Do dark matter/energy really exist?

[Rdivine]

hey guys!

I know that the KSP community consists of many smart geniuses and actual physicists. Can anyone inform me why dark matter must exist, and any assumptions made in the process?

Here are some of my hypothesis on dark matter. Please try to tell me why they won't/might work.

1. Dark matter and energy doesn't actually exist.

Since gravity travels at the speed of light, fast-moving galaxies or large bodies of mass may leave behind "trails" of gravity. Hence, when you see a star 1 billion light-years away, you are actually feeling the very,very,very small gravitational influence from that star 1 billion years ago.

Since trails of gravity are possible, can it be that, all the effects of dark matter are actually caused by gravity trails?

Spinning galaxies may hence, be able to hold itself together, as the individual stars leave trails of gravity that tug any escaping star back in.

2. Dark matter is really just jumps of interstellar dust.

We may all know that space is vacuum. But, in intergalactic distances, the space in between may consist of intergalactic lumps of dust. Those are remnants of stars and nebulaes being tugged around the galaxies. Is it possible that those dust is constantly exerting a gravitational influence on galaxies, and thus exhibiting properties of dark matter?

-lastly, i would like to point out,my views on why there are more matter than antimatter in the observable universe.

-perhaps our observable universe is only a fraction of a bigger, undiscovered universe. Which means that huge pockets of antimatter galaxies and such exist in other areas of the universe. This balances out matter and antimatter perfectly.

If anyone has knowledge on why my hypothesis can/can't be a possibility, please leave your comments below! Thanks!

[baggers]

1. Dark matter and energy doesn't actually exist.

Since gravity travels at the speed of light, fast-moving galaxies or large bodies of mass may leave behind "trails" of gravity. Hence, when you see a star 1 billion light-years away, you are actually feeling the very,very,very small gravitational influence from that star 1 billion years ago.

Since trails of gravity are possible, can it be that, all the effects of dark matter are actually caused by gravity trails?

Spinning galaxies may hence, be able to hold itself together, as the individual stars leave trails of gravity that tug any escaping star back in.

The asomption made by Einstein is that Gravity, unlike matter or light, don't "travel": it's a geometrical spacetime curvature related to mass:

https://en.wikipedia.org/wiki/General_relativity

So, the propagation of gravity isn't limited by the speed of light: it's an "instant" process: it's not a mechanical force like Newton previously think.

https://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation

Edited June 25, 2015 by baggers

[BlueCosmology]

The asomption made by Einstein is that Gravity, unlike matter or light, don't "travel": it's a geometrical spacetime curvature related to mass:

https://en.wikipedia.org/wiki/General_relativity

So, the propagation of gravity isn't limited by the speed of light: it's an "instant" process: it's not a mechanical force like Newton previously think.

https://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation

You've got that the wrong way round. Newton's gravitation is instantaneous. General relativity is limited by the speed of light.

[baggers]

You've got that the wrong way round. Newton's gravitation is instantaneous. General relativity is limited by the speed of light.

?

I'm not very good in english, sorry ^^ But Newton itself stated that the effective experimental observation of "instant" gravity was a major problem for it's own theories:

https://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation#Newton.27s_reservations

Problem only solved by the general relativity that say "gravity" is not a force, but a space-time curvature, so it's action is instant, and not delayed (by the speed of ligth).

Maybe you speak about of the effective power of this gravity, an equation that, yes, include "c" in General relativity, and not in newton equations:

https://en.wikipedia.org/wiki/General_relativity#Einstein.27s_equations

But the gravity action itself is instant and not delayed, and this can't be explained by a "force": a spacetime curvature explain that.

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This mean that a "gravity trail" can't be "left behind" by some galaxy, as the "gravity field" is a space-time curvature alway "under" the galaxy mass and has no "gravity trail" to left behind like a fold in the sheets: gravity "instantly travel" with the mass.

Edited June 25, 2015 by baggers

[PakledHostage]

I understand that Einstein rings, and in particular double Einstein rings like the one in the image below, are evidence for the existence of dark matter surrounding distant galaxies. Maybe someone with more of a physics background can explain further?

[baggers]

This is a gravity lense: I love this one:

It's the effect of a curvature of distant light passing in a heavy gravity field before reaching Earth:

An evidence for the existence of very heavy objects that curve light by gravity. A Dark Hole maybe. An abundance of Dark Hole is one of the possibility for Dark Matter.

[NFUN]

Dark matter (aka WIMPs, Weakly Interacting Massive Particles) also fits the math best between it, MACHOs (MAssive Compact Halo Objects, basically regular matter that doesn't emit light) and MOND (MOdified Newtonian Dynamics) in solving some discrepancies in cosmic phenomena, particularly in the rotation speed of galaxies. Their angular velocities appear so great as to be able to tear them apart without some fudge factor to fix the math, so these theories were created in order to explain the issue. Dark matter works well at doing this while keeping other physics unaffected, and with mysterious gravitational lensing existing, it seems to be the best candidate all around.

Dark energy is a bit more nefarious. Scientists don't really have a mechanism to explain it, and its need could be an error with the math, or perhaps the accelerating universal expansion is caused by something else entirely.

[Ralathon]

1. Dark matter and energy doesn't actually exist.

Since gravity travels at the speed of light, fast-moving galaxies or large bodies of mass may leave behind "trails" of gravity. Hence, when you see a star 1 billion light-years away, you are actually feeling the very,very,very small gravitational influence from that star 1 billion years ago.

Since trails of gravity are possible, can it be that, all the effects of dark matter are actually caused by gravity trails?

Spinning galaxies may hence, be able to hold itself together, as the individual stars leave trails of gravity that tug any escaping star back in.

Why would gravity operating at the speed of light prevent fast stars from escaping? If I look at the other side of the milky way galaxy I am seeing stars as they were 80 thousand years ago yes. But it is still the same number of stars, with the same mass. So I experience the exact same gravitational force regardless of the time delay.

2. Dark matter is really just jumps of interstellar dust.

We may all know that space is vacuum. But, in intergalactic distances, the space in between may consist of intergalactic lumps of dust. Those are remnants of stars and nebulaes being tugged around the galaxies. Is it possible that those dust is constantly exerting a gravitational influence on galaxies, and thus exhibiting properties of dark matter?

Nah, whatever Dark matter is, it isn't just gas and dead stars. We have clear evidence of this through gravitational lensing. When 2 galaxies collide all the normal matter slows down thanks to friction, light pressure et cetera. The dark matter only interacts via gravity, so it just flies on without slowing down. This velocity difference separates the galaxy from its dark matter halo. So when you look at the gravitational lensing of 2 colliding galaxies you see 4 spots with lots of mass: The 2 galaxies themselves and the 2 blobs of dark matter. That's why dark matter can't just be normal stuff, that would've slowed down together with the rest of the galaxy.

-lastly, i would like to point out,my views on why there are more matter than antimatter in the observable universe.

-perhaps our observable universe is only a fraction of a bigger, undiscovered universe. Which means that huge pockets of antimatter galaxies and such exist in other areas of the universe. This balances out matter and antimatter perfectly.

Could be, but then you have to figure out some kind of mechanism to explain how the antimatter and matter got separated during the big bang. You have to remember that things happened ridiculously fast back then. It took less than 10^-12 seconds for baryogenesis to end. Your light cone is only about a millimeter wide at that point. How do you propose to separate the matter and antimatter on a galaxy wide scale when every particle can move less than a millimeter?

Besides, we know it is possible for particle interactions to violate CP conservation. Neutral Kaons and B mesons are good examples of particles with a bias towards matter. So its much more likely that those (and processes like it) are responsible.

[YNM]

I know that reading some books about cosmology might get your head spinning, but at least, try it. This should answer most of your question, as well as helping you understand most of the logics and assumptions often used in cosmology.

For those who don't like to read a book :

• Dark Matter : Dark Matter gains their name out of their invisible-yet-present observation (mind, invisible means no form of EM radiation). Galactic dark matter are quite dubious, for that their existence can be described (uh, substituted) by standard matter like interstellar gasses, a lot of red dwarfs, or stellar black holes. Worse are cluster's and supercluster's dark matter, for that intergalactic space is really empty (put another way, the gases are really tenuous), and there's no star there (only a handful), so standard matter have to be ruled out. Galactic dark matter are deduced from gravitational mass minus luminous mass, and cluster / supercluster dark matter are deduced from gravitational mass (often via gravitational lensing) minus luminous mass. Best part of gravitational lensing is, you can tell the distribution of the dark matter, and it's pretty bizarre...
  
• Dark Energy : First introduced by Einstein to ensure his model of Universe are stable (even so, he stated that it's a menace, something that makes his equations looks bad), later retracted after Hubble's discovery of Universe's expansion. Yet later again, are re-introduced to explain the acceleration of Universe's expansion. Nobody knows what is it for sure, in the past (before computational power rose really quick) it's assumed to have a certain limiting properties and are called cosmological constant / Λ (Lambda), now many seeks to get a few types of them in one Universe, the same for dark matter (in the past only CDM is used). Pretty possible, considering that even the 5% matter in the Universe (that is, "standard" matter) are pretty diverse, why not the 95% ?

TL;DR one is anomalous amount of gravitational matter, the other anomalous energy required to keep the world the way it is.

Edited June 25, 2015 by YNM

[RainDreamer]

It seems to be similar to the Aether way back when the greeks looks up to space and figured, "there has to be something to fill up all that void up there". We are just repeating history here, but this time with (hopefully) much more knowledge about the cosmos.

[YNM]

It seems to be similar to the Aether way back when the greeks looks up to space and figured, "there has to be something to fill up all that void up there". We are just repeating history here, but this time with (hopefully) much more knowledge about the cosmos.

Yeah, pretty much they're an aether for our current understanding. Who knows that they're actually ordinary matter, waiting to be discovered ? Or some corrections can make them less important ? Or they're actually a matter of their own kind, with their own kind of objects ?

Though, proof of dark matter annihilation (even, the creation of ordinary matter out of them) have been proposed, types of dark matter have been tried (and tested to a limit), and dark energy are accepted as more like something that's embedded to spacetime.

Edited June 25, 2015 by YNM

[78stonewobble]

For purely subjective aesthetic reasons I find the "dark fluid theory" interesting. I like the idea of unifying gravity, dark matter and dark energy and the fluidity of spacetime.

That doesn't make it true tho...

[Unheld]

Your ideas are actually not that far off...

Since gravity travels at the speed of light, fast-moving galaxies or large bodies of mass may leave behind "trails" of gravity. Hence, when you see a star 1 billion light-years away, you are actually feeling the very,very,very small gravitational influence from that star 1 billion years ago.

Since others have already posted quite good answers about dark energy/matter. I thought I throw some interesting things in about gravity:

You're right about gravity exerting its effects with the speed of light. There are many experiments devised in order to test that with the help of gravitational waves. The idea being, that for example two dense stars or even black holes circling each other in close proximity and therefore very fast should send out disturbances very like waves. Consider the famous rubber sheet, but this time two dents close two each other, or maybe for better visualisation you put not a ball (or two in this case) but a stick in the middle, so that the dent looks elongated. If that stick rotates, the elongated dent changes direction, which an observer at the side can feel. Let's call that observer LISA

For large masses to leave "trails", spacetime has to be somewhat inert or sluggish. In the rubber sheet analogy a ball or dent rolling on the sheet wouldn't leave a trail, but let's say rolling a ball on rubber foam or foamed plastic (like a memory foam mattress) would leave a trail in which other small probe balls could roll.

Interestingly you're theory is not that far off. Space time has a certain sluggishness as the Lense–Thirring precession perfectly shows. Again with the rubber sheet: Imagine you put you're index finger on the rubber sheet (and make a dent) and then rotate you're hand. The rubber sheet would twirl around your finger. Suppose you had painted radial lines from your fingertip outwards before twisting your hand. These lines were straight and are now curved, spiralling to the center.

That is basically what the Lense-Thirring effect means, large rotating masses "twirl" the space time.

But this "sluggishness" is too small or too weak for "gravity trails" like you propose, or at least for the effects you want to explain with it.

-lastly, i would like to point out,my views on why there are more matter than antimatter in the observable universe.

-perhaps our observable universe is only a fraction of a bigger, undiscovered universe. Which means that huge pockets of antimatter galaxies and such exist in other areas of the universe. This balances out matter and antimatter perfectly.

Yes, that's a perfectly reasonable assumption. But for now it seems a bit ... useless. I'm very sorry, if this sounds harsh, it isn't meant that way, but any assumption like that doesn't solve anything. Obviously it can't be directly tested, and so long as there doesn't exist a (mathematical) concept, mechanism, or what have you, that has measurable and testable consequences inside the observable universe, it can't be much more than a side note in the sciences, which it surely is, since it isn't disprovable either.

The scientific principle leads to the assumption in cosmology that our universe is homogeneous and isotropic. That means that our universe is everywhere the same (in terms of laws) and that every observer looking in any direction sees the same, basically it says that our place in the universe is not anything special, unlike the assumption of antimatter pockets outside but not inside of our observable universe.

Hope that helps a bit, and sorry if my writing style may be a bit weird, I'm an english-not-first-languager

Edited June 25, 2015 by Unheld

[ZetaX]

Problem only solved by the general relativity that say "gravity" is not a force, but a space-time curvature, so it's action is instant, and not delayed (by the speed of ligth).

No, gravity's speed is probably c for all we know. General relativity has it move at the finite speed of c; please stop making this claim that it does otherwise, because it does definitely not. If it were instant, you would not have things such as gravitational waves. If someone right now stops the sun with a huge thruster, we would only feel the gravitational change in ~8 minutes, at the same time we see it happen.

Claiming that it is faster because it curves spacetime is an unfounded claim; there is no reason the latter implies the former. Going further, any information transmission faster than c would allow to break causality using special relativity (has been posted so many times, please just search for elaborations); moving something and detecting that change in gravity is obviously a way to transmit information. So your proposal, which as I now repeat is not what general relativity says, has very severe implications and is probably wrong.

See also https://en.wikipedia.org/wiki/Speed_of_gravity#General_relativity.

But for most purposes, gravity behaves as if it were instant ("no aberration"): up to a high accuracy, a force spreading at the speed of light and being proportional to 1/r² behaves like an instantaneous force. Accuracy becomes even higher for the specific properties gravity has (but the former also applies e.g. to electromagnetism).

This does not mean it travels that fast. It only means that as long as gravity is just doing its thing, we won't see a notable difference to such behaviour. But as soon as other things come into play, e.g. a huge thruster on the sun, we will; or more accurately, we won't see the difference for 8 minutes as mentioned above.

Funnily, this instantaneous-like behaviour is what keeps orbits stable, they would only last some hundreds to tens of thousands years otherwise.

See https://en.wikipedia.org/wiki/Speed_of_gravity#Aberration_of_field_direction_in_general_relativity.2C_for_a_weakly_accelerated_observer for some further details.

This mean that a "gravity trail" can't be "left behind" by some galaxy, as the "gravity field" is a space-time curvature alway "under" the galaxy mass and has no "gravity trail" to left behind like a fold in the sheets: gravity "instantly travel" with the mass.

Sure, the "bulge" is always where the mass is. But the gravitational influence on objects further away can point to the old location, barring the effect I mentioned above.

[Unheld]

a huge thruster on the sun

And I cudgel my brain with rubber sheets...

But ZetaX is absolutely right.

[ko1Stein]

1. Concerning "gravity trails": Gravity can only travel at the speed of light because anything else would violate causality and makes even more sense if the (debateable) theories about gravitons would be taken into consideration. This does mean that it would lag behind the actual object, but to create a recogniseable change you would have to travel near or above light speed (which not even the fastest stars accomplish).

This does not, however, change the strength of the object's gravity in any way or else it would be attracted by its own gravity trail, slowly brake and possibly create a perpetuum mobile (which IS possible with newtonian physics).

2. The theory about dark matter is not too far off, but it can't be regular dust since this would interact with light, etc.

3. It is true that we only know a small part of our universe, since all our knowledge about it comes from the light everything emits and this takes a while to travel to us at light speed, we can, logically, only see as far as light can travel since the universe was created, about 13.5 billion light years if I remember it correctly. So the theory would be possible, especially since matter and antimatter were originally distributed randomly, but it is very unlikely that NONE would have survived and be observed today. (This is still one of the big mysteries of physics, though)

Short note on Dark Energy: It could be exotic matter inside our universe, meaning matter that creates antigravity and thus pushes, or even a force pulling from outside our universe. Nobody knows for sure, though a lot of theories have been made and many proven wrong.

[Unheld]

3. It is true that we only know a small part of our universe, since all our knowledge about it comes from the light everything emits and this takes a while to travel to us at light speed, we can, logically, only see as far as light can travel since the universe was created, about 13.5 billion light years if I remember it correctly.

Fun fact: although you are right about the age of the universe (since big bang) being about 13.5 billion years, the radius of the observable universe is (thanks to expansion) about 46 billion light years.

[justidutch]

Fun fact: although you are right about the age of the universe (since big bang) being about 13.5 billion years, the radius of the observable universe is (thanks to expansion) about 46 billion light years.

So this means the speed of light is faster than the speed of time?

[78stonewobble]

Yes, that's a perfectly reasonable assumption. But for now it seems a bit ... useless. I'm very sorry, if this sounds harsh, it isn't meant that way, but any assumption like that doesn't solve anything. Obviously it can't be directly tested, and so long as there doesn't exist a (mathematical) concept, mechanism, or what have you, that has measurable and testable consequences inside the observable universe, it can't be much more than a side note in the sciences, which it surely is, since it isn't disprovable either.

The scientific principle leads to the assumption in cosmology that our universe is homogeneous and isotropic. That means that our universe is everywhere the same (in terms of laws) and that every observer looking in any direction sees the same, basically it says that our place in the universe is not anything special, unlike the assumption of antimatter pockets outside but not inside of our observable universe.

Hope that helps a bit, and sorry if my writing style may be a bit weird, I'm an english-not-first-languager

So far atleast, they have not found areas of the universe where widespread matter/antimatter annihilation occurs. So yeah, no evidence

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So this means the speed of light is faster than the speed of time?

It means the distance that light has to travel is expanding as the light travels.

[NFUN]

So this means the speed of light is faster than the speed of time?

The expansion is greater than what would seem possible due to the speed of light because of the incredibly rapid inflation of the early universe, as well as the faster-than-light expansion of space theoretically, tentatively due to dark energy.

[justidutch]

My I wish I was drunk already stupid thought of the day...

The universe is actually fixed in size, and always has been. It's just that every single observable entity in existence (especially our egos) started out infinitely large, and are now (except maybe our egos) decreasing in size, giving us the illusion that other galaxies are expanding away from us.

I really don't know where this came from, it just popped into my head. I don't seriously think this, but it's a Friday and I'm almost off work and I tend to get goofy at these times.

[78stonewobble]

My I wish I was drunk already stupid thought of the day...

The universe is actually fixed in size, and always has been. It's just that every single observable entity in existence (especially our egos) started out infinitely large, and are now (except maybe our egos) decreasing in size, giving us the illusion that other galaxies are expanding away from us.

I really don't know where this came from, it just popped into my head. I don't seriously think this, but it's a Friday and I'm almost off work and I tend to get goofy at these times.

Haha, this is a great one... Alternatively our ego's are growing faster and faster and pushing / pulling the universe apart.