this post was submitted on 20 Sep 2025
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Science Memes

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[–] Kolanaki@pawb.social 42 points 4 days ago (4 children)

So blue is moving toward you?

I was always a bit confused with the explanations in high school. But I also got a D in physics so maybe I'm just dumb.

[–] Agent641@lemmy.world 49 points 4 days ago* (last edited 4 days ago) (1 children)

Blue shift, it's moving towards you, the photons are being "compressed" to a higher, bluer frequency. Redshift, the light is being "stretched" to a lower, redder frequency. Both only noticeable at significant fractions of the spped of light, relativistic speed.

Something ominous about the post is that a cosmic object that is moving towards you at a steady rate is consided "blueshifted" in the past tense, it's velocity is steady. If a galaxy is "Blueshifting" in the present tense, then that galzy is somehow accelerating at you, which is impossible unless it's under direct control by an entity, presumably a kardeshev level 3 civilization.

[–] Kolanaki@pawb.social 6 points 4 days ago* (last edited 4 days ago) (2 children)

Isn't the rate of expansion in the universe increasing, and at an uneven rate at that? Meaning it could just as easily be an as of yet unknown natural phenomenon?

[–] deltapi@lemmy.world 14 points 4 days ago (2 children)

The expansion is supposed to be happening everywhere at the same time, not just at the edges.
For example, tomorrow there should be more space between the Sol and Alpha Centauri systems than there was yesterday.
Our present understanding suggests that 'normal' universal expansion should not (in and of itself) result in anything moving towards us.

[–] Kolanaki@pawb.social 8 points 4 days ago (3 children)

Aren't the milky way and andromeda on a collision course with each other?

[–] Zink@programming.dev 11 points 4 days ago

Yep. Space is expanding everywhere at once, but the effect is minuscule at the scales we're used to. And even at galactic scales the "speed" of expansion might seem like a lot to us, but it still isn't enough to overcome the motion of objects. I looked up some rough numbers to give you an idea:

The rate of expansion of space is 73 km/s/Mpc. So for every 3.26 million light-years between you and a distant galaxy, the space between you and that galaxy is expanding by 73 kilometers per second.

Andromeda's blue shift indicates it's headed towards us at 110 km/s. And in my non-expert head I'm thinking that blueshifted light must have already been redshifted by the millions of years traveling through space to reach us. So the galaxy's speed through space towards us when the light was emitted was considerably higher.

Andromeda is 2.5 Million light-years away, btw. So the cumulative distance of space between here and there is expanding at something like 73 km/s/Mpc * 2.5 Mly * 1Mpc/3.26Mly = 57 km/s.

But when talking about relativistic distances and speeds, basic terms regarding time and location don't always make sense.

[–] psud@aussie.zone 4 points 4 days ago* (last edited 4 days ago)

Milky way and Andromeda are close enough that expansion is too small to overpower gravity

Our local group is racing toward The Great Attractor but will never reach it as expansion is pulling it away faster than we're falling toward it

[–] Starski@lemmy.zip 2 points 4 days ago (1 children)

Yes, but not because of universal expansion really, they're just headed in a direction that is going to intersect at some point, likely combining the two galaxies together. This'll be so far in the future we'll all be long dead though, on top of it being unlikely that anything is going to even come close to our solar system

[–] Kolanaki@pawb.social 2 points 4 days ago (1 children)

That's the other part I find fascinating; these two galaxies are gonna intersect, but our solar system probably won't hit anything. That's so gnar.

[–] Zink@programming.dev 2 points 4 days ago

It's not just that, but it is unlikely that any star in our galaxy will collide with any star in Andromeda.

I think it's easy to think of galaxies as individual things, like these nodes in the universe where all the stuff is stored. But galaxies are incredibly vast and incredibly empty.

I love the video this guy did on the subject: https://www.youtube.com/watch?v=VsRmyY3Db1Y

The part that stuck with me is that if you made the Milky Way the size of the United States, our gigantic sun holding 99.86% of the matter in the solar system would be microscopic -- the size of a red blood cell. And iirc, the planet earth would be all the way down to the size of a virus.

[–] Davel23@fedia.io 5 points 4 days ago (1 children)

For example, tomorrow there should be more space between the Sol and Alpha Centauri systems than there was yesterday.

Galaxies are gravitationally bound, they do not expand in the same way as the universe.

[–] deltapi@lemmy.world 2 points 4 days ago

Fair enough, I should have been thinking bigger.

Expansion red-shifts light.

Yes which would be very odd for a far away object

[–] tetris11@feddit.uk 6 points 4 days ago (1 children)

No, it's just depressed and that makes us feel sad and worried for it.

[–] Kolanaki@pawb.social 6 points 4 days ago

I'm blue shifting-daba-dee-di

[–] Sterile_Technique@lemmy.world 2 points 4 days ago

It's the doppler effect, but with light instead of sound, and for the same reason.

Thing emits sound/light waves at a constant rate: sound/light waves hit you at a constant rate.

Thing continues to emit the same sound/light at the same rate, but starts to move toward you: sound/light waves hit you at a faster rate, causing the sound/light to turn higher-pitched/bluer.

Thing continues to emit the same sound/light at the same rate, but starts to move away from you: sound/light waves hit you at a slower rate, causing the sound/light to turn lower-pitched/redder.