It can be very confusing, but as I said comoving objects (for our purpose we may as well call comoving objects galaxies because galaxies generally speaking are very close to comoving) are constantly entering the particle horizon. Indeed they will always be in any model with a particle horizon for so long as the particle horizon exists because our particle horizon travels away from us at the maximum speed anything can travel away from us at, so any object not travelling away from us at the maximum possible speed will be overtaken by it.
However the Hubble sphere is not the same thing as the particle horizon and it can be bigger or smaller than the particle horizon, depending on the model. In fact the significance of the Hubble sphere is not all obvious, though are two examples when it coincides with actual horizons: 1) in a radiation-dominated Einstein-de Sitter Universe where it coincides with the particle horizon and 2) in de Sitter space, where it coincides with the cosmic event horizon. Neither of those models could describe our Universe though (the first one lacks ‘matter’ and the second one lacks matter and radiation).
Not entirely sure what you’re trying to show here or what your calculations are based on.
This is a fundamental misunderstanding of big bang theory. The big bang is everywhere, so there is no centre. I don’t know why we would even be discussing particle horizons if we weren’t talking about the big bang theory. The existence of a particle horizon depends on the (conformal) age of the Universe being finite.
I’d pay top dollar for a 72-points of dexterity of a 1/12 Scale model of Neil deGrasse Tyson enshrouded in his half-mirrored, Bobba-Fettesque Cosmoship of Wonder™.
Top Dollar. Especially if it makes a ponderous swhooooossshhhh… noise.
Okay, wow…it took until post 30 to get to someone who is on my wavelength. Tough crowd! Really glad to read this one though. (Who knows, maybe the tide turned later in the thread and people will read this and be like “oh, that’s right, a lot of negativity on the first page”.)
I guess the rest of your post is a little more negative than me though.
I love Carl Sagan and am glad that some of his most profound soliloquoys have found a new life on YouTube. I just don’t expect anyone to be able to replace Sagan, although I am glad his widow Ann Druyan is the first named executive producer in the credits. I have seen just the first episode so far, but have watched that twice with each of my oldest two kids. Both times I wept profusely through the elegy (and torch passing) of the concluding few minutes.
I personally liked the way the scale kept expanding out, until we were at the scale that groups of galaxies (if I have it right) are what is representing basically pixels or “motes of dust”. Freaky to see that at that huge scale, there was a visible pattern to the structure, like osteoporosis or something. And my mind was totally blown by the zoom out to where our entire universe is just a bubble in an infinitely large multiverse.
Anyone notice the subtle optimism in saying we can’t see back to what happened before the Big Bang, “yet”? Even if we ever can, isn’t it basically that the “Big Bang” really wasn’t so big but rather a little local pop of a continuous effervescence of bubbles?
Looked at that way, aren’t we already *living *in the microverse? (Whoa, man, far out…but yeah, I think it’s cool that Sagan smoked a lot of dope and theorised on its effects in terms of left vs. right hemisphere of the brain.)
I also thought it was cool that he was willing to just state flat out that the origin of life is one of science’s great unanswered questions, without being defensive about it. Religions try to attack on points like that as a weakness, but it is in fact a strength (imagine a major religion being secure enough to admit they don’t know something, especially the answer to a major metaphysical question).
reaction to item #221 of asympotically fat:
The real meaning of particle - and cosmic event horizon is not yet clear to me, so I’m going to study that. The calculation example I mentioned in #220 is 100% based on Hubble’s constant H0, with which you can calculate the recession speed at a given distance from earth. If you know the redshift value of an object, you first calculate the object-speed and then, with the help of H0, you calculate at which distance from earth that speed is valid. At the moment the object released those measured photons, that object was at that distance, so in the past.
That was confirmed to me by astronomers. However, like I said, I don’t know about the horizons you mentioned, so I’m going to spend time to find information about it. Thanks!
The whole subject of various horizons and surfaces in cosmological models can be very confusing, you can read a lot of incorrect and misleading things about them because they can be a source of confusion even among astronomers and astrophysicists. The best way to understand the particle horizon is that if a galaxy is outside of your (current) particle horizon it means that you can’t currently see it and the best way to understand the meaning of the cosmic event horizon is that if a galaxy is currently outside of your current cosmic event horizon you will never at any point in the future be able to see the galaxy as it currently appears.
However even these descriptions are not the complete picture, for example it is technically possible (though not possible for practical purposes) to be able to observe objects that never enter the observable Universe as defined by the particle horizon if they are receding from us with a velocity additional to their recession velocity.
In practice you can only approximately calculate approximate distances from red shifts using the current value of Hubble’s constant for smaller red shifts, for larger red shifts it becomes completely inaccurate.
