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Disambiguating the ‘Observable Universe’ (samenright.com)
76 points by MajesticFrogBoy 15 days ago | hide | past | favorite | 55 comments

I can’t believe they call it the Hubble sphere and not the Hubble bubble.

I can’t believe they call it the Planck length and not the Planck plank.

“ Future astronomers would gaze upon a barren universe.”

This is one of my favorite tidbits.

I love explaining that far into the future the sky will be empty and those that think the earth is the center of the universe will be entirely forgiven;)

Right, but why doesn't this observation lead to the next logical conclusion? If we can prove that in the future we would draw incorrect conclusions, why is it that we can pretend to understand the past at all? This is a stronger statement than the typical brain in the vat thought experiment because we (presumably) know it would have happened.

[...] why is it that we can pretend to understand the past at all?

We don't. We have a current best understanding that explains our observations, we don't claim that this understanding is the ultimate truth.

And there will be no way to disprove a future conspiracy theory that historical scientific records and observations documenting other stars and galaxies are fake.


Guvf wbxr qrfreirf nyy gur qbjaibgrf gung vg jvyy trg.

As they say “we have yet to find a problem that blockchain doesn’t solve”

Now we can add heat death of the universe to the list!

In about 5 billion years the Sun will become a red giant. So those future astronomers won't be gazing from earth because earth will no longer exist.

Earth and at least the 'inner' solar system will be inhabitable long before that.

Inhabitable means habitable? What a country!

Sorry, uninhabitable. (errNEC(not enough coffee))

edit: Anyway, long before the Sun expands, its radiation alone will make everything too hot, while it is believed it will be more unstable in the process. Which means more solar storms, coronal mass ejections and the like. Not good.

Star lifting [1] to the rescue. By mining mater from stars it might be possible to influence its life cycle and extend its lifespan by billions of years.

[1] https://en.wikipedia.org/wiki/Star_lifting

Uhm. Let's first try to tame Yellowstone by using it as geothermal power plant, ok? :-)

Of course, step by step.

that's this sun. the sun next to the last earth might also be 5 billion years from growing but that's 100 billion years from now.

Just nitpicking but there is no other sun than this sun. Sun is the name of the star that the earth orbits.

When talking about other solar systems, the term "sun" (not "Sun" with a capital S) is often used to refer to the central star or stars. For example, Tatooine is often mentioned as being in a two-sun system.

This is not dissimilar to using the word moon, lowercase M, to refer to bodies orbiting planets other than Earth. But the earth has The Moon (capital M).

Maybe true, but... I thin the real "names" of the sun is sol, and of the moon is Luna.

Just mankind is so pre-occupied with particular sun/moon(s) that the language heavily leans into the context where "moon" refers to Luna, and "sun" refers to sol; unless the context explicitly implies otherwise. Hence "the moon" isn't just the proper title for Luna, but rather a reference to "some moon in particular", which has always been Luna for so long, that the term is now an idiom. In contrast, I think if you where on a planet in a different solar-system, "the sun" might reasonably refer to the local star, and "the moon" to the local moon, assuming just one prominent moon.

After a quick bit of searching around I think Sun, Sol, Sonne, and many more are the name of our star in various languages. On the other hand language usages is what defines the meaning of words, so if other stars are commonly called suns, those two words are synonyms.

Just nitpicking but there is no categorical evidence that no astronomer on this earth or elsewhere has named any other star "sun" in English or any other language; and also, proper names can serve as generics, c.f. "hoover", "Karen"

Hmm, I think "sun" predates the revelation that stars and "the sun" are the same kind of thing. I think Sun can mean both/either the (terran) sun, as well as other "sun"(s).

> Sun is the name of the star that the earth orbits.

I thought it was "Sol"?

> Every second, 20,000 stars become newly unobservable from Earth.

Inquiring minds want to know.

1) If we had an instrument that measures the gravitational field due to one of these stars, would it drop discontinuously from a small positive value to zero, implying something like a gravitational wave shaped like a step function, or is there some reason for it to smear out?

2) If there are observable stars far enough away from us that these disappeared stars are still observable at their positions, should we detect the observable stars being affected by gravitational forces appearing to come from nowhere?

3) If the remaining observable universe is always ascending from a gravitational well, does relativistic time dilation imply that the rate of expansion we perceive is always faster than the actual rate but gradually slowing down to it?

edit: time dilation was the wrong way around

1. I don't think so. I think this is because the "moments" are different. A star becomes unobservable once it passes from the region within the particle horizon to the region outside the particle horizon. But passing into this region can't affect all the information on the way to us. That's because the region doesn't exist until we trace all light path backward. So start right now and trace out 14B years of light paths. You will find that star is within the region.

However some point in the future you notice that the star passed beyond the horizon. The star still exists it still emits light, but that light will never reach us.

We don't see the star "go away" as it passes beyond the horizon. Instead one day the future we will receive the last piece of information about the star redshifted to infinity. I think this means we observed a decay to some constant and that constant should be zero. This argument feels very similar to black hole arguments, so rings true to me.

2. I think this is trying to state something paradoxical. But it's not really a paradox. Everything we observe is within our particle horizon. Everything has it's own particle horizon. These regions don't overlap.

I think if I restate what you said without the objects the paradox dissolves.

> If there are points (P) in our particle horizon far enough away from us that these newly unobservable (U) points are still within the respective particle horizon of the points (P) in our horizon, should we detect the points (P) our particle horizon as being affected by points in their own horizons?

Of course, why should what we can observe in future affect what had happened in the past at some distant point.

I think if the information can't travel to you by the shortest, most direct path through vacuum without being sublimed away by spacetime expansion, then there's no other path for that information to get to you. The observed positions of faraway stars are also delayed by a proportional amount of time, so I think you can see the continued past effects of the unobservable star on an observable star, but nothing from after it became unobservable to you directly.

I think that's the case in an expanding universe, but it might be the other way around in a contracting universe!

The 46.5 billion light-year distance is TODAY's distance to the observable Universe's limit.

The Universe has been expanding in size since it first appeared 13.8 billion years ago. Since the Universe was smaller in the past, light could travel a further distance in the same time interval. For example, when the Universe was half its current size, a photon traveled 2 light-years every year. Light's "total travel distance" is deduced from its observable redshift z .

If the Universe stopped expanding today, it would take 45.6 billion years for Earth's expanding electromagnetic bubble to reach the edge of today's observable Universe, more than triple the Universe's current age.

The a(t)=3.4 (=46.5/13.8) scale multiplier, which accounts for the Universe's expansion, scales as ~ t^{⅔} where t is a time interval parameter since the Big Bang. See Ned Wright's comprehensive cosmology site for more details. <https://www.astro.ucla.edu/~wright/cosmology_faq.html#z>

Someone linked it in the comments over there, but I think it's worth reposting here, "The Edges of Our Universe" by Toby Ord (https://arxiv.org/abs/2104.01191), which goes over a number of different radii of this form that one could define.

We are told that spacetime undergoes metric expansion. Can it undergo metric contraction too?

That's what happens in the vicinity of masses.

> And the Hubble constant is decreasing: the relative velocities of celestial objects are (on average) growing more slowly than the distances between them.

You keep using that word. I do not think it means what you think it means. [0]

[0] https://www.youtube.com/watch?v=dTRKCXC0JFg

You can also call it the Hubble parameter. But I think even then the current value of the Hubble parameter is still called the Hubble constant and it is still not constant. But don't worry, those are just labels, it doesn't really matter that a constant is not constant. We also didn't rename atoms once we realized we could rip electrons out of them or even smash them altogether. There is no tin in tin foil, no lead in pencil leads, no chalk in chalk sticks, and not all blackboards are black.

This was a bit over my head, but if I’m reading this correctly, there are regions in space that are expanding faster than the speed of light. Therefore, we’ll never be able to observe those regions because the light will never reach us. That’s pretty fascinating. The definition of “observable universe” is more profound than I initially thought.

No, that's not correct.

The rate of expansion in the observable universe is believed and observed to be the same everywhere. We don't know if this is true for entire universe but it appears so in the part that we can observe.

The rate of expansion is believed and tested to be directly related to the distance between the points (Hubble's Law https://en.wikipedia.org/wiki/Hubble%27s_law). When points are far enough, the rate of expansion becomes faster than light.

The constant is 70 (km/s)/Mpc.

"The Hubble length or Hubble distance is a unit of distance in cosmology (...) — the speed of light multiplied by the Hubble time. It is equivalent to 4,550 million parsecs or 14.4 billion light years. (...) The Hubble distance would be the distance between the Earth and the galaxies which are currently receding from us at the speed of light (...)"

May I ask a dumb question?

What is meant by the phrase "expansion of space"? Is new space being created? Are the physical distances between things changing?

From what I understand: all points in space are being pushed away from each other; from two atoms in your hand to two atoms across the universe. The reason objects aren't getting bigger is because atomic/nuclear/gravitational forces hold nearby objects together: if you stretched the earth to be a millimeter wider (massive amount), the earth would just collapse down to the same size it was before. Objects are held together locally, but pushed apart globally.

Not quite, expansion doesn't happen on such small scales.

Expansion is a property of the Friedman-Lemaitre-Robertson-Walker metric, which is a solution to the Einstein equations if you assume everything is isotropic and homogeneous. Spacetime around a star or a galaxy is obviously not homogeneous. It looks much more like a Schwarzschild metric, which features no expansion.

It may help to think of the universe as a cake with raisins in it. When you bake the cake, the dough expands uniformly, but the raisins do not.

Small caveat: If dark energy has a certain property, it changes things and a "big rip" will occur, tearing apart everything but elementary particles.

If expansion is indeed happening, wouldn't it play a role everywhere, even if it's eclipsed by whatever else happens to be going on?

from my understanding, it happens at ever scale. But the other forces (weak, strong, gravity, electromagnetism) are MUCH more powerful than that at small scales, and thus not observable at small scales.

Don't take my word for it, but I think of it as magnets on a stretchy scarf. Three magnets that are already clumped together stay together but other clumps become further and further removed.

My dumb answer: the universe is an expanding balloon being inflated by The Big Bang. If you were a standing on one point on the surface of the balloon, objects relative to your observable distance would be moving away from you. This observation can be measured by a phenomenon known as the redshift.

I'm lost from here..

I think more like you’re in the middle of a balloon being blown up and all points are accelerating away from you.

Except every point in the universe is like the middle of the balloon with space accelerating away from it in all directions.

I've read about the fruitcake metaphor. When you bake a fruitcake in the oven it expands and thus all the little pieces of fruit in it move away from each other.

But if that is the proper metaphor then my question is: Is there any difference between everything "moving away from everything else" (like pieces of fruit in expanding fruitcake) vs. "space expanding"?

Why do we say "space is expanding" rather than saying "all stars are moving away from each other"?

What is the difference between "space expanding" and "things moving away from each other"?

> What is the difference between "space expanding" and "things moving away from each other"?

Not an astrophysicist, but I think both are happening.

Space is the medium we live in. It’s the cake in the fruit cake. The fruit wants to stay in the same place, but the cake is expanding which is causing the fruit to appear to move away from each other even though the fruit wants to stay in one place. Same with the universe, space is expanding which is causing objects to become farther apart, even if they could somehow have 0 speed.

But in our universe the planets, stars, galaxies, etc. (the fruit) do have speed. Some are getting closer to us, like the Andromeda galaxy, while many others are moving away from us.

However, given the vast distances and how fast space is expanding (how fast the cake is rising), even those distant objects that want to move towards us are actually moving away from us.

Imagine a swimmer swimming against a tsunami. You can be swimming any direction you want but the power and speed of the tsunami will offset that and dictate where you end up.

Space is expanding at incredible speeds, so it’s quite the tsunami: https://www.forbes.com/sites/startswithabang/2020/06/12/ask-...

Let's assume that at Bing Bang all the matter was in a very small area. Then it exploded, in other words all the pieces of matter started moving away from the starting area, presumably at an accelerating speed. That would simply mean that everything started moving away from everything else and that movement seems to be even accelerating. Fine. That is a good and easy to understand description.

Why then do I have to add "BTW. space is expanding"? Things are moving away from each other I get it. Why do they need to add to that "Space itself is expanding"?

I don't doubt there is a reason since best scientist are thinking about it. I just haven't seen a good explanation for it, yet.

AIUI, “Started moving away from the starting area” is wrong.

The starting area is everywhere.

It isn’t that there is some point where the center of the Big Bang was.

The idea is that, stuff was basically evenly distributed everywhere , and very dense, and then later, without needing the amount of stuff to have decreased, stuff is nevertheless more spread out.

Now, you might just explain this with like, “well, space is infinite in extent, so you can just spread it all out, without running out of room, by moving things out towards infinity” but you can describe the same sort of situation even if space is compact, turning back around in itself (as in the analogy of “imagine space is the surface of a balloon”)

I find it difficult to wrap my head around as well. One way you might think of it is not space expanding, but all objects shrinking. If everything shrank to half size, including wave-lengths and velocities, the size of the universe would double in size, and objects would appear to move apart from each other above the speed of light at large enough distances. However, you could never use this fact to travel above the speed of light.

Not sure if this comparison is valid though.

  > What is the difference between "space expanding" and "things moving away from each other"?
The difference is the distance, and that is measured by looking at light. Let us take as true the fact that light moves at a constant speed in a vacuum. We observe that light from very distant objects is more redshifted than light from closer objects, when looking at objects on the order of millions to hundreds of millions of lightyears away. The expanding universe theory explains this and other phenomenon as well.

There are other evidence as well, but the redshifted light I believe was the first hint.

One difference is that space can expand at whatever velocity it wants to, but objects can't move faster than light.

> Are the physical distances between things changing?


> Is new space being created?

I’m not sure what you mean by this question.

Imagine ants living on the surface of a balloon which is inflating, and, I think that’s not that far off? (Except, the balloon is compact and wraps around, and the universe likely doesn’t.)

Here's a rabbit hole you may wish to go down:


From a basic perspective: Yes, new space is being created. And, yes, the physical distance between things are changing.

The [Hubble constant](https://www.wolframalpha.com/input/?i=hubble+constant ) is roughly (2.2 * 10^-18)/s -- often written as, e.g., 68 km/s/Mpc (same thing).

Which basically asserts that, every second, each unit of space is increasing by 2.2*10^-18.

For example, if we suppose that there's a ball 1 meter away from you, then in a second, it'ld be ~1.0000000000000000022 meters away from you. Not that it moved away from you, but rather that ~2.2*10^-18 meters of space was created between you and it.

At least, in basic theory. There're a lot of reasons to not take it too literally. For example, at local-scales, forces may resist expansion, instead pulling things back together. For another, the idea that the value is constant seems dubious. But, it's really simplified/approximate/speculative/etc..

Note that this does imply that things can, in a sense, move faster than the speed of light without violating Einstein's Relativity. Because Einstein's Relativity asserts that things can't move faster than light locally; however, the expansion of space isn't considered to be a factor in that. Which is relevant to the main-article above: when something's so far that space is being created between you and it faster than the speed-of-light, it's theoretically outside of the universe that's relevant to you, and thus outside of one (disfavored) notion of "the observable universe".

For example, say that two aliens are separated by such a distance that the expansion-of-space between them is greater than the speed-of-light. Then, even if both aliens blasted off at nearly the speed-of-light toward each other, they'd still approach each other more slowly than the distance is expanding, such that they wouldn't reach each other. (At least, under a lot of assumptions.)

That distance-of-separation is currently [estimated at about 14-billion light-years](https://www.wolframalpha.com/input/?i=%28speed+of+light%29+%... ), called the [Hubble length](https://en.wikipedia.org/wiki/Hubble%27s_law#Hubble_length ).

I think i get it. Thank you for the great reply.

tldr, energy density of the universe remains same while the universe is expanding, so yes space is being created, and yes physical distances between things are changing (outside of the local influence of gravity that is, so no distance between earth and moon is not changing for example).

Now a longish answer, space is like a stage or a backdrop, it's not made of anything, yet we can model the behavior of radiation and matter within space, also we cannot talk only about space, we have to talk about both space and time since they are intertwined. So spacetime has geometry, ie it can be curved by presence of matter, and space tells matter how it can move (geodesics),

As for the expansion of space itself, Hubble observations found that every star outside the local group is moving away from earth and not only are they moving away from earth but they are speeding up as they move away from us, also we observe that the acceleration is proportional to the distance to those stars, for example a star 100 light years away is racing away 2x faster than a star 50 light years away (we inferred this based on the redshifting of the light coming from these stars that are moving away from us), based on these observations, we can conclude that the space between stars itself is expanding at rate proportional to the distance between stars, ie something like the opposite of positive pressure is pushing stars away and the further they are from each other the stronger the push away, we have some sort of always accelerating negative pressure affecting everything. This negative pressure is referred to as dark energy and it is responsible for this expansion of space. To put it another way outside of local cluster of galaxies, gravity is losing the battle against this negative pressure expanding force we call dark energy and eventually everything would be so far away that light from these distinct racing away stars won't ever make it to earth no matter how long we wait, which is why the author ends with future astronomers would see a barren universe.

disclaimer: I am no physicist, just another nerd who's watched a lot of documentaries and thinks he understands a thing or two about this stuff but could totally be drawing the wrong analogy or incorrect in some technical way for the above explanation, so I am happy to be corrected and learn.

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