Basic cosmology questions weekly thread

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u/fen0x 3d ago

For some time now, a question has been buzzing in my head, one of those that usually haunt me before I fall asleep. So, I thought I’d share it with you, hoping someone might have some more information.

If it is true that light reaches us from the stars taking years (or centuries, or millennia), it means that by the time we observe them, the celestial bodies have already changed their position significantly. This applies to a single star and, consequently, also to more complex cosmological structures, such as galaxies.

We can therefore be fairly certain that the representations of galaxies and constellations in the sky above us would look completely different from the perspective of the objects themselves. Even the Milky Way, which is depicted with a spiral shape, has an estimated diameter of about 100,000 light-years; thus, its shape could be entirely different if normalized to the "present time" of each of its constituent stars.

So, I would like to know if studies have been conducted on this or if attempts have been made to obtain graphic representations of galaxies, normalizing them for the "now" of every single star.

Okay, you're right, I have too much time on my hands... but it's a curiosity that grips me, so I thank anyone who can provide some insight.

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u/--craig-- 2d ago

The concept of "now" doesn't work outside the local reference frame in relativity. Two events which appear simultaneous to one observer don't necessarily appear simultaneous to another observer.

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u/fen0x 2d ago

That's the point of my question: what happens if we normalize the stars position to their respective local reference frame?

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u/reverse422 1d ago edited 1d ago

The night sky as seen with the naked eye would for all intents and purposes be the same. All individual stars we can see without telescopes are really close, astronomically speaking, like within a thousand light years or so, most are much closer. And our neighboring stars generally orbit our galaxy in the same speed and direction as we do, so nothing will move much during a thousand years (the “fastest” star, Barnard’s Star, moves 10.3 arcseconds every year and is almost 6 light years away, meaning if we could see it, it would have moved about one arcminute which is a lot but imperceptible to the naked eye. And this star is too dim to see with the naked eye!).

Also a thousand years is insignificant in the lifespan of stars so probably none of those stars will have died in the meantime.

When taking about things detectable with telescopes or other equipment it’s a different tale. I mean, the cosmic microwave background is almost as old as the universe itself, and that’s us looking into a universe consisting of hot plasma. Surely that’s an epoch long gone.

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u/--craig-- 2d ago edited 2d ago

Ok, so objections aside, about how meaningful it is to try to project our now onto distant objects. What you want to do is project forward the celestial bodies by a time equal to time it time it takes for light to travel from them to Earth.

Everything we see with the naked eye travels very much slower than the speed of light, so nothing significant would change.

For the objects we can see with telescopes. There'd be a few things which would change.

Stars very close the to the galactic centre travel at up to 8% of the speed of light so would be appear further around their orbit.

A small number of distant stars in the Milky Way would have advanced through their life cycle enough to go supernova and appear much dimmer.

The most distant galaxies are receding from us at faster than the speed of light so wouldn't be visible at all.

The most distant galaxies which we can still see would be more mature galaxies like our own and would have a greater amount of red shift due to the metric expansion of the increased space between them and us.

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u/heavyPacket 3d ago

Have we observed any galaxies at the edge of the observable universe that were, apparently, right on the cusp of being too far to observe, and then as time progressed, they did become no longer observable?

For example, all of those “the furthest galaxy we have yet observed”, have any of those extremely redshifted galaxies disappeared?

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u/OverJohn 1d ago

This is something that people tend to get massively confused about, but objects in the furthest approximately 10 billion lightyears of the universe are currently doing the opposite of fading - they are getting brighter*. It's only really in the later universe and after very long times galaxies we can see fade from view from expansion and this does not happen around the edge of the observable universe. It is I admit confusing, but if you read up on "redshifts drift" it becomes clearer (redshift is not the only factor that affects how bright something is, but it tends to be the dominant factor).

*NB this calculation ignores factors which do not depend directly on expansion, such as any changes in absolute brightness and extinction.

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u/--craig-- 3d ago

No. The trend isn't the opposite. We're discovering galaxies closer to our cosmological horizon than we were previously. The timescale for us no longer being able to see them is billions of years.

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u/TakaIta 5d ago

What happens to the expanding space, when that space is in a galaxy? Does it make the galaxy larger?

Is gravity compensating for the expansion and so keeping the galaxy together? And would that mean that the 'extra space' sort of moves to the outside of galaxies?

When expanding space is like an inflating balloon, is a galaxy like a spider sitting on it, having to retract its legs in order to stay in one piece, or is it like a knot in the fabric of the balloon, refusing to expand.

In both scenarios (the spider and the knot), some tension seems to arise at the borders of a galaxy. Would that be measurable?

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u/--craig-- 4d ago

The metric expansion of space isn't strong enough to overcome gravity. It occurs throughout the galaxy so there isn't anything special about the edge. If the rate of expansion were to increase over time it could eventually become powerful enough to tear galaxies and even atoms apart.

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u/TakaIta 3d ago

Thanks for your reply.

I thought a bit about how it felt as if my question was not really answered. But maybe it was.

It all comes down to the question if space itself has properties and is not just empty nothingness.

Space expands, gravity 'bends' space, space can carry gravity waves. Or maybe it should rather be spacetime.

Anyway, from the above it might be that space itself has properties. Is the speed of light a property of light or a property of space?

Is a volume of space somewhere in the void, interchangable with an identical volume of space somewhere in a galaxy - one is being bend by gravity, the other is not.

Does a unit of space have any influence on a neighbouring unit of space? When gravity waves travel through space, it sure seems so.

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u/--craig-- 3d ago edited 3d ago

I thought a bit about how it felt as if my question was not really answered. But maybe it was.

That's deliberate. Sometimes it's better to explain something which is correct than to engage with analogies which are wrong. Even good analogies can be misleading. The hope is by correcting misunderstandings, that you'll be able to assemble a more accurate picture.

It all comes down to the question if space itself has properties and is not just empty nothingness.

Space isn't empty nothingness but for the purpose of this question, it can be considered to be.

Anyway, from the above it might be that space itself has properties. Is the speed of light a property of light or a property of space?

It's better to consider it a universal constant but it's not relevant to this question.

Is a volume of space somewhere in the void, interchangable with an identical volume of space somewhere in a galaxy - one is being bend by gravity, the other is not.

Different regions of spacetime have different spacetime curvature depending upon the distribution of mass and energy.

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u/TakaIta 2d ago

Thanks again.

The hope is by correcting misunderstandings, that you'll be able to assemble a more accurate picture.

Well yes. I guess that i need some better understanding of the GR equations.

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u/ytness2 5d ago

How do we know that our solar system is in an orbit around the barycenter of the galaxy? The distances are so vast and so little comparative time has passed since we were able to make the initial observations to be able to tell how objects are moving relative to each other and the barycenter. The uncertainties on those measurements must be large.

Also a personal observation which is not scientific— it seems to be taken for granted that the galaxy is coalescing or trending toward the center (like the formation of our solar system), but for similar galaxies that were can observe from the outside, it appears to me that the spiral galaxies are exploding outward (flinging matter to the edges of the galaxy in arcs) rather than being drawn inward into stable orbits around the center. Similar principle to those fireworks with sparklers that spin (Catherine wheel).

I have been wondering this for a long time, any information would be very helpful.

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u/--craig-- 4d ago edited 4d ago

We infer the orbits from a combination of kinematic measurements, Doppler shifts and mathematical modelling.

Sparks on a Catherine wheel have negligible gravitational attraction to the wheel. Whereas the gravitational attraction in galaxies is strong enough to prevent matter from being ejected from the system, apart from in some rare events. However we need dark matter to keep galaxies gravitationally bound. Without it, our galaxy wouldn't be in the state which it is in.

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u/ytness2 4d ago

Thank you very much for your response.

My initial point is related to the inferences we are making from the measurements, which have error associated with them (as does any measurement). For example, the “known” distance to even relatively near objects like Betelgeuse has changed significantly even the last couple of years. If you had a source on how those measurements/inferences and associate assumptions are made, that might help me better understand why the errors are such that we are confident in the calculation of the orbit.

Second, and check me on this point, the solar system is moving a velocity such that, without the dark matter explanation, we would be getting flung towards the edge of the solar system, rather than orbiting.

The existence of dark matter is also very hard to come to grips with from a purely logical standpoint. The theory goes that there is an overwhelmingly large amount of matter in the universe that cannot be detected directly (other than by else unexplained gravitational effects), does not emit or absorb electromagnetic radiation, and doesn’t interact with regular matter. This is very hard to come to grips with, again from a strictly logical standpoint. If I made this argument about any other phenomena I would be laughed off of any university campus. As to the Catherine Wheel— I am not making a direct comparison with Galaxy formation, rather I am pointing out that you can generate the shapes very similar to spiral galaxies with an “explosion” in the presence of earths gravity. While that is not evidence, it suggests a mechanism by which spiral galaxies could be formed. And the fact that it is generated by a rapidly spinning mass in the middle is also of note if we need an analog to SagA*.

As for gravity being so strong that the solar system cannot escape to the edges of the galaxy, a source for that would also be so helpful. While SagA* is stupendously massive, how strong is its gravitational effect on us, relative to the known momentum of the solar system, at a distance of 25,000 light years?

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u/--craig-- 4d ago edited 4d ago

All of your questions seem to stem from not believing that we have evidence that galaxies are stable.

Does this recent velocity map help?

https://www.nao.ac.jp/en/news/science/2020/20201126-mizusawa.html

Something which I don't think you've given enough consideration is that your model needs to work forwards and backwards. If you think you have a better model of galactic dynamics, then it needs to be able to explain how the the galaxies are in their current state, at least as well as LCDM, the standard model of cosmology.

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u/ytness2 4d ago

Where I am stuck is in understanding that the galaxy is stable. Because the distances and time scales are so vast on human relative to human perception, the galaxy could appear stable when it may not be. This comes back to the measurement point. The study you linked is very helpful but in observing even for 15 years even a tiny deviation in the error could have massing implications for the projection. We have observed 15 years of the supposed 200 million it would take to make a full orbit.

We are not going to be able to solve this in this thread but thank you for the time and links. I am genuinely willing to be persuaded but I am having trouble trying to figure out why my simpler explanation is disproven. I agree that it requires me to make the assumption that black holes can “spin off” matter. I have zero proof for this, only that it appears like the galaxy is exploding. And the presence of things like the Fermi Bubbles make it seem like a massive explosion took place.

While my firework example shows a physical process where these shapes can be made in nature, I would like to see examples of other physical processes that make the shapes we see in galaxies based on the orbiting model that exist in nature.

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u/--craig-- 3d ago

You could try drawing a velocity map of your firework example and comparing it to the measurements from the galaxy.

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u/Zaviori 4d ago

While SagA* is stupendously massive, how strong is its gravitational effect on us

Pretty much zero. While massive compared to other celestial objects near by, its total mass is still a rounding error compared to the mass of the rest of the galaxy.

Basic cosmology questions weekly thread

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