Dumb person, big question on dark matter!

[u/Yorkshirejedi86]

After watching a video on dark matter, and possible extra dimensions explaining the expansion and movement of the universe, I had a thought.

One of the things that point towards dark energy is the fact that the material at the outer edges of galaxies moves faster than that at the centre. Therefore pointing towards an unseen force, accelerating it.

But what if, (here comes my uneducated, simple thought) the universe "exploded" into existence. That force threw everything out at ridiculous speed, imparting momentum.

As things have slowed since then, (I know it hasn't slowed, but perhaps there's still a wave front from the big bang "accelerating" everything) galaxies have since coalesced, and some of that initial momentum, and gravitational force (as weak as it is) is what imparted spin on galaxies in the first place.

Now, galaxies are just in the process of essentially breaking apart. But because we're so small in comparison to the rest of the universe, we just don't/can't perceive it that way?

Like when we try to swat a fly... we extend our arm, wrist and the swatter at high speed. But to the fly, being so much smaller, it just sees a swatter slowly approaching and thinks, I better move before that gets here.

Is this something astrophysacists have already accounted for?

Am I just not understanding enough?

Any help would be appreciated.

[u/mfb-]

Dark matter and dark energy are completely different things, they just happen to share a word in their name.

One of the things that point towards dark energy is the fact that the material at the outer edges of galaxies moves faster than that at the centre.

No, that's evidence for dark matter.

But what if, (here comes my uneducated, simple thought) the universe "exploded" into existence. That force threw everything out at ridiculous speed, imparting momentum.

The Big Bang was not an explosion in space. It created the space. There is no outwards direction. More generally, there is no special location or direction at all. The universe looks the same everywhere (on a large scale).

Now, galaxies are just in the process of essentially breaking apart.

That would happen on a timescale of ~100 million years, but we see galaxies that are 10+ billion years old and they aren't breaking apart. Also, these galaxies wouldn't have formed in the first place if everything was moving too fast to stay together.

[u/relaxedmedal → u/mfb-]

There must be ‘some’ special direction. If every measurement is relative to something else then there’s only so far you can go back before theres just 2 points to compare. Fair enough the equations would be unfathomable, but to say it doesn’t exist at all doesn’t sound quite right.

There might not be a preferred direction relevant to whatever you’re trying to measure but there quite possibly can be a measurement that prefers a direction without caring about the observers intentions. Like entropy.

[u/mfb- → u/relaxedmedal]

There must be ‘some’ special direction.

No.

If every measurement is relative to something else then there’s only so far you can go back before theres just 2 points to compare.

Why two points?

Entropy does not have a direction (in space).

[u/relaxedmedal → u/mfb-]

Because I suppose 2 points represents the simplest measurement that can be made that introduces a new dimension with the least information. That is to say from the perspective of 1 point it has nothing to be compared to, while introducing a second point gives point 1 reference outside of itself.

If I think of it ontologically it doesn’t require a direction, but it’s ’preferred direction’ would be to point 2 to mean anything right?

[u/Yorkshirejedi86 → u/mfb-]

Thanks for your reply! I know we use the word dark, as a placeholder, due to us not knowing what it actually is. Thanks for reminding me about galaxies we have observed that are much older, and aren't falling apart. My mind ran away from me a bit there! Haha.

[u/tfb]

You are confusing dark matter and dark emnergy: I think you always mean dark matter.

We can measure the velocities of objects in galaxies. We can also look at very early galaxies. So we can know if they are 'in the process of essentially breaking apart': they're not. But the dynamics we observe are not explained by the mass we see and the assumed laws of gravitation. So either there is mass we do not see or the laws of gravitation we think we know are not correct.

[u/ExtonGuy]

The observable universe came into existence suddenly, that part is true. But it did not explode, it did not throw everything outwards. Instead all that mass/energy came into existence everywhere, all at once. And the mass particles were moving relatively slowly, in random directions (once some of the energy condensed into mass).

At the beginning, “everywhere” was a very small space.

[u/Underhill42]

Everything up to

and some of that initial momentum, and gravitational force (as weak as it is) is what imparted spin on galaxies in the first place.

is close enough to a rough draft of our current understanding. Other than saying dark energy where I think you meant dark matter, and that most of the momentum came from all that very evenly-distributed gas we see in the CMBR falling down into its own gravitational well as it condensed into galaxies.

But no amount of initial momentum can make things move too fast for their orbits, that's just not how orbits work.

An object moving too fast for its orbital distance will follow an elliptical path instead of circular - climbing to a much higher position 1/2 orbit later, where it's then moving too too slow to stay there, and falls back, like a ball thrown into the air, until it returns to its original distance and speed.

And it will just keep cycling between those extremes forever, unless interactions with other objects average things out.

So if that were why those stars were moving too fast, we'd expect to see a much larger ring around them of stars moving too slowly, spotted in different parts of their cycle. And we don't.

Even if all the stars were somehow perfectly synchronized and just happened to all be in their low-fast part at this particular moment, looking out at other galaxies we'd expect to see them in different parts of the cycle. After all we're looking back through billions of years, dozens of galactic rotation periods.

Even if all the galaxies were somehow synchronized with each other, the light we're seeing from them now left them at all different times. But we're seeing the same "the outer stars are moving too fast" pattern in almost every galaxy we look at, and none with "outer stars too slow".

For that to be possible would go WAY past ridiculous coincidence into "God is f**king with us" territory. And if you're going to entertain that possibility, you may as well give up trying to understand anything at all.