# B Dark Matter -- Could dark matter be standard matter from the past?

#### ryanbeem

Summary
Could dark matter be standard matter from the past.
On the premise that gravity transcends time, is there any reason that the effects of gravity that we observe as dark matter simply be the effects of gravity from the mass of standard matter from the past? For example the mass of our galaxy may not be enough to explain the effects of gravity observed on the stars within it, but what if some of the mass of the galaxy from yesterday, last week and last year was also exerting some gravitational pull on present matter as well.

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#### Dale

Mentor
Summary: Could dark matter be standard matter from the past.

what if some of the mass of the galaxy from yesterday, last week and last year was also exerting some gravitational pull on present matter as well.
The Milky Way is about 53000 light years across, so that is already considered.

#### ryanbeem

The Milky Way is about 53000 light years across, so that is already considered.
already considered in that by the time the effect of gravity reaches a point, the source is already in the past... but I'm suggesting that the the effect of gravity from mass 'A' is augmented by the effect of gravity from mass 'A' from where it was yesterday (because in a 4d model it is still there). As the universe is expanding the effects of past mass is less effective since it becomes 'further away' but nonetheless the effects could explain why there would appear 'dark matter' in the galaxy that cannot otherwise be observed.

#### PeterDonis

Mentor
On the premise that gravity transcends time
Where are you getting this premise from, and what does it mean?

is there any reason that the effects of gravity that we observe as dark matter simply be the effects of gravity from the mass of standard matter from the past?
Any such effects are already taken into account in our model of the universe. Remember that in GR, spacetime is a 4-dimensional geometry that already contains the entire history of whatever is being modeled, including all effects of "past" things on "future" things. The reason we hypothesize dark matter is that we don't get the right 4-dimensional geometry if we only take into account the matter we can see.

I'm suggesting that the the effect of gravity from mass 'A' is augmented by the effect of gravity from mass 'A' from where it was yesterday (because in a 4d model it is still there)
You are misunderstanding what a 4-d model is. It's a 4-dimensional geometry in which the effect of each piece of matter at each particular time is counted once. What you are describing is double counting, and that's not how it works. The past is not "still there" in the present; the entire 4-d history is "there" in the model, and each particular moment in it is counted once.

#### Dale

Mentor
I'm suggesting that the the effect of gravity from mass 'A' is augmented by the effect of gravity from mass 'A' from where it was yesterday
I don’t know of any workable theory of physics that resembles that. I doubt such a theory is possible

#### Ibix

For example the mass of our galaxy may not be enough to explain the effects of gravity observed on the stars within it, but what if some of the mass of the galaxy from yesterday, last week and last year was also exerting some gravitational pull on present matter as well.
In addition to comments others have made, this is a modified gravity theory, so has the same challenge all such theories have - why would you expect such effects not to show up in the motion of bodies in the solar system? We have never seen the slightest evidence that general relativity is inaccurate in the solar system.

#### ryanbeem

Where are you getting this premise from, and what does it mean?

Any such effects are already taken into account in our model of the universe. Remember that in GR, spacetime is a 4-dimensional geometry that already contains the entire history of whatever is being modeled, including all effects of "past" things on "future" things. The reason we hypothesize dark matter is that we don't get the right 4-dimensional geometry if we only take into account the matter we can see.

You are misunderstanding what a 4-d model is. It's a 4-dimensional geometry in which the effect of each piece of matter at each particular time is counted once. What you are describing is double counting, and that's not how it works. The past is not "still there" in the present; the entire 4-d history is "there" in the model, and each particular moment in it is counted once.
I meant "still there" speaking in four dimensional terms if I could assume to. I'm assuming my thought of the formula for gravity being F = G*((m sub 1*m sub 2)/r^2) doesn't seem to take in to account a 4d model, but I'm sure my experience is fairly limited. What formula for gravity would account for a 4d model, or does the same formula do so in a way I don't see.

#### weirdoguy

What formula for gravity would account for a 4d model

#### ryanbeem

In addition to comments others have made, this is a modified gravity theory, so has the same challenge all such theories have - why would you expect such effects not to show up in the motion of bodies in the solar system? We have never seen the slightest evidence that general relativity is inaccurate in the solar system.
I would assume the mass of something as small as a solar system doesn't have enough mass for it's extra 3d counterparts to exert any gravitational effect, but in thinking about it further, has anyone considered that the faster orbits of matter near the outside of galaxies could be due to the time dilation exhibited by matter closer to the center only appearing to be moving slower than it actually is, and not actually from dark matter at all?

#### Ibix

What formula for gravity would account for a 4d model,
The Einstein Field Equations, $G^{ab}=8\pi T^{ab}$. Despite the compact notation, these are sixteen second order differential equations that must be solved simultaneously. They do involve both space and time as one combined entity called spacetime.

The formula you quoted is Newton's law of gravity. It can be derived from the Einstein field equations as an approximation valid under certain circumstances.

#### Ibix

I would assume the mass of something as small as a solar system doesn't have enough mass for it's extra 3d counterparts to exert any gravitational effect
You can't just assume. You need to come up with a consistent mathematical model of what you are talking about - which given your demonstrated level of knowledge lies about ten years of hard studying in the future. PF can help you with the study of existing knowledge, but we don't do theory development here.
has anyone considered that the faster orbits of matter near the outside of galaxies could be due to the time dilation exhibited by matter closer to the center only appearing to be moving slower than it actually is, and not actually from dark matter at all?
Yes. Time dilation is utterly negligible for the study of galaxy rotation curves. Edit: back of the envelope, gravitational time dilation for a clock one light hour from the black hole at the centre of the Milky Way means it ticks 1% slower than a distant clock. The galaxy is about 850 billion light hours across. Time dilation is irrelevant here.

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#### Dale

Mentor
I would assume the mass of something as small as a solar system doesn't have enough mass for it's extra 3d counterparts to exert any gravitational effect
I think you dramatically underestimate how difficult it is to come up with workable alternative theories. While you can certainly assume what you like, the way you do so is mathematically, not by simply mouthing words in English. This is incredibly difficult because doing it in math requires you to be both exquisitely precise and completely self consistent. You need a mathematical model that:

A) has a term involving the effect of previous matter

B) the effect disappears at solar system scales

C) the effect arises at galactic scales and is exactly the right magnitude to explain the rotation curves of all observed galaxies

D) the effect disappears at cosmological scales

E) the theory explains all other observed gravitational effects, including tests on frame dragging, gravitational lensing, and time dilation.

F) the theory makes predictions that differ from GR + dark matter in some untested regime

G) Not required, but desirable would be that the theory has very few tunable parameters, explains other things besides the galactic rotation curves, and is based on physically justifiable assumptions

I doubt such a theory is possible.

#### PeterDonis

Mentor
I meant "still there" speaking in four dimensional terms if I could assume to.
If you're speaking in 4-dimensional terms, then, as I said, each piece of matter at each instant of time can only get counted once. Saying "matter in the past still exerts gravity on observers in the present" would be double counting.

my thought of the formula for gravity being F = G*((m sub 1*m sub 2)/r^2) doesn't seem to take in to account a 4d model
No, because that's Newtonian gravity, not General Relativity. You posted this in the relativity forum; that means you need to use the relativistic theory of gravity, GR. And if you're trying to explain all our observations of the universe, you need to use GR anyway because Newtonian gravity gives wrong answers.

What formula for gravity would account for a 4d model
The Einstein Field Equation of General Relativity. GR is the 4D model I have been describing. You posted this in the relativity forum: what theory of gravity were you expecting us to use?

Mentor

#### Nugatory

Mentor
The original question has been answered so this thread will stay closed.

As with all thread closures, if you believe that the closing is premature and that there is the possibility of continued productive discussion, PM me or any other mentor to request that it be reopened.

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