PDA

View Full Version : Dark Matter and Gravitons



chadkman
2010-Jul-15, 06:09 PM
So I was reading about gravitons and was wondering if they are a viable candidate for dark matter or are we able to detect their mass, ect.? Also another question. Could dark matter be the force that causes gravity to remain constant, even though the space between galaxies in increasing at greater and greater rates? Kinda like how time can change but the speed of light is constant no matter how fast something is moving. I'm only 18 so there's a lot I do not know about astronomy and physics, but any explanation would be appreciated.

Chandler Kendall

pzkpfw
2010-Jul-15, 07:20 PM
Hi - and welcome. As a defence against spammers, the first few posts of a new user can be held in a queue from where they need to be approved before appearing in public. Please don't post the same thing again if you don't see it after submitting - just be patient a little while.

caveman1917
2010-Jul-15, 09:47 PM
So I was reading about gravitons and was wondering if they are a viable candidate for dark matter or are we able to detect their mass, ect.? Also another question. Could dark matter be the force that causes gravity to remain constant, even though the space between galaxies in increasing at greater and greater rates? Kinda like how time can change but the speed of light is constant no matter how fast something is moving. I'm only 18 so there's a lot I do not know about astronomy and physics, but any explanation would be appreciated.

Chandler Kendall

Hi Chandler.

Let me first say that our current theory of gravity - general relativity - doesn't feature gravitons. The graviton as a particle is still a hypothetical concept.
But let us assume one of the theories which proposes gravitons is correct, and gravitons do indeed exist.

A thing to be noticed here is that the graviton moves at c (light speed), just like a photon. Therefore it is massless (an object with mass can never accelerate to the speed of light). This doesn't mean they don't contribute to gravity, but it does mean that the increase in mass/energy in the form of a graviton is offset by an equal decrease in the mass/energy of the object emitting the graviton. In general though, the contribution to gravity from gravitons would be exceedingly small.

There have however been attempts to explain dark matter in a similar sense as to how you are. These attempts are called 'hot dark matter' (which means made of particles moving very close to the speed of light) - in contrast to 'cold dark matter' (made up of particles moving relatively slowly compared to the speed of light). Particles called neutrino's are an example of the former.

It has been found that, to explain the formation of small scale structure ('small' in this sense still being galaxy size), the fast movement of hot dark matter prohibits it 'clumping' together too much to explain this structure formation. Therefor any model for dark matter must at least include a 'cold' component, and that is where a lot of research in the subject matter is heading.


Could dark matter be the force that causes gravity to remain constant

It is important to understand that dark matter is not a (new) force. While we really don't know what it is exactly, we do know some things about it.
It is a substance which doesn't participate in the electromagnetic force, it doesn't emit or absorb [ETA: or reflect] photons. In other words, it can't be seen. What it does do, like everything else in the universe, is 'curve spacetime'. A phenomenom more commonly explained as gravity. So while it does attract everything else, we can only detect it by exactly that - its gravity. As a sidenote it is very much possible it does participate in the so-called weak nuclear force, the force responsible for radioactivity.


even though the space between galaxies in increasing at greater and greater rates

Here you seem to be referring to what is known as "dark energy". Even less is known on this than is known on dark matter.
First of all there is the concept of cosmic expansion. This is the reason galaxies seem to be receding away from us in all directions, with galaxies further apart moving at faster velocities.
Recently it has been shown that the rate of this expansion occurs at an ever increasing rate, exactly as you say. The reason for this increase is named 'dark energy'. While nothing has yet been conclusively shown about this dark energy, there is up to date no reason to believe it is related to gravity - or dark matter. Gravity doesn't seem to 'care' at all about this dark energy.

chadkman
2010-Jul-15, 10:02 PM
Ok that makes sense. So I have another question. So at the beginning of the Big Bang things moved away from each other at faster than the speed of light, not because they were moving through space at faster than the speed of light, but because the space itself was expanding faster than the speed of light. Does space still expand like it did in the first moments of the Big Bang, or is space more or less constant? Does this have anything to do with the expansion of the Universe?

caveman1917
2010-Jul-16, 01:16 AM
Ok that makes sense. So I have another question. So at the beginning of the Big Bang things moved away from each other at faster than the speed of light, not because they were moving through space at faster than the speed of light, but because the space itself was expanding faster than the speed of light. Does space still expand like it did in the first moments of the Big Bang, or is space more or less constant? Does this have anything to do with the expansion of the Universe?

This depends on what exactly you mean by the 'first moments' of the Big Bang. It seems to me you are thinking of what is commonly called the 'inflationary epoch', a period between 10-36 and 10-32 seconds after the Big Bang. During that period the expansion of space was exponential (ie a lot faster than 'normal'). In that time the volume of the universe increased by a factor of about 10+78 (that's a one with 78 zeroes behind it). This is not the same as the expansion which we are experiencing now (and presumably before inflation) which is linear.

While the cause of inflation is not known, it is postulated to be the phase transition that occured when the strong force decoupled from the electroweak force.
Think of it this way, before the end of the 19th century, it was thought that electricity and magnetism where two seperate forces. Then Maxwell showed that they were just different manifestations of a single force, the electromagnetic force. Recently it has been shown that at high temperatures, electromagnetism and the weak force are also 'united'. While it hasn't been shown, it is postulated that at even higher temperatures, the electroweak force and the strong force are also united in the electronuclear force.

So when the universe expanded and cooled off to the temperature where the strong and electroweak forces 'decoupled' (about 10-36 seconds), this decoupling (a 'phase transition') is thought to have caused a field to come into existence (the inflaton field) which 'pushed' space to expand more and more rapidly. At the end of the inflation period this field is though to have 'transformed' into (most of) the particles we observe today. After which 'normal' expansion took over once again.

Where dark energy fits into this picture is not known. It is presumed to have existed since at least the end of the inflation period, though evidence to date only supports its existence since around 9 billion years ago. There have been thoughts about linking dark energy to inflation, but none of those has yet come into the mainstream thinking.

If you're interested, wikipedia has a nice overview of the timeline of the big bang here (http://en.wikipedia.org/wiki/Timeline_big_bang).

chadkman
2010-Jul-16, 07:50 PM
Ok that helps a lot. Thank you for explaining this to me.

caveman1917
2010-Jul-16, 11:18 PM
Ok that helps a lot. Thank you for explaining this to me.

You're welcome. If you have other questions, feel free to ask.
If you have new questions that would be further removed from the original dark matter-graviton question, it might be good to start a new thread for those, just to keep things a bit organized.