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Observable universe

I just understood one interesting thing: the notion of "observable universe" is not limited to light (in the general sense). Gravity is also moving at the speed of light so whatever the masses laying outside of our observable universe, they do not affect us, will not affect us, will never affect us whatever they do.

Even weirder: the notion of "observable universe" is centered on the observer. So imagine a galaxy at the limits of our observable universe, but inside. Imagine it's affected by something lying just outside of OUR observable universe because that "something" is inside ITS observable universe. We'll never see how it is affected either. Because the total time of gravity application from source to galaxy and then of light travel between the galaxy and us is greater than the age of the universe.

In short, it's impossible, for all our observable universe, to observe any effect on anything inside it from outside it, even if that "anything" is closer to us. This drives me crazy, it's another event horizon.

Comments

1. On Friday 1 May 2020, 14:17 by le hollandais volant

« We'll never see how it is affected either. Because the total time of gravity application from source to galaxy and then of light travel between the galaxy and us is greater than the age of the universe. »

That’s not totally true: the observable universe is growing constantly: every second, we get light from 1 light-second farther beyond the observable universe.
If we ignore the expanding of the universe, the observable universe would have a radius of exactly one age-of-the-universe light-years, all the time. Since the universe is ageing at a rate of one second every second, our observable universe is expanding too, constantly.

However: the universe is expanding: space-time itself is stretching. A kilometre now will be slightly so longer in a minute from now.
Does that mean that some events at the border from our observable universe will get lost? No. Actually it’s the opposite: even if the universe is about 13.8 gigayears old, our observable universe has a radius of about 46.5 gigalight-years.

The expansion (as of today) rates about 96 km per megaparsec, every second (1 parsec = 3.6 light-years). That means that two immobile objects separated by 3.6 millions light-years now, will be separated by 3.6 Mly + 96 km in a second.
Even if 96 km is ridiculously small in comparison to 3.6 Mly, it is enough to create considerable amounts of distance between objects of billions of years.

Besides, this rate of expansion seems to accelerate. There will be a time when this rate be 100 km/Mpc/s, and then 1000, or 10000 km/Mpc/s… We do not know what causes this acceleration, but we call it dark-energy.

In an unforeseeable future, the rate of expansion might be enormous, and might be faster than the speed of light. Any objet we see now will be expanding so rapidly that its light will never reach us ever again.

First, the most distant galaxies will forever become invisible. Then, some closer ones… Only our so-called Local Group of galaxies are close enough to be gravitationally bound to us and not faint in the forever dark sky.
When that happens, if we look at the sky, we will once again think that we are the only galaxy in existence.

There will come a day when some distant cosmic objects vanishes in the darkness. But this means also that some already have as of today. My biggest unanswerable question is: what did *we* miss?

What unknoweable stuff lies ahead of our cosmological horizon? This, we will never ever know…