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I understand that the so called observable universe has a limit we cannot see through.

My question is whether we might get information through an intermediate hypothetical actor located halfway to the limit. This actor would have, I assume, a limit of the observable universe further away than ours.

We, in turn, might interact with this actor in order to get information about a nova we cannot see from the Earth.

Information about such a star might perhaps have reached the Earth earlier at a time before it changed into a nova. The information about the nova would therefore be new.

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The simple answer is no, but it might be easier to understand if we look at the difference between the different cosmological horizons.

Since no information can travel faster than light, and nothing can have travelled for longer than the age of the Universe, then the limit of the observable universe is calculated by multiplying the speed of light by the conformal time since the Big Bang (the age of the Universe, adjusted for the scale factor representing the expansion of the Universe). This limit is also known as the particle horizon.

The current limit is a sphere of ~46.5 billion light-years in radius. If we could detect any signals emitted from matter that far away, we'd be seeing the Big Bang itself! However, the most distant light (EMR) signal we will ever be able to detect is the cosmic microwave background radiation (CMBR), which represents the earliest light after photon decoupling, emitted from a shell ~45.7 billion light-years away.

The CMBR was emitted about 380,000 years after the Big Bang, while the earliest stars didn't form for at least another 100 million years, and the earliest galaxy we've observed so far is GN-z11 which dates back to about 400 million years after the Big Bang. Nonetheless, there's been enough time since then for the light from those earliest structures – albeit highly redshifted and extremely weak – to reach us directly, so there's no need for an "intermediary".

While the particle horizon defines the current limit of the observable universe, what about future observations? In a Universe with accelerating expansion, the outer parts of the observable universe are now receding from us faster than the speed of light, and information from those areas may never reach us at all. The point at which light emitted now can never reach us is called the cosmic event horizon. The current cosmic event horizon is around 16 billion light-years away, which is a lot less than the ~46b ly to the edge of the observable universe.

To make this a bit clearer, let's look at the path the information would take via the intermediary. To avoid confusion, let's call your distant object a supernova rather than just a nova, and let's assume the intermediary is at the midpoint of a straight line between us and the supernova.

The information – whether light waves or gravitational waves – leaves the supernova and travels at the speed of light to the intermediary, where (let's say) the information is immediately relayed in a boosted signal – also travelling at the speed of light – towards Earth. Apart from the momentary delay in relaying the signal at the midpoint, the information will be following exactly the same path as if it had left the supernova and travelled directly to Earth. In other words, if we can receive the information about the distant supernova from the intermediary, we must also be able to receive the information straight from the supernova itself.

Now, in the case of a supernova that's just beyond our cosmic event horizon, the intermediary can receive the light from the supernova, but the space between us and the intermediary will have expanded in the billions of years it takes for the supernova's light to get to them. By that time, the Earth is receding from the intermediary so fast that their relayed signal can never overcome the accelerating expansion of space on the second leg of its journey. We never receive their signal, as they too have receded over the cosmic event horizon.

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This can't happen. One of the fundamental principles of relativity is "there is no overtaking of light by light". So the light carrying the message from the intermediary cannot overtake the light from the nova, which never gets to Earth thanks to the accelerating expansion of the universe.

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  • $\begingroup$ My question was based on a few seemingly simple ideas, i) the intermediary observer could be seen by us, ii) the intermediary had its own observable universe different from ours , where the nova could be seen (which we couldn’t see), and iii) the intermediary signals us the information (following from the first point). Where am I wrong? $\endgroup$ – Mikael Jensen Apr 14 '19 at 22:54
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    $\begingroup$ The observable universe changes over time. By the time the light from the nova reaches them, the intermediary observer will no longer be in our observable universe. $\endgroup$ – Steve Linton Apr 14 '19 at 23:05
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    $\begingroup$ @SteveLinton and, if the expansion of the universe wasn't accelerating, by the time we'd get the message from the intermediary, the observable universe would have grown enough to allow direct observations of the nova as well. $\endgroup$ – John Dvorak Apr 15 '19 at 1:51

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