2 questions: Dark Energy, Dark Sky at night

Hi, I've been hearing a lot about how dark energy is moving the galaxies away from each other. How can something be so powerful as to move whole galaxies yet not tear apart the galaxies themselves? Not even picking away at the small planets on the fringe of each galaxy. Why? What's protecting them?

A question was raised as to why the sky gets dark at night when there are so many galaxies with trillions of stars shooting light our way. The answer given was that the universe is expanding, so a lot of the galaxies are moving at a rate that prevents their light from reaching us. My question is: How can galaxies move even close to the speed of light?

reply to post by jiggerj

I've been hearing a lot about how dark energy is moving the galaxies away from each other. How can something be so powerful as to move whole galaxies yet not tear apart the galaxies themselves? Not even picking away at the small planets on the fringe of each galaxy. Why? What's protecting them?

Gravity holds the space round massive objects together.

A question was raised as to why the sky gets dark at night when there are so many galaxies with trillions of stars shooting light our way. The answer given was that the universe is expanding, so a lot of the galaxies are moving at a rate that prevents their light from reaching us. My question is: How can galaxies move even close to the speed of light?

They are being pushed apart by the metric expansion of space, which some people attribute to dark energy. Very distant galaxies (more accurately, galaxy clusters) could even be moving faster than the speed of light relative to us, or to each other. This is possible because the expansion is occurring in space, not matter. Space, unlike matter and ‘normal’ energy, is not subject to relativistic speed limits. If space wants to expand faster than light, it can.

Incidentally, it isn’t so much a question of galaxies moving too fast for their light to ever reach us (this only applies to objects beyond the 13.7-billion-light-year radius of the visible universe); rather, that inside the visible portion of the universe, light from very distant objects is redshifted down to invisibility, moving from the visible spectrum down to infrared, microwave or radio frequencies.

At relatively close cosmic distances, most redshift is due to the good old Doppler Effect, but as you move farther outward from our terrestrial point of view, the main contributor becomes cosmological redshift, which is due to the expansion of space itself.

If we never get to identify the source of the accelerative force pushing the universe ever-wider (the conjectured mysterious 'dark matter') could it be simply because it doesn't exist?

I'm imagining a source of attraction far beyond our limited perception could create the same effect with further away objects which are actually closer to that outside source of gravity experiencing greater acceleration via simple inverse-square principles. That outside influence could be so weak in comparison to more local gravity sources within our solar system or even from our galaxy to be undetectable (for us) and still be enough to have huge effects over periods of billions of years on relative locations of clumps of matter like galaxies in a virtually frictionless environment like the vacuum of space. It's as if our universe is within a container and matter is being attracted toward the container walls.

It's all conjecture and when we find out - we'll know.

There is also a theory that suggests that the color of stars represents their age.

Until someone shows me a falsifiable experiment that can be used to test the hypothesis of dark matter, I will continue to consider it the purview of funding wars for research dollars, and of little practical use.

I did the math.
Inventing dark matter instead of revisiting the theory of gravity beyond our solar system is the height of speculation.



David Grouchy

I created a thread a while back looking for a discussion on dark energy, but it turned into a philosophical slap fest, and it quickly got passed by without a single reply about the actual physics.

In the first post there are links to more than 50 hours of classroom lectures from Berkeley and Yale you might find useful. Rather than posting them again here, I'll just point you to that. A fascinating subject indeed.

Are we Space Itself?

Originally posted by lernmore
I created a thread a while back looking for a discussion on dark energy, but it turned into a philosophical slap fest, and it quickly got passed by without a single reply about the actual physics.

Are we Space Itself?

Dude!
That thread is a great intro to the official story on Dark Energy.
Well worth looking at.


David Grouchy

reply to post by lernmore

Thanks for the link to your earlier thread. It’s a pity it took the direction it did; I hope this one doesn’t follow. Most people on this board aren’t really as big on science as they are on what they would call philosophy or spirituality. And sometimes they get the three confused. Threads that stick to the science are rare on ATS.

As to your proposition in that thread, it’s very likely indeed. Though, judging by your familiarity with the scientific material, I’m sure you’re aware of that already. Loop-quantum-gravity theories look on fundamental particles as kinks or braids in spacetime. As for string theories, what is a string or a brane but an oscillation in spacetime?