Disambiguating the ‘Observable Universe’
samenright.com
samenright.com
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;)
We don't. We have a current best understanding that explains our observations, we don't claim that this understanding is the ultimate truth.
Guvf wbxr qrfreirf nyy gur qbjaibgrf gung vg jvyy trg.
Now we can add heat death of the universe to the list!
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.
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).
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.
I thought it was "Sol"?
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
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 that's the case in an expanding universe, but it might be the other way around in a contracting universe!
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>
You keep using that word. I do not think it means what you think it means. [0]
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 (...)"
What is meant by the phrase "expansion of space"? Is new space being created? Are the physical distances between things changing?
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.
I'm lost from here..
Except every point in the universe is like the middle of the balloon with space accelerating away from it in all directions.
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"?
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-...
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.
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”)
Not sure if this comparison is valid though.
> What is the difference between "space expanding" and "things moving away from each other"?
There are other evidence as well, but the redshifted light I believe was the first hint.
Yes.
> 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.)
https://www.physicsforums.com/threads/what-is-the-fabric-of-...
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 ).
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.