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# DARK MATTER

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Loving HOW THE UNIVERSE WORKS episodes on the SMITHOSONIAN CHANNEL, but bewilered by new episode tonight

on DARK MATTER in which astronomers throw up their hands when asked how much of it there is?  They don't know.

They don't know how to "measure it".  Why wouldn't this work:

+  In a given area of the universe, carve out an imaginary sphere.

+  Calculate the mass of the sphere.

+  Calculate the mass of everything within the sphere.

+  Determine the difference

Shouln't this render a resasonable estiamte on the amount of DARK MATTER  (maybe also DARK ENERGY?) in the sphere?

As a cross-check:

+  Now carve out, say, 20 other imaginary spheres in the universe

+  Conduct the same calculations as above

+  If there is consistence in this experiment, then we should be onto something, shouldn't we?

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• Mr Spock changed the title to DARK MATTER

Welcome to SGL.

19 minutes ago, PennyRiver said:

+  In a given area of the universe, carve out an imaginary sphere.

+  Calculate the mass of the sphere.

+  Calculate the mass of everything within the sphere.

Sorry, but I am not sure what you mean by this.

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33 minutes ago, PennyRiver said:

+  Determine the difference

I could definitely do this bit. If someone else could do the first two we are on to a winner.

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I think they may have been talking to the wrong astronomers?

It can be measured, eg through galaxy rotation profiles and gravitational lensing. There is some uncertainty when extrapolating the measurements to cover the whole observable universe, but current estimates are

27% Dark Matter

68% Dark Energy

Of course what we have really no idea about is - what they are.

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8 hours ago, PennyRiver said:

They don't know how to "measure it".  Why wouldn't this work:

+  In a given area of the universe, carve out an imaginary sphere.

+  Calculate the mass of the sphere.

+  Calculate the mass of everything within the sphere.

+  Determine the difference

Because you can't calculate mass of anything - you can just measure it somehow - directly or indirectly.

How do you propose one measures mass of such a sphere?

How do you propose one measures mass of everything in such a sphere that is not dark matter?

Last part is trivial as outlined above - we subtract two numbers.

2 + 2 = 4, or 22

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Posted (edited)

I think the main issue is, no one really knows what the "missing" mass of the universe is so it's been labelled under dark matter. Another way to see it is, if you were completely surrounded by said matter and couldn't see through or out of it/into it or see the effects of it how would you know it's there, let alone measure it (think if you were forever underwater, how would you know what water was/is). Scientists still don't know what causes gravity (how it works, otherwise we'd have the technology for it) other than knowing there's a force at play there and can be used in calculations and are looking further into the unknown.

Edited by Elp
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1 minute ago, Elp said:

Scientists still don't know what causes gravity

It is curvature of space/time

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Posted (edited)

That's a way to "observe" what it does (ie see its effect), not how it comes to create that effect (ie we know mass has something to do with it, but don't know how it generates that force (whether it's a derivative of strong nuclear force or something), otherwise we'd have figured out how to make anti gravity/gravity engines by now for example).

Edited by Elp
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Just now, Elp said:

That's a way to "observe" what it does (ie see its effect), not how it comes to create that effect (ie we know mass has something to do with it, but don't know how it generates that force, otherwise we'd have figured out how to make anti gravity engines by now for example).

We do know how to manipulate gravity.

Mass/energy density is responsible for curvature of space time. We can concentrate large amount of energy in tiny space and change local space/time curvature there.

I'm not sure that existence of "anti gravity" engines is proof that "we finally know how it generates force".

It's a bit like saying - we don't understand energy because we have not made perpetual motion machine. No where in the universe we have not observed "inverse gravity" phenomenon - and there is no reason to believe it is possible - much like creating energy out of nothing.

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But the universe is expanding in an unexplained way, this could be your inverse gravity, or as scientists have attributed somewhat to dark matter.

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6 minutes ago, Elp said:

But the universe is expanding in an unexplained way, this could be your inverse gravity, or as scientists have attributed somewhat to dark matter.

Depends what you mean by "in an unexplained way".

Currently most widely accepted cosmological theory - Lambda CDM - explains expanding of the universe quite nicely - and not as inverse gravity - but rather within frame work of general relativity as vacuum energy - which has negative pressure and thus causes space to stretch when its devoid of matter and energy (here energy is any form of non zero point vacuum energy).

We know from quantum mechanics that vacuum is not empty and that it indeed must contain some amount of zero point energy in form of random quantum fluctuations (there is however discrepancy in calculation of said energy by different means).

Dark matter is not the cause of accelerated expanse of universe - quite the opposite it is one form of matter that together with luminous matter - counter acts above negative pressure.

Dark energy is the name for above vacuum energy that is causing accelerated expansion of the universe.

Above Lambda CDM model is one of things why we believe there is dark matter present. It fits quite well in observable facts - except that amount of matter that we can detect via EM radiation - or stuff that shines (which is mostly stars and large black holes in centers of galaxies that we observe via release of energy) is only about 1/5 of mass needed to fit observational data of structure of the universe and rates of expansion.

But this is not the only "proof" of dark matter. There are other phenomena that are easily explained by additional matter (which does not "shine") - like rotational curves of galaxies and gravitational lensing.

You can see nice list of things that point towards the dark matter here:

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As @vlaiv said dark matter has a normal positive gravitational effect.

In  GR the simplest explanation of dark energy is the Lamda in Lamda CMD. That is a very small residual curvature of space time.

As we don't have a theory of quantum gravity it is difficult to relate "vacuum " energy and dark energy.  Attempts to do give a miss match by 100s of orders of magnitude.

Regards Andrew

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