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Dark Matter


Danilo_Rocha2
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Dark Matter is approximatly 85% of our universe, we don't know what it is so we can´t say that is matter.

But imagine if Dark Matter was a distortion in the space-time fabric and those distrotions were grooves left by stars, planets or even galaxies, that were doing their thing and just did it.

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1 hour ago, Danilo_Rocha2 said:

What is dark matter for you?

I don’t understand the question. This is objective, not subjective.

3 hours ago, Danilo_Rocha2 said:

Dark Matter is approximatly 85% of our universe,

85% of the matter in the universe

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

Will you explain what you mean by “grooves left by stars”?

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4 hours ago, Danilo_Rocha2 said:

Dark Matter is approximatly 85% of our universe, we don't know what it is so we can´t say that is matter.

But imagine if Dark Matter was a distortion in the space-time fabric and those distrotions were grooves left by stars, planets or even galaxies, that were doing their thing and just did it.

If dark matter was an effect caused by visible matter like stars and planets etc. , then its effects would follow a pattern that reflects that. 

Galaxies would act as if they had extra mass, but that extra mass would be concentrated where we see stars, etc.   However, what we see is that galaxies behave as if most of the mass is located in regions where we see mostly nothing. (Such as the areas above and below the visible disks of spiral galaxies.) So it the distribution of the extra gravitational effect and not just an increased magnitude.

In addition, we have seen examples where the gravitational effect attributed to dark matter has been dislodged from its parent galaxy cluster by a collision between two such clusters. Again if dark matter was just an additional effect caused by the stars, etc by themselves, you wouldn't be able to " knock it loose" from those stars.

We also have found galaxies that are nearly identical in all other respects, but one behaves like it has dark matter, while the other behaves like it has little to no dark matter.  If dark matter was an effect caused by the stars in the galaxy, then two identical looking galaxies would behave exactly alike.

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6 hours ago, Danilo_Rocha2 said:

But imagine if Dark Matter was a distortion in the space-time fabric and those distrotions were grooves left by stars, planets or even galaxies, that were doing their thing and just did it.

The concept of the "fabric" of spacetime is for instruction only, usually combined with a heavy ball to represent a planet curving spacetime with its mass. There is no actual fabric in space, nor is there anything for a star to leave grooves in, as swansont mentioned.

I think Douglas Adams would've quite liked the idea of the fabric of spacetime being corduroy, though.

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8 hours ago, Danilo_Rocha2 said:

Dark Matter is approximatly 85% of our universe, we don't know what it is so we can´t say that is matter.

But imagine if Dark Matter was a distortion in the space-time fabric and those distrotions were grooves left by stars, planets or even galaxies, that were doing their thing and just did it.

All matter/energy warps, curves, twists spacetime, and we see that deformation of flat spacetime as gravity. Dark Matter though differs from normal baryonic matter, in that it does not interact with light, neither absorbing or reflecting or emitting any part of the electromagnetic spectrum. Hence we don't see it, and can only infer its existence by its gravitational effects.

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So far as I can see there are roughly four basic approaches in the literature as to the nature of DM:

1. It is what it says on the tin - a form of matter that does not interact with light. This will likely require a new addition to the Standard Model, since none of the hitherto known particles readily appear to have the properties required of DM. This appears to be the most popular option that most scientists in this field pursue.

2. A new fundamental interaction. This postulates an as yet unknown additional fundamental interaction which acts on ordinary matter-energy. Hence, the motion of bodies we detect is the net result of both gravity and that new interaction

3. A modification of the laws of gravity. The idea here is that in actual fact there is no DM, but that gravity on larger scales is not well described by the GR field equations, requiring some amended (scale-dependent?) law of gravity on those scales. The appearance of DM is then simply the difference between what GR predicts, and what the actual motion of test particles under the amended gravity law is like.

4. DM is neither a new form of matter, nor a new interaction, nor the result of a new law of gravity. Rather, it arises because we are not using the standard (unmodified) laws of GR correctly. The idea here is that any calculation in GR relies on some form of simplified approximation that allows us to actually perform the computation - we must choose to ignore some boundary conditions, and introduce extra symmetries that in reality are not actually there, otherwise the equations are simply too complicated to be solvable, even in principle. For example, when modelling a galaxy, we might choose to describe it as a disk-shaped continuous gas distribution of roughly the right shape, which allows us to find some kind of solution to the field equations. In actual reality though a galaxy isn’t continuous like that, it’s a collection of a very large number of discrete objects that all interact gravitationally, so it’s really a general relativistic n-body problem with n being on the magnitude of ~100’s of millions. We assume that our continuous approximation to an actual galaxy yields a gravitational metric that is sufficiently similar to that of the (unsolvable) case of having 100 million discrete objects - but how do we actually know this, since we cannot derive an actual solution for the latter case? The Einstein equations are highly non-linear, so it is notoriously difficult to mathematically determine what kind of error arises from a given choice of simplification, and how this error evolves (kind of similar to varying initial conditions in chaos theory). So in this proposal, DM is precisely the error that arises from our choice of simplifications in the ansatz of our model - the idea being that if we were able to accurately model the gravitational source and the relevant boundary conditions, this error would simply disappear, and GR would produce the correct motion of all objects. So DM is an artefact of our own computational limits, and not a real aspect of the world at all.

Out of all these, option (1) is probably the most popular and perhaps also the most likely, based on current knowledge. But we shall see.

 

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