# controllable universe?

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If I choose a galaxy near the edge of the observable universe, and I send a ray of light toward this galaxy, will the light ever reach the galaxy?

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1 hour ago, Danijel Gorupec said:

If I choose a galaxy near the edge of the observable universe, and I send a ray of light toward this galaxy, will the light ever reach the galaxy?

No.

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what then is the approximate size of this 'controllable' universe? (That is, the part of the universe that we can affect in any way... I don't know if this universe is already named somehow; borrowed the term 'controllable' from the control theory).

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2 hours ago, Danijel Gorupec said:

If I choose a galaxy near the edge of the observable universe, and I send a ray of light toward this galaxy, will the light ever reach the galaxy?

Let me rephrase the question. If I send a ray of light in a direction exactly opposite this galaxy, will the light ever reach the galaxy? The universe is curved so it has no "edge".

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Posted (edited)

Galaxies at the "edges" of the observable universe are apparently  receding from us (due to space expansion) at faster then the speed of light. So that means that any light signal we send in their direction will never reach them.

That does not mean that Einstein's SR is invalidated for two reasons, (1) the law of "c" being the universal speed limit only applies to anything with mass, and (2) is harder to explain, so..... "A key feature of this expansion is how uniform it is. Imagine a bunch of folks standing around the edges of a stretchy piece of fabric, tugging at it. Let us assume they're choreographed well and are able to walk backward and pull at the same rate. You, standing in the middle, would correctly observe that your "universe" is expanding: any objects placed on that fabric would slowly move away from you.

Because stretchy stuff is stretchy, the objects on the fabric close to you would appear to move away with some speed, but the farther objects would appear to move faster. Even though the folks doing the pulling are moving at a constant speed, the apparent stretch changes with distance. I swear this is true; you can even try it for yourself at home!

Now, let's jump to the universe. It's as if a bunch of folks are at the edge of the cosmos, gently tugging at the fabric of space-time, stretching it. Edwin Hubble was the first to measure the expansion rate. The number he got was way wrong, so I won't bother mentioning it, but good on him for trying. The more modern value is 68 kilometers per second per megaparsec, plus or minus a couple, but close enough"

The other point is that we cannot yet say with any confidence whether the universe is finite or infinite, just that it appears flat, (as opposed to open and closed) according to latest scientific instruments like WMAP. It appears flat (which normally denotes infinite) within small error bars.

Edited by beecee
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4 hours ago, Danijel Gorupec said:

If I choose a galaxy near the edge of the observable universe, and I send a ray of light toward this galaxy, will the light ever reach the galaxy?

How powerful is the ray? Unless extraordinarily powerful no photons from it will hit that place - they'll scatter too far... and that specific galaxy will have moved, both after the light we detect from it was emitted and after the ray we send in return was emitted.

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Posted (edited)

The term for the volume of the universe in causal connection is 'observable' universe.
It is the sphere that defines how far we can possibly 'see' at any specific time.
Beyond that 'edge' light/signals/information will never reach us, and so they cannot affect us ( causally disconnected ).
At the time, before inflation, the universe must have been causally connected, to account for isotropy and homogeneity, otherwise it could not be in thermal equilibrium.
But after inflation, and as time progressess, the portion of the universe that is causally connected expands out of the observable universe, even as the observable universe is continuously getting larger, and eventually ( billions and billions of years into the future ) we may only be able to see galaxies that are gravitationally bound to ours, like the two Magellanic clouds and Andromeda galaxy.

Edited by MigL
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8 hours ago, Ken Fabian said:

How powerful is the ray? Unless extraordinarily powerful no photons from it will hit that place - they'll scatter too far... and that specific galaxy will have moved, both after the light we detect from it was emitted and after the ray we send in return was emitted.

All powerful. I want to know the theoretical distance that I can still 'touch' by any means possible.

9 hours ago, beecee said:

The more modern value is 68 kilometers per second per megaparsec, plus or minus a couple, but close enough"

I was playing with this number a little, and if my back-of-envelope calculations are any good, then my light ray will not be able to reach an object that is currently farther than cca 5.3 billion light years (and the ray will reach an object at this limiting distance in about 14 billion years).

If my calculations are correct, then this 'controllable' universe is much smaller than the observable universe.

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9 hours ago, MigL said:

we may only be able to see galaxies that are gravitationally bound to ours, like the two Magellanic clouds and Andromeda galaxy.

By that time, we will be merged into one larger galaxy anyway. 😉

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