dark Matter is either:
(a) An Axion. A still unobserved particle predicted by some versions of the theory of the Strong Force.
(b) A WIMP. Most of these are predicted by SuperSymmetry, so if this is the case it'll be Supersymmetry's first success.
(c) Some random yoke we have no idea about.

It's come a long way since the old days because Dark Matter is made of unusual new particles, meaning we'll need a lot of Quantum Field Theory to figure it out and it's the cause of extra gravity, meaning you also need General Relativity. So now, you have to be an expert in both.

With the LHC turning on in CERN we'll get more data on the possible particles it could be and combined with further study of the bullet cluster, hopefully we'll crack this thing.

Kevster

So - out of the two - Dark Matter has been proven to exist but Dark Energy has not?

Also, you said that Dark Matter can be 10,000 degrees Kelvin - Is that really true? I would have thought it was much colder but I honestly had nothing concrete to base that on.


Thanks though - great explanations.

Take care,
Kevin

Son Goku

Kevster wrote:

Also, you said that Dark Matter can be 10,000 degrees Kelvin - Is that really true? I would have thought it was much colder but I honestly had nothing concrete to base that on.

So did most people. It was a pretty odd result. Although to be fair it only makes sense now that we know more about Dark Matter. Since it interacts so weakly, it has little chance to give off heat to other objects.

Kevster wrote:

So - out of the two - Dark Matter has been proven to exist but Dark Energy has not?

Both yes and no. Dark Matter has been proven to exist, but the postulation of its existence was always dodgy, because if it didn't exist General Relativity would be wrong.
Basically if you artificially assert Dark Matter exists and feed it into the equations of General Relativity then it gives the required Galactic rotations.
Fortunately it has now been observed.

Dark Energy on the other hand is naturally in General Relativity and predicted by it, giving rise to the expansion of the universe.
General Relativity naturally has solutions that involve an expanding universe, but they all have a non-matter energy density field (Dark Energy).
Since we've observed the universe expanding exactly the way General Relativity predicts it would, we infer that Dark Energy exists. The problem is General Relativity doesn't tell you what Dark energy is. It could be the cosmological constant or it could be quintessence.
Neither of these have been shown to exist directly.
However in recent years there has been some indirect evidence that the cosmological constant does exist, particularly from the recent WMAP* (Wilkinson Microwave Anisotropy Probe) studies conducted from 2001 to 2006.

Now what if they don't exist? Then you have a few questions to answer.

You have to create a theory of gravity that predicts everything General Relativity predicts, gives the same Hubble Constant, the same laws of motion, also predicts black holes(since they've been observed now).
You also have to make your theory produce the same expanding universe solution that General Relativity does, since it's the correct one.
It's very hard to this without accidentally making General Relativity, as you'd imagine because you're trying to make a theory that predicts everything General Relativity does, except without Dark Energy.

*The WMAP study was very important in that it gathered data in favour of cosmic inflation, another idea (not really related to General Relativity) concerning the early universe.

If there is any other questions then feel free to ask.

Kevster

Son Goku wrote:

If there is any other questions then feel free to ask.

No questions that I can think of. That's the sign of a good, thorough, explanation.

Thanks,
Kevin.

The Hill Billy

Kevster - Here's another article regarding Dark stuff from New Scientist that may interest you.

Kevster

I like the 'dark stuff' term

I'll print the article off and give it a read later.